Renewable Energy Sources - 5 Units Important Notes

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 UNIT – I

GLOBAL AND NATURAL ENERGY SCENARIO

Over view of conventional & renewable energy sources

Conventional energy sources such as natural gas, oil, coal, or nuclear are finite but still hold the majority of the energy market. However, renewable energy sources like wind, fuel cells, solar, biogas/biomass, tidal, geothermal, etc. are clean and abundantly available in nature and hence are competing with conventional energy sources. Among the renewable energy sources wind energy has a huge potential of becoming a major source of renewable energy for this modern world. Wind power is a clean, emissions-free power generation technology. As per the Global Wind Energy Council (GWEC) 2013 statistics, cumulative global capacity has reached to a total of 318 GW, which shows an increase of nearly 200 GW in the past 5 years. GWEC predicts that wind power could reach nearly 2000 GW by 2030, supply between 16.7% and 18.8% of global electricity and help save over 3 billion tons of CO2 emissions annually. From this scenario, it is clear that wind power is going to dominate the renewable as well as the conventional energy market in the not too distant future. Wind energy is the only power generation technology that can deliver the necessary cuts in CO2 emissions from the power sector in the critical period up to 2020, when greenhouse gas emissions must peak and begin to decline if we are to have any hope of avoiding the worst impacts of climate change. However, grid integration, voltage, and power fluctuation issues should adequately be addressed due to the huge penetration of wind power to the grid.

Need & Development of renewable energy sources:

The term renewable energy refers to energy sources that are in nature and are renewed in whole or in part, in particular, the energy of watercourses, wind, non-accumulated solar energy, biomass, geothermal energy, and so on. The use of these sources contributes to the more efficient use of their own potentials in energy production, reduction of greenhouse gas emissions, reduction of fossil fuel imports, development of local industry and job creation. Renewable energy technologies are clean, which have a much less environmental impact than conventional energy technologies.

Types of renewable energy systems

A renewable energy source means energy that is sustainable - something that can't run out, or is endless, like the sun. When you hear the term 'alternative energy' it's usually referring to renewable energy sources too. It means sources of energy that are alternative to the most commonly used non-sustainable sources like coal.

Types of renewable energy

Alternative or renewable energy comes from natural processes that (unlike those listed above) can reliably produce cheap energy with minimal impact to the environment.

The most popular renewable energy sources currently are:

1. Solar energy

2. Wind energy

3. Hydro energy

4. Tidal energy

5. Geothermal energy

6. Biomass energy

1. Solar energy:

Sunlight is one of our planet’s most abundant and freely available energy resources. The amount of solar energy that reaches the earth’s surface in one hour is more than the planet’s total energy requirements for a whole year. Although it sounds like a perfect renewable energy source, the amount of solar energy we can use varies according to the time of day and the season of the year as well as geographical location. 

2. Wind energy:

Wind is a plentiful source of clean energy. To harness electricity from wind energy, turbines are used to drive generators which then feed electricity into the National Grid. Although domestic or ‘off-grid’ generation systems are available, not every property is suitable for a domestic wind turbine. 

3. Hydro energy:

As a renewable energy resource, hydro power is one of the most commercially developed. By building a dam or barrier, a large reservoir can be used to create a controlled flow of water that will drive a turbine, generating electricity. This energy source can often be more reliable than solar or wind power (especially if it's tidal rather than river) and also allows electricity to be stored for use when demand reaches a peak. Like wind energy, in certain situations hydro can be more viable as a commercial energy source (dependant on type and compared to other sources of energy) but depending very much on the type of property, it can be used for domestic, ‘off-grid’ generation. 

4. Tidal energy:

This is another form of hydro energy that uses twice-daily tidal currents to drive turbine generators. Although tidal flow unlike some other hydro energy sources isn’t constant, it is highly predictable and can therefore compensate for the periods when the tide current is low. 

5. Geothermal energy:

By harnessing the natural heat below the earth’s surface, geothermal energy can be used to heat homes directly or to generate electricity. Although it harnesses a power directly below our feet, geothermal energy is of negligible importance in the UK compared to countries such as Iceland, where geothermal heat is much more freely available.

 

 

6. Biomass energy:

This is the conversion of solid fuel made from plant materials into electricity. Although fundamentally, biomass involves burning organic materials to produce electricity, this is not burning wood, and nowadays this is a much cleaner, more energy-efficient process. By converting agricultural, industrial and domestic waste into solid, liquid and gas fuel, biomass generates power at a much lower economical and environmental cost.

Future of Energy use:

Fossil fuels like coal, oil and natural gas supply 80 percent of the world’s energy to warm homes, charge devices and power transportation. They are also the primary human source of greenhouse gas emissions. Stanford scientists broadly agree that curtailing our use of fossil fuels would have significant benefits – like improving health and reducing the number and severity of natural disasters – but it’s not yet clear what can replace them.

Wind and solar are increasingly popular sources of energy, but the sun does not always shine, and the wind doesn’t always blow. Batteries to store their intermittent energy are not yet cheap and powerful enough to fill the gaps. Nuclear energy produces no greenhouse gases directly, but the current generation of reactors has other problems. Solutions like storing carbon dioxide underground or turning it into clean fuel are promising, but they also need much development. None of the possible solutions is without challenges.

Energy for sustainable development:

Energy is one of the major inputs for the economic development of any country. In the case of the developing countries, the energy sector assumes a critical importance in view of the ever increasing energy needs requiring huge investments to meet them.

Energy can be classified into several types based on the following criteria:

• Primary and Secondary energy

• Commercial and Non-commercial energy

• Renewable and Non-Renewable energy

1. Primary and Secondary Energy:

Primary energy sources are those that are either found or stored in nature. Common primary energy sources are coal, oil, natural gas, and biomass (such as wood). Other primary energy sources available include nuclear energy from radioactive substances, thermal energy stored in earth's interior, and potential energy due to earth's gravity. The major primary and secondary energy sources are shown in Figure 1.1 Primary energy sources are mostly converted in industrial utilities into secondary energy sources; for example coal, oil or gas converted into steam and electricity. Primary energy can also be used directly. Some energy sources have non-energy uses, for example coal or natural gas can be used as a feedstock in fertiliser plants.

2. Commercial Energy and Non Commercial Energy:

Commercial Energy

The energy sources that are available in the market for a definite price are known as commercial energy. By far the most important forms of commercial energy are electricity, coal and refined petroleum products. Commercial energy forms the basis of industrial, agricultural, transport and commercial development in the modern world. In the industrialized countries, commercialized fuels are predominant source not only for economic production, but also for many household tasks of general population.

Examples: Electricity, lignite, coal, oil, natural gas etc.

Non-Commercial Energy

The energy sources that are not available in the commercial market for a price are classified as non-commercial energy. Non-commercial energy sources include fuels such as firewood, cattle dung and agricultural wastes, which are traditionally gathered, and not bought at a price used especially in rural households. These are also called traditional fuels. Non-commercial energy is often ignored in energy accounting.

Example: Firewood, agro waste in rural areas; solar energy for water heating, electricity generation, for drying grain, fish and fruits; animal power for transport, threshing, lifting water for irrigation, crushing sugarcane; wind energy for lifting water and electricity generation.

3. Renewable and Non-Renewable Energy:

Renewable energy is energy obtained from sources that are essentially inexhaustible. Examples of renewable resources include wind power, solar power, geothermal energy, tidal power and hydroelectric power .The most important feature of renewable energy is that it can be harnessed without the release of harmful pollutants. Non-renewable energy is the conventional fossil fuels such as coal, oil and gas, which are likely to deplete with time.

Energy for sustainable development:

Helps curb global warming and contributes to the protection of human health caused by air pollution; and Enhances energy security through reliance on domestic energy sources.

Potential of renewable energy sources:

The most sustainable energy sources are renewable bioenergy (wood, biomass, energy crops), geothermal (deep or shallow), solar energy (photovoltaic, solar thermal), hydro and wind energy. Since much more, orders of magnitudes more, solar energy hits the earth than is required for human needs, the total potential of renewable energies seems to be almost infinite. It should be noted that, with respect to our discussion about energy here, the term "potential" is not the same as in physics a better term would be "availability". Also, the terms "renewable energy" and "energy sources" do not make sense physically, since in physics the energy conservation law prohibits a source or renewal of energy; only transformations are allowed. From a physical point of view, it would be better to formulate this as "availability of sustainable energies" instead of "potential of renewable energies".

Renewable electricity and key concepts:

Economic growth, automation, and modernization mainly depend on the security of energy supply. Global energy demand is rapidly growing, and, presently, the worldwide concern is on how to satisfy the future energy demand. Long-term projections indicate that the energy demand will rapidly increase worldwide. To supply this energy demand, fossil fuels have been used as primary energy sources. Fossil fuels emit greenhouse gases that highly affect the environment and the future generation .The emissions largely depend on the emission factor of primary energy sources (i.e., input fuel of the plant). Among all energy sources, the emission factor of fossil fuels (i.e., coal, natural gas, and oil) is very high, as shown in Table 1. Fossil fuels are widely used as the main fuel in power generation. In Malaysia, fossil fuels (i.e., natural gas [53.3%] and coal [26.3%]) serve as major power generation sources, as shown in Figure 1. Large-scale use of fossil fuels, however, greatly affects the environment. Based on the global CO2 distribution in 2013, the emission breakdown is as follows: coal (43%), oil (33%), gas (18%), cement (5.3%), and gas flare (0.6%).

Fuel Emission factor (kg/kWh) CO2 SO2 Coal 1.1800 0.019 0.0052 Petroleum 0.8500 0.0164 0.0025 Gas 0.5300 0.0005 0.0009

Table 1 

Emission factors of fossil fuels for electricity generation.

Figure 1 

Share of installed capacity.

Meanwhile, renewable energy sources (solar, wind, hydro, geothermal, biomass, etc.) are emission-free energy sources in the world. Renewable energy technologies are an ideal solution because they can contribute significantly to worldwide power production with less emission of greenhouse gases [8–11]. The “sustainable future” scenario of the International Energy Agency (IEA) shows 57% of world electricity being provided by renewable energy sources by 2050. Long-term forecast and planning is required to achieve this ultimate target. Renewable energy-based power generation and supply to the national grid for a specific zone are necessary. The conventional grid aggregates the multiple networks, and the regulation system consists of various levels of communication and coordination, in which most of the systems are manually controlled. A smart grid is a new concept that leads to the modernization of the transmission and distribution grid. The smart grid system is the digital upgrade of transmission and new markets for the alternative energy generation of renewable energy sources. Presently, smart grid is an often-cited term in the energy generation and distribution industry.

Smart grid connected with distributed power generation is a new platform that significantly generates reliable security of supply (SOS) and quality of electric energy. This concept is practical and reliable as numerous types of energy sources become available, such as solar, wind, biomass, and hydropower. Renewable and nonconventional energy sources are allowed to integrate with the distributed power generation link that has a smart grid. This study therefore highlights the role of renewable energy sources in generating electricity and the integration with the smart grid system for energy security.

Global Climate Change:

Global climate change has already had observable effects on the environment. Glaciers have shrunk, ice on rivers and lakes is breaking up earlier, plant and animal ranges have shifted and trees are flowering sooner.

Effects that scientists had predicted in the past would result from global climate change are now occurring: loss of sea ice, accelerated sea level rise and longer, more intense heat waves.

CO2 reduction potential of renewable energy:

The Indian power sector is experiencing a lot of pressure to supply sustainable electricity at affordable cost due to heavy demand especially in the summer peak season. Most of India's electricity is produced by fossil fuelled power plants, which are the source of CO2 emissions. In this case, renewable energy sources play a vital role in securing sustainable energy without environmental emissions. This paper examines the effects of renewable energy use in electricity supply systems and estimates the CO2 emissions by developing various scenarios under the least cost approach. The LEAP energy model is used to develop these scenarios. The results show that in an ARET (accelerated renewable energy technology) scenario, 23% of electricity is generated by renewables only, and 74% of CO2 reduction is possible by 2050. If the maximum energy savings potential is combined with the ARET scenario, the renewables share in electricity supply rises to 36% as compared to the reference scenario, while the CO2 emission reduction in this case remains at 74%.

Hybrid Systems:

A hybrid system is a dynamical system that exhibits both continuous and discrete dynamic behaviour – a system that can both flow (described by a differential equation) and jump (described by a state machine or automaton). Often, the term "hybrid dynamical system" is used, to distinguish over hybrid systems such as those that combine neural nets and fuzzy logic, or electrical and mechanical drivelines. A hybrid system has the benefit of encompassing a larger class of systems within its structure, allowing for more flexibility in modelling dynamic phenomena.

 

UNIT – II

SOLAR ENERGY

Solar energy system:

A solar power system is made up of multiple photovoltaic (PV) panels, a Dc to AC power converter (called inverter) and a rack system that holds the PV panels in place.

Solar Photovoltaic (PV) panels are generally fitted on the roof. They should face in an easterly, northerly or westerly direction. The panels should be tilted at particular angles to maximize the amount of sunlight that hits the panels.

Solar PV panels on the roofs of homes and businesses generate clean electricity by converting the energy in sunlight. This conversion takes place within solar panels of specially fabricated materials that make up the solar panels. It is a process that requires no moving parts. In most cases solar panels are connected to the mains power supply through a device called a solar power inverter.

Solar panels are different to solar hot water systems, which are also mounted on household roof-tops but use the heat from the sun to provide hot water for household uses, in a similar principle like a hose in summer contains hot water after a few hours in the sun.

The Solar System is the gravitationally bound system of the sun and the objects that orbit it, eight directly or indirectly. Of the objects that orbit the sun directly, the largest are the eight planets, with the remainder being smaller objects, the draft planets and small solar system bodies.

Solar Radiation:

A solar radiation sensor measures solar energy from the sun. Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum.
The units of measure are Watts per square meter.
The device is typically used in agricultural applications, and is used in the calculation of Evapotranspiration. Evapotranspiration is the potential for evaporation of moisture from the soil (or the reverse of rainfall) and is a function solar energy, wind and temperature.

Availability, Measurement and Estimation:

Solar energy is emitted in huge amounts from the sun's surface but only a fraction of that reaching the earth's surface can be converted into useful forms of energy for utilization either as thermal, electrical or mechanical. For desalination purposes solar energy can be used either directly to distil water in solar stills or can be utilized indirectly to drive conventional desalination plants. In indirect use of solar energy solar radiation has to be collected and converted into either heat or electricity. The transformed solar radiation can then be applied to drive one of the conventional desalination systems. Thus in an indirect utilization of solar energy two separate plants are involved: a solar thermal or electricity plant and the desalination plant. Collectors or concentrators are used to convert the radiation into thermal energy in the form of hot water or steam. The steam generated is of low or high temperature, depending from the type of the solar collector field. Low and medium temperature steam is used as process heat in industry, including desalination plants, or as space heating. Electricity is produced either by high temperature steam in solar power systems or by solar cells in photovoltaic fields.

The irregularity of solar radiation intensity affects the normal and smooth operation of the desalination plants, which operate in a discontinuous and unsteady state basis if storage is not provided. Storage devices are necessary for continuous operation, during night time or during cloudy days, but are also capital intensive enterprises adding to the already high cost of solar energy conversion systems. Although there are these disadvantages, thermal solar energy systems are suitable for providing energy to medium or small size desalination plants in sites where there is abundant and intensive solar radiation. Normally all solar powered plants offer energy, at a reasonable cost, suitable to use in desalination facilities. Solar energy may be available in some form everywhere but it is not available at all times anywhere on the earth. To determine whether and how solar energy is available and, or, usable on an economic basis precise meteorological data must be known, and all data concerning the incident solar radiation of the region. To operate all solar plants and desalination solar systems, the spatial and the temporal, i.e. the hourly, daily and seasonal, variability data of solar radiation is of importance these data are also indispensable for the proper design of a solar driven system, and for the proper selection of devices and methods to be used. Their most important utilization is in the evaluation of the performance of both plants. A detailed understanding of the spatial, temporal and the spectral characteristics of the solar radiation is attained through detailed and exact measurements, detailed analysis of the data selected, and computer modelling of the variations in solar radiation. It is thus very important for an engineer dealing with solar desalination to know all about the sun and its global radiation emitted around the year.

Thermal Conversion Device and Storage:

Energy is not a .good unto itself; it is valued rather as a means of satisfying important needs of a society. In classical thermodynamics, energy is defined as the capacity to do work; but from a more practical point of view, energy is the main stay of any industrial society. In the United States, energy is currently provided by seven primary sources: petroleum, natural gas, coal, hydro-power, nuclear fission, geothermal, and wood and waste. The first three of these sources are fossil fuels. They are stored forms of solar energy that received their solar input eons ago, have changed their characteristics over time, and now are in a highly concentrated and convenient form. It is apparent, however, that these stored forms of solar energy are being used so rapidly that they soon will be depleted. To maintain our present social structure, it is desirable, therefore, that we supply an increasing portion of our energy needs from renewable sources. The radiative solar energy reaching the earth during each month is approximately equivalent to the entire world supply of fossil fuels. Thus, from a purely thermodynamic point of view, the global potential of solar energy is many times larger than the current energy use. However, many technical and economic problems must be solved before large-scale use of solar energy can occur. The future of solar power deployment depends on how we deal with these constraints, which include scientific and technological problems, marketing and financial limitations, and political and legislative actions including equitable taxation of renewable energy sources. Approximately 30 percent of the solar energy impinging on the earth is reflected back into space. The remaining 70 percent, approximately 120,000 terawatts [l terawatt is equal to 1012 watts], is absorbed by the earth and its atmosphere. Solar radiation reaching the earth consists of the beam radiation that casts a shadow and can be concentrated and the diffuse radiation that has been scattered along its path in space from sun to earth. The solar radiation reaching the earth degrades in several ways. Some of the radiation is directly absorbed as heat by the atmosphere, the ocean, and the ground. Another component produces atmospheric and oceanic circulation. A third component evaporates, circulates, and precipitates water in the hydrologic cycle. Finally, a very small fraction is captured by green plants and drives the photosynthetic process. 1 For solar energy to be used in meeting the demands of a society, it must be converted into heat, mechanical power, or electricity. The conversion methods can be divided into natural and technological conversion systems (see Figure O. In natural conversion, the biosphere, i.e, earth, wind or water, serves as a solar energy collector and storage. Since no man-made collectors are needed, the cost of energy from natural systems is largely determined by the conversion equipment, such as a wind turbine. In technological conversion systems, solar energy must be absorbed by man-made structures or collectors; the amount of insolation intercepted is determined by the total area and orientation of the collecting surface at a given geographic location (Kreith and Kreider 1978). The source of the sun's energy is a hydrogen-to-helium thermonuclear reaction. The outer layer of the sun, from which the solar radiation emanates, has an equivalent black body temperature of about 5760 K (5487° C). The solar energy reaching the earth, called insolation, is in the form of photons, or radiation, covering a range of wavelengths corresponding approximately to a 5760 K black body. To convert this radiation into useful energy, one may either use photons in the appropriate wavelength range of the spectrum to generate electricity directly by photovoltaic conversion devices; or one may use-the thermal part of the radiation spectrum to heat a working fluid by thermal conversion in a solar collector. The following discussion is concerned only with solar thermal conversion systems. The thermal conversion process of solar energy is based on well-known phenomena of heat transfer (Kreith 1976). In all thermal conversion processes, solar radiation is absorbed at the surface of a receiver, which contains in contact with flow passages through which a working fluid passes. As the receiver heats up, heat is transferred to the working fluid which may be air, water, oil, or a molten salt. The upper temperature that can be achieved in solar thermal conversion depends on the insolation, the degree to which the sunlight is concentrated, and the measures taken to reduce heat losses from the working fluid. Since the temperature level of the working fluid can be controlled by the velocity at which it is circulated, it is possible to match solar energy to the load · requirements, not only according to the amount but also according to the temperature level, Le., the quality of the energy required. In this manner, it is possible to design conversion systems that are optimized according to both the first and the second laws of thermodynamics. 2 The collection and conversion of the solar radiation to thermal energy depends on the collector design and the relative amounts of direct beam and diffuse radiation absorbed by the collector (Krieger and Keith 1981). As indicated in the following discussion of solar thermal collectors, the collectors used for higher temperature applications can collect only the direct radiation from the sun. Figure 2 shows the annual average daily direct normal solar radiation for the contiguous United States, Alaska, and Hawaii; values range from under 2.78 kW/hr./m2 (10 MJ/m2) to over 7.22 kW/hr./m2 (26 MJ/m2) (Solar Energy Research Institute 1981). Peak direct solar radiation at noon during a clear day averages about I kW/m2• Generally speaking, the south-western and western regions of the country receive direct normal solar radiation levels sufficiently high for most high temperature solar thermal conversion applications. High temperature heat is needed by industry for process heat and by utilities for electricity. In 1980, the last year for which statistics are available, industry and utilities accounted for approximately 73 percent of the 76.3 quads of energy consumed in the United States (Energy Information Administration 1980). The industrial process heat portion alone was 20.6 quads (17 percent). Figure 3 displays a recent analysis by the Solar Energy Research Institute (Krawiec et al 1981) of the distribution of industrial process heat requirements by process temperature. It can be seen that 48.9 percent of the process heat total falls below 500° F (260° C) and 34.0 percent is above 10aaoF (538°C).

Applications Solar Photovoltaic Conversion Solar Photovoltaic:

Applications of solar photovoltaic cells:

• When a number of modules are connected to the grid PV system via an inverter, they transform the DC current generated by the solar PV modules to AC current. The electricity generated can be used for lighting purposes and powering household appliances. The excess electricity can be sold to the grid directly.

• Individual solar PV modules can be used for powering torches, flashlights, wrist watches, etc. in remote and rural locations.

 

 

Features of solar photovoltaic cell:

• Solar PV modules have aluminium frames that are attached with tapes directly on to the silicon or laminate. These frames are useful for increasing the mechanical strength of PV modules and making the installation process easier.

• Manufacturers conduct a series of tests for measuring the electricity generated by PV modules using a sun simulator. The sun simulator is designed to produce specific light conditions for measuring the peak power generated by the solar module.

• Thin-film solar modules are made up of layers whose thickness is only 2 microns; they are up to 40 times thinner than a human hair strand.

• Polycrystalline solar modules are made of PV cells comprising of numerous small silicon crystals. Polycrystalline modules are more cost-effective but less efficient than monocrystalline solar PV modules.

• Monocrystalline solar panels occupy very less space and are space efficient. The cells present in them are manufactured from single silicon crystals to form wafers that are about 0.2 mm in thickness. The cells in monocrystalline solar panels are highly efficient in their electricity bearing capacity.

Solar Thermal:

Solar thermal technologies capture the heat energy from the sun and use it for heating and/or the production of electricity. This is different from photovoltaic solar panels, which directly convert the sun’s radiation to electricity.

There are two main types of solar thermal systems for energy production – active and passive.  Active systems require moving parts like fans or pumps to circulate heat-carrying fluids. Passive systems have no mechanical components and rely on design features only to capture heat (e.g. greenhouses).  The technologies are also grouped by temperature - low, medium or high.

• Low-temperature (<100°C) applications typically use solar thermal energy for hot water or space heating (Boyle, 2004). Active systems often consist of a roof-mounted flat plate collector through which liquid circulates. The collector absorbs heat from the sun and the liquid carries it to the desired destination, for example a swimming pool or home heating system. Passive heating systems involve intelligent building design practices, which cut back on the need for heating or cooling systems by better capturing or reflecting solar energy. 

• Medium-temperature (100-250°C) applications are not common. An example would be a solar oven, which uses a specially-shaped reflector to focus the sun’s rays on a central cooking pot. Similar systems could be used for industrial processes, but are not widely used.

• High-temperature (250°C >) solar thermal systems use groups of mirrors to concentrate solar energy onto a central collector. These concentrated solar power (CSP) systems can reach temperatures high enough to produce steam, which then turns a turbine, driving a generator to produce electricity.

Applications of solar energy systems:
Some of the major application of solar energy are as follows:

1. Solar water heating

2.  Solar heating of buildings

3. Solar distillation

4. Solar pumping

5. Solar drying of agricultural and animal products

6. Solar furnaces

7. Solar cooking 

8.  Solar electric power generation

9. Solar thermal power production

10. Solar green houses

(a)Solar Water Heating:

A solar water heating unit comprises a blackened flat plate metal collector with an associated metal tubing facing the general direction of the sun. The plate collector has a transparent glass cover above and a layer of thermal insulation beneath it.

The metal tubing of the collector is connected by a pipe to an insulated tank that stores hot water during cloudy days. The collector absorbs solar radiations and transfers the heat to the water circulating through the tubing either by gravity or by a pump.

This hot water is supplied to the storage tank via the associated metal tubing. This system of water heating is commonly used in hotels, guest houses, tourist bungalows, hospitals, canteens as well as domestic and industrial units.

(b) Solar Heating of Buildings:

Solar energy can be used for space heating of buildings in many ways namely:

(a) Collecting the solar radiation by some element of the building itself i.e. solar energy is admitted directly into the building through large South-facing windows.

(b) Using separate solar collectors which may heat either water or air or storage devices which can accumulate the collected solar energy for use at night and during inclement days.

When the building requires heat then from these collectors or storage devices, the heat is transferred by conventional equipment such as fan, ducts, air outlets, radiators and hot air registers etc. to warm up the living spaces of a building.

When the building does not require heat, the heated air or water from the collector can be moved to the heat storage device such as well insulated water tank or other heat holding material. For inclement days, an auxiliary heating system using gas, oil or electricity is required as a backup system.

(c) Solar-Distillation:

In arid semi and or coastal areas there is scarcity of potable water. The abundant sunlight in these areas can be used for converting saline water into potable distilled water by the method of solar distillation. In this method, solar radiation is admitted through a transparent air tight glass cover into a shallow blackened basin containing saline water.

Solar radiation passes through the covers and is absorbed and converted into heat in the blackened surface causing the water to evaporate from the brine (impure saline water). The vapours produced get condensed to form purified water in the cool interior of the roof.

The condensed water flows down the sloping roof and is collected in the troughs placed at the bottom and from there into a water storage tank to supply potable distilled water in areas of scarcity, in colleges, school science laboratories, defines labs, petrol pumps, hospitals and pharmaceutical industries. Perliter distilled water cost obtained by this system is cheaper than distilled water obtained by other electrical energy-based processes.

(d) Solar – Pumping:

In solar pumping, the power generated by solar-energy is utilized for pumping water for irrigation purposes. The requirement for water pumping is greatest in the hot summer months which coincide with the increased solar radiations during this period and so this method is most appropriate for irrigation purpose. During periods of inclement weather when solar radiations are low then the requirement for water pumping is also relatively less as the transpiration losses from the crops are also low.

(e) Solar Drying of Agricultural and Animal Products:

This is a traditional method of utilising solar energy for drying of agricultural and animal products. Agricultural products are dried in a simple cabinet dryer which consists of a box insulated at the base, painted black on the inner side and covered with an inclined transparent sheet of glass.

At the base and top of the sides ventilation holes are provided to facilitate the flow of air over the drying material which is placed on perforated trays inside the cabinet. These perforated trays or racks are carefully designed to provide controlled exposure to solar radiations.

Solar drying, especially of fruits improves fruit quality as the sugar concentration increases on drying. Normally soft fruits are particularly vulnerable to insect attack as the sugar content increases on drying but in a fruit dryer considerable time is saved by quicker drying —minimizing gap the chances of insect attack.

The present practice of drying chilies by spreading them on the floor not only requires a lot of open space and manual labour for material handling but it becomes difficult to maintain its quality and taste unless drying is done in a controlled atmosphere. Moreover, the products being sun dried very often get spoiled due to sudden rains, dust storms or by birds. Besides, reports reveal that it is not possible to attain very low moisture content in the sun-dried chilies.

(f) Solar Furnaces:

In a Solar furnace, high temperature is obtained by concentrating the solar radiations onto a specimen using a number of heliostats (turn-able mirrors) arranged on a sloping surface. The solar furnace is used for studying the properties of ceramics at extremely high temperatures above the range measurable in laboratories with flames and electric currents.

Heating can be accomplished without any contamination and temperature can be easily controlled by changing the position of the material in focus. This is especially useful for metallurgical and chemical operations. Various property measurements are possible on an open specimen. An important future application of solar furnaces is the production of nitric acid and fertilizers from air.

(g) Solar Cooking:

A variety of fuel like coal, kerosene, cooking gas, firewood, dung cakes and agricultural wastes are used for cooking purposes. Due to the energy crisis, supply of these fuels are either deteriorating (wood, coal, kerosene, cooking gas) or are too precious to be wasted for cooking purposes (cow dung can be better used as manure for improving soil fertility). This necessitated the use of solar energy for cooking purposes and the development of solar cookers. A simple solar cooker is the flat plate box type solar cooker.

It consists of a well-insulated metal or wooden box which is blackened from the inner side. The solar radiations entering the box are of short wavelength. As higher wavelength radiations are unable to pass through the glass covers, the re-radiation from the blackened interior to outside the box through the two glass covers is minimised, thereby minimising the heat loss.

The heat loss due to convection is minimised by making the box airtight. This is achieved by providing a rubber strip between the upper lid and the box for minimising the heat loss due to conduction, the space between the blackened tray and outer cover of the box is filled with an insulting material like glass wool, saw-dust, paddy husk etc.

When placed in sunlight, the solar rays penetrate the glass covers and are absorbed by the blackened surface thereby resulting in an increase in temperature inside the box. Cooking pots blackened from outside are placed in the solar box.

The uncooked food gets cooked with the heat energy produced due to increased temperature of the solar box. Collector area of such a solar cooker can be increased by providing a plane reflector mirror. When this reflector is adjusted to reflect the sun rays into the box, then a 15°C to 25°C rise in temperature is achieved inside the cooker box.

The solar cooker requires neither fuel nor attention while cooking food and there is no pollution, no charring or overflowing of food and the most important advantage is that nutritional value of the cooked food is very high as the vitamins and natural tastes of the food are not destroyed.

Maintenance cost of the solar cooker is negligible. The main disadvantage of the solar cooker is that the food cannot be cooked at night, during cloudy days or at short notice. Cooking takes comparatively more time and chapattis cannot be cooked in a solar cooker.

(h) Solar Electric Power Generation:

Electric energy or electricity can be produced directly from solar energy by means of photovoltaic cells. The photovoltaic cell is an energy conversion device which is used to convert photons of sunlight directly into electricity. It is made of semi-conductors which absorb the photons received from the sun, creating free electrons with high energies.

These high energy free electrons are induced by an electric field, to flow out of the semiconductor to do useful work. This electric field in photovoltaic cells is usually provided by a p-n junction of materials which have different electrical properties. There are different fabrication techniques to enable these cells to achieve maximum efficiency.

These cells are arranged in parallel or series combination to form cell modules. Some of the special features of these modules are high reliability, no expenditure on fuel, minimum cost of maintenance, long life, portability, modularity, pollution free working etc.

Photovoltaic cells have been used to operate irrigation pumps, rail road crossing warnings, navigational signals, highway emergency call systems, automatic meteorological stations etc. in areas where it is difficult to lay power lines.

They are also used for weather monitoring and as portable power sources for televisions, calculators, watches, computer card readers, battery charging and in satellites etc. Besides these, photovoltaic cells are used for the energisation of pump sets for irrigation, drinking water supply and for providing electricity in rural areas i.e. street lights etc.

(i)Solar Thermal Power Production:

Solar thermal power production means the conversion of solar energy into electricity through thermal energy. In this procedure, solar energy is first utilised to heat up a working fluid, gas, water or any other volatile liquid. This heat energy is then converted into mechanical energy m a turbine. Finally a conventional generator coupled to a turbine converts this mechanical energy into electrical energy.

Production of Power through Solar Ponds:

A solar pond is a natural or artificial body of water utilised for collecting and absorbing solar radiation and storing it as heat. It is very shallow (5-10 cm deep) and has a radiation absorbing (black plastic) bottom. It has a curved fibre glass cover over it to permit the entry of solar radiation but reduces losses by radiation and convection (air movement). Loss of heat to the ground is minimised by providing a bed of insulating material under the pond.

Solar ponds utilise water for collecting and storing the solar energy which is used for many applications such as space heating, industrial process heating and to generate electricity by driving a turbine powered by evaporating an organic fluid with a low boiling point.

(j) Solar Green Houses:

A green house is a structure covered with transparent material (glass or plastic) that acts as a solar collector and utilises solar radiant energy to grow plants. It has heating, cooling and ventilating devices for controlling the temperature inside the green house.

Solar radiations can pass through the green house glazing but the thermal radiations emitted by the objects within the green house cannot escape through the glazed surface. As a result, the radiations get trapped within the green house and result in an increase in temperature.

As the green house structure has a closed boundary, the air inside the greenhouse gets enriched with CO2 as there is no mixing of the greenhouse air with the ambient air. Further, there is reduced moisture loss due to restricted transpiration. All these features help to sustain plant growth throughout the day as well as during the night and all year round.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UNIT – III

WIND ENERGY

Wind Energy Conversion:

A wind energy conversion system (WECS) is powered by wind energy and generates mechanical energy that sends energy to the electrical generator for making electricity. Fig. 3.1 shows the interconnection of a WECS. The generator of the wind turbine can be a permanent magnet synchronous generator (PMSG), doubly fed induction generator, induction generator, synchronous generator, etc. Wind energy acquired from the wind turbine is sent to the generator. To achieve maximum power from the WECS, the rotational speed of the generator is controlled by a pulse width modulation converter. The output power of the generator is supplied to the grid through a generator-side converter and a grid-side inverter. A wind farm can be distributed in onshore, offshore, seashore, or hilly areas. The WECS might be the most promising DG for future SG.

Figure 3.3. Wind energy conversion system.

Wind energy is an alternative to fossil fuels, it is plentiful, renewable, widely distributed, clean, low cost, produces no emissions during operation, and uses a tiny land area . The effects on the environment are generally less problematic than those from other conventional power sources. Due to the variable wind speed, the output power of the WECS fluctuates and may create a frequency deviation of the power grid. To solve this problem, much research has already been conducted.

Potential, Wind energy Potential Measurement:

Solar and wind energy potential depends on climate, that is, on the availability of solar radiation and wind, but also on the amount of suitable surfaces to be covered with solar panels and on the texture of the settlement, which affects wind velocity. Biomass potential depends on the neighbourhood design, as it includes wood and leaves derived from trees being pruned and bushes being trimmed in the parks, green spots, tree-lined streets, and on the existence and size of plots dedicated to urban agriculture.

The use of renewable energy technologies is a very challenging issue in neighbourhood design, as it may impose significant constraints. PV systems, for example, are affected by the roofs’ size, which constrains the production potential. The possible consequence is that the maximum buildings’ height is limited by the amount of electricity needed by its inhabitants, if the goal is energy self-sufficiency.

PV systems could be used for supplying electricity to electric cars fleets, and the ideal would be to park these cars in dedicated outdoor parking plots equipped with PV canopies; in this case, the challenge is to optimize the size and the position of the parking lot in relation to the number of cars and of the PV area needed to charge them.

In windy areas, rooftop wind turbines up to 3–4 kW can be used. Wind turbines up to 20 kW are consistent with the urban context and may contribute to the energy system, if they are positioned in open spaces such as service areas or urban agriculture plots.

Biogas production from wastewater requires an appropriate design of the sewerage and the provision for the necessary space to host the anaerobic digestion plant.

Syngas production requires space allocation not only for the gasifier, but also for wood storage and pre-processing.

The main problem with solar and wind energy is that electricity production is not programmable, as PV systems cannot produce by night, and both PV and wind systems produce according to the meteorological conditions; thus it is very unlikely that demand and supply will match.

The easiest solution is to be connected to the main grid, which provides the lacking power when the renewable production is insufficient, and absorbs power when the production exceeds the demand.

If a connection to the main grid is not available, or the power supply is unreliable, there are two options, used often in combination. The first option is electricity storage by means of batteries, or other storage technology. The second is a generator supplied with programmable energy sources, such as fossil and biomass energy. A control system is necessary for the management of both the storage and the generator, to regulate their output so that instantaneous power demand is met by the corresponding instantaneous power production.

Mini-grids, or micro-grids, derive from this approach, and they are defined as a local energy system of distributed energy resources, distributed consumers, and optionally.

Distributed generation located close to demand delivers electricity with minimal losses. This power may, therefore, have a higher value than power coming from large, central conventional generators through the traditional utility transmission and distribution infrastructure, especially when—as it is common in many developing countries, particularly in Sub-Saharan Africa—transmission losses are very high (World Bank, 2013). In addition to the economic loss, the CO2 emissions are not balanced by any benefit.

A microgrid designed for a sustainable neighborhood includes programmable and non-programmable renewable generation, energy storage facilities, and/or optional fossil-fueled generation and load control. This system is scalable, which means that a growing load may require the installation of additional generators without any negative effect on the stable and reliable operation of the existing micro-grid. Typical distributed energy resources for micro-grids are wind and solar-powered generators, and biomass-powered systems .

Diversity of building functions (land-use mix) and socio-economic diversity give a positive, very important, contribution to the development of cost-efficient mini- and micro-grids, and to their resilience. The increased cost-efficiency is due to the fact that such diversity allows smoothing the daily electricity load patterns and thus the size of the necessary storage, as the load moves from productive uses (offices, shops, etc.) to residential uses, when people go home from work. Also, the socio-economic diversity helps, as a variety of behaviours correspond.

A neighbourhood supplied with on-site produced renewable energy from different sources can be very resilient, because of the variety of the renewable energy sources and technologies used, which increases the system’s redundancy.

In new urban developments, especially in developing countries, where most part of the infrastructure for electricity production, transmission, and distribution has to be built, the mini-grids (or smart grids, as mini-grids are often named because of a “smart” control system managing them) are an almost obliged technical option. It would be very odd, in fact, to develop the settlements having in mind that kind of obsolete centralized system that developed countries are correcting or abandoning, as it is not consistent with an energy system based on renewable energy sources.

Site Selection:

 The power available in the wind increases rapidly with the speed, hence wind energy conversion machines should be located preferable in areas where the winds are strong and persistent. Although daily winds at a given site may be highly variable, the monthly and especially annual average are remarkably constant from year to year.

The major contribution to the wind power available at a given site is actually made by winds with speeds above the average. Nevertheless, the most suitable sites for wind turbines would be found in areas where the annual average wind speeds are known to be moderately high or high.

The site choice for a single or a spatial array of WECS is an important matter when wind electrics is looked at from the systems point of view of aero turbine generators feeding power into a conventional electric grid.

If the WECS sites are wrongly or poorly chosen the net wind electrics generated energy per year may be sub optimal with resulting high capital cost for the WECS apparatus, high costs for wind generated electric energy, and low Returns on Investment. Even if the WECS is to be a small generator not tied to the electric grid, the sitting must be carefully chosen if inordinately long break even times are to be avoided. Technical, Economic, Environmental, Social and Other actors are examined before a decision is made to erect a generating plant on a specific site.

Some of the main site selection consideration are given below:

1.    High annual average wind speed:

2.    Availability of anemometry data:

3.    Availability of wind V(t) Curve at the proposed site:

4.    Wind structure at the proposed site:

5.    Altitude of the proposed site:

6.    Terrain and its aerodynamic:

7.    Local Ecology

8.    Distance to road or railways:

9.    Nearness of site to local centre/users:

10.  Nature of ground:

11.  Favourable land cost:

 

1.           High annual average wind speed:

The speed generated by the wind mill depends on cubic values of velocity of wind, the small increases in velocity markedly affect the power in the wind. For example, doubling the velocity, increases power by a factor of 8. It is obviously desirable to select a site for WECS with high wind velocity. Thus a high average wind velocity is the principle fundamental parameter of concern in initially appraising WESCS site. For more detailed estimate value, one would like to have the average of the velocity cubed.

2.           Availability of anemometry data:

It is another improvement sitting factor. The aerometry data should be available over some time period at the precise spot where any proposed WECS is to be built and that this should be accomplished before a sitting decision is made.

3.           Availability of wind V(t) Curve at the proposed site:

This important curve determines the maximum energy in the wind and hence is the principal initially controlling factor in predicting the electrical output and hence revenue return o the WECS machines.

It is desirable to have average wind speed ‘V’ such that V>=12-16 km/hr (3.5 – 4.5 m/sec) which is about the lower limit at which present large scale WECS generators ‘cut in’ i.e., start turning. The V(t) Curve also determines the reliability of the delivered WECS generator power, for if the V(t) curve goes to zero there be no generated power during that time.

If there are long periods of calm the WECS reliability will be lower than if the calm periods are short. In making such reliability estimates it is desirable to have measured V(t) Curve over about a 5 year period for the highest confidence level in the reliability estimate.

4.           Wind structure at the proposed site:

The ideal case for the WECS would be a site such that the V(t) Curve was flat, i.e., a smooth steady wind that blows all the time; but a typical site is always less than ideal. Wind specially near the ground is turbulent and gusty, and changes rapidly in direction and in velocity. This departure from homogeneous flow is collectively referred to as “the structure of the wind”.

5.           Altitude of the proposed site:

It affects the air density and thus the power in the wind and hence the useful WECS electric power output. Also, as is well known, the wind tend to have higher velocities at higher altitudes. One must be carefully to distinguish altitude from height above ground. They are not the same except for a sea level WECS site.

6.           Terrain and its aerodynamic:

One should know about terrain of the site to be chosen. If the WECS is to be placed near the top but not on the top of a not too blunt hill facing the prevailing wind, then it may be possible to obtain a ‘speed-up’ of the wind velocity over what it would otherwise be. Also the wind here may not flow horizontal making it necessary to tip the axis of the rotor so that the aero turbine is always perpendicular to the actual wind flow.

It may be possible to make use of hills or mountains which channel the prevailing wind into a pass region, thereby obtaining higher wind power.

7.           Local Ecology

If the surface is base rock it may mean lower hub height hence lower structure cost. If trees or grass or vegetation are present, all of which tend to destructure the wind, the higher hub heights will be needed resulting in larges system costs that the bare ground case.

8.           Distance to road or railways:

This is another factor the system engineer must consider for heavy machinery, structure, materials, blades and other apparatus will have to be moved into any choosen WECS site.

9.           Nearness of site to local centre/users:

This obvious criterion minimizes transmission line length and hence losses and cost. After applying all the previous string criteria, hopefully as one narrows the proposed WECS sites to one or two they would be relatively near to the user of the generated electric energy.

10.       Nature of ground:

Ground condition should be such that the foundation for a WECS are secured. Ground surface should be stable. Erosion problem should not be there, as it could possibly later wash out the foundation of a WECS, destroying the whole system.

11.       Favourable land cost:

Land cost should be favourable as this along with other siting costs, enters into the total WECS system cost.

12. Other conditions such as icing problem, salt spray or blowing dust should not present at the site, as they may affect aero turbine blades or environmental is generally adverse to machinery and electrical apparatus.

 

Types of Wind Turbine:

A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy.

Windmills:
If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is usually called a windmill.

Wind Turbines:

If the mechanical energy is then converted to electricity, the machine is called a wind generator.

Types of Wind Turbines:
Wind turbines are classified into two general types: horizontal axis and vertical axis. A horizontal axis machine has its blades rotating on an axis parallel to the ground. A vertical axis machine has its blades rotating on an axis perpendicular to the ground. There are a number of available designs for both and each type has certain advantages and disadvantages. However, compared with the horizontal axis type, very few vertical axis machines are available commercially.

Vertical Axis Wind Turbines:

Although vertical axis wind turbines have existed for centuries, they are not as common as their horizontal counterparts. The main reason for this is that they do not take advantage of the higher wind speeds at higher elevations above the ground as well as horizontal axis turbines.

Darrius Wind Turbine:
The Darrius turbine is the most famous vertical axis wind turbine. It is characterised by its C-shaped rotor blades which give it its eggbeater appearance. It is normally built with two or three blades.

Giro mill Wind Turbine:

The giro mill is typically powered by two or three vertical aerofoils attached to the central mast by horizontal supports. Giro mill turbines work well in turbulent wind conditions and are an affordable option where a standard horizontal axis windmill type turbine is unsuitable.

Horizontal Axis Wind Turbines:

A horizontal Axis Wind Turbine is the most common wind turbine design. In addition to being parallel to the ground, the axis of blade rotation is parallel to the wind flow.

Up-Wind Turbines:

Some wind turbines are designed to operate in an upwind mode (with the blades upwind of the tower). Large wind turbines use a motor-driven mechanism that turns the machine in response to a wind direction. Smaller wind turbines use a tail vane to keep the blades facing into the wind.

Down-Wind Turbines:

Other wind turbines operate in a downwind mode so that the wind passes the tower before striking the blades. Without a tail vane, the machine rotor naturally tracks the wind in a downwind mode.

Shrouded Wind Turbines:

Some turbines have an added structural design feature called an augmenter. The augmenter is intended to increase the amount of wind passing through the blades.

Wind Farms:

A wind farm or wind park, also called a wind power station or wind power plant, is a group of wind turbines in the same location used to produce electricity. Wind farms vary in size from a small number of turbines to several hundred wind turbines covering an extensive area. Wind farms can be either onshore or offshore.

Wind farms tend to have much less impact on the environment than many other power stations. Onshore wind farms are also criticized for their visual impact and impact on the landscape, as typically they need to take up more land than other power stations and need to be built in wild and rural areas, which can lead to "industrialization of the countryside", habitat loss, and a drop in tourism. Critics have linked wind farms to adverse health effects (see wind turbine syndrome). Wind farms have also been criticized for interfering with radar, radio and television reception.

 

Wind Generation and Control:

Wind turbine control is necessary to ensure low maintenance costs and efficient performance. The control system also guarantees safe operation, optimizes power output, and ensures long structural life. Turbine rotational speed and the generator speed are two key areas that you must control for power limitation and optimization. The “Control Methods” and “Control Strategies” sections of this document explain which techniques to use and how to manage these areas.

A wind turbine is a revolving machine that converts the kinetic energy from the wind into mechanical energy. This mechanical energy is then converted into electricity that is sent to a power grid. The turbine components responsible for these energy conversions are the rotor and the generator. 

The rotor is the area of the turbine that consists of both the turbine hub and blades. As wind strikes the turbine’s blades, the hub rotates due to aerodynamic forces. This rotation is then sent through the transmission system to decrease the revolutions per minute. The transmission system consists of the main bearing, high-speed shaft, gearbox, and low-speed shaft. The ratio of the gearbox determines the rotation division and the rotation speed that the generator sees. For example, if the ratio of the gearbox is N to 1, then the generator sees the rotor speed divided by N. This rotation is finally sent to the generator for mechanical-to-electrical conversion.

The amount of surface area available for the incoming wind is key to increasing aerodynamic forces on the rotor blades. The angle at which the blade is adjusted is referred to as the angle of attack, α. This angle is measured with respect to the incoming wind direction and the chord line of the blade. There is also a critical angle of attack, αcritical, where air no longer streams smoothly over the blade’s upper surface. Figure 1 shows the critical angle of attack with respect to the blade.

Figure 1. The Critical Angle of Attack (αcritical) with Respect to the Blade

Power and Efficiency

This section explains what affects the power extracted from the wind and the efficiency of this process.   Consider Figure 3 as a model of the turbine’s interaction with the wind. This diagram indicates that wind exists on either side of the turbine, and the proper balance between rotational speed and the velocity of wind are critical to regulate performance.  The balance between rotational speed and wind velocity, referred to as the tip speed ratio, is calculated using Equation 1.

                  

   Where : is the blades frequency of rotation (Hz)

 is the length of a blade (m)

 

Equation 1. Calculating the Tip Speed Ratio

 

The efficiency of a wind turbine is called the power coefficient, or . Theoretically, the power coefficient is calculated as the ratio of actual to ideal extracted power. You can find this calculation in Equation 2. Also, you can adjust  by controlling the angle of attack, α, and the tip speed ratio, .  The calculation for this case is shown in Equation 3. In Equation 3, c1-c6 and x are coefficients that wind turbine manufacturer should provide. Note that the maximum power coefficient that you can achieve with any turbine is .59, or the Betz limit.

 

Equation 2. The power coefficient is calculated as the ratio of actual to ideal extracted power.

 

Equation 3.You can adjust the  by controlling the angle of attack, α, and the tip speed ratio.

Finally, you can calculate the usable power from the wind using Equation 5. From this equation, you can see that the main drivers for usable power are the blade length and wind speed.

Where:              = density of air (1.2929 kg/m3)

Equation 5. Calculating Usable Power from the Wind

 

Figure 3. Model of the Turbine’s Interaction with the Wind

The Power Curve

It is important to understand the relationship between power and wind speed to determine the required control type, optimization, or limitation. The power curve, a plot you can use for this purpose, specifies how much power you can extract from the incoming wind. Figure 3 contains an ideal wind turbine power curve.

Figure 3. Ideal Wind Turbine Power Curve

The cut-in and cut-out speeds are the operating limits of the turbine. By staying in this range, you ensure that the available energy is above the minimum threshold and structural health is maintained. The rated power, a point provided by the manufacturer, takes both energy and cost into consideration. Also, the rated wind speed is chosen because speeds above this point are rare. Typically, you can assume that a turbine design that extracts the bulk of energy above the rated wind speed is not cost-effective.

From Figure 3, you can see that the power curve is split into three distinct regions. Because Region I consists of low wind speeds and is below the rated turbine power, the turbine is run at the maximum efficiency to extract all power. In other words, the turbine controls with optimization in mind. On the other hand, Region III consists of high wind speeds and is at the rated turbine power. The turbine then controls with limitation of the generated power in mind when operating in this region. Finally, Region II is a transition region mainly concerned with keeping rotor torque and noise low.

Control Methods

You can use different control methods to either optimize or limit power output. You can control a turbine by controlling the generator speed, blade angle adjustment, and rotation of the entire wind turbine. Blade angle adjustment and turbine rotation are also known as pitch and yaw control, respectively. A visual representation of pitch and yaw adjustment is shown in Figures 4 and 5. 

                          

Figure 4. Pitch Adjustment                                           Figure 5. Yaw Adjustment

The purpose of pitch control is to maintain the optimum blade angle to achieve certain rotor speeds or power output. You can use pitch adjustment to stall and furl, two methods of pitch control. By stalling a wind turbine, you increase the angle of attack, which causes the flat side of the blade to face further into the wind. Furling decreases the angle of attack, causing the edge of the blade to face the oncoming wind. Pitch angle adjustment is the most effective way to limit output power by changing aerodynamic force on the blade at high wind speeds.

Yaw refers to the rotation of the entire wind turbine in the horizontal axis. Yaw control ensures that the turbine is constantly facing into the wind to maximize the effective rotor area and, as a result, power.  Because wind direction can vary quickly, the turbine may misalign with the oncoming wind and cause power output losses. You can approximate these losses with the following equation:

EQ 6: ∆P=α cos(ε)                 

Where ∆P is the lost power and ε is the yaw error angle

The final type of control deals with the electrical subsystem. You can achieve this dynamic control with power electronics, or, more specifically, electronic converters that are coupled to the generator. The two types of generator control are stator and rotor. The stator and rotor are the stationary and no stationary parts of a generator, respectively. In each case, you disconnect the stator or rotor from the grid to change the synchronous speed of the generator independently of the voltage or frequency of the grid. Controlling the synchronous generator speed is the most effective way to optimize maximum power output at low wind speeds. 

Figure 7 shows a system-level layout of a wind energy conversion system and the signals used. Notice that control is most effective by adjusting pitch angle and controlling the synchronous speed of the generator.

Figure 7. System-Level Layout of a Wind Energy System

Control Strategies

Recall that controlling the pitch of the blade and speed of the generator are the most effective methods to adjust output power. The following control strategies use pitch and generator speed control to manage turbine functionality throughout the power curve: fixed-speed fixed-pitch, fixed-speed variable-pitch, variable-speed fixed-pitch, and variable-speed variable-pitch. Figure 8 shows the power curves for different control strategies explained below, with variable-speed variable-pitch, VS-VP, being the ideal curve.

Figure 8. Power Curves for Different Control Strategies (Variable-speed variable-pitch, VS-VP, is the ideal curve.)

Fixed-speed fixed-pitch (FS-FP) is the one configuration where it is impossible to improve performance with active control. In this design, the turbine’s generator is directly coupled to the power grid, causing the generator speed to lock to the power line frequency and fix the rotational speed. These turbines are regulated using passive stall methods at high wind speeds. The gearbox ratio selection becomes important for this passive control because it ensures that the rated power is not exceeded. Figure 8 shows the power curve for FS-FP operation.

From the figure, it is apparent that the actual power does not match the ideal power, implying that there is lower energy capture. Notice that the turbine operates at maximum efficiency only at one wind speed in the low-speed region. The rated power of the turbine is achieved only at one wind speed as well. This implies poor power regulation as a result of constrained operations.

Fixed-speed variable-pitch (FS-VP) configuration operates at a fixed pitch angle below the rated wind speed and continuously adjusts the angle above the rated wind speed. To clarify, fixed-speed operation implies a maximum output power at one wind speed. You can use both feather and stall pitch control methods in this configuration to limit power. Keep in mind that feathering takes a significant amount of control design and stalling increases unwanted thrust force as stall increases. Figure 8 shows the power curve for FS-VP using either feather or stall control. 

Below the rated wind speed, the FS-VP turbine has a near optimum efficiency around Region II.  Exceeding the rated wind speed, the pitch angles are continuously changed, providing little to no loss in power.

Variable-speed fixed-pitch (VS-FP) configuration continuously adjusts the rotor speed relative to the wind speed through power electronics controlling the synchronous speed of the generator. This type of control assumes that the generator is from the grid so that the generator’s rotor and drive-train are free to rotate independently of grid frequency. Fixed-pitch relies heavily on the blade design to limit power through passive stalling. Figure 8 shows the power curve for VS-FP. 

Figure 8 shows that power efficiency is maximized at low wind speeds, and you can achieve rated turbine power only at one wind speed. Passive stall regulation plays a major role in not achieving the rated power and can be attributed to poor power regulation above the rated wind speed. In lower wind speed cases, VS-FP can capture more energy and improve power quality.

Variable-speed variable-pitch (VS-VP) configuration is a derivation of VS-FP and FS-VP. Operating below the rated wind speed, variable speed and fixed pitch are used to maximize energy capture and increase power quality. Operating above the rated wind speed, fixed speed and variable pitch permit efficient power regulation at the rated power. VS-VP is the only control strategy that theoretically achieves the ideal power curve shown in Figure 8.

Nature of the Wind:

The wind is the vertical and horizontal motion of air masses in the atmosphere. Global winds are caused by pressure differences, due to the non-uniform heating of the Earth surface by solar radiation. The different pressure zones are due to the vertical movement of the air. For example, at the Equator the strong solar radiation warms the ground surface. The air in contact with the ground surface gets warmer an lighter, rising up, generating a low pressure zone. At cold places, for examples the poles, the air is falling generating (relatively) high pressure zones (Figure 3.1). Usually high pressure zones are characterized by good weather conditions while low pressure zones are more rainy.

The difference in pressure between geographical areas gives rise to a pressure gradient force. The pressure gradient force is perpendicular to the isobar lines, hence the air mass start the motion in the opposite direction of the pressure gradient. This large scale motion is also influenced by the Earth rotation which causes, among others, the Coriolis force. For example, a particle of air moving from the latitude 1 towards latitude 2 (figure 3.2, valid for northern hemisphere) have a velocity component parallel to the equator which is greater than the same velocity component of the particles at the latitude 2. This determine that the moving air particle is leading the other particles. For an external observer this looks like that the particle turns right. A particle moving in the other direction (from latitude 1 towards latitude 2) will be lagging the other particles at latitude 1, hence it is, again, turning right respect to the direction of motion (Figure 3.2).

The Coriolis force deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere until the equilibrium between the pressure gradient force and the Coriolis force is reached (point c of figure 3.3).

When there is a balance between the pressure gradient force and the Coriolis force the wind is called the geostrophic wind. This happens often above the ABL (Atmospheric Boundary Layer), but not always since the isobars are curved contours (Figure 3.3), which also gives rise to centrifugal force.

The circulation from the Equator to the poles is in the reality divided in several cells (Hadley cells, figure 2.5) because the air that rise at the equator, on the way to the poles cools down (thanks to the expansion and to heat exchange) and fells towards the ground.

In the atmospheric boundary layer the Coriolis force also gives rise to a change of direction with height (wind veer), clockwise moving upwards: at the surface the roughness of the terrain generates a drag force that alter the equilibrium between pressure gradient force and Coriolis force. In the new equilibrium condition, the drag force and the Coriolis force are counterbalanced by the pressure gradient force, and the wind direction is to the left of the wind direction at the top of the ABL.

Hills and mountains generates local increases and reductions of the wind speed. The sea has a large heat capacity, therefore the land warms up much more rapidly than the sea. Ocean streams move heat from one region to another. Local heating or cooling can produce seasonal winds (like monsoons in India) or daily winds (like katabatic wind in Chile), or other variations of wind speed.

Power in the Wind:

Wind power or wind energy is the use of wind to provide the mechanical power through wind turbines to turn electric generators and traditionally to do other work, like milling or pumping. Wind power is a sustainable and renewable energy, and has a much smaller impact on the environment compared to burning fossil fuels.

Wind farms consist of many individual wind turbines, which are connected to the electric power transmission network. Onshore wind is an inexpensive source of electric power, competitive with or in many places cheaper than coal or gas plants. Onshore wind farms also have an impact on the landscape, as typically they need to be spread over more land than other power stations and need to be built in wild and rural areas, which can lead to "industrialization of the countryside" and habitat loss. Offshore wind is steadier and stronger than on land and offshore farms have less visual impact, but construction and maintenance costs are higher. Small onshore wind farms can feed some energy into the grid or provide electric power to isolated off-grid locations. 

Wind is an intermittent energy source, which cannot make electricity nor be dispatched on demand. It also gives variable power, which is consistent from year to year but varies greatly over shorter time scales. Therefore, it must be used together with other electric power sources or storage to give a reliable supply. As the proportion of wind power in a region increases, more conventional power sources are needed to back it up (such as fossil fuel power and nuclear power), and the grid may need to be upgraded.[10][11] Power-management techniques such as having dispatchable power sources, enough hydroelectric power, excess capacity, geographically distributed turbines, exporting and importing power to neighbouring areas, energy storage, or reducing demand when wind production is low, can in many cases overcome these problems. Weather forecasting permits the electric-power network to be readied for the predictable variations in production that occur. 

Factors influencing wind:

Since pressure differences are mainly caused by unequal heating of the earth’s surface, solar radiation may be called the ultimate driving force of the wind. If the earth were stationary and had a uniform surface, air would flow directly from high pressure areas to low pressure areas. Because none of these conditions exist, the direction and speed of wind are controlled by a number of factors. These are pressure gradient, the Coriolis Effect, the centripetal acceleration and friction.

 

 

1. Pressure Gradient Force:

This is the force generated due to the differences in horizontal pressure, and it operates from the high pressure area to a low pressure area. Since a closely spaced gradient implies a steep pressure change, it also indicates a strong wind speed. The wind direction follows the direction of change of pressure, i.e. perpendicular to the isobars.

2.Coriolis Force:

Due to the earth’s rotation, winds do not cross the isobars at right angles as the pressure gradient force directs, but get deflected from their original path. This deviation is the result of the earth’s rotation and is called the Coriolis Effect or Coriolis force. Due to this effect, winds in the northern hemisphere get deflected to the right of their path and those in the southern hemisphere to their left, following Farrel’s Law. The Coriolis force changes wind direction but not its speed. This deflection force does not seem to exist until the air is set in motion and increases with wind velocity, air mass and an increase in latitude. (Fig. 2.16)

3.Centripetal Acceleration:

Due to inward acceleration of air towards the centre of rotation on the rotating earth, it is possible for the air to maintain a curved path (parallel to the isobars), about a local axis of high or low pressure. It is known as centripetal acceleration.

4.Frictional Force:

The irregularities of the earth’s surface offer resistance to the wind motion in the form of friction. This force determines the angle at which air will flow across the isobars, as well as the speed at which it will move. It may also alter wind direction. Over the relatively smooth ocean surface, the friction is minimum, so the air moves at low angles to the isobars and at a greater speed. Over uneven terrain, however, due to high friction, the wind direction makes high angles with, isobars and the speed gets retarded.

Wind data and Energy Estimation:

WAsP is a software program for predicting wind climate and energy yield of wind turbines. The predictions are based on wind data measured on site or from stations in the same region and considers the effects of the surrounding terrain to the wind flow (topography, surface description, obstacles)

 

Wind Speed Monitoring:

When thinking of installing a small wind turbine at your home or farm, there are a few things to consider, but probably the most important can be summarised in three key points:

• Height

• Location

• Size of the wind turbine

 Height

This is simple, the taller the wind turbine tower the better. The higher, the fewer obstacles and therefore the less turbulence on the wind and usually means higher wind speeds

Above shows the comparison with two anemometer on the same wind mast, one at 40 meters and the other at 30 meters. The 40m anemometer is showing an average wind speed of 1m/s more than the 30m one.

Location

Location, location, location, like the property market this point is probably one of the most important things to consider when thinking about installing a wind turbine. We may be in a windy area but the particular spot we may be looking at could be surrounded by obstacles like trees, houses or even towns a few miles away. These could have an unwanted effect not just on the increase of wind turbulences but also on the wind speed itself which could be severely reduced.

As you can see above, the higher the hub height of the wind turbine and the better located the wind turbine, the less turbulence on the wind and more steady flow of wind.

 

Size of the wind turbine

Think about the sails of a ship, the bigger they are the more wind they capture and therefore the more energy they can harvest from it. Wind turbines are not much different, the bigger the rotor the more wind they will capture.

Classification of Wind:

Wind in simple terms is nothing but moving air. We all enjoy wind rustling through the leaves in our garden. It has also expanded the range of transport and has provided a power source in terms of mechanical energy for the generation of electricity in windmills and recreation purposes in hot air balloons. Wind power was also used in voyages by sailors to direct their ships. When the winds are strong, they lead to the destruction of life and property in the form of cyclones and storms, causing forest fires, landslides etc. In this article, we will learn about the causes of wind and the destruction caused by winds.

Types of Wind

Wind blowing above the earth surface may be classified into five major types:

• Planetary winds

• Trade winds

• The westerlies

• Periodic winds

• Monsoon winds

• Land breeze

• Sea breeze

• Mountain and valley breeze

• Local winds

Planetary Winds

Planetary winds comprise winds distributed throughout the lower atmosphere. The winds blow regularly throughout the year confined within latitudinal belts, mainly in north-east and south-east directions or from high-pressure polar-regions to low-pressure regions.

Trade Winds:

These winds are also known as tropical easterlies and blow from the right in Northern hemisphere and to the left in the Southern hemisphere due to Coriolis Effect and Ferrell’s law. They start blowing from the sub-tropical high-pressure areas towards the equatorial low-pressure belt. In the Northern hemisphere, they blow as northeaster trades and in the Southern hemisphere, they blow as southeaster trades.

The Westerlies

These winds are also known as Shrieking Sixties, Furious Fifties, and Roaring Forties. They blow from the subtropical high-pressure belts towards sub-polar low-pressure belts. The westerlies of Southern hemisphere are stronger and constant than the westerlies of Northern hemisphere.

Periodic Winds

These winds change their direction periodically as there is a change in the seasons. Following are the types of periodic winds:

• Monsoon winds: The temperature difference created due to the Indian Ocean, Arabian Sea and Bay of Bengal on one side and the Himalayan wall on the other forms the basis of monsoons in the Indian subcontinent.

• Land breeze: These winds blow from land to sea, carrying no moisture but dry and warm.

• Sea breeze: These winds blow from sea to land, carrying some moisture.

• Mountain and valley breeze: Valley breeze is the hot air blowing from the valley which flows up to the slopes of mountain slopes. While mountain breeze is the reverse of the valley breeze that is the cold air from the mountain flow towards the valley.

Characteristics:

To successfully deploy wind energy, we need to have the resources and the transmission to carry the electrical energy product to the load centres. Abundant resources can be profitable only if we can generate low-cost wind energy. The transmission line, the strength of the grid, and the proximity of the wind resource to the load centre are all important factors in successful wind deployment. In an interconnected grid, the power system network and the wind power plant are interrelated. Knowing the characteristics of the wind power plant and the transmission and distribution systems is very important for identifying the problems and finding the resources to resolve the issues. When we discuss transmission lines, we are concerned about, among other things, the stability, power transfer capability, and the losses. The stability and power transfer capability are determined partly by the impedance between the sending and the receiving ends. In the transmission lines, the resistive part of the impedance is generally much smaller than its reactance. In the sections that follow, the resistive part of the impedance is often ignored to simplify the discussion. The reactance between the two points consists of the reactance presented by the lines, the transformer, and the electric machine impedances when the line reactance is the major component of the total impedance. The line reactance is mostly determined by the line length—the geometrical distance between the wires and the ground. Raising the voltage will significantly increase the transfer capability of the systems and reduce the losses, the voltage drop, and the per unit (p.u.) value of the reactance; however, higher voltage transmission requires significant investment. A large wind power plant (> 100 MW) is commonly connected to a transmission line. At the main substation, the voltage is stepped down from the transmission level to the sub transmission level through the main transformer. From the main substation, a collector system collects the power generated by the wind turbines through line feeders. The collector system impedance of many turbines can be represented by its equivalence and as a single impedance. One method to equivalence the collector system of a large wind power plant can be found in reference.  The single-line diagram represents a wind power plant. Each wind turbine has a pad-mounted transformer to step down the voltage from the sub transmission level to the low voltage generated by the wind turbine. In the future, as wind turbines become larger, the voltage at the point of generation may be increased to medium. The transmission line connects the wind power plant to the grid or to a very large network, which can be treated as an infinite bus. The short circuit current at the point of interconnection plays an important role in the behaviour of the wind power plant. The dynamic analysis is this paper is simulated by using Power System Simulator for Engineers [2], a commonly used program in utility planning. References and provide a good background on dynamic analysis for wind turbines. For a more detailed discussion of the dynamic model used in this section, see reference. A single line diagram that describes the nature of the fault and the network topology in pre-fault, fault, and post-fault conditions. The transmission lines consist of two parallel paths. The fault occurs at the middle of line A, and the circuit breakers disconnect line A. The remaining line B continues to deliver the output of the wind power plant. The short circuit current (SCC) of the transmission line was originally at 10 p.u.; in the post-fault condition after line A was removed, the SCC dropped to 5 p.u. 

Applications of Wind Turbine:

Primarily wind energy is used for the power generation and wind energy plays an important role in majority of the applications such as

• Flour mill

• Lift water

• To lightning purpose

• Pump water to higher level

• To lift the load

The main attractive benefit of the win energy is that generate electricity. It could be used to propel sailboats in the river and seas for transporting material and men from one place to another. Wind energy is considered as the world’s fastest growing energy source. It might become most environmental friendly and economical source of the electricity in many countries. Lifetime cost of the wind energy system could be categorized into decommission cost, operating and maintenance cost and initial cost for system manufacturing and installation. Initial upfront cost of the wind energy system includes foundation, turbine, and connection to grid, electrical equipment and transportation. Typically it is used in charging of the batteries which is useful to store energy captured by wind turbines. Water pumping is the key historical application of the wind energy. Main key competitive area of the wind energy is that remote off grid power applications. You are always advisable to know about wind energy technologies that are beneficial to you. Wind turbine is the best power generating machine which is using wind as the source of energy for generating power.

In fact wind power energy definition, energy of the wind could be converted into the useful form. Windy and remote site is required to construct wind turbines. Wind power is cost effective and it could be built on the existing ranches and farms. It might not generate any kinds of toxic emissions like fossil fuels so it can offer clean source of the power. This energy is offering excellent numbers of the benefits to people such as renewable energy source, eco-friendly, reduce overdependence on the traditional source of electricity, low operational costs and incredible domestic potential.

Offshore wind energy:

Offshore wind power or offshore wind energy is the use of wind farms constructed in bodies of water, usually in the ocean, to harvest wind energy to generate electricity. Higher wind speeds are available offshore compared to on land, so offshore wind power’s electricity generation is higher per amount of capacity installed, and NIMBY opposition to construction is usually much weaker.

Unlike the typical use of the term "offshore" in the marine industry, offshore wind power includes inshore water areas such as lakes, fjords and sheltered coastal areas as well as deeper-water areas. Most offshore wind farms employ fixed-foundation wind turbines in relatively shallow water. As of 2020, floating wind turbines for deeper waters are in the early phase of development and deployment.

 

Hybrid systems:

The term wind hybrid system describes any combination of wind energy with one or more additional sources of electricity generation (e.g. biomass, solar or a generator using fossil fuels). Hybrid system are very often used for stand-alone applications at remote sites. For this reason the article focusses on stand-alone hybrid systems containing storage or diesel-backup.

The combination of renewable energy technologies allows a more balanced electricity supply during day/night and seasonal changes: At most sites wind speed is low, when the sun is shining and reaches higher values on cloudy days. Thus the amount of energy generated by wind energy reaches its maximum in the winter months, while the output of PV-cells is significantly higher in the summer. Other important examples are Wind-Diesel systems often used in remote areas. A diesel generator will be used as backup, if the electricity demand cannot be covered by the installed wind turbines. Regulation and conversion of the available energy sources is a central issue planning a wind hybrid system. Many hybrid systems are uses as stand-alone off-grid applications.

 

Wind resource assessment:

Wind resource assessment is the process by which wind power developers estimate the future energy production of a wind farm. Accurate wind resource assessments are crucial to the successful development of wind farms.

 

Betz Limit:

The Betz limit is the theoretical maximum efficiency for a wind turbine, conjectured by German physicist Albert Betz in 1919. Betz concluded that this value is 59.3%, meaning that at most only 59.3% of the kinetic energy from wind can be used to spin the turbine and generate electricity. In reality, turbines cannot reach the Betz limit, and common efficiencies are in the 35-45% range. 

Wind turbines work by slowing down passing wind in order to extract energy. If a wind turbine was 100% efficient, then all of the wind would have to stop completely upon contact with the turbine—which isn't possible by looking at a wind turbine .In order to stop the wind completely, the air wouldn't move out of the way to the back of the turbine, which would prevent further air from coming in—causing the turbine to stop spinning.

For Further Reading

• Wind turbine

• Efficiency

• Carnot efficiency

• Law of conservation of energy

• Or explore a random page

Site Management:

The power available in the wind increases rapidly with the speed, hence wind energy conversion machines should be located preferable in areas where the winds are strong and persistent. Although daily winds at a given site may be highly variable, the monthly and especially annual average are remarkably constant from year to year.

The major contribution to the wind power available at a given site is actually made by winds with speeds above the average. Nevertheless, the most suitable sites for wind turbines would be found in areas where the annual average wind speeds are known to be moderately high or high.

The site choice for a single or a spatial array of WECS is an important matter when wind electrics is looked at from the systems point of view of aero turbine generators feeding power into a conventional electric grid.

If the WECS sites are wrongly or poorly chosen the net wind electrics generated energy per year may be sub optimal with resulting high capital cost for the WECS apparatus, high costs for wind generated electric energy, and low Returns on Investment. Even if the WECS is to be a small generator not tied to the electric grid, the sitting must be carefully chosen if inordinately long break even times are to be avoided. Technical, Economic, Environmental, Social and Other actors are examined before a decision is made to erect a generating plant on a specific site.

Some of the main site selection consideration are given below:

1. High annual average wind speed

2. Availability of anemometry data

3. Availability of wind V(t) Curve at the proposed site

4. Wind structure at the proposed site

5. Altitude of the proposed site

6. Terrain and its aerodynamic

7. Local Ecology

8. Distance to road or railways

9. Nearness of site to local centre/users

10. Nature of ground

11. Favourable land cost

Wind Energy conversion devices:

Wind energy conversion devices can be broadly categorized into two types according to their axis alignment. They are as follows

1. Horizontal axis wind turbines

2. Vertical axis wind turbines

 

Horizontal axis wind turbines:

It can be further divided into three types:

• Dutch type grain grinding wind mills

• Multiblade water pumping windmills

• High speed propeller type windmills

1. Dutch wind mill:

Man has used Dutch windmills for a long time. In fact the grain grinding windmills that were widely used in Europe since the middle ages were Dutch. These windmills were operated on the thrust exerted by the wind. The blades, generally four, were inclined at an angle to the plane of rotation. The wind being deflected by the blades exerted a force in the direction of rotation. The blades were made of sails or wooden slats.

2. Multiblade water pumping windmill:

Modern water pumping windmills have a large number of blades- generally wooden or metallic- driving a reciprocating pumps. As the mill has to be placed directly over the well, the criterion for site selection concerns about water availability & not windiness. Therefore the mill must be able to operate at slow winds. The large number of blades gives a high torque, required for driving a centrifugal pump, even at low wind speeds. Hence sometimes these are called as fan mills. As these windmills are supposed to be installed at remote places, mostly as single units, reliability, sturdiness, and low cost are the prime criteria and not efficiency. The blades are made of flat steel plates, working on the thrust of wind. These are hinged to a metal ring to ensure structural strength, and the low speed of rotation adds to the reliability. The orientation is generally achieved by tail vane.

3.High speed propeller type windmill:

The horizontal axis wind turbines that are used today for electricity generation do not operate on thrust force. They depend mainly on the aerodynamic forces that develop when wind flows around a blade of aerofoil design. Windmills working on thrust force are inherently less efficient. So all the modern wind turbine blades are designed based on aerofoil section.

Vertical axis wind turbines

It comes in two different designs

• The savonius rotor

• The darrieus rotor

1.The savonious rotor:

The savonius rotor is extremely simple vertical axis device that works entirely because of the thrust force of wind. The basic equipment is a drum cut in two halves vertically. The two parts are attached to the two opposite sides of a vertical shaft. As the wind blowing into the structure meets with two dissimilar surfaces – one convex and the other concave – the forces exerted on the two surfaces are different, which gives the rotor a torque. By providing a certain amount of overlap between the two drums, the torque can be increased. This is because the wind blowing into the concave surface turn around and give a push to the inner surface of the other drum, partly cancelling the wind thrust on the convex side. It has been found that an overlap of about one third the drum diameter gives optimum result.

2.The darrieus rotor:

The particularity of Darrieus rotor is that its working is not at all evident from its appearance. Two or more flexible blades are attached to a vertical shaft. The blades bow outwards, taking approximately the shape of a parabola and are of symmetrical air foil section. Here the torque is zero when the rotor is stationary. It develops a positive torque only when it is already rotating. This means that such a rotor has mo. starting torque and has to be start using some external means.

Wind mill component Design:

Wind turbine design is the process of defining the form and specifications of a wind turbine to extract energy from the wind.[1] A wind turbine installation consists of the necessary systems needed to capture the wind's energy, point the turbine into the wind, convert mechanical rotation into electrical power, and other systems to start, stop, and control the turbine.

This article covers the design of horizontal axis wind turbines (HAWT) since the majority of commercial turbines use this design.

In 1919, the physicist Albert Betz showed that for a hypothetical ideal wind-energy extraction machine, the fundamental laws of conservation of mass and energy allowed no more than 16/27 (59.3%) of the kinetic energy of the wind to be captured. This Betz' law limit can be approached by modern turbine designs which may reach 70 to 80% of this theoretical limit.

In addition to aerodynamic design of the blades, design of a complete wind power system must also address design of the hub, controls, generator, supporting structure and foundation. Further design questions arise when integrating wind turbines into electrical power grids.

Generator torque

Modern large wind turbines are variable-speed machines. When the wind speed is below rated, generator torque is used to control the rotor speed in order to capture as much power as possible. The most power is captured when the tip speed ratio is held constant at its optimum value (typically 6 or 7). This means that as wind speed increases, rotor speed should increase proportionally. The difference between the aerodynamic torque captured by the blades and the applied generator torque controls the rotor speed. If the generator torque is lower, the rotor accelerates, and if the generator torque is higher, the rotor slows down. Below rated wind speed, the generator torque control is active while the blade pitch is typically held at the constant angle that captures the most power, fairly flat to the wind. Above rated wind speed, the generator torque is typically held constant while the blade pitch is active.

One technique to control a permanent magnet synchronous motor is Field Oriented Control. Field Oriented Control is a closed loop strategy composed of two current controllers (an inner loop and outer loop cascade design) necessary for controlling the torque, and one speed controller.

Constant torque angle control

In this control strategy the d axis current is kept zero, while the vector current is align with the q axis in order to maintain the torque angle equal with 90o. This is one of the most used control strategy because of the simplicity, by controlling only the Iqs current. So, now the electromagnetic torque equation of the permanent magnet synchronous generator is simply a linear equation depend on the Iqs current only.

So, the electromagnetic torque for Ids = 0 (we can achieve that with the d-axis controller) is now:

Te= 3/2 p (λpm Iqs + (Lds-Lqs) Ids Iqs )= 3/2 p λpm Iqs

Machine Side Controller Design

So, the complete system of the machine side converter and the cascaded PI controller loops is given by the figure in the right. In that we have the control inputs, which are the duty rations mds and mqs, of the PWM-regulated converter. Also, we can see the control scheme for the wind turbine in the machine side and simultaneously how we keep the Ids zero (the electromagnetic torque equation is linear).

Yawing:

Modern large wind turbines are typically actively controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), the power output is maximized and non-symmetrical loads minimized. However, since the wind direction varies quickly the turbine will not strictly follow the direction and will have a small yaw angle on average. The power output losses can simply be approximated to fall with (cos(yaw angle))3. Particularly at low-to-medium wind speeds, yawing can make a significant reduction in turbine output, with wind direction variations of ±30° being quite common and long response times of the turbines to changes in wind direction. At high wind speeds, the wind direction is less variable.

Electrical braking:

Braking of a small wind turbine can be done by dumping energy from the generator into a resistor bank, converting the kinetic energy of the turbine rotation into heat. This method is useful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit.

Cyclically braking causes the blades to slow down, which increases the stalling effect, reducing the efficiency of the blades. This way, the turbine's rotation can be kept at a safe speed in faster winds while maintaining (nominal) power output. This method is usually not applied on large grid-connected wind turbines.

Mechanical braking:

A mechanical drum brake or disk brake is used to stop turbine in emergency situation such as extreme gust events or over speed. This brake is a secondary means to hold the turbine at rest for maintenance, with a rotor lock system as primary means. Such brakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed as the mechanical brakes can create a fire inside the nacelle if used to stop the turbine from full speed. The load on the turbine increases if the brake is applied at rated RPM.

Turbine Size:

Figure 1. Flow diagram for wind turbine plant

There are different size classes of wind turbines. The smallest having power production less than 10 kW are used in homes, farms and remote applications whereas intermediate wind turbines (10-250 kW) are useful for village power, hybrid systems and distributed power. The world's largest wind turbine, an 8-MW turbine located at the Burbo Bank Extension wind farm in Liverpool Bay, United Kingdom, was installed in 2016. Utility-scale turbines (larger than one megawatt) are used in central station wind farms, distributed power and community wind. 

A person standing beside 15 m long blades.

For a given survivable wind speed, the mass of a turbine is approximately proportional to the cube of its blade-length. Wind power intercepted by the turbine is proportional to the square of its blade-length. The maximum blade-length of a turbine is limited by both the strength, the stiffness of its material, and transportation considerations.

Labour and maintenance costs increase only gradually with increasing turbine size, so to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting requirements.

Typical modern wind turbines have diameters of 40 to 90 metres (130 to 300 ft) and are rated between 500 kW and 2 MW. As of 2017 the most powerful turbine, the Vestas V-164, is rated at 9.5 MW and has a rotor diameter of 164m 

Increasingly large wind turbines are being designed, manufacturers have not yet come close to the maximum size. The largest turbines will be 265 metres or more. 

Nacelle:

The nacelle is housing the gearbox and generator connecting the tower and rotor. Sensors detect the wind speed and direction, and motors turn the nacelle into the wind to maximize output.

Gearbox:

In conventional wind turbines, the blades spin a shaft that is connected through a gearbox to the generator. The gearbox converts the turning speed of the blades 15 to 20 rotations per minute for a large, one-megawatt turbine into the faster 1,800 revolutions per minute that the generator needs to generate electricity. Analysts from Global Data estimate that gearbox market grows from $3.2bn in 2006 to $6.9bn in 2011, and to $8.1bn by 2020. Market leaders were Winery in 2011. The use of magnetic gearboxes has also been explored as a way of reducing wind turbine maintenance costs. 

Generator:

For large, commercial size horizontal-axis wind turbines, the electrical generator  is mounted in a nacelle at the top of a tower, behind the hub of the turbine rotor. Typically wind turbines generate electricity through asynchronous machines that are directly connected with the electricity grid. Usually the rotational speed of the wind turbine is slower than the equivalent rotation speed of the electrical network: typical rotation speeds for wind generators are 5–20 rpm while a directly connected machine will have an electrical speed between 750 and 3600 rpm. Therefore, a gearbox is inserted between the rotor hub and the generator. This also reduces the generator cost and weight. Commercial size generators have a rotor carrying a field winding so that a rotating magnetic field is produced inside a set of windings called the stator. While the rotating field winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the generator output voltage.

Older style wind generators rotate at a constant speed, to match power line frequency, which allowed the use of less costly induction generators. Newer wind turbines often turn at whatever speed generates electricity most efficiently. The varying output frequency and voltage can be matched to the fixed values of the grid using multiple technologies such as doubly fed induction generators or full-effect converters where the variable frequency current produced is converted to DC and then back to AC. Although such alternatives require costly equipment and cause power loss, the turbine can capture a significantly larger fraction of the wind energy. In some cases, especially when turbines are sited offshore, the DC energy will be transmitted from the turbine to a central (onshore) inverter for connection to the grid.

Blades:

Blade design:

The ratio between the speed of the blade tips and the speed of the wind is called tip speed ratio. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7. Modern wind turbines are designed to spin at varying speeds (a consequence of their generator design, see above). Use of aluminium and composite materials in their blades has contributed to low rotational inertia, which means that newer wind turbines can accelerate quickly if the winds pick up, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speed ratio during energetic gusts of wind allows wind turbines to improve energy capture from sudden gusts that are typical in urban settings.

In contrast, older style wind turbines were designed with heavier steel blades, which have higher inertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertia buffered the changes in rotation speed and thus made power output more stable.

It is generally understood that noise increases with higher blade tip speeds. To increase tip speed without increasing noise would allow reduction the torque into the gearbox and generator and reduce overall structural loads, thereby reducing cost. The reduction of noise is linked to the detailed aerodynamics of the blades, especially factors that reduce abrupt stalling. The inability to predict stall restricts the development of aggressive aerodynamic concepts. Some blades (mostly on Emerson) have a winglet to increase performance and/or reduce noise. 

A blade can have a lift-to-drag ratio of 120, compared to 70 for a sailplane and 15 for an airliner. 

The hub:

In simple designs, the blades are directly bolted to the hub and are unable to pitch, which leads to aerodynamic stall above certain wind speeds. In other more sophisticated designs, they are bolted to the pitch bearing, which adjusts their angle of attack with the help of a pitch system according to the wind speed to control their rotational speed. The pitch bearing is itself bolted to the hub. The hub is fixed to the rotor shaft which drives the generator directly or through a gearbox.

Blade count:

The number of blades is selected for aerodynamic efficiency, component costs, and system reliability. Noise emissions are affected by the location of the blades upwind or downwind of the tower and the speed of the rotor. Given that the noise emissions from the blades' trailing edges and tips vary by the 5th power of blade speed, a small increase in tip speed can make a large difference.

Wind turbines developed over the last 50 years have almost universally used either two or three blades. However, there are patents that present designs with additional blades, such as Chan Shin's Multi-unit rotor blade system integrated wind turbine. Aerodynamic efficiency increases with number of blades but with diminishing return. Increasing the number of blades from one to two yields a six percent increase in aerodynamic efficiency, whereas increasing the blade count from two to three yields only an additional three percent in efficiency.[27] Further increasing the blade count yields minimal improvements in aerodynamic efficiency and sacrifices too much in blade stiffness as the blades become thinner. 

Theoretically, an infinite number of blades of zero width is the most efficient, operating at a high value of the tip speed ratio. But other considerations lead to a compromise of only a few blades. 

Component costs that are affected by blade count are primarily for materials and manufacturing of the turbine rotor and drive train. Generally, the lower the number of blades, the lower the material and manufacturing costs will be. In addition, the lower the number of blades, the higher the rotational speed can be. This is because blade stiffness requirements to avoid interference with the tower limit how thin the blades can be manufactured, but only for upwind machines; deflection of blades in a downwind machine results in increased tower clearance. Fewer blades with higher rotational speeds reduce peak torques in the drive train, resulting in lower gearbox and generator costs.

System reliability is affected by blade count primarily through the dynamic loading of the rotor into the drive train and tower systems. While aligning the wind turbine to changes in wind direction (yawing), each blade experiences a cyclic load at its root end depending on blade position. This is true of one, two, three blades or more. However, these cyclic loads when combined together at the drive train shaft are symmetrically balanced for three blades, yielding smoother operation during turbine yaw. Turbines with one or two blades can use a pivoting teetered hub to also nearly eliminate the cyclic loads into the drive shaft and system during yawing. A Chinese 3.6 MW two-blade is being tested in Denmark. Mingyang won a bid for 87 MW (29 * 3 MW) two-bladed offshore wind turbines near Zhuhai in 2013. 

Finally, aesthetics can be considered a factor in that some people find that the three-bladed rotor is more pleasing to look at than a one- or two-bladed rotor.

Blade materials:

In general, ideal materials should meet the following criteria:

• wide availability and easy processing to reduce cost and maintenance

• low weight or density to reduce gravitational forces

• high strength to withstand strong loading of wind and gravitational force of the blade itself

• high fatigue resistance to withstand cyclic loading

• high stiffness to ensure stability of the optimal shape and orientation of the blade and clearance with the tower

• high fracture toughness

• the ability to withstand environmental impacts such as lightning strikes, humidity, and temperature

This narrows down the list of acceptable materials. Metals would be undesirable because of their vulnerability to fatigue. Ceramics have low fracture toughness, which could result in early blade failure. Traditional polymers are not stiff enough to be useful, and wood has problems with repeatability, especially considering the length of the blade. That leaves fibre-reinforced composites, which have high strength and stiffness and low density, as a very attractive class of materials for the design of wind turbines. 

Wood and canvas sails were used on early windmills due to their low price, availability, and ease of manufacture. Smaller blades can be made from light metals such as aluminium. These materials, however, require frequent maintenance. Wood and canvas construction limits the airfoil shape to a flat plate, which has a relatively high ratio of drag to force captured (low aerodynamic efficiency) compared to solid airfoils. Construction of solid airfoil designs requires inflexible materials such as metals or composites. Some blades also have incorporated lightning conductors.

New wind turbine designs push power generation from the single megawatt range to upwards of 10 megawatts using larger and larger blades. A larger area effectively increases the tip-speed ratio of a turbine at a given wind speed, thus increasing its energy extraction.[35] Computer-aided engineering software such as HyperSizer (originally developed for spacecraft design) can be used to improve blade design. 

As of 2015 the rotor diameters of onshore wind turbine blades are as large as 130 meters,[38] while the diameter of offshore turbines reach 170 meters. In 2001, an estimated 50 million kilograms of fibreglass laminate were used in wind turbine blades. 

An important goal of larger blade systems is to control blade weight. Since blade mass scales as the cube of the turbine radius, loading due to gravity constrains systems with larger blades.[41] Gravitational loads include axial and tensile/ compressive loads (top/bottom of rotation) as well as bending (lateral positions). The magnitude of these loads fluctuates cyclically and the edgewise moments (see below) are reversed every 180° of rotation. Typical rotor speeds and design life are ~10 and 20 years, respectively, with the number of lifetime revolutions on the order of 10^8. Considering wind, it is expected that turbine blades go through ~10^9 loading cycles. Wind is another source of rotor blade loading. Lift causes bending in the flatwise direction (out of rotor plane) while airflow around the blade cause edgewise bending (in the rotor plane). Flaps bending involves tension on the pressure (upwind) side and compression on the suction (downwind) side. Edgewise bending involves tension on the leading edge and compression on the trailing edge.

Wind loads are cyclical because of natural variability in wind speed and wind shear (higher speeds at top of rotation).

Failure in ultimate loading of wind-turbine rotor blades exposed to wind and gravity loading is a failure mode that needs to be considered when the rotor blades are designed. The wind speed that causes bending of the rotor blades exhibits a natural variability, and so does the stress response in the rotor blades. Also, the resistance of the rotor blades, in terms of their tensile strengths, exhibits a natural variability. 

In light of these failure modes and increasingly larger blade systems, there has been continuous effort toward developing cost-effective materials with higher strength-to-mass ratios. To extend the current 20 year lifetime of blades and enable larger area blades to be cost-effective, the design and materials need to be optimized for stiffness, strength, and fatigue resistance. 

The majority of current commercialized wind turbine blades are made from fibre-reinforced polymers (FRPs), which are composites consisting of a polymer matrix and fibres. The long fibres provide longitudinal stiffness and strength, and the matrix provides fracture toughness, delamination strength, out-of-plane strength, and stiffness. Material indices based on maximizing power efficiency, and having high fracture toughness, fatigue resistance, and thermal stability, have been shown to be highest for glass and carbon fibre reinforced plastics (GFRPs and CFRPs). 

Fiberglass-reinforced epoxy blades of Siemens SWT-2.3-101 wind turbines. The blade size of 49 meters is in comparison to a substation behind them at Wolfe Island Wind Farm.

Manufacturing blades in the 40 to 50-metre range involves proven fibreglass composite fabrication techniques. Manufactures such as Nordex SE and GE Wind use an infusion process. Other manufacturers use variations on this technique, some including carbon and wood with fibreglass in an epoxy matrix. Other options include pre-impregnated ("prepare") fibreglass and vacuum-assisted resin transfer moulding. Each of these options use a glass-fibre reinforced polymer composite constructed with differing complexity. Perhaps the largest issue with more simplistic, open-mould, wet systems are the emissions associated with the volatile organics released. Preimpregnated materials and resin infusion techniques avoid the release of volatiles by containing all VOCs. However, these contained processes have their challenges, namely, the production of thick laminates necessary for structural components becomes more difficult. As the preform resin permeability dictates the maximum laminate thickness, bleeding is required to eliminate voids and ensure proper resin distribution. One solution to resin distribution a partially impregnated fibreglass. During evacuation, the dry fabric provides a path for airflow and, once heat and pressure are applied, the resin may flow into the dry region resulting in a thoroughly impregnated laminate structure. 

Epoxy-based composites have environmental, production, and cost advantages over other resin systems. Epoxies also allow shorter cure cycles, increased durability, and improved surface finish. Prepare operations further reduce processing time over wet lay-up systems. As turbine blades pass 60 metres, infusion techniques become more prevalent; the traditional resin transfer moulding injection time is too long as compared to the resin set-up time, limiting laminate thickness. Injection forces resin through a thicker ply stack, thus depositing the resin wherein the laminate structure before gelation occurs. Specialized epoxy resins have been developed to customize lifetimes and viscosity. 

Carbon fibre-reinforced load-bearing spars can reduce weight and increase stiffness. Using carbon fibres in 60-metre turbine blades is estimated to reduce total blade mass by 38% and decrease cost by 14% compared to 100% fibreglass. Carbon fibres have the added benefit of reducing the thickness of fibreglass laminate sections, further addressing the problems associated with resin wetting of thick lay-up sections. Wind turbines may also benefit from the general trend of increasing use and decreasing cost of carbon fibre materials.[40]

Although glass and carbon fibres have many optimal qualities for turbine blade performance, there are several downsides to these current fillers, including the fact that high filler fraction (10-70 wt %) causes increased density as well as microscopic defects and voids that often lead to premature failure. 

Recent developments include interest in using carbon nanotubes (CNTs) to reinforce polymer-based nano composites. CNTs can be grown or deposited on the fibres or added into polymer resins as a matrix for FRP structures. Using nanoscale CNTs as filler instead of traditional micro scale filler (such as glass or carbon fibres) results in CNT/polymer nanocomposites, for which the properties can be changed significantly at very low filler contents (typically < 5 wt%). They have very low density and improve the elastic modulus, strength, and fracture toughness of the polymer matrix. The addition of CNTs to the matrix also reduces the propagation of interlinear cracks which can be a problem in traditional FRPs. 

Further improvement is possible through the use of carbon nanofibers (CNFs) in the blade coatings. A major problem in desert environments is erosion of the leading edges of blades by wind carrying sand, which increases roughness and decreases aerodynamic performance. The particle erosion resistance of fibre-reinforced polymers is poor when compared to metallic materials and elastomers, and needs to be improved. It has been shown that the replacement of glass fibre with CNF on the composite surface greatly improves erosion resistance. CNFs have also been shown to provide good electrical conductivity (important for lightning strikes), high damping ratio, and good impact-friction resistance. These properties make CNF-based nano paper a prospective coating for wind turbine blades. 

Another important source of degradation for turbine blades is lightning damage, which over the course of a normal 25-year lifetime is expected to experience a number of lightning strikes throughout its service. The range of damage caused from lightning strikes goes from merely surface level scorching and cracking of the laminate material, to ruptures in the blade or full separation in the adhesives that hold the blade together. It is most common to observe lightning strikes on the tips of the blades, especially in rainy weather due to the copper wiring within attracting lightning. The most common method to combat this, especially in non-conducting blade materials like GFRPs and CFRPs, is to add lightning "arresters", which are merely metallic wiring that provides an uninterrupted path to the ground, skipping the blades and gearbox entirely to eliminate the risk of damage in those components.

Blade recycling:

The Global Wind Energy Council (GWEC) predicts that wind energy will supply 15.7% of the world's total energy needs by the year 2020, and 28.5% by the year 2030. This dramatic increase in global wind energy generation will require installation of a newer and larger fleet of more efficient wind turbines and the consequent decommissioning of aging ones. Based on a study carried out by the European Wind Energy Association, in the year 2010 alone, between 110 and 140 kilotons of composites were consumed by the wind turbine industry for manufacturing blades. The majority of the blade material will eventually end up as waste, and in order to accommodate this level of composite waste, the only option is recycling. Typically, glass-fibre-reinforced-polymers (GFRPs) compose of around 70% of the laminate material in the blade. GFRPs hinder incineration and are not combustible.[52] Therefore, conventional recycling methods need to be modified. Currently, depending on whether individual fibres can be recovered, there exists a few general methods for recycling GFRPs in wind turbine blades:

• Mechanical Recycling: This method doesn't recover individual fibres. Initial processes involve shredding, crushing, and/or milling. The crushed pieces are then separated into fibre-rich and resin-rich fractions. These fractions are ultimately incorporated into new composites either as fillers or reinforcements. 

• Chemical Processing/Pyrolysis: Thermal decomposition of the composites is used to recover the individual fibres. For pyrolysis, the material is heated up to 500 °C in an environment without oxygen, thus causing it to break down into lower weight organic substances and/or gaseous products. The glass fibres will generally lose 50% of their initial strength and can now be down cycled for fibre reinforcement applications in paints or concrete.[54] Research has shown that this end of life option is able to recover up to approximately 19 MJ/kg.[55] However, this method has a relatively high cost and requires similar mechanical pre-processing. In addition, it has not yet been modified to satisfy the future need of large scale wind turbine blade recycling. 

• Direct Structural recycling of composites: Developed to combat the inefficiencies and costs associated with chemical, thermal and mechanical recycling processes, which either reduce the performance properties or only act as filler for other composites. The general idea to this method is to reuse the composite as is, which can be achieved especially in larger composite materials as it can be partitioned in several pieces which can be used in other applications as is, without altering the chemical properties of the composite component.

Tower:

Tower height:

Wind velocities increase at higher altitudes due to surface aerodynamic drag (by land or water surfaces) and the viscosity of the air. The variation in velocity with altitude, called wind shear, is most dramatic near the surface. Typically, the variation follows the wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%. To avoid buckling, doubling the tower height generally requires doubling the diameter of the tower as well, increasing the amount of material by a factor of at least four.

At night time, or when the atmosphere becomes stable, wind speed close to the ground usually subsides whereas at turbine hub altitude it does not decrease that much or may even increase. As a result, the wind speed is higher and a turbine will produce more power than expected from the 1/7 power law: doubling the altitude may increase wind speed by 20% to 60%. A stable atmosphere is caused by radiative cooling of the surface and is common in a temperate climate: it usually occurs when there is a (partly) clear sky at night. When the (high altitude) wind is strong (a 10-meter wind speed higher than approximately 6 to 7 m/s) the stable atmosphere is disrupted because of friction turbulence and the atmosphere will turn neutral. A daytime atmosphere is either neutral (no net radiation; usually with strong winds and heavy clouding) or unstable (rising air because of ground heating—by the sun). Here again the 1/7 power law applies or is at least a good approximation of the wind profile. Indiana had been rated as having a wind capacity of 30,000 MW, but by raising the expected turbine height from 50 m to 70 m, the wind capacity estimate was raised to 40,000 MW, and could be double that at 100 m. 

For HAWTs, tower heights approximately two to three times the blade length have been found to balance material costs of the tower against better utilisation of the more expensive active components.

Sections of a wind turbine tower, transported in a bulk carrier ship

Road size restrictions makes transportation of towers with a diameter of more than 4.3 m difficult. Swedish analyses show that it is important to have the bottom wing tip at least 30 m above the tree tops, but a taller tower requires a larger tower diameter. A 3 MW turbine may increase output from 5,000 MWh to 7,700 MWh per year by going from 80 to 125 meter tower height. A tower profile made of connected shells rather than cylinders can have a larger diameter and still be transportable. A 100 m prototype tower with TC bolted 18 mm 'plank' shells has been erected at the wind turbine test centre Høvsøre in Denmark and certified by Det Norske Veritas, with a Siemens nacelle. Shell elements can be shipped in standard 12 m shipping containers, and 2½ towers per week are produced this way. 

As of 2003, typical modern wind turbine installations use towers about 210 ft (65 m) high. Height is typically limited by the availability of cranes. This has led to a variety of proposals for "partially self-erecting wind turbines" that, for a given available crane, allow taller towers that put a turbine in stronger and steadier winds, and "self-erecting wind turbines" that can be installed without cranes. 

Tower materials:

Currently, the majority of wind turbines are supported by conical tubular steel towers. These towers represent 30% – 65% of the turbine weight and therefore account for a large percentage of the turbine transportation costs. The use of lighter materials in the tower could greatly reduce the overall transport and construction cost of wind turbines, however the stability must be maintained. Higher grade S500 steel costs 20%-25% more than S335 steel (standard structural steel), but it requires 30% less material because of its improved strength. Therefore, replacing wind turbine towers with S500 steel would result in net savings both in weight and cost. 

Another disadvantage of conical steel towers is that constructing towers that meet the requirements of wind turbines taller than 90 meters proves challenging. High performance concrete shows potential to increase tower height and increase the lifetime of the towers. A hybrid of prestressed concrete and steel has shown improved performance over standard tubular steel at tower heights of 120 meters. Concrete also gives the benefit of allowing for small precast sections to be assembled on site, avoiding the challenges steel faces during transportation. One downside of concrete towers is the higher CO2 emissions during concrete production as compared to steel. However, the overall environmental benefit should be higher if concrete towers can double the wind turbine lifetime. 

Wood is being investigated as a material for wind turbine towers, and a 100 metre tall tower supporting a 1.5 MW turbine has been erected in Germany. The wood tower shares the same transportation benefits of the segmented steel shell tower, but without the steel resource consumption. 

Connection to the Electric Grid:

All grid-connected wind turbines, from the first one in 1939 until the development of variable-speed grid-connected wind turbines in the 1970s, were fixed-speed wind turbines. As recently as 2003, nearly all grid-connected wind turbines operated at exactly constant speed (synchronous generators) or within a few percent of constant speed (induction generators). As of 2011, many operational wind turbines used fixed speed induction generators (FSIG). As of 2011, most new grid-connected wind turbines are variable speed wind turbines—they are in some variable speed configuration. 

Early wind turbine control systems were designed for peak power extraction, also called maximum power point tracking—they attempt to pull the maximum possible electrical power from a given wind turbine under the current wind conditions. More recent wind turbine control systems deliberately pull less electrical power than they possibly could in most circumstances, in order to provide other benefits, which include:

• spinning reserves to quickly produce more power when needed—such as when some other generator suddenly drops from the grid—up to the max power supported by the current wind conditions. 

• Variable-speed wind turbines can (very briefly) produce more power than the current wind conditions can support, by storing some wind energy as kinetic energy (accelerating during brief gusts of faster wind) and later converting that kinetic energy to electric energy (decelerating, either when more power is needed elsewhere, or during short lulls in the wind, or both). 

• damping (electrical) sub synchronous resonances in the grid

• damping (mechanical) resonances in the tower 

The generator in a wind turbine produces alternating current (AC) electricity. Some turbines drive an AC/AC converter—which converts the AC to direct current (DC) with a rectifier and then back to AC with an inverter—in order to match the frequency and phase of the grid. However, the most common method in large modern turbines is to instead use a doubly fed induction generator directly connected to the electricity grid.

A useful technique to connect a permanent magnet synchronous generator to the grid is by using a back-to-back converter. Also, we can have control schemes so as to achieve unity power factor in the connection to the grid. In that way the wind turbine will not consume reactive power, which is the most common problem with wind turbines that use induction machines. This leads to a more stable power system. Moreover, with different control schemes a wind turbine with a permanent magnet synchronous generator can provide or consume reactive power. So, it can work as a dynamic capacitor/inductor bank so as to help with the power systems' stability.

Reactive power regulation consists of one PI controller in order to achieve operation with unity power factor (i.e. Qgrid = 0). It is obvious that IdN has to be regulated to reach zero at steady-state (IdNref = 0).

Economics and Demand side management:

Energy demand management, also known as demand-side management (DSM) or demand-side response (DSR), is the modification of consumer demand for energy through various methods such as financial incentives and behavioural change through education.

Usually, the goal of demand-side management is to encourage the consumer to use less energy during peak hours, or to move the time of energy use to off-peak times such as night time and weekends. Peak demand management does not necessarily decrease total energy consumption, but could be expected to reduce the need for investments in networks and/or power plants for meeting peak demands. An example is the use of energy storage units to store energy during off-peak hours and discharge them during peak hours. A newer application for DSM is to aid grid operators in balancing intermittent generation from wind and solar units, particularly when the timing and magnitude of energy demand does not coincide with the renewable generation. 

Energy wheeling and Energy banking concepts:

In 1978, the Public Utility Regulatory Policies Act (PURPA) required regulated electric utilities to buy power from non-utility generators using cogeneration, renewable, or other sources at the utilities’ avoided cost of generation. But because of transmission challenges and other factors, renewable generators sold power to the nearest utility instead of selling it to the most favourable market. The Energy Policy Act of 1992 (EPAct) further removed barriers to the market entry of IPPs by requiring well-established competitive generators to be given rates and terms comparable to non-IPPs. To carry out these goals, FERC issued Order 888 in 1996, requiring transmission owners to mitigate undue discrimination in transmission networks and to provide open access of their systems to wholesale customers under a regulated Open Access Transmission Tariff (OATT).  The order required public utilities to file a single wholesale open access tariff for point-to-point and network services. This order was critical in promoting competitive wholesale electricity markets as a part of a larger restructuring effort. With larger wholesale electricity markets, generators need to rely less on wheeling from one balancing area to another, thereby minimizing traditional wheeling transactions.

Safety and Environmental aspects:

Safety:

Some turbine nacelle fires cannot be extinguished because of their height, and are sometimes left to burn themselves out. In such cases they generate toxic fumes and can cause secondary fires below. Newer wind turbines, however, are built with automatic fire extinguishing systems similar to those provided for jet aircraft engines. These autonomous systems, which can be retrofitted to older wind turbines, automatically detect a fire, shut down the turbine unit, and extinguish the fires. 

During winter, ice may form on turbine blades and subsequently be thrown off during operation. This is a potential safety hazard, and has led to localised shut-downs of turbines. A 2007 study noted that no insurance claims had been filed, either in Europe or the US, for injuries from ice falling from wind towers, and that while some fatal accidents have occurred to industry workers, only one wind-tower related fatality was known to occur to a non-industry person: a parachutist. 

Given the increasing size of production wind turbines, blade failures are increasingly relevant when assessing public safety risks from wind turbines. The most common failure is the loss of a blade or part thereof.

Environmental Aspects:

"Wind turbines emit low frequency noise, which can enter the home with little or no reduction in energy, potentially resulting in.annoyance."

Regarding the comparison of low frequency wind turbine noise annoyance to transportation noise annoyance, the Health Canada study summary states: "Studies have consistently shown. That, in comparison to the scientific literature on noise annoyance to transportation noise sources such as rail or road traffic, community annoyance with (low frequency) wind turbine noise begins at a lower sound level and increases more rapidly with increasing wind turbine noise."

 

Wind Energy potential and installation in India:

Wind power generation capacity in India has significantly increased in recent years. As of 31 December 2019 the total installed wind power capacity was 37.505 GW, the fourth largest installed wind power capacity in the world. Wind power capacity is mainly spread across the Southern, Western and Northern regions. 

Wind power costs in India are decreasing rapidly. The levelised tariff of wind power reached a record low of ₹2.43 (3.4¢ US) per kWh (without any direct or indirect subsidies) during auctions for wind projects in December 2017. In December 2017, union government announced the applicable guidelines for tariff-based wind power auctions to bring more clarity and minimise the risk to the developers. 

 

 

 

UNIT – IV

BIOGAS

Properties of Biogas(calorific value and composition):

Biogas typically refers to a gas produced by the anaerobic digestion of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions. Biogas is comprised primarily of methane and carbon dioxide. It also contains smaller amounts of hydrogen sulphide, nitrogen, hydrogen, methylmercaptans and oxygen.

Biogas originates from bacteria in the process of bio-degradation of organic material under anaerobic (without air) conditions. The natural generation of biogas is an important part of the biogeochemical carbon cycle. Methanogens (methane producing bacteria) are the last link in a chain of micro-organisms which degrade organic material and return the decomposition products to the environment. In this process biogas is generated, a source of renewable energy.

The gases methane, hydrogen and carbon monoxide can be combusted or oxidized with oxygen. Air contains 21% oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a low-cost fuel in any country for any heating purpose, such as cooking. It can also be utilized in modern waste management facilities where it can be used to run any type of heat engine, to generate either mechanical or electrical power. Biogas is a renewable fuel and electricity produced from it can be used to attract renewable energy subsidies in some parts of the world.

Below is the biogas equivalent to different fuels:

• 1 Kg firewood => 0.2 m³ biogas

• 1 Kg dried cow dung => 0.1 m³ biogas

• 1 Kg Charcoal => 0.5 m³ biogas

• 1 Litre Kerosene => 2.0 m³ biogas

Composition and properties of Biogas:

The composition of biogas varies depending upon the origin of the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55-75% CH4.

 

Component	Content [%]
Methane, CH4	50-75
Carbon dioxide, CO2	25-50
Nitrogen, N2	0-10
Hydrogen, H2	0-1
Hydrogen sulphide, H2S	0-3
Oxygen, O2	0-2

Like those of any pure gas, the characteristic properties of biogas are pressure and temperature-dependent.

They are also affected by the moisture content. The factors of main interest are:

• change in volume as a function of temperature and pressure,

• change in calorific value as a function of temperature, pressure and water-vapor content, and

• Change in water-vapour content as a function of temperature and pressure.

 

The calorific value of biogas is about 6 kWh/m3 - this corresponds to about half a litre of diesel oil. The net calorific value depends on the efficiency of the burners or appliances. Methane is the valuable component under the aspect of using biogas as a fuel.

 

Biogas plant technology and status:

Biogas is the mixture of gases produced by the breakdown of organic matter in the absence of oxygen (anaerobically), primarily consisting of methane and carbon dioxide. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. Biogas is a renewable energy source. In India, it is also known as "Gobar Gas".

Biogas is produced by anaerobic digestion with methanogen or anaerobic organisms, which digest material inside a closed system, or fermentation of biodegradable materials. This closed system is called an anaerobic digester, bio digester or a bioreactor. 

Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of hydrogen sulfide (H2S), moisture and siloxanes. The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat. 

 

 

Bio Energy System:

Bioenergy is renewable energy made available from materials derived from biological sources. Biomass is any organic material which has stored sunlight in the form of chemical energy. As a fuel it may include wood, wood waste, straw, and other crop residues, manure, sugarcane, and many other by-products from a variety of agricultural processes. By 2010, there was 35 GW (47,000,000 hp) of globally installed bioenergy capacity for electricity generation, of which 7 GW (9,400,000 hp) was in the United States. 

In its most narrow sense it is a synonym to biofuel, which is fuel derived from biological sources. In its broader sense it includes biomass, the biological material used as a biofuel, as well as the social, economic, scientific and technical fields associated with using biological sources for energy. This is a common misconception, as bioenergy is the energy extracted from the biomass, as the biomass is the fuel and the bioenergy is the energy contained in the fuel.

Solid Biomass:

One of the advantages of biomass fuel is that it is often a by-product, residue or waste-product of other processes, such as farming, animal husbandry and forestry. In theory this means there is no competition between fuel and food production, although this is not always the case. Land use, existing biomass industries and relevant conversion technologies must be considered when evaluating suitability of developing biomass as feedstock for energy. 

Biomass is the material derived from recently living organisms, which includes plants, animals and their by-products. Manure, garden waste and crop residues are all sources of biomass. It is a renewable energy source based on the carbon cycle, unlike other natural resources such as petroleum, coal, and nuclear fuels. Another source includes Animal waste, which is a persistent and unavoidable pollutant produced primarily by the animals housed in industrial-sized farms.

There are also agricultural products specifically being grown for biofuel production. These include corn, and soybeans and to some extent willow and switch grass on a pre-commercial research level, primarily in the United States; rapeseed, wheat, sugar beet, and willow (15,000 ha or 37,000 acres in Sweden) primarily in Europe; sugarcane in Brazil; palm oil and miscanthusin Southeast Asia; sorghum and cassava in China; and jatropha in India. Hemp has also been proven to work as a biofuel. Biodegradable outputs from industry, agriculture, forestry and households can be used for biofuel production, using e.g. anaerobic digestion to produce biogas, gasification to produce syngas or by direct combustion. Examples of biodegradable wastes include straw, timber, manure, rice husks, sewage, and food waste. The use of biomass fuels can therefore contribute to waste management as well as fuel security and help to prevent or slow down climate change, although alone they are not a comprehensive solution to these problems.

Biomass can be converted to other usable forms of energy like methane gas or transportation fuels like ethanol and biodiesel. Rotting garbage, and agricultural and human waste, all release methane gas—also called "landfill gas" or "biogas." Crops, such as corn and sugar cane, can be fermented to produce the transportation fuel, ethanol. Biodiesel, another transportation fuel, can be produced from left-over food products like vegetable oils and animal fats. Also, Biomass to liquids (BTLs) and cellulosic ethanol are still under research.

Sewage Biomass:

The use of municipal and household waste is on the forefront of new sources for biomass, and is a largely discarded resource on which new research is being conducted for use of energy production. A new bioenergy sewage treatment process aimed at developing countries is now on the horizon; the Omni Processor is a self-sustaining process which uses the sewerage solids as fuel to convert sewage waste water into drinking water and electrical energy. Sewage sludge is a point of focus in current research for developing bioenergy from biomass. The large quantity being produced by households at a continuous rate presents an opportunity to extract valuable compounds contained within it which can be then used to produce bioenergy. The main form of bioenergy being produced from sewage is methane, but producing other forms is still being researched. The use of sewage to produce methane reduces the amount of waste put into landfills, its costs of transportation and disposal, and also keeps a larger amount of gas out of the atmosphere, as more is able to be captured.

Electricity generation from Biomass:

The biomass used for electricity production ranges by region. Forest by-products, such as wood residues, are popular in the United States. Agricultural waste is common in Mauritius (sugar cane residue) and Southeast Asia (rice husks). Animal husbandry residues, such as poultry litter, is popular in the UK.

Electricity from electrogenic micro-organism:
Another form of bioenergy can be attained from microbial fuel cells, in which chemical energy stored in wastewater or soil is converted directly into electrical energy via the metabolic processes of electrogenic micro-organisms. The power generation capability of this technology has not been found to be economically viable till date, however, this technology has been found to be more useful for chemical treatment processes and student education. 

Biomass conversion process:

There are many different types of biomass that include crop wastes, forestry residues, purpose-grown grasses, woody energy crops, algae, industrial and municipal organic wastewaters and sludges, non-recyclable municipal solid waste, urban wood waste, and food waste.  Biomass is considered renewable as either a feedstock or waste and due to government incentives, corporate sustainability goals and climate change initiatives, a majority of the conversion technologies use biomass to produce various forms of renewable energy.  The type of energy includes electrical power, thermal energy, renewable natural gas, biodiesel, jet fuel, and ethanol.

Biomass also can be used as a substitute for fossil fuels in the manufacturing of high value products including plastics, lubricants, industrial chemicals, and many other products derived from petroleum or natural gas. The US Department of Energy’s Bioenergy Technologies Office also is promoting the existing “petroleum refinery” model, where these “bio products” can be produced alongside biofuels at an integrated “biorefinery.” This co-production strategy offers a more efficient, cost-effective, and integrated approach to the utilization of our nation’s biomass resources. Revenue generated from bio products provides added value, improving the economics of biorefinery operations and creating more cost-competitive biofuels.

There are four types of conversion technologies currently available that may result in specific energy.and.potential.renewable.products:

Thermal conversion is the use of heat, with or without the presence of oxygen, to convert biomass into other forms of energy and products.  These include direct combustion, pyrolysis, and torrefaction.  

• Combustion is the burning of biomass in the presence of oxygen. The waste heat is used to for hot water, heat, or with a waste heat boiler to operate a steam turbine to produce electricity.  Biomass also can be co-fired with existing fossil fuel power stations.

• Pyrolysis convert biomass feed stocks under controlled temperature and absent oxygen into gas, oil and biochar (used as valuable soil conditioner and also to make grapheme). The gases and oil can be used to power a generator and some technologies can also make diesel and chemicals from the gases.

• Torrefaction is similar to pyrolysis but in a lower operating temperature range. The final product is an energy dense solid fuel often referred to as “bio-coal”.

Thermochemical conversion is commonly referred to as gasification. This technology uses high temperatures in a controlled partial combustion to form a producer gas and charcoal followed by chemical reduction. A major use for biomass is for agriculture residues with gas turbines. Advanced uses include production of diesel, jet fuel and chemicals.

Biochemical Conversion involves the use of enzymes, bacteria or other microbes to break down biomass into liquids and gaseous feedstocks and includes anaerobic digestion and fermentation. These feedstocks can be converted to energy, transportation fuels and renewable chemicals.

Chemical Conversion involves the use of chemical agents to convert biomass into liquid fuels which mostly is converted to biodiesel.

Direct Combustion:

Direct combustion is a thermochemical technique in which the biomass is burned in open air or in the presence of excess air. In this process, the photo synthetically stored chemical energy of the biomass will be converted into gases. Generally, direct combustion is carried out inside a furnace, steam turbine, or boiler at a temperature range of 800–1000°C. This process is suitable for all types of biomass, which has low moisture content (<50%). The cost of energy production from direct combustion is slightly higher as compared to pyrolysis and gasification due to the need of pre-treatment of biomass, such as dehydrating, cutting, and crushing, before introducing into the combustion chamber. There is another possibility for sustainable operation of microalgae biomass in direct combustion is coal–algae coffering. Kadam proposed that the idea of coal–algae coffering could reduce the emission of CO2 by recycling the CO2 from the combustion process to microalgae cultivation. This leads to lower emission of GHGs into the atmosphere and a golden opportunity for carbon credit program . However, there is still no extensive study that has been carried out to determine the feasibility of this technology.

 

Biomass Gasification:

Biomass gasification is a process of converting solid biomass fuel into a gaseous combustible gas (called producer gas) through a sequence of thermo-chemical reactions. The gas is a low-heating value fuel, with a calorific value between 1000- 1200 kcal/ Nm3 (kilo calorie per normal cubic metre). Almost 2.5-3.0 Nm3 of gas can be obtained through gasification of about 1 kg of air-dried biomass. Since the 1980's the research in biomass gasification has significantly increased in developing countries, as they aim to achieve energy security.

TERI independently began research work in gasifier technology in the mid-1980s. Since, the gasifier technology has been customized for a range of direct-heat application and tested successfully in the field. Silk processing, large-cardamom drying and gasifier-based crematoria are a few examples of the applications worked on at TERI. This technology is slowly replacing both traditional biomass use and gas-powered systems, as it provides an excellent de-centralized source of energy at an affordable cost. Apart from rural households, biomass fuels are the main source of energy to a large number of small, rural and cottage industries.

Salient Features of TERI's biomass gasifier

• Throat-less patented design

• Multi-fuel capability

• Low initial investment

• Better conversion (solid gas) efficiency (>75%)

• Production of clean gases in the exhaust

• Available in both, downdraft and updraft mode

• Can be customized for a variety of applications

• Thermal application to meet the process heat requirement

• Power application for rural electrification and captive use

• Shaft powder

• Reduced deforestation through fuel wood savings

• Substantial reduction in diesel/kerosene/furnace oil cost (since 3-4 kg of biomass can replace 1 litre of petroleum fuel)

• Use of castable insulation material in the fire box capable of withstanding high temperatures (upto 1860°C)

• Since biomass is a carbon neutral fuel, the net emission of CO2  would amount to zero

 

Biochemical Conversion:

Biomass biochemical conversion technologies refer to the conversion of biomass into corresponding products through certain physical, chemical, and biological pretreatments. Pretreatments in the biochemical conversion technologies of biomass aim to help reach ideal conversion effects, not to produce final products, which is the essential difference between the aforementioned physical and chemical conversion of biomass. In addition, biochemical conversion technologies of biomass are more moderate than the other two.

Biomass can be turned into different products, such as hydrogen, biogas, ethanol, acetone, butanol, organic acids (pyruvate, lactate, oxalic acid, levulinic acid, citric acid), 2,3-butanediol, 1,4-butanediol, isobutanol, xylitol, mannitol, and xanthan gum by selecting different microorganisms in the process of biochemical conversion (Chen, 2010). On the one hand, such products can synthetize replacements of petroleum-based products. On the other hand, the products can replace products derived from grains, such as ethanol.

Compared with other conversion technologies, biomass biochemical conversion technologies are moderate, pure, clean, and efficient. Moreover, biomass can be turned into various intermediates by screening different enzymes or microorganisms through biochemical conversion technologies, thus providing many platform substances for the conversion of renewable materials, fuels, and chemicals. As a result, people pay much attention to biochemical conversion technologies of biomass.

 

Anaerobic digestion:

The treatment of any slurry or sludge containing a large amount of organic matter utilizing bacteria and other organisms under anaerobic condition is commonly referred as anaerobic digestion or digestion. Anaerobic digestion consists of the following three stages. The three stages are (i) the enzymatic hydrolysis, (ii) acid formation and (iii) methane formation.

 

 

Application:

Biogas can be used for electricity production on sewage works, in a CHP gas engine, where the waste heat from the engine is conveniently used for heating the digester; cooking; space heating; water heating; and process heating. If compressed, it can replace compressed natural gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells and is a much more effective displacer of carbon dioxide than the normal use in on-site CHP plants.

Alcohol production from biomass:

Biodiesel is a liquid fuel produced from renewable sources, such as new and used vegetable oils and animal fats and is a cleaner-burning replacement for petroleum-based diesel fuel. Biodiesel is nontoxic and biodegradable and is produced by combining alcohol with vegetable oil, animal fat, or recycled cooking grease.

Like petroleum-derived diesel, biodiesel is used to fuel compression-ignition (diesel) engines. Biodiesel can be blended with petroleum diesel in any percentage, including B100 (pure biodiesel) and, the most common blend, B20 (a blend containing 20% biodiesel and 80% petroleum diesel).

 

Biodiesel production:

Biodiesel production is the process of producing the biofuel, biodiesel, through the chemical reactions of transesterification and esterification. This involves vegetable or animal fats and oils being reacted with short-chain alcohols (typically methanol or ethanol). The alcohols used should be of low molecular weight. Ethanol is the most used because of its low cost, however, greater conversions into biodiesel can be reached using methanol. Although the transesterification reaction can be catalyzed by either acids or bases, the base-catalyzed reaction is more common. This path has lower reaction times and catalyst cost than those acid catalysis. However, alkaline catalysis has the disadvantage of high sensitivity to both water and free fatty acids present in the oils.

Urban waste to energy conversion:

Waste-to-energy is the use of modern combustion and biological technologies to recover energy from urban wastes. There are three major waste to energy conversion routes – thermochemical, biochemical and physico-chemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. On the other hand, biochemical technologies are more suitable for wet wastes which are rich in organic matter.

Thermochemical Conversion:

The three principal methods of thermochemical conversion are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. The most common technique for producing both heat and electrical energy from household wastes is direct combustion.

Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.

Combustion technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine. Plasma gasification, which takes place at extremely high temperature, is also hogging limelight nowadays.

Biochemical Conversion:

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biofertilizer and biogas.

Anaerobic digestion is a reliable technology for the treatment of wet, organic waste.  Organic waste from various sources is biochemically degraded in highly controlled, oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat.

In addition, a variety of fuels can be produced from waste resources including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking.

Physico-chemical Conversion:

The physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation. The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter are then mechanically separated before the waste is compacted and converted into fuel pellets or RDF.

Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.

 

Biomass energy programme in India:

Biomass has always been an important energy source for the country considering the benefits and promises it offers. It is a carbon neutral fuel source for the generation of electricity; and apart from providing the much needed relief from power shortages, biomass power projects could generate employment in rural areas.

About 32% of the total primary energy use in the country is derived from biomass and more than 70% of the country’s population depends upon it for their energy needs. The Ministry of New and Renewable Energy (MNRE), Government of India has realized the potential and role of biomass energy in the Indian context and has initiated a number of programmes for the promotion of efficient biomass conversion technologies to be used in various sectors of the economy.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UNIT – V

OCEAN ENERGY

Ocean Wave Energy Conversion:

Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC).

Wave power is distinct from tidal power, which captures the energy of the current caused by the gravitational pull of the Sun and Moon. Waves and tides are also distinct from ocean currents which are caused by other forces including breaking waves, wind, the Coriolis effect, cabbeling, and differences in temperature and salinity.

Wave-power generation is not a widely employed commercial technology compared to other established renewable energy sources such as wind power, hydropower and solar power. However, there have been attempts to use this source of energy since at least 1890 mainly due to its high power density. As a comparison, the power density of the photovoltaic panels is 1 kW/m2 at peak solar insolation, and the power density of the wind is 1 kW/m2 at 12 m/s. whereas, the average annual power density of the waves at e.g. San Francisco coast is 25 kW/m2. 

Principles of ocean thermal energy conversion (OTEC):

The basic working principle of OTEC is quite simple. The warm water is used to evaporate a working fluid with a low boiling point. The high pressure vapour that is produced drives a turbine-generator to produce electricity. The cold deep seawater is used to condense the working fluid vapour back into a liquid.

Tidal power is taken from the Earth's oceanic tides. Tidal forces are periodic variations in gravitational attraction exerted by celestial bodies. These forces create corresponding motions or currents in the world's oceans. Due to the strong attraction to the oceans, a bulge in the water level is created, causing a temporary increase in sea level. As the Earth rotates, this bulge of ocean water meets the shallow water adjacent to the shoreline and creates a tide. This occurrence takes place in an unfailing manner, due to the consistent pattern of the moon's orbit around the earth. The magnitude and character of this motion reflects the changing positions of the Moon and Sun relative to the Earth, the effects of Earth's rotation, and local geography of the seafloor and coastlines.

Tidal power is the only technology that draws on energy inherent in the orbital characteristics of the Earth–Moon system, and to a lesser extent in the Earth–Sun system. Other natural energies exploited by human technology originate directly or indirectly with the Sun, including fossil fuel, conventional hydroelectric, wind, biofuel, wave and solar energy. Nuclear energy makes use of Earth's mineral deposits of fissionable elements, while geothermal power utilizes the Earth's internal heat, which comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).

A tidal generator converts the energy of tidal flows into electricity. Greater tidal variation and higher tidal current velocities can dramatically increase the potential of a site for tidal electricity generation.

Because the Earth's tides are ultimately due to gravitational interaction with the Moon and Sun and the Earth's rotation, tidal power is practically inexhaustible and classified as a renewable energy resource. Movement of tides causes a loss of mechanical energy in the Earth-Moon system: this is a result of pumping of water through natural restrictions around coastlines and consequent viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since its formation. During the last 620 million years the period of rotation of the earth (length of a day) has increased from 21.9 hours to 24 hours;[8] in this period the Earth has lost 17% of its rotational energy. While tidal power will take additional energy from the system, the effect is negligible and would only be noticed over millions of years. 

Ocean Thermal Power Plants:

Ocean Thermal Energy Conversion (OTEC) is a process that can produce electricity by using the temperature difference between deep cold ocean water and warm tropical surface waters. OTEC plants pump large quantities of deep cold seawater and surface seawater to run a power cycle and produce electricity.

 

Tidal Energy Conversion:

Tidal energy or tidal power is a form of renewable energy obtained due to alternating sea levels. The kinetic energy from the natural rise and fall of tides is harnessed and converted into electricity. Tides are caused by the combined gravitational forces of the moon, sun, and earth.

Small hydro power plant

 

Importance of small hydro power plant and their elements:

Hydropower is a method of generating electricity that uses moving water (kinetic energy) to produce electricity. Small-scale hydropower has been used as a common way of generating electricity in isolated regions since end of 19th century. Small-scale hydropower systems can be installed in small rivers, streams or in the existing water supply networks, such as drinking water or wastewater networks. In contrast with large-scale hydropower systems, small-scale hydropower can be installed with little or negligible environmental impact on wildlife or ecosystems, mainly because the majority of small hydropower plants are run-of-river schemes or implemented in existing water infrastructure. Due to its versatility, low investment costs, and as a renewable energy source, small-scale hydropower is a promising option for producing sustainable, inexpensive energy in rural or developing areas.

Types of turbines for small hydro

There are two main types of hydro turbines: impulse and reaction. The type of hydropower turbine selected for a project is based on the height of standing water—referred to as "head"—and the flow, or volume of water, at the site. Other deciding factors include how deep the turbine must be set, efficiency, and cost.

IMPULSE TURBINE

The impulse turbine generally uses the velocity of the water to move the runner and discharges to atmospheric pressure. The water stream hits each bucket on the runner. There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner. An impulse turbine is generally suitable for high head, low flow applications.

REACTION TURBINE:

A reaction turbine develops power from the combined action of pressure and moving water. The runner is placed directly in the water stream flowing over the blades rather than striking each individually. Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines.

Geothermal Power Plant:

Geothermal energy is thermal energy generated and stored in the Earth. ... The adjective geothermal originates from the Greek roots γη (geo), meaning earth, and θερμος (thermos), meaning hot. Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation.

Various Types:

All geothermal power plants use steam to turn large turbines, which run electrical generators. In the Geysers area, dry steam from below ground is used directly in the steam turbines. In other areas of the state, super – hot water is “flashed” into steam within the power plant, and that steam turns the turbine.

Direct Dry Steam:

Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine.

Flash and Double Flash Cycle:

Hydrothermal fluids above 3600F(1820C) can be used in flash plants to make electricity. Fluid is sprayed into a tank held at a much lower pressure that the fluid, causing some of the fluid to rapidly vaporize, or “flash”. The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank (double flash) to extract even more energy.