6

8. RENEWABLE ENERGY SOURCES8.1 INTRODUCTIONIncreasing the use of domestic renewable energy resources is one of the primary goals of U.E. and U.S.A. energy policy makers. Unfortunately there is an underlying problem to this growth target. Renewable energy market penetration continues to lag expectations, and significant efforts on many fronts will be needed to expand the level of renewable energy generation to the point where it can begin to account for such a significant segment of the supply base. From these sources, power production will continue to expand especially small scale hydro, solar and wind power, because are quite flexible and can be integrated most easily into the existing infrastructure. The main renewable energy plants are the followings:

fuel cell plants,

small scale hydro,

solar plants,

wind plants.

8.2 FUEL CELL PLANTSThe base element of a fuel cell plant is the fuel cell. A complete fuel cell plant comprises a fuel delivery system, a stack, various controls over the plant operation and output power conditioning equipment.8.2.1 Fuel cells

Fuel cells are energy conversion devices, which by combining hydrogen and oxygen into water convert chemical energy into electricity and heat. A fuel cell works much like a battery. In both batteries and fuel cells two electrodes, an anode and a cathode, are separated by an electrolyte. Whereas a storage battery contains all the substances in the electrochemical oxidation-reduction reactions involved and has, therefore, a limited capacity. A fuel cell is supplied with its reactants externally and operates continuously as long as it is supplied with fuel. There are five basic types of fuel cell being commercially developed, a classification being based on the electrolytes used. Low temperature types include the alkaline fuel cell (AFC) and the solid polymer fuel cell (SPFC), to which belong the proton exchange membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC), the medium temperature type is the phosphoric acid fuel cell (PAFC) and the two high temperature types are the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC). An idealized schematic diagram illustrating the structure, electron and ion flow for the various types of fuel cell is shown in Fig. 8.1.Phosphoric acid (PAFC) and proton exchange (PEMFC) fuel cells both use acid electrolytes, alkaline (AFC) and molten carbonate (MCFC) fuel cells use liquid alkaline based electrolytes while solid oxide (SOFC) fuel cells use a zirconia-based ceramic. The direction of the ion flow depends on whether the ion is positively or negatively charged and also determines the site of water formation and subsequent removal. Table 8.1 summarizes the different characteristics of these fuel cells.

Fig. 8.1: Principle of fuel cell operation.

Alkaline fuel cells (AFC): the application of AFCs in space has been especially noteworthy. They are ideally suited to closed environments containing their own supplies of hydrogen and oxygen and have also been demonstrated in a variety of automotive applications. AFC performance is particularly sensitive to contaminants in the gas supplies, notably carbon dioxide, which reacts with the electrolyte to form a carbonate and reduces the conductivity. With the chemical reaction occurring at the cathode and the low operating temperature the start-up time of the AFC is very fast and the cell yields high power generation efficiencies.Proton exchange membrane fuel cells (PEMFC): the low operating temperatures and the solid electrolyte (an acid-based ion conducting plastic membrane) also make PEMFCs suited to a wide array of uses from low and medium to high power applications. Such a list would include power tools, compressors, recreational applications in camping and boats, heat and electricity to dwellings and electricity to commercial buildings, schools and hospitals. They are most widely known at present for their potential in automotive applications where much evaluation is underway. The low temperature electrolyte requires platinum as the catalyst applied to either side of the membrane to accelerate the dissociation of hydrogen and oxygen.

Direct methanol fuel cell (DMFC): this cell is similar to the PEMFC except that hydrogen is extracted from a methanol/water solution. This gives it an advantage in as much as the hydrogen is extracted by the catalyst and not by the addition of complex reforming plant. The initial markets for DMFCs are considered to be in the small and medium power applications such as occupied by batteries in consumer and military electronic products. The power to weight ratio is theoretically of the order of 10:1 compared with batteries. The main outstanding problem is the cross-over of methanol from the anode to the cathode through the membrane drastically reducing the cell performance. Table 8.1: Fuel cell types and characteristics.

Phosphoric acid fuel cell (PAFC): these fuel cells have given much durable service over many years with a very stable electrolyte in medium to high power stationary applications. Typically 200 kW units are found in hospitals, schools, hotels and various military installations. With its higher temperature carbon monoxide poisoning is not such

a severe problem as in the PEMFCs, but with poor ionic conduction this comparatively large and weighty cell technology appears to be relatively expensive.

Molten carbonate fuel cell (MCFC): multi-megawatt plants have been demonstrated with obvious applications in commercial buildings, especially those requiring high quality heat such as in commercial buildings, hospitals and hotels. Developments include cells for use in natural gas and coal-based power plants in heavy industries. The high operating temperatures allow the use of fuels such as natural gas and coal gas without the need of noble metal catalysts and allow the fuel to be internally reformed without the need of complex reforming equipment. The quality of the heat allows for the possibility of higher efficiencies when coupled to CHP or combined cycle plant.

Solid oxide fuel cell (SOFC): here the biggest problem in the development of these cells is working with the high operating temperatures above 850oC where significant material problems occur. Below this temperature ionic conduction is a problem and the cell's performance deteriorates rapidly. Designs and demonstrations are being carried out to supply a wide variety of stationary needs from a few kilowatts to multi-megawatt industrial and power system plants as well as automotive power units. The high temperatures confer the same advantages onto this cell as for the MCFC design with the high temperatures adding further to CHP and combined cycle plants. As a completely solid state device, however, the management problems inherent in liquid electrolyte designs are avoided and, in principle, there are no constraints on cell configuration allowing flexibility in design. The theoretical limit on voltage developed by a single cell is about 1.23 V with typical operation being at about 0.7 V. Current (d.c.) delivered is approximately 0.5 amps/cm2 of cell surface area giving an output power of about 0.35 watts/ cm2. To generate more power the cells are connected together in stacks.8.2.2 Fuel Delivery SystemThe fuel delivery system can range from a simple flow control unit to a fuel pre- processing unit. In practice hydrogen always occurs in combination with other elements. It is necessary, therefore, to produce it either by electrolysis in the case of water or by separation in a reformer if a hydrocarbon fuel is used as a primary source. A fuel cell system, which includes a fuel reformer, utilizes, in principle, the hydrogen from any hydrocarbon fuel from natural gas to methanol. The two primary types of reformers being developed for transportation are steam reformers and partial oxidation reformers. Steam reformers have higher efficiency but partial oxidation reformers are simpler.8.3.3 Plant Control SystemPlant control includes water management control and appropriate temperature control. As exhaust product water is seen as providing a potentially important added benefit in fuel cell operation in future, given concerns about the adequate local supplies of pure water for human consumption and industrial purposes. With regard to temperature control, a preheating stage particularly for high temperature fuel cells maybe required for start-up. Heat exchangers are required to ensure the reactants enter the cells at appropriate temperatures for operation, again a particularly important requirement for high temperature cells. The flow rate of the oxidant generally controls the stack temperature.8.3.4 Plant Conditioning Equipment

Finally the power conditioning equipment converts the generated electricity, which is in the form of direct current, into the form required for use, usually alternating current at specified voltage and frequency. Figure 8.2 illustrates the structure of a fuel cell plant with heat reformer to convert fossil fuel such as natural gas or methanol into hydrogen, or a hydrogen rich gas for supply to the cells.

Fig. 8.2: Fuel cell power-generation system.Fuel cell systems incorporating the collection of the water exhaust, its dissociation by electrolysis using an external source of electricity with storage and subsequent use of the hydrogen and oxygen produced, form a regenerative system. Such systems could be independent of established fuel/energy infrastructures. The pure forms of the gases would be a further benefit in cells susceptible to carbon monoxide or carbon dioxide. Applications are seen initially in uninterruptible power supplies and in remote power requirements. A different type of regenerative fuel cell has been developed known as the Regenysis system which could also be considered as a flow cell type of battery using fuel cell technology. Its chemistry is not based on combining hydrogen and oxygen into water and converting chemical energy into electricity and heat, but instead stores or releases energy by means of a reversible electrochemical reaction between two electrolyte solutions.

One design comprising one hundred 100 kW stacks or modules provides 120 MWh of energy storage capable of being released at a rate of 12 MW for 10 h with a peak output of 14.75 MW. In common with all electrochemical systems, maximum efficiency is achieved below the maximum power rating. Power response is fast with zero to full discharge being achieved in 10 15 ms, being limited by the performance of the power conversion unit.8.3 SMALL-SCALE HYDRO PLANTS

A small-scale hydroelectric facility requires that a sizable flow of water and an adequate head of water areavailable without building elaborate and expensive facilities. Small hydroelectric plants can be developed at existing dams and have been constructed in connection with water level control ofrivers, lakes and irrigation schemes.By using existing structures, only minor new civil engineering works are required, which reduces the cost of this component of a development.In other, more rugged regions of the country, it is possible to develop relatively higher heads without elaborate or expensive civil engineering works so that relatively smaller flows are required to develop the desired power. In these cases, it may be possible to construct a relatively simple diversion structure and obtain the highest drop by diverting flows at the top of a waterfall or steeply falling water course.

8.3.1 Types of Small-Scale Developments

Small-scale hydro stations are classified in the Table 8.2:Table 8.2: Classification of small-scale hydro stations.Size of hydroelectric facilityPower output

Micro100 kW or less typical supply for one or two houses

Mini100 kW to 1 MW typical supply for a small factory or isolated community

Small1 MW to 30 MW typical NUG development and low end of range for supply to a regional or provincial power grid

In large facilities, custom design detailed engineering is required. Small-scale hydroelectric developments have to be approached quite differently to achieve economical feasibility.Over the last twenty-five years, efforts have been made to reduce development costs by improving all phases of project development. Some of the innovations produced by these efforts are:- improved methodologies for hydro resource assessment and project identification, - improved methods of hydrologic assessment, - standardized designs of turbines and generators, - standardized requirements for connection to grid, - new contracting methods turnkey, - improvements in computational technology, - standardized civil designs and partial development.8.3.2 Benefits of Small Hydroelectric EnergyThe benefits of small hydroelectric energy are the followings: Environmental benefitsSmall-scale hydroelectric developments do not take up much space and they rarely cause significant shoreline flooding or required river diversions. Large-scale projects, however, can create adverse environmental impacts such as shoreline flooding.Most of the negative environmental impacts of small-scale hydroelectric developments can be avoided in part or in whole by a good design and appropriate construction and operating practices.

Reducing riskof transporting fuel supplies (fossil fuel generation)Fuel supplies must be transported over long distances. The risk of fuel spills is significant, especially in remote areas where the roads can be ice covered and the environment is ecologically fragile.In urban areas, the risks to public safety from collisions or derailments in crowded road or rail corridors are also significant. Socio-economic benefitsThe most obvious social benefit of small hydroelectric developments is the supply of reliable low-cost electric energy to provide the comforts of modern living.Small-scale hydroelectric developments can provide a competitive source of reliable and inflation- proof energy.Small-scale hydroelectric energy is an especially attractive alternative to traditional high-cost diesel generation that currently provides electric energy in most remote communities. Compared with diesel generation, small-scale hydroelectric developments offer other interesting advantages such as:- using a local resource and therefore produce electricity at a stable price that is not subject to the fluctuations of the international oil market,- providing more economic benefits to the region by way of construction employment and use of local services, 10% to 25% of capital cost,- providing greater opportunities for local residents to learn and upgrade their construction skills. Business benefits creation of new economic opportunities (equipment manufacturers and engineering firms employing working people),

important contributions in manufacturing and services, new opportunities for consultants and financiers, also export to overseas customers.8.3.3 Constraints imposed on Small-Hydro DevelopmentsConstraints that challenge small-scale hydroelectric development include the following issues: Cold climate requirementsSmall hydroelectric design must provide for control of frazil ice and pipeline freezing factors that add to capital expenses and operating costs Fish protectionAt sites where fish migration is a concern, small hydroelectric developers may have to provide expensive preventative measures such as fish guidance or habitat compensation. InstitutionalOften, small-scale hydroelectric developers sell the output of their plants to regional or provincial grids.Because purchase contracts and interconnection requirements are not standardized, project preparation and design costs are higher. Regulation(norme, directive)The regulations focus more on large-scale hydroelectric issues than on small-scale hydroelectric issues that sometimes impose disproportionate demands on small hydroelectric developers.8.3.4 Understanding the Types of Hydroelectric Facilities

To generate electricity, hydroelectric facilities must haveadequate river flow and a sufficient head(height for the water to fall). The best locations are waterfalls, rapids, canyons, deep valleys or river bends.Often these locations make it easier to build compact and economic plant layouts.Its also an advantage if the site is close to a location where the electricity is consumed and there is good accessibility for construction activities.Run-of-river facilities use low dams to provide limited storage of water - at most daily pondage.Run-of-river facilities produce minimal reservoir flooding and cause few alterations to the natural river.Because run-of-river facilities have limited storage, water must either be turbined or spilled.A partial development places an intake on a riverbank instead of on a dam. These types of development can only use a portion of the available river flow.This is a special type of run-of-river plant.Natural river flows vary significantly from season to season. Usually, rivers have the highest flows in spring, or early summer, and the lowest toward the end of winter.During the high periods, surplus water can be stored in a reservoir that is part of the hydroelectric facility. The water in the reservoir is used during low flow periods. The surplus water can be stored in the forebay/reservoir or at a remote, upstream location.This control of flow is called flow regulation. Depending on the volume of storage, flow regulation can be used on a seasonal, annual or multiple-year basis.In systems where hydroelectric facilities and thermal facilities are used together, the two types complement each other.During periods when the water flow is low, thermal facilities can boost the low hydroelectric production. Hydroelectric facilities can be used when demand is high because hydro units can respond to rapid load changes, which is not possible with steam-powered thermal facilities.A water resource project may have multiple purposes, such as electricity generation, irrigation, flood control, recreation or environmental sites.These types of facilities must find an optimal balance between competing uses of water.8.4 SOLAR PLANTS

Solar plants are based on solar energy and are designed to provide electrical power on the same scale as plants that rely on nuclear or fossil fuel. 8.4.1 Solar EnergySolar energy is generally emitted by a star (the sun). Energy from the sun travels to the earth in the form of electromagnetic radiation similar to radio waves, but in a different frequency range.Figure 8.3 shows the anatomy of the star. The energy emitted by it is generated by nuclear fusion.

Fig. 8.3: Anatomy of a star.

The fusion process occurs in the core (center), of the star. Energy released by the fusion process propagates away from the core by radiating from one atom to another in the radiation zone of the star. As the energy moves away from the core and passes through the radiation zone, it reaches the part of the star where energy continues its journey toward the surface of the star as heat associated with thermal gradients. This part of the star is called the convection zone. The surface of the star, called the photosphere, emits light in the visible part of the electromagnetic spectrum. The star is engulfed in a stellar atmosphere called the chromosphere. The chromosphere is a layer of hot gases surrounding the photosphere. The luminosity of a star is the total energy radiated per second by the star. The luminosity of the sun is approximately 3.8 1026 W. Radiation from the sun is comparable to the radiation emitted by a black body at 6000oK. The amount of radiation from the sun that reaches the earths atmosphere is called the solar constant and is approximately equal to 1370 W/m2. The solar constant varies with time because the earths axis is inclined and the earth follows an elliptical orbit around the sun. The distance between a point on the surface of the earth and the sun varies throughout the year. To account for the time dependence, we write the solar constant as a function of time S(t). The amount of solar radiation that reaches the surface of the earth depends on the factors illustrated in Fig.8.4.

Fig. 8.4: Solar radiation and the earth-atmosphere system.

The flux of solar radiation incident on a surface placed at the edge of the earths atmosphere depends on the time of day and year, and the geographical location of the surface. The geographical location of the surface can be identified by its latitude lat and longitude long. The incident flux of solar radiation (Einc) at the edge of the atmosphere is: (8.1)where the angle (t, lat, long) is the angle between the incident solar flux at time t and the normal to the surface at latitude lat and longitude long. Some incident solar radiation is reflected by the earths atmosphere. The fraction of solar radiation that is reflected back into space by the earth atmosphere system is called the albedo. The albedo (a) is approximately 0.35, which is due to clouds (0.2), atmospheric particles (0.1), and reflection by the earths surface (0.05). The flux that enters the atmosphere is reduced by the albedo, thus. (8.2)Once in the atmosphere, solar radiation can be absorbed in the atmosphere or scattered away from the earths surface by atmospheric particulates such as air, water vapor, dust particles, and aerosols. Some of the scattered light eventually reaches the surface of the earth as diffused light. Solar radiation that reaches the earths surface from the disk of the sun is called direct solar radiation if it has experienced negligible change in its original direction of propagation. As a result, on a clear day the amount of solar energy available at the Earths surface in the direction of the sun is typically 1000 W/m2.At any particular time, the available solar energy is primarily dependent upon how high the sun is in the sky and current cloud conditions.On a monthly or annual basis, the amount of solar energy available also depends upon the location.Furthermore, useable solar energy is depends upon available solar energy, other weather conditions, the technology used and the application.Solar energy has the following advantages over conventional energy:- the energy from the sun is virtually free after the initial cost has been recovered;

- depending on the utilization of energy, paybacks can be very short when compared to the cost of common energy sources used;

- solar and other renewable energy systems can be stand-alone; thereby not requiring connection to a power or natural gas grid;

- the sun provides a virtually unlimited supply of solar energy;

- the use of solar energy displaces conventional energy; which usually results in a proportional decrease in green house gas emissions.

- the use of solar energy is an untapped market.

8.4.2 Global WarmingSolar radiation heats the earth. The average temperature at the earths surface is approximately 287oK, and typically varies from 220oK to 320oK. Daily variations in temperature are due to the rotation of the earth around its axis. Seasonal variations in temperature are due to the rotation of the earth around the sun and the angle of inclination

of the earths axis relative to the ecliptic plane.

The global warming effect consists in increasing of the average temperature of the earths atmosphere. The increase in atmospheric temperature can be traced to the beginning of the twentieth century and is associated with the combustion of fossil fuels. When a carbon-based fuel burns, carbon can react with oxygen and nitrogen in the atmosphere to produce carbon dioxide, carbon monoxide, and nitrogen oxides (often abbreviated as NOx). The combustion by-products, including water vapor, are emitted into the atmosphere in gaseous form. Some of the gaseous by-products are called greenhouse gases because they capture the energy in sunlight that is reflected by the earths surface and re-radiate the energy in the form of infrared radiation. Greenhouse gases include carbon dioxide, methane, and nitrous oxide, as well as other gases such as volatile organic compounds and hydro-fluorocarbons. Global warming due to the absorption of reflected sunlight and subsequent emission of infrared radiation is called the greenhouse effect because greenhouse walls allow sunlight to enter the greenhouse and then trap reradiated infrared radiation. Some of the incident solar radiation from the sun is absorbed by the earth, some is reflected into space, and some is captured by chemicals in the atmosphere and re-radiated as infrared radiation (heat). The re-radiated energy would have escaped the earth as reflected sunlight if greenhouse gases were not present in the atmosphere.The concentration of CO2 that would establish an acceptable energy balance is considered to be 550 parts per million. To achieve the acceptable concentration of CO2 through the next century, society would have to reduce the volume of greenhouse gases entering the atmosphere. The Kyoto Protocol is an international treaty that was negotiated in Kyoto, Japan in 1997 to establish limits on the amount of greenhouse gases a country can emit into the atmosphere. The Kyoto Protocol has not been accepted worldwide. Some countries believe the greenhouse gas emission limits are too low, and would adversely affect national and world economies without solving the problem of global warming. Research is underway to develop the technology needed to capture and store greenhouse gases in geologic formations as an economically viable means of mitigating the increase in greenhouse gas concentration in the atmosphere. The storage of greenhouse gases in an environmentally acceptable manner is called sequestration.

8.4.3 Active Solar Systems

Active solar energy refers to the design and construction of systems that collect and convert solar energy into other forms of energy such as heat and electrical energy. Active solar energy technologies are typically mechanical systems that are used to collect and concentrate solar energy.8.4.3.1 Solar heat collectors and systemsSolar heat collectors capture sunlight and transform radiant energy into heat energy. Figure 8.5 is a diagram of a solar heat collector.

Fig.8.5: Solar heat collector.Sunlight enters the collector through a window made of a material like glass or plastic. The window is designed to take advantage of the observation that sunlight is electromagnetic radiation with a distribution of frequencies. The window in a solar heat collector is transparent to incident solar radiation and opaque to infrared radiation. The heat absorber plate in the solar heat collector is a dark surface, such as a blackened copper surface, that can be heated by the absorption of solar energy. The surface of the heat absorber plate emits infrared radiation as it heats up. Sunlight enters through the window, is absorbed by the heat absorber plate, and is reradiated in the form of infrared radiation. Greenhouses work on the same principle: the walls of a greenhouse allow sunlight to enter and then trap reradiated infrared radiation. The window of the solar heat collector is not transparent to infrared radiation, so the infrared radiation is trapped in the collector. The solar heat collector must have a means of transferring collected energy to useful energy. A heat transfer fluid such as water is circulated through the solar heat collector in Fig.8.5 and carries heat away from the solar heat collector for use elsewhere.Figure 8.6 illustrates a solar heating system for residential or commercial use.

Fig. 8.6: Solar heating system.The solar heating system sketched in Fig.8.6 uses solar energy to heat a liquid coolant such as water or anti-freeze. The heat exchanger uses heat from the liquid coolant in the primary circulation system to heat water in the secondary circulation system. The control valve in the lower right of the figure allows water to be added to the secondary circulation system. An auxiliary heater in the upper right of the figure is included in the system to supplement the supply of heat from the solar collector. It is a reminder that solar energy collection is not a continuous process. A supplemental energy supply or a solar energy storage system must be included in the design of the heating system to assure continuous availability of heat from the solar heating system.8.4.3.2 Energy conversion efficiency

The temperature of a solar heat collector does not increase indefinitely because the window and walls of the solar heat collector cannot prevent energy from escaping by conduction and radiation. The collector will emit thermal radiation according to the StefanBoltzmann law when its temperature is greater than ambient temperature. This law says that the net energy Qrad radiated through a surface area A by an object at absolute temperature T with surroundings at absolute temperature Te during a time interval t is, (8.3)where is the Boltzmann constant, and e is the thermal emissivity of the object at absolute temperature T. Thermal emissivity is a dimensionless quantity, and the thermal emissivity of a black body is 1. The temperature of the solar heat collector will increase until thermal equilibrium is established. The energy balance for thermal equilibrium must include energy output as well as energy loss, thus. (8.4)The energy conversion efficiency shc of the solar heat collector is then given by:

(8.5)The efficiency shc depends on the increase in temperature relative to ambient temperature, the intensity of solar radiation, and the quality of thermal insulation. An example of an expression for the efficiency shc for a solar heat collector with commercial insulation is, (8.6)where a0, b0 are empirical constants, Tamb is ambient temperature in degrees Celsius, T is the temperature in degrees Celsius of the solar heat collector at a given time, Is is incident solar intensity at a given time, and Ismax is the maximum solar intensity observed at the location of the solar heat collector.The incident solar intensity Is can decrease by more than 50% in cloudy (or smoggy) conditions relative to Ismax. The efficiency of converting solar energy to heat decreases because there is less solar energy impinging on the collector. An increase in solar heat collector temperature T relative to ambient temperature Tamb causes a decrease in intensity because of energy losses associated with convection and thermal radiation. The loss of energy by convection and radiation causes a decrease in energy conversion efficiency.8.4.3.3 Solar power plantSociety is beginning to experiment with solar power plants, and a few are in commercial operation. They use reflective materials like mirrors to concentrate solar energy.As solar power plant applications we mention the followings two ones:

- the Solar Power Tower (SPT),

- the Solar Electric Generating Station (SEGS). Solar Power TowerFigure 8.7 presents a solar power tower with a heliostat field. The heliostat field is a field of large, sun-tracking mirrors called heliostats arranged in rings around a central receiver tower. The heliostats concentrate sunlight on a receiver at the top of the tower. The solar energy heats a fluid inside the receiver.

Fig. 8.7: Solar power tower schematic.Figure 8.8 is a sketch of the geometry of the sun-tracking mirrors relative to the central receiving station.

Fig. 8.8: Sketch of the solar tower (sun-tracking mirrors geometry).

The heliostats must be able to rotate to optimize the collection of light at the central receiving station. Computers control heliostat orientation. As a ring of heliostats gets farther away from the tower, the separation between the ring and adjacent, concentric rings must increase to avoid shading one ring of mirrors by an adjacent ring.Example: the first solar power plant based on the solar power tower concept was built in the Mojave Desert near Barstow, California, in the 1980s. The solar-thermal power plant at Barstow used 1900 heliostats to reflect sunlight to the receiver at the top of a 300-foot tall tower. The sunlight generates heat to create steam. The steam is used to drive a turbine or it can be stored for later use. The first solar power tower, Solar One, demonstrated the feasibility of collecting solar energy and converting it to electrical energy. Solar One was a 10-MWe plant. Solar Electric Generating SystemA Solar Electric Generating System (SEGS) consists of a large field of solar heat collectors and a conventional power plant. The SEGS plants usually use rows of parabolic trough solar heat collectors. The collectors are sun-tracking reflector panels, or mirrors. The sunlight reflected by the panels is concentrated on tubes carrying heat transfer fluid. The fluid is heated and pumped through a series of heat exchangers to produce superheated steam. The steam turns a turbine in a generator to produce electricity. For extended periods of poor weather, solar power plants must use auxiliary fuels in place of sunlight. A prototype SEGS plant used natural gas as an auxiliary fuel. Is reported that, on average, 75% of the energy used by the plant was provided by sunlight, and the remaining 25% was provided by natural gas. In this case solar collection efficiencies ranged from 40% to 50%, electrical conversion efficiency was on the order of 40%, and the overall efficiency for solar to electrical conversion was approximately 15%. The overall efficiency SEGS of a solar electric generating system is the product of optical efficiency o, thermal conversion efficiency c, and thermodynamic efficiency t:. (8.7)The optical efficiency is a measure of how much sunlight is reflected into the system. The thermal conversion efficiency is a measure of how much sunlight entering the system is converted to heat in the system. The thermodynamic efficiency is a measure of how much heat in the system is converted to the generation of electricity. SEGS plants are designed to supply electrical power to local utilities during peak demand periods. For example, a peak demand period would be a hot summer afternoon when the demand for air conditioning is high. This is a good match for a SEGS plant because solar intensity is high. Peak demand periods also correspond to periods of high pollution. One benefit of a SEGS plant is its ability to provide electrical power without emitting fossil fuel pollutants such as nitrous oxide (a component of smog) and carbon dioxide (a greenhouse gas).8.4.4 Synthesis of Solar ApplicationsA Synthesis of Solar Applications is presented in the Table 8.3, as follows:Table 8.3: Solar system applications.

What is solar energy used for?Where is it used?Which solar technologies are used?Which secondary technologies are used? (where applicable)

Heating - Water Homes Glazed flat plate collectors Batch collectors Vacuum tube collectorsHeat exchanger

Hot water tank

Commercial Liquid-based collectorsHeat exchanger

Medium-large water tank

Agriculture Glazed flat plate collectors Unglazed flat plate collectorsHeat exchanger

Medium-large water tank

Aquaculture Unglazed flat plate collectorsMedium-large water tank

Heating - Swimming Pools Outdoor pools Unglazed flat plate collectors

Indoor pools Glazed flat plate collectorsHeat exchanger

Heating - Ventilation Air All building types all Air-based collectorsBy-pass dampers

Make-up air handling units

Heating - Buildings Homes / Commercial Advanced windows Transparent insulationAppropriate building materials

Building design

Trombe wall

Liquid-based collectorswith home heating systemHeat exchanger

Advanced thermal storage

Community-wide

Liquid-based collectors

large-scale arraysSeasonal thermal storage

District heating network

Greenhouses Transparent insulation

Cooling - Buildings Commercial Vacuum tube collectors Glazed flat plate collectors Cooling cycles - various

Daylighting -Buildings Homes & Commercial Advanced windows Transparent insulationBuilding design

Crop Drying Agriculturalall Air-based collectors

Electricity Generation - Off Grid Cottages / Seasonal homesPhotovoltaics small arraysBatteries

Power Invertors

Small wind turbines or microhydro

Power for remote equipmentPhotovoltaics small-medium arrays

Electricity Generation -Distributed Power All building typesPhotovoltaics building integrated medium-scale arraysPower Invertors

Remote communitiesPhotovoltaics medium-scale arrays in a hybrid systemcombined with diesel generators on local grid

Electric UtilitiesPhotovoltaics - large-scale arraysPower Invertors

Sun trackers

Power tower Parabolic troughSteam turbine

High temperature thermal storage

Detoxifying - WaterIndustrial / Municipal Photocatalystsfor oxidationUV lamps for backup

Thermal catalystsfor oxidation with concentrating collectors

Detoxifying - Air Commercial / Homes Photocatalystsfor oxidationUV lamps for backup

Cooking food & H2O treatmentHomes Solar cookers

8.5 WIND POWER PLANTSWind energy converts kinetic energy that is present in the wind into more useful forms of energy such as mechanical energy or electricity. Wind energy is a pollution-free, infinitely sustainable form of energy. It doesnt use fuel; it doesnt produce greenhouse gasses, and it doesnt produce toxic or radioactive waste.Modern uses of wind energy include generation of electricity and pumping water. Current wind energy machines are called wind turbine generators, wind pumps, or more generally, wind turbines.The amount of energy depends mainly on wind speed, but is also affected slightly by the density of the air, which is determined by the air temperature, barometric pressure, and altitude.Wind speed is affected by the local terrain and increases with height above the ground, so wind turbines are usually mounted on tall towers.Any wind turbine that is installed in a very windy area generates less expensive electricity than the same unit installed in a less windy area. So its important to assess the wind at the potential site. Modern wind turbine generators cost between $1500 and $2000 per kilowatt for wind farms that use multiple-units of large machines. Smaller individual units cost up to $3000 per kilowatt. In good wind areas, the costs of generating electricity range between five and ten cents per kilowatt hour. Wind energy costs are decreasing every year, whereas most conventional generation costs continue to increase.When compared to the money that is charged by electrical companies, wind energy costs are nearly competitive. And that is without accounting for the environmental and health benefits of using a non-polluting source of energy.8.5.1 Wind TurbinesModern wind turbines are classified as either horizontal axis turbines or vertical axis turbines (see Fig.8.9). A vertical axis turbine has blades that rotate around a vertical axis, and its visual appearance has been likened to an eggbeater. A horizontal axis turbine has blades that rotate around a horizontal axis.

a) horizontal axis turbine b) vertical axis turbine Fig. 8.9: Modern wind turbines.A wind turbine consists of six major components (Fig. 8.9):

- a rotor that aerodynamically converts the wind energy into mechanical energy on a slowly turning shaft;

- a gearbox that increases the rotor-shaft speed for the generator. Some specially designed generators run at rotor-shaft speed and do not need a gearbox.

- a generator that produces electricity;

- a control and protection system that optimizes performance and keeps the machinery operating within safe limits;

- a tower that raises the rotor high off the ground where the wind speed is greater and the effects of local obstructions are less;

- a foundation that supports the wind turbine system, sometimes with the aid of guy wires.Wind turbine generators produce a range of electricity. Rotors that have diameters of about 1m produce a few hundred watts of electricity. Rotors that have diameters that approach 75m can produce over one megawatt.The angle between the rotor blade and the plane of rotation of the rotor blade is the pitch angle. The pitch angle of the rotor blade can be used to control the rotation rate of the rotor blade. Horizontal axis turbines are the most common turbines in use today. A detailed view of such turbine is presented in the Fig. 8.10.

Fig.8.10: View of a horizontal wind turbine.

Figure 8.11 illustrates a cylinder of air approaching a rotating horizontal axis wind turbine.

Fig. 8.11: Schematic horizontal wind turbine.The cylinder of air shown in has the volume:, (8.8)where A is the cross-sectional area and L is the length of the cylinder of air. Let us assume that the density of air air is approximately constant. The air mass of the cylinder is:

. (8.9)Suppose the cylinder of air is moving with speed vair directly at the turbine. The air speed vair is the speed of the wind. The kinetic energy of the moving air is:

. (8.10)The length L of the air cylinder that reaches the wind turbine in a time interval t is:

. (8.11)Substituting eq. (8.11) into eq. (8.10) gives:. (8.12)

The rate of arrival of air is the wind power, or

(8.13)

Wind power is proportional to the cube of wind speed. The area A is the surface area of the circle formed by the rotating tip of the rotor blade. If the rotor blade has radius R, the area is:

. (8.14)We can use eq. (8.14) to write wind power in the form:. (8.15)

Equation (8.15) shows that wind power is proportional to the square of the radius of the fan created by the rotating rotor blade. We have assumed that the wind direction is perpendicular to the plane of rotation of the rotor blade, and that the wind speed is constant. Wind power is at maximum if the wind direction is perpendicular to the plane of rotation of the rotor blade, otherwise the wind power will be less than maximum. If the wind direction is parallel to the plane of rotation of the rotor blade for an infinitesimally thin rotor blade, the wind turbine will not provide any wind power. A change in wind direction can put stress on wind turbines because of the gyroscope effect. A gyroscope is a symmetrical rigid body that is free to turn about a fixed point and is subjected to an external torque. The wind turbine is a symmetrical rigid body that is free to turn about the fixed post. The change in wind direction subjects the wind turbine to a torque that causes the wind turbine to behave like a gyroscope. In addition, wind speed is seldom constant; it can vary from still to tornado or hurricane speed. The speed of rotation of the tip of the wind turbine is:

, (8.16)where is the angular frequency of the turbine. If vtip is sufficiently large, it can be lethal to animals entering the fan area of the rotor blade. This creates an environmental hazard that must be considered when selecting locations for wind turbines. Electrical power output from a wind turbine is a product of the efficiency wind times the input wind power. The optimum power output is approximately:. (8.17)

The efficiency wind depends on several factors. One factor that affects the efficiency of a wind turbine is the efficiency of converting mechanical energy of the rotor blade into electrical energy. Another factor is the reliability of the wind turbine. The rate of rotation of the rotor blade depends on wind speed. If the wind speed is too large, the rotor blade can turn too fast and damage the system. To avoid this problem, wind turbines may have to be taken off-line in high wind conditions.The analysis of wind power above presented assumes that all of the kinetic energy of the wind incident on the turbine is converted to rotational energy of the rotor blades. In reality, the wind speed vupwind upwind of the turbine is reduced to a wind speed vdownwind downwind of the turbine. According of Shepherd analysis (1998), we can estimate wind power for a more realistic system. In this aim, we again consider the cylinder of air in Fig. 8.11 and the eq. 8.10. The kinetic energy of the moving air that is extracted for power production is:. (8.18)

The mass of the air that is needed to move the rotor blade in a time interval t, is:, (8.19)

where vactuate is the wind velocity that actuates the rotor and A is the cross sectional

area shown in Fig.8.11. If we assume that the reduction in kinetic energy in the upwind air stream is transferred to the wind turbine, conservation of energy and the continuity of the wind flow, vactuate results as a mean value, given by:

. (8.20)Substituting eqs. (8.19) and (8.20) into eq. (8.18), gives. (8.21)The extracted wind power is:

. (8.22)

This last equation, it can be written in a simplified form, as follows: (8.23)

where wind = vdownwind/vupwind is the ratio of downwind velocity to upwind velocity, and Cp is the dimensionless power coefficient. (8.24)

The power coefficient is typically in the range 0 Cp 0.4 for actual wind turbines. If we compare eq. (8.23) to eq. (8.17), we see that the power coefficient Cp serves as an efficiency factor to convert the input wind power to output rotor blade power. A theoretical maximum power coefficient is obtained by solving the extremum condition:

(8.25)

for wind. The physically meaningful solution to Equation (8.25) is. (8.26)

Substituting eq. (8.26) into eq. (8.24) gives the power coefficient

. (8.27)

Equation (8.27) is the theoretical maximum power coefficient and is called Betz limit, after Albert Betz, the person who first made the calculation in 1928. Betz limit says that approximately 59.3% of the wind power is the maximum percentage that can be extracted of it. It is obtained when the downwind velocity is one third of the upwind velocity, as shown in eq. (8.26).8.5.2 Applications of Wind Energy

Some applications of the wind energy, are shown in the Table 8.4.

Table 8.4: Applications of the wind energy.What is wind energy used for?Who uses wind energy?How is wind energy obtained?

Using wind to generate electricity National and Provincial networks

Communities

Homes and farms

HYPERLINK "http://www.canren.gc.ca/tech_appl/Individual%20houses.doc"

Process industries

Remote communities The wind powers awind turbinethat produces electricity.

Using wind to pump water Farms The wind powers awind pumpthat pumps water.

Wind farmsA wind farm or wind park is a collection of wind turbines. The areal extent of the wind farm depends on the radius R of the rotor blades (see Fig. 8.12). A wind turbine must have enough space around the post to allow the fan of the rotor blade to face in any direction. If we neglect the finite extent of the machine cabin and assume the rotor blade with radius R is horizontal, the blade will sweep out a surface area R2 if the rotor blade is directly above the post of the wind turbine and the machine cabin is rotated through 360. If we include the finite extent of the machine cabin, the surface area will be R2eff R2.

Fig. 8.12: Wind turbine spacing.

The minimum spacing between the posts of two equivalent wind turbines must be 2Reff to avoid collisions between rotor blades. If we consider the aerodynamics of wind flow, which is the factor that controls turbine spacing, the turbine spacing in a wind farm increases to at least 5 to 10 times rotor diameter 2R behind the plane of the rotor blade. The additional distance between posts is designed to minimize turbulence between wind turbines and enable the restoration of the wind stream to its original undisturbed state after it passes by one turbine on its way to the next turbine. Wind turbine spacing is an important factor in determining the surface area, or footprint, needed by a wind farm.

8.5.3 Wind Energy Benefits1. Wind energy is an ideal renewable energy because:

- it is a pollution-free, infinitely sustainable form of energy,

- it doesnt require fuel,

- it doesnt create greenhouse gasses,

- it doesnt produce toxic or radioactive waste.

2. Wind energy is quiet and does not present any significant hazard to birds or other wildlife.

3. When large arrays of wind turbines are installed on farmland, only about 2% of the land area is required for the wind turbines. The rest is available for farming, livestock, and other uses.

4. Landowners often receive payment for the use of their land, which enhances their income and increases the value of the land.

5. Ownership of wind turbine generators by individuals and the community allows people to participate directly in the preservation of our environment.

Each megawatt-hour of electricity that is generated by wind energy helps to reduce the 0.8 to 0.9 tons of greenhouse gas emissions that are produced by coal or diesel fuel generation each year.THERMINOLOGY stack: system de stocare

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