Mechanical Engineer''''s Reference Book 2011 Part 12 ppt

Solar chemistry 12/17 solutions, while those which are stable in water have too wide a band gap. 12.6.4 Fuels Environmental considerations are increasingly becoming more important in the utilization of solar energy to obtain new fuels at a competitive cost. One of the ideal cycles is water splitting to produce hydrogen. Hydrogen can be made by following any of the paths outlined above. e.g. thermochemical, photochemical, electrochemical or in a pairwise hybrid process. The main features of the use of hydrogen as an energy carrier, the ‘Hydrogen Energy’ concept, were first outlined by Bockris and Triner8’ in 1970 and are shown in Figure 12.11. Hydroglen forms the intermediate link between the primary energy sources and the energy-consuming sectors. ’ It is also independent of the primary energy sources used for its production. Even if these change, the intermediary energy systems of transmission, storage and conversion can remain unaltered. The hydrogen economy system is completely cyclic. Water from lakes, rivers or oceans is converted into hydrogen and oxygen. Its combustion product is water vapour: which is returned to the biosphere, and it is the least polluting of all the synthetic fuels. The well-known solar furnace at OdeilloX2 was first used to produce hydrogen and oxygen in the early 1980s, but the challenge is to make the process cost-effective, some- thing which could come with the increasing emphasis on the true costs of fossil-fuel-induced environmental pollution. Hydrogen can also be applied in the traditional reactions of the chemical industry, or can be reacted with carbonaceous material, such as carbon dioxide, carbonates, or biomass to produce o’rganic fuels and chemicals. ” Energy input water Conversion system oxygen hydrogen production water Figure 12.11 The main features of the use of hydrogen as an energy carrier (after McVeigh’) Another route for the thermochemical transport of solar energy is through the CO2 reforming of methane. The endo- thermic reaction of C02 with CH4 to yield CO and H2 was suggested over ten years ago.83 Methane and carbon dioxide are reacted in a solar receiver (reformer) at high temperature to produce energy-rich products. These can be stored at ambient temperatures and can be transported over distances of some hundreds of kilometres to the consumer site. Here the reverse reaction takes place in a thermal reactor (methanator), releasing the chemical energy.s4 The process could take place on one site, with the reformer-storage-methanator linked by pipework. 12.6.5 Chromogenic materials Chromogenic materials offer the possibility of developing advanced glazings which combine variable control of solar gain with efficient thermal insulation.” In a major review of the technical properties and merits of known electrochromic phenomena in 1984, Lamperts6 pointed out that most of the wealth of technical literature and patents dealing with electrochromic materials and devices was primarily for electronic information display or other small-scale applications. Conse- quently, only minor attention had been paid to electrochromic devices as transmissive devices. Since then, transparent aper- tures employing photochromic, thermochromic or electrochromic materials have been the focus of intensive world- wide research by many gro~ps~~.~~ concerned with the efficient use of energy in buildings. The electrochromic window is the most advanced example of these efforts. It is basically a multilayer thin-film device which performs as an electric cell, and consists of an electrochromic layer and a counter-electrode, or ion-storage layer, separated by an ion conductor. For window applications these layers are commonly sandwiched between two transparent electronic conductors which are deposited onto transparent substrates (e.g. glass or polymeric materials). In operation a d.c. electric field is applied across the transparent conductors and ions are driven either into or out of the electrochromic layer, causing reflectance and/or absorptance modulation of visible and near-infrared electromagnetic radiation and hence changes in the optical properties of the device. The electrochromic layer may be caused to colour or bleach in a reversible way under the influence of the external electric field. The principal aim of current research is the development of a stable, durable, all solid-state electrochromic device -the ‘smart window’. E7 Liquid-crystal-based chromogenic materials have also been successfully used as electrically activated devices. s9 Two transparent electrodes provide an electric field to change the orientation of liquid-crystal molecules interspersed between the electrodes. The orientation of the liquid crystals alters the optical properties of the device. Two main types of liquid crystal systems, the guest-host and polymer-dispersed or encapsulated devices, have been identified for large areas. Their disadvantages are that their unpowered state is diffuse, haze remains in the activated (transparent) state and ultravio- let stability is poor. A third approach uses suspended particle devices: but various technical problems such as long-term stability and cyclic durability have slowed their development. Two non-electrically activated devices use photochromic or thermochromic materials. Their research histories date back at least 100 years.87 When photochromic materials are exposed to light they change their optical properties, only reverting to their original properties in the dark. Photochro- mic plastic has been developed for ophthalmic use and could become useful for regulating solar glazings. E7 Thermochromic materials display a large optical property change when a 1211 8 Alternative energy sources particular temperature is exceeded. Above this critical temperature, transmittance is reduced and if this temperature is close to a comfort temperature thermochromism could be used for automatic temperature control in buildings. *' 12.6.6 Transparent insulation materials A new technology is emerging from European initiatives which will bring about revolutionary changes in the building ind~stry.'~ Transparent (or translucent) insulation materials are a relatively new class of materials which combine the uses of glazing and insulation in the traditional design of any solar thermal system. While the primary use of glazing in buildings has also been to allow light to enter, its ability to transmit radiation gives it the subsidiary function of providing solar heat. Insulation suppresses conduction and convection losses from buildings, but the traditional opaque materials such as polystyrene granules or foam which have been developed for this purpose are equally effective in suppressing solar gain from the outside of the building. For large energy gains both high irradiation levels and high values for the product of the solar transmittance and absorptance of the absorber are essential. The influence of insulation (U-value) depends on the temperature level of the system and the desired heat storage period. For low U-values very good absorption is needed in the thermal wavelengths, although infrared select- ive coatings can also be used on the front cover or the absorber plate to reduce infrared radiation losses. 91 Convection losses can be greatly reduced by the use of structured materials such as capillaries and honeycombs or low-pressure systems. Until recently no natural or man-made product could offer both high-transmission, low-conduction and strong convection suppressant characteristics, but by the mid-1960s it was possible to conceive the potential benefits of such a material. 92 Following rapid advances made in Germany during the 1980s the term 'transparent insulation' was accepted as best describ- ing the goal of the te~hnology.~~ The first ex erimental results were presented in 1985 by Wittwer et a,.' and the following four generic types which displa different physical properties were first proposed in Type Examples Absorber-parallel Multiple glazing, plastic films Absorber-perpendicular Honeycombs, capillaries Cavity structure Duct plates, foam (Quasi)-homogeneous Glass fibres, aerogels By 1990 several systems had become commercially available and considerable potential for more scientific research work had been identified. 1986: B 12.6.7 Solar detoxification The ability of sunlight to detoxify waterborne chemicals is well known through the cleansing of polluted streams as they flow through areas open to direct solar radiation. Solar detoxification uses this natural process of degradation to produce non-hazardous substances from hazardous organic chemicals. As conventional detoxification methods do not always deal adequate1 with chemical wastes, the solar route is attracting attention.'6 Among the advantages are that, by using the sun as the energy source, there is no added airborne pollution or use of conventional fossil fuels. Solar detoxification breaks down the hazardous chemicals into an environmentally benign or easily treated end-product in one step. However, conventional processes often remove the wastes from the water and take them elsewhere for treatment, thus increasing the possibility of further contamination. The detoxification of water is a photochemical process which destroys contaminants by the chemical action of light and a semiconductor catalyst. 97 When exposed to sunlight, the catalyst absorbs the high-energy photons and reactive chemicals, the hydroxyl radicals, are formed. These radicals are powerful oxidizers and break down the contaminant molecules, typically forming carbon dioxide, water and dilute mineral acids (e.g. hydrochloric acid), which can be neutra- lized in a post-treatment process before the treated water is discharged. Among the common toxic chemicals which could be solar treated are trichloroethylene and other chlorinated solvents, 96 pesticides. wood preservatives, dyes typically found in textile mill effluent and leakages or spills of various liquid fuels.96 Dioxins in PCBs can now be destroyed by solar energy and these laboratory techniques should reach the market by 1995.9x 12.6.8 Chemical heat storage Thermal energy storage is essential for many solar thermal applications. The pro erties of suitable salt hydrates were first mixed with 34% borax as a nucleating agent if complete crystallization is to be obtained, was the most-tried material, with a transition temperature close to 30°C. The problem of a barrier being formed between the liquid and solid phases proved very difficult to solve and numerous polymeric stabi- lizers were tried. Many different salt-phase change materials have been tried in the past two decades. including calcium chloride hexahydrate and sodium acetate trihydrate, and modified varieties covering a range of transition temperatures from about 8°C to 58"C, several of which were commercially available in 1990.97 For high temperatures, molten nitrate salt receivers have been designed for the 10 MWe Solar One electricity generating pilot plant discussed in Section 12.4. discussed by Telkes' 8. in 1974. Sodium sulphate decahydrate, 12.6.9 Other applications 12.6.9.1 Surface transformation of materials Highly concentrated solar energy, typically greater than 1 MW m-2, provides a controlled method for delivering large nux densities of broadband radiation to solid surfaces, thus creating the solar-induced surface transformation of materials. Candidate technologies identified by Pitts et al. lo' are shown in Table 12.2. 12.6.9.2 Thermochemical heat pump A thermochemical heat-pump system, consisting of a fixed- focus parabolic solar collector, a stationary thermochemical metal hydride storage unit and various sub-systems has been investigated at the laboratory stage. lo' It could provide a small, independent solar-powered home-energy centre for countries with good solar radiation conditions. The basic principles are that sunlight is concentrated in the fixed-focus solar concentrator and heats the Stirling engine generator, producing electricity, and dehydrogenates the high- temperature magnesium hydride storage unit. The freed hydrogen is transferred to a second low-temperature hydride storage unit, and may be recycled to the high-temperature unit (hydrogenation) through a valve. The chemical systems are as follows: Storage: MgH+Mg+H-75kJmol-' Release of stored heat: Mg + H + MgH + 75 kJ mol-' Hydropower 1211 9 Table 12.2 Teckndogy Present uses (1) Chemical vapour deposition (2) Diffusion coatings (3) Layered thin films (4) Melted powdered coatings (5) Rapid thermal annealing (6) Self-propagating high temperature (7) Transformation hardening (8) Zone-melting recrystallization Electronics, hard-facing corrosion Pack cementation Electronics, photovoltaics Corrosion, hard-facing, ceramics Electronics, photovoltaics Ceramics, refractory powders synthesis Steel industry Electronics As yet, a Stirling engine for remote, maintenance-free applications has not been developed, but this remains one of the main development goals of the project team.lo2 Main parameters for an aperture area of 3 m2 are 4 kWh of high-temperature energy (heat) for cooking + 3.4 kWh of electricity + 3 kWh of heat for domestic hot water + 3 kWh cooling energy for a refrigerator. 12.7 Hydropower 12.7.1 Introduction Hydroelectric power is the world's largest commercially available renewable energy source, accounting for about 6.7% of the total primary energy consumption. lo3 Water has been used as an energy source for thousands of years, but the various traditional designs of watermill used until the nineteenth century could only lead to a technical dead end.'04 None of them were capable of using a head of water much greater than their own diameter. Further progress followed with the development of the water turbine which was subsequently linked to an electric generator. Although credit for the world's first hydroelectric plant is often attributed to the US plant which started in the autumn of 1882 at Appleton, Wisconsin, two plants were already operating in the UK at that time. lo' The earliest was Sir William Armstrong's small hydroelectric plant, rated at just under 5 kW, which was constructed in 1880 to light his picture gallery at Cragside, Northumberland, some 1.5 km away. The first public supply of electricity was reported from Surrey in 1881, when electric current generated from the waters of the River Wey was used to light the streets of Godalming. The cables had to be laid in the gutters as there was no legal authority to dig up the streets. The world's first large hydroelectric plant was built in 1895 at the Niagara Falls in the United States, with two turbines each rated at 4100 kW.'06 The subsequent development of alternating current by George Westinghouse in 1901 allowed electric power to be transmitted over long distances. lo' By 1903 Canada had a 9.3 MW plant, also at Niagara Falls, and the era of modern hydropower had commenced. The first reliable survey of water turbines manufactured and installed throughout the world in the late 1920s108 suggested that about 40% of the world's electricity was generated by hydropower, with the United States and Canada having a combined operating potential capacity of over 13 000 MW and five other countries (France, Japan, Norway, :Sweden and Switzerland) with operating potential capacities greater than 1000 MW. A few of the earlier hydropower plants, known as run-of-the- river plants, could not generate any power when the river was low during the dry season. but by the 1930s the use of large dams had been established in the United States. The creation of the Tennessee Valley Authority in 1933 with their compre- hensive approach to the planning and development of river basins set a pattern which has been widely followed in other countries. lo' Since then there has been a steady growth in hydropower throughout the world although the percentage share of hydropower in meeting world electricity demand had fallen to about 25% by the early 1990s. During the 1980s. the total output from North America and Europe remained unchanged. but their share of the total world output dropped from just under 60% to 45%. Among the developing countries, Brazil, Ghana, Mozambique, Zaire and Zambia obtained over 85% of their electricity from hydropower. lo3.109 The potential for development of hydropower over the next 40 years is so great that it could provide an output equivalent to the total electricity generated in the world from all sources in the early 1980s. Most of this potential is in the developing countries, some of whom could, in theory, increase their present use of hydropower by a factor of ten or more. 12.7.2 The basic hydropower plant The basic principles of hydroelectric power generation are shown in Figure 12.12. Water at a high level, often stored behind a dam, falls through a head z. Its gravitational potential energy is converted to kinetic energy and the flowing water drives a water turbine. The rotating turbine shaft drives the electric generator to produce electricity. The theoretical maximum velocity is obtained by equating the gravitational and potential energies as follows: gz = 112 vz If the volumetric flowrate is Q (m3 s-l), density p (kg m-'), then the power output in watts is given by 12.7.3 Types of turbine Turbines can be classified according to the direction of the water flow through the blades, e.g. radial, axial or combined- flow turbines, or as reaction, impulse or mixed-flow turbines. In reaction turbines there is a change of pressure across the turbine rotor, while impulse turbines use a high velocity jet impinging on hemispherical buckets to cause rotation. There are three basic types of turbine broadly related to low, medium or high heads. 12/20 Alternative energy sources Turbo- generator . \ . j Figure 12.12 The basic principles of hydroelectric power generation (after McVeigh’) Propeller or axial flow turbines are used for low heads in the range from 3 to 30 metres. They can have relatively inexpensive fixed blades, which have a high conversion efficiency at the rated design conditions but a poorer par€-load efficiency, typically 50%, at one third of full rated output. Alternatively, the more expensive Kaplan turbine has variable-pitch blades which can be altered to give much better part-load efficiency, perhaps 90% at one third of full rated output. The Francis turbine is a mixed-flow radial turbine and is used for medium heads in the range from 5 to 400 m. It has broadly similar performance characteristics to the fixed-blade propeller type and its speed is controlled by adjusting the guide vane angle. The best-known impulse turbine is the Pelton wheel. Each bucket on the wheel has a centrally placed divider to deflect half the flow to each side of the wheel. It is normally used for heads greater than 50 m and has good performance characteristics over the whole range, very similar to the Kaplan turbine, reaching 60% efficiency at one-tenth of full rated output. The speed is controlled by a variable inlet nozzle, so that with a constant head, the delivered torque to the generator is proportional to the flowrate and the turbine speed can be held at that required for synchronous generation at the particular grid frequency. This type of installation is known as a constant- speedkonstant-frequency system and optimization of the power output is relatively easy. I1O In smaller installations, optimum power cannot be obtained at constant speed where the hydraulic head is both relatively low and variable over a wide range. A detailed description of methods which can be used for optimizing electric power from small-scale plant has been given by Levy.”’ He points out that small hydroelectric systems will become more financially attractive through developments of low-cost power converters (from 100 W upwards), special variable-speedkonstant-frequency generators and cheap computing units for on-line power measurement and optimizing control. This means that many run-of-the-river sites that were considered in the past to be unsuitable for electricity generation can now be used. 12.7.4 Hydropower potential The Earth’s energy flow diagram (Figure 12.1) shows that just over 4 x W flows in the hydrological cycle of evaporation, rain, other precipitation and storage in water and ice. A very small proportion of this hydrological energy flow, probably between 0.01 and 0.015%0, is considered to be theoretically available for conversion into hydropower. 109~111 This theoretical world hydropower potential is calculated as the total energy potential of river discharges relative to a datum of sea level or the base level of erosion for closed basins and is widely quoted as 44.28 x 10” kWh per annum. 109*111 How ever, this figure does not seem to include the 3.94 x lo1* kWh for the former USSR, which was separately listed by the 1980 World Energy C~nference,“~ and there is also some doubt as to whether the 6 X 10” kWh estimated for the People’s Republic of China has been included.’” A better assessment is the ‘technically usable hydropower potential’, which allows for the unavailability of certain river reaches, mainly those near estuaries. This is less than half the theoretical value. The ‘economic potential’ includes all hydropower resources which are regarded as economic compared with alternative sources of electric power at the time of the assessment. These can be classified into three categories: operating, under construction and planned, as shown in Table 12.3. The economic operating hydropower potential of 372.1 GW represented just under 16% of the technically usable potential. The world operating potential of some 372.1 GW could, in theory, have provided 372.1 X lo9 X 365 X 24 watt-hours or 3.26 X 10’* kWh. The actual energy generated was 1.65 X 10” kWh.7 This represents 50.6% of the potential, a typical figure for most hydroelectric plant. Not only are there seasonal fluctuations in water availability, but the demand for electricity fluctuates and plants need to close for maintenance. In the United States and Canada the figure of (actual energy generated) divided by (theoretically available potential) was 47.7% in 1979. This ratio is known as the load factor. As the electrical power from a hydroelectric plant can be used directly without the conversion losses and wasted heat associated with conventional fossil fuel power plant, the primary energy equivalent of Hydropower 12/21 Table 12.3 Hydropower potential (GW) [after reference 109) Region Technically usable Econonzic Operating Under construction Planned Asiaa Latin America Africa USA and Canada Former USSR Chinab Europe Rest of world 609.6 431.5 358.4 356.2 250.0 216.9 163.2 44.5 53.1 34.1 17.2 128.9 30.3 5.7 96.1 6.7 9.1 40.5 5.4 34.6 21.8 5.9 10.7 2.3 42.0 92.4 22.9 39.0 19.4' Unknown 22.5 3.6 Total 2430.3 372.1 130.3 241.8 Figures from Asia probably do not include data from the People's Republic of China."' Figures may not include all small hydropower plant. Estimated. hydroelectricity is usually taken as about three times its actual output. A common conversion is that 4000 kWh of 'electricity generated' is considered to have the primary energy equivalent of one tonne of oil. '03 By 1980 there had been a steady growth in hydropower for many years at about 3.5% per annum, representing a doubling period every 20 years. This figure was used by the 1980 World Energy Conference to estimate that hydropower could be quadrupled by 2020, reaching a total of over 1600 Mtoe. A more realistic figure was suggested by McVeigh' with a logistic equation approach giving a growth rate of just under 3%. By the early 1990s, however, it could be seen that growth in the decade of the 1980s had only averaged 2.0%, and that the doubling period had stretched to about 35 years. Reasons for this could include the gradual reduction in performance of some of the older hydro schemes due to silting, and the changing patterns of rainfall, which, in turn, could be due to global warming. 12.7.5 Pumped storage Pumped storage systems are used at times of peak demand for electricity. The water can be pumped to an upper storage reservoir usually at night when the demand is low, and then allowed to flow down through the turbines, generating electricity when it is required. Although small pumped storage schemes were first built in the 1890s. the first large system in the UK was built at Ffestiniog, Wales, in 1963, with four 90 MW generators each coupled to separate pumps and turbines on the same vertical shaft. This was soon followed by a 4 x 100 MW system at Cruachan in Scotland. The largest pumped storage system in Europe was completed iin 1984 at Dinorwig, near Llanberis in North Wales. During its construction 3 million tonnes of rock were exca- vated from the heart of the mountain between two reservoirs and 16 km of shafts and tunnels were created."2 The upper reservoir is 568 m above the underground power station and the horizontal distance between the upper and lower reservoirs is 3200 m. There are six turbogenerator units, each rated at a nominal 300 MW. It can generate at full output for about 5 hours. The overall efficiency of any pumped storage system is less than the 'once-through' conventional plant, as the pumping efficiency during the return flow to the upper reservoir must be included. This pumping efficiency, typically about go%, reduces the overall efficiency to about 70-75%. However, the economics are quite different. Pumping to the upper reservoir only occurs when the electricity tariffs are iow. Electricity is supplied to meet peak demands when tariffs are usually at their highest. Further, the use of a pumped storage system reduces the need for additional conventional plant which would only be needed for very short periods each year. The pumped storage plant at Dinorwig can be generating electricity within 10 seconds of requirement. Other benefits, apart from the reduction in utilization of both high-cost oil-fired and low-efficiency coal-fired plant during the peak demand periods, include a reduction of both the extent and the duration of frequency excursions arising from large losses of generation output or the sudden increase in consumer demand ex erienced at the end of many popular television programmes. This is often up to 2000 MW in a few minutes. Dinorwig has given the system the ability to pump prior to the impact of the television 'pick-up', thus creating an artificial demand. As the real demand increases, the pumps can be reversed to generate within 90 seconds. 12.7.6 Small-scale hydropower One of the needs in many parts of the world is for electrical power in remote regions far from a conventional transmission system. Small-scale hydropower is again becoming considered for an increasing number of these applications. Recently, the energy policies adopted by the different Member States of the European Economic Community to reduce their dependence on third countries as suppliers of energy, together with the technical improvements outlined above, have made it possible for small hydropower plants to become competitive in many parts of Europe. 'I3 The early history of hydropower up to the 1930s was largely dominated by small plants, less than 1 MW in capacity, but then the economies of scale began to favour large-scale development. Until fairly recently it was necessary to match the turbine design very carefully to the particular site. This resulted in an expensive special 'one-off' hydropower generator. The smaller the application, the greatsr the installed 12/22 Alternative energy sources cost per kilowatt of capacity. The need for these specially designed systems has been largely overcome by the use of standardized turbines and associated equipment, with the acceptance of some loss in overall plant efficiency and performance. As outlined above, one of the major factors which could favourably influence the economics of small-scale hydropower is the development of microprocessor-based electronic load governors. These can overcome problems of instability in matching waterflow to a variable demand and can also reduce costs as expensive mechanical controls are no longer necessary. Definitions of the size of any hydro scheme into Large, Small, Mini and Micro appear in the literature, but there seems to be no agreement on what these sizes represent, as Table 12.4 shows. Bazaga'I3 points out that the definition of small hydropower plant is 'not exactly the same in the different Member States of the EEC' and bases his analyses on a power capacity less than or equal to 10 MW. Among the distinctive features of small hydropower plants which he identifies are: 1. They are usually run-of-the-river and the energy produced depends on the available flow. 2. They rarely contaminate the environment and do not give off heat. 3. They can be built in a short period of time, with standard equipment and well-known construction processes. 4. Projects can be developed which combine electricity generation with other uses. 5. The power source is reliable, within its hydrological limita- tions. The equipment and facilities involved have a long life. require little maintenance and seldom break down. 6. The technology involved is well developed and overall efficiency is over 80%. 7. Operating systems are often automatic, leading to low operation and maintenance costs. By the 1980s the country with the greatest experience in small-scale hydropower development was the People's Republic of China, where near1 100 000 plants have been have a rated output of some 300 kW and much of their projected increase in hydropower over the next 20 years will also be small-scale. constructed in the past 20 years.' 7 ' Most of their recent plants 12.7.7 Economic, social and environmental issues The costs and benefits of hydropower plant are usually evaluated by an economic comparison with conventional thermal or nuclear power stations. The main factors which must be considered, in addition to increases in construction costs, are changes in the cost of fossil fuels and in environmental protection regulations. Although there has been a steady growth in power station construction costs in all countries over the past two decades, thermal and nuclear power station costs have risen at a greater rate than those of hydropower plants. There are two reasons for this. The technology and management of the construction of hydropower plants has improved relative to conventional power station construction and new environmental protection and safety regulations have adverselv affected the cost of nuclear and coal-fired power stations. ''' These new regulations have resulted in greatly increased expenditure for the control of air and water pollution with coal-fired stations, and for radiation monitoring and control together with improved safety stan- dards in nuclear installations. Some of the adverse effects of hydropower schemes, such as the essential reinforcement of river banks or compensation for moving and resettling whole communities from flooded land, have always been included in the overall construction costs. An economic advantage when considering the later stages in any hydropower scheme is that dams with existing hydropower schemes can be raised to provide both additional storage capacity and a potentially increased output. Turbine generators can be added to some existing storage reservoirs to create new generating capacity. The economics of any hydropower system are absolutely site-specific, depending critically on the topology, geology and hydrology of the site.64 These factors influence the power capacity and developments costs, which, in turn, depend on what is required from the system (e.g. a high or low load factor) or whether storage is required or not. Hydropower, like tidal power. is highly capital-intensive and can have a very long life, often over a hundred years for the basic civil engineering work. With the low operation and maintenance costs, together with the other advantages outlined by Bazaga above, the main economic problems arise from the financial requirements of hi h interest rates and the demand for short fuel cost, and the UK Watt Committee also commented64 that it is paradoxical that investment in hydro schemes looks extremely favourable in retrospect. Rivers and streams are regarded in the great majority of countries throughout the world as a public resource. Their use in potential hydropower schemes is subject to government control. Hydropower development may be socially acceptable to some sectors of the community and have quite disastrous effects on others. For example, the construction of the Aswan High Dam in Egypt resulted in the destruction of the sardine fishing industry in the Eastern Mediterranean, but this was balanced by the development of a new fishing industry on the newly created Lake Nasser. '07 There have been many studies on the adverse impacts on health which can result from the large dams associated with hydropower projects107 and it would appear that there is still a need for major health- education programmes to be associated with these projects, so that diseases such as bilharzia and malaria could be eliminated. Other associated environmental problems include the need for extensive drainage systems on newly irrigated land and the threats to new dams caused by widespread deforesta- 'payback' periods. P' Again, as with tidal power, there is a zero Table 12.4 Power output ranges Source UK Watt Committee (1990)64 Hurst and Barnett (1990)'14 Bazaga (1 988) 'I3 Large 50 MW Greater than 1 MW Small 5-50MW 0.5 - 1 MW 10 MW or less Micro Less than 500 kW Less than 100 kW Mini 0.5 - 5 MW 100 - 500 kW Wind power 12/23 tion and soil erosion many kilometres upstream. Some existing aquatic and terrestial ecosystems have been disrupted and there may have been a loss of visual amenities in scenic areas. On the other hand. the United Nations Hydropower Panel"' has also drawn attention to the positive effects of hydropower reservoirs on the environment. The creation of regulating reservoirs has been shown to make a substantial improvement in the water supply for domestic, industrial and agricultural purposes in many cases. The danger of catastro- phic floods has often been eliminated. The overall effects of hydropower schemes throughout the world have been bene- ficial, although there have been some largely unanticipated adverse reactions with the environment. These could either be reduced or eliminated through careful resource planning. 12.7.8 Summary Hydropower is the only renewable energy resource with a fully developed technological base and a relatively predictable growth rate over the next few decades. Its indlustrial infra- structure is well established in many countries, and it provides very subs,tantial proportions of the electricity demand in a number of countries. Although it accounted for only 6.7% of the worldl's primary energy consumption in 1990, this figure could easily rise to over 10% by the middle of the next century. It is particularly suitable for the needs of remote communities in the developing countries. 62.8 Wind power 12.8.1 Iintroduction Energy from the wind is derived from solar energy, as a small proportiomn of the total solar radiation reaching the Earth causes movement in the atmosphere which appears as wind on the Earth's surface.' The wind has been used as a source of power for thousands of years and the traditional horizontal axis tower mill for grinding corn, with sails supported by a large tower; rather than a single post, had been developed by the beginning of the fourteenth century in several parts of Europe. Its use continued to expand until the middle of the nineteenth century, when the spread of the steam engine as an alternative. cheaper, source of power started its decline. Nevertheless, before the end of the nineteenth century several countries used the windmill as one of their main sources of power. In the Netherlands"' there were about 10 000 windmills giving power outputs of up to 50 kW. In Denmark housemills were often mounted on the roofs of barns and, together with industrial mills, were estimated to be producing about 200 MW from over 30 000 units.*16 In the United States'lj an estimated 6 million small multi-bladed windmills for water pumping were manufactured between 1850 and 1940. Work on the development of wind-generated electricity started in Denmark in 1890 when Professor P. La Cour obtained substantial support from the Danish government, which not only enabled him to erect a windmill at Ashov but provided a fully instrumented wind tunnel and laboratory. Between 1890 and his death in 1908, Professor La Cour developed a more efficient. faster-running windwheel, incor- porating a simplified means of speed control, and pioneered the generation of electricity. The Ashov windmill had four blades 2;!.85 m in diameter. mounted on a steel tower 24.38 m high. Power was transmitted. through a bevel gearing, to a vertical shaft which extended to a further set of bevels at ground level, and the drive was connected to two 9 kW generators - the first recorded instance of wind-generated electricity. By 1910 several hundred windmills of up to 25 kW capacity were supplying villages with electricity. The use of wind-generated electricity continued to increase in Denmark and a peak of 481 785 kWh was obtained from 88 windmills in January 1944. Large-scale modern windpower dates from the designs of an American engineer, Palmer C. Putnam118 in the 1930s. He was responsible for the Smith-Putnam windmill which was erected at Grandpa's Knob in central Vermont in 1941. It had two blades with a diameter of 53.34 m. and at that time it was the world's largest ever windmill, a record it was to hold for the next 35 years. The synchronous electric generator and rotor blades were mounted on a 33.54 m tower and electricity was fed directly into the Central Vermont Public Service Corporation network. The windmill was rated at 1.25 MW and worked well for about 18 months until a main bearing failed in the generator, a failure unconnected with the basic windmill design. It proved impossible to replace the bearing for over two years because of the war and during this period the blades were fixed in position and exposed to the full force of the wind. Also, in 1942, cracks had been noticed around some rivet holes, but these were considered to be so small that they could be ignored. On 26 March 1945, less than a month after the bearing had been replaced, the cracks widened suddenly and a spar failed, causing one of the blades to fly off. The S. Morgan Smith Company, who had undertaken the project, decided that they could not justify any further expenditure on it, apart from a feasibility study on the installation of other units in Vermont. This indicated that the capital cost per installed kilowatt would be some 60% greater than conventional systems. Although sceptics have tended to regard this experiment as an expensive failure, it was the most significant advance in the history of windpower. For the first time. synchronous generation of electricity had taken place and been delivered to a transmission grid. Both mechanical failures were due to a lack of knowledge of the mechanical properties of the materials at that time. Bearing design and the problems of fatigue in metals have been studied extensively since then and similar failures are less likely to occur in modern windmills.'19 Their research programme included an extensive series of on-site measurements, which proved that the actual site at Grandpa's Knob had a mean wind velocity of only 70% of the original estimated velocity and that many other sites should have been selected. The technical problems of converting wind energy into electricity had been largely overcome and the possibility of developing wind power as a national energy resource in any country with an appropriate wind climate has been established. However, very few wind turbines were to be built over the next 30 years. 12.8.2 Wind-energy potential Wind has a dependable annual statistical energy distribution but a complete analysis of how much energy is available from the wind in any particular location is rather complicated. It depends, for example, on the shape of the local landscape, the height of the windmill above ground level and the climatic cycle. Somewhat surprisingly, the British Isles have been studied more extensively than practically any other country in the world'20,'2' and the west coast of Ireland, together with some of the western islands of Scotland, have the best wind conditions with mean average wind speeds approaching 9 ms-I. The kinetic energy of a moving air stream per unit mass is iV2 and the mass flow rate through a given cross-sectional 12/24 Alternative energy sources area A is pAV, where p is the density. The theoretical power available in the air stream is the product of these two terms: IpA V3 If the area A is circular, typically traced by rotor blades of diameter D, then a/4D2 = A, and the power available be- comes 7F - pD2V3 8 The actual power available can be conveniently expressed as C $pAV3 where Cis the coefficient of performance or power coefficient. The maximum amount of energy which could be extracted from a moving airstream was first shown by the German engineer Betz, in 1927, to be 16/27 or 0.59259 of the theoretical available power. This efficiency can only he approached by careful blade design, with blade-tip speeds a factor of six times the wind velocity, and is known as the Betz limit. Modern designs of windmills for electricity generation operate with power coefficient values (C) of about 0.4, with the major losses caused by drag on the blades and the swirl imported to the air flow by the rotor.'22 Any aerogenerator will only operate between a certain minimum wind velocity, the starting velocity Vs, and its rated velocity VR. Typically, VR/V/S lies between 2 and 3. If the pitch of the blades can be altered at velocities greater than V,, the system should continue to operate at its rated output, the upper limit depending only on the design. In some systems the whole rotor is turned out of the wind to avoid damage at high wind speeds. An annual velocity duration curve for a continuously generating windmill is shown in Figure 12.13. The effect of the height of the windmill tower on the performance can be significant and empirical power law indices have been e~tablished'~~ relating the mean wind velocity V to the height H, in the equation V = Ha. A value of a = 0.17 is the accepted value in the UK for open, level ground, but this rises to 0.25 for an urban site and 0.33 for a city site. An ideal site is a long, gently sloping hill. The mean annual wind velocity is normally used to describe the wind regime at any particular location, but the output from a windmill is proportional to V3. Since a transient arithmetic increase in wind velocity will contribute much more energy to the rotor than an equal arithmetic decrease will deduct, the mean of V3, which is always much greater than the cube of the mean annual wind velocity, should be used. For example, if the mean wind velocity is 8 ms-' the most common variation Hwrs per arnum Figure 12.13 Annual velocity duration curve for a continuously generating windmill (after McVeigh') in wind velocity occurs at frequent short intervals between 6 ms-' and 10 ms-' and 83 = 512, whereas 1(63 + lo3) = 608. A useful concept is the velocity exceeded for 50% of the year (4380 hours), shown in Figure 12.13 as Vs0. This is quite close to the mean annual wind speed and has been used to give the annual extractable eneg E, if the rotor shaft is attached to an electrical generator as E, = 3.2289 D2 Vjo3 kWh The Betz limit, outlined above, is purely theoretical, and in practice the power extraction efficiency will be reduced if either: 12j 1. The blades are so close together or rotating so rapidly that a following blade moves into the turbulent air created by a preceding blade; or 2. The blades are so far apart or rotating so slowly that much of the air passes through the cross section of the device without interfering with a blade. The rotational frequency of the wind turbine must be matched to particular wind speeds to obtain the optimum efficiency. The power extraction is therefore a function of the time taken by a following blade to reach the position occupied by the preceding blade, and the time taken for the normal airflow to become re-established once the disturbed air has left that position. This has resulted in a very important parameter - the tip speed ratio - defined as the speed of the turbine blade tip divided by the speed of the normal airstream, or oncoming wind. A more detailed analysis can be found in the standard literature. For the great majority of wind-power applications, however, it is more important to know the probability that a minimum site wind velocity will be exceeded. Long periods of no wind or only light winds are obviously unacceptable. Matching the wind turbine to the characteristics of any particular site has needed the use of probability functions, the best known being the Weibull function. 12.8.3 Small to medium-range windmills Multi-bladed windmills for water pumping are still being manufactured in several countries and an estimated one million were in use in the early 1980~."~ These windmills have a high solidity, or area of blade relative to total swept area. This gives a high starting torque but a relative low power coefficient, typically about 0.2. Wind energy was considered to have a significant role in pumping water in the developing countries by the United Nations Technical Panel,'" hut they also identified three problems with existing designs: they were too complicated for local manufacture, too expensive and too difficult to maintain and repair. Several new designs appeared in the late 1970s and early 1980s. These could be made locally and were relatively inexpensive, but a wider educational programme was still needed before the technology could be disseminated. Small low-solidity wind turbines for generating electricity in the range up to 10 kW are widely available in many countries. Windmills in Sri Lanka, for example, locally developed in the early 1980s, could give an output of up to 400 W and would cost no more than $200 to build.126 Prototypes are used to charge locally manufactured lead-acid batteries which power low-energy consumption fluorescent tubes. This provides an electric lighting system at about half the cost of conventional kerosene lamps. Isolated communities in good wind areas, especially in mountain regions, on islands or in coastal areas, can meet their power needs in the 10-1000 kW range by a combination of wind power and a suitable back-up system. Wind power 12/25 By the mid-1980s the combination of wind and diesel generators was attracting very considerable international research and development activity. The results of much of this work were summarized by Lipman in 1990, 127 who pointed out that a wind power system may be fully meeting an autonomous load at one moment and be in considerable power deficit a few seconds later. Strategies which were being tried included various types of load control, both long- and short-term energy storage, hybrid systems using flywheels and multiple diesels'27 or a pumped hydroelectric system. Among ?he smaller UK companies the Northumbrian Ener- gy Workshop (NEW) have helped the government of the Seychelles with wind-resource assessment using data loggers they have installed under a United Nations Development Programme (UNDP). NEW is also continuing to support a UNDP project in the very different climatic conditions of Mongolia for which they supplied 27 Marlec WG910 50 W windchargers and four Dyna Technology 200 W windchargers. NEW, together with the National Centre for Alternative Technology in Wales, and Marlec. have also supplied some small solar-photovoltaic-wind hybrid systems for projects in Tanzania and Kenya. Most of the Marlec WG510 windchargers are exported to remote parts of both developing and developed countries. A particularly interesting user is the 'Footsteps of Scott Expedi- tion', which reported using their Marlec aerogenerator at temperatures down to -40°C and windspeeds exceeding force 12 and averaging 40 mph over 12 h. They re orted 'faultless' operation under these extreme conditions. 128129 12.8.4 The vertical-axis windmill The mo'dern vertical-axis windmill is a synthesis of two earlier inventions. These are the Darrieus13' windmill with blades of symmetrical aerofoil cross section bowed outward at their mid-poiint to form a catenary curve and attached at each end to a vertical rotational axis perpendicular to the wind direction and the Savoni~s'~' windmill or S-rotor, in which the two arcs of the 'S' are separated and overlap, allowing air to flow through the passage. The Darrieus windmill is the primary power-producing device, but, like other fixed-pitch high- performance systems, is not self-starting. The blades rotate as a result of the high lift from the aerofoil sections, the S-rotor being used primarily to start the action of the Darrieus blades. The wind-energy conversion efficiency of the Darrieus rotor is approxiimately the same as any good horizontal system'32 but its potential advantages are claimed to be lower fabrication costs and functional simplicity. 133 In 1981 the largest Ameri- can Darrieus machine, with three blades, had developed 500 kW. A 4 MW machine jointly funded by the Canadian National Research Council and the Institut de Recherche d'Energie du Quebec was completed on a site in the St Lawrence river valley, Quebec, in 1985. An earlier feasibility study concluded that Darrieus machines up to 8 MW in size could be built. In the UK, an analysis of the Darrieus rotor suggested to Musgrove'34 that straight-bladed H-shaped rotors. with the central horizontal shaft supporting two hinged vertical blades, could tie a more effective system. A variety of designs based on Musgrove's work in the UK during the 1970s and early 1980s have been studied and a small, 6 m diameter, three- bladed version was commercially available by 1980. This work was followed by a 25 m diameter 130 kW machine at Carmarthen Bay, which started a test and monitoring programme in November 1986. Full details of the development of this design are a~ai1able.I~~ Following the highly successful trials, a larger version, known as VAWT 850, was inaugurated in August 1940. The '850' refers to the swept area of the blades. Its rated capacity is 500 kW, with a cut-in windspeed of 6 m s-' and a shutdown windspeed of 23 m s-1.136 Musgrove also considered the possibility of siting groups or clusters of windmills in shallow offshore locations in the UK such as the Wash. Two advantages of this proposal are the higher mean windspeeds and the greatly reduced environmental objections. 12.8.5 The development of large horizontal-axis wind turbines and some national programmes Details of the largest horizontal-axis wind turbines built or planned in Europe during the period from the late 1970s to the mid-1980s showed that four countries. Denmark, Germany, Sweden and the UK, had major programmes.' In 1979: the Danish machine at Twind, rated at 2 MW with a blade diameter (three blades) of 54 111, became the largest in the world since the Smith-Putnam machine. 137 This was a private venture and it never achieved the full rated power. The official Danish programme for large electricity producing wind energy systems started in 1977 with a joint programme directed by the Energy Ministry and the Electricity Utilities. Their major project was the design, construcrion and operation of two machines, Nibe A and B, which were erected in 1979. These turbines are sited close to each other and are identical, apart from their rotor blades. Those for the A machine are supported by stays while the blades of the B machine are self-supporting. Construction of a 2 MW wind turbine near Esbjerg, in Western Jutland, was completed in 1988, with grants from the EC.138 The main parameters were a blade diameter (three blades) of 61 m; a hub height of 60 m and a rated windspeed of 15 m s-'. The estimated annual output was 3.5 GWh y-' and the estimated capacity of Danish windfarms was approaching 100 MW at the same time. 139 The German wind programme, known as the Growian programme, had some 25 projects in operation during the early 1980s, ranging from some small, low-cost units rated at 15 kW for production in developing countries and a medium- sized 25 m diameter twin-bladed 265 kW machine, the Voith- Hutter commissioned in 1981, to the large Growian 1 machine, rated at 3 MW, with the world's largest blade diameter of 100 m. The rated capacity of the German Research, Development and Demonstration programme was 8 MW towards the end of the 1980s. The main feature of the Swedish programme was relared to the design, construction and operation of two large-scale prototypes, located at Maglarp in the province of Skane in southern Sweden, and Nasudden on the island of Gotland. These projects, with rated capacities of 3 MW and 2 MW, respectively, formed the main basis of Swedish work during the decade. In the United States the first major project in the official wind energy programme was the ERDA Model Zero (MQD- 0) 100 kW windmill which consisted of a two-bladed, 38.10 m diameter, variable-pitch propeller system driving a synchronous alternator through a gearbox, mounted on a 30.48 m high steel tower. 140 The blades were located downstream from the tower and a powered gear-control system replaced the traditional tail fin of earlier designs. This initial test programme was designed to establish a database concerning the fabrication, performance, operating and economic characteristics of propeller-type wind turbine systems for providing electrical power into an existing power grid. The next in the series, the MOD-1 windmill, became the world's largest machine in May 1979, when it was commissioned. This was also a twin-bladed downwind horizontal axis [...]... with all the active members of what is described as 'the wave energy community' participating, to produce a 'forwardlooking review based on best current knowledge' Figure 12. 15 Schematic diagram of an oscillating water column device (afler Review 7"') 12/ 36 Alternative energy sources 12. 12 Biomass and energy from wastes 12. 12.1 Introduction The development of human life can be directly traced through... (1976) 121 Golding E W and Stodbart, A H., The potentialities of windpower for electricity generation, British Electrical and Allied Industries Research Association, Tech Rep WIT16 (1949) 122 Taylor, R H., Alternative Energy Sources, Adam Hilger, Bristol (1983) 123 Davenport, A G Proceedings of the (1963) Conference on Wind Effects on Building and Structure Vol I HMSO (1965) ~ References 12/ 43 124 Caton,... for high-performance racing cars and was subsequently stadied as an additive in many laboratories It is now considered to be an essential part of the future automobile fuel The typical energy content of some of the products in Figure 12. 16 is shown in Table 12. 9 12. 12.5 Cooking - the major application of biomass x The United Nations have warned for many ears that over 90% of wood cut in Africa is burnt... Data and Results, SAND 77 -120 4, Sandia Laboratories Albuquerque, New Mexico (1977) 29 Com,mission of the European Communities European Passive Solar Handbook, Brussels (1989) 30 Department of Energy, ‘Gains all round through passive solar design‘, Review, 2, London (1988) 31 Department of Energy Renewable Energy in the UK: The Way Forward, Energy Paper 55 HMSO, London (1988) 32 Department of Energy ‘EDAS... geothermal heat as part of a larger 12 MW system The maximum pumping rate is limited to 12 1 s-' to ensure an operating life of 20 years.'j8 A very small warm water scheme (ca 22°C) is operating from a 250 m experimental borehole in Cornwall for a horticultural application.(j4 The main features of a typical geothermal district heating system are shown in Figure 12. 14, based on information from reference 1.59... (1975) 125 Twidell, J W and Weir A D Renewable Energy Resources, E & F N Spon London (1986) 126 McVeigh J C ‘When nuclear power is not the answer’, Electrical Review International, 1, 1, February (1984) 127 Lipman N H ‘Overview of windidiesel systems’ in Energy and the Environment into the 1990s, Volume 3 pp 1547-1569 Pergamon Press, Oxford (1990) 128 lVindirections, 5, No 2, October (1985) 129 h‘ational... of valuable nutrients.*09 In parts of Africa crop residues and stubble are uprooted and used for fuel Cooking is a very cultural-specific acrivity However, the most common means of cooking throughout the developing "' 12/ 38 Alternative energy sources Table 12. 10 Estimated UK waste and its energy content Gross weight (t x 10-61 Weight of combustible content (t x 10-6) 18.0 12. 0 3.8 1.6 Industrial refuse... route is through algae in the sea or grown in inland ponds 12. 12.4 Conversion of biomass to fuels and other products A selection from some of the main conversion processes is illustrated in Figure 12. 16, which shows that there are often several different routes to the same end product Combustion is by far the simplest and best-known technique, particularly with forestry residues and industrial and urban... exploit this conversion of fertile to fissile material particularly using neutrons around 1 MeV to cause fission at an energy that produces about three neutrons per fission 13.3 Mechanical engineering aspects of nuclear power stations and associated plant In this section the broad outline of the nuclear electricity industry is described with reference to mechanical engineering aspects that embrace: pressure... in non-thermal areas but are only worth exploiting if they are located fairly close to an appropriate application, such as space heating in a town or city 12. 9.3 Thermal applications 8 200 180 36 Data derived from references 154 and 155 figure Table 12. 6 suggests that the convertible geothermal resource is some forty times greater than the world annual production of electricity This figure may be of . -40°C and windspeeds exceeding force 12 and averaging 40 mph over 12 h. They re orted 'faultless' operation under these extreme conditions. 128 129 12. 8.4 The vertical-axis windmill. hydropower by a factor of ten or more. 12. 7.2 The basic hydropower plant The basic principles of hydroelectric power generation are shown in Figure 12. 12. Water at a high level, often stored. turbine broadly related to low, medium or high heads. 12/ 20 Alternative energy sources Turbo- generator . . j Figure 12. 12 The basic principles of hydroelectric power generation