WIND ENERGY:
World Energy Council, 2001:
World wind energy capacity has been doubling every three years during the last decade and growth rates in the last two years have been even faster. It is doubtful whether any other energy technology is growing, or has grown, at such a rate. Total world wind capacity at the end of 2000 was around 17 500 MW and generation from wind now approximately equates to annual consumption of electricity in Chile or Singapore. Germany, with over 6 000 MW, has the highest capacity but Denmark, with over 2 000 MW, has the highest level per capita and the production accounts for about 12% of Danish electricity.
The attractions of wind as a source of electricity which produces minimal quantities of greenhouse gases has led to ambitious targets for wind energy in many parts of the world. More recently, there have been several developments of offshore wind installations and many more are planned. Although offshore wind-generated electricity is generally more expensive than onshore, the resource is very large and there are few environmental impacts.
Whilst wind energy is generally developed in the industrialised world for environmental reasons, it has attractions in the developing world as it can be installed quickly in areas where electricity is urgently needed. In many instances it may be a cost-effective solution if fossil fuel sources are not readily available. In addition there are many applications for wind energy in remote regions, worldwide, either for supplementing diesel power (which tends to be expensive) or for supplying farms, homes and other installations on an individual basis.
Types of Modern Wind Turbine
Early machines - less than twenty years ago - were fairly small (50-100 kW, 15-20 m diameter) but there has been a steady growth in size and output power. Several commercial types of wind turbine now have ratings over 1 MW and machines for the offshore market have outputs up to 3 MW.
Machine sizes have increased for two reasons. They are cheaper and they deliver more energy. The energy yield is improved partly because the rotor is located higher from the ground and so intercepts higher velocity winds, and partly because they are slightly more efficient. The higher yields are clearly shown in Figure 13.2, which shows data from machines in Denmark; the productivity of the 600 kW machines is around 50% higher than that of the 55 kW machines. Reliability has improved steadily and most wind turbine manufacturers now guarantee availabilities of 95%.
The majority of the world's wind turbines have three glass-reinforced plastic blades. The power train includes a low speed shaft, a step-up gearbox and an induction generator, either four or six-pole. There are numerous other possibilities, however. Wood-epoxy is an alternative blade material and some machines have two blades. Variable speed machines are becoming more common and most generate power using an AC/DC/AC system. Variable speed brings several advantages - it means that the rotor turns more slowly in low winds (which keeps noise levels down), it reduces the loadings on the rotor and the power conversion system is usually able to deliver current at any specified power factor. A few manufacturers build direct-drive machines, without a gearbox. These are usually of the variable speed type, with power conditioning equipment.
Towers are usually made of steel and the great majority are of the tubular type. Lattice towers, common in the early days, are now rare, except for very small machines in the range 100 kW and below.
As the power in the wind increases with the cube of the wind speed, all wind turbines need to limit the power output in very high winds. There are two principal means of accomplishing this, with pitch control on the blades or with fixed, stall-controlled blades. Pitch-controlled blades are rotated as wind speeds increase so as to limit the power output and, once the "rated power" is reached, a reasonably steady output can be achieved, subject to the control system response. Stall-controlled rotors have fixed blades which gradually stall as the wind speed increases, thus limiting the power by passive means. These dispense with the necessity for a pitch control mechanism, but it is rarely possible to achieve constant power as wind speeds rise. Once peak output is reached the power tends to fall off with increasing wind speed, and so the energy capture may be less than that of a pitch-controlled machine. The merits of the two designs are finely balanced, which accounts for the roughly equal numbers of machines.
Energy Production
Contrary to popular opinion, energy yields do not increase with the cube of the wind speed, mainly because energy is discarded once the rated wind speed is reached. To illustrate a typical power curve and the concept of rated output, Figure 13.3 shows a typical performance curve for a 1.65 MW machine. Most machines start to generate at a similar speed - around 3 to 5 m/s - and shut down in very high winds, generally around 20 to 25 m/s.
Annual energy production from the turbine whose performance is charted in Figure 13.3 is around 1 500 MWh at a site where the wind speed is 5 m/s, 3 700 MWh at 7 m/s and 4 800 MWh at 8 m/s. Wind speeds around 5 m/s can be found, typically, away from the coastal zones in all five continents, but developers generally aim to find higher wind speeds. Levels around 7 m/s are to be found in many coastal regions and over much of Denmark; higher levels are to be found on many of the Greek Islands, in the Californian passes - the scene of many early wind developments - and on upland and coastal sites in the Caribbean, Ireland, Sweden, the United Kingdom, Spain, New Zealand and Antarctica.
Wind speed is the primary determinant of electricity cost, on account of the way it influences the energy yield so, roughly speaking, developments on sites with wind speeds of 8 m/s will yield electricity at one third of the cost for a 5 m/s site. Offshore wind speeds are generally higher than those onshore. Offshore wind farms have been completed, or are planned, in Denmark, Sweden, Germany, the United Kingdom, Ireland and elsewhere. Offshore wind is attractive in locations such as Denmark and the Netherlands where pressure on land is acute and windy hill top sites are not available. In these areas offshore winds may be 0.5 to 1 m/s higher than onshore, depending on the distance. The higher wind speeds do not usually compensate for the higher construction costs but the chief attractions of offshore are a large resource and low environmental impact.
Wind Energy Costs
As wind energy is not generally cost-competitive with the thermal sources of electricity generation, the pattern of development has been largely dependent on the support mechanisms provided by national governments.
Wind costs have declined steadily and a typical installed cost for onshore wind farms is now around US$ 1 000/kW, and for offshore around US$ 1 600/kW. The corresponding electricity costs vary, partly due to wind speed variations and partly due to differing institutional frameworks. Wind prices are converging with those from the thermal sources but it is not easy to make objective comparisons, as there are few places where totally level playing fields exist. Two examples may be given. Until recently, the UK operated a competitive tender market for renewable energy sources which guaranteed payments for 15 years. Vigorous competition drove prices down rapidly and the prices realised in the last round of the Non-Fossil Fuel Obligation may be compared with prices for new gas and coal-fired plant. These comparisons, show that wind prices are very similar to those for coal-fired plant and only a little more than those of gas-fired plant. The second set of comparisons, has been drawn from two US sources: a Department of Energy projection for 2005 and a recent analysis for the State of Oregon in 2000. This comparison shows a bigger gap between wind and gas although wind is significantly cheaper than nuclear. Other US data suggest that wind prices down to around 4 US cents/kWh can be realised in some areas.
Wind farms
The way in which wind energy has developed has been influenced by the nature of the support mechanisms. Early developments in California and subsequently in the UK, for example, were mainly in the form of wind farms, with tens of machines, but up to 100 or more in some instances. In Germany and Denmark the arrangements favoured investments by individuals or small cooperatives and so there are many single machines and clusters of two or three. Economies of scale can be realised by building wind farms, particularly in the civil engineering and grid connection costs and possibly by securing "quantity discounts" from the turbine manufacturers. Economies of scale deliver more significant savings in the case of offshore wind farms and many of the developments involve large numbers of machines. Figure 13.6 gives an indication of typical parameters for offshore and onshore wind farms. It may be noted that the offshore project uses machines with three times the power rating of the onshore project.
Small wind turbines
There is no precise definition of "small", but it usually applies to machines under about 10 kW in output. In developing countries small wind turbines are used for a wide range of rural energy applications, and there are many "off-grid" applications in the developed world as well - such as providing power for navigation beacons. Since most are not connected to a grid, many use DC generators and run at variable speed. A typical 100 W battery-charging machine has a shipping weight of only 15 kg.
A niche market, where wind turbines often come into their own as the costs of energy from conventional sources can be very high, is in cold climates. Wind turbines may be found in both polar regions and in northern Canada, Alaska, Finland and elsewhere. To illustrate the point about economic viability, data from the U.S. Office of Technology Assessment quotes typical costs of energy at 10 kW capacity in remote areas:
Micro-Hydro ~ US$ 0.21/kWh
Wind ~ US$ 0.48/kWh
Diesel ~ US$ 0.80/kWh
Grid Extension ~ US$ 1.02/kWh
Environmental Aspects
No energy source is free of environmental effects. As the renewable energy sources make use of energy in forms that are diffuse, larger structures, or greater land use, tend to be required and attention may be focused on the visual effects. In the case of wind energy, there is also discussion of the effects of noise and possible disturbance to wildlife - especially birds. It must be remembered, however, that one of the main reasons for developing the renewable sources is an environmental one - to reduce emissions of greenhouse gases.
Noise
Almost all sources of power emit noise, and the key to acceptability is the same in every case - sensible siting. Wind turbines emit noise from the rotation of the blades and from the machinery, principally the gearbox and generator. At low wind speeds wind turbines generate no noise, simply because they do not generate. The noise level near the cut-in wind speed (see Figure 13.3) is important since the noise perceived by an observer depends on the level of local background noise (the masking effect) in the vicinity. At very high wind speeds, on the other hand, background noise due to the wind itself may well be higher than noise generated by a wind turbine. The intensity of noise reduces with distance and it is also attenuated by air absorption.
The exact distance at which noise from turbines becomes "acceptable" depends on a range of factors. As a guide, many wind farms with 400-500 kW turbines find that they need to be sited no closer than around 300-400 m to dwellings.
Television and Radio Interference
Wind turbines, like other structures, can scatter electro-magnetic communication signals, including television. Careful siting can avoid difficulties, which may arise in some situations if the signal is weak. Fortunately it is usually possible to introduce technical measures - usually at low cost - to compensate.
Birds
The need to avoid areas where rare plants or animals are to be found is generally a matter of common sense, but the question of birds is more complicated and has been the subject of several studies. Problems arose at some early wind farms that were sited in locations where large numbers of birds congregate - especially on migration routes. However, such problems are now rare, and it must also be remembered that many other activities cause far more casualties to birds, such as the ubiquitous motor vehicle.
In practice, provided investigations are carried out to ensure that wind installations are not sited too near large concentrations of nesting birds, there is little cause for concern. Most birds, for most of the time, are quite capable of avoiding obstacles and very low collision rates are reported where measurements have been made.
Visual effects
One of the more obvious environmental effects of wind turbines is their visual aspect, especially that of a wind farm comprising a large number of wind turbines. There is no measurable way of assessing the effect, which is essentially subjective. As with noise, the background is also vitally important. Experience has shown that good design and the use of subdued neutral colours - "off-white" is popular - minimises these effects. The subjective nature of the question often means that extraneous factors come into play when acceptability is under discussion. In Denmark and Germany, for example, where local investors are often intimately involved in planning wind installations, this may often ensure that the necessary permits are granted without undue discussion. Sensitive siting is the key to this delicate issue, avoiding the most cherished landscapes and ensuring that the local community is fully briefed on the positive environmental implications.
Integration into supply networks
Electricity systems in the developed world have evolved so as to deliver power to the consumers with high efficiency. One fundamental benefit of an integrated electricity system is that generators and consumers both benefit from the aggregation of supply and demand. On the generation side, this means that the need for reserves is kept down. Consumers benefit from a high level of reliability and do not need to provide back-up power supplies. In an integrated system the aggregated maximum demand is much less than the sum of the individual maximum demands of the consumers, simply because the peak demands come at different times.
Wind energy benefits from aggregation; it means that system operators simply cannot detect the loss of generation from a wind farm of, say, 20 MW, as there are innumerable other changes in system demand which occur all the time. Numerous utility studies have indicated that wind can readily be absorbed in an integrated network until the wind capacity accounts for about 20% of maximum demand. Beyond this, some modest changes to operational practice may be needed, but there are no "cut-off" points. Practical experience at these levels is now providing a better understanding of the issues involved.
Future Developments
Recent rapid growth in Denmark, Spain and Germany shows no sign of slowing and there are plans for further capacity in the United States, Canada, the Middle East, the Far East and South America. If the current growth rate continues, there may be about 150 GW by 2010. The rate of development will depend on the level of political support from the national governments and international community. This, in turn, depends on the level of commitment to achieving the carbon dioxide reduction targets now internationally agreed. Although the technology has developed rapidly during the past ten years, further improvements can be expected both in performance and cost.
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