Wind Energy
Wind energy is the conversion of wind power into a useful form, such as electricity, using wind turbines. At the end of 2007, worldwide capacity of wind-powered generators was 94.1 GW. Although wind currently produces about 1% of world-wide electricity use, it accounts for approximately 19% of electricity production in Denmark, 9% in Spain and Portugal, and 6% in Germany and the Republic of Ireland (2007 data). Globally, wind power generation increased more than fivefold between 2000 and 2007.
Most wind power is generated in the form of electricity. Large-scale wind farms are connected to electrical grids. Individual turbines can provide electricity to isolated locations. In windmills, wind energy is used directly as mechanical energy for pumping water or grinding grain.
Wind energy is plentiful, renewable, widely distributed, and clean, and it reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity. The intermittency of wind seldom creates problems when using wind power to supply a low proportion of total demand, but it presents extra costs when wind is to be used for a large fraction of demand. However, these costs—even for quite large percentage penetrations—are considered to be modest.
The siting of turbines has become a controversial issue among those concerned about the value of natural landscapes, particularly since the best sites for wind generation tend to be in scenic mountain and oceanside areas.
This 3-bladed wind turbine is the most common modern design because it minimizes forces related to fatigue.
History
The earliest historical reference describes a windmill used to power an organ in the 1st century AD. Windmills were used extensively in northwestern Europe to grind flour beginning in the 1180s, and many Dutch windmills still exist.
In the United States, the development of the "water-pumping windmill" was the major factor in allowing the farming and ranching of vast areas that were otherwise devoid of readily accessible water. These windmills also contributed to the expansion of rail transport systems throughout the world, by pumping water from wells to supply the needs of the steam locomotives of those early times.
The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America.
The modern wind turbine was developed beginning in the 1980s, although designs are still under development.
Wind Energy
The origin of wind is complex. The earth is unevenly heated by the sun, resulting in the poles receiving less energy from the sun than the equator does. Also the dry land heats up (and cools down) more quickly than the seas do. The differential heating drives a global atmospheric convection system reaching from the earth's surface to the stratosphere, which acts as a virtual ceiling. Most of the energy stored in these wind movements can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the earth's surface and the atmosphere.
There is an estimated 72 TW of wind energy on the earth that potentially can be commercially viable. Not all the energy of the wind flowing past a given point can be recovered.
Distribution of Wind Speed
Distribution of wind speed (red) and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado. The histogram shows measured data, while the curve is the Rayleigh model distribution for the same average wind speed. Energy is the Betz limit through a 100 m diameter circle facing directly into the wind. Total energy for the year through that circle was 15.4 GWh.
Windiness varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To assess the frequency of wind speeds at a particular location, a probability distribution function is often fit to the observed data. Different locations will have different wind speed distributions. The Rayleigh model closely mirrors the actual distribution of hourly wind speeds at many locations.
Because so much power is generated by higher windspeed, much of the energy comes in short bursts. The 2002 Lee Ranch sample is telling; half of the energy available arrived in just 15% of the operating time. The consequence is that wind energy does not have as consistent an output as fuel-fired power plants; utilities that use wind power must provide back-up generation for times when the wind is weak. Making wind power more consistent requires that storage technologies be used to retain the large amount of power generated in the bursts for later use.
Worldwide installed capacity in 2006 and prediction 1997–2010, Source: WWEA
Grid Management
Induction generators often used for wind power projects require reactive power for excitation, so substations used in wind power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modeling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behavior during system faults. In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators (however, properly matched power factor correction capacitors along with electronic control of resonance can support induction generation without grid). Doubly fed machines, or wind turbines with solid-state converters between the turbine generator and the collector system, have generally more desirable properties for grid interconnection. Transmission systems operators will supply a wind farm developer with a grid code to specify the requirements for interconnection to the transmission grid. The grid code will include power factor, constancy of frequency, and dynamic behavior of the wind farm turbines during a system fault.
Capacity Factor
Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favorable sites. For example, a 1 MW turbine with a capacity factor of 35% will not produce 8,760 MWh in a year, but only 0.35 x 24 x 365 = 3,066 MWh, averaging to 0.35 MW. Online data is available for some locations, and the capacity factor can be calculated from the yearly output.
Unlike fueled generating plants, the capacity factor is limited by the inherent properties of wind. Capacity factors of other types of power plants are based mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants have low incremental fuel cost, and so are run at full output and achieve a 90% capacity factor. Plants with higher fuel cost are throttled back to follow load. Gas turbine plants using natural gas as fuel may be very expensive to operate and may be run only to meet peak power demand. A gas turbine plant may have an annual capacity factor of 5–25% due to relatively high energy production cost.
According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting ten or more wind farms allows 33 to 47% of the total energy produced to be used as reliable, baseload electric power, as long as minimum criteria are met for wind speed and turbine height.
Intermittency and Penetration Limits
Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. Because instantaneous electrical generation and consumption must remain in balance to maintain grid stability, this variability can present substantial challenges to incorporating large amounts of wind power into a grid system. Intermittency and the nondispatchable nature of wind energy production can raise costs for regulation and incremental operating reserve, and at high penetration levels they could require energy demand management, load shedding, or storage solutions. At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units require reserve capacity that can also regulate for variability of wind generation.
Pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed. Stored energy increases the economic value of wind energy since it can be shifted to displace higher cost generation during peak demand periods. The potential revenue from this arbitrage can offset the cost and losses of storage; the cost of storage may add 25% to the cost of wind energy.
Peak wind speeds may not coincide with peak demand for electrical power. In California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. In the UK, however, winter demand is higher than summer demand, and so are wind speeds. Solar power tends to be complementary to wind: On most days with no wind there is sun, and on most days with no sun there is wind. A demonstration project at the Massachusetts Maritime Academy shows the effect. A combined power plant linking solar, wind, biogas, and hydrostorage is proposed as a way to provide 100% renewable power. The 2006 Energy in Scotland Inquiry report expressed concern that wind power cannot be a sole source of supply, and recommends diverse sources of electric energy. A report from Denmark noted that their wind power network was without power for 54 days during 2002. Wind power advocates argue that these periods of low wind can be dealt with by simply restarting existing power stations that have been held in readiness. The cost of keeping a power station idle is in fact quite low, since the main cost of running a power station is the fuel.
Penetration
Wind energy penetration refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted "maximum" level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. An interconnected electricity grid will already include reserve generating and transmission capacity to allow for equipment failures; this reserve capacity can also serve to regulate for the varying power generation by wind plants. Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty. These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed. Beyond this level, there are few technical limits, but the economic implications become more significant.
At present, few grid systems have penetration of wind energy above 5%: Denmark (values over 18%), Spain and Portugal (values over 9%), Germany and the Republic of Ireland (values over 6%). The Danish grid is heavily interconnected to the European electrical grid, and it has solved grid management problems by exporting almost half of its wind power to Norway. The correlation between electricity export and wind power production is very strong.
A study commissioned by the state of Minnesota considered penetration of up to 25%, and concluded that integration issues would be manageable and would have incremental costs of less than one-half cent ($0.0045) per kWh.
Predictability
Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled." The nature of this energy source makes it inherently variable. Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term operation.
Turbine Placement
Good selection of a wind turbine site is critical to economic development of wind power. Aside from the availability of wind itself, other significant factors include the availability of transmission lines, value of energy to be produced, cost of land acquisition, land use considerations, and environmental impact of construction and operations. Offshore locations may offset their higher construction cost with higher annual load factors, thereby reducing the cost of energy produced. Wind farm designers use specialized wind energy software applications to evaluate the impact of these issues on a given wind farm design.
Economics and Feasibility
Erection of an Enercon E70-4 in Germany
Growth and Cost Trends
Global Wind Energy Council (GWEC) figures show that 2007 recorded an increase of installed capacity of 20 GW, taking the total installed wind energy capacity to 94 GW, up from 74 GW in 2006. Despite constraints facing supply chains for wind turbines, the annual market for wind continued to increase at an estimated rate of 31% following 32% growth in 2006. In terms of economic value, the wind energy sector has become one of the important players in the energy markets, with the total value of new generating equipment installed in 2007 reaching €25 billion, or US$36 billion.
In 2004, wind energy cost one fifth of what it did in the 1980s, and some expected that downward trend to continue as larger multi-megawatt turbines are mass-produced. However, installed cost averaged €1,300 per kilowatt in 2007, compared to €1,100 per kilowatt in 2005. Not as many facilities can produce large modern turbines and their towers and foundations, so constraints develop in the supply of turbines, resulting in higher costs.
Wind and hydro power have negligible fuel costs and relatively low maintenance costs; in economic terms, wind power has a low marginal cost and a high proportion of capital cost. The estimated average cost per unit incorporates the cost of construction of the turbine and transmission facilities, borrowed funds, return to investors (including cost of risk), estimated annual production, and other components, averaged over the projected useful life of the equipment, which may be in excess of twenty years. Energy cost estimates are highly dependent on these assumptions, so published cost figures can differ substantially. A British Wind Energy Association report gives an average generation cost of onshore wind power of around 3.2 pence per kWh (2005). Cost per unit of energy produced was estimated in 2006 to be comparable to the cost of new generating capacity in the US for coal and natural gas: Wind cost was estimated at $55.80/MWh, coal at $53.10/MWh, and natural gas at $52.50/MWh. Other sources in various studies have estimated wind to be more expensive than other sources.
Similar methods apply to other electrical energy sources. Existing generation capacity represents sunk costs, and the decision to continue production will depend on marginal costs going forward, not estimated average costs at project inception. For example, the estimated cost of new wind power capacity may be lower than that for "new coal" (estimated average costs for new generation capacity) but higher than for "old coal" (marginal cost of production for existing capacity). Therefore, the choice to increase wind capacity will depend on factors including the profile of existing generation capacity.
Research from a wide variety of sources in various countries shows that support for wind power is consistently between 70 and 80% amongst the general public.
Theoretical Potential
Wind power available in the atmosphere is much greater than current world energy consumption. The most comprehensive study to date found the potential of wind power on land and near-shore to be 72 TW, equivalent to 54,000 MToE (million tons of oil equivalent) per year, or over five times the world's current energy use in all forms. The potential takes into account only locations with mean annual wind speeds ≥ 6.9 m/s at 80 m. It assumes 6 turbines per square km for 77 m diameter, 1.5 MW turbines on roughly 13% of the total global land area (though that land would also be available for other compatible uses such as farming). The authors acknowledge that many practical barriers would need to be overcome to reach this theoretical capacity.
The practical limit to exploitation of wind power will be set by economic and environmental factors, since the resource available is far larger than any practical means to develop it.
Direct Costs
Many potential sites for wind farms are far from demand centers, requiring substantially more money to construct new transmission lines and substations.
Since the primary cost of producing wind energy is construction and there are no fuel costs, the average cost of wind energy per unit of production is dependent on a few key assumptions, such as the cost of capital and years of assumed service. The marginal cost of wind energy once a plant is constructed is usually less than 1 cent per kWh. Since the cost of capital plays a large part in projected cost, risk (as perceived by investors) will affect projected costs per unit of electricity.
The commercial viability of wind power also depends on the pricing regime for power producers. Electricity prices are highly regulated worldwide, and in many locations may not reflect the full cost of production, let alone indirect subsidies or negative externalities. Customers may enter into long-term pricing contracts for wind to reduce the risk of future pricing changes, thereby ensuring more stable returns for projects at the development stage. These may take the form of standard offer contracts, whereby the system operator undertakes to purchase power from wind at a fixed price for a certain period (perhaps up to a limit); these prices may be different than purchase prices from other sources, and may even incorporate an implicit subsidy.
In jurisdictions where the price for electricity is based on market mechanisms, revenue for all producers per unit is higher when their production coincides with periods of higher prices. The profitability of wind farms will therefore be higher if their production schedule coincides with these periods. If wind represents a significant portion of supply, average revenue per unit of production may be lower as more expensive and less efficient forms of generation, which typically set revenue levels, are displaced from economic dispatch. This may be of particular concern if the output of many wind plants in a market have strong temporal correlation. In economic terms, the marginal revenue of the wind sector as penetration increases may diminish.
External Costs
Most forms of energy production create some form of negative externality—costs that are not paid by the producer or consumer of the good. For electric production, the most significant externality is pollution, which imposes social costs in increased health expenses, reduced agricultural productivity, and other problems. In addition, carbon dioxide, a greenhouse gas produced when fossil fuels are burned, may impose even greater costs in the form of global warming. Few mechanisms currently exist to internalize these costs, and the total cost is highly uncertain. Other significant externalities can include military expenditures to ensure access to fossil fuels, remediation of polluted sites, destruction of wild habitat, loss of scenery/tourism, etc.
If the external costs are taken into account, wind energy may be competitive in more cases. Wind energy costs have generally decreased due to technology development and scale enlargement. Wind energy supporters argue that, once external costs and subsidies to other forms of electrical production are accounted for, wind energy is amongst the least costly forms of electrical production. Critics argue that the level of required subsidies, the small amount of energy needs met, and the uncertain financial returns to wind projects make it inferior to other energy sources. Intermittency and other characteristics of wind energy also have costs that may rise with higher levels of penetration, and may change the cost-benefit ratio.
Incentives
Some of the over 6,000 wind turbines at Altamont Pass, in California. Developed during a period of tax incentives in the 1980s, this wind farm has more turbines than any other in the US, producing about 125 MW. Considered largely obsolete, these turbines produce only a few tens of kWs each.
Wind energy in many jurisdictions receives some financial or other support to encourage its development. A key issue is the comparison to other forms of energy production, and their total cost. Two main points of discussion arise: direct subsidies and externalities for various sources of electricity, including wind. Wind energy benefits from subsidies of various kinds in many jurisdictions, either to increase its attractiveness or to compensate for subsidies received by other forms of production which have significant negative externalities.
In the US, wind power receives a tax credit for each kWh produced; at 1.9 cents per kWh in 2006, the credit has a yearly inflationary adjustment. Another tax benefit is accelerated depreciation. Many American states also provide incentives, such as exemption from property tax, mandated purchases, and additional markets for "green credits." Countries such as Canada and Germany also provide incentives for wind turbine construction, such as tax credits or minimum purchase prices for wind generation, with assured grid access (sometimes referred to as feed-in tariffs). These feed-in tariffs are typically set well above average electricity prices.
Secondary market forces also provide incentives for businesses to use wind-generated power, even if there is a premium price for the electricity. For example, socially responsible manufacturers pay utility companies a premium that goes to subsidize and build new wind power infrastructure. Companies like the Borealis Press print millions of greeting cards every year using this wind-generated power, and in return they can claim that they are making a powerful "green" effort, in addition to using recycled, chlorine-free paper, soy inks, and safe press wash. The organization Green-e monitors business compliance with these renewable energy credits.
Environmental Effects
Wind power consumes no fuel for continuing operation, and has no emissions directly related to electricity production. Operation does not produce carbon dioxide, sulfur dioxide, mercury, particulates, or any other type of air pollution, as do fossil fuel power sources. Wind power plants consume resources in manufacturing and construction. During manufacture of the wind turbine, steel, concrete, aluminum, and other materials will have to be made and transported using energy-intensive processes, generally using fossil energy sources. The initial carbon dioxide emissions "pay back" is within about 9 months of operation for offshore turbines.
Danger to birds is often the main complaint against the installation of a wind turbine. However, studies show that the number of birds killed by wind turbines is negligible compared to the number that die as a result of other human activities such as traffic, hunting, power lines, high-rise buildings, and the environmental impacts of non-clean power sources. For example, in the UK, where there are several hundred turbines, about one bird is killed per turbine per year, while 10 million per year are killed by cars alone.
Migratory bat species appear to be particularly at risk, especially during key movement periods (spring and more importantly in fall). Lasiurines such as the hoary bat, the red bat, and the silver-haired bat appear to be most vulnerable at North American sites. Almost nothing is known about current populations of these species or the impact on bat numbers of mortality at windpower locations. Offshore wind sites 10 km or more from shore do not interact with bat populations.
Aesthetics have also been a concern. The Massachusetts Cape Wind project was delayed for years mainly because of aesthetic concerns.
External Links
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Wind power." |
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