Wind Power
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==Wind Power== | ==Wind Power== | ||
Revision as of 08:34, 18 July 2007
Contents |
Wind Power
Wind power is the conversion of wind energy into more useful forms, usually electricity, using wind turbines. At the end of 2006, worldwide capacity of wind-powered generators was 74,223 megawatts; although it currently produces less than 1% of world-wide electricity use, it accounts for approximately 20% of electricity use in Denmark, 9% in Spain, and 7% in Germany. Globally, wind power generation more than quadrupled between 2000 and 2006.
Most modern wind power is generated in the form of electricity by converting the rotation of turbine blades into electrical current by means of an electrical generator. In windmills (a much older technology), wind energy is used to turn mechanical machinery to do physical work, such as crushing grain or pumping water.
Wind power is used in large scale wind farms for national electrical grids as well as in small individual turbines for providing electricity to rural residences or grid-isolated locations.
Wind energy is plentiful, renewable, widely distributed, clean, and reduces toxic atmospheric and greenhouse gas emissions if used to replace fossil-fuel-derived electricity. The intermittency of wind seldom creates problems when using wind power at low to moderate penetration levels.
Wind Energy
An estimated 1% to 3% of energy from the Sun that hits the earth is converted into wind energy. This is about 50 to 100 times more energy than is converted into biomass by all the plants on Earth through photosynthesis.[citation needed] Most of this wind energy 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.
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 powers a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling.
Wind variability and turbine power
The power in the wind can be extracted by allowing it to blow past moving wings that exert torque on a rotor. The amount of power transferred is directly proportional to the density of the air, the area swept out by the rotor, and the cube of the wind speed.
The power P available in the wind is given by:
The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15°C (59°F) day at sea level, air density is 1.225 kilograms per cubic metre. An 8 m/s breeze blowing through a 100 meter diameter rotor would move almost 77,000 kilograms of air per second through the swept area.
The kinetic energy of a given mass varies with the square of its velocity. Because the mass flow increases linearly with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed. The power of the example breeze above through the example rotor would be about 2.5 megawatts.
As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out and diverts it around the wind turbine to some extent. Albert Betz, a German physicist, determined in 1919 (see Betz' law) that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine.
Key Issues
Wind power can be a controversial issue, and several main areas of dispute are debated between supporters and opponents.
Growth and cost trends
Global Wind Energy Council (GWEC) figures show that 2006 recorded an increase of installed capacity of 15,197 megawatts (MW), taking the total installed wind energy capacity to 74,223 MW, up from 59,091 MW in 2005. Despite constraints facing supply chains for wind turbines, the annual market for wind continued to increase at an estimated rate of 32% following the 2005 record year, in which the market grew by 41%. 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 2006 reaching €18 billion, or US$23 billion.
The countries with the highest total installed capacity are Germany (20,621 MW), Spain (11,615 MW), the USA (11,603 MW), India (6,270 MW) and Denmark (3,136). Thirteen countries around the world can now be counted among those with over 1,000 MW of wind capacity. In terms of new installed capacity in 2006, the US lead with 2,454 MW, followed by Germany (2,233 MW), India (1,840 MW), Spain (1,587 MW), China (1,347 MW) and France (810 MW).
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, installation costs have increased significantly in 2005 and 2006, and according to the major U.S. wind industry trade group, now average over 1600 U.S. dollars per kilowatt, compared to $1200/kW just a few years before. A British Wind Energy Association report gives an average generation cost of onshore wind power of around 3.2 pence per kilowatt hour (2005). Cost per unit of energy produced was estimated in 2006 to be comparable to the cost of new generating capacity in the United States for coal and natural gas: wind cost was estimated at $55.80 per MWh, coal at $53.10/MWh and natural gas at $52.50.[38] Other sources in various studies have estimated wind to be more expensive than other sources (see Economics of new nuclear power plants, Clean coal, and Carbon capture and storage).
Most major forms of electricity generation are capital intensive, meaning that they require substantial investments at project inception, and low ongoing costs (generally for fuel and maintenance). This is particularly true for wind and hydro power, which have fuel costs close to zero and relatively low maintenance costs; in economic terms, wind power has an extremely low marginal cost and a high proportion of up-front capital costs. The estimated "cost" of wind energy per unit of production is generally based on average cost per unit, which incorporates the cost of construction, borrowed funds, return to investors (including cost of risk), estimated annual production, and other components. Since these costs are averaged over the projected useful life of the equipment, which may be in excess of twenty years, cost estimates per unit of generation are highly dependent on these assumptions. Figures for cost of wind energy per unit of production cited in various studies can therefore differ substantially. The cost of wind power also depends on several other factors, such as installation of power lines from the wind farm to the national grid and the frequency of wind at the site in question.
Estimates for cost of production use similar methodologies for other sources of electricity generation. 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 by building new facilities will depend on more complex factors than cost estimates, 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 per cent amongst the general public.
Scalability
A key issue debated about wind power is its ability to scale to meet a substantial portion of the world's energy demand. There are significant economic, technical, and ecological issues about the large-scale use of wind power that may limit its ability to replace other forms of energy production. See, for example, the annual report of the Independent Electricity System Operator in Ontario, Canada . Most forms of electricity production also involve such trade-offs, and many are also not capable of replacing all other types of production for various reasons. A key issue in the application of wind energy to replace substantial amounts of other electrical production is intermittency; see the section below on Economics and Feasibility. At present, it is unclear whether wind energy will eventually be sufficient to replace other forms of electricity production, but this does not mean wind energy cannot be a significant source of clean electrical production on a scale comparable to or greater than other technologies, such as hydropower. Most electrical grids use a mix of different generation types (baseload generating capacity and peaking capacity) to match demand cycles by attempting to match the variable nature of demand to the most economic form of production; with the exception of hydropower, most types of production capacity are not used for all production (hydropower usage is limited by the presence of appropriate geographical sites). For example, nuclear power is effective as a baseload technology, but cannot be easily varied in short timeframes, and gas turbine plants are most economically used as peaking capacity; coal generation is primarily considered appropriate for baseload generation with some capacity to cycle to meet demand.
A significant part of the debate about the potential for wind energy to substitute for other electric production sources is the level of penetration. With the exception of Denmark, no countries or electrical systems produce more than 10% from wind energy, and most are below 2% (of course, this is in large part because wind power is a relatively new technology, with the vast majority of installations having taken place within the last 10 years). While the feasibility of integrating much higher levels (beyond 25%) is debated, significantly more wind energy could be produced worldwide before these issues become significant. In Denmark, wind power now accounts for close to 20% of electricity consumption and a recent poll of Danes show that 90% want more wind power installed.
