The sun’s beaming rays heat the Earth, but not evenly. Many attributes of the Earth – such as its atmosphere, topography, bodies of water, and rotation – contribute to uneven heat distributions, which create air movement, or wind . Windmills use wind-generated kinetic motion to perform useful work, such as pumping water or grinding grains, whereas wind turbines use it to generate electricity .
Wind turbine nuts and bolts: from wind-powered movement to electricity
The most common wind turbine design is the horizontal axis wind turbine. These turbines contain a central hub fastened to two or three blades. The blades, some equal to a football field in length, tilt at an angle to split the airflow, making them rotate at sufficient wind speeds. Blade rotation causes gears to spin inside a transmission system contained within the turbine. A generator then converts the spinning gear motion into electricity, which travels down the wind turbine tower through cables to an electrical grid for storage or distribution (Figure 1) [1-3]. State of the art wind turbines can even sense the wind direction and adjust the blades accordingly to maximize blade rotation speed for optimal energy output.
Figure 1. Anatomy of a horizontal axis wind turbine. Wind turbines can be broken down into three primary components: rotor (central hub and blades), nacelle (gearbox, generator, high and low speed shafts, controller, and brake), and tower. The blades are mounted to the central hub at a pitch, or angle. Incoming wind generates a force called lift on the blades, causing the rotor to rotate. The spinning rotor then turns a low-speed shaft connected to a gearbox, a mechanical component used to increase the rotational speed of a smaller, but faster shaft. The mechanical energy created by the high speed shaft can be converted to AC electricity through the generator. Wind turbines have anemometers to measure the wind speed and a wind vane to detect the direction. The controller turns the turbine on when wind speeds are between 8 and 16 mph, and shuts the system off using the brakes at damagingly high wind speeds (about 55mph). The yaw drive and motor allow the nacelle and rotor to rotate in the direction of incoming wind to maximize energy output. Wind-generated electricity travels through wires down the tower, along transmission lines to an electrical grid (www1.eere.energy.gov/wind/wind_animation.html).
Turbulent winds shake and bend the wind turbine blades, leading to hastened blade failure and replacement. In order to extend blade life, engineers design wind turbines that reach high into the sky to capture the stronger and less turbulent winds at elevated altitudes. Increased blade length also helps to generate more electricity. In addition to the hefty size, each turbine contains over 8000 parts, all of which must function in sync for the turbine to work . New computational algorithms are being developed by engineers not only to monitor the status of the wind turbine, but also to predict failure .
Problems of existing wind turbines: sources of active research
Two major challenges are inherent to wind as an energy source: wind blows intermittently, and not all locations have sufficiently strong wind speeds to generate energy. Furthermore, optimal wind farming lands often reside far from densely populated areas with high energy demands . To meet energy demands when it is not windy, researchers are developing large capacity, megawatt storage systems in a variety of forms, including batteries and fly wheels. These apparatuses hoard the harvested wind energy in the form of potential energy such that it can be used when needed instead of just when the wind is blowing. In addition, current research endeavors seek to integrate multiple renewable energy systems—such as wind and solar panel farms—to switch between energy sources as needed . To address the concern about winds not being strong enough, new wind turbine designs function at lower wind speeds, thereby expanding the regions of wind-farmable land.
Beyond land use problems that can be partially solved through the development of off-shore wind farms, wind turbine lifespans average 20 to 25 years, and blade replacement occurs multiple times prior to decommission . The blades are non-recyclable and a current trend of increasing wind turbine size has driven intense research on new lightweight, durable, and recyclable materials sturdy enough to withstand the heavy loads placed on the turbine blades . To keep the blade cost at bay, cheaper manufacturing methods are being developed as alternatives to the expensive techniques currently in use, which are derived from the aerospace industry .
Wind farms sprawl across several square miles. For each turbine on a wind farm to collect ample air flow, there must be a distance of ten times the blade length between each turbine. One way to use less land for wind energy is to develop off-shore wind farms. Offshore wind farms have the added advantage that winds above the water tend to blow stronger with less turbulence, and have the highest wind speeds during the afternoon, which is the peak time for energy use. For years, both Europe and China have derived energy from offshore wind farms, but none have been built in the U.S. Only states along the east coast or bordering the Great Lakes may build offshore wind farms because in these locations, the water along the coast is shallow enough for wind turbine construction. Turbines could be placed in these regions close enough to shore to relay the electricity to electrical grids without large, high power transmission lines and far enough from shore to minimize visual impact .
Cape Wind: America’s first offshore wind farm
Massachusetts will lead the US offshore wind energy endeavor by housing the country’s first offshore wind farm in Horseshoe Shoal, nestled within Nantucket Sound. At completion, the wind farm will include 130 wind turbines, generating up to 420 megawatts (MW) of power to meet the energy demands of approximately 75% of Cape Cod’s homes and businesses. By harvesting the wind to produce electricity, Cape Cod will save 113 million gallons of oil per year. Cape Wind will reduce greenhouse gas emissions by about 734,000 tons/year, and will also limit fossil fuel burning, which harms air quality and human health. The first offshore wind turbines will be installed in 2013 .
Future outlook on wind energy policy in the US
Support for wind energy typically waxes and wanes with the price of non-renewable energy sources. In addition, special interest groups can impede the construction of new farms when they vocalize concerns about potential negative impacts on local businesses or wildlife. Opposition continues to hinder wind energy efforts, and had delayed Cape Wind offshore wind farm development for nearly a decade, prior to receiving local and state approval in 2009 and federal permits in 2010 .
Each wind farm requires considerable financial investment, requiring start-up costs from several million to up to a billion dollars. To offset these high start-up costs, the US government spends about 1 billion dollars per year to subsidize wind energy and keep it cost-competitive with other non-renewable energy sources. Furthermore, businesses that produce turbine components face tough competition from overseas, particularly with China, which exports turbines at prices less than manufacturing costs due to significant subsidies by the Chinese government and stiff competition within their own country. In May of this year, however, the US Department of Commerce placed tariffs on Chinese-made turbines to protect US-housed wind energy companies.
Because of bipartisan support for these government support measures, the wind energy industry has blossomed over the years, spurring intense research and providing 75,000 jobs . Continued support for wind energy research and use from both the government and consumers will help the US achieve its goal of deriving 20% of its total energy from the wind by 2030 [4, 8].
Eileen Sun is a graduate student at Harvard University in the Program in Virology.
 U.S. Department of Energy. “How Do Wind Turbines Work?”, (July 27, 2012) http://www1.eere.energy.gov/wind/wind_how.html
 European Wind Energy Association. “Wind Energy FAQ” http://www.ewea.org/wind-energy-basics/wind-energy-faq/
 National Renewable Energy Laboratory. “Wind Energy Basics: How Wind Turbines Work”, (May 30, 2012) http://www.nrel.gov/learning/re_wind.html
 “Wind Power”, The New York Times (December 12, 2012) http://topics.nytimes.com/top/news/business/energy-environment/wind-power/index.html
 Avitabile, T, Dougherty, P, Lackner, M, et al. (2011) “Identifying Research Gaps and Future Directions.” Wind Energy Research Workshop Final Report. http://www.uml.edu/Research/centers/Wind-Energy/Workshop/Workshop.aspx
 “Wind Energy Guide” Wind Energy Development Programmatic Environmental Impact Statement (January 7, 2009) http://windeis.anl.gov/guide/index.cfm
 “Cape Wind” http://www.capewind.org/index.php
 American Wind Energy Association. “Explore the Issues”, (November 21, 2012) http://www.awea.org/issues/