# Wind energy

Wind energy is a form of solar energy.[1] Wind energy (or wind power) describes the process by which wind is used to generate electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. A generator can convert mechanical power into electricity[2]. Mechanical power can also be utilized directly for specific tasks such as pumping water. The US DOE developed a short wind power animation that provides an overview of how a wind turbine works and describes the wind resources in the United States.

## Wind Energy Basics

Wind is caused by the uneven heating of the atmosphere by the sun, variations in the earth's surface, and rotation of the earth. Mountains, bodies of water, and vegetation all influence wind flow patterns[2], [3]. Wind turbines convert the energy in wind to electricity by rotating propeller-like blades around a rotor. The rotor turns the drive shaft, which turns an electric generator. Three key factors affect the amount of energy a turbine can harness from the wind: wind speed, air density, and swept area.[4]

### Equation for Wind Power

$P = {1\over2} \rho A V^3$

• Wind speed
The amount of energy in the wind varies with the cube of the wind speed, in other words, if the wind speed doubles, there is eight times more energy in the wind ($2^3 = 2 x 2 x 2 = 8$). Small changes in wind speed have a large impact on the amount of power available in the wind [5].
• Density of the air
The more dense the air, the more energy received by the turbine. Air density varies with elevation and temperature. Air is less dense at higher elevations than at sea level, and warm air is less dense than cold air. All else being equal, turbines will produce more power at lower elevations and in locations with cooler average temperatures[5].
• Swept area of the turbine
The larger the swept area (the size of the area through which the rotor spins), the more power the turbine can capture from the wind. Since swept area is $A = pi r^2$, where r = radius of the rotor, a small increase in blade length results in a larger increase in the power available to the turbine[5].

## DOE Wind Programs and Information

• DOE's Wind Program works to improve the performance, lower the costs, and accelerate the deployment of innovative wind and water power technologies. Greater use of the nation's abundant wind and water resources for electric power generation will help stabilize energy costs, enhance energy security, and improve our environment[6].
• WINDExchange is a nationwide initiative designed to increase the use of wind energy across the United States by working with regional stakeholders. The WINDExchange program illustrates the Department of Energy's commitment to dramatically increase the use of wind energy in the United States. The WINDExchange website provides a wide range of wind-related information, including: State-by-state breakdowns of wind resource potential, success stories, installed wind capacity, news, events, and other resources, which are updated regularly[7].
• The National Wind Technology Center (NWTC) is the nation's premier wind energy technology research facility. The goal of the research conducted at NWTC is to help industry reduce the cost of energy so that wind can compete with traditional energy sources, providing a clean, renewable alternative for our nation's energy needs.

## Worldwide Installed Capacity

Country Total Capacity, end of 2014 (MW)[8] Total Capacity, June 2010 (MW)[9] Total Capacity, end of 2009 (MW)[10]
U.S. 65,900 36,300 35,159
China 114,600 33,800 25,853
Germany 40,000 26,400 25,813
Spain 23,000 19,500 18,748
India 22,500 12,100 10,827
France 9,300 5,000 4,775
U.K 12,200 4,600 4,340
Portugal 4,953 3,800 3,474
Denmark 4,883 3,700 3,408

### United States Installed Capacity

In the U.S., installed wind energy capacity has advanced significantly over the past ten years, with the United States increasing its wind power capacity 30% year over year. As of the third quarter of 2015, the U.S. now has an installed wind capacity of 69,471 MW with over 13,250 MW of wind currently under construction and an additional 4,100 MW in the advanced stages of planning[11].

## Wind Farm Development

Siting a wind farm varies from one location to another, but there are some important matters for land owners to consider:[12]

2. Evaluate distance from existing transmission lines
3. Determine benefits of and barriers to allowing your land to be developed
5. Identify reliable power purchaser or market
6. Address siting and project feasibility considerations
7. Understand wind energy’s economics
8. Obtain zoning and permitting expertise
9. Establish dialogue with turbine manufacturers and project developers
10. Secure agreement to meet O&M needs

### Land Requirements

The amount of land required for a wind farm varies considerably, and is particularly dependent on two key factors: the desired size of the wind farm (which can be defined either by installed capacity or the number of turbines) and the characteristics of the local terrain[13]. Typically, wind turbine spacing is determined by the rotor diameter and local wind conditions. Some estimates suggest spacing turbines between 5 and 10 rotor diameters apart. If prevailing winds are generally from the same direction, turbines may be installed 3 or 4 rotor diameters apart (in the direction perpendicular to the prevailing winds); under multi-directional wind conditions, spacing of between 5 and 7 rotor diameters is recommended[13].