EME 444
Global Energy Enterprise

Wind Power


A picture is worth a thousand words! Below are three examples of wind turbine of varying scales.

Residential-scale installation
A typical residential-scale installation. This is a Skystream 3.7, which has a rated capacity of 2.4 kW. It has a rotor diameter of 12 feet and is mounted on a tower that is probably about 30 to 45 feet. The manufacturer’s published “energy potential” is 400 kWh/month, based on 12 mph winds.
Credit: National Renewable Energy Laboratory Photographic Information Exchange, 15337
100-kw wind turbine
A 100-kW Northern Power Northwind 100A turbine with 19-meter diameter blades, mounted on a 30-meter tower. This installation is located at the National Wind Technology Center in Golden, Colorado.
Credit: National Renewable Energy Laboratory Photographic Information Exchange, 16392
3.6 MW wind turbine installed off shore
A GE Wind 3.6-MW wind turbine, located about 10 kilometers off the coast of Arklow, Ireland. It is one of seven in the Arklow Bank Offshore Wind Power Plant. Each blade is about 165 feet long for a rotor diameter of 341 feet. Each tower weighs 160 tons and is 230 feet tall. Airtricity, a partner in the project, estimates that the 25-MW facility (7 turbines at 3.6 MW each) will generate enough electricity to power about 16,000 Irish households.
Credit: National Renewable Energy Laboratory Photographic Information Exchange, 13043

To Read and View Now

To View Now

Please watch the following (4:18) video from First Wind, Where does Wind Power come from? Climbing Inside a Wind Turbine:

Please watch the following (2:38) video from Puget Sound Energy, Wind Turbine Tour:

Wind Turbine Output

As described in the videos above, wind turbines convert the kinetic energy of the wind into mechanical energy that rotates a rotor, which then spins a generator, which generates electricity. This process (from wind to electricity) has a theoretical maximum efficiency of 59.3% (this is called the Betz Limit), but in practice turbines operate a significantly lower efficiency.

So where does the energy in the wind come from, and how much is there? Wind is caused by differences in pressure - air from high pressure areas will naturally move toward areas of lower pressure. Pressure differences are caused by differential heating of the surface of the earth. All else being equal, cold air has a higher pressure than warmer air. There are many localized wind sources, but global wind circulation is caused by cold air from polar regions (relatively high pressure) moving toward warm air (relatively low pressure) toward the equator.

The power in the wind is given by the following equation:

Power (W) = 1/2 x ρ x A x v3

  • Power = Watts
  • ρ (rho, a Greek letter) = density of the air in kg/m3
  • A = cross-sectional area of the wind in m2
  • v = velocity of the wind in m/s

Thus, the power available to a wind turbine is based on the density of the air (usually about 1.2 kg/m3), the swept area of the turbine blades (picture a big circle being made by the spinning blades), and the velocity of the wind. Of these, clearly the most variable input is wind speed. However, wind speed is also the most impactful variable because it is cubed, whereas the other inputs are not.

Turbines are rated in terms of capacity, usually in kW or MW. As with other energy sources, this is not the amount of power that a turbine generates at all times - it is the peak output. At peak output, a 100 kW wind turbine will generate 100 kWh of energy over 1 hour (100 kW x 1 h = 100 kWh). To determine the output at different speeds, you need to look at the power curve. The power curve for the 95 kW Northern Power turbine (similar to the turbine in the picture above) is below. As you can see, the turbine will only generate its rated 95 kW with a very limited range of wind speeds. Note also that the turbine has a startup speed of 2 m/s.

Power curve of the Northwind
Power curve of the Northwind 100C, 95 kW wind turbine.
Source: Northern Power Systems, turbine spec sheet)

Distributed Wind Generation

Energy.gov's Wind Program gives this description of distributed wind generation:

The Wind Program defines distributed wind in terms of technology application, based on a wind plant's location relative to end-use and power distribution infrastructure, rather than size. The following wind system attributes are used by the Wind Program to characterize them as distributed:

  • Proximity to End-Use: Wind turbines that are installed at or near the point of end-use for the purposes of meeting onsite energy demand or supporting the operation of the existing distribution grid.
  • Point of Interconnection: Wind turbines that are connected on the customer side of the meter, directly to the distribution grid, or are off-grid in a remote location.

Distributed wind energy systems are commonly installed on, but are not limited to, residential, agricultural, commercial, industrial, and community sites, and can range in size from a 5 kilowatt turbine at a home to a multi-megawatt turbine at a manufacturing facility. Small wind turbine technology, which includes turbines that have a rated capacity of less than or equal to 100 kilowatts, is the primary technology type used in distributed wind energy applications and is the focus of the Wind Program's technology R&D efforts for distributed applications.

Not required, but for more information on distributed wind generation see Distributed wind energy systems and OpenEI's Small Wind Guidebook.

IEA Wind is the International Energy Agency's (IEA) Implementing Agreement for Co-operation in the Research, Development, and Deployment of Wind Energy Systems. "Founded in 1974, the IEA Wind Agreement sponsors cooperative research tasks and provides a forum for international discussion of research and development issues" (IEA Wind).

Visit International Energy Agency (IEA) Wind and open the most recent report, the IEA Wind 2015 Annual Report.

In the Executive Summary, read:

  • Section 1.0 Introduction
  • Section 2.0 National Objectives and Progress, these portions:
    • Section 2.1 National targets
    • Section 2.2 Progress
    • Section 2.3 National policies
    • Section 2.4 Issues affecting growth
    • Section 3.3 Operational Details

Wind Resources in the U.S.

Average wind speeds vary widely by geographical location. Take a few minutes to inspect the wind speed charts from the National Renewable Energy Laboratory below. Note the location of the greatest and wind speeds, and think about the physical characteristics of those areas (e.g. flat, mountainous, on-shore, off-shore, etc.).  Click here for a larger version of the 30m wind speed image and click here for the 80m image.

In addition to variability being a barrier to wind deployment, location of wind resources is as well. In general - and certainly in the U.S. - the best onshore wind resources are not located near major population centers. Approximately 50% of the U.S. population lives within 50 miles of the coast, but as you can see in the maps below, this is generally not where the greatest onshore wind is located.  This is a problem because transporting electricity over power lines results in energy loss (as heat) due to electrical resistance in wires. The longer the electricity has to travel, the more energy is lost.  To minimize this loss, large (and very expensive) power lines must be built. As you can imagine, this type of infrastructure is lacking in areas of the country that do not have large populations.

Average wind speed at 30 m height, U.S.


Onshore and offshore average wind speeds in the U.S.
Average annual wind speeds at 30 m height (top image) and 80 m height (bottom image) in the U.S. 
Credit: National Renewable Energy Laboratory Dynamic Maps, GIS Data, & Analysis Tools.

To Read Now

For an idea of how expensive building high voltage lines can be ($560 million to $720 million for 224 miles!) and to gain some insight on some interesting issues related to wind, hydro, and international energy issues, read the summary below.