Into the Wind - the AWEA Blog

A recent report being circulated in the United Kingdom casts some doubt on wind's ability to serve as a reliable source of electricity generation. Not surprisingly, its assertions seem largely questionable or off the mark. Stated in general terms, they are as follows:

Allegation: Wind turbines will generate on average 30% of their rated capacity over a year.

Fact: First, A conventional utility power plant uses fuel, so it will normally run much of the time unless it is idled by equipment problems or for maintenance.  An average capacity of 40% to 80% is typical for conventional plants.

A wind plant is "fueled" by the wind, which blows steadily at times and not at all at other times.  Although modern utility-scale wind turbines typically operate 65% to 90% of the time, they often run at less than full capacity.  Therefore, an average capacity of 25% to 40% is common, although they may achieve higher average capacities during windy weeks or months.

While average capacity is almost entirely a matter of equipment reliability for a fueled power plant, it is not for a wind plant—for a wind plant, it is a matter of economical turbine design.  With a very large rotor and a very small generator, a wind turbine would run at full capacity whenever the wind blew and would have a 60-80% average capacity—but it would produce very little electricity.  The most electricity per dollar of investment is gained by using a larger generator and accepting the fact that the average capacity will be lower as a result.  Wind turbines are fundamentally different from fueled power plants in this respect.

Second, the average capacity for wind turbines in the U.S. has improved consistently (15% every two years on average) and is in the 30% + range--see analysis by utility consulting firm Black & Veatch here (see specifically figures 5-4 and 5-5).

Allegation: While wind proponents say "the wind is always blowing somewhere," sometimes it is calm throughout the United Kingdom.

Fact: First, the statement "the wind is always blowing somewhere" applies to geographic areas quite a bit larger than the United Kingdom. The area covered by the Midwest Independent System Operator, which runs the utility system throughout several states in the upper Midwest, for example, is five times the size of the U.K.  A number of studies by the government, system operators, and utilities in the United States indicate that when wind resources are aggregated over a large geographic area, nearly the entire energy output of the wind plants can be counted on for meeting power system capacity needs.

Second, no individual power plant operates 100% of the time. Predictability is what is needed for the utility system as a whole to deliver electricity reliably, and wind farms are generally quite predictable, because their output can be forecast and because that output changes slowly over time compared with conventional power plants that can suffer instantaneous outages. A noteworthy example of this advantage of wind's predictability occurred in February 2011, when millions of electricity consumers in the state of Texas experienced rolling blackouts because of the sudden loss of over 50 fossil-fired power plants totaling over 7,000 megawatts (MW).  At the same time, wind plants continued to produce the approximately 3,500 MW they had been forecast and scheduled to produce, enough to power millions of typical homes.

Large coal and nuclear power plants frequently break down with no warning, taking 1,000 MW or more offline instantaneously. That is very difficult for grid operators to deal with--they must maintain expensive, fast-acting reserves 24/7/365, as such an outage can occur at any time. In contrast, changes in wind output occur gradually and are usually predictable, allowing grid operators to use slow-acting, non-spinning reserves that typically cost 40 times less than the fast-acting reserves needed for other power plants. Additionally, because the variability of wind energy is smaller than other sources of variability on the power system, particularly the large variability of electric demand, most of wind’s variability is canceled out so that adding wind to the grid only slightly increases aggregate power system variability.

Allegation: Periods of low wind are frequent in the United Kingdom, occurring on average every six days and lasting for an average of five hours at a time.

Fact: That data indicates that low winds occur about 3% of the time.  That’s amazingly low–certainly not worth complaints!  The 3% low wind time is considerably less than expected from a wind plant in a high-performing project in a great wind resource area, which is estimated to be at least 10% of the time.

Allegation: During periods of peak demand in the U.K., wind farms were producing only 6% of their rated capacity (due to low winds).

Fact: This may be true--in the U.S., wind farms normally generate somewhere between 10% and 40% of their rated capacity during periods of peak electricity demand.  As mentioned earlier, though, the power system is designed to draw on a variety of different types of power plants, and no power plant operates 100% of the time.

Allegation: The U.K.'s pumped hydro storage capacity is not enough to rely on during periods when the winds are low.

Fact: This is essentially irrelevant. European countries like Denmark, Spain, Ireland, and Germany have successfully integrated very large amounts of wind energy without having to install new energy storage resources. In the U.S., numerous peer-reviewed studies have concluded that wind energy can provide 20% or more of our electricity without any need for energy storage.

How is this possible? The secret lies in using the sources of flexibility that are already present on the electric grid. Every day, grid operators constantly accommodate variability in electricity demand and supply by increasing and decreasing the output of flexible generators – power plants like hydroelectric dams or natural gas plants that can rapidly change their level of generation. Thus, the water kept behind a dam or the natural gas held in a pipeline may be thought of as a form of energy storage, with operators using this energy when it is needed and "storing" it when it is not. Grid operators use these same flexible resources to accommodate any variability introduced by wind energy.

A tremendous amount of flexibility is already built into the power system. Demand for electricity can vary by a factor of three or more depending on the time of day and year, which nationwide translates into hundreds of thousands of megawatts of flexibility that are already built into the power system. Because these power plants and other sources of flexibility have already been built, it is almost always much cheaper to use this flexibility than to build new sources of flexibility like energy storage facilities. While continuing advances in energy storage technology can make it more economically competitive as a provider of utility system flexibility, it is important to remember that resources like wind energy can already be cost-effectively and reliably integrated with the electric utility system without energy storage.


Additional reading:

Fact sheet: Wind Energy and Energy Storage
Fact sheet: Wind Power: Clean AND Reliable
Fact sheet: Wind, Backup Power, and Emissions
Wind Power Myths Debunked, article by Milligan et al from Power Engineering magazine

AWEA Manager of Transmission Policy Michael Goggin and Senior Outreach Engineer John Dunlop provided valuable assistance on this article.