• The wind doesn't blow all the time. How much can it really contribute to a utility's generating capacity?
• How much energy can wind realistically supply to the U.S.?
• What is needed for wind to reach its full potential in the U.S.?
• I've heard that Denmark is pulling back on wind development. Does that mean wind is a failure?
• Since wind is a variable energy source, doesn't its growing use present problems for utility system managers?
• Since wind is a variable energy source, doesn't it cost utilities extra to accommodate on a system that mostly uses fueled power plants with predictable outputs?

The wind doesn't blow all the time. How much can it really contribute to a utility's generating capacity?
Utilities must maintain enough power plant capacity to meet expected customer electricity demand at all times, plus an additional reserve margin. All other things being equal, utilities generally prefer plants that can generate as needed (that is, conventional plants) to plants that cannot (such as wind plants).

However, despite the fact that the wind is variable and sometimes does not blow at all, wind plants do increase the overall statistical probability that a utility system will be able to meet demand requirements. A rough rule of thumb is that the capacity value of adding a wind plant to a utility system is about the same as the wind plant's capacity factor multiplied by its capacity.

Thus, a 100-megawatt wind plant with a capacity factor of 35% would be similar in capacity value to a 35-MW conventional generator. For example, in 2001 the Colorado Public Utility Commission found the capacity value of a proposed 162-MW wind plant in eastern Colorado (with a 30% capacity factor) to be approximately 48 MW.

The exact amount of capacity value that a given wind project provides depends on a number of factors, including average wind speeds at the site and the match between wind patterns and utility load (demand) requirements. It also depends on how dispersed geographically wind plants on a utility system are, and how well-connected the utility is with neighboring systems that may also have wind generators. The broader the wind plants are scattered geographically, the greater the chance that some of them will be producing power at any given time.

More reading:
What Happens When the Wind Stops Blowing?, http://www.bwea.com/ref/stop.html

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How much energy can wind realistically supply to the U.S.?
Wind energy could supply about 20% of the nation's electricity, according to Battelle Pacific Northwest Laboratory, a federal research lab. Wind energy resources useful for generating electricity can be found in nearly every state.
U.S. wind resources are even greater, however. North Dakota alone is theoretically capable (if there were enough transmission capacity) of producing enough wind-generated power to meet more than one-third of U.S. electricity demand. The theoretical potentials of the windiest states are shown in the following table.

Experience also shows that wind power can provide at least up to a fifth of a system's electricity, and the figure could probably be higher. Wind power currently provides nearly 25% of electricity demand in the north German state of Schleswig Holstein. In western Denmark, wind supplies 100% of the electricity that is used during some hours on windy winter nights.

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What is needed for wind to reach its full potential in the U.S.?
A number of factors are needed, including:

Consistent policy support. Over the past five years (1999-2003), the federal production tax credit has been extended twice, but each time Congress allowed the credit to expire before acting, and then only approved short durations. The PTC expired again December 31, 2003, and as of March 2004 had still not been renewed. These expiration-and-extension cycles inflict a high cost on the industry, cause large lay-offs, and hold up investments. Long-term, consistent policy support would help unleash the industry's pent-up potential.

Nondiscriminatory access to transmission lines. Transmission line operators typically charge generators large penalty fees if they fail to deliver electricity when it is scheduled to be transmitted. The purpose of these penalty fees is to punish generators and deter them from using transmission scheduling as a "gaming" technique to gain advantage against competitors, and the fees are therefore not related to whether the system operator actually loses money as a result of the generator's action. But because the wind is variable, wind plant owners cannot guarantee delivery of electricity for transmission at a scheduled time. Wind energy needs a new penalty system that recognizes the different nature of wind plants and allows them to compete on a fair basis.

New transmission lines. The entire transmission system of the wind-rich High Plains, which cover the central one-third of the U.S., needs to be extensively redesigned and redeveloped. At present, this system consists mostly of small distribution lines-instead, a series of new high-voltage transmission lines is needed to transmit electricity from wind plants to population centers. Such a redevelopment will be expensive, but it will also benefit consumers and national security, by making the electrical transmission system more reliable and by reducing shortages and price volatility of natural gas. Transmission will be a key issue for the wind industry's future development over the next two decades.

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I've heard that Denmark is pulling back on wind development. Does that mean wind is a failure?
No. As this is being written (mid-2003), Denmark is revisiting its current wind policy. The degree to which that means the U.S. should reexamine its own policy revolves around the degree to which our situation is similar to Denmark's. In fact, a brief analysis of some major differences suggests that there are strong reasons for continuing to support wind development in the U.S. rather than back away from it:

Denmark is small, the U.S. is not:

(1) Wind supplies 20% of national electricity demand in Denmark. Although the U.S. has nearly twice as much installed wind equipment as Denmark, wind generates only 0.4% of our electricity, far below the 10% threshold identified by most analysts as the point at which wind's variability becomes a significant issue for utility system operators.
(2) Denmark is also so small geographically (half the size of Indiana) that high winds can cause many of its wind plants to shut down almost at once--in the U.S., wind plants are much more geographically dispersed (from California to New York to Texas) and do not all experience the same wind conditions at the same time.

Denmark has transformed its national power system, the U.S. has not:

Rapid development of wind and new small-scale power plants within the past five years has brought Denmark to the point where power produced by so-called non-dispatchable resources in the country's West exceeds 100% of demand in the region. At many times, this excess generation leaves the country scrambling to increase electricity export capabilities to handle the surplus. This situation is essentially unimaginable in the U.S.

Danish wind plants are typically small, U.S. wind plants are not:

Denmark's approach encourages community involvement, but places particular stress on low-capacity distribution networks (at the "end of the line" on transmission systems). In the U.S., our larger wind plants require advance transmission planning, but feed into main transmission lines and do not affect the customer distribution network.

In summary, Denmark's situation should not cause concern in the U.S. Denmark's problem is that wind has been too successful too quickly in a small country, and it must now take steps to manage that success; it is unfortunate that the U.S. has not dealt with its energy problems so decisively.

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Since wind is a variable energy source, doesn't its growing use present problems for utility system managers?
At current levels of use, this issue is still some distance from being a problem on most utility systems. The rule of thumb (admittedly rough) is:

• Up to the point where wind generates about 10% of the electricity that the system is delivering in a given hour of the day, it's not an issue. There is enough flexibility built into the system for reserve backup, varying loads, etc., that there is effectively little difference between such a system and a system with 0% wind. Variations introduced by wind are much smaller than routine variations in load (customer demand).
• At the point where wind is generating 10% to 20% of the electricity that the system is delivering in a given hour, it is an issue that needs to be addressed, but that can probably be resolved with wind forecasting (which is fairly accurate in the time frame of interest to utility system operators), system software adjustments, and other changes.
• Once wind is generating more than about 20% of the electricity that the system is delivering in a given hour, the system operator begins to incur significant additional expense because of the need to procure additional equipment that is solely related to the system's increased variability.

These figures assume that the utility system has an "average" amount of resources that are complementary to wind's variability (e.g., hydroelectric dams) and an "average" amount of load that can vary quickly (e.g., electric arc furnace steel mills). Actual utility systems can vary quite widely in their ability to handle as-available output resources like wind farms. However, as wholesale electricity markets grow, fewer, larger utility systems are emerging.

Therefore, over time, more and more utility systems will look like an "average" system.

For detailed information on this topic, see "Grid Impacts of Wind Power: A Summary of Recent Studies in the United States," Milligan et al, http://www.nrel.gov/docs/fy03osti/34318.pdf

More reading:
What Happens When the Wind Stops Blowing?, http://www.bwea.com/ref/stop.html

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Since wind is a variable energy source, doesn't it cost utilities extra to accommodate on a system that mostly uses fueled power plants with predictable outputs?
Yes, but as the previous answer suggests, the added cost is modest. Three major studies of utility systems with less than 10% of their electricity supplied by wind have found the extra or "ancillary" costs of integrating it to be less than 0.2 cents per kilowatt-hour. Two major studies of systems with wind at 20% or more have found the added cost to be 0.3 to 0.6 cents per kilowatt-hour.

For detailed information on this topic, see "Grid Impacts of Wind Power: A Summary of Recent Studies in the United States," Milligan et al, http://www.nrel.gov/docs/fy03osti/34318.pdf

More reading:
What Happens When the Wind Stops Blowing?, http://www.bwea.com/ref/stop.html

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