Wind has many of the same limitations as solar power—location, intermittency, relatively high cost, and footprint. The location of wind resources is different from that of solar resources. This means that the two types of energy can potentially be complementary. Probably the best example of this is in California, which has both good solar and wind resources. For much of the densely populated part of the country, however, wind resources—similar to solar resources—are not where the highest concentrations of people are. This requires an extensive system of high voltage transmission lines to be built to carry the power from the Midwest to the population centers.

Offshore wind resources are excellent and the wind tends to be less variable, but for years the concern over ruining the views from Cape Cod prevented the development of the Cape Cod Wind Project, the first offshore wind power devel­opment being built in the United States. The opposition from politicians and many environmentalists was finally overcome, and the US Department of the Interior approved the project in 2010. It will cost at least $2 billion to provide 468 MW peak power, which could be up to 75% of the electrical usage of Cape Cod,

F igure 4.5 Wind resources in the United States.

source: Reproduced by permission from the National Renewable Energy Laboratory.

Martha’s Vineyard, and Nantucket. Opponents say it could cost up to $10 billion to upgrade the grid and build transmission lines (42, 43).

INTERMITTENCY

Intermittency is a big problem for wind—the wind doesn’t always blow, so there has to be backup sources of power that can adjust rapidly to make up the shortfall. The capacity factor for wind is the proportion of the total installed power that is actually available to be used in the electrical grid when it is needed. Because of variations in wind on hourly, daily, and monthly time frames, the average capacity factor for the US wind power in 2010 was 27% (44). In other words, of the 50 GW installed in the US, only slightly over one-quarter of that is available on average, or about 13.5 GW. The actual capacity factor depends on the wind power classi­fication, so wind farms in Class 3 areas might have a capacity factor of only 10%, while wind farms in Class 7 areas might be near 40% (see wind map in Figure 4.5). As the best areas for wind get developed, the capacity factor will inevitably go down. According to the EIA (Energy Information Administration), the capacity factor for wind turbines built in 2016 is expected to average 34% (45).

Wind varies throughout the day, but in many areas it is stronger at night than during the day when the peak electrical demand occurs. Thus it is not well matched with the demand curve. This can be a particular problem when there is a large demand for air conditioning on a hot day but the wind is not blowing strongly enough. As an example, grid operators in Texas count on just 8.7 MW for each 100 MW of installed wind capacity to be available during the peak demand on hot days, a capacity factor of just 8.7% (46). This might be partly balanced with solar power that is highest during the day, but most states do not have both good wind and good solar resources.

The Rawhide Energy Station that powers Fort Collins is largely dependent on coal, as I discussed in Chapter 3. However, it also owns an 8.3 MW wind farm in Medicine Bow, Wyoming—one of the windiest places in the country. If you have any doubt about that, just drive across Wyoming pulling a trailer (or look at the wind map)! It is an area rated 7 for wind resources, so it is as good a wind resource as it gets. If the wind turbines were running at full capacity for 24 hours a day, the wind farm should generate 6.0 million kWh per month.

But what does it really generate? I have plotted the actual energy generated per month over a five-year period in Figure 4.6. It is immediately obvious that the wind blows much more strongly in the winter months than it does during the sum­mer months, generating about three times as much energy during the winter as during the summer. This is fairly typical of wind resources, and it is a problem because much of the additional energy demands are for air conditioning during the summer. The graph also illustrates how variable wind energy is, even when averaged over a monthly basis. Given that the maximum generation capacity is 6.0 million kWh per month, the actual output (capacity factor) was half of that for only 6 months out of 5 years! In the summer the capacity factor is frequently about 15%, while in the winter it averages closer to 40%. The overall monthly output averaged over 5 years is 1.8 million kWh or a capacity factor of 30%, slightly better than the US average.

The variation on an hourly or daily basis is even greater and does not match the actual demand for electricity very well. Actual output from an installed capacity of 1,500 MW in Minnesota over a two-week period shows how variable wind output is compared to the load demand (Figure 4.7). The peaks in the wind output curve frequently come during the lull in demand because the wind frequently blows stronger during the night when it is not needed than during the day (44).

Because of the issue of intermittency, it is not the total availability of wind that limits its use but how much an electrical network can effectively use, since an elec­trical grid has to deliver power whether the wind is blowing or not. Large-scale electricity is not easily stored, so the supply has to match the demand at any given time. An oversupply means that the tightly regulated voltage and frequency would rise. An undersupply would lead to a voltage dip—a brownout—and reduced frequency. Either of these would raise havoc with electrical devices, since they require very accurate voltages and frequencies (41).

The Bonneville Power Authority has had problems with oversupply of wind power in the Pacific Northwest when big storms come through. Most of the power comes from hydropower sources, but there are strict limits on how much water can be diverted from the turbines to the spillways because the water absorbs nitro­gen and poisons fish. They are trying to cope with the problem by increasing water heater temperatures and heating ceramic blocks in houses of volunteers to soak up the excess energy (47). But, you ask, why not just reduce the amount of wind power when it is in excess? That could be done, but it is uneconomical for the wind power generators. They have invested a lot of money in the wind turbines and want to get everything out of them that they can. This kind of issue will be increasingly common if utilities get too much power from wind sources because it is so unstable.

It is easy to get the impression from advocates of renewable energy that solar, wind, and hydropower can completely power the United States, and even the world (2). More realistic estimates put the fraction of electricity generated by wind at a maximum of about 20%, which has never actually been accomplished by any country (38, 48-50). South Dakota, the state with the best overall wind resources in the country, generated 22.3% of their electricity from wind in 2011. Iowa came in second with 18.8% of its electricity coming from the wind (39). The bulk of the electricity to meet demand comes from baseline power (mostly coal and nuclear), augmented with more variable sources such as natural gas and hydropower that can meet the rapid fluctuations in wind power.

The NREL modeled the effects of up to 30% wind and 5% solar in the West on electrical grid operations. Two months were modeled. During the windy month of April, with highly variable wind, it becomes very difficult for a grid operator to meet the net load requirements. In July—when winds are low, wind contributes only 10-15% of the power, and solar matches the load much better—it is much easier for the grid operator to match the load requirements. Overall, a combined contribution of 23% solar and wind was feasible but increasing that to 35% caused severe problems in matching the load because of the large fluctuations in wind power. Furthermore, the main impact of the wind and solar power is to reduce the most efficient and least carbon intensive fossil fuel plants—natural gas turbine and combined cycle plants—rather than reducing coal-fired power plants, which is the biggest problem for CO2 and other emissions. And you still have to have most of the natural gas power plant capacity available to meet the summer demand (51).

Denmark made a major commitment to wind energy beginning in the late 1980s and has the highest proportion of electricity generated by wind of any country in the world. Currently, the country of 5.5 million people has 5,500 wind turbines that provide 19% of its electrical demand. A detailed analysis of the elec­trical usage shows that Denmark actually has to sell a lot of the power to Norway and Sweden when the wind blows strongly. The reason is complicated and some­what unique to Denmark. Denmark uses fossil fuel plants not only for electricity but also to heat homes. When the wind is blowing hard, the best thing to do would be to shut down some of the fossil fuel plants, but they can’t do that because they need them to provide heat and thus can’t use all of the wind-generated power. As a result, Denmark sells its very expensive wind power at (subsidized) cheap prices when it is in excess because Norway and Sweden have ample cheap power from hydro that they can quickly shut down and use Denmark’s excess wind power. But Denmark then buys back expensive electricity from Norway and Sweden when wind does not supply enough power. In effect, Denmark has hydropower storage available in Norway and Sweden to modulate the variability of the wind power. So in the end, the overall amount of electricity produced by the wind in Denmark that is actually used by the Danish people is about 10%. And for that, the Danes pay the highest cost of electricity in Europe (41).

Germany has also made a major commitment to wind power as well as solar power. By the end of 2010, Germany had installed wind capacity of 27 GW com­ing from 21,607 wind turbines. This is in a country slightly smaller than the state of Montana. In spite of this large number of turbines and installed capacity, wind generates only 6.2% of Germany’s electricity (52). The problem for Germany is that its wind resources—like its solar resources—are not very good, with an aver­age capacity factor of 15% for wind power, compared to 25% in Denmark, 30% in Britain, and 27% in the United States (53).

Can pumped storage4 solve the problem for the intermittency of wind power? Not really. The main problems are the volume of water necessary and the high energy cost of pumping water against gravity. Assuming an overall efficiency of 75% for the pumps to raise the water and the turbines to generate electricity, 5.4 tons of water would have to be pumped 100 meters high to store 1 kWh of elec­tricity. To store the output from an 800 MW power plant for a week would require a reservoir of 25 square miles fluctuating in height by 10 meters (54). And, of course, it would take another reservoir to hold the water on the downhill side. Reservoirs are not highly popular among environmentalists, so it is hard to imag­ine this amount of water storage being acceptable to the public. Add to this the fact that much of the best wind resources in the United States are in the Midwest, which is relatively flat, and it would be difficult to find a high elevation to locate the reservoir. Other types of storage, such as vanadium batteries or compressed air energy storage, also have serious limitations (54). In general, the added cost of energy storage systems make them uneconomical for utilities, so they are unlikely to make major contributions to the use of renewable energy (53).