January 11, 2011

Interaction between Wind Turbines and Coal Plants

This piece from the American Scientist argues that "Clean components do not necessarily mean a clean system" and looks to the interaction of wind turbines and coal plants as an example:

Wind turbines produce power at 100 percent of their rated capacity when a storm front passes through. Without wind, no power. Even at windy locations, wind turbines on average generate only 20 percent of their full capacity. Shortfalls must be met with dispatchable (available on demand) energy, likely generated by fossil fuels. (If we had nonfossil-fuel generators that could respond quickly enough to back up wind, why have wind at all?)

Once the number of wind farms on a grid reaches about 5 percent of the average load, we encounter periods when the grid has too much power—typically in the middle of the night when load is low and the wind is high. Current law, which is the basis for much current investment in wind power, requires the grid to buy all the wind power that is produced. This forces grid operators to interrupt operations at coal plants that are designed to operate at constant power. Coal units that stop and start emit much more CO2 and nitrogen and sulfur oxides than units that operate at constant power. Think of a Formula 1 race car in stop-and-go traffic. The excess emissions of integrating wind can wipe out the savings.

Before governments approve a wind farm, they should ask “What is the system impact? Will the system emit more or less pollution?”

Wind advocates argue that grid-scale storage (which does not exist today) can improve the average capacity. Perhaps it can, a bit. Advocates also argue that long-distance transmission increases average capacity. It does, a bit. Even if it increases average capacity by 50 percent, from 20 percent to 30 percent, 70 percent of the power must still come from backup generators.

There has been no disciplined effort to verify that wind can reduce CO2 emissions systemwide. We do know that Denmark with 20 percent wind has not reduced its consumption of coal. A study of the Irish electrical grid showed that CO2 reductions due to wind may have been only 60 percent of expectations. A study from Bentek Energy, a leading analyst of natural-gas market fundamentals, suggests that wind farms in Colorado are increasing, not decreasing, pollution by forcing coal plants to cycle on and off and spew pollution.

Once coal plants are forced to cycle, all bets are off. A responsible engineer would not deploy wind systems without evidence that it satisfies its purpose. The purpose is not to deploy wind but to reduce CO2 emissions by a specified amount.

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