Wind Energy Storage for the Grid

Case Study 1.

"Cost Effective Seasonal Storage of Wind Energy"
Presented at:
The Energy-Sources Technology Conference and Exhibition
Houston, Texas
January 29-February 1, 1995
by
Alfred J. Cavallo and Matthew B. Beck, Center for Energy and Environomental Studies, Princeton University, Princeton, New Jersey
Electric distribution designers have a problem combining the varying wind power generated at a large windfarm with fossil fuel powered plants. The designer must assure a steady supply of power to customers from the unsteady supply of the prevailing wind. The wind is gusty from moment to moment, and there is a large seasonal variation in winds. On the Great Plains of the central U.S. the springtime wind is the greatest and the summer wind the least. However summer demand is highest. If wind power is to supply a significant portion of the demand, energy storage is needed.

The authors of this paper say this problem can be overcome by using an oversized wind farm and a compressed air energy storage system (a baseload wind energy system). A minimum volume storage reservior is needed to transform the intermittent wind energy to baseload power, while a larger reservoir can be used to store excess power produced during the sping for either peak power or baseload output during the summer. The yearly average cost of energy increases by about 3 percent for the largest storage reservoir indicating that seasonal storage of wind energy is economically feasible.

The authors propose a windfarm in North Dakota, a state called "The Saudi Arabia of Windpower". Not only would the windfarm harvest the abundant wind resource, but also use convenient geologic resources. Excess springtime wind energy could be stored as compressed natural gas in salt domes. Salt domes are underground geologic formations that are hollowed out by mining. Salt is impermeable to natural gas making a large, safe, storage container. In times of excess winds a compressor pumps natural gas into the salt formation to a pressure of over 70 bar (1000 psi). When demand is beyond the wind generated power, the natural gas is brought out of storage and burned on site in a gas fired power plant.

North Dakota does not have a large electric demand in state so new transmission lines must be built to carry power to population centers like the Twin Cities of Minnesota. Temporary energy storage would result in savings in transmission line construction. The transmission line can be sized to carry a steady amount of power from the windfarm. It would be much more expensive to carry the high intermittent peak power the windfarm produces on a windy day.

The authors' proposed system is:

  1. An oversized wind farm of 23,000 wind turbines, with maximum output power of 225 kiloWatts per turbine. The maximum power output of the windfarm is 5, 175 MW.
  2. A Compressed Gas Energy Storage system with a charge rate of 2,250 MW, a discharge rate of 1,500 MW and between 20 and 250 hours of storage.
  3. A High Voltage Alternating Current transmission line with a 2,000 MW capacity.
A goal of this system is to increase the Capacity Factor of the wind power system. A large windfarm such as those in California have a capacity factor of less than 30 percent. That means the wind turbines only supply their rated output a third of the time. And that third is literally at the whim of the wind. The system described in this paper could have a 90 percent capacity factor full time, providing a dependable, predictable, substantial source of electric power.

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