Since wind is so variable, one problem is how to smooth out the fluctuations. This means storing the energy. The method depends on how long the energy has to be stored. The capacitors in the nacelles and the turbine’s transmission station need to store energy only from one AC cycle to the next, and those capacitors are already very large. Storing enough energy to last an hour would do a lot of smoothing.22 Batteries can do this, but they are too expensive. The best batteries are lithium-ion (as in laptop computers) and sodium-sulfide (NaS). Enough Li-ion batteries to service a large turbine would cost as much as the turbine itself. A 1-MW bank of NaS batteries would be the size of three shipping containers. A 34-MW NaS dem­onstration plant in Rokkasho, Japan, occupies 16 large buildings.22 This does not seem practical either. Storing mechanically in large flywheels is not yet taken seriously.

There is also day-night storage for 8 hours or longer. If there is a hill, pumped hydro can be used. The excess energy is used to pump water into a reservoir uphill. The energy is then regained quite efficiently by hydroelectric power. A scheme that is being taken seriously is compressed-air storage. Excess wind energy is used to pump air into underground salt domes or porous sandstone topped by shale. These sites, also usable for CO2 storage, can be found over 85% of the USA.23 The energy is recovered by bringing the compressed-air backup to help spin a natural-gas tur­bine driving an electric generator. The scheme is shown in Fig. 3.17. The turbines there are gas turbines, not wind turbines. A gas turbine is shown in Fig. 3.18. When natural gas is burned, the expanding air blows through the fan blades and turns the shaft, which then turns an electric generator. The compressed air from underground can add to this push, increasing the efficiency of the turbine by 60% or more. However, there is a heat cycle involved. When the air is first compressed, it heats up, and that heat is lost to the rock. Then when the air is decompressed, it cools down and has to be reheated to help drive the turbine. If you examine Fig. 3.17 closely, you will see that the heat for this is recovered from the hot air leaving the turbines after it has done its work. The loss in efficiency is more than 50%.22 Nonetheless, large projects for such storage are planned in Iowa, Minnesota, Texas, and the Dakotas.23