The Magazine of The Sierra Club
How to Store Renewable Energy For Later
Wind turbines often catch their strongest, most consistent breezes at night—when, inconveniently, demand for electricity is low. Some wind farms invest in large-scale batteries to store that surplus energy until daytime, but compressed-air energy storage (CAES) may offer a cheaper solution.
In Texas, where wind power is booming, Apex CAES plans to use excess electricity to operate compressors that will pump air at high pressure into an underground salt cavern. This impermeable geological formation can store 45 million cubic feet of compressed air. When energy demand is high, pressurized air can be sent back up to expansion turbines, which generate electricity that then flows into the grid.
One challenge that all compressed-air energy storage systems face is that air heats up considerably as it's compressed and cools dramatically when it expands. The Apex project will let some of the heat dissipate before the air descends to the cavern. When the air rises up again, a natural gas turbine will warm the air enough to prevent it from freezing the expansion turbines.
In Toronto, a start-up called Hydrostor is storing air in underwater bags in Lake Ontario, using the weight of the water to keep the air pressurized. The problem of too-cold stored air? Hydrostor uses the heat generated during compression to warm up water, which is stored in tanks. When the air is called back into service, Hydrostor adds the heat back again. This procedure improves the overall efficiency of the system and removes fossil fuels from the process entirely.
Illustrations by Brown Bird Design
