Flow batteries are a unique kind of battery — they have tanks that hold hundreds of gallons of the chemicals, called electrolytes. Flow batteries just happen to be very good at storing solar or wind power. Renewable power sources suffer from being intermittent, and less reliable for continuous power supply than coal or gas. Neither resource is a good match to handle the load of a typical grid.
Wind tends to be strongest at night and in the winter, while peak load is usually on summer afternoons. Solar production is strongest in the afternoon but ends long before the peak does, since high temperatures persist when the sun is very low in the sky or below the horizon.
Storage technologies are emerging, though none are in common use and all have disadvantages.
One method is batteries. They may use technology similar to car batteries, using chemicals that react either to absorb electrons or give them off. But these batteries are set up differently, with vastly larger quantities of those chemicals.
VRB Power Systems, of Vancouver, B.C., sells “flow batteries,” with tanks to hold hundreds of gallons of the chemicals, called electrolytes. The company has sold one installation at a solar power farm in Germany, and has announced sales to Australia.
The battery system costs $500 to $600 to store a kilowatt-hour, the quantity of electricity that sells for an average of 10.5 cents in the United States. And the system has a “round-trip efficiency” of 65 to 75 percent, meaning that it loses 25 to 35 percent of the electricity put into it. At a solar thermal installation, that has the potential to raise the price per kilowatt-hour, already a multiple of the market price, by another 50 percent or more.
Still, said Simon Clark, a spokesman for the company, the technology can be useful if it makes the output of a wind or solar farm more steady. A battery can be a “shock absorber” for a system that is a mix of solar, wind and diesel, he said.
Two other storage systems are in use in the United States, although at first glance a visitor might not recognize the thing being stored as energy.
One is compressed air. In 1991 the Alabama Electric Cooperative opened a plant in McIntosh, Ala., consisting mostly of an underground cavern, created by using water to hollow out a natural salt formation, and a modified gas-fired power plant on the surface. At night, when power is cheap, the cooperative buys power to run compressors and pump up the cavern.
In the afternoon, the compression becomes an ingredient of making electricity. Conventional natural gas turbines, like jet engines on airplanes, work by burning gas under pressure, and they typically compress that gas by using some of the power they produce. If something else does the compression, the gas can produce far more energy.
Samir Succar, a researcher at Princeton University, said the McIntosh plant can make a kilowatt-hour of electricity on a peak day from two-thirds of a kilowatt-hour delivered the night before, plus 4,000 B.T.U. of natural gas. This is only about two-thirds as much natural gas as a conventional plant requires.
But the technology has not found wide commercial acceptance.
Another form of energy storage is ice, typically a 500-gallon block in the basement of a large building with a big cooling load. The idea, said Frank R. Ramirez, the chief executive of a company called Ice Energy, is that all air conditioners gather heat from within a building and dump it outside, but that moving it outside gets progressively harder as the outdoor temperature rises.
His company installs the ice storage system and runs it at night, when electricity is cheap and when making ice is easy, because the outdoor temperature is lower. Then during the day, the compressor in the building air conditioning system rejects its heat to the cold block, instead of to hot air, sharply lowering the electric demand on hot afternoons.
Unlike the battery, ice storage can break even, or better, Mr. Ramirez said. For every kilowatt-hour put in at night, the system will return a kilowatt-hour of savings the next day, assuming the nighttime temperature is at least 17 degrees cooler than the daytime. In many places, though, the daily temperature swing is larger; if the swing is 35 degrees, which is common in some climates, then three-quarters of a kilowatt-hour deposited will yield a full kilowatt-hour the next day.
Ice Energy markets the system in California, where electricity generated at night creates fewer smog-forming pollutants than electricity made during the day. But if the system saves electricity, it also saves greenhouse gases, he said, and it reduces peak-hour load on transmission and distribution systems.
“What we don’t do is store electrons,” he said. His system sells for $166 a kilowatt-hour, less than a third of the battery system.
Like the battery and the compressed air, the ice system does not care where the off-peak power comes from.
The compressed air system in Alabama runs on electricity from coal, according to officials of the cooperative. That has an ambiguous effect on carbon emissions. It could allow coal plants to run at night and substitute for conventional gas plants during the day. Since coal plants emit twice the carbon dioxide of natural gas plants for a given amount of power, greenhouse gas emissions could rise.
In California, the Ice Energy system runs on natural gas, and some of the nighttime production is more efficient than daytime production. In many parts of the country, wind is strongest at night, and could be used for storage, experts say.