Image courtesy of AUSRA

Renewable energy has a critical role to play in reducing greenhouse gases and leading the United States toward energy independence. That role should soon be getting bigger: The U.S. government is pushing for a 100 percent increase in renewable energy by 2012. The two biggest sources are the wind and the sun. But the variable nature of wind and solar energy can cause problems with matching supply to demand—problems that would be greatly eased if only we had a really good way of storing electricity on an industrial scale. Currently there are several storage systems vying for dominance.

Compressed-Air Energy Storage
At night, when the strongest winds blow and customers are sleeping, unused wind-generated electricity can run giant compressors, forcing large amounts of air into sealed underground spaces. When demand rises during the day, the compressed air can be used to spin turbines, turning the energy back into electricity. Georgianne Peek, a mechanical engineer at Sandia National Laboratories in New Mexico, says this technology can provide a lot of power over long periods of time at a relatively low cost. The technology is also well established: Two compressed-air storage plants have been in operation for decades. The McIntosh Unit 1 plant in McIntosh, Alabama, went online in 1991; a similar plant in Germany has been running since the 1970s. McIntosh 1 can reliably put out 110 megawatts for 26 hours. (One megawatt is enough power to supply roughly 600 to 1,000 typical American homes.)

The compressed-air system does have its drawbacks. For one, it does not completely eliminate the need for fossil fuels, because the associated electric generators use natural gas to supplement the energy from the stored compressed air. Compressed-air storage systems also require an airtight underground space, limiting the locations where they can be installed. The two existing compressed-air plants use natural salt domes. Engineers flushed the domes with water to dissolve the salt, then pumped out the brine to create a nicely sealed cavern. But salt dome formations are not plentiful, so researchers are investigating other inexpensive ways to create storage chambers. A facility proposed for Norton, Ohio, would use an abandoned limestone mine. Another, in Iowa, would pump air into drained natural aquifers. Abandoned oil wells and depleted natural gas reservoirs might also work, Peek says, as long as they are not too remote to be hooked into the electrical grid.


Molten Salt Heat Exchanger
The sun, like the wind, is a variable source of energy, disappearing at night and ducking behind clouds at inconvenient moments. Thermal storage systems, such as molten salt heat exchangers, mitigate those problems by making solar power available anytime.

Right now only one example exists: Spain’s Andasol Power Station, which began operating last fall. Andasol has about 126 acres’ worth of trough-shaped solar collectors that focus the sun’s heat onto pipes full of synthetic oil. The hot oil is piped to a nearby power plant, where it is used to generate steam. During the day, some of the oil is used to heat a mixture of liquid nitrate salts (made by combining elements like sodium and potassium with nitric acid) to temperatures above 700 degrees Fahrenheit. These liquid salts can retain their heat for weeks in insulated tanks. When the collectors cannot generate enough power to meet demand, the salts are drawn out from the tanks and their heat is tapped to run the power plant. A full stockpile of molten salts can keep the Andasol plant running at top capacity—50 megawatts of electricity—for up to seven and a half hours.

Molten salt backup systems make solar power more flexible and reliable, says Frank Wilkins of the U.S. Department of Energy’s Solar Energy Technologies Program. Wilkins says that thermal storage systems can increase a solar plant’s annual capacity factor (the percentage of time, on average, that the plant is operational) from 25 percent to up to 70 percent. Expense is the biggest drawback. The Andasol Power Station cost about $400 million, and that was just for phase one of a planned three-phase project. But costs may come down as more plants are built. This past February, the Arizona Public Service power utility announced plans to construct a power station similar to Andasol. It is expected to go online in 2012.

Sodium-Sulfur Batteries
Sodium-sulfur batteries work much the same way as the lead-acid battery that starts your car; both use chemical reactions to store and produce electricity. The difference lies in the materials used. Lead-acid batteries contain a lead plate and a lead dioxide plate (the electrodes) in a bath of sulfuric acid (the electrolyte). A reaction between the lead and the acid creates the electric current. Lead-acid batteries are simple and reliable, but they are impractical to use on wind farms because of the amount of space and power electronics they would require.

The sun is a variable source of energy, disappearing at night. Thermal storage systems make solar power available at any time.

Sodium-sulfur batteries, which use molten sodium and sulfur as electrodes and a solid ceramic electrolyte, have a higher energy density. “Lead-acid batteries are cheaper,” Peek says. “But you can get the same amount of energy in a smaller amount of space with sodium-sulfur—and that’s important, because real estate costs money too.” Sodium-sulfur batteries can also be charged up to the maximum and discharged completely, which makes them more efficient. And they last about 20 years, versus three to five years for lead-acid.

Some U.S. utility companies, including Xcel Energy, have installed small-scale combinations of wind farms and sodium-sulfur batteries. (American Electric Power’s is not yet operational.) Excess electricity from the wind farms can be stored in the batteries and fed into the system later, when wind is low and demand is high. Each battery system, which is roughly the size of a semitrailer, can store about one megawatt and discharge it over six to eight hours. The downside, again, is cost, which is high in part because there are no American companies making sodium-sulfur batteries; the only manufacturers are in Japan.

Zinc bromide and vanadium redox flow batteries are other promising technologies. Although not as far along in development as sodium-sulfur, they may be easier to scale up. Vanadium batteries may also charge and discharge more quickly than sodium-sulfur, so they might be better suited to smoothing out power fluctuations caused by rapidly changing weather.

Hydrogen
Hydrogen-based energy storage looks great on paper: Use electricity to split hydrogen out of water, then convert the hydrogen back into electricity in a fuel cell when needed. Alas, the underlying technology is expensive and complicated, but MIT chemist Daniel Nocera may have found a better way. His hydrogen-ion-creating system uses an indium tin oxide electrode and a container of water with cobalt and potassium phosphate mixed in. Put the electrode in the water and add voltage. Cobalt, potassium, and phosphate migrate to the electrode, forming a catalyst that begins splitting water molecules into oxygen gas and hydrogen ions. Unlike most existing systems, the materials are fairly inexpensive, and the catalyst renews itself so it lasts a long time.

Nocera is still seeking a cheap way to convert hydrogen ions into hydrogen gas and an efficient way to get electricity from photovoltaic panels to the catalyst. But he thinks his approach will help other pieces of the hydrogen infrastructure fall into place. “The discovery opens doors we haven’t been able to walk through before,” Nocera says. “I don’t think this will be as hard.”