|Image courtesy of Austra|
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.
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.
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 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.
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-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."
Americans may be ready to embrace the electric car, but can the technology catch up?
It has taken a long, long time, but financial chaos, environmental concerns, and wild gyrations in oil prices - along with $2.4 billion in government funding—may finally bring practical electric cars to the American market. Virtually every major automaker is preparing to sell a battery-powered vehicle over the next few years. But a big question remains: Will battery technology finally be good enough to take the place of gasoline? Engineers see three ways it could happen.
A successful automotive battery must provide long driving range from a single charge and release its energy quickly enough for brisk acceleration. Lithium-ion batteries - similar to what powers your laptop or cell phone - satisfy both requirements, making them a big step up from the nickel-metal hydride cells used in gas-electric hybrids like the Toyota Prius. But the technology still has limitations: It is costly, it delivers about 1/40 as much energy per unit weight as petroleum, and if overheated or overcharged, it could burst into flames.
Nevertheless, it exists today, and carmakers are putting money into some 14 improved designs that should make lithium-ion batteries smaller, safer, and more efficient. One line of research adds manganese or iron phosphate to the technology, increasing energy capacity while protecting against runaway heating. Stanford University scientists recently showed that embedding silicon wires in batteries could increase their storage capacity tenfold, while researchers at MIT have reengineered the battery material to allow much faster charging. If these innovations make it to the market, plug-in cars like the Chevrolet Volt could recharge in minutes instead of hours and drive 400 miles on a charge.
But it will take time for such advances to make their way into the extreme environment under the hood. Price could also present a barrier. A recent Carnegie Mellon University study suggests that hybrid plug-in vehicles would be more expensive over a lifetime of use than comparable gas-powered cars due to the battery's hefty cost. For instance, the Chevy Volt's 200-lithium-cell battery pack would cost about $16,000, according to estimates.
A truly successful electric car may need significantly better electricity storage technology. Toyota has shown interest in metal-air batteries, which store electricity from zinc or aluminum reacting with oxygen. Metal-air would offer far greater range than lithium-ion, but it is not rechargeable with simple electric current, so drivers would have to clean out the battery regularly and replenish it with metal "fuel."
A more fundamental breakthrough could come from switching to capacitors, devices that use electric fields to trap electrons. Although capacitors cannot store as much energy as batteries, they are far better at releasing rapid pulses of electricity (for fast acceleration) and collecting electricity (recovered during braking, for instance). Engineers are experimenting with dual systems of batteries and capacitors that capitalize on each system's strengths.
Given the shortcomings of both batteries and capacitors, some engineers say the true solution lies in better infrastructure: They want to make electric-charging spots as ubiquitous as gas stations.
One proposal comes from Better Place, a company that envisions a system in which consumers would pay a fee to get access to a national network of plug-in parking lots and automated exchange stations that would swap out a rundown battery for a fresh one, providing a quick fix. Israel has already signed on to create such a network.
For now, automakers are jousting to develop as many electric vehicles as possible and seeing what sells. Ahmad Pesaran of the National Renewable Energy Laboratory predicts that over the next decade lithium-ion will rule. GM, Ford, Nissan, and Mercedes are developing lithium-battery vehicles; even Toyota, which has had tremendous success with its nickel-battery Prius, is set to release a lithium-ion version later this year. Of course, all that could change quickly - as happened at the turn of the 20th century, when the quiet, reliable electric car, powered by primitive lead-acid batteries, seemed destined to sweep the market. Instead, Henry Ford's gasoline-powered Model T transformed the industry, enabling lower cost, longer distances, and higher speeds. History may yet repeat itself. "These are all big, expensive bets," says Ted Miller, senior manager of energy storage strategy and research at Ford. "I guess you have to have a little bit of a gambler's mentality."