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Renewable power is inspiring clever new ways to store electricity—and to uncork it exactly when and where it is needed.
by Maggie Koerth-Baker
From the June 2009 issue, published online June 17, 2009
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.”
Assault on the Battery
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.
Refining the Battery
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.
Banking on the Breakthrough
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.
Sticking to the Infrastructure
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.”
Source: http://discovermagazine.com/2009/jun/18-forget-lightning-how-do-we-catch-sunshine-in-a-bottle
A new MIT invention turns shock absorbers into electric generators.
by Andrew Grant
Image courtesy of Zack Anderson
From the May 2009 issue, published online May 3, 2009
Engineers at MIT have built a shock absorber that, in large vehicles, can generate enough energy to charge the battery and run electronics like headlights and a stereo. In conventional vehicles, shocks convert the vertical motion caused by running over a bumpy road into heat. That heat energy is then wasted. The MIT team’s shock absorber, called GenShock, harnesses the energy generated by traversing rough roads and converts it into useful electricity, easing the workload of the gasoline-dependent alternator. That in turn improves the vehicle’s fuel economy. Drivers will also benefit from a smoother ride due to an electronic system that monitors and responds to stress on the shocks.
“We identified the suspension as a significant source of energy loss, especially in heavy trucks,” says Zack Anderson, a student engineer on the team and chief operating officer of Levant Power, which will commercialize the MIT technology. “This is the solution for a big problem.” GenShock is most useful in vehicles that rack up a lot of miles, such as tractor-trailers used for shipping, and in military vehicles that must barrel over rough terrain.
Although GenShock is not yet cost-effective to produce for smaller cars, Anderson says he hopes to combine it with other energy-harvesting systems to maximize the efficiency of consumer automobiles. Some of this technology is already in use in hybrids: Regenerative braking converts the car’s motion into electricity as the vehicle comes to a stop, and BMW and Honda are designing devices that recover heat from engine exhaust. The ability to capture vehicle energy creatively is not limited to the car itself: London and Israel plan to embed generators in roads to harvest energy dissipated by the traffic running over them.
Source: http://discovermagazine.com/2009/may/03-next-source-of-green-energy-your-car-itself
Researchers say this longtime bane of offshore drilling is more cost-efficient than wind and solar.
by Andrew Grant
From the March 2009 issue, published online February 25, 2009
A prototype underwater generator shows the
fluid dynamics that will produce power from
slow-moving currents using metal
rods suspending near the ocean
or river floor.
Image courtesy of NOAA
The latest frontier for renewable energy is the ocean floor. A novel method of generating power uses a network of metal rods to tap into the currents that flow along the bottom of the ocean (and along riverbeds as well). Water swirls as it flows past the rods, making them vibrate. This phenomenon is painfully familiar to oil companies, which spend large sums of money minimizing such vibrations in order to stabilize offshore drilling equipment. “Everyone was obsessed with suppressing this motion,” says Michael Bernitsas, the University of Michigan engineer who developed the technology. “At some point it dawned on me that maybe we can do the opposite: Enhance it and harness the energy.”
Many proposed ocean energy projects rely on turbines that require sustained strong currents, but Bernitsas’s device can run efficiently on water flows of just a few miles per hour. He says that the cost of water-flow power production is less than that of solar or wind and that current-based generators can be arranged in large networks to power thousands of homes.
Although his technology has proved itself in the lab, Bernitsas still needs to find the best materials to withstand the elements in a real underwater environment. Vortex Hydro Energy, Bernitsas’s company, plans to install a prototype—about the size of a large car—in the Detroit River by the end of the year.
Source: http://discovermagazine.com/2009/mar/25-weirdest-new-source-alternative-energy-underwater-vibrations
GM says that tightly controlling how the battery charges will keep it alive for 10 years/150,000 miles.
by Stephen Cass
From the April 2009 issue, published online April 9, 2009
General Motors is hoping to gain an advantage over its competitors’ hybrid car offerings with the Chevy Volt, an extended-range electric vehicle. The Volt promises to be so fuel efficient, GM says, that it will let some drivers almost completely stop visiting gas stations.
Since cars started puttering around on America’s streets early in the 20th century, there have been several attempts to replace gasoline-powered internal combustion engines with all-electric motors and batteries. These attempts failed, in no small part because it was difficult to recharge the vehicles when they ventured outside their local range. In recent years the solution has been hybrid vehicles that recharge their batteries on board, thanks to a generator connected to a gasoline-powered internal combustion engine. Although this is more fuel efficient than a pure internal combustion vehicle, all the energy these hybrids use to move the vehicle ultimately comes from gasoline.
The Chevy Volt’s lithium-ion battery pack, in addition to being rechargeable on board by an internal combustion engine, can also be charged from an external power source—specifically, regular household current. This means that drivers can plug in their vehicles to recharge overnight and get up to 40 miles of driving the next day from about 80 cents’ worth of electricity. When the vehicle goes beyond the 40-mile mark, the internal combustion engine kicks in, powering the car and partially recharging the batteries, giving the car a total range of about 400 miles. If a driver is, say, a commuter living within 20 miles of her workplace, she can go a long while between gas tank refills. (Even in such a case, however, the Volt would consume some fuel; periodically the car’s internal combustion engine automatically starts up and burns some gas in order to perform self-diagnostic tests and keep itself lubricated.)
General Motors hopes to start selling the Volt in 2010 for $30,000 to $40,000 (before a tax credit, the automaker says, of up to $7,500).
How it Works
Image: GM
Lithium-ion (Li-ion) batteries are familiar to anyone who owns a cell phone or laptop computer. Relative to other rechargeable battery technologies, they offer a good combination of cost and energy density, a measure of how much electricity can be stored in the battery per pound. Unfortunately, also familiar to many cell phone and laptop owners is the fact that Li-ion batteries tend to deteriorate over time and die after a few hundred charge-discharge cycles. In consumer electronics, it is a relatively minor inconvenience to replace an old battery with a new one every couple of years, but this would be unacceptable in a hybrid car, especially given the large size of the battery pack and the difficulty involved in replacing it.
Chevy got around this problem by carefully managing how the battery pack is charged and discharged. Enough spare capacity is built into the pack so that it has to be charged to only 80 percent of its theoretical capacity to provide a 40-mile driving range. By not charging the battery pack to the max, its life is prolonged. For similar reasons, the pack is never discharged to less than 30 percent of its capacity. GM claims the result is a battery lifetime of 10 years or 150,000 miles without any noticeable deterioration.
Source: http://discovermagazine.com/2009/apr/09-can-smart-tech-keep-chevy-volt.s-battery-running
Using the principles of photosynthesis, scientists create more efficient storage for solar power.
by Stephen Ornes
From the January 2009 issue, published online December 18, 2008
One of the biggest obstacles to widespread use of solar energy is the lack of a low-cost, effective way to store it when the sun is not shining. To get around this, two MIT chemists have devised a method [subscription required] of hoarding solar energy that functionally mimics what plants do during photosynthesis. “We spent a lot of time understanding how a leaf works,” Daniel Nocera says, “and then built something that looks totally different but operates in the same way.”
During photosynthesis, plants convert solar energy into chemical form. A key aspect of this process involves splitting water into oxygen and hydrogen. The oxygen is given off, and the hydrogen is ultimately incorporated into sugars that the plant stores as fuel. Nocera and his postdoctoral student, Matthew Kanan, discovered that cobalt (a widely available metal) can be used to create a catalyst that similarly splits water molecules—in this case, in the presence of an electric current. This process could form the basis of a practical solar-energy storage system, Nocera says, in which electric current from a solar cell passes through water to the catalyst, breaking the water into oxygen and hydrogen through electrolysis. Those gases could be stored and later turned back into electricity in a fuel cell. Cobalt is both easier to engineer and less expensive than metals currently used in electrolyzers.
Source: http://discovermagazine.com/2009/jan/021
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