The challenge for green energy: how to store excess electricity

One possible answer? In Japan, so-called “flow” batteries have been used for years to store backup power at industrial plants. Conventional batteries store energy in chemical form. With flow batteries, charged chemicals are pumped into storage tanks, allowing still more chemical to be charged and pumped away, then pumped back into the active portion of the battery and drawn down as needed. One big advantage: battery “size” can be expanded by simply adding more chemicals and more storage tanks. In 2003, the local utility on small King Island, off the coast of Australia, installed a large flow battery to sop up and later release excess power from a wind farm.

As with the alternative generation technologies, cost will be key for determining which battery or other storage technologies might prevail. Aside from such typical economic concerns as raw material and maintenance costs and durability, storage technologies all face some losses in “round-trip efficiency”. Inevitably, some energy is lost as it goes into storage, and more is lost as it comes out.

Right now, hopes are riding high on lithium ion batteries, because they have impressive round-trip efficiencies, can pack in high densities of energy, and can charge and discharge thousands of times before becoming degraded. Because of those attributes, lithium-ion battery technology has become increasingly dominant in laptop computers and cell phones. On a far larger scale, a powerful lithium ion battery pack powers the pricey all-electric Tesla Roadster, and is slated to power the plug-in hybrid Chevy Volt next year.

On the grid, lithium ion experiments are already underway. One company, General Electric-backed A123 Systems, announced late in 2008 that it had been contracted to install a two-megawatt lithium ion storage unit at a California power plant owned by global utility giant AES.

Still, lithium ion remains a relatively expensive technology - 10 times more expensive than lead acid batteries with equivalent capacity. Technological improvements and manufacturing scale should bring lithium costs down over time, but by the time that happens, the world could be beating a path to the door of someone who’s found a way to build an even better battery.

Early this year, IBM revealed that it was launching a major research program into what looks like an even more promising technology - the lithium metal-air battery. Last month, a company called PolyPlus announced that it had already succeeded in developing one.

The PolyPlus battery and the IBM technology deliver an astonishing 10 times more energy density than even today’s best lithium ion technology. That means that, pound for pound, they offer about the energy density of gasoline. The key reason they can store so much energy is that they use oxygen, drawn from the air, in place of some of the chemical reactants used along with lithium in their lithium ion cousins.

There’s one big rub: air isn’t just oxygen. Notably, it also contains humidity, and the lithium has a bad habit of acting like ignited gasoline when exposed to moisture, creating a real risk of fire and explosion. Chandrasekhar Narayan, manager of science and technology at IBM’s Almaden Research Center near San Jose, California, has suggested that it will take five to 10 years to develop an effective membrane that will let oxygen into the battery while keeping moisture out.

Still in pie-in-the-sky mode, there’s “vehicle to grid” storage, or “carbitrage.” This enticing notion relies on idled storage in the batteries of the millions of plug-in hybrid or all-electric automobiles that will be in use in the future. There’s reason to believe this scheme could work. More than 90 per cent of the time cars sit idled, and aside from days they’re used for long trips, most of their full energy storage capacity goes unused.

A single idle, electric-powered car could generate as much as 10 kilowatts of power, enough to meet the average demand of 10 houses, according to Willett Kempton, director of the Center for Carbon-free Power Integration at the University of Delaware. With vehicle-to-grid technology, controlled by an array of smart meters, car owners plugged in at home or work could allow the grid to draw off unused chunks of power at times when short-term demand is high. Conversely, cars could be recharged when demand is low.

The stored power in those electric cars, or anywhere on the grid, might not come from batteries after all. In March, Texas-based EEStor announced that it had received third-party verification of its “ultracapacitor” technology. The company claims the lightweight device, which was awarded a US patent last December, can bottle up huge amounts of electricity far more quickly than any battery and can do so at lower cost.

Like batteries, capacitors store and mete out electricity. Small conventional capacitors have been ubiquitous in electronic devices as far back as the early days of radio. But capacitors, so far, haven’t been able to store electricity for long enough to come close to competing with batteries. They have found use as devices that level out fluctuations in voltage or that briefly store power for near-instant release.