When Joel Schindall '63, SM '64, PhD '67, came back to MIT in 2002 after 35 years in industry, he knew, as all electrical engineers knew, that there was no way the wimpy charge-storage devices called capacitors would ever be able to power a car. But when a colleague handed him a canister the size of a soda can and told him it could store more than a million times as much charge per volt as common capacitors on the market, he rapidly became a convert. Ultracapacitors, as these next-generation capacitors are called, became a major focus of his research; he was determined to come up with a way to make them store even more energy.

Finding better technologies for energy storage will be crucial as we work to switch from fossil fuels to more environmentally friendly power sources, says Schindall, a professor of the practice in electrical engineering and computer science. "When we reach the point where renewables are the main forms of energy, we're stuck if we don't have a place to store it for when it's needed," he says. Unlike coal and gasoline, which can be burned whenever they're needed, renewable energy sources are intermittent. Solar power, for example, can be captured only when the sun is shining. Wind power peaks at night in most locations, whereas energy demand typically peaks in the afternoon. If energy from these sources can't be stored, it will be lost. It must be integrated into the electrical grid, where it can help even out discrepancies between supply and demand. And better energy storage is also needed to make clean electricity a more widely useful power source for transportation.

Today's energy-storage devices won't work for these purposes, because they are too expensive, too cumbersome, or too limited in capacity. Take batteries, the best-known storage technology. Sodium-sulfur batteries have the capacity to store wind power that can't be used immediately, but adding them to a wind farm would quintuple the price of electricity per kilowatt-hour, according to one estimate. State-of-the-art rechargeable lithium-ion batteries, which are used in laptop computers and plug-in hybrid cars, are likewise too expensive to be incorporated into the grid in bulk. For that matter, since they wear out and need to be replaced every few years, they're too expensive for widespread use in cars. Lead-acid batteries--the kind used in ordinary gas-fueled cars--are cheaper but too heavy, and too short-lived, to serve as a vehicle's power source. As for ultracapacitors, they can improve efficiency when used in tandem with batteries, but they don't yet have the capacity to solve the storage problem.

At MIT, materials scientists and engineers have been working on better, cheaper energy storage since long before the current vogue for all things green. The Institute has been behind some of the most significant developments of the past several years, including the batteries that will power Chrysler's line of electric vehicles: made by A123 Systems, a Massachusetts company cofounded by Professor Yet-Ming Chiang, they're 10 times as powerful as conventional lithium-ion batteries. People at MIT have also developed new designs for ultracapacitors as well as some more unusual technologies, including battery materials made by viruses. Here's a look at five ongoing projects that could make green energy viable by improving the performance of energy-storage devices while significantly reducing their price.

Storage Basics

Researchers measure the performance of an energy-storage device according to two main criteria: energy density and power. Think of the device as a bucket. The energy density tells you how much energy the bucket can hold, and the power tells you how fast it can be filled and emptied. In general, batteries have higher energy density than ultracapacitors--they can store more total energy. And capacitors tend to be more powerful than batteries--they can take in and release energy more quickly. That's because the two technologies operate in different ways.