As the heat of summer fades into the cool of fall in the U.S., air-conditioning becomes more luxury than necessity. Yet demand for cool, dry air during hot spells is on the rise—and not just because of global warming. The U.S. expends roughly 185 billion kilowatt-hours of energy each year on home cooling, the most of any nation in the world. Plus, air-conditioner sales worldwide are growing by roughly 20 percent per year, with the newly affluent in China and India leading the way.

So this is the question: How do we beat the heat without increasing that heat through global warming caused by burning fossil fuels to power the air-conditioner? The U.S. Advanced Research Projects Agency for Energy, more commonly known as ARPA–E, hopes to cut this hot forecast by reducing the energy required for air-conditioning.

"Air-conditioning is inherently inefficient," says Cheryl Martin, ARPA-E's deputy director. "It's everywhere and it's a huge energy sink."

Conventional air-conditioners employ refrigerants such as chlorofluorocarbons to absorb heat from the room to be cooled. That heat is then expelled outside, requiring electrically powered pumps and compressors. One idea to conserve energy is to replace coolant fluids and gases—which are often super-powered greenhouse gases capable of trapping more than 1,000 times more heat than CO2—with solid materials, such as bismuth telluride. A new device from Sheetak, developed in part with ARPA-E funding, uses electricity to change a thermoelectric solid to absorb heat, and could lead to cheaper air-conditioners or refrigerators. Such refrigerators, which lack moving parts and are therefore less likely to break down, can be lifesavers in remote, rural areas for keeping medicines cool or food fresh.

Another approach is to employ specialty membranes to cool air by condensing water. These technologies are being developed by companies such as ADMA and Dais Analytic Corp. Such systems, after initial funding through ARPA-E, have now acquired backing from the U.S. Navy, which requires efficient air-conditioners and dehumidifiers for both troops and equipment in hotspots such as Iraq and Afghanistan. "A 30 percent improvement in efficiency means 30 percent less fuel to drag to the front," Martin notes, adding that the Navy program aims for units that use 20 to 50 percent less fuel. "The cost savings is important but, honestly, it's the convoys, it's the life-saving, that's what they're in this for."

Once such membrane technologies are developed with military funding, then they may become more cost-effective for the big cooling systems used in commercial buildings and eventually offices and homes. As a result, after a two-year push, ARPA-E is no longer funding air-conditioner efforts, not even the more radical technical innovations that still require further research and development, such as cooling with sound waves or magnets. "It's still an important area," Martin says. "I wouldn't be surprised to come back to it in the future."

More efficient air-conditioners can provide cooling that could prove vital for people trying to adapt to more extreme heat waves in the future, whether in the U.S. or India. In the meantime, even the ARPA-E office building near Washington, D.C., labors to cool its inhabitants efficiently, instead often making the building too cold. "We're the energy efficiency people and we've got lights that automatically turn on and off but we struggle like any other building with heating and cooling," Martin says. "I'll be very grateful when these things are available and we can retrofit easily."