"The technology we have today is nearly a hundred years old," says Eric Kozubal, a senior engineer at NREL. Kozubal and colleagues have come up with an air conditioner that combines evaporative cooling with a water-absorbing material to provide cool, dry air while using up to 90 percent less energy. The desiccant-enhanced evaporative, or DEVap, air conditioner is meant to addresses the old complaint, "It's not the heat; it's the humidity," more efficiently.

Evaporative cooling--blowing air across a wet surface to promote evaporation--has long been used in so-called swamp coolers. A method called indirect evaporative cooling improves on this design, dividing air into two streams, which separated by a polymer membrane. Water is passed through one airstream, making it cooler and wetter; the cool air cools the membrane, which in turn cools the air on the other side without adding water.

But air can only hold so much water vapor, so in humid climates the effect is limited. On a 32 ºC day in Houston, Kozubal says, evaporative cooling may only bring the temperature down to about 27 ºC. Ideally, to provide a comfortable building, an air conditioner should cool air to 13 or 16 ºC.