Cool it

US engineers have developed a system to keep houses cool without turning up the aircon.

Bill Miller and his team from Oak Ridge National Laboratory in Tennessee have come up with a roofing system that soaks up heat during the day and then re-radiates it out to space at night, keeping homes cool in the process.

There are four parts to the new system. The first three are concerned with reflecting heat off the roof surface using a combination of reflective tile surfaces, modified tiles that channel warm air out from within the roof cavity, and further reflective coatings beneath the rafters. These are fairly standard techniques, but the fourth line of defence is highly innovative. It consists of sheets of a material which melts at temperatures above 23 degrees Celsius. Melting a solid provokes a phase change, which consumes energy, so the material remains at the same temperature until it has all melted, which can take all day. In this way the material behaves as an energy sponge, preventing the heat from entering the living space below. Later, when the temperature falls at night, the material reverts to a solid again, releasing the energy it soaked up during the day and radiating it out into space.

During tests the material reduced attic temperatures on sunny days by more than 5 degrees Celsius. "We're able to intercept 90% of the heat energy that would otherwise penetrate into the living space through the attic floor. This could reduce the cooling bill for houses by up to 8%" says Miller.