AC killers: These acrylic microcapsules are filled with a paraffin wax that can absorb heat from buildings.   Credit: Peter Schossig

Developers think these phase-change materials could reduce the need for air-conditioning.

Building materials that absorb heat during the day and release it at night, eliminating the need for air-conditioning in some climates, will soon be on the market in the United States. The North Carolina company National Gypsum is testing drywall sheets–the plaster panels that make up the walls in most new buildings–containing capsules that absorb heat to passively cool a building. The capsules, made by chemical giant BASF, can be incorporated into a range of construction materials and are already found in some products in Europe.

The “phase-change” materials inside the BASF capsules keep a room cool in much the same way that ice cubes chill a drink: by absorbing heat as they melt. Each polymer capsule contains paraffin waxes that melt at around room temperature, enabling them to keep the temperature of a room constant throughout the day. The waxes work best in climates that cool down at night, allowing the materials inside the capsules to solidify and release the heat they’ve stored during the day.

In some southern European climates, for example, the materials absorb enough heat during the day to save 20 percent of the electricity needed for air-conditioning. In northern Europe, where nighttime temperatures are cooler, a building incorporating the materials may not need an air conditioner at all, says Peter Schossig, an engineer at the Fraunhofer Institute in Munich, Germany, whose research group worked with BASF to develop the capsules.

The work is part of a push in the construction industry toward greener building materials that help maintain comfortable temperatures without using electricity. According to the U.S. Energy Information Administration, buildings consume more than 70 percent of the electricity generated in America, and about 8 percent of that is used for air-conditioning in homes and offices. Widely used lightweight construction materials including wooden framing and drywall enable contractors to put up buildings rapidly, but they don’t store much heat, so temperatures inside fluctuate throughout the day.

Phase-change materials offer a way to add thermal mass to lightweight building materials, says Leon Glicksman, professor of building technology and mechanical engineering at MIT. Since the 1950s, several companies have tried to develop passive cooling systems that take advantage of phase-change materials. But they had limited success because it’s difficult to incorporate these new materials into existing building substances.

BASF makes the microcapsules by rapidly beating melted wax into hot water. Since wax and water repel one another, the wax forms small droplets. When the researchers add acrylic precursors to the mix, the repulsion between wax and water drives them to coat the droplets’ surface. Finally, they add a catalyst to form an acrylic polymer shell around the wax. The resulting wet mixture can then be added to the powder that’s used to make drywall or dried out and incorporated into other construction materials, including concrete and plasters.