A building material that can absorb heat during the day and release heat at night to reduce the use of air conditioning will soon be available in the United States, according to the Massachusetts Institute of technology's "Technology Review" magazine.

According to reports, at present, most of the walls of buildings need to use gypsum board. National gypsum, based in North Carolina, is testing a new type of gypsum board containing capsules that can passively cool buildings by absorbing heat. These capsules are made by BASF, a German chemical giant, and can be added to many construction products. In Europe, BASF's achievements have been used in some construction products.

BASF's capsules contain phase change materials, substances that change shape with temperature and provide latent heat. This makes the principle of cooling a house the same as that of ice absorbing heat when melting to help cool drinks. Each polymer capsule contains solid paraffin that melts at room temperature, so they keep the room temperature basically constant during the day. Paraffin wax has the best effect in cool climate at night. After cooling down at night, the capsule can harden again and release the heat stored during the day.

Peter schossig, an engineer at the Fraunhofer Institute in Munich, Germany, and his team worked with BASF to develop the capsules. In some countries in southern Europe, he said, the material can absorb enough heat during the day to save up to 20% of air conditioning power demand. In northern Europe, where nighttime temperatures are cooler, buildings using this material may not need air conditioning at all.

This new achievement will help drive the "green" trend in the construction industry - the use of green building materials that can maintain a comfortable indoor temperature without using electricity. According to the U.S. energy information administration, buildings in the United States consume about 70 percent of the nation's electricity generation, of which 8 percent belongs to air conditioning for homes and offices. At present, lightweight building materials are widely used in the United States, such as wood frame and gypsum board. They can make a building complete quickly, but these materials don't absorb much heat, so room temperature fluctuates a lot throughout the day.

Leon Glicksman, a professor of building technology and mechanical engineering at MIT, said phase change materials provide a way to add heat storage to lightweight building materials. Since the 1950s, several companies have tried to develop passive cooling systems that can take advantage of the properties of phase change materials, but their achievements have been limited because it is difficult to add new materials to existing building materials.

BASF obtained microcapsules by rapidly stirring the melted wax with hot water. Because wax and water repel each other, wax will form many tiny wax droplets in water. When the researchers add acrylic acid precursor to the wax water mixture, the repulsion force between the wax and water causes the acrylic acid precursor to wrap around the surface of the wax drop. The resulting moist mixture can be added to the powder used to make gypsum boards, or to other building materials such as cement and plaster after drying.

DuPont, another chemical giant, is also making encapsulated phase change materials and has added them to heat absorbing plates for sale in Europe. BASF's strategy is slightly different from DuPont's: they sell the capsules to other companies to add to building materials, including ceilings, aerated concrete blocks and gypsum boards. A study by BASF predicts that, taking electricity prices and climate conditions in Germany as an example, if 360 grams of the phase change material is added to gypsum used in a residential building (about $4883), the electricity cost saved can be recovered within five years.