Researchers from the Ecole Polytechnique Fédérale de Lausanne, in Switzerland, have developed a coating capable of absorbing heat as well as repelling it. Invisible to the naked eye, this coating can prevent over-production of energy and overheating of facilities. Solar thermal collectors are used to produce hot water and contribute to home heating. In summer, however, thermal collectors become overheated and deliver excess heat, and until recently, this overproduction has remained an unresolved problem that can even damage facilities. The researchers, led by Andreas Schüler, developed a smart material that changes its properties depending on temperature. In the case of overheating during summer, this new material would allow the collector to get rid of excess energy by radiating it. Their research has just been published in the journal Solar Energy. Tanks and all the other elements of solar panels have to withstand very high temperatures – sometimes up to 180 deg. C – several times during each summer. Over time, the heat transfer fluid degrades. Sensors, thermal insulation and the absorbent layer suffer and become less efficient. An ideal solar panel should be able to absorb heat up to a point and then repel the sun’s rays – like a mirror – to prevent overheating. “A mirror doesn’t absorb heat,” Schüler says. “That’s why mountain rescue blankets have aluminum coating. But we also need absorbent elements.” EPFL’s Solar Energy and Building Physics Laboratory (LESO-PB) focuses on optimizing the transition temperature through ‘doping’ adapted to the material. The material must behave as a ‘good’ semiconductor at lower temperatures and as a ‘bad’ metallic conductor at higher temperatures. “With a coating of this material on a metallic substrate,” Schüler adds, “one can get a surface that has a low thermal emissivity in a cold state and high thermal emissivity in a hot one.” The LESO-PB team works specifically on materials that are absorbent in the solar spectrum and reflective in the infrared range. This is called the selective effect. The innovation in this process is the successful combination of a selective effect with a thermochromic function – the color reacts and changes with heat, a phase change that occurs at 68 deg. C. The new material developed at the laboratory enables efficient absorption of solar energy while reducing the impact of overheating.