Low-cost nanostructured coatings for thermal solar collectors
15.12.07 - We spend more than 80 % of our time indoors. Buildings shield us from the elements, but at a relatively high environmental cost. Taken as a sector, which includes their operation as well as the embodied energy in their construction (manufacturing of cement, carpet, tile, and glass, for example), buildings are among the largest energy consuming and greenhouse-emitting things on the planet, responsible for more that 40 % of humanity's CO2 emissions.
In Switzerland however, and thanks to the progress of building research and technology, the overall heating energy demand for buildings has remained stable since the first oil crisis in 1974 : this, despite a considerable increase in the building stock and living standard. The major remaining challenge is the generation and dissemination of construction technologies, such as the Minergie standard, which would enable a four-fold reduction in building-related energy consumption. These energy-efficiency strategies go hand-in-hand with a substantial increase of the use of renewable energy in buildings, such as taking full benefit of daylight and passive or active solar energy.
Professor Jean-Louis Scartezzini uses a physicist's approach to tackle these architectural and engineering challenges. In his Solar Energy and Building Physics laboratory at EPFL, researchers focus on renewable energy, building science and urban physics. He has several different projects on the drawing board at once ; renewable energy and sustainable development in cities, building technologies that mimic nature, development of new solar collectors, the health and quality of indoor environment and the intensive use and perception of day light.
In one of the lab's research groups, Dr Andreas Schüler is exploring how nanometer-scale structures, in the form of ultra-thin coatings or tiny quantum wells, can improve solar energy conversion. At the nanoscale, Schüler explains, a material's optical and electronic properties change, and this opens up all kinds of possibilities. They're developing color coatings with high solar transmittance that could be used for solar active façades, photoluminescent " quantum dots " for planar concentrators to convert solar energy, novel "smart" thermochromic coatings, and optically selective absorber coatings for thermal solar collectors. Solar energy is quite efficient when used in space heating, domestic hot water production, industrial process heat, desalination, and thermal electricity generation. All solar thermal systems have one thing in common : a coating on the solar absorber, which should absorb a maximum of incoming solar radiation and avoid energy losses by infrared radiation.
The lab has just started a new project to develop a low-cost process for producing nanostructured coatings. They're trying to come up with an environmentally-friendly manufacturing process for selective absorbers of solar collectors. Preliminary results are promising : the novel coatings resist corrosion and are stable at high temperatures. The lab is also developing nanocomposite film coatings for solar thermal façade collectors. These coatings combine a colored reflection with a high solar transmittance, and open up new possibilities for architecturally integrating solar collectors in building facades.