Advanced coatings boost the competitiveness of solar thermal energy

Researchers at Umeå University have developed new sustainable coatings that improve the performance of solar thermal collectors—strengthening solar heat’s position as a climate-smart energy source. Using nanomaterials and simple manufacturing methods, the technology can become both more efficient and more accessible.

“Solar thermal has great potential to contribute to the green transition, especially as a source of industrial process heat. But the technology needs to become even more competitive to gain broader traction,” says Erik Zäll, doctoral student in experimental physics at the Industrial Doctoral School at Umeå University.

In his doctoral thesis, he demonstrates how optical coatings—thin films that control how light interacts with surfaces—can be tailored to improve both efficiency and durability. His work focuses on two key components of solar thermal collectors: the cover glass that allows sunlight to enter, and the receiver that absorbs the light and converts it into heat.

For the glass, Zäll has developed an anti-reflective coating made of silica with small, hexagonally ordered pores. By adjusting the size and shape of the pores, as well as the thickness of the coating, he has succeeded in increasing the light transmitted through the glass. At the same time, the coating’s resistance to scratches, dirt and moisture is improved—factors that would otherwise reduce performance over time.

For the receiver, the thesis presents two solutions. One is an electroplated cobalt-chromium coating that absorbs light thanks to its surface structure. It is produced using a type of chromium that is far more environmentally friendly than those previously used. The second solution is a composite film made of carbon nanotubes and silica, spray-coated on annealed stainless steel using ultrasonic technology. The thermal treatment of the steel creates a thin oxide layer that improves both optical properties and heat resistance.

Both coatings absorb most of the sunlight while emitting very little thermal radiation. They can be manufactured using low-cost, environmentally friendly methods suitable for large-scale production.

The research has been carried out in close collaboration with Swedish solar energy company Absolicon Solar Collector and is directly adapted to their collector technology. The results have led to two patent applications and may eventually lead to more efficient solar thermal collectors, boosting Absolicon’s competitiveness in the global solar thermal market.

“Our work shows that it’s possible to combine sustainability, cost-effectiveness and high performance in optical coatings—a key to making solar heat a viable alternative to fossil fuels on a larger scale,” says Zäll.