A major issue for the solar power industry is that solar cells are less effective as they heat due to having to convert sunlight into electricity – the hotter solar cells get, the less efficient they become at converting the photons in light into useful electricity. However, a new study has developed a transparent coating that improves their efficiency by cooling the cells even when they are in direct sunlight, helping to improve solar array performance.
Based on the concept of heat loss from emitting of energy that radiates outwards as infrared light, researchers from Stanford University developed a transparent overlay that pushes away the heat generated by a solar cell in sunlight and cools it in a process that turns more photons into electricity. As reported in Proceedings of the National Academy of Sciences [Zhu et al. Proc. Natl. Acad. Sci. (2015) DOI: 10.1073/pnas.1509453112], the team used a thin, patterned silica material placed on top of a standard solar cell. As the material is transparent to the visible sunlight that works the solar cells, it is able to capture and emit thermal radiation from infrared rays.
They team had previously produced an ultrathin material that radiated infrared heat back toward space without warming the atmosphere, a process called radiative cooling. Here, they experimented with a homemade solar absorber covered with a micron-scale pattern designed to maximize the potential to push heat, in the form of infrared light, into space. Tests demonstrated that the overlay let visible light pass through to the solar cells while cooling the underlying absorber by as much as 23oF, showing perhaps for the first time that radiative cooling can be used in combination with the utilization of sunlight.
The transparent thermal overlays operate most effectively when in a dry and clear location, similar to that already used for large-scale solar arrays. As researcher Shanhui Fan said “Our thermal overlay allows sunlight to pass through, preserving or even enhancing sunlight absorption, but it also cools the cell by radiating the heat out and improving cell efficiency.”
The concept also has potential applications in outdoor systems that requires cooling but also that the visible spectrum of sunlight is preserved, so the overlay needs to be transparent or tuned to absorb only light outside the visible spectrum. It is hoped the technology can be scaled up to be commercially viable, maybe through nanoprint lithography to produce nanometer-scale patterns, and with new techniques for manufacturing such patterns continuing.
“Our thermal overlay allows sunlight to pass through, preserving or even enhancing sunlight absorption, but it also cools the cell by radiating the heat out and improving cell efficiency”Shanhui Fan