Passively Cooling Solar Panels

Surfaces which passively cool themselves by re-emitting thermal radiation into space could enable an additional way to deal with global warming. Our research looks at creating these cooling surfaces with polymers, potentially facilitating their scalable production as solar panel coatings.

Coverage of our warming planet in the media inevitably covers themes of ‘greenhouse gas emissions’ and ‘green energy’. And for good reason; moving away from highly polluting sources is a key goal of climate strategies. It is inevitable, therefore, that the scientific community’s response is rallied predominately around these themes of green energy production and carbon capture. However, a broader approach to the problem might equally help to reduce our environmental impact further.

At this years Photovoltaic Specialist Conference (a conference dedicated to solar panels), there was a talk titled ‘Global Warming as a Solar Converter Problem’ given by Christina Honsberg. The concept reframes how we think about global warming. Instead of treating it just as a carbon emission and carbon capture problem, we can assess our environmental impact in terms of thermal absorption from the sun and thermal re-emission from the planet into space.

Solar panels are optimised to absorb the incoming solar energy at the earth’s surface, however not all of this energy is transferred to electrical energy. The rest is either reflected back towards the atmosphere or absorbed as heat at the surface, of which some subsequently gets re-emitted. The atmosphere, however, absorbs much of the radiation which is reflected or emitted at the surface, and therefore energy remains trapped inside the earth’s atmosphere. This contributes to a net heating effect. It is observed for many materials, for example tarmac is particularly problematic. Solar panels are interesting to study as we will require large areas of the planet to be covered by them for green energy production.

Passive Daytime Radiative Cooling materials are a class of material which can mitigate this heating effect. By careful tuning of the absorption, reflection and emission properties across a wide spectrum (ranging from frequencies in the Ultraviolet to the Infrared) they achieve cooling of a surface without the need for any additional energy input (for example, compare this to how a fridge requires energy to cool). How is this achieved? The atmosphere is transparent to certain frequencies, and we target this transparent part of the spectrum to ‘couple’ with the cold temperatures of space and cool the material. For solar panel coatings, we additionally need to be careful not to reduce the efficiency of the solar panels by blocking out any of the light from the sun.

Creating these cooling materials usually requires ceramic particles and fabrication methods which are complicated and expensive, and so are not commercially scalable for use on top of solar panels. This is because careful control of the surface structure and composition is demanded to achieve an appropriate tuning effect.

Our research looks at using polymers instead. Conveniently, certain classes of polymer which have been identified as potential cooling materials have also been investigated, separately, for the ability to control their surface structure. We aim to bring together the research from these two to optimise a passively cooled solar panel coating.

Thomas Williamson

NanoDTC PhD Student, c2023