Unlocking the potential of the dark side of solar

Most of today's solar panels capture sunlight and convert it to electricity only from the side facing the sky, but there is a limit to their efficiency. If the dark underside of a solar panel could also convert sunlight reflected off the ground, even more electricity might be generated.

Experts estimate that by 2030, bifacial solar cells will account for nearly half of the market share for solar panels worldwide. Now a new formula developed by two Purdue University physicists reveals exactly how much more electricity double-sided panels can generate compared to conventional single-sided panels.

The new thermodynamic formula reveals that the bifacial cells making up double-sided panels generate on average 15–20% more sunlight to electricity than the monofacial cells of today’s one-sided solar panels, taking into consideration different terrain such as grass, sand, concrete and dirt.

The formula is designed to be used for calculating, in minutes, the most electricity that bifacial solar cells could generate in a variety of environments, as defined by a thermodynamic limit. It is hoped that this formula will help solar designers take full advantage of bifacial cells.

In the example above, a petrol station in Atlanta, Georgia, has installed double-sided solar panels that also function as a canopy. (Lumos Solar photo)

“The formula involves just a simple triangle, but distilling the extremely complicated physics problem to this elegantly simple formulation required years of modelling and research. This triangle will help companies make better decisions on investments in next-generation solar cells and figure out how to design them to be more efficient,” said Muhammad ‘Ashraf’ Alam, Purdue’s Jai N. Gupta Professor of Electrical and Computer Engineering.

In a paper published in the Proceedings of the National Academy of Sciences, Alam and co-author Ryyan Khan, now an assistant professor at East West University in Bangladesh, also show how the formula can be used to calculate the thermodynamic limits of all solar cells developed in the last 50 years. These results can be generalised to technology likely to be developed over the next 20–30 years.

The researchers use the formula to recommend better bifacial designs for panels on farmland and the windows of buildings in densely populated cities. Transparent, double-sided panels allow solar power to be generated on farmland without casting shadows that would block crop production. Meanwhile, creating bifacial windows for buildings would help cities to use more renewable energy.

The paper also recommends ways to maximise the potential of bifacial cells by manipulating the number of boundaries between semiconductor materials, called junctions, that facilitate the flow of electricity. Bifacial cells with single junctions provide the largest efficiency gain relative to monofacial cells.

“The relative gain is small, but the absolute gain is significant. You lose the initial relative benefit as you increase the number of junctions, but the absolute gain continues to rise,” Khan said.

The formula, detailed in the paper, has been validated and is ready for companies to use as they decide how to design bifacial cells.

This research was partially supported by the National Science Foundation.

Image credit: ©stock.adobe.com/au/Uve