Current solar cells can turn sunlight into electricity with an efficiency of about 20%. Much of the remaining energy turns into heat, which can harm the solar cell.
Now, researchers from Stanford University have developed and tested a new material that can cool a solar cell by up to 13°C. Because heat makes solar cells less efficient, the researchers predict their cooling layer could help solar cells convert approximately 1% more sunlight into electricity, a big boost from a relatively simple modification. The cooler temperatures also mean the solar cells are likely to last longer due to greatly reduced efficiency degradation rates.
The researchers will present their results at the Conference on Lasers and Electro-Optics (CLEO), which is being held San Jose, California, US, on 5–10 June 2016.
One way to keep objects cool in the sun is to reflect the incoming light back into the atmosphere. This approach works for white cars and mirrored rooftops, but it wouldn’t work for solar cells, because they need to absorb as much light as possible to generate electricity. An alternative approach is to make it easier for heat to escape — an approach known as radiative cooling.
“What’s unique about our work is that we demonstrate radiative cooling while preserving the amount of solar absorption,” said Linxiao Zhu, a graduate student in the research group of Shanhui Fan, a professor of electrical engineering. In other words, the new material keeps the solar cell cooler even while it absorbs the same amount of sunlight.
The researchers achieved this combination of cooling and sunlight absorption with a wafer made of silica, into which the researchers etched tapered holes about 6µm across and 10µm deep. The holes are designed to smooth the path the thermal radiation takes to escape.
“What’s unique about our work is that we demonstrate radiative cooling while preserving the amount of solar absorption.”Linxiao Zhu, Stanford University
The team tested the silica layer by placing it on top of a solar cell mimic – a polished silicon wafer with an antireflection surface and aluminum back that has similar absorption characteristics to standard solar cells, but wasn’t actually wired to produce electricity.
This testing verified that, because the silica layer is transparent, approximately the same amount of sunlight still reached the solar cell mimic. In fact, there was a slight increase in absorption because of antireflection and light-trapping effects produced by the etched silica. Even so, the researchers found that the etched silica layer lowered the temperature by 13°C compared to a bare solar cell mimic.
Cold solar cells function better than hot ones, so the cooler the better, Zhu said. The researchers estimate that the 13°C cooling would result in an absolute efficiency improvement of more than 1%. Aaswath Raman, a co-author of the study, also noted that heat can speed up the degradation of solar cell parts, so cooling could lengthen their lifespan and likely save costs.
Ultimately, radiative cooling relies on the coldness of the universe, which is a mostly untapped thermodynamic resource, Zhu said. And solar cells aren’t the only applications that could benefit from this cooling approach, especially since the new research shows it can work without significantly altering the sunlight absorption characteristics of an underlying material. According to Zhu, cooling cars, clothing and outdoor equipment are all possible applications.
The next step for Zhu and his colleagues is to test the etched silica layer with a real solar cell to demonstrate the predicted efficiency improvements. The team is also talking to industry partners who could be interested in commercializing the approach.
This story is adapted from material from The Optical Society, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.