A report published by Stanford details the development of a novel cooling technology that its engineers claim can improve solar panel performance.
The outcome represents a solution to a long-standing problem in the field of photovoltaics; that at higher temperatures resultant from prolonged exposure to sunlight, solar cells become increasing inefficient at converting sunlight’s photons into electricity.
In response to this problem the engineers developed a visibly transparent overlay — more technically a silica photonic crystal overlay — that increases solar cell efficiency by radiating the heat of cells away from them — much like how we naturally radiate heat from our bodies to prevent overheating.
The overlay itself was made from patterned silica, fashioned into a thin, transparent material. This design means that the overlay can be laid on top of an unaltered solar cell — this is important since it infers that the new solution requires no additional tampering with the solar cells themselves to achieve cooling effects and enhanced cell performance.
The critical feature of the silica overlay is found in its micron-scale pattern, which is designed to maximise the radiating of heat, in the form of infrared light, out and away from the cell into space. For this reason the overlay is considered a form of blackbody material.
Being transparent, however, the material allows visible sunlight through to the cell, thereby preserving the energy that’s required to produce electricity. But as the researchers noted in a statement, the overlay may actually provide an enhancing effect on the cell’s capacity to absorb sunlight in addition to increasing cell efficiency through the cooling mechanism.
Importantly, the technology works in the face of direct sunlight and was tested as such when it was installed on a Stanford rooftop.
During testing, an overlay was fitted to a solar absorber — a device that provides an analogue to the properties of a solar cell and measures absorption of solar radiation, but doesn’t actually produce electricity. Several other absorbers were also set up in control conditions.
The photograph above — kindly provided by Linxiao Zhu, one of the study’s authors — shows the rooftop setup, and was sent to me with the following explanation:
“Samples from left to right are the [solar] absorber structure with the planar silica layer on top, the absorber structure with ‘overlay’ on top, and two bare solar absorbers, respectively. The bare solar absorber here has structure similar as solar cell, and is bluish.”
“One can observe that the structure with ‘overlay’ (the second from left) has same color as the other samples, while it near-optimally cools the absorber structure underneath it.”
Analysis of the data revealed that the overlay was able to cool the underlying solar absorber by up to 13°C (23°F). That might not sound much, but as the researchers point out, in a typical solar cell that difference would confer a significant improvement in overall cell efficiency. Zhu explained: “The experimentally demonstrated temperature reduction of 13°C (23°F) would translate to an absolute solar cell efficiency improvement larger than one percentage point — a significant solar cell efficiency improvement.”
The Stanford Report additionally noted that such a gain in energy production is predicted for a typical crystalline silicon solar cell with an efficiency of 20%.
So what’s next?
Aware that there is often-times a large difference between experimental demonstration and real-world application, I asked the researchers what would be next for their investigations and the technology.
“For the next stage, we are actively looking into directly demonstrating solar cell efficiency improvement from our cooling strategy, by measuring the generated electricity,” explained Zhu.
Clearly this is an important step. The overlay has successfully demonstrated its ability to cool a solar absorber, and by all indications it should achieve comparable results when paired with a solar cell that is at the same time generating electricity. But still, this must be shown under controlled conditions before we may imagine the technology being applied to the production of solar PV cells.
Nevertheless the group are confident, and are already considering matters of mass production. They believe their methods are scalable to the extent that commercial and industrial applications are feasible. In this regard they point toward employing nanoprint lithography — a common technique for producing nanometer-scale patterns in larger quantities — to produce silica overlays.
Investigating radiative cooling has been the primary focus of the researchers’ aims — with the solar panel demonstration merely one, especially persuasive example of how the concept can be utilised in a real-world setting. The scientists also consider their findings to hold great potential for application over a wide range of electronic products which could benefit from passive cooling.
“Besides solar cell, the optically transparent thermal blackbody overlay has potential for cooling any outdoor electronics and outdoor structures, where sunlight absorption is required either for functional (such as solar cells) or aesthetic reasons (such as maintaining the colors for cars, clothes etc.).”
NB. Link to Proceedings of the National Academy of Sciences publication
My thanks to Linxiao Zhu (Department of Applied Physics, Stanford University) and Shanhui Fan (Ginzton Laboratory, Department of Electrical Engineering, Stanford University) for their time and correspondence.
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