Self-cooling PV cells offer mass-market appeal

A team of researchers led by Shanhui Fan, an electrical engineering professor at Stanford University in California, has discovered a way to let solar cells cool by themselves. They were able to do this by adding a specially patterned layer of silica glass to the surface of ordinary solar cells.

Solar cells are among the most promising and widely used renewable energy technologies on the market today. Though readily available and easily manufactured, even the best designs convert only a fraction of the energy they receive from the sun into usable electricity.

This drawing demonstrates how solar cells cool themselves by shepherding away unwanted thermal radiation. The pyramid structures made of silica glass provide maximal radiative cooling capability. Credit: L. Zhu, Stanford University.

Part of this loss is the unavoidable consequence of converting sunlight into electricity. A surprisingly vexing amount, however, is due to solar cells overheating.

Under normal operating conditions, solar cells can easily reach temperatures of 55掳C or more. These harsh conditions quickly sap efficiency and can markedly shorten the lifespan of a solar cell. Actively cooling solar cells, however, either by ventilation or coolants, would be prohibitively expensive and at odds with the need to optimise exposure to the sun.

The proposed design avoids these problems by taking a more elegant, passive approach to cooling. By embedding tiny pyramid- and cone-shaped structures on an incredibly thin layer of silica glass, the researchers found a way of redirecting unwanted heat, in the form of infrared radiation, from the surface of solar cells, through the atmosphere, and back into space.

"Our new approach can lower the operating temperature of solar cells passively, improving energy conversion efficiency significantly and increasing the life expectancy of solar cells," said Linxiao Zhu, a physicist at Stanford and lead author of the paper. "These two benefits should enable the continued success and adoption of solar cell technology."

Solar cells work by directly converting the sun's rays into electrical energy. As photons of light pass into the semiconductor regions of the solar cells, they knock off electrons from the atoms, allowing electricity to flow freely, creating a current. The most successful and widely used designs, silicon semiconductors, however, convert less than 30 per cent of the energy they receive from the sun into electricity, even at peak efficiency.

The solar energy that is not converted generates waste heat, which inexorably lessens a solar cell's performance. For every 1掳C increase in temperature, the efficiency of a solar cell declines by about half a per cent.

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