News Column

Solar Cells Cool Down to Power Up

August 1, 2014


Solar panels are not immune to the warming effect of the sun's incident rays. While the sun is a necessary ingredient for photovoltaics (PV) to produce electricity, it is also detrimental to the PV cells.

Scientists at Stanford Univ. have developed a coating that enables solar panels to cool themselves by directing unwanted heat away from the cell.

"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," says Linxiao Zhu, a physicist at Stanford Univ. "These two benefits should enable the continued success and adoption of solar cell technology."

Photovoltaic cells directly convert the sun's rays into electrical energy. The most widely used PV cells, which are made of crystalline-silicon semiconductors, convert less than 30% of the sunlight that hits them into electric- ity. The rest of the sunlight generates heat, which decreases the solar panel's performance. For every 1C (1.8F) increase in temperature, the efficiency of a solar cell declines by about 0.5%. Actively cooling solar panels with ventilation or coolants, however, would be prohibitively expensive.

The Stanford researchers took a different approach to cooling. They designed a coating that consists of a layer of silica glass covered with tiny pyramidand cone-shaped structures. When applied to the surface of silicon solar cells, the patterned glass redirects the unwanted heat (in the form of infrared light) away from the cell surface and into the atmosphere.

Zhu and his team exploited the fundamental properties of light to design a coating that would enable the solar cells to give off excess heat without affecting performance. For example, the way light interacts with solar cells depends on its wavelength. While visible light is best for electricity generation, infrared light is more efficient at carrying heat. Different wavelengths also bend and refract differently when they pass through the solar cell.

"Silica is transparent to visible light, but it is also possible to finetune how it bends and refracts light of very specific wavelengths," says Shanhui Fan, a professor of electrical engineering at Stanford Univ. "A carefully designed layer of silica would not degrade the performance of the solar cell, but it would enhance radiation at the predetermined thermal wavelengths to send the solar cell's heat away more effectively."

Using computer simulations, the scientists developed a pattem of pyramids and cones on top of a very thin layer of silica glass. By adjusting the size of the structures, they came up with a pattern that effectively redirected infrared light away from the surface but did not affect the visible light. A coating of 100-pm thick silica with pyramids 20 pm high and 4 pm wide lowered the operating temperature of the solar cell by 18.3 K.

"The goal was to lower the operating temperature of the solar cell while maintaining its solar absorption," Fan says. "We were quite pleased to see that while the flat layer of silica provided some passive cooling, the patterned layer of silica considerably outperforms the 5-pm-thick uniform silica design."

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Source: Chemical Engineering Progress

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