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Scientists create solar cells that keep working in the dark

In a new study published by the American Institute of Physics, researchers from Stanford University have developed a photovoltaic cell that can harvest energy from the surrounding environment after dark. The solar device is powered at night by heat that is leaking back into space. It could potentially provide power to some of the 750 million people who lack access to electricity at night.

“A large fraction of the world’s population lacks access to the electric grid. Standard photovoltaic (PV) cells can provide a renewable off-grid source of electricity but only produce power from daytime solar irradiance and do not produce power at night ” wrote the study authors.

“While there have been several theoretical proposals and experimental demonstrations of energy harvesting from the radiative cooling of a PV cell at night, the achieved power density is very low. Here, we construct a device, which incorporates a thermoelectric generator that harvests electricity from the temperature difference between the PV cell and the ambient surrounding.”

Overnight, solar cells radiate and lose heat to the sky. The new device uses a thermoelectric module to generate voltage and current from the temperature gradient between the cell and the air, explained the researchers. The thermal design of the system includes both a hot and cold side. 

“You want the thermoelectric to have very good contact with both the cold side, which is the solar cell, and the hot side, which is the ambient environment,” said study co-author Sid Assawaworrarit. “If you don’t have that, you’re not going to get much power out of it.”

During the day, the system operates in a reverse mode which contributes additional power to the conventional solar cell. The experts report that their device is inexpensive and could theoretically be incorporated within existing solar cells. 

“What we managed to do here is build the whole thing from off-the-shelf components, have a very good thermal contact, and the most expensive thing in the whole setup was the thermoelectric itself,” said study co-author Zunaid Omair.

Going forward, the researchers will work to optimize the thermal insulation and thermoelectric components of the device. They hope to improve the solar cell to enhance its radiative cooling performance without affecting its capacity to harvest solar energy.

The study is published in the journal Applied Physics Letters.

By Chrissy Sexton, Staff Writer

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