Around 750 million people in the world do not get electricity at night. Solar cells provide electricity during the day, but require sufficient battery storage to save energy for later use.
In applied physics paper, by AIP Publishing, Stanford University researchers have built a photovoltaic cell that harvests energy from the environment during the day and night, avoiding the need for batteries altogether. The device uses heat leaking from Earth back into space – energy that is on the same order of magnitude as incoming solar radiation.
At night, solar cells radiate and lose heat to the sky, reaching temperatures a few degrees below ambient air. The device in development uses a thermoelectric module to generate voltage and current from the temperature gradient between the cell and the air. This process depends on the thermal design of the system, consisting of a hot side and a cold side.
“You want the thermoelectric to have great contact with both the cold side, which is the solar cell, and the hot side, which is the ambient environment,” said author Sid Asvorait. “If you don’t have that, you’re not going to get much power from it.”
The team demonstrated power generation in their device during the day, when it runs in reverse and contributes to a conventional solar cell and additional power at night.
The setup is cheap and, in theory, could be incorporated within existing solar cells. It is also simple, so construction is possible in remote locations with limited resources.
“What we managed to do here is make it entirely of off-the-shelf components, have a very good thermal contact, and the most expensive thing in the whole setup was the thermoelectric itself,” said author Junaid Omeyer.
Lighting requires a few watts of electricity to use at night. The current equipment generates 50 milliwatts per square meter, which means that the lighting system would require about 20 square meters of photovoltaic area.
“None of these components were specifically engineered for this purpose,” said author Shanhui Fan. “So, I think there’s room for improvement, in the sense that, if someone actually engineered each of these components for our purpose, I think the performance could be better.”
The team aims to optimize the thermal insulation and thermoelectric components of the device. They are exploring engineering improvements in solar cells to enhance radiative cooling performance without affecting solar energy harvesting efficiency.
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