American Institute of Physics researchers have constructed a photovoltaic cell that works both day and night by making use of the heat leaking from Earth back into space.

At night, solar cells radiate and lose heat to the sky, reaching temperatures a few degrees below the ambient air. The device under development uses a thermoelectric module to generate voltage and current from the temperature gradient between the cell and the air. The setup is inexpensive and, in principle, could be incorporated within existing solar cells. This process depends on the thermal design of the system, which includes a hot side and a cold side.

Currently, about 750 million people in the world do not have access to electricity at night. Solar cells provide power during the day, but saving energy for later use requires substantial and costly battery storage.

In a paper published in Applied Physics Letters, researchers from Stanford University constructed a photovoltaic cell that harvests energy from the environment during the day and night, avoiding the need for batteries altogether. The device makes use of the heat leaking from Earth back into space – energy that is on the same order of magnitude as incoming solar radiation.

The device generates electricity at night from the temperature difference between the solar cell and its surroundings. Image Credit: Sid Assawaworrarit, Stanford University. Click the press release link above for a larger image.

Author Sid Assawaworrarit said, “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. If you don’t have that, you’re not going to get much power out of it.”

The team demonstrated power generation in their device during the day, when it runs in reverse and contributes additional power to the conventional solar cell, and at night.

The setup is inexpensive and, in principle, could be incorporated within existing solar cells. It is also simple, so construction in remote locations with limited resources is feasible.

Author Zunaid Omair said, “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.”

Using electricity at night for lighting requires a few watts of power. The current device generates 50 milliwatts per square meter, which means lighting would require about 20 square meters of photovoltaic area.

Author Shanhui Fan noted, “None of these components were specifically engineered for this purpose. So, I think there’s room for improvement, in the sense that, if one really 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 to the solar cell itself to enhance the radiative cooling performance without influencing its solar energy harvesting capability.


This technology should get some market access and may well get popular in low income areas. Meanwhile there has to be improvement for the night power generation. 20 square meters is a fairly large installation for residential use (4 x 5 meters for example or about 12 feet by about 16.5 feet). That would provide quite a lot of daytime power.

Thermoelectric has a long way to go and deserves a lot more attention. That market, based on heat now simply lost, has immense potential.


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