Researchers at Stanford University claimed to have developed a solar photovoltaic cell that harvests energy from the environment during the day and night using radiative cooling, avoiding the need for batteries altogether.

In a research paper published in the Applied Physics Letters by AIP Publishing, researchers from Stanford University claim that their device uses the heat leaking from the earth back into space – energy that is on the same order of magnitude as incoming solar radiation.

Standard solar PV cells provide a renewable off-grid source of electricity but only produce power from daytime solar irradiance and do not produce power at night. The researchers at Stanford claim that they have built a device that incorporates a thermoelectric generator that harvests electricity from the temperature difference between the PV cell and the ambient surrounding.

At night, solar cells radiate and lose heat to the sky, reaching temperatures a few degrees below the surrounding air. The device uses a thermoelectric module to generate voltage and current from the temperature gradient between the cell and the air. This process depends on the system’s thermal design, which includes a hot side and a cold side.

                                                                   Image Credit: Sid Assawaworrarit

One of the research paper authors, 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.”

According to the researchers, using electricity at night for lighting requires a few watts of power. The current device generates 50 milliwatts per square meter at nighttime with a clear night sky, with an open-circuit voltage of 100 mV, which means lighting would require about 20 square meters of photovoltaic area.

The thermoelectric generator provides additional power on top of the electric power generated directly from the PV cells during the daytime. The system can be used as a continuous renewable power source for both day and nighttime in off-grid locations.

Developing a means to extract energy from existing PV cells at night would alleviate the daytime limitation of PV power generation and reduce or eliminate the need for battery storage in electrical power systems.

The researchers also claim that the setup is inexpensive and, in principle, could be incorporated within existing solar cells, and their construction in remote locations with limited resources is also feasible.

“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 another author Zunaid Omair.

The team aims to optimize the device’s thermal insulation and thermoelectric components. They intend to explore engineering improvements to the solar cell to enhance the radiative cooling performance without influencing its solar energy harvesting capability.

In 2020, researchers at the University of California came up with a solar cell that can work at night. designed a unique solar cell that can generate up to 50 W of solar power per square meter under ideal conditions at night. This is about a quarter of what a conventional solar cell can generate during the daytime.

Recently, a team of researchers from the University of Surrey and Imperial College London collaborated with Amsterdam’s research institute AMOLF to develop a method to help achieve a 25% increase in energy levels absorbed by wafer-thin solar photovoltaic (PV) panels.