Electrical engineer Sid Assawaworrarit leads a research team from Stanford University in the United States, developing a device that makes photovoltaic electricity at night. The system uses a standard solar panel paired with a thermoelectric generator, harvesting a small amount of power from the temperature difference between the ambient air and the panel’s surface pointed toward space.
The breakthrough hinges on a surprising aspect of solar panels. While daylight delivers light from the Sun to the cells, night reverses that flow. Solar panels emit infrared radiation as anything above absolute zero, effectively cooling by radiating heat into the night sky.
“There is light coming from the solar panel itself, and we use it to generate electricity at night. The photons that rise into the night sky carry away heat from the panel,” explains the team’s lead scientist.
As those photons depart the panel and head skyward, they carry heat with them. On a clear night, when clouds are absent to reflect infrared light back to Earth, the panel’s surface can be several degrees cooler than the surrounding air.
Assawaworrarit and colleagues harness this temperature difference with a thermoelectric generator. This device captures some of the heat flowing from warmer air to the cooler solar panel and converts it into electricity.
On a clear night, the device tested on Stanford’s roof produced about fifty milliwatts per square meter of solar panel (50 mW/m2).
The engineer notes that with additional refinements and favorable locations, the system could double its output.
The theoretical limit is likely one or two watts per square metre. It’s not a large amount, but there are many practical applications where that level of power at night could be valuable.
Because the thermoelectric generators used in these panels are solid state, their lifetime is essentially endless.
One notable application is powering the vast network of environmental sensors researchers use to monitor weather and ecosystems, including invasive species in remote areas. By producing a small amount of electricity at night, these panels can reduce reliance on batteries and the associated maintenance and replacement costs.
If achieving one watt per square metre proves feasible, it would be very cost-effective.
An Easily Overlooked Power Source
The Earth receives an enormous inflow of energy from the Sun, about 173,000 terawatts. Clouds, atmospheric particles, and snowy surfaces reflect roughly 30 percent of this energy back into space. The rest warms land, oceans, clouds, and the atmosphere.
That energy does not stay put. Earth emits as much energy as it receives, with greenhouse gas buildup since the Industrial Revolution trapping additional heat. In infrared, a form of light with wavelengths too long for the eye to see, Earth releases a staggering amount of energy back into space. This is a natural cooling process that must continue for the planet to stay in balance.
What Assawaworrarit and his team are doing is a new way to capture this outgoing energy. While thermoelectric generators have been used before to harvest energy of this type, integrating the technology with daytime solar panels marks a meaningful step toward enabling everyday people to harness this energy themselves.
Reference: Stanford University and coverage of the research on night-time photovoltaic generation.