Researchers efficiently design inexpensive system for producing night power

Researchers have designed an off-grid, inexpensive modular energy source that can efficiently produce power at night. The system uses commercially available technology and could ultimately help meet the need for night lighting in urban areas or provide lighting in developing countries.

Although solar energy brings many benefits, its use depends very much on the distribution of sunlight, which can be limited in many locations and completely unavailable at night. Systems that store energy produced daily are typically expensive, thus increasing the cost of using solar energy.

To find a less expensive alternative, researchers led by Shanhui Fan of Stanford University looked at radiative cooling. This approach uses the temperature difference due to heat absorbed from the ambient air and the radiant cooling effect of cold space to generate electricity.

In The Journal of The Optical Society (OSA) Optics Express, the researchers theoretically demonstrate an optimized radiative cooling approach that can generate 2.2 watts per square meter with a rooftop device that does not require a battery or external energy. This is about 120 times the amount of energy shown experimentally and enough to support modular sensors like those used in safety and environmental applications.

“We are working to develop high-performance, sustainable light generation that enables anyone – including those in developing and rural areas – to access reliable and sustainable energy sources for lighting costs,” said Lingling Fan, first author of the paper. “A modular energy source could also stream off-grid sensors that are used in a variety of applications and are used to convert car waste heat into usable power.”

Maximize power generation

One of the most efficient ways to generate electricity with radiative cooling is to use a thermoelectric power generator. These devices use thermoelectric materials to generate power by converting the temperature differences between a heat source and the cool side of the device, as a radiative cooler, into an electrical voltage.

In the new work, the researchers optimized each step of the thermoelectric energy generation to maximize night generation from a device that would be used on a roof. They improve the harvest of energy so that more heat flows into the system from the ambient air and incorporate new commercially available thermoelectric materials that improve how well that energy is used by the device. They also calculated that a thermoelectric power generator occupying one square meter of a roof could achieve the best alternation between heat loss and thermoelectric conversion.

“One of the most important innovations was the design of a selective emitter that is attached to the cool side of the device,” said Wei Li, a member of the research team. “This optimizes the radiant cooling process so that the power generator can escape hot heat more efficiently.”

The researchers demonstrated the new approach by using computer models to simulate a system with realistic physical parameters. The models faithfully reproduced previous experimental results and revealed that the optimized system designed by the researchers could come close to what is calculated as the maximum efficiency with thermoelectric conversion.

In addition to conducting experiments, the researchers are also investigating optimal designs for operating the system during the day, as well as at night, which could expand the practical applications of the system.