Can solar energy generate power at nighttime too? Australian scientists are recently adding functions to solar panels through the so-called “radiative cooling” mechanism, and are hoping to exert 1/10 the power of solar power during daytime.
Solar panels do have potential during nighttime. N.J. Ekins-Dauks, Associate Professor at the University of New South Wales (UNSW) School of Photovoltaics & Renewable Energy Engineering, pointed out that sunlight hits the ground and warms the surface during daytime, and the ground would reflect the same level of energy and radiation back to the space at night.
This is referred to as radiative cooling in meteorology, where the solar thermal energy absorbed by the surface of the earth would emit longwave radiation to the sky at night, and the surface will cool down rapidly under a sunny weather, breeze, or dry temperature, thus leading to a sudden temperature drop eventually. The UNSW research team believes that if radiation flow can be converted into electrical energy through cells, there would be a massive amount of unused energy for utilization.
After all, Australia is one of the countries in the world with the largest volume of rooftop solar users. The country has seen more than 3 million users with solar panels since 2001, and the capacity sat at more than 3GW during 2021.
With that being said, Ekins-Daukes emphasized that the “nighttime solar” is still currently at the preliminary phase. The research has proved its feasibility, though the power generation remains relatively low at 1/100,000 of general solar energy, and the power density is at 2.26mW/㎡ under a temperature difference of 12.5°C, with a radiant efficiency of 1.8%.
Traditional solar cells are essentially formed with the junctions of P-type and N-type semiconductors, and the particular structure is known as the P-N junction. The P-N junction, when semiconductors are absorbed with sunlight, would generate electron-hole pair, where the stimulated electrons and the electron-deprived hole will move in the opposite direction under the effect of the built-in electric field, which further derives current and voltage. The area that is close to the junction would form a region without portable carriers, and is referred to as the depletion region.
The equipment researched and developed by the team is called thermoradiative diode, and has an opposite working principle compared to that of solar cells, as it absorbs thermal energy emitted upward from the surface of the earth (or other heat sources), then converts the difference of temperature, with materials bearing similarities to that of infrared night-vision goggles.
Michael Nielsen, lecturer of the University of New South Wales (UNSW) School of Photovoltaics & Renewable Energy Engineering, pointed out that thermoradiative diode does indeed function opposite compared to traditional solar cells, though the semiconductor P-N junction is still the core equipment, and it’s just that the operation orientation is reversed. Ekins-Daukes commented that existing thermoradiative diode is still relatively low in power, and one of the challenges is the inspection method, though the theory indicates that this technology may eventually yield approximately 1/10 of solar cell power.