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Tuesday, 4 February 2020

Anti-solar cell: a photovoltaic cell that works at night

One of the major drawbacks of photovoltaic solar panels is that they do not produce electricity at night. The energy generated during the day must therefore be stored for use in the evening. What if we could develop solar panels that generate electricity at night? Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis, says it is entirely possible. A specially designed photovoltaic cell could generate up to 50 watts of energy per square meter under ideal conditions at night, about a quarter of what a conventional solar panel can generate during the day.

Munday, who recently joined UC Davis, is developing prototypes of these nocturnal solar cells capable of generating small amounts of energy. The researchers now hope to improve the power output and the efficiency of the system.

The operation would be similar to that of a normal solar cell, but involves a reverse process. An object that is warm relative to its surroundings will emit heat in the form of infrared light. A conventional solar cell is cold (compared to solar radiation), so it absorbs light.

Space is an extremely cold place, so if a hot object is pointed at the sky, it will radiate heat towards it. This phenomenon has been used in particular for night cooling for hundreds of years. "Over the past five years, there has been a lot of interest in devices that can generate energy during the day (by harnessing sunlight)," said Munday.

A conventional photovoltaic cell (left) absorbs photons from sunlight and generates an electric current. A thermoradiative cell (on the right) generates an electric current when it radiates infrared light (heat) towards the extreme cold of deep space. UC Davis engineers suggest that such cells could generate a significant amount of energy and help balance the power grid over the day-night cycle. Credits: Tristan Deppe / Jeremy Munday, UC Davis

Generate energy by radiating heat

There is another type of device called a “thermoradiative cell”, which generates energy by radiating heat to its environment. Researchers have notably explored its use to capture residual heat from engines.

"We said to ourselves, what if we took one of these cells and placed it in a hot area with the sky pointing at it," said Munday. This thermoradiative cell pointed towards the night sky would emit infrared radiation because it is hotter than outer space.

“An ordinary solar cell generates energy by absorbing sunlight, which causes voltage to appear across the device and the flow of current. In these new devices, the light is rather emitted and the current and voltage go in the opposite direction, but it still generates energy," said Munday. "It requires different materials, but the physics is the same."

The device would also work during the day, as long as direct sunlight is blocked. Because this new type of solar cell could potentially operate 24 hours a day, it is an attractive option for balancing the electrical network on the day-night cycle.


Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space

Tristan Deppe Jeremy N. Munday*

ACS Photonics 2020, 7, 1, 1-9

Publication Date:November 20, 2019

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