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solar power, Solar Cell, conventional cell, University of California Davis, UC Davis

New Solar Cell Can Generate Power at Night

Device can generate about a quarter of the energy of a conventional cell by reversing the typical process of creating solar power.

Researchers have developed a new solar cell that can generate electricity at night by reversing the same process that generates energy during the day.

A conventional photovoltaic or solar cell (left) absorbs photons of light from the sun and generates an electrical current. A thermoradiative cell (right) generates electrical current as it radiates infrared light (heat) toward the extreme cold of deep space. Engineers at the University of California Davis propose that such cells could generate a significant amount of energy and help balance the power grid over the day-night cycle. (Image Source: Tristan Deppe/Jeremy Munday)

A team of electrical and computer engineers at the University of California (UC) Davis developed the photovoltaic cell, which can generate about a quarter of the energy—up to 50 wats of power per square meter—that a typical cell can generate when the sun is shining, said Jeremy Munday, a professor in UC Davis’ Department of Electrical and Computer Engineering who led the research.

The primary stumbling block to more widespread use of renewable energy like solar is how to generate electricity even when the source of the power—in this case, sunlight—isn’t available.

Until recently most of the research has focused on storing energy for use later; however, some scientists aim to crack the code of how to develop solar panels that can also generate electricity when the sun goes down.

Reverse Engineering

The UC Davis team is a part of this latter effort. Researchers there achieved their results by applying the process of how a typical solar cell works in reverse and using heat instead of light as the source of energy, Munday explained.

“A regular solar cell generates power by absorbing sunlight, which causes a voltage to appear across the device and for current to flow,” he said in a press statement. “In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power. You have to use different materials, but the physics is the same.” 

The concept behind the cell is this: An object that is hot compared to its surroundings will radiate heat as infrared light. A conventional solar cell is cool compared to the sun, so it absorbs light. However, space itself is extremely cold, so if you have a warm object and point it at the sky, it will radiate heat toward it.

This concept has been used for centuries as a cooling mechanism at night in hot places, Munday observed. In the last five years scientists have literally warmed to the idea as well and are exploring the development of devices that can do this in the daytime by filtering out sunlight or pointing away from the sun, he said.

“We were thinking, what if we took one of these devices and put it in a warm area and pointed it at the sky,” Munday said of his team’s work.

Using Heat, Not Light

This type of cell, which is called a thermoradiative cell, emits infrared light because it is warmer than outer space, researchers wrote in an abstract for a paper on their work in the journal ACS Photonics.

“In order to produce electrical power after the sun has set, we consider an alternative photovoltaic concept that uses the earth as a heat source and the night sky as a heat sink, resulting in a ‘nighttime photovoltaic cell’ that employs thermoradiative photovoltaics and concepts from the advancing field of radiative cooling,” researchers wrote.

In addition to this nighttime generation of energy, the device also could work during the day if steps are taken to either block direct sunlight or to point the device away from the sun.

The team believes that their device could be a viable option to existing solar cells because it can potentially work around the clock to help balance the power grid and provide energy during gaps in power that solar energy may experience, Munday said.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.

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