What if solar cells worked at night?
That’s no joke, according to Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis. In fact, a specially designed photovoltaic cell could generate up to 50 watts of power per square meter under ideal conditions at night, about a quarter of what a conventional solar panel can generate in daytime, according to a concept paper by Munday and graduate student Tristan Deppe.
The article was published in, and featured on the cover of, the January 2020 issue of ACS Photonics.
Munday, who recently joined UC Davis from the University of Maryland, is developing prototypes of these nighttime solar cells that can generate small amounts of power.
The researchers hope to improve the power output and efficiency of the devices.
Munday said that the process is similar to the way a normal solar cell works, but in reverse. 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.
Space is really, really cold, so if you have a warm object and point it at the sky, it will radiate heat toward it. People have been using this phenomenon for nighttime cooling for hundreds of years. In the last five years,
Munday said, there has been a lot of interest in devices that can do this during the daytime (by filtering out sunlight or pointing away from the sun).
Generating power by radiating heat
There’s another kind of device called a thermoradiative cell that generates power by radiating heat to its surroundings. Researchers have explored using them to capture waste heat from engines.
“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.
This thermoradiative cell pointed at the night sky would emit infrared light because it is warmer than outer space.
“A regular solar cell generates power by absorbing sunlight, which causes a voltage to appear across the device and for current to flow.
In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power,” Munday said.
“You have to use different materials, but the physics is the same.”
The device would work during the day as well, if you took steps to either block direct sunlight or pointed it away from the sun. Because this new type of solar cell could potentially operate around the clock, it is an intriguing option to balance the power grid over the day-night cycle.
There’s a stark contrast between the freezing temperatures of space and the relatively balmy atmosphere of Earth, and that contrast could help generate electricity, scientists say – utilising the same optoelectronic physics used in solar panels.
The obvious difference this would have compared with solar energy is that it would work during the night time, a potential source of renewable power that could keep on going round the clock and regardless of weather conditions.
Solar panels are basically large-scale photodiodes – devices made out of a semiconducting material that converts the photons (light particles) coming from the Sun into electricity by exciting electrons in a material such as silicon.
In this experiment, the photodiodes work ‘backwards’: as photons in the form of infrared radiation – also known as heat radiation – leave the system, a small amount of energy is produced.
Diagram showing the two directions a current can go in a photodiode. (Santhanam & Fan, Phys. Rev. B, 2016)
This way, the experimental system takes advantage of what researchers call the “negative illumination effect” – that is, the flow of outgoing radiation as heat escapes from Earth back into space. The setup explained in the new study uses an infrared semiconductor facing into the sky to convert this flow into electrical current.
“The vastness of the Universe is a thermodynamic resource,” says one of the researchers, Shanhui Fan from Stanford University in California.
“In terms of optoelectronic physics, there is really this very beautiful symmetry between harvesting incoming radiation and harvesting outgoing radiation.”
It’s an interesting follow-up to a research project Fan participated in last year: a solar panel that can capture sunlight while also allowing excess heat in the form of infrared radiation to escape into space.
In the new study, this “energy harvesting from the sky” process can produce a measurable amount of electricity, the researchers have shown – though for the time being it’s a long way from being efficient enough to contribute to our power grids.
In the team’s experiments they were able to produce 64 nanowatts per square metre (10.8 square feet) of power – only a trickle, but an amazing proof of concept nevertheless. In theory, the right materials and conditions could produce a million times more than that: 4 watts per square metre.
“The amount of power that we can generate with this experiment, at the moment, is far below what the theoretical limit is,” says one of the team, Masashi Ono from Stanford.
A schematic of the experimental infrared photodiode setup. (Masashi Ono)
When you consider today’s solar panels are able to generate up to 100-200 watts per square metre, this is obviously a long way behind. Even in its earliest form, though, it could be helpful for keeping low-power devices and machines running at night: not every renewable energy device needs to power up a city.
Now that the researchers have proved this can work, the challenge is to improve the performance of the experimental device. If it continues to show promise, the same idea could be applied to capture energy from waste heat given off by machinery.
“Such a demonstration of direct power generation of a diode facing the sky has not been previously reported,” explain the researchers in their published paper.
“Our results point to a pathway for energy harvesting during the night time directly using the coldness of outer space.”
More information: Tristan Deppe et al, Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space, ACS Photonics (2019). DOI: 10.1021/acsphotonics.9b00679
