Power from the dark side

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Last year, almost three-quarters of new electricity generation capacity installed across the world was renewable, to the point where more than one third of the world’s power is now generated by renewable sources.

Although solar power provided 55 per cent of this new capacity1, and continues to show a lot of promise in helping the global push toward carbon neutrality, the photovoltaic (PV) technology involved still has one fundamental flaw. It only works during the day. This means that throughout the night there is a continual need to switch over to other sources of energy, most notably fossil fuels. 

Professor Jeremy Munday and graduate student Tristan Deppe of the Department of Electrical and Computer Engineering at University of California, Davis, have therefore taken it upon themselves to design a new PV technology that can be deployed at night. Together, the two researchers have proposed a device that they say under ideal conditions could generate up to around 50W of power per square metre, even after the sun has set – about a quarter of what conventional solar panels are able to generate during the day. Described in January’s ACS Photonics2, the device, known as a thermoradiative cell, would work in a similar way to a normal photovoltaic cell, but in reverse.

Power through cooling

A standard silicon photovoltaic cell operates by absorbing photons from solar radiation throughout the day. These absorbed photons create electron−hole pairs across the semiconductor bandgap and establish a working voltage. The researchers’ proposed thermoradiative cell, which also employs concepts from the advancing field of radiative cooling, is instead designed to absorb heat from a source and then emit thermal radiation in the infrared. In doing this, as electron−hole pairs recombine across the semiconductor bandgap, a negative voltage is established.

‘In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power,’ said Munday. ‘You have to use different materials, but the physics is the same.’

Such thermoradiative cells have previously been explored as an option for capturing waste heat from industrial sources and turning it into power, for example from an engine’s exhaust pipe or a generator’s cooling towers. The cell simply needs to be at a higher temperature than the object toward which it radiates.

‘We were thinking, what if we took one of these devices and put it in a warm area and pointed it at the sky?’ said Munday. In this way, the earth, at a temperature of about 300K, could act as the heat source, while the darkness of space, at 3K, would act as a heat sink, facilitating the transfer of energy required to generate a voltage.

Coupled with the cosmos

In order for the proposed thermoradiative cells to work, they must effectively be able to ‘see’ through the earth’s atmosphere and be optically coupled with deep space, enabling them to transmit heat energy there which has first been absorbed from the earth. 

The thermoradiative cell must therefore emit thermal radiation at a wavelength that can pass through the atmosphere without being absorbed. Such wavelengths fall within a region of the infrared portion of the electromagnetic spectrum known as the ‘atmospheric transparency window’, which occurs roughly between 8 and 13µm, with a secondary window falling at slightly longer wavelengths. 

The proposed nighttime thermoradiative photovoltaic cell absorbs heat from the earth and radiates it into deep space. 

The earth is therefore indeed an ideal heat source for these cells, as its surface, which ranges in temperature from about 220 to 320K, emits strongly within this infrared window. It is this that enables the planet to regulate its temperature and remain habitable.

In order to absorb the radiation, the bottom surface of the proposed thermoradiative cell must therefore be thermally conductive, the researchers report in their paper. The cell should also contain a back-reflector and an infrared window on the front to direct its radiation toward the sky. In addition, the cell would also have to be encapsulated to protect itself from the environment and restrict conductive and convective heat loss.

How low can the bandgap go?

The proposed cells should also be made using a material with an ultra low bandgap in order to maximise their power output, according to the researchers. While most modern commercial solar cells are made from silicon, which has a 1.1eV bandgap; this is too high for use in the nighttime thermoradiative cells. If an ultra low bandgap material were instead used to make these devices, then in ideal conditions (an extraterrestrial cell held at 300K coupled with deep space at 3K) an output of up to 54W/m2 would be possible, the researchers report, while for a terrestrial cell in typical sky conditions, more than 10W/m2 may be possible.

The proposed devices could add 12 per cent to the 24-hour power generation cycle of photovoltaics in certain areas.

‘For the active material, there are several possible low bandgap semiconductors that could serve as starting points for investigation,’ the researchers state in their paper. ‘InSb [indium antimonide] can reach a bandgap below 0.1eV, which can be useful in proof of principle devices. However, for optimal power, even lower bandgaps are needed.’ The researchers therefore suggest that Hg1−xCdxTe, a heavily characterised material used in the infrared sensing industry, could be suitable, or alternatively, newer materials such as graphene−hBN heterostructures. ‘Although difficult to fabricate, it has been shown that alternately stacking sheets of graphene with hBN can open a bandgap of 0.04eV in the graphene layer, which would be an excellent bandgap for a nighttime PV cell,’ they confirmed. ‘However, any chosen material may likely need additional engineering in order to suppress nonradiative generation/recombination.’

Around-the-clock power

In the paper, the researchers suggest that one possibility with the proposed thermoradiative technology would be to use it in a tandem system with a standard solar cell operating during the day – producing higher total around-the-clock power. The nighttime device could, for example, be rolled out on top of the standard solar modules after sunset.

‘For example, in an average US climate, such as Boulder, Colorado, the National Renewable Energy Laboratory database records an average solar irradiance of about 5kWh/m2 per day, of which a commercial solar cell could harvest 1kWh/m2,’ the researchers say. ‘A nighttime PV cell in this climate could produce an average of 120Wh/m(if operated during only 12 hours), adding approximately 12 per cent more power to the 24-hour cycle.’

In conclusion, as the global push toward carbon neutrality continues, the sun is seemingly not the only sky-facing option for power generation. Through the clever use of photonics, optics, and materials science, thermoradiative photovoltaic devices exhibiting strong absorption and emission at thermal wavelengths offer the possibility of nighttime power generation by optically coupling with the cold of deep space.

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References
[1] www.irena.org/publications/2020/Mar/Renewable-Capacity-Statistics-2020
[2] ACS Photonics 2020, 7, 1-9: Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space - Tristan Deppe and Jeremy Munday.

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