To move from today’s world to a world where renewable energy dominates our energy economy, some additional technologies are needed. These can include storage, improved grid management and demand-driven power management, but they can also include something entirely new. Recently, a paper took a look at a technology I didn’t even know existed.
The paper evaluates the potential of what the authors call “night-time photovoltaics,” and the easiest way to think about it is to “make solar panels work in reverse”—generating electricity by beaming energy away into space. Its efficiency is nothing compared to standard photovoltaic cells and can’t even get there except in unusual conditions. But as the name implies, it can continue to generate power long after the sun sets.
Photovoltaic at night
The easiest way to understand this technology is to think of a photovoltaic device that is in balance with its environment. Here, incoming photons will occasionally release an electron, leaving behind a positively charged hole. These can then combine and radiate a photon back from the device. When it works as a photovoltaic device, a large excess of photons come in, producing a corresponding excess of electrons and holes that can then be harvested as electricity.
Now imagine setting up the reverse situation: there are no incoming photons, and instead we cause an excess of photons to be radiated out as infrared radiation. There is now an excess of electrons and holes, which can be harvested again. Except in this case, we don’t need any incoming light – instead we need something to ensure that photons are extracted from the device with high efficiency.
This last requirement is something that researchers in a seemingly unrelated field of research have been working on. There is a “window” of infrared wavelengths that are not absorbed by any of the gases in the atmosphere. A carefully crafted material can absorb radiation of different wavelengths, but only re-emit it at wavelengths that pass through this atmospheric window. People have built devices that work this way to passively cool buildings, and there are even ways to treat wood to make it work this way.
By coupling a photovoltaic material that emits photons that correspond to a passive-radiating device, it is possible to build a “thermoradiative cell” that can extract some electricity as photons are radiated into space. In contexts where there is a high demand for cooling or a lot of residual heat, this can be an additional advantage for the cooling that must be done anyway. But can it really work in a practical way?
Run the numbers
That’s what Tristan Deppe and Jeremy Munday decided to find out. Their paper analyzes everything from the semiconductor bandgaps needed for this to work to the wavelengths normally present in the atmosphere.
The atmosphere is a problem because no material is completely transparent to all the wavelengths of light typically present in our atmosphere. That means all the materials involved — the one emitting the infrared and the one generating spare electrons — will absorb some of the stray photons, decreasing the overall efficiency of the device. The bandgap is a problem because the infrared wavelengths needed to make this work have relatively little energy, so they require a much smaller bandgap than the silicon used in most photovoltaic cells. The researchers suggest that a family of mercury-cadmium-tellurium compounds appears to have the right properties.
For all their initial calculations, Deppe and Munday assume that the Earth will provide a constant source of 27°C (about 80°F) of heat to power the system. Under these conditions, they estimate that these systems built with current technology can produce somewhere in the neighborhood of two to eight watts per square meter, with the value changing a bit based on season and location.
Compared to a photovoltaic panel, which typically has a range of 200 watts/meter2, that’s pretty pathetic. But this could potentially generate 24 hours a day, which a photovoltaic panel certainly cannot, so that makes up for some of the drawback. Since much of the infrastructure would be identical for photovoltaic and thermoradiative cells, they also estimated the benefits of a system that can automatically switch panels when the sun goes down. They calculate that this would increase the capacity of a solar park by 12 percent.
Low, low prices
For those kinds of incremental gains, the thermoradiative cells would have to have incredibly low prices to make sense economically. But when there are additional benefits, such as lowering cooling costs for buildings in hot climates, the economy may shift a bit.
The really interesting thing, however, comes when Deppe and Munday explore what can be done with waste heat. At a starting heat source of 170°C, a thermoradiative cell could produce approximately the same number of watts/meter as a solar panel. While that temperature is high – it is well above the boiling point of water – it falls within the range of what might be considered waste heat for industrial plants and steam generators. And when coupled with things like geothermal or solar thermal power plants, it would be a way to get more energy from an existing renewable resource.
For now, this is all purely hypothetical; many of the numbers in the paper come from idealized situations, and we have no idea how close we can get to those in the real world or how cheaply these devices could be manufactured. But the idea is a smart extension of existing technology that could potentially integrate with a variety of existing systems.
ACS Photonics, 2020. DOI: 10.1021/acsphotonics.9b00679 (About DOIs).