Two devices have been proposed to turn the Earth's night-time infrared emissions into a source of power. At the moment the idea is a series of calculations rather than one prototype, let alone two, but the thought experiments alone were enough to win publication in the Proceedings of the National Academy of Sciences. A crucial component of one idea is work forgotten since the late 1960s.
A team at the Harvard School of Engineering and Applied Sciences set out to take on the challenge of how to produce clean, renewable energy at night for regions dependent on solar power. Theoretically speaking it is possible to generate energy wherever there is a difference in temperature. This includes the heat radiated back to space at night in the infrared part of the spectrum.
"Sunlight has energy, so photovoltaics make sense; you're just collecting the energy. But it's not really that simple, and capturing energy from emitting infrared light is even less intuitive," says lead author Dr Steven J. Byrnes.
Clouds complicate the picture further, but Byrnes and his colleagues decided to focus on conditions in deserts where this is seldom an issue.
Nevertheless, co-author Professor Frederico Capasso says, “It’s not at all obvious, at first, how you would generate DC power by emitting infrared light in free space toward the cold. To generate power by emitting, not by absorbing light, that's weird.”
One concept Byrnes and Capasso propose consists of two plates, a hotter one on the ground and a “cold” plate above it. In fact the temperature difference between the two would be very small. The hotter plate would be the temperature of the Earth – substantial at the end of a sunny day, but cooling down in the course of the night. The cold plate would be made of a material that cools very efficiently, radiating the heat it absorbs from the hotplate away almost as soon as it gets it.
“This approach is fairly intuitive because we are combining the familiar principles of heat engines and radiative cooling,” says Byrnes, although his definition of intuitive may refer more to that of a physicist or engineer than the population as a whole.
In theory, if the cold plate can be kept cool the team show that it should be possible to generate a few watts per square meter – tiny compared to photovoltaic panels during daytime. However, even this may be a problem since the cold plate would tend to warm towards the temperature of the surrounding environment.
The team's second proposal is even more exotic. They suggest using tiny diodes and antennas shrunk to the nanoscale to harvest really tiny energy differentials.
“The key is in these beautiful circuit diagrams,” says Capasso (see image below). “We found they had been considered before for another application—in 1968 by J.B. Gunn, the inventor of the Gunn diode used in police radars—and been completely buried in the literature and forgotten. But to try to explain them qualitatively took a lot of effort.”
The electrical noise that can pollute circuits occurs when components push current in either direction. Diodes act like valves, letting current flow one way but not the other. If a diode is at a higher temperature than a resistor it will push the current forwards, but not allow it to flow back. If the resistor is made of the same efficiently emitting material used in the cold plate above the electrons within it will be cooler than those in the rest of the circuit. “You get an electric current directly from the radiation process, without the intermediate step of cooling a macroscopic object,” says Byrnes.
If thousands of tiny circuits generating minute amounts of power in this way are added together the result might be something we can use.
Not long ago, such an idea would have been entirely theoretical, but developments in small-scale electronics, nanofabrication and new materials means that it should now be possible to build such a device. Nevertheless, two classes of problems remain.
The first is technical “The more power that’s flowing through a single circuit, the easier it is to get the components to do what you want. If you’re harvesting energy from infrared emissions, the voltage will be relatively low,” says Byrnes. “That means it’s very difficult to create an infrared diode that will work well.”
Byrnes thinks the solution here may lie in a mix of new diode devices suited to low voltages and higher impedance circuit components that raise the voltage of operation.
Beyond this lies the economic challenge. The cost of battery storage has started to fall, and some observers expect it to undergo the same sort of dizzying drop in price solar cells have experienced in recent years. Moreover, the majority of solar thermal projects – ideally suited to the desert conditions in which Byrnes and Capasso's system might operate – now come with hot salt tanks that allow them to generate power through the night. Any design the Harvard team produces will have to compete on price with such solutions, which may prove harder even than the astonishing physics required to make it happen.
More exotic applications may also be a consideration. For example the Moon's lack of atmosphere makes for much more dramatic heating and cooling, and storing solar power through the two week lunar night could be a challenge, changing the economics for future bases.