The big problem with solar power generation here on the Earth's surface is that the sun isn't there all the time. Batteries and other storage technologies can hold power from daytime for use at night.
But wouldn't it be nice to have something that generates power when the sun isn't available? Something like "negative-" or "anti-solar power".
How about, "Generating Power on Earth From the Coldness of Deep Space"? [1]
Whoa!
This fall, Stanford Professors Assawaworrarit and Fan discuss the other half of Earth's energy balance, the "outgoing" cooling, heat flowing out to the deep cold of space [1]. Even on the Earth's surface, even in high daylight, heat can flow out towards space. If we can tap that flow, we can get more free energy, anti-solar energy.
"While incoming radiation has become a mainstay for renewable energy in the form of solar energy, outgoing radiation has largely remained untapped for energy generation."
(From [1])
Down here at the surface, outgoing radiation is often swamped by absorption of the atmosphere and flows of heat from objects and processes. But at some wavelengths and in good conditions, ordinary objects radiate out to space.
How do you tap this radiative cooling?
"Plastic"? No, semiconductors. The right magical materials form thermoelectric generators. Heat flows from the hot part (e.g., the ground) to the cold part (space). Put in a semiconductor where this heat flows, and the heat moves the electrons and holes around, potentially generating current.
When an emitter radiates heat at a wavelength within the atmospheric transmission window, it cools down, creating a cold reservoir. A thermoelectric generator can then use the ambient air as its hot side and the emitter as its cold side to produce electricity. Image credit: Chris Philpot (From [1])
This is a delicate process, but researchers have developed "a library of materials, each with a specific shape and size, to adjust the wavelength band and direction for heat radiation." [1]
The good news is, this actually works.
The bad news is, there isn't a lot of energy available, at least not in practical densities.
But what about making a real "anti-solar" material.
Photovoltaic materials work because the sunlight floods the material with photons at certain energies which hit the band gap of the material. This generates a lot of electrons and holes, and selective materials let one or the other bleed off to harvest as power.
So, can this concept work with the heat flowing out, instead of in? The theory is actually so obvious that even a dunce like me groks it.
"In such a case, the electrons and holes in the semiconductor recombine and radiate photons, reversing the process of light absorption."
(from [1])
To make this work, the material needs a band gap that matches the "transmission window" that passes through the atmosphere (13 μm). This corresponds to 0.09 electron volts, which is tiny. Silicon cannot do it, but there are semiconductor materials that can.
But small band gap materials are finicky, and it is hard to get much of the theoretical power in practices.
But, hey, the dog does actually dance!
The researchers calculate that it may be practical to get something like 1 W/m2, which no where near what solar power generates. (Though it is infinitely more than the zero W/m2 my PV array generated inthe dark.) But this trickle is enough to charge a battery, light a LED, or run a sensor[3]. I would take that, especially if it was a layer on my regular daytime PV panel.
- Sid Assawaworrarit and Shanhui Fan, Generating Power on Earth From the Coldness of Deep Space, in IEEE Spectrum - Energy, November 25, 2023. https://spectrum.ieee.org/energy-from-cold
- Zhen Chen, Linxiao Zhu, Wei Li, and Shanhui Fan, Simultaneously and Synergistically Harvest Energy from the Sun and Outer Space. Joule, 3 (1):101-110, 2019/01/16/ 2019. https://www.sciencedirect.com/science/article/pii/S2542435118304719
- Aaswath P. Raman, Wei Li, and Shanhui Fan, Generating Light from Darkness. Joule, 3 (11):2679-2686, 2019. https://doi.org/10.1016/j.joule.2019.08.009
- Parthiban Santhanam and Shanhui Fan, Thermal-to-electrical energy conversion by diodes under negative illumination. Physical Review B, 93 (16):161410, 04/25/ 2016. https://link.aps.org/doi/10.1103/PhysRevB.93.161410
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