On a clear night you’ll feel your body cool; some of that cooling is heat radiating into space:
Removing heat this way can cool that object down tens of degrees below the temperature of its surroundings.
We can exploit the temperature difference by turning it into electricity through thermoelectric power generation. The working principle behind a thermoelectric generator is the Seebeck effect, which describes how a material develops a voltage difference in response to a temperature differential across it. We can manipulate the Seebeck effect in semiconductors by the controlled addition of impurities, or dopants.
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With the ambient environment as a hot reservoir, we can use the coldness from deep space to create the cold reservoir. To do this, we send heat out to space using what we call an emitter, which cools itself to a lower temperature than its surroundings. That’s a phenomenon known as radiative cooling. Then, a thermoelectric generator situated between the cold emitter and the now-hotter ambient surroundings can produce electricity.
The emitter’s job is to radiate the heat out beyond Earth’s atmosphere. But the atmosphere is transparent only to photons of certain wavelengths. Within the mid-infrared range, which is where heat radiation from typical earthbound objects is concentrated, the most applicable atmospheric transmission band is in the 8- to 13-micrometer-wavelength range. Even some simple emitters send out heat radiation at these wavelengths. For example, if it’s insulated from ambient surroundings, black paint emits enough radiation within that band to cool a surface down by 10 degrees Celsius when exposed to the night sky.
In the wavelength range outside 8 to 13 mm, the atmosphere bounces back a substantial amount of radiation. During the daytime, solar radiation comes into the equation. More-advanced emitter designs aim to avoid the incoming radiation from the atmosphere and sunlight by ensuring that they absorb and emit only within the transparency window. The idea of using such a wavelength-selective emitter for radiative cooling dates back to the pioneering work of Claes-Göran Granqvist and collaborators in the 1980s. Just as an engineer designs a radio antenna with a specific shape and size to transmit over a certain wavelength in a certain direction, we can design an emitter using a library of materials, each with a specific shape and size, to adjust the wavelength band and direction for heat radiation. The better we do this, the more heat the emitter ejects into space and the colder the emitter can get.
Glass is a great material for an emitter. Its atomic vibrations couple strongly to radiation around the 10-micrometer wavelength, forcing the material to emit much of its heat radiation within the transmission window. Just touch a glass window at night and you’ll feel this cooling. Adding a metallic film to help reflect radiation skyward makes the emissions—and the cooling—even more effective. And structures can be specifically designed to strongly reflect the wavelengths of sunlight.
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Putting all these optimizations together, we calculated that the maximum achievable power density for this technology is 2.2 W/m2. This power density is a lot lower than what can be generated with solar cells under sunlight. However, when sunlight isn’t readily available, this is pretty good; it’s significantly higher compared to what can be achieved with many other ambient energy-harvesting schemes. For example, it’s orders of magnitude more than the less than 1 mW/m2 that can be harvested from ambient radio waves.
These technologies are promising not just as energy sources, but as good ways to dump excess heat from buildings in humid tropical environments. One study promised five degrees of cooling even for a building in a humid jungle at 29 degrees Celsius.
https://opg.optica.org/oe/fulltext.cfm?uri=oe-27-22-31587&id=422391
This has been tried so many times, so many ways. It always ends with the two sides of the thermocouple reaching temperature equilibrium and producing no power.
If this was truly a promising technology there would already be living organisms that evolved to exploit the phenomenon.
I am not a physical engineer, but I read some of their journals. Thus I cannot claim any personal experience with operating cost-effective thermoelectric generators. However, from my reading, I note:
>…the article discusses recent thermoelectric material advancements. It also compares thermoelectric generator and photovoltaic efficiency and cost. Results reveal that wearable thermoelectric generators have lower power density (This has been tried so many times, so many ways. It always ends with the two sides of the thermocouple reaching temperature equilibrium and producing no power.
I wonder whether McChuck is arguing mostly from academic reading, informal hearsay, professional experience, personal experience, or some other source of experience.
Peter: “If this was truly a promising technology there would already be living organisms that evolved to exploit the phenomenon.”
Natural selection is much like “the free market” in that sometimes it works but mostly it’s a blind idiot god at best.
Peter: “If this was truly a promising technology there would already be living organisms that evolved to exploit the phenomenon.”
Radio?
But that is an interesting question: What useful things have humans invented/found that evolution otherwise has not?
Lasers? Which for the first 20-30 years were the hopeless “product in search of a market”.
Felix, Peter, regarding temperature differential, perhaps there are living organisms that exploit temperature differential to create electrical energy, and that use radio waves. IRO the latter it would be extremely surprising if there were not. Naturally the entire sensory nervous system converts a sensation into an electrical signal, but I think we’re talking metabolic electrical energy here? A fascinating question.