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You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing
by skosch 7y ago
You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing.
As a rule, everything around you glows – in other words, it emits photons. Some infrared ones, some visible ones, some ultraviolet (or even higher-frequency) ones. How much of each depends on the temperature of the material. Most things (e.g. you) are too cold to be visibly glowing, but they still emit plenty of infrared radiation.
Now, standard solar cells convert visible photons into electricity. Any incoming infrared photons, however, aren't strong enough to kick any electrons loose, so they go to waste. Here, the researchers used carbon nanotubes to absorb the wasted infrared photons. This heats up the nanotubes, which then emit visible photons, which then increase electricity generation.
There is no way to convert heat to electricity directly, unfortunately. However, you can get small amounts of electricity out of temperature differences: https://en.wikipedia.org/wiki/Thermoelectric_effect https://en.wikipedia.org/wiki/Thermoelectric_effect
- reaperducer 7y agoThis heats up the nanotubes, which then emit visible photons, which then increase electricity generation Once a solar cell is built with this technology, do you think there will be enough light emitted by the nanotubes to make the solar panels appear to glow, themselves? Because that would be pretty interesting.
- dlivingston 7y agoIf the solar panels appear glowing, it's because visible light is being dispersed away from the solar panel itself. The consequence of that would be wasted photons that could otherwise be converted into electricity. So, from a pragmatic perspective, I would say that no, they wouldn't glow - proper engineering would ensure these particles are collected.
- skosch 7y agoThe idea is to direct all of the nanotube-emitted light onto the PV cells, so hopefully you won't see anything. Citing the Rice article: > Because electrons in nanotubes can only travel in one direction, the aligned films are metallic in that direction while insulating in the perpendicular direction, an effect Naik called hyperbolic dispersion. Thermal photons can strike the film from any direction, but can only leave via one.
- p1esk 7y agoI see, thanks! Why are visible light photons "stronger" than infrared photons?
- fengazer 7y agoVisible light photons have more energy each. The shorter the wavelength, the higher the energy of the photon. Light with a short enough wavelength (UV-B, for example) has enough energy to damage the DNA in our skin and increase our risk of skin cancer.
- p1esk 7y agoIf that's the case, how is it possible that low energy photons absorbed by carbon nanotubes can generate high energy photons?
- skosch 7y agoBecause it's not a one-to-one conversion. See e.g. https://en.wikipedia.org/wiki/Second-harmonic_generation https://en.wikipedia.org/wiki/Second-harmonic_generation
- tim333 7y ago>This heats up the nanotubes, which then emit visible photons I'm a bit skeptical that that would work. Otherwise you could build a perpetual motion machine by having the nanotubes in a hot room and running your perpetual motor off a solar cell powered by the light. My guess is when the article says emit light they mean emit infra red light / radiation rather than visible. The article doesn't really make sense to me - I imagine someone got confused by the physics rather than it being an actual breakthrough in energy generation. I note the article says "propose a way to build" rather than having actually built something that works.
- skosch 7y agoThe article doesn't provide very much detail, but it's absolutely possible to absorb multiple low-energy photons and emit fewer higher-energy ones, see e.g. https://en.wikipedia.org/wiki/Second-harmonic_generation https://en.wikipedia.org/wiki/Second-harmonic_generation – and apparently carbon nanotubes can do this as well.
- Retric 7y ago> This heats up the nanotubes, which then emit visible photons Critically these photons are not heating up the material. It’s like saying photons reflecting off a mirror heat it. No, the photons that are not reflected do heat the mirror, but not the reflected ones. (At least mostly.)
- skosch 7y agoYou're correct, that was sloppy wording on my part.
- sdenton4 7y ago"There is no way to convert heat to electricity directly, unfortunately." Well, if you have enough heat, you can boil water and then use the steam to turn a turbine... I suppose in some sense this is dependent on a temperature difference, but I wouldn't call the amount of electricity we get by this method 'small.'
- Terr_ 7y ago> I suppose in some sense this is dependent on a temperature difference It is absolutely dependent on a temperature difference, because it relies on cooler water that isn't already flashing into steam.