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Rice University researchers propose a way to boost solar efficiency

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Re: Rice University researchers propose a way to boost solar efficiency

#111

Post by amluto »

TTPrograms wrote: post #108
amluto wrote: post #32

As far as I can tell, this is functionally just a coating with low emittance in the IR outside a narrow band. A matched PV device that is kept cool can, in principle, approach the Carnot efficiency for the temperature difference between the emitter and the PV junction. If the emitter is the sun, then the source temperature is very high and the Carnot efficiency isn’t a major limit. If the source is a solar panel, I’m…

One sketch of such a system is to physically mate an effective high T solar absorber to an effective narrow spectrum photon emitter (as described in this work). Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives. As I understand the emissivity of these materials can exceed blackbody radia…

> Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives.

One way or another, once you've converted sunlight to heat, you are limited by the Carnot efficiency. For the 80% efficiency they claim, if all of it comes from thermophotovoltaics, they need a hot side temperature at least 5x ambient, which is over 1000 C. I wish them luck getting anything resembling a solar panel up to 1000 C. (I'm not, in any respect, saying it's impossible -- I'm saying it's very hard. You'd need excellect spectrally or directionally specific absorption to avoid re-radiating all that heat out the top of your panel, and you'd need conventional transparent insulation to stop conduction.)

On top of that, super-Plankian emission or no, if it's limited to the near field, then the PV cell is very, very close to the hot surface. That PV cell needs to be kept near room temperature to get that efficiency.

This whole thing seems extraordinary complex for something that wants to be cost-effective.

Re: Rice University researchers propose a way to boost solar efficiency

#112

Post by TTPrograms »

amluto wrote: post #111
TTPrograms wrote: post #108

Earlier quoted context omitted.

One sketch of such a system is to physically mate an effective high T solar absorber to an effective narrow spectrum photon emitter (as described in this work). Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives. As I understand the emissivity of these materials can exceed blackbody radia…

> Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives. One way or another, once you've converted sunlight to heat, you are limited by the Carnot efficiency. For the 80% efficiency they claim, if all of it comes from thermophotovoltaics, they need a hot side temperature at least 5x ambient,…

The absorber/emitter assembly doesn't really resemble a solar cell. Demonstrated tungsten emitters have exceeded 1500K - it's not really that wild, it's what's in incandescent light bulbs. You concentrate sunlight typically.

Re: Rice University researchers propose a way to boost solar efficiency

#114

Post by skosch »

tim333 wrote: post #101
skosch wrote: post #75

Earlier quoted context omitted.

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 co…

>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…

The 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 – and apparently carbon nanotubes can do this as well.

Re: Rice University researchers propose a way to boost solar efficiency

#115

Post by skosch »

reaperducer wrote: post #88
skosch wrote: post #75

Earlier quoted context omitted.

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 co…

This 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.

The 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.

Re: Rice University researchers propose a way to boost solar efficiency

#116

Post by skosch »

p1esk wrote: post #105
fengazer wrote: post #103

Earlier quoted context omitted.

Visible 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.

If that's the case, how is it possible that low energy photons absorbed by carbon nanotubes can generate high energy photons?

Because it's not a one-to-one conversion. See e.g. https://en.wikipedia.org/wiki/Second-harmonic_generation

Re: Rice University researchers propose a way to boost solar efficiency

#117

Post by elwell »

dredmorbius wrote: post #82
elwell wrote: post #73

Earlier quoted context omitted.

What is the accurate model of infrared light vs. heat?

It's a bit complicated, but taking a stab at it: All physical objects emit electromagnetic radiation corresponding to their temperature (as opposed to reflected, reaction-induced, or stimulated radiation, more below), with a corresponding blackbody radiation curve. Note that blackbody emissions don't have a specific frequency (say, unlike laser light), though the curve has a peak emission. Traditional incandescent li…

Thanks, very informative!

Re: Rice University researchers propose a way to boost solar efficiency

#118

Post by infogulch »

Terr_ wrote: post #28

> turning heat into light > channel mid-infrared radiation (heat energy) into light energy > absorbs thermal photons and emits light. Goddamnit, no! None of this is "heat", it's just one range of light being converted into another! This is shitty "science"-journalism parroting a common misconception. After all, the only reason we associate infrared with "heat" in the first place is that it's useful for detecting thin…

Yeah it seems more like it.. fluoresces (?) around the infrared spectrum. Fluorescence might actually the right term for what is happening.

Re: Rice University researchers propose a way to boost solar efficiency

#119

Post by ThomPete »

philipkglass wrote: post #70
ThomPete wrote: post #50

Earlier quoted context omitted.

There are many types of fuel cells. I am talking about the kind of fuel cells which can be used by wind and solar to store energy and distributed economically feasible. We are not even close to that. If you know of anything, by all means, please let me know as the group I am part of would invest in it in a heart beat.

I am talking about the kind of fuel cells which can be used by wind and solar to store energy and distributed economically feasible. I don't know of anything like that either. Small fuel cells rely on expensive precious metal catalysts and large ones have too much capacity to be a good match for the distributed generation sector. A technology that is commercially available but too expensive at present for large scale…

Again no. Fuel cells that can be used for wind and solar to store their energy isn't close at all it's not just cost it's also the physics.
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