Doubling the amount of energy generated by solar cells

The problem with solar energy has always been efficiency, since solar panels generate power far less efficiently than other options. In fact, it takes about 40 square meters of solar panels to generate enough energy to power just one American home for a day, based on average use.
Progress is slowly being made in labs around the world, though you might not know it by looking at the recent performance of solar stocks. But now, a breakthrough in the way solar energy is collected could double the amount of power generated by solar panels without dramatically increasing cost.
Researchers at the Massachusetts Institute of Technology published a paper this week in which they describe how they built a working solar thermophotovoltaic device (STPV). Using this revolutionary new setup, the researchers think they can dramatically increase the amount of energy solar panels generate by harnessing some of the energy current panels waste.

http://bgr.com/2016/05/25/solar-panels-efficiency-doubled-mit/

 

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Let me see if I understand this: a conventional photovoltaic cell can only use light of a narrow band of shorter wavelengths, so all the longer wavelength radiation just heats up the device to no purpose. And this thing absorbs all the radiation as heat, but then re-radiates internally, at a particular band of wavelengths, tuned to suit the photovoltaic conversion. Sounds very ingenious. But I see the overall conversion efficiency is still only 6.8%!

 

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Their proof-of-principle demo was admitted to being inherently low efficiency owing to the PV used. So let's assume with the right materials etc., 'more than doubling' of base PV efficiency is actually achievable. There is imo a big question mark over the parallel claim of 'at not much greater cost'. Some key passages from: http://news.mit.edu/2016/hot-new-solar-cell-0523

[does that scream 'cheap and easy' to you?]

[Goodbye relatively cheap and convenient-to-install flat solar panels on rooftops. Hello elaborate arrangements of pricey high energy density PV chips + reflector arrays.]

 

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But how ? Assuming one electron is absorbing two photons(heat/longer wavelength) and then re-radiates one photon(shorter wavelength) tuned to suit the photovoltaic conversion ?

 

Statistically, the overwhelming trend is for higher energy photons to convert to lower energy photons, in keeping with the 2nd Law. What a PC (photonic crystal) does is act as a frequency selective filter, passing only frequencies in a narrow band. [This neglects consideration of direct conduction of heat through the PC lattice material] Since the Planck distribution at ~ 1000 K is heavily biased to the infrared, only a tiny fraction of incident mostly-infrared radiation is passed through a PC filter at a given band of optical wavelengths. I'm not prepared to pay for payware, so can't further comment on viability of the proposed heat-to-light conversion idea.

 

It apparently works by heating carbon nanotubes, which are close to black bodies, up to 1000K (very hot! ) and then these photonic crystals, somehow embedded in the nanotubes, or surrounding them, I suppose, will emit defined bands of frequencies only. To answer your question properly therefore, we need to understand the principles of a photonic crystal. I found this article on Wiki on the subject: https://en.wikipedia.org/wiki/Photonic_crystal

It seems to be a question of transmitting or reflecting light, depending on its frequency. It passes the light in a certain frequency range and I suppose reflects back onto the nanotubes any other frequencies. So the only way the heat can radiate away from the nanotubes is via the portion that is in the frequency range to pass though these crystals. Or something like that.

But as Q-reeus says, it does not at first glance look particularly simple or cheap!

 

But it probably got the MIT guys an extension of their funding, so it was an undoubted success.

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The more I think about this whole idea of turning incident sunlight to heat then via PC filtering back to 'ideal' light band, the less sense it makes. In fact, I'm prepared to call it wrongheaded BS at best. Incident sunlight is mostly already in the optical spectrum, so direct conversion of that spectrum to electricity via conventional PV makes sense. Solar concentration PV allows multi-layer multi-wavelength enhanced overall capturing and conversion, but with no use of up-conversion from lower to higher frequencies. The OP article premise is, overall waste heat somehow gets recycled as light. To be viable, mostly-light-to-heat-back-to-light implies the 2nd law would have to be violated. Somehow the absorbed IR layer needs to wholesale up-convert to narrow-band optical. The power balance doesn't make sense otherwise. Doesn't anyway. That team needs a clued-up physicist to go over it all with a fine-tooth comb imo.

 

Good point about any process that back-converts from heat being limited by Carnot cycle. But I suspect the efficiency of photocells is so low that this does not matter. If you irradiate an object with sufficient intensity of long wavelength radiation, you can raise it to a temperature at which it radiates to some extent at shorter wavelengths. I suspect this is all they are claiming to do.

 

It is hard to know exactly what they are doing; but decades ago, concentrated sunlight heated a target (dense carbon, I think) dull red hot in vacuum and its IR radiation was more correctly matched the the band gap of a silicon PV cell. The inner walls of the vacuum chamber, except where the convergent sun light entered, was covered with solar cells (and any spaces between them, was reflective of the hot body's IR.

I forget how much better this system was than just directly exposing the PV cells, but they were getting more than 30% conversion efficiency as I recall. I. e. >450% better than MIT's complex system! But the cost of the electrical energy was far from competitive (even back when PV energy was much more expensive than today). The sun must be tracked, and the not-cheap vacuum chamber turned to keep the convergent sunlight entering thru the small glass window.

They speak of a plasma, but not where it is or how made. If they have an economical way to control the emissivity (Make it high at the wavelengths that match or at slightly shorter than the band gap energy and in the visible so the sunlight is well absorbed) yet very low elsewhere, that is interesting, but I think Ophiolite's post 7 comment is what this is all about - no prospect of being competitive.

 

Maybe they know something game-changing special we don't, but I can't see how. Consider the following:
http://www.bentham.co.uk/pdf/PV_Technical_Note.pdf (check out plots of spectral response for silicon PV)
http://solarcellcentral.com/limits_page.html (similar to above, but a nice pic showing how suited to solar spectrum is standard silicon PV)
Got the hang of where wavelength band has to be for silicon PV? Now take a fun interactive exercise here:
https://phet.colorado.edu/en/simulation/legacy/blackbody-spectrum (Plays well in Chrome browser but not my Firefox.)
Setting aside absolute intensities, just concentrate on the relative intensities for say 0.4-1.1um range which Silicon PV responds broadly well to. Given the stipulated ~ 1000 K heat environment for MIT device, of the current crop of PV materials, basically only silicon has the capacity to take such punishment.
Compare for Sun temperature of ~ 5778 K, and 1000 K as per MIT device. In the latter case, use the lhs magnifier to lift profile to visible level. A picture can be worth a thousand words. Remember, we are talking relative, not absolute values.