[math-fun] stupid solar mirror tricks, and solar energy
The TV program "mythbusters" did a show about Archimedes / Clarke ship-roasting legend. The participants aimed their mirrors with the aid of a net 1-2 meters in front of them (by shining thru the net, and seeing the lit-up part of the net, they could aim decently) However, I believe their conclusion was, it was not an effective weapon. They could char the ship a bit, and annoy the sailors with the light and heat, at best. For solar energy, mirror-based systems are best used in non-cloudy high altitude locations while photovoltaics are better in cloudy or lower-altitude locations since the latter can use scattered light. Question: what about a combined photovoltaic plus heat-engine/thermal based solar system? Couldn't that achieve greater efficiency than either alone, and why is it not heavily used?
On 7/17/2013 7:56 AM, Warren D Smith wrote:
The TV program "mythbusters" did a show about Archimedes / Clarke ship-roasting legend. The participants aimed their mirrors with the aid of a net 1-2 meters in front of them (by shining thru the net, and seeing the lit-up part of the net, they could aim decently)
However, I believe their conclusion was, it was not an effective weapon. They could char the ship a bit, and annoy the sailors with the light and heat, at best.
For solar energy, mirror-based systems are best used in non-cloudy high altitude locations while photovoltaics are better in cloudy or lower-altitude locations since the latter can use scattered light.
Question: what about a combined photovoltaic plus heat-engine/thermal based solar system? Couldn't that achieve greater efficiency than either alone, and why is it not heavily used?
That's an interesting question. A photovoltaic only uses about 10% of the photons because they have to be in the right frequency band. The others are mostly just absorbed, since the panels look black or dark blue. But maybe the absorbtion is at the electrodes. A mesh electrode and a reflective backing might allow most of the photons that are not absorbed by the photovoltaic to be reflected to a boiler. Brent
________________________________ From: meekerdb <meekerdb@verizon.net> To: math-fun <math-fun@mailman.xmission.com> Sent: Wednesday, July 17, 2013 12:31 PM Subject: Re: [math-fun] stupid solar mirror tricks, and solar energy
On 7/17/2013 7:56 AM, Warren D Smith wrote:
The TV program "mythbusters" did a show about Archimedes / Clarke ship-roasting legend. The participants aimed their mirrors with the aid of a net 1-2 meters in front of them (by shining thru the net, and seeing the lit-up part of the net, they could aim decently)
However, I believe their conclusion was, it was not an effective weapon. They could char the ship a bit, and annoy the sailors with the light and heat, at best.
For solar energy, mirror-based systems are best used in non-cloudy high altitude locations while photovoltaics are better in cloudy or lower-altitude locations since the latter can use scattered light.
Question: what about a combined photovoltaic plus heat-engine/thermal based solar system? Couldn't that achieve greater efficiency than either alone, and why is it not heavily used?
That's an interesting question. A photovoltaic only uses about 10% of the photons because they have to be in the right frequency band. The others are mostly just absorbed, since the panels look black or dark blue. But maybe the absorbtion is at the electrodes. A mesh electrode and a reflective backing might allow most of the photons that are not absorbed by the photovoltaic to be reflected to a boiler.
Brent
The ideal solar panel will look black. Reflected light is wasted light. Photons above the band gap are OK, but only yield 1 electron per photon, and that electron settles to the bottom of the conduction band, so the full photon energy is unavailable. Solar cells are under development with multiple band gaps to more efficiently use solar energy and with subwavelength surface structure to minimize reflection. I've heard at a talk a few years ago that 40% efficiency is the limit with current ideas.
-- Gene
I recall seeing a lecture by someone at Caltech doing 'synthetic photosynthesis'. If I remember correctly, he said that one of the tricks plants use is to 'downconvert' some of the high-frequency light into lower-frequency light that was easier to capture. That way, you get (at least) 2 frequencies for the price of one. I think that someone else is trying to genetically engineer plants to absorb slightly different frequencies than normal, so that eventually, a plant could be engineered to absorb a wider spectrum. If I remember correctly, I think that the absorbing molecule was 'tunable' by adjusting some side-chains. Someone at UCSB was working on photovoltaic 'paint'; it might not be the most efficient, but it could be spread cheaply over much larger areas. The whole area of 'quantum dots' is interesting, precisely because they can be engineered to have whatever band energies (within reason) that you want. I.e., you no longer have to search the periodic table for weird/expensive elements that have a particular band gap; you engineer a quantum dot with the right properties. At 12:31 PM 7/17/2013, meekerdb wrote:
That's an interesting question. A photovoltaic only uses about 10% of the photons because they have to be in the right frequency band. The others are mostly just absorbed, since the panels look black or dark blue. But maybe the absorbtion is at the electrodes. A mesh electrode and a reflective backing might allow most of the photons that are not absorbed by the photovoltaic to be reflected to a boiler.
participants (4)
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Eugene Salamin -
Henry Baker -
meekerdb -
Warren D Smith