In reply to Stephen A. Lawrence's message of Mon, 06 Jul 2009 15:42:15 -0400: Hi,
I think you may be reaching a bit to far. I think it's just a matter of economics. By going to 500 Suns, they reduce the size of the solar cell required, and hence also the cost. Of course the temperature would be prohibitive without cooling, so they may also be extracting useful energy from the cooling fluid. (Energy concentrators are much cheaper / m^2 than solar cells). A trough only concentrates in 1 dimension as it were, whereas a circular parabolic mirror of Fresnel lens will concentrate in 2 dimensions, which is why a trough will only achieve about 80 Suns, while the others can get up into hundreds of Suns, depending on the accuracy and quality of the reflective surface. [snip] >>> 500 times seems too high. I gather the parabolic troughs used at SEGS in >>> California only concentrate sunlight by a factor of 80 and yet they heat the >>> devices up to 400 deg C. >>> >>> How do you concentrate sunlight 500 times? With a Fresnel lens? Parabolic >>> trough? It seems to me if you are going to the trouble to do that, the thing >>> will be pretty hot and you might as well go with large scale solar thermal >>> steam generator or Stirling engine. I expect they are also ~30% efficient >>> but lot simpler and cheaper. [snip] >> Cumulative effect? I would think that would only apply to increasing >> the amount of concentrated heat, not sunlight. >> >> These are solar cells, not solar collectors. Not clear on this: Is >> heat a contributing factor in the generation of electricity in solar >> cells? >> >I think the issue is not heat at all. I think it's entirely an issue of >light intensity. > >The cells respond nonlinearly as the intensity goes up. Thus, >concentrating the same sunlight on a smaller conversion surface >increases the power you get out. > >There could be any number of mechanisms for this. Just kind of wildly >speculating, the one that comes to mind first is that maybe the first >photon which hits an "active spot" on the cell boosts an electron to a >higher energy state, but not high enough to actually move it off to form >part of the output current. Then a second photon comes along and hits >the same electron again, and then the electron finally goes off to >work. But if no second photon comes along for "too long", the electron >falls back to its earlier, inactivated state, and just reradiates the >photon it had absorbed. So, an active site must be hit by two separate >photons in rapid succession to actually make current -- and the >probability of that goes up nonlinearly as the light intensity increases. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
