From: Bill Gosper <billgosper@gmail.com> To: math-fun@mailman.xmission.com Subject: Re: [math-fun] one crazy idea Message-ID: <CAA-4O0FQFOA9+hNtR-+=pNNpnuk1F-9FP-UXeQXBZpEWzPE7zQ@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1

The 43 yr old, 2000m^2 solar furnace at Odeillo, France is still operating: http://commons.wikimedia.org/wiki/File:Four_solaire_001.jpg Thermodynamics limits its spot temperature to that of the solar "surface". The installation is visitor-friendly and probably has personnel who could enrich this thread.

Re double duty solar collectors: Unless you're short of real estate, trying to recover waste heat from a photovoltaic just imposes engineering constraints that cost more than collecting heat separately. Also, electrical energy is much more valuable per watt than heat.

This reminds me of my dumb idea to reduce disk seek times with multiple heads per surface. Surface is cheap. --rwg

--well, pretty much totally wrong. For use on house rooftops, surface is NOT cheap, and solar panels NOT cheap, and hot water is highly demanded (washing clothes, showers) not just electricity. So I do not see why no combined hot-water and electricity panels for housetops. Incidentally, one thing I learned during Hurricane Sandy -- power out all around here for 2 weeks -- but there were some houses with a ton of solar volt panels on roof, so they were fine, right? Wrong. Actually, essentially every such system REFUSES to generate power when your house stops getting power form the utility company. This is done to protect utility linemen. That means one of the major reasons to buy such a system -- self sufficiency -- is flushed down the toilet. About mirror farms like http://upload.wikimedia.org/wikipedia/commons/a/ac/Four_solaire_001.jpg http://en.wikipedia.org/wiki/Themis_(solar_power_plant) achieving "up to 3500C"; this is well short of solar surface temp, due to inefficiencies. This site (it says) gets 300 sun days/year and is at 1-2km altitude (wasn't precisely stated) and on sloping ground, all positive factors. The power plant generated a (tiny) 2 MW using molten salts heat bath at 450C. Ran for 3 years then closed due to high maintenance expense. The molten salts used were potassium & sodium nitrates mainly, which sounds like safety and corrosion nightmare. The largest in the world is http://en.wikipedia.org/wiki/Solar_Energy_Generating_Systems with 354 MW peak capacity, but annual generation is 662 GW-hours which I compute is on average 75 MW, i.e. averages 21% of that. Also can burn natural gas to generate power at night. 1600 acres of mirrors which is equivalent to a square 2.54 km on a side. The solar power incident on that area (normal to beam, above atmosphere) is 8780 MW, so the efficiency of the entire plant is 354/8780 = 4.0%. It uses tracking parabolic trough mirrors focusing on a black pipe with oil-fluid flow thru, heats to 400C. This site in in Mojave desert, flat, low cloud site (340 sunny days / year), at altitude about 620 meters. I would expect they'd have done a good deal better at higher altitude. About 3000 of their mirrors break per year (0.3%) and one time their oil tank exploded. The company running the place went bankrupt. A 30 MW sub-plant costs about $3 million/year maintenance, plus it cost $90M to build and lasts 20 years which is an additional $4.5M/year cost. The net effect is they produce electricity at 14 cents / kW-hour, which is above the US average price but not hugely above. If their black pipes had magic photovoltaic wrap layer generating electricity directly at 10% efficiency, that'd be maybe 30% extra juice for them... except if the pipe as a consequence got less-black, that'd cost them. Anyway they do not try to do that.