________________________________ 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