Originally posted by twhitehead
But mirrors are cheap. It would presumably be fairly easy to set up a tin foil reflector on your roof that concentrates the sunlight onto one of these. The actual area of solar cells could then be even smaller than usual and thus even cheaper for a given amount of sunlight captured.
Here in Africa we frequently use corrugated galvanized Iron as roofing ...[text shortened]... to save costs on standard solar cells is that they are less efficient at higher temperatures.
More than less efficient at higher temps, they deteriorate quickly at those temps. That's why they use gallium arsenide, they can take that kind of heat by design.
They are also used in high temperature environments as computers which normal silicon would quickly break down. The problem with silicon at 200 degrees C is the junctions leak current badly and that contributes to the breakdown of the junctions.
Junctions are the heart of computers, having literally billions of them. A couple of junctions breaking down can be bypassed but not when millions of them die at once.
The problem with using a corrugated structure like you are talking about is the small surface area of each corrugation. A couple of cm across but meters long. There would by design have to be a large area of pv cells at each focal line and there would not be enough concentration to get up to 200 C.
You mention the solar cookers, I saw one at a festival here and like you say, they are about 1 meter in diameter. On the top of the atmosphere, the sun deposits about 1300 watts on each square meter, but you would be very lucky to get 1000 watts on the ground, probably more like 800 watts max.
The problem with the circular one meter diameter mirror is it is only about .8 square meter (circle is 78% of the square size, 1 meter by 1 meter=1 square meter but a circle of 1 meter diameter is only 0.78 square meter actual concentration area)
Did anyone put a temperature probe in in food in the pot on that cooker? I doubt it got up to 200 C. I know the one I saw was a cubical structure based on lining a big box with aluminum foil and shaping the flaps of the box into reflectors that fed heat inside to a pot.
It was a hot day but the thing didn't even boil water. Your box may have been more efficient than that one, I was not impressed by that particular cooker design.
My electric stove has burners that can apply about 2 kilowatts to a pot on the small burners and one triple burner about 30 cm across puts out at least 3 kilowatts to heat a larger pot but even that takes a long time to bring a pot of about 10 liters to a boil.
To get 3 kilowatts on a pot would take at least 4 square meters of collection area and if you needed to keep it hot for say 2 hours, a mechanism to track the sun. That is one of the weaknesses of parabolic reflectors, the absolute need to track the sun.
That is not as hard as if it were a radio telescope which has to be able to aim in all directions, because the sun tracks a fixed path across the sky, fixed at least for a single day.
That fact allows the design of parabolic reflectors that only has to track in one dimension which simplifies the design but some means of keeping the reflector pointed right at the sun MUST be designed in, maybe like the tilting wheel of a Dobson telescope mount.
A design like that could be made simply so you could manually track the sun and keep the focal point in one place by having a kid stand there and move the reflector every few minutes or maybe some kind of wind up spring assembly that would be tied to some kind of temperature sensor that would keep the temperature as high as possible using the angle of the parabola track as the variable.
The bottom line is, no matter how you go about it, a parabolic reflector MUST track the sun or you get no concentration of solar radiation.
There is another design, accidentally discovered by a guy named Miller, the Miller Trough I think it's called where you track the sun at least in one direction for a significant angle of sun movement across the sky.
His design was discovered one day when he had some aluminum or stainless steel thin sheets stored on the side of his garage but outside. Picture putting a thin sheet of this stuff just resting against the side of the garage, it would bend.
The difference that he discovered, is a variation on a parabolic shape, more like a spiral, a changing angle as it rested on the outside wall at some angle, maybe 30 degrees away from the vertical wall, just a guess since I don't know the exact details.
The gist of it is one day he noticed where the sun concentrated the light on the ground, he came back a few hours later and to his surprise, the sun of course was in a much higher position than 4 hours earlier but the pattern of concentration was the same, heating more or less the same spot on the ground!
The guy's name might be Wilson. Have to goog it.
Couldn't find it, but here is a link to a real solar energy plant using parabolic trough collectors:
Of course that is an industrial sized system but it showed the trough collector can generate 500 degrees C at the focal line! Still needs to track the sun but only in one direction. Also of course these are very large troughs. It would be quite an engineering challenge to build one for cooking though, since you would have to be able to have a fluid go through the focal line tube and then transfer that to an insulated cooking box.
That could be done, maybe not even that difficult on the larger scale of things!
1: track the sun, 2: transfer fluid to an insulated box in a closed loop flow. My guess is 3 square meters of collection for real cooking. Of course it could be used to just generate hot water for bathing also.
That would be a collector 1 meter across and 3 meters long. That would concentrate something like 2500 watts to whatever you are heating, food or water.
The trough could be a balloon covered on the inside with mylar, a long sausage shape, half coated with reflective mylar which would be the reflective surface, since the balloon would be light weight, the mechanism to move the reflective part would be very light weight also.
Just thinking off the top of my head here.