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  1. Standard member sonhouse
    Fast and Curious
    02 Aug '10 16:27
    http://www.physorg.com/news199962208.html

    Potentially 2X the efficiency of regular silicon cells!
  2. 03 Aug '10 16:11
    With increasing efficiency at high temperatures sounds like it would be useful in a concentrator cell.
  3. Standard member sonhouse
    Fast and Curious
    03 Aug '10 22:58
    Originally posted by jonevery
    With increasing efficiency at high temperatures sounds like it would be useful in a concentrator cell.
    Sounds like it would be pretty useless WITHOUT one! It seems to love 200 degrees C or near 400 degrees F. The ones on the space station don't even get that hot.

    I wonder how close to the sun you would have to go with a space probe to get that temperature on exposed PV cells?
  4. 04 Aug '10 09:19 / 1 edit
    Originally posted by sonhouse
    Sounds like it would be pretty useless WITHOUT one! It seems to love 200 degrees C or near 400 degrees F. The ones on the space station don't even get that hot.

    I wonder how close to the sun you would have to go with a space probe to get that temperature on exposed PV cells?
    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. They are highly reflective. Now all we need is some clever architect to redesign them to act as reflectors so as to keep costs down.
    Suppose the corrugations were instead parabolic 'u' shapes, with rows of these new solar cells suspended upside down in the center of each dip?

    My mum imported some solar cookers for use in an orphanage. The are about a metre in diameter and you place the pot at the focus point. They don't need to be perfectly parabolic, even an approximation is fine as the collection area (the pot) is not an exact point.

    I am sure that a meter or two diameter reflector would get a solar array suspended at its focus up to 200 degrees quite easily.

    If this device is not only more efficient, but costs the same per unit area, it would be much cheaper overall (for the concentrator plus cells.

    The reason we do not use mirror to save costs on standard solar cells is that they are less efficient at higher temperatures.
  5. Standard member sonhouse
    Fast and Curious
    04 Aug '10 12:25
    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:

    http://www.solarpaces.org/CSP_Technology/docs/solar_trough.pdf

    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.
  6. 04 Aug '10 14:23
    Originally posted by sonhouse
    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.
    If the width is not enough, then larger corrugations would be needed, but that would reduce the strength (which is what the corrugations are for). So that idea might have to be scrapped in favor of a dish shaped roof!

    Did anyone put a temperature probe in in food in the pot on that cooker? I doubt it got up to 200 C.
    I do know that they work. You can place a large pot of water in the center and it will boil. I think it takes a while though I don't know how long. So approx 100 degrees is achievable with 1m diameter dish
    I see no real reason why we cant have 4m diameter or larger dishes.

    That is one of the weaknesses of parabolic reflectors, the absolute need to track the sun.
    But because the target can be fairly large, the focus does not have to be accurate. Only approximate tracking would be required - possibly none at all at the expense of efficiency, but then the same applies to current solar panels.
    I think a North-South trough would work better than a dish, but I could be wrong. Maybe an East-West trough would be better?
  7. Standard member sonhouse
    Fast and Curious
    04 Aug '10 17:26 / 1 edit
    Originally posted by twhitehead
    If the width is not enough, then larger corrugations would be needed, but that would reduce the strength (which is what the corrugations are for). So that idea might have to be scrapped in favor of a dish shaped roof!

    [b]Did anyone put a temperature probe in in food in the pot on that cooker? I doubt it got up to 200 C.

    I do know that they work. Y would work better than a dish, but I could be wrong. Maybe an East-West trough would be better?[/b]
    Since the sun moves on an east-west path, the trough would have to be oriented north-south. There would have to be some kind of tracking still but only in one dimension, tilting the trough to follow the sun. After a tilting mechanism has been built, it could be just run by hand, move it to a better position after 15 minutes or whatever.

    I have seen clever non-motorized non-electrical designs that track the sun using fluids that heat up going through small sized tubing going to two plates, could concentrate light, not sure.

    The idea there is it tries to equalize the heat in both plates, if one gets hotter than the other it tilts to the colder side till the heat is equal. No electronics, no motors, at least no electric motors. I guess it could have a few pv cells to self power itself if it had to have electronics and motors. It probably for a home unit only need 20 watts or less.

    It would be fairly simple to make a couple of light sensors, (photodiodes mabye)
    where the same thing would happen, it would seek a position in which the light coming to each sensor is equal. That would be the point at which the solar radiation would be maximally focused.

    However you do it, tracking would be needed but even a 1X2 meter trough would collect a lot of heat. Especially since you are not worried about converting to electricity, where you lose 80 percent to heat anyway.

    The one mentioned in my last post, the pro units giving 500 C are highly polished and optimized and for sure tracking and a lot larger than 1X2 or 3 meters! They also would have means of insulating the top of the pipes in the focal line so minimum heat would escape there and then of course the pipes leading to the power converters would also be highly insulating. You wouldn't need much of that in a simple home system. Of course everything helps.
  8. 05 Aug '10 07:03
    Originally posted by sonhouse
    Since the sun moves on an east-west path, the trough would have to be oriented north-south.
    I am not convinced. I think that a East to West trough tilted to the angle of the suns arc would loose a little light at the ends, but otherwise remain in focus for most of the day along most of its length.

    After a tilting mechanism has been built, it could be just run by hand, move it to a better position after 15 minutes or whatever.
    Obviously nobody wants to have to tilt it by hand. I am sure it would be relatively easy to set up a mechanism based either on a time or light sensors.

    I have seen clever non-motorized non-electrical designs that track the sun using fluids that heat up going through small sized tubing going to two plates, could concentrate light, not sure.
    I like that idea. If plants can do it, why cant we.

    The one mentioned in my last post, the pro units giving 500 C are highly polished and optimized and for sure tracking and a lot larger than 1X2 or 3 meters! They also would have means of insulating the top of the pipes in the focal line so minimum heat would escape there and then of course the pipes leading to the power converters would also be highly insulating. You wouldn't need much of that in a simple home system. Of course everything helps.
    The difference is we are not trying to heat pipes, we are trying to heat a solar array. The target in our case could be much larger - say a strip 50 - 100cm wide. The focus would not need to be very accurate.
    All we are trying to do is concentrate the sunlight so that a) the necessary temperature is achieved for greater efficiency, b) the total cost of materials is reduced (mirrors are cheaper per square meter than solar panels).
    The design doesn't even need to be a dish. A whole array of flat mirrors that reflect the sun to one area - say a panel mounted at the end of the house. Of course the mechanism for tilting the mirrors might get complicated.
  9. Standard member sonhouse
    Fast and Curious
    05 Aug '10 18:43
    Originally posted by twhitehead
    I am not convinced. I think that a East to West trough tilted to the angle of the suns arc would loose a little light at the ends, but otherwise remain in focus for most of the day along most of its length.

    [b]After a tilting mechanism has been built, it could be just run by hand, move it to a better position after 15 minutes or whatever.

    Obviousl ...[text shortened]... t the end of the house. Of course the mechanism for tilting the mirrors might get complicated.[/b]
    If you go east-west orientation, it would do like you say but I think the actual concentration would be a lot lower overall. You would get a lot more bang for the buck to go with the north south idea. Flat mirrors certainly work but they would have to be individually aimed, I don't think you would want the job of engineering that.

    If you are talking about concentrating sunlight to power a super pv cell at 200 degrees C, the best idea would be the dish. There are some really sneaky designs for dishes, using mylar sheets on a large lightweight hoop affair where a vacuum cleaner sucking on a tube inside pulls the mylar into a parabola and it is arranged so when the vacuum pulls the mylar against the tube it stops sucking and thus maintains a constant focus. Of course that takes some energy, but it wouldn't have to be some 10 amp vacuum, something a lot cheaper and weaker would work fine for that. You could probably build a dish a few meters diameter that weighs in at 20 kg or less with the right materials.
  10. Standard member uzless
    The So Fist
    05 Aug '10 22:11
    should just place solar panels close to the sun and then beam the electricity wirelessly back to earth
  11. Standard member sonhouse
    Fast and Curious
    06 Aug '10 02:49
    Originally posted by uzless
    should just place solar panels close to the sun and then beam the electricity wirelessly back to earth
    That way you can lose half the energy which has been beamed back to Earth and then the system performs exactly as good as the non heat converting cells
  12. 06 Aug '10 06:50
    Originally posted by uzless
    should just place solar panels close to the sun and then beam the electricity wirelessly back to earth
    What sort of wireless energy beam would you use? Light?
    Surely the most efficient would be a large magnifying glass between the earth and the sun rather than pv cells to electricity then back to light.
    Or are other beams like microwaves more efficient?
  13. Standard member sonhouse
    Fast and Curious
    07 Aug '10 02:03 / 1 edit
    Originally posted by twhitehead
    What sort of wireless energy beam would you use? Light?
    Surely the most efficient would be a large magnifying glass between the earth and the sun rather than pv cells to electricity then back to light.
    Or are other beams like microwaves more efficient?
    Microwaves are the energy of choice. It's really the same energy, just a lot lower frequency. The main thing is, the atmosphere is more transparent to UHF radiowaves or microwaves.

    During a solar maximum cycle (we are in a dip right now), the ionosphere gets attacked by huge ionized mass ejections from the sun, and when it hits Earth's atmosphere, it causes, among other things like northern and southern lights, the ionosphere, about 50 to 100 miles up, it turns into a radio mirror.

    But not for microwaves. Lower frequencies, like 200, 300 Mhz, can bounce back to Earth after being beamed up into the ionosphere, also way lower frequencies than that, 10, 20, 30 Mhz. So these signals bounce around under the ionosphere and you can pick up that signal 12,000 miles or more away with ease.

    Microwaves just slice right through the ionosphere like it wasn't there, solar minimum or solar maximum, those waves could give a crap about that, the ionosphere is transparent to those frequencies, 1 Ghz, 2 Ghz and the like.

    Japan already has on the drawing board plans to do exactly that. Orbit large PV cell arrays, turn that energy into microwave beams, aim them at the Earth from a geostationary orbit, and have maybe a couple square kilometers of receive antennas that turn that energy back into electricity with high frequency diodes. So they think the idea will work fine. It would for sure be green energy, no CO2, etc.

    If you worry about birds getting fried, or people, the density of the beam is spread out over a very large area and as long as you don't set up shop and build a house there, you can walk through such a beam and get a lot less energy in your body than say a cell phone glued to your ear
  14. 07 Aug '10 07:27
    Originally posted by sonhouse
    Japan already has on the drawing board plans to do exactly that. Orbit large PV cell arrays, turn that energy into microwave beams, aim them at the Earth from a geostationary orbit, and have maybe a couple square kilometers of receive antennas that turn that energy back into electricity with high frequency diodes. So they think the idea will work fine. It would for sure be green energy, no CO2, etc.
    What are the benefits? You implied earlier that about half the sunlight is lost on entry through the atmosphere. Does this mean that you only need half the area if the array is in space? Is it really worth the extra effort of putting it in space and microwaving the energy down etc?
  15. Standard member sonhouse
    Fast and Curious
    07 Aug '10 09:36 / 3 edits
    Originally posted by twhitehead
    What are the benefits? You implied earlier that about half the sunlight is lost on entry through the atmosphere. Does this mean that you only need half the area if the array is in space? Is it really worth the extra effort of putting it in space and microwaving the energy down etc?
    The advantage of putting PV cells in space (geostationary orbit 22,400 miles high)(35840 kilometers up) is you almost always have sunlight, the amount of dark time is very short compared to being in sight of the sun.

    PV cells on Earth of course has to deal with not only night time where you get zero energy from the sun but even in the day hours, only about 4 hours or so gets maximum energy even if you track the sun because at sunrise and several hours after, the sun's energy is traveling through a large slice of atmosphere and after noon towards sunset, the same thing.

    So the gist of it is, orbiting PV cells get around three times the energy on a 24/7 basis than the same cells on Earth.

    Orbiting in high orbits like Geostationary altitude. In low orbits, there is regular day/night cycles. Even there, as soon as the sun comes up there is no atmosphere so tracking PV cells get maximum light all the time the sun is visible.

    The higher up you go, the less dark time you get and at geostationary orbits you get something like 95% daylight.

    You can figure that % # pretty easily using geometry.