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  • Solar Generated Steam

    I've been thinking about alternative energy. I realize that almost any alternative energy plan will be criticized as "crazy" and it has been proved so many times. The oil industry can lower the cost of fuel drastically and low enough to crush competion, and they have done so many times.

    Here is an idea that I've been playing with. It is for a solar steam generator on the cheap. Long frames would be set up in an East/West orientation. Long sheets of clear and thin plastic would be held in these frames and water would be on top of the plastic. The plastic would deform in a parabolic shape by the weight of the water in its cross section. Another sheet of plastic might be required on top to prevent wind from reducing the focusing effect of the water on these channels of water. I've been thinking scales of 1 foot wide and 3 foot wide but I haven't experimented.

    These long frames of plastic would effectively become lenses that would heat water contained in tubes underneath the frames at the focal point. The goal would be to have the tubes underneath reach temperatures higher than 212 degrees and to generate a working steam pressure. These panels could be on roof tops or on open land.

    There are certain problems with the introduction of the water in the tubes and the handling of the steam pressure, but I think it could work. It wouldn't cost very much to set up. Certain modifications would be required to avoid "slugging."

  • #2
    Sorry to burst your bubble but it isn't feasible for one simple reason. Water is nearly opaque at deep red frequencies and near infrared as well as ultraviolet and above. That is why water looks blue. It transmits almost no heat.

    Here is the absorption spectrum for pure water. The higher the curve the higher the absorption. Unfortunately the majority of the useful thermal energy from the sun lies in the strong absorption bands. This is the opposite from solar cells which depend on the visible spectrum because of the bandgap energy of the photons needed to generate electricity.

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    • #3
      Besides that, the "water laying in a trough" shape is backward to focus energy- the convexity would need to face the sun, with the flat facing the "target". Light passing through the water trough would be distributed, not focused.

      And, of course, the fact the sun would rarely be straight overhead, and liquid lenses would be difficult to "aim" in order to maximize efficiency.

      Doc.
      Doc's Machine. (Probably not what you expect.)

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      • #4
        Ok, now that you've shot down the OBVIOUS problems with the idea, how about something positive on the up side?

        My original idea was for a water filled plastic tube column that was stretched in a frame. It would be nearly symmetrical and stood up to face the sun.

        I decided that it would be better to have a long tubular lens than have to maneuver something to mechanically orient it to the sun.

        It wouldn't be much different of an idea to have a long flat sheet of glass with a contained body of water above it. The prime factors being the focusing of the light and the most inexpensive means of doing it.

        I wonder about the problems of wavelength absorbtion. The idea here is volume and I think it would be substancially effective even with regard to some infrared absorbtion.

        Thanks for the input.

        Spence

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        • #5
          Spence, don't let these "experts" sway your ideas. You know, it's a good thing Edison didn't believe people like that either. Keep up the good work, this is why I started the other thread.

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          • #6
            I always thought that a parabolic mirror, focused at a tube filled with sodium or some other agent that would hold the heat well. The tube would make a loop around a heat exchanger (radiator?)which would frovide steam which in turn would run a motor attached to a generator. Old large satillite dishes might be used.

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            • #7
              Originally posted by Rustybolt
              I always thought that a parabolic mirror, focused at a tube filled with sodium or some other agent that would hold the heat well. The tube would make a loop around a heat exchanger (radiator?)which would frovide steam which in turn would run a motor attached to a generator. Old large satillite dishes might be used.

              Yes this is a proven method, and if you want to have a zero maintenence version you dont use steam you use a sterling engine, they are out in space right now humping away 24-7 (no clouds out there!)

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              • #8
                Mirrors work just fine for two reasons. They can be up to 98% efficient at nearly all wavelengths and they also don't exhibit chromatic abberation as lenses do. You can't focus all the required wavelengths to a single focal point with a simple lens because the refractive index of any material is frequency dependent. This means that if the visible light is focused the infrared isn't and vice-versa. You may notice that some manual focus camera lenses have a red dot beside the the infinity focus position. That is the infinity focus position for infrared.

                That really doesn't matter with a water lens as the infrared won't pass through anyway.
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                • #9
                  another problem with that is that your water won't be at the focal point of your parabola (assuming your plastic does form a parabola, haven't checked that, but I'm not sure I believe it). Not to mention reflection from the surface of the water, and aiming etc.

                  I believe the sane way to do it is a bunch of mirrors aimed at a collection point, which is backed up by some blackbody like material. Kinda like what you see in the movie GATCA. Which, I believe is a real solar farm somewhere out in the Mojave.

                  -Justin

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                  • #10
                    It won't form a parabola but instead a catenary which is similar but not the right shape.

                    [edit for sp]
                    Last edited by Evan; 05-16-2006, 12:36 PM.
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                    • #11
                      Solar steam

                      I don't know about going directly to steam in one stage, but you can get water damn hot with regular solar collectors. Maybe a two stage approach where you pre-heat the water though an array of conventional collectors, run it into an insulated holding tank, an old hot water heater. Then pump it into a "boiler" tube array (metal), through a check valve, and use a focused mirror array to heat it to steam. There is no doubt, that with enough focused sunlight, that you could make steam. Wasn't there a solar farm in California that used molten salts of some sort that recently shut down?

                      My question is how are you going to harness the steam? Traditional piston engine?
                      James Kilroy

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                      • #12
                        Sorry to burst your bubble but it isn't feasible for one simple reason. Water is nearly opaque at deep red frequencies and near infrared as well as ultraviolet and above. That is why water looks blue. It transmits almost no heat.
                        To millman's comment:

                        The above is true, but may be irrelevant. Turn it around, and make the "downfall" a feature!

                        The water lense idea is feasible, IF the lense is assumed to be used as a 'feedwater heater", then anything it absorbs is pure gain.... it deposits heat in the water deirctly.

                        The water might be used to focus the remaining light energy on a pipe with a black coating to finish the absorbing of energy.

                        Now, I won't say there is no better way than this, I am sure there is. But this does neatly answer the "nope, aint a gonna work" response.

                        BTW, I believe any glass front mirror, or glass protected assembly will also waste some infrared, you need to use something transparent to infrared, or use a front surface mirror and no guard/cover.
                        1601

                        Keep eye on ball.
                        Hashim Khan

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                        • #13
                          What won't work is trying to generate steam this way. You can certainly capture the solar energy as low grade heat and it would work as a pool heater. To make steam requires a very efficient concentrator. You don't just need to boil the water, you need to superheat it. This requires temperatures of at least 1000 degrees F. It just won't happen with a water "lens", too many physical principles are against it. It absorbs the heat as low grade thermal energy, it doesn't focus properly and the shape will be wrong even if abberation wasn't a problem to name just a few.

                          There is a reason you have never seen such a solar collector making steam and I am afraid it isn't because it's a new idea. There are a number of patents relating to this idea and they have not been successfully exploited.
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                          • #14
                            BTW, I believe any glass front mirror, or glass protected assembly will also waste some infrared, you need to use something transparent to infrared, or use a front surface mirror and no guard/cover.
                            An absolutely trivial way to make a nearly perfect, cheap first surface concentrating mirror of high efficiency is to stretch aluminized mylar over a circular sealed frame. When a small vacuum is pulled in the frame the mylar assumes the perfect shape and as a bonus the focal length is adjustable. Reflectivity of good quality aluminized mylar is around 88% including the infrared spectrum and replacement is cheap and easy. I have thought of using my old 8' satellite dish this way to make a small solar boiler and all I need is a large bag of round tuits.
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                            • #15
                              Oh..............and lets not forget the fact that water weighs over 60 lbs per cubic foot. If it "did" work, might take a lot to do any productive good........

                              Will your "plastic" hold the weight (of say 100 gallons) and withstand the 212+ temp at the same time???

                              Me thinks that we need some new materials that might not yet be invented..........but then again.......maybe they are......???
                              RPease

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