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Another Design For A Cheap Solar Thermal Collector
  • Inspired by the response I received for my previous idea-- A Simple Design For A Cheap Solar Thermal Collector, i am posting a follow-up to the idea.

    The system shown in the first attached image can be used as a concentrating solar thermal system. The object is to achieve a concentration ration of 100:1 using as little expensive materials as possible.
    --It uses an array of transparent plastic or glass spherical containers that act like a spot lenses when filled with water. Each lens focuses a small amount of sunlight at its own focus with a concentration ratio of around 10:1.
    --The focus of this lens would be a small spot, just like a parabolic dish. We have to place a very small dual-axis adjustable mirror controlled by a microprocessor at this focus.
    --An array of such collectors would be used to focus light at another single spot. The effective concentration ratio at this point depends on how many lenses we are able to place in an array.
    --Since the focus lies below the lens, one needs to take care that the lens is placed at a sufficient elevation so that there is enough room for the mirrors to move throughout the day and still focus light at the same spot.
    --This system acts as a cheaper alternative to expensive dish-sterling systems.

    The sun-tracking of this system is shown in the second attached image.

    Advantages of the lens collector over the Dish Collector:-
    --It does not eliminate the need for reflective glass surface completely. But this design will significantly reduce total surface area of expensive glass required, thus reducing the the cost of construction.
    --In the Dish collector, it is required that the entire dish has to track the movement of the sun continuously. This requires giant heliostats. But in the lens collector, only the small mirror at the focus has to be adjusted according to the movement of the sun. It requires cheap microprocessors and small motors that can be programmed easily.
    --The round shape of the lenses provide better protection against strong winds than the large dish shape.
    --With clever design and engineering, I am sure that several lenses can be placed in an array to provide a significantly high concentration ratio. In my earlier design the size of the cylinder was a constraint. It can be overcome with this design and thus provide significantly higher conversion-efficiency.
    --The mirrors and their control system are an additional cost, but this can be justified if very high temperatures are attained because it will greatly reduce the cost of the conversion unit (to convert heat to electricity).
    Collector Design.JPG 67K
    SunTracking.JPG 44K
  • 12 Comments sorted by
  • I like the lens collector. It'd definitely be cheaper than the dish collector - cheaper water containers, fewer mirrors, fewer electronics.

    You could make spherical lenses by putting water in a bag made of flexible, transparent plastic - possibly neoprene. Basically make a clear water balloon; it will naturally form a sphere when you fill it. It would be tricky to get glass blown to the right size and shape.
  • Another advantage of having an array of independent lens-mirror combination is that you could adjust the temperature of the absorber according to your requirement. If your requirement is low, you can take a few mirrors off the focus.
  • Here's a question I have. Take a look at the image i have attached. It consists of 2 concentric hollow transparent spheres. The space between them is filled with water. Will this still work like a lens? We can significantly reduce the weight if it does. If so, how large can we make it? Here are the specifications of a sterling engine system developed by Infinia Solar. It generates 3.2 KW at 4.7 m diameter. Can we make a lens of around 10 sq.m or larger and get around 2 horsepowers per unit? Any suggestions?
    Lens.JPG 19K
  • Why is this tagged as "Announcement"?
  • I'm using the "announcement" feature to temporarily highlight some discussions so that more people can take a look at it.
  • The problem with this is that we need to come up with cost estimates.  It can easily be the case that it is not so cheap after all.  Plus, if you are going to use a lens you really can't beat a fresnel lens for small amount of material usage.

    Here's an idea: (stir) welding, metal spinning and aluminum sheets.  A 4 by 8 sheet of Al is only 30 bucks.  That's less than bathroom mirror and it could turn out to be easier to make.

    .lookup stir welding on wikipedia. Wire welding might be just as suitable though. Basically you weld the sheets together, carefully including leveling the bead to the same thickness as the surrounding sheet.  

    Then put it in a giant metal spinning machine, which can be constructed at low cost from mostly wood.  Spin the sheet to a parabola and then it can either be used right away, or it you want it disassemblable, you can then cut it into segments.

    Another idea might be to use cone-segments of reflective aluminum to approximate a parabola.  An alloy like what is used in aluminum cans could be nice, it is quite durable and springy but also quite cheap (about 4 cents per can I think).  A strip of aluminum can be made into a cone-segment easily.

    The segment shape I am thinking of is to segment the parabola along the axis of rotational symmetry.  The aluminum would be much like rings, then.  If you think about it, actually it may be possible to produce a parabola-cone segment- approximation from a single (big) sheet of aluminum welded together like above (the advantage being that the parabolic dish provides most of it's own structural strength if is a single piece.)  The circular line, if you will, where one cone segment meets another is the same length for each of the cone segments.

    Trying this with a piece of paper confirms that the bending resistance of the material is the only impediment to making a cone-segment parabola approximation out of a single sheet (with a sort of wedge removed from the sheet of course, much like making a cone out of paper by rolling it up if you see what I mean).   The material can of course be sort of creased at the connections between the cone segments to overcome this, i.e. use a little bit of bending to get the paper to become permanently bent at the angle desired in that region.

    A machine which could take the large piece of aluminum sheet and bend it into the approximation of a parabola shape could be pretty easy to make.  A bunch of rollers basically.  Cut out the wedge and put the sheet in the machine.  I wonder if large rolls of aluminum sheet are available? That could save some of the welding needed to produce the initial sheet.

    I think you may be able use a spiral shaped parabolic approximation too.  That might have some manufacturing advantages but I doubt it.

    Of course the aluminum would have to be polished but that is easy to do by machine in a semi automatic way. 

  • This combination of lenses with mirror idea may not be feasible. Once the light rays pass through the lenses they will not be parallel. So the mirror cannot focus it on a spot. It will merely diffuse it in different directions. I had not thought about that before posting this.
  • One potential problem I see with these lens designs is how they scale. To capture a larger cross-section of sunlight, it grows as a cubic function (x^3), whereas the parabolic systems grow proportional to the surface area they're capturing (only x^2). When trying to boil water by concentrating sunlight, a critical minimum amount must be collected for it to work, otherwise heat will escape from the focal point before the temperature can increase fast enough.

    In order to determine the viability of this, it must first be determined what volume of water is necessary to create a enough of a concentration, and what the tolerance is away from a perfect sphere or cylinder.

    I would be curious what it would take to melt down glass and make our own reflectors.

    It would also be interesting if we could make these into smaller modules by somehow insulating the collection point. For example, what if the collector was in an evacuated glass chamber, with all surfaces silvered except for the portion receiving the reflected/focused light?
  • @rajeshbhatsmailbox
    While you're right they will no longer be parallel, it may be possible to have another lens before the mirror that could make them parallel. They'd have to be designed to match the size and distance from the sphere to work.
  • I have been thinking about the collector in evacuated glass chamber idea. How bout using a stirling engine. Paint the surface that receives the sunlight black and then enclose it in an evacuated glass chamber. That may not be possible in the regular applications of stirling engines, since the source of heat is fire. But here we are only using sunlight. If this design increases the output of the stirling engine, mirrors would not really be necessary. We can use just one big water lens with 2 meter diameter.
    Or two lenses if it is really necessary, as explained in my other idea--Two lens design for a cheap solar thermal collector:-

    I don't believe efficiency is a concern since the fuel is free. Efficiency is only a problem if the materials used are very expensive. A water lens should be very cheap and easy to manufacture.
  • Jason, your idea definitely violates the laws of thermodynamics.  The entropy of a system of photons is related to its density x angular spread of direction.  You cannot concentrate parallel rays and then bend them back to parallel while concentrated.

    OP's drawing with the spheres concentrating to a mirror that also turns the light into a perfect laser is not possible, even with lenses, fiber-optics, etc.  Larger (probably accurately curved) reflectors are needed and the tracking will be a nightmare.
  • I like the Auroville spherical solar dish design. The dish remains static and can be set in the ground or hillside and the collector moves to stay with the sun. The focus is a line in line with the sun rays so a tubular collector in evacuated glass would get seriously hot, and the bit of the dish that gets the wind is small. A fluid would be needed to transfer to the sterling motor. Concentration of energy is more at one end than the other, so move the fluid that way.

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