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Help forming concave solar dish...

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  • Help forming concave solar dish...

    I ended up with several sheets of mirror polished stainless, about 22-26 ga., I saw once a focuser for a solar array that was cut into a spiral and pulled into a distance that focused the sun on a center point kinda like a concave dish.

    ANy clue how they figured the spiral cut?

    I have already bent a hot-dog cooker and it works great. Just a light bend with tig rod holding hot-dog.. The grease from the dog spoils the mirror thou..


  • #2
    Get a low fat dog.

    John S.

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


    • #3
      John.. ya crack me up..

      I got about 50 of these sheets thou... I don't eat 50 hotdogs at a time.

      Solar collector? Solar air conditioner?



      • #4
        Make a frame to hold some of them (you didn't say what size they are) and set them outside so they will reflect light into the house in winter. This works especially well for rooms with north facing windows. I do that with a 2' X 4' mirror on an adjustable stand about 40' outside of my living room. The mirror catches the sun for over an hour each sunny day and contributes about 500 watts worth of light. It really helps keep the housplants happy during the winter months.
        Free software for calculating bolt circles and similar: Click Here


        • #5
          Actually, I was thinking of a air conditioner. Expanding a freon replacement type gas. Perhaps a solar fan in the other end. No outside power used.

          We built a swimming pool heater using a fresnel lens from edmund scientific. I put a 3/8 or 1/2 plate (don't remember) and coiled a copper line up on it. It had less forward resistance then the filter so it flowed.

          Johnny used two freon tanks with shutters to track the sun.

          Evan, they are 24"x48" and thin.


          [This message has been edited by ibewgypsie (edited 04-26-2004).]


          • #6
            Figure out the power requirement for what ever you intend. You can start with the assumption of approx 1000 watts per sq metre at noon on a clear sunny day. Unfortunately, stainless steel has poor reflectivity, even when mirror finished it is only around 63% so there is a considerable effciency loss there.
            Free software for calculating bolt circles and similar: Click Here


            • #7

              I would suggest using a plywood frame to build up the shape. Create a number of radial ribs with a cross section in the shape of a parabola. Check with some drafting books for how to lay out a parabola. If I remember correctly, you just need a large framing square and a straight edge.

              I've been thinking about building something similar for some time. Your idea of using freon, or something similar, for a working fluid is interesting. My original idea was to use water to make steam, but I think you're on to something better.

              You will need quite a bit collector to generate any significant power, however. The strenght of sunshine is about 1000W/m^2 at the earth. The atmosphere absorbs a significant amount of this solar power, with only a few hours a day where useful amounts can be collected. My rough estimate is that you can collect about 1kw/day/m^2 with a thermal setup. More in the high desert SW, less up in NW foggy Canada.

              A solar air conditioner may be a use that proves to be a good match as air conditioning use is maximum during the middle of the day when the sun is brightest.

              I think if you examine this problem in detail you will find that it is quite involved. There are issues of support stuctures, tracking mechanisms, maintence, etc.


              • #8
                Grab one of those 10 ft satellite dishes at the dump. Tile the curve with 2" squares of foil tape. Set up the equitorial mount, find the focus, and build a thingy for the hot dog cooker. If you really want to get fancy, drive it with a 24 hour movement. A 10 foot dish in full sun might develop 6 kW of heat.


                • #9
                  You guys never seen a refrigarator run on heat?

                  The compresser is just a tank with one-way and orifice, the pressure drop is where the heat exchange is taking place.

                  All Rv refrigarators (I have saw) work like this. When running on electricity, it has a heater on the bottom of the "compressor" tank. No real compressor.

                  The Fresnel lens we had in Florida cost $150 back in the late 80s.. it would make 2300(oops, typo) degrees in focus point.. Not enough collection area thou.. BTU per square foot. Hence more than one..

                  You got some interesting math formulas, I'll have to look into.

                  I still want a spiral cut concave reflector.


                  [This message has been edited by ibewgypsie (edited 04-26-2004).]


                  • #10
                    I once glued a piece of space blanket to a hoop, put a backing on it, and used vacuum to pull it into shape. I wonder if you could get the stainless to do the same without buckling, and maybe it would retain some concavity once the vacuum is lost. That would get you a long focal point, though. I kind of have to agree that the stainless sheet is not the best material of choice for your application. It needs good reflectivity across the visible and into the infrared spectrum to gather enough useful energy. Space blanket beats it by a mile, although if you factor in the problem of keeping the surface clean and reflective, the stainless might come out ahead in the long run. I think you're better off using some greenhouse reflective mylar sheet, same as space blanket basically, and using the stainless for something else.
                    By the way, marking out the spiral cut is easy. Attach a marking tool (pencil) to a string, attach the string to a central dowel pin. As you draw going around in a circle, the string will wind up on the dowel, and decrease the radius of the spiral smoothly. The constant width of the spiral will be equal to the circumference of the dowel. Of course, you still have the problem of neatly cutting along that mark without bending up the sheet into unuseability. If you were to affix the dowel to the stainless disc, and pivot that assembly on a nail in a temporary near-vertical framework, you could rotate the sheet by hand while the string, or wire, slowly raises a cutting torch tip. I would certainly do a dry run to see that this makeshift mechanism operates without a glitch. The string or wire would have to wind up in a smooth layer without overlapping.

                    [This message has been edited by darryl (edited 04-26-2004).]
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                    • #11
                      I'm not sure from your description but this may already be what you made.

                      Instead of making a parabolic mirror like a satellite dish, consider making a trough shaped mirror so that the cross section is a parabola then run a black rubber hose, black pipe, or your favorite brand of hot dogs along the focal points of the curve in a straight line supported by standoffs.

                      Heat won't be as intense at any one point but you have a more efficient way to collect the heat. It's also a heck of a lot easier to make than pounding out a 3-D parabolic section.


                      [This message has been edited by pgmrdan (edited 04-26-2004).]


                      • #12
                        I like the 10 foot dish idea but I would line with your material, find the focus point and use it to heat water.

                        Paul G.
                        Paul G.


                        • #13
                          Go sparse array. Make a frame and sun-tracking system for each of your remaining sheets, set them up individually in a large field, and have each focus on your collector.

                          Or just make your own chrome-look DeLorean. Whichever.

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


                          • #14
                            There is a plant in the desert that has a tracking array of mirrors to heat a boiler high on a tower. You could build your own power plant with a steam turbine.


                            • #15
                              Geez David, you're gonna turn into one of those metalshaper guys. Find someone with an English Wheel. You could whip those sheets into a dish in less than 1/2 hour. You could use mine but you're a little far away.

                              To calculate the focal distance, all you need is the diameter and the depth of the dish. Focal distance is equal to the diameter squared (D x D) divided by 16 times the depth (16 x d) or:

                              D x D
                              ------ = Focal Distance
                              16 x d

                              This is what I used to calculate the FD for my satellite dish. (Got it off the web somewhere) It should work for any parabolic dish.