Parabolic Trough Hompage
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Video


(Not the final version ;-))

Building a parabolic trough mirror


It is not that hard to build a parabolic trough collector, even not for me not being all that handy with tools. I got started a few days ago and am trying to just make one and figure out optimizations as I go while online and playing .

Step 1. Calculating the Parabolic Shape

You can use the formula Y=(X*X)/4*f where Y and X are real coordinates and f is the focal length. You will end up with a graph like this:


This is the parabolic shape the mirror should have. If you can manage to make a support for a flexible mirror in this shape you'll be almost done.

Materials
I initialy thought ribbed plastic would be a good idea. It is light, cheap and easy to work with. I came back from that because although processing it is easy, the plastic does not glue easily (lots of chemicals) and is less versatile than wood. Therefore I changed to triplex. I used transparent plastic (PVC) to make the shape to copy.

I recieved mirror material of Alanod, two sheets of 137 cm (4,5 ft) by 150 cm (5 ft) and 210 cm (7 ft). My goal was to make a mirror about 120 cm (4 ft) wide by combining two identical shapes based on the parabolic curve. You can see below that that means you can use the material quite efficiently.


Step 2. Drawing and making the shape
First you graph out the shape on a regular X- and Y-axis. I improvised paper so the measurements will not have been to precise, you have graphing paper that you can tape together (in general I expect the aluminium to correct the curve somewhat)..


Drawing the curve on XY paper



Tracing the shape (easy with transparent plastic)


Example of the plastic shape (that did not make it).



To connect the two halves I first drew a piece of the graph that overlaps both. THis I used to check alignment of the two halves. Then I used smal screws to fix them (this could be done with glue and tacks, quicker and stronger). Then I used bolts and these metal rods to connect the curves. This is easier doen with straight strips of solid wood, but I did not make sure the curves where equally thick, so maybe next time.



This is the end result today. I have not removed the protective plastic of the mirror. You can see the mirror is smaller (I left a margin). This means I can use straight wooden strips to reinforce the ribs some more.

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Next up will be to test the mirror focus and to find a way to rock the trough (as it needs to follow the sun (1 axis tracking)..

Backing up a little..

I found the metal to be costly, and I had a concept of a building kit with mostly wooden parts, so I replaced the metal rods with strips of wood. First I had to make sure the uneven parabolic shapes could be connected in a regular way. That required making right angle wooden supports. They had to be at a precise right angle, and have a line on them showing the same distance from the mirror curve on all of them. I used glue and screws to put them on the curves, but it seems glue would have done the trick (common waterproof wood glue).

With some struggle I got the whole thing together. I may glue on four more strips of wood, but the structure is quite rigid this way. The difficult part is to get started, this would be easier if you can fix the planks in some way first (guess there are lessons to learn from wooden boatbuilding).

Below the result with the mirror on it. The lines show what needs to be done still..The red lines are where additional strips will go, I will probably glue them on. The function to keep the aluminium in place. It might be necessary to add a strip along the curve just to keep the metal to the wood under windy conditions. The blue line is where beams will go to that will support the reciever tube (may be vacuum glass or copper). The idea is to rotate the mirror around the tube, so the connection between the tube and where the water/heat goes does not have to be flexible (the tube will be stationary). The blue supports will also make the whole thing stronger. The yellow line shows where the reciever tube goes.

I will get some electronics that I will try to use to control the whole thing (TI msp430 and triacs of picaxe). To be continued.

Cost estimation

(preliminary)
1. Triplex for curves 80*120 cm 16 Euro
2. Wooden strips 8 * 180 cm 8*3 = 24 Euro 30 cm
3. 24 * 8 cm pieces 0 Euro (taken from rest material)
4. Glue 5 Euro
5. Mirror 30 Euro
6. Tube support 20 Euro
7. Vacuum tube (heat pipe) 40 Euro

Total 135 Euro

Power estimation

Apperture area 1,5 m2
Insolation 600 Watt/m2 (Holland)
Insolation 1400 Kwh/m2/year

Thermal output 900 Watts/hr
Thermal output 2100 Kwh/year

Cost 0,15 ct/Watt

If the thermal energy would be converted into electricity using a turbine (more troughs aligned) with an efficiency of 15% (very conservative) then the cost would be about 1 euro per watt. Using 33% efficiency achievable with state of the art turbines this would be ,5 euro/watt





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