Friday, March 27, 2015

Devising an optimal arrangement of black tubes

March 2015: We intend to make the solar dehydrator work better using a bit of Thermal Storage, a bit of Eutectics. The solar tube needs a thermal absorber, and it needs to be black, and we have been thinking about some steel rods (too thin), an old radiator (like in my Sunbox), some steel mesh (bothersome to paint black) and some black bricks (too heavy). One of my colleagues has kindly donated some Rubitherm thermal wax from an old project of his. I would have liked wax with a phase change of 25-40 degrees, but this wax is graded 58º.... a bit too high but we do not turn down donations. I decided to buy some 40mm black plastic waste pipe. Easy to cut and join, using push-fit joints.

I have 50 kilos or wax here, and the quantity is enough for two solar dehydrators. Each of these sacks will do about 25 metres of 40mm piping (using a simple calculation of PI*R^2 on the pipe).

The design problem was how to get 25m of piping inside a cuboid volume of 250mm x 900mm x 1400mm. There must be access to sunlight for the pipes, air space around the pipes, and transportability. It must be refillable, and leakproof. Using a combination of Tee-joints, Elbows and 135º joints, I devised a 3-dimensional space frame - but it was too difficult to do on the computer, so I bought a load of joints and some piping, and spent an evening experimenting.

Having devised some sort of piping for one end, it then became clear how to do the other end, and I needed to buy enough joints and straight piping to make the final length correctly.

Still a bit confused and needing yet more joints to make it complete. Another requirement is that when the granules of wax are poured in through the top pipes, they will efficiently fill the whole tube-grid without air gaps. I am not clear about whether the wax melts into a single liquid body, or whether it stays as granules. 

Here is the finished 3-D grid when it lies flat for transportation.  To this has now been added a small aluminium subframe. The final length of pipe is near enough to 25 metres.Here is the grid standing up in 3-D to give a finished height of 260 mm.  The length is designed to allow the bottom louvre to open and close.

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