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.

Monday, March 16, 2015

Making a rabbit trap for the organic farm

March 2015: My farming friend John Macdonald has a problem of a rabbit in his polytunnel on the PIP organic farm, so I resolved to make a large scale model of the mouse catcher. This is entirely made from plywood from a skip, except for the duct tape and the small quantities of aluminium and polycarbonate which are all recycled from previous solar panel projects.

Forming the basic box shape using 8th inch ply and black duct tape.

 A tilting door at the end composed of polycarbonate and small length of hardwood with aluminium brackets.

When a rabbit walks in to the box, it tilts and closes the door which then locks using small aluminium catches. The window allows one to check to see if there is anybody inside the box ha ha.

As an April afterthought, I thought I would ask John whether he has actually caught his rabbit from this. I will add it to this posting, although when I last asked, he thought the rabbit had actually left the polytunnel. With this one spring weather, it is possibly getting too hot for the rabbit and there is no supply of water in the polytunnel.

Tuesday, March 10, 2015

Solar Dehydrator

9 Mar 2015: For some weeks now, I have been working on building a solar dehydrator. This is far too big for my garden, but it is being built in my garden and will be transported to Woodbro', to the organic farm (PIP) of John Macdonald. The whole device stands 3 m high and is designed for drying out herbs and tomatoes. As the solar tube warms up it draws in from the bottom louvre and that rises up through the drying chamber and out through the roof. As the air rises it passes through as many as 30 perforated trays which can contain herb leaves or drying fruit or drying tomatoes and then exit through the roof.
   In this image, it is fully modelled in ArchiCAD and GDL and is being built precisely according to the model, And if I make small on-site variations in the real thing, I go back to the model and make sure that it represents fully the final object. 
    This will be on a turntable so that it can be rotated to track the sun. The folding mirror will ensure that maximum sunlight enters the solar tube. The tube will contain black painted absorbers of thermal mass phase change wax (Rubitherm) which will cool the tube in hot weather and produce warmth when the sky is grey or the sun has set. The top panel of the solar tube is designed to be an ETFE panel which we are hoping to get from Holscot or Evolve. Holscot kindly donated the front panels for my Surya 3 solar panels. The improved performance of my solar Sunbox has proved the efficacy of ETFE in a solar thermal panel. 

Below is an early 2D drawing of the design, but it has since been 3D modelled, down to the last bolt and bracket, and the final 2D drawings for construction are taken from the model.

I have taken progress photos during the construction and will post a fuller set as a different article in the blog.

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