There’s something very cool about parabolas. What I really want is a solar furnace to heat water and incinerate weeds.  

Rob over at cockeyed made a “light sharpener”  with a ton of mirrors.

I thought I’d take a simpler tack and start with a parabolic trough.  Because it focuses light along a single line rather than a single point it’s not as lethal as a dish – but initial results are just swell.

Step one was to work out the size parabola to use.  Most materials come in 600mm or 1200mm wide so I figured I’d go for around 600mm wide.  I wanted the focal height to be 50mm so just used a standard calculation of

x²/4p where p = focal height.

In retrospect this made a reasonably thin parabola and I think a more shallow trough wouldn’t require the same degree of critical alignment with the sun.

At 300mm out from the very bottom of the parabola, the height is 450mm.

 

 

 

focus point	50
x	y
25	3.13
50	12.5
75	28.13
100	50
125	78.13
150	112.5
175	153.13
200	200
225	253.13
250	312.5
275	378.13
300	450

 

Step two was to mark out a pattern using these dimensions.  I plotted out the parabola on a piece of 3mm MDF we had lying around.

Next I used the pattern to mark out some flooring board and got my partner to cut 3 identical “negatives” of the pattern.

Step three was assembly – hang the whole thing together with some fence palings.  Luckily I have a good friend in the engineering trade and I was able to source some mirror stainless.  We screwed this into place and put a length of black PVC tube through the focal point – attached the hose and started it up.  Unfortunately the PVC tube melted and disintegrated before I got to take pics of this step.  Now before you think I’m a complete idiot -  I do realise that it was going to get very hot and plastic melts – but I was running cold water through the pipe and it was after 6pm at night… I thought it would last the 10 mins or so I wanted to run the test for.  Nevermind – I took the meltdown as a complete success!

We found a length of copper tube and replaced the PVC pipe:

I’m going to paint this black in the future.  Now with the trough angled at the sun that copper tube got VERY hot.  Too hot to touch.  Excellent.

We hooked up the hose with a very makeshift fitting:

and angled it towards the sun again.  

Experiment One: I first turned the tap on at a dribble – the water was VERY hot – so hot that it was hard to let it dribble over your hand.

Experiment Two: Turn up the tap to a slow stream – the water was tepid.  By this time the water coming out of the tap was pretty cold, but after racing through the parabolic heat sleeve – it’s definitely warm.

Upcoming experiments -

Build a tank to hold the water and pump it through the trough in a circuit.

Build an oil tank and run a copper coil inside an old water cylinder to act as a heat exchanger.  If this works it might be worth hooking it up to a pretank before our instantaneous water supply.

If nothing else, this awesome free hot water will be great in the greenhouse this winter to heat the nutrient mix.

Have you built one of these?  Any pointers?