Monday, 20 May 2013

DIY Solar Furnace

It has been about 5 years since I have played about with renewable energy at the house. That first project had started out as more of an engineering exercise come garden feature, than anything else.

The water wheel:
I designed it as a Poncelet undershot wheel as the stream in the river had a very low gradient. The wheel is 12' in diameter and 24" wide and was supplied from the bottom of a dammed sluice gate with a depth of 3'. Looked nice, but only produced about 0.3kw at best. With the odd flash flood, I felt it was too much of a bottleneck within the stream and so was removed & sold. It is now installed where it should be: Supplied by a 4m overshot head. This installation is at Hewletts Mill, Somerset, now in reversed form & internally shuttered to catch the flow. Output is now stated at 2.8kw. Nice!

Having mapped the fall of the stream across my site, I now have foundations in place for a 6' overshot wheel. This I plan to use to drive a ground source heat pump in the future.

The latest project has been the installation of a solar furnace to the house. This is heating the living room at the back of the house which faces NNE, so never benefits from direct solar heating. The panel I built consists of 221 Guinness cans which had the bottom removed and placed in 17 columns, 13 cans tall, within a 8'x4' box. The can tops were cut & twisted to create a swirl effect within and were joined using o-rings. The box was lined internally with insulation and the the whole assembly sprayed matt black.
The flow of air passes down through 9 columns of cans & then returns back up through 8 columns. The upper & lower manifolds were capped off with sheets of matt black aluminium. The front was then covered with a sheet of clear Lexan & trimmed with black pvc soffit board. The assembly is mounted at 25deg from vertical to maximise solar gain during the spring & autumn months.
The air is sucked from the room at the rear of the house through 10m of insulated foil duction by a 20 watt inline fan. This then pushes the air through the furnace and back through another 10m of ducting to return it to the same room. Flow rate is 1m^3/min and the room volume is 80m^3. The fan is operated by a temperature controller via a thermocouple mounted within the exit manifold. There are significant heat losses within the foil ducting which increase as the temperature increases. Here is a photo of the controller showing the temperature at furnace exit & the air temperature entering the room:

The highest temperature I have noted to date was at the end of April with a furnace temperature of 92deg.C & the air entering the room at 68deg.C. Running costs for the inline fan are currently £0.01 for every 3 hours of operation.

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