Solar Warm Air Panel: Part 1

An Eccentric Anomaly: Ed Davies's Blog

About a fortnight ago I decided a little bit of practical making-things activity would be good for me.

The house I'm staying in has very thick stone walls so the temperature is pretty stable; particularly the downstairs stays around 12°C whatever. The LPG central heating warms the air quite nicely and the stone's all boarded over so the surface layers warm quickly giving reasonable thermal comfort but as soon as the heating is switched off the temperature drops back rapidly. Over the last few weeks it's been abnormally sunny and the upstairs, which is less well thermally connected to the stone, has been quite comfortable, around 20°C, most days with not more than a quick boost of electric heating first thing in the morning in the bedroom I use as an office.

The downstairs, with its high thermal mass and small windows, has not followed along. This is moderately unpleasant with the bathroom down there. Of course, the heating can deal with that but using it just feels wrong when there's bright sunlight falling on the outside of the building. What I've therefore decided to do is make a small solar warm-air collector to lean up against the bathroom window to bring in a trickle of heat in the hope of increasing the average temperature quicker than it naturally would.

In the mean time there have been a few distractions. Not least has been the digging of a much needed drainage ditch immediately behind the house:

It'll be filled with pipe and rocks when there's a bit more confidence that the water flows are fully understood.

The bathroom window in question is the taller one to the right of the vent pipe.

Anyway, yesterday afternoon the planets were suitably aligned, I didn't need to do anything else, I had the materials to hand and it wasn't too windy or threatening to shower so I could get on with at least making the basic box for the collector:

The box is 1.2 m on a side. Plan A was for it to be 8'x4' but I had a brain fart and bought 18 mm ply rather 9 or 12 mm. A big 18 mm panel would be too unwieldy to move around so I decided to make it half size for this prototype. The top edge is slightly taller than the others to allow a lip over the transparent front panel to make a reasonable degree of water-tightness a little less problematic.

While cutting the ply backing for this the other day I had an audience:

I've chosen to use Corolux corrugated polycarbonate PVC sheeting for the front because a) it's what the agricultural suppliers had to hand and b) for excellent technical reasons: the panel will be east facing so will receive sunlight at quite an oblique angle for most of the morning. Once the angle of light approaching a surface gets much beyond about 60° a lot is reflected (see my reflection calculator). Using the corrugations will mean that most of the arriving light will find itself meeting a surface more nearly perpendicular to its path with the result that more gets inside the box. Also, more of the light which is reflected will then strike another surface giving a second bite. I hope.

Of course, having got that far (and having added a strip of wood across the top to support the top edge of the polycarbonate PVC) I had to give it a quick try:

Ambient air temperature was flickering between 10 and 11°C. With no absorber, no sealing and quite a breeze blowing the temperature in the upwind corner of the box was 14°C:

You can see from the shadows the angle of the sun. At the diagonally opposite corner things looked a little better; the temperature was 22 or 23°C:

The glove is performing the important function of ensuring that the thermocouple is in the shade.

Next time it's not so breezy that handling sheet materials is a bit of a hazard and it doesn't look like it's going to bucket down in the next half hour I'll silicone seal the corners of the box and screw them together fully then apply some protective stain. Then it'll be a matter of thinking about the airpaths and the solar absorber within the box. More soon, I hope, but the forecast isn't looking too good for the next few days.