Solar Panel Refurbishment

[Vortex]

A few months ago I was given 4 solar heating panels by the father of a friend. He built them in the late 70's to a CAT (Centre for Alternative Technology) design, basically:

• 1m^2 collector made of sheet copper to which about 10m of 15mm pipe is soldered.

• Collector backed with aluminium foil

• 3/4" plywood box, insulated with Fibre Glass

• A glass window on the front

Pix here:

On a sunny day we reckoned each panel could deliver about 1kW. They were used for hot water for 25 years (obviously backed up by conventional heating). I want to refurbish them with a view to getting another 10 years use.

Q1. The boxes are basically in good shape. The plywood has in places been damp but is not delaminated. What is the best outdoor treatment for this? My inclination is to good ol' creosote (I have a secret stash) unless anybody has a better suggestion.

Q2 I need to repaint the collectors with the blackest matt black paint I can find. Any ideas? Is there paint made specially for such purposes?

Q3. I do not want a glass window for safety reasons (though I suppose it could be toughened glass). I imagine some kind of acrylic or polycarbonate sheet would be sensible. any comments?

BTW the aluminium foil in the original design will be replaced with horticultural mylar film. The water circuit will be autonomous and pressurised, feeding a separate "loop" in an as yet unpurchased "heatbank" type thermal store.

Off to watch the Ice Hockey now....

David.

[David Hansen]

On Sat, 11 Feb 2006 14:36:17 -0000 someone who may be "Vortex" wrote this:-

You might want to ask on uk.environment as well, where some people involved with such things congregate.

[meow2222]

1m^2 might receive insolation of upto 800w. With cold water flow, most of this would end up in the water. As water temp increases, efficiency falls. By the time the water is at the stagnation temp of the panel, the efficiency has dropped to zero. So in short you wont get 1kW, 400w would be a closer guesstimation. You dont need to of course, the 2.4-3kW rating of immersions is to get quick heatup. If they ran all day they'd be rated in the hundreds of watts, like your panels.

The efficiency tail off complicates systam design, with an assortment of options there. Another issue is that your hot water is ready not in the morning, but mid afternoon. Good insulation is wanted to keep it hot till next am.

Alt.solar.thermal is the definitive ng for this sort of thing.

A selective coating would be a bit better than black paint, but matt black certainly works.

One simple way to get round a host of these issues is to use an external reflector to put 2 suns onto the panel. This roughly doubles your max power output, moves your tail off to zero efficiency point (stagnation temp) to a much higher value, thus ensuring reasonable efficiency while heating hot water. It also allows the panels to be of use in poorer weather conditions, thus extending the season of use. If doing this of course the stagation temp will be much higher, so check your plastic glazing is upto that.

Your metal piping will need antifreeze to survive the winter, unless run in draindown mode.

wired glass is an option.

al foil is rather more reflective of IR. I cant see much point in using a non-reflective layer there, if thats what you mean.

heat exchanger near the bottom of the heatbank will help maximise efficiency.

I wouldnt want to pressurise it. No sense encouraging the possibility of leaks on the rooftop. And I cant think why you'd pressurise it.

NT