You are unlikely to get 200 deg without a selective coating. Asssuming you are using heat-resistant black paint, 150ish is probably nearer the mark. I have had 140, empty. I think I had seen that AES are now using 2-wall polycarb (they used to use 2 sheets of Teflon/Tedlar 'cling film'), and Solartwin certainly used to. The solartwin I installed 10 years ago has slightly discoloured, but by no means milky, 2-wall polycarb on. I hope that helps.

Nick

Edit: Yes, AES say:


''Utilising a high efficiency, selectively coated aluminum absorber plate that is metallurgically bonded to a rhombic copper waterway, the solar collector fins provide superior heat absorption and conductivity, as well as long-term durability.

This absorber is combined with a light-weight, twin-walled glazing that possesses excellent optical properties while creating an unsurpassed outer thermal insulation layer, ensuring that solar energy is retained within the panel for water heating.''

This suggests that 2-wall polycarb would be fine for your self-build panel since (I am guessing) yours does not have a selective coating, and theirs definitely does.

The US site is brilliant. Thanks. Once I've got all the windows and doors done, that's my next project.

Is the question about materials or number of layers.
As for the glazing material, the clearer the better, even allowing for reflection, as this is to do with surface finish, shape makes no difference in practice. Flat and clear is all you need.
As for single, double or even triple, this makes little difference too unless your in an extreme climate (very cold and clear) for two reasons.
1, they only work during the daylight, so no need to limit losses during the night, unlike a house.
2, as the panel temperature rises, so do the losses and it is a non linear equation (actually the sum of two differential exponent equations with the greatest temperature rise at low temperature and the greatest temperature losses at high temperature).

The important thing to remember when dealing with water heating is that you are limited to a narrow band of 100 K, but you need water at the tap at about 320 to 335 K or 50 to 65 C.
The main thing to know is how much water you need to heat and how often you need to heat it. Then work out if you have a large enough area to heat it all, if not you can work out the best way to 'top up' the heat in the tank to the desired temperature.
You may well find that you are better off with a secondary tank that is used to preheat the cold mains water before it enters the water cylinder. Really does depend on your usage and expectations.
Ideally at dawn, you will have a tank that is totally cold and you can reach maximum by the time you need it, start the day with a hot tank and you gain nothing.

SteamyTea - It's about materials, but thanks for comments re layers too.

joe90 - I couldn't find that thread, but would hope to sidestep mould growth by sealing the open ends of the cavities with silicone.

I would use single glass for mine. (an old patio door glass with one skin smashed out)

Conservatory glazing here...

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=4548&page=2

Think it's the one anyway!

''Why would anyone not use selective black coating, as instant prime efficiency booster?''

Because it used to be hideously expensive. Maybe it is less so now?

Thanks Joiner, this was the thread and your comments ref mould but correct that full sun should avoid this problem.

ok fostertom, thanks, but a bit vague. Let's get specific. If you were building your own panels, what would you use ?

Thanks VH. I've been sourcing alu sheeting from the local scrap yard so I suspect that the DIY route will still be cheaper, if not as efficient, but then the budget is tight.
But, by "tubes" do you mean evacuated tubes, and by flat plate do you mean water-bearing pipes resting on a flat collector plate, as distinct from alu fins placed over the pipes ?

By tubes I mean evacuated tubes, by flat plate I mean 8mm copper pipes welded to copper painted fins with 4mm glass 25mm away from the fins.
If you use a drain-back system that stops the pump at higher temperatures or speeds up the pump as the collector gets hotter then you should be fine with a 2-wall polycarb cover, how much is that per m2? or you may be even able to make a PEX pipe collector?
Norwegian company Solarnor http://kbam.geampod.com/KBAM/Reflection/Assets/Thumbnail/17945_3.pdf makes plastic Solar Collectors.

ST - By "SHC" do you mean Spontaneous Human Combustion?

Nick - thanks for the thread link - that's a very pleasing salvage cycle ! Will look out for caravans.
VH - thanks for clarifying. I will be using a drain-back system, and the PEX approach sounds interesting. Have just googled it & it seems to be about 84% efficient compared to copper. Will weigh that against material cost savings.
ST - I think I must have been off sick when they did simple polynomials at my school, but I also spotted the insulant as one of the trickier bits to spec. Am thinking of forming a rigid cavity behind the collector & filling it with vermiculite.

Great! Thanks ST!

Posted By: Taigh CruinnIf you were building your own panels, what would you use ?

That's commercial! but anyone can work it out - get researching and thinking.

VH as usual talks much sense - thanks.