Green Building Forum - Glazing of DIY solar collectors

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 13:03:13 +0100

I'm planning to construct some roof-mounted solar panels for heating water, on a drain-back system. I've been advised that the temperature of the absorber plate & pipes could reach around 200 deg C, and that a design temp of 250 deg C should be allowed for.
We are quite close to some trees so I'd prefer to glaze the panels with UV-stable perspex or polycarbonate, rather than glass, to avoid breakage in high winds, and also to allow the possibility of curving the panels for optimum collecting efficiency. Many of the DIY guidelines on websites seem to advocate using perspex or polycarbonate, with the only qualifiers being the tendency to scratch or discolour. However, neither material is stable much above 150 deg C, which is hard to reconcile with the potential temperature within the panel.
So, if temps of around 200 deg C really can be reached, and if the non-glass glazing really does work without melting/slumping, this can only be because the air temp within the panel is substantially lower than the surface temp of the absorber plate & pipes.
I would welcome any informed challenges to these assumptions, or comment from anyone with experience of using a non-glass glazed panel on a drain-back system.
Thanks !

Glazing of DIY solar collectors

Nick Parsons — Fri, 05 Aug 2011 13:33:05 +0100

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.

Glazing of DIY solar collectors

djh — Fri, 05 Aug 2011 13:51:32 +0100

For building your own panels, I'd suggest looking at the following sites if you haven't already:
http://www.builditsolar.com/
http://www.solarverein-trier.de/

And obviously, build a test panel first. :)
BTW, I think it is possible to buy selective absorbers or coating material if you want it.

Glazing of DIY solar collectors

Joiner — Fri, 05 Aug 2011 14:54:50 +0100

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

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 15:10:13 +0100

Thanks Nick & DJH - that's v encouraging.
I'm planning to use heat resistant matt black paint, but nothing more sophisticated so, as you suggest, the twin-wall polyc. should be fine in terms of temperature.
Thanks for the links too. The builditsolar one seems particularly comprehensive. The solarverein site will have to wait until my in-house translator gets home !

Glazing of DIY solar collectors

SteamyTea — Fri, 05 Aug 2011 16:05:04 +0100

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.

Glazing of DIY solar collectors

joe90 — Fri, 05 Aug 2011 16:06:13 +0100

The only problem with twinwall poly is mould growth inside (see thread on conservatory roofing)

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 16:30:25 +0100

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.

Glazing of DIY solar collectors

djh — Fri, 05 Aug 2011 17:06:41 +0100

I don't think you need to worry too much about mould growth, since it'll hopefully be in full sun and at quite a high temperature with no source of moisture. There are special tapes for sealing the ends of the polycarb. I think the one at the top may really seal it, but the one at the bottom is a breather to allow for expansion and contraction as the air inside heats up and cools.

I couldn't find a thread either.

Glazing of DIY solar collectors

tony — Fri, 05 Aug 2011 17:08:40 +0100

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

Glazing of DIY solar collectors

fostertom — Fri, 05 Aug 2011 17:16:17 +0100

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

Posted By: SteamyTea2, 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)

I'm sure there's an important truth there - but can't you teach your students (e.g. me) in a way they may grasp?

Posted By: SteamyTeaAs 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

It's often v cold daytime too. Re-radiation loss (not lack of insolation) is what makes winter collection a waste of time, according to conventional wisdom. Max insulation of front glass, plus selective coating, plus varying flow rate so as to run the panel always at lowest poss temp, plus drainback as soon as the sun goes in and the pump stops - all help.

Of course increased resistance to outward re-radiation by multiple glazing also increases resistance to incoming radiation - but not necessarily pro rata, so careful selection of glass/coating spec reqd. Maybe that's what 2, above was about?

Glazing of DIY solar collectors

Joiner — Fri, 05 Aug 2011 17:17:39 +0100

Conservatory glazing here...

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

Think it's the one anyway!

Glazing of DIY solar collectors

skyewright — Fri, 05 Aug 2011 17:19:33 +0100

Posted By: SteamyTeaAs for single, double or even triple, this makes little difference too unless your in an extreme climate (very cold and clear) for two reasons.

The important thing to remember when dealing with water heating

Would the same logic re. single or double not making much difference apply if it were solar air heating rather than solar water heating?

There are some very interesting air heating projects on BuildItSolar, and for at least some of the designs air that passes next to the glazing is used.

Quite a number of the designs on BuildItSolar use single skin corrugated polycarbonate, but others use twin wall.

Glazing of DIY solar collectors

Nick Parsons — Fri, 05 Aug 2011 17:19:53 +0100

''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?

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 17:36:38 +0100

Joiner - thanks for link. Hopefully not a problem then, on south facing bit of roof.
fostertom - re selective black coatings - are they really effective & can you recommend one ? This is an interesting experiment http://www.builditsolar.com/Experimental/PaintAbsorbancy/GlossvsFlat.htm which suggests maybe there's not much between them & standard matt black.
- re Steamy Tea's erudition - well done - I thought I was being thicker than usual

Glazing of DIY solar collectors

joe90 — Fri, 05 Aug 2011 18:58:43 +0100

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

Glazing of DIY solar collectors

fostertom — Fri, 05 Aug 2011 20:53:18 +0100

Posted By: Taigh Cruinnselective black coatings - are they really effective & can you recommend one ?

Posted By: Nick Parsonst used to be hideously expensive. Maybe it is less so now?

They range from aerospace-grade electroplating, down to aerosol spray for exhaust manifolds. Lots on the net about it, incl comparative studie(s) - check it out.

Not true that they're 'little better than matt black'. Ordinary matt black has mid-range emissivity, 40-60%, like most building materials incl ordinary white paint. Photographic optical black is in the 90-98% range. Selective coatings similar.

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 20:59:06 +0100

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

Glazing of DIY solar collectors

Viking House — Fri, 05 Aug 2011 21:17:46 +0100

Using ready made factory produced absorbers @â‚¬55/m2 I found is cheaper than making your own and you get a very high quality product. These can be made any lenght and are fitted together in racks so the complete roof can be covered. I should have some photos soon as we are making 4 solar roofs at the moment in sizes ranging from 20m2 to 60m2. We're covering the collectors with 4mm tempered low iron glass which has a G-value of over 90%, costing â‚¬27/m2, this should be resistant to falling branches! If you collect at low temperatures (35-45 deg C) then flat plate collectors are more efficient than tubes and you can cover the complete roof.

Glazing of DIY solar collectors

Taigh Cruinn — Fri, 05 Aug 2011 21:46:12 +0100

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 ?

Glazing of DIY solar collectors

Nick Parsons — Fri, 05 Aug 2011 22:30:25 +0100

If you are sourcing alu from the scrap yard you obviously have not heard of the Nick Parsons caravan to solar panel virtuous circle! I think it does appear in another thread

Edit: Yes, it's this one: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=5868

Glazing of DIY solar collectors

Viking House — Fri, 05 Aug 2011 22:41:21 +0100

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.

Glazing of DIY solar collectors

SteamyTea — Fri, 05 Aug 2011 23:58:31 +0100

Have devised an experiment and will report back in morning, too late for me now to explain summation let alone differentiation of exponents (let's face it we all struggled with it on simple polynomials at school).
But as for materials, the main one that will give trouble is insulation, tends to soften at temperatures above 60C if using PU or Polystyrene. Timber, metals and glass do not give a problem though. Dissimilar metals can cause expansion and contraction issues, hence never solder ST pipework and make sure it is flexible somewhere.
As for heating the air via solar, this is a very valid method, it does have storage issues as air has a low SHC and is 'stored' in the house that has a large surface area. It is also free to move about willy nilly and will transfer heat from the hottest to the coolest parts quite rapidly if left to its own devices. Water has a high SHC and is stored in a small cylinder that can be easily insulated and controlled. See the difference?
If you use acrylic sheet (Perspex) make sure it is the cast variety and not the extruded or you will have 'waviness' problems.
Finally for tonight never use ordinary or laminated glass, toughened is best just in case if gets broken in a storm, then only tiny bits fall to the ground.

Glazing of DIY solar collectors

Joiner — Sat, 06 Aug 2011 07:22:05 +0100

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

Glazing of DIY solar collectors

SteamyTea — Sat, 06 Aug 2011 07:39:44 +0100

Yes Joiner, it is all magic, smoke and mirrors

Glazing of DIY solar collectors

Taigh Cruinn — Sat, 06 Aug 2011 08:47:37 +0100

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.

Glazing of DIY solar collectors

SteamyTea — Sat, 06 Aug 2011 08:50:55 +0100

Here is a chart of my kettle boiling up 1.5kg of water and the subsequent cooling after. You can see that it heats up very fast (the positive gradient) and cools very slowly (the negative gradient). The important thing is that as it is heating up there is also an element of cooling, or heat loss. This cooling as it is heating can be calculated (would be easier if I could have logged at a1 second interval, but no matter) from the cooling side of the chart (the negative gradient).
Take any fixed temperature rise, say 5 C and calculate the time taken.
Then calculate the time taken to cool over that exact same temperature range.
Take the Heating time from the Cooling time and you have calculated the net energy gain.
Do this for all sequential temperature rises and you can calculate the temperature gradient, you will find that it is not a straight line (where y=mx+c) but an exponent function where y=e^-kt, where k is the system properties and t is the time.

Alternatively you can just look at the chart and get a feeling for what is happening, as the temperature reaches maximum the temperature rises slows because the heat losses are greatest.

Glazing of DIY solar collectors

Viking House — Sat, 06 Aug 2011 09:22:08 +0100

Great! Thanks ST!

Glazing of DIY solar collectors

Taigh Cruinn — Sat, 06 Aug 2011 10:26:03 +0100

ST - please be patient with a pedestrian here, but what would you say are the practical implications of the temperature gradient for the design and/or use of a simple solar collector ?
The one thing I can spot is confirmation of your earlier point about it being better to be starting with a cold tank.

Glazing of DIY solar collectors

fostertom — Sat, 06 Aug 2011 11:02:35 +0100

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.