I'd been thinking about a water-based garage solar heating system and come to the conclusion that overheating in the summer was the big problem. But this circulating air idea looks as if it has legs. Surely all you need to do to stop overheating in summer is to close the vents into the garage whilst opening vents into the unit so all the unwanted heat can escape. What would be really k3wl though would be to use all that waste heat to drive a refrigerator/air conditioner.
Nick
I dont see that causing any problem, it just means you'll need a fan, plus a thermostat, or better a garage thermostat plus a diffrential stat.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D| =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 = | =A0 =A0 =A0|------------------------------| =A0 =A0=3D=3D=3D: > :: > :: air flow > :: > ::=3D=3D=3D =A0 =A0 =A0|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| =A0 =A0 =A0|''''''''''''''''''''''''''''''| double-glazing panel, the air space and the plastic sheet should create
NT
I wonder how well it would work to simplify it further. Simply replace some of the siding with the plastic sheet, matt the inside face of it, and hang a silvered plastic film curtain inside. Open curtain for heat, close it for cool.
NT
I meant, it's why I think the angled cloth won't work for me. Because this box is more horizontal, as soon as any air warms up it will rise - towards the glazing above it. I don't see how a wire or cloth mesh will prevent that.
Basically, as soon as the angled mesh layer is the wrong side of vertical, any warm air will be rising through it towards the glazing.
What's the significance of a stagnation temperature?
I want to bounce heat back into the mesh/air flow.
Daniele
The fan means air will be blown through the collector 100x as fast as it could ever rise, so not a problem.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D| > > =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 = =A0 | > > =A0 =A0 =A0|------------------------------| > > =A0 =A0=3D=3D=3D: > :: > :: air flow > :: > ::=3D=3D=3D > > =A0 =A0 =A0|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| > > =A0 =A0 =A0|''''''''''''''''''''''''''''''|
The collector has to survive the stagntion temp on a hot 95F day when youre not running the fan. With triple glazing thats going to be wickedly high, and probably rules out the use of wood or polystyrene.
Well, you'd be better off not doing so.
NT
I haven't understood how, in my scheme, mounting the mesh at an angle will make any difference.
I think it needs some of those expanding-wax-controlled louvres that up-market greenhouses have.
Because?
Daniele
Wickes have a combo heat pump / air con / dehumidifier 830x450x450 on offer at =A3189 currently. It is a mobile unit, needs an 'ole in a wall etc. Just a benchmark costs against.
You may need quite a bit of insulation - Dec-2010 whilst exceptional was a long period of crystal clear nights allowing temperatures to drop away severely. Anything with water in will need protecting against such scenarios and insulation to keep inside what solar energy did fall. There was a lot of daytime solar energy in that period to harvest too.
Ah. I'll try and ascii it clearly.
. _______ . :| . | : | . | : | . | : | . | : | . |: | . _______|
Notice how the cloth goes from corner to corner. All the air goes through the cloth, not just past it. And the slight pressure differential across teh cloth keeps t he 2 volumes of air separate, unheated and heated.
If the air merely goes past the cloth, its less effective at cooling the cloth, which then radiates more heat out. Also the hot air reaches the glazing, and more is conducted out.
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D| > > > > =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 = =A0 =A0 | > > > > =A0 =A0 =A0|------------------------------| > > > > =A0 =A0=3D=3D=3D: > :: > :: air flow > :: > ::=3D=3D=3D > > > > =A0 =A0 =A0|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| > > > > =A0 =A0 =A0|''''''''''''''''''''''''''''''|
I presume there are cheaper temp control options
its better to keep the heat on the hot side of the cloth, where it wont lose heat. If you reflect the light back, some will get reflected right out.
NT
OK, I get it - the top layer of mesh will be hottest, so we especially want the air to travel through this top layer in particular (but through the others too).
A concertina folded mesh might work too to achieve some of this:
air flow ---> /\/\/\/\/\/\/\ --->
but would be hard to make.
Thanks for the explanation.
Daniele
Before you invest too heavily in a complex solar powered winter heating system for your garage you might want to look out of the window and count the number of decent sunny days in a UK winter.
Since you want this to be nearly optimum for the winter months it needs to be south facing and at 60-70 degrees to the horizontal to try and catch as much winter sun as possible. This will also help a bit with the summer stagnation overheating problem. On the roof at a typical pitched roof angle it will be optimised for the summer sun track.
I have obtained my double-glazing panels, so I am going to start experimenting this weekend.
I'm wondering whether layers of shade-cloth or layers of metal mesh would be better as the collector for the system. Metal, I feel, would do a better job, with more thermal mass.
Also, I think that I should aim to get as much air moving through the collector as possible - in which case, perhaps instead of a single collector running corner to corner I'd do better with two or more in series, each arranged so that the air must be drawn through them.
Daniele
Thermal mass in the collector is of no use to you. I didnt understand the 2nd bit.
NT
It will help smooth out the output from the system - by how much I don't know.
Anyway, I got some shade cloth this morning from a hardware store, so I will experiment with that.
Instead of a single set of three layers running corner to corner, three sets of three layers, each one taking care of one-third of the surface area, so that each molecule of air has to pass through nine layers.
But I won't bother; it seems unnecessarily complex and would be extra work for the fan.
Daniele
It wont, but even if it did it wouldnt be an advantage. You get most out and lose least out of a solar panel when thermal capacity is minimised, and the fan or passive airflow stops and starts with the available sun's heat. Delaying cool off by 2 or 3 seconds wont make a noticeable difference in practice. Shadecloth collectors are already an exceptionally good design, I dont think anyone's managed to improve on them yet.
NT
You don't want much thermal mass in the collector. You want something that gets warm enough to be at least faintly useful even in the feeble winter sun. The thing is barely going to work adequately even on the sunniest winter days so you may as well configure it so that it does produce some warm air out at ~20C as soon as is reasonably possible (rather than taking forever to warm up from overnight frosts).
You have got just about everything wrong in what you plan to do. The only thing you have right is the double glazing panel and even that is wasted in this application. Far better to have it output a modest amount of air at a useful temperature - then with a bit of ducting you might at least be able to keep your feet warm. All forcing too much air through it will do is give you a not quite so cold draft!
And you may as well take your air intake for this contraption from the highest point in your garage which is where any warm air you blow into it will end up (assuming here that the garage door is closed).
A 1W 12v battery top up solar panel and a cheap and nasty computer fan will probably blow more air through it than the panel can ever heat in winter unless it is very large indeed.
Regards, Martin Brown
Maximum return with minimum loss occurs when the panel's fed with the coldest air, which is at floor level. That also minimises hot & cold spots.
Shadecloth panels are often used passively, with plastic film dampers to stop airflow when the panel's cold.
NT
It really depends on whether the purpose is to extract as much thermal efficiency as possible from the collector or to warm up the relatively finite volume of air inside the garage to a comfortable temperature.
You could well be right that there is no advantage to recycling the warm air that collects at the top of the garage. I was thinking that it would give the device more chance of working in weak hazy sun. The purpose being to move such warm air as it can produce to somewhere near where you want it.
At our UK latitude and with our damp foggy winters I suspect it is an exercise in futility to build a solar powered garage heater. YMMV
Regards, Martin Brown
I may not get anything very practical out of it in the end, but it will have cost me little and I'll have learned something by doing something.
Daniele
In any given sun conditions, the collector has a certain stagnation temperature. The collector ceases giving any heat when the interior air approaches that T_stagn. The colder the air intake, the more conditions under which it outputs at least some warmth.
Output will always go to the same level, from where the air then stratifies a bit. When heat is needed, the worst offender comfortwise is the coldest air near the floor.
Some of the time yes, and some of the time it'll make a big difference. The proposed triple glazing willl extend its usefulness.
NT
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