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What to do with my garage roof?

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Posted by NickW on July 2, 2003, 7:23 am
I have a flat roofed garage next to my house. It's about 50 square
meters, made of corrugated metal (probably galvanised steel). This is
crying out to be some kind of solar collector. I live in the UK so
house heating is required about 8 months of the year. I also have a
pool (although I already have a black pipe coil to heat that).

I thought I could:
- Paint the metal black
- Buy some transparent corrugated sheets and build these above the
metal, leaving an airgap.
- rig up two large diameter air pipes going from the house to opposite
corners of the 'collector'
- have a fan to suck the air out of the collector, possibly activated
by a temperature sensor inside the collector

Does it sound like this would work?

Suggest any refinements?

In the Summer, could it be used to heat the pool if you bubble warm
air through the water?



Posted by Steve Spence on July 2, 2003, 10:15 am
at 51 degrees north latitude, you need a tilt from 50 to 65 degrees in order
to collect the suns heat efficiently.

Steve Spence

Posted by NickW on July 2, 2003, 12:33 pm
Yeah but if we are including cost and visual impact into the equation,
making some sort of raised surface to get the right angle of tilt may
not be worth it. At the moment, part of the collector surface already
exists, just needs to be painted black.

A rough estimate is that the polycarbonate and fixings would cost
about 300 pounds. I would use an existing extractor fan in my
downstairs bathroom to suck the air through the system.

I calculate that on a sunny day I'm getting up to 6kw from my 25sqm
solar collector for the pool (which lies flat on the ground BTW). In
the winter, if the available sunlight is halved, the garage roof
system should produce about the same because it's double the area.
That's not a bad contribution to the central heating is it?


Posted by Nick Pine on July 2, 2003, 12:57 pm

How about a greenhouse aka polytunnel with a transparent south side and
a whitewashed north side? Water-filled poly film ducts might store heat
and make a good ballast "foundation" with no need for holes in the roof.

Sounds too small. Removing say 100 Btu/h-ft^2 with a 10 F temp rise requires
about 10 cfm/ft^2.

...76 Btu/h-ft^2 peak? A solar collection efficiency of about
30% in full sun at a better right angle with warmer air?


Posted by NickW on July 4, 2003, 8:35 am
 Bah, after looking at my site yesterday evening, I realised that the
house is taller in relation to the garage than I thought... with low
winter sun, the garage roof might be at least partially shaded for a
large portion of the day.

I still want to do the forced air heating. Maybe I'll make some free
standing panels that can go on the ground (on frames at the correct
angle this time) on the south side of the house and stowed away in the

Anyone got any inxepensive designs for such panels?

a 10' x 26'' sheet of clear polycarbonate is ~10 pounds sterling.

Is it best if the absorber is metal? Another corrugated roofing sheet
would do the trick.

I presume the back should be insulated? Some sort of fibreboard to
also give it rigidity?

The airgap should contain baffles to direct the air over the whole
absorber surface - best to make the width of these air channels to be
the same cross sectional area as the intake vents?

Thanks in advance


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