Responses interspersed:
So, I've just had a 32X60 foot steelclad pole building erected.
Congratulations on your erection
Thanks :>)
I intend to insulate it with fiberglass batts (6" walls), and to put in
a concrete floor.
With insulation under the floor, or at least the perimeter?
--Floor is yet to be poured. If it is advisable to add insulation, I
can do so. I expect that rigid polystyrene foamboard could be used?
It will be a storage barn/vehicle workshop.
It may not need much heat at night, when the floor stays warm and
wastes
energy. C = 4"/12"x25x32x60 = 16K Btu/F and G = 32x60/R20 = 96 Btu/h-F
for the ceiling + 1840/R20 = 92 for the walls makes, RC = C/G = 85
hours.
A slab that's 70 F at 5 PM on a 30 F day might cool to
30+(70-30)e^-15/85
= 65.3 by 8 am, but it won't store much heat for a few cloudy days.
...I want to use the south-facing side of the roof as the solar collector.
What's the slope? Walls are better for winter heating. Where do you
live?
--Roof is a 4 in 12 slope, IIRC. I live in central Alberta, latitude
is about 51° N. We tend to have a lot of sunny days in Winter, but it
can be windy. If the solar heat system can take some of the chill off
the building, and make it seem somewhat warmer within than without,
I'd be happy. I don't have to live in there.
It is galvanized steel sheet, with ribs in it...
Shiny? You might paint it dark.
--Kind of tough to get paint to stick to galvanized metal. What about
mixing pigment in the fluid itself?
I propose to simply glaze it over with clear plastic...
What kind? Polycarbonate is fairly expensive and may not last long in
the presence of warm water vapor. Greenhouse polyethylene film is
cheap
and comes in large pieces, with a 4-year guarantee. You might inflate
it over the roof, under a 4'x4' rope mesh to contain it.
--I haven't settled on what kind of plastic, but it will have to be
rigid sheet material. The wind here would destroy plastic film, rope
containment or not.
and use a pump to trickle glycol/water solution over the roof...
You may not need glycol.
--True. It would be easier in some respect without glycol, since rain
leaking into the sytem would not cause grief. But I likely would have
to provide some kind of backup heat source in the event of a long cold
spell with heavy overcast. It wouldn't do to allow the floor piping to
freeze.
and circulate it through tubing embedded in the concrete floor...
I have on hand a large quantity of 1/4" polyethylene tubing in 50
meter lengths. Could this be used to make the heating loops in the floor?
I would plan to run multiple loops, so that the flow resistance would be
comparable to that of the tubing normally used for this purpose.
The Hazen-Williams equation says L' of d" smooth pipe with a G gpm
flow
has a 0.0004227LG^1.852d^-4.871 psi pressure loss. You might replace a
piece of 1/2" PEX tubing with 2^4.871 = 29 pieces of 1/4" PE tubing
--Then I will go with the 1/2" PEX tubing. The concrete slab is
expensive enough that I don't want to do it twice. :>)
A new barn might have clear Dynaglas corrugated polycarbonate
greenhouse
roofing material as the south wall. This comes in 4'xN' sheets and
costs
$1.50/ft^2 and has a 10-year guarantee. Hang 80% greenhouse shadecloth
inside to reduce the light and heat to comfortable levels during the
day.
Cover the underside of the roof with foil and install a fan-coil unit
(eg
a $35 used auto radiator and fan) under the roof to transfer heat from
warm air to water, with an unpressurized boxful of water on the floor,
and heat the air in the barn as needed by running the fan coil unit
with
a ceiling fan and thermostat to move warm air down into the barn.
If you've already covered the south wall with metal and insulation,
you
might add a sunspace with a similar heat collection system and more
storage
space, but less natural light inside the barn. Where I live near
Phila,
the barn would need about (70-30)188 = 2500 Btu/h or 60K Btu of heat
on
an average 30 F January day when 1000 Btu/ft^2 falls on a south wall
and
610 falls on the ground. A 800 Btu/h-F fan-coil could make 2500 Btu/h
with 70+2500/800 = 73 F water. The barn needs 300K Btu for 5 cloudy
days
in a row, eg a 4'x4'x8' boxful of water cooling from
73+300K/(4x4x8x62)
= 111 to 73 F. We might warm an R20 box that loses
24h(110-50)160ft^2/R20
= 11.5K Btu/day warm with air near the ceiling at 111+11.5K/(6hx800)
= 113 F for 6h/day, which raises its loss by 6h(114-70)32x60/R20 =
25.3K
Btu/day. With air at 114 F (worst-case), a square foot of R1 vertical
south sunspace glazing with 90% solar transmission would gain 900
Btu/day
and lose 6h(114-30)1ft^2/R1 = 500, for a net gain of 400. If 8'xN' of
glazing supplies 60K+25.3K = 85.3K Btu/day of heat, N = 26'. Half the
south wall would do, with the box in the 8'x32' sunspace.
Nick
Thanks, Nick, for taking the time to respond. I like the idea of a
sunspace. I will look into the Dynaglass material for my rooftop
collector. When I go to pour a slab, I will ensure it's insulated, and
that it has the PEX tubing installed. The radiant slab is an asset to
the building, even if it ultimately gets heated by gas (say, if I sell
the property, or begin working in there full-time).