# Solar collector to radiant floor, will this work?

Discussion in 'Home Power and Microgeneration' started by Gordon Richmond, May 18, 2005.

1. ### Gordon RichmondGuest

So, I've just had a 32X60 foot steelclad pole building erected. I
intend to insulate it with fiberglass batts (6" walls), and to put in
a concrete floor. It will be a storage barn/vehicle workshop.

to my gas bill. I'd like to try the following, and would welcome any
informed opinion on the viability of my plan.

1. I want to use the south-facing side of the roof as the solar
collector. It is galvanized steel sheet, with ribs in it, and I
propose to simply glaze it over with clear plastic, and use a pump to
trickle glycol/water solution over the roof, collect it in a gutter at
the eaves, and circulate it through tubing embedded in the concrete
floor. The system would be "closed" as to rainfall getting into it,
but would not necessarily be airtight.

2. 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.

I don't have to heat this building to a "shirt=sleeve" environment in
-40° weather, but if I can take some of the chill off it, I'll be
delighted.

Gordon Richmond

2. ### Guest

With insulation under the floor, or at least the perimeter?
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.
What's the slope? Walls are better for winter heating. Where do you live?
Shiny? You might paint it dark.
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.
You may not need glycol.
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

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

3. ### Gordon RichmondGuest

Responses interspersed:

Thanks :>)
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 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.
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.
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?
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.
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.
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).

4. ### Gordon RichmondGuest

Responses interspersed:

Thanks :>)
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 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.
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.
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?
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.
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.
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).

5. ### Guest

You might quantify that.
Investigate further.
Maybe not.
See *** above.

Nick

6. ### daestromGuest

So in the winter soltice, the sun only rises (51 - 23.5) = 27.5 degrees
above the horizon. A '4 in 12' roof has a pitch of only ~18.5 degrees. So
the sunlight will hit the roof at a very, *very* shallow angle of only
(27.5 - 18.5) = 9 degrees. This is terrible.

Like Nick said, you'll probably be much better off if you have a wall you
can use instead. A vertical wall at 90 degrees will have sunlight hit at
(90-27.5) = 62.5 degrees at noon. Even if none of the walls are perfectly
east-west, you would do better than that 4 in 12 roof. The only problem
would be if the southern side of the building is shaded (although deciduous
trees wouldn't be a problem).

And in snow climates, you can get quite a bit of secondary radiation from
reflection from the snow. Living in Alberta, you undoubtedly know the value
of sunglasses on a sunny day in wintertime

daestrom

7. ### Guest

Or maybe 90-51-23.5 = 15.5 degrees at noon on the winter solstice.
Or maybe only ~18.4...
Or maybe 15.5 + 18.4 = 33.9, with cos(90-33.9) = 0.56.
Or maybe 15.5, with cos(15.5) = 0.96.
Then again, a roof would collect more sun on an overcast day.

Nick

8. ### daestromGuest

You're right of course, don't know where my mind was this morning :-(
Obviously my 'angles' are all wet today, glad you caught them.
Why would that be? Even on a an overcast day, the reflection off of snow is
intense. Do you think there is more radiation coming straight down on an
overcast day? If he has a sizeable area to the south of the wall in
question, I think he would still get quite a bit of secondary radiation from
the reflection.

daestrom

9. ### Gordon RichmondGuest

Fortunately, I do have a long south-facing wall, with no significant
shading from the winter sun. About a third of the wall would be
unavailable for solar collectors since there is a sliding door at the
extreme east end of the wall. And there are two small windows.

assymetrical design, and discovered that it would run quite a lot more
money than to go with a standard design.

Gordon Richmond

10. ### Guest

Perhaps you are right. If a flat roof receives I W/m^2 diffuse isotropic
sky radiation and far-field snow reflects 60% of that to a south wall
that receives 0.5I directly, the wall gets a total of total of 1.1I.

Nick

11. ### Derek BroughtonGuest

Can you explain that to me? I always assumed I'd get more sun where I have
reflection off the snow and ice to the southwest. What is it about the
roof that makes it better on an overcast day? Just the fact that
everything is diffuse, and it simply has more total exposure?

12. ### Guest

It "sees" 100% of the sky hemisphere, vs a wall which only sees 50%,
so it beats the wall unless the ground is more than 50% reflective,
on an overcast day with uniform (isotropic) radiation from every
part of the hemisphere.

Nick