Congratulations on your erection :-)
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
$.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 $5 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.