Craig Davis wrote:

I live in Central Texas and want to heat a 12' W X 28' L X 4' 4" D exercise
pool for year around use as economically as possible. I have the roof of
a 24' X 45' RV barn located next to the pool that is available for solar
collectors. The sides of the barn face North and South and have no shade.
Can I maintain 84 degrees F in the winter using solar heat if I use a cover
for the pool when it is not being used?

Sure. Is the pool itself shaded? NREL data imply January is the worst-case
month for solar heating in Austin, when 940 Btu/ft^2 of sun falls on the
ground and 1200 falls on a south wall on an average 48.8 F day with a 58.8
daily max temp. An R1 pool cover with 80% solar transmission would make
0.8x940 = 24h(T-48.8)1ft^2/R1, for a pool temp T = 81 F. An R2 cover with
3 layers of plastic film and 70% solar transmission would make T = 104.

A square foot of roof with a 4/12 (18.4 degree) slope would get 940cos(18.4)
= 892 Btu/day of overhead sun + 1200sin(18.4) = 379 Btu of south sun, for
a total of 1271. If 80% of that enters a solar collector glazing with 84 F
water inside and the outdoor temp is (48.8+58.8)/2 = 53.8 for 6 hours, the
net gain is 0.8x1271-6h(84-53.8)1ft^2/R1 = 836 Btu/day. A shaded pool with
an R1 cover would lose 24h(84-48.8)12'x28'/R1 = 283.9K Btu/day. You might
heat it with 283.9K/836 = 340 ft^2 of roof, about the same size as the pool.

So you might buy 2 $75 No. 441428 14'x28' clear vs blue solar pool covers
from solarcovers.com (800) 433-4701 and put one on the roof over wires to
keep it from gluing itself to the roof when there's no water beneath it.

Would I need a booster heat source for cold snaps?

Probably not, if you are willing to swim in 72 F water. On an average day,
a square foot of pool would lose 24h(84-48.8)1ft^2/R1 = 845 Btu. Water
weighs 62.33 lb/ft^3, so a 1ft^2x4'4" column of water weighs 270 pounds,
and 1 pound of water stores 1 Btu/F, and the pool walls and bottom and
earth add more thermal mass, so the pool might cool by less than 845/270
= 3 F on a cloudy day.

Nick