Subject: A high-performance, cost-effective, solar heated pool

From: [email-masked] (Nick Pine)
Date: 10 May 1995 06:29:52 -0400

Solar pool heating should be easy, since temperatures are low, it is usually
done only in mild weather, and there is a large inherent water heat battery.
The May/June issue of Solar Today has a primer on conventional solar pool
heating that says: "In many areas of the country, solar pool heating is the
most economically attractive solar technology available today."

But there is still room for improvement, especially for year-round heating.
Yesterday, a physics teacher told me about a remarkable house that was next
to a swimming pool, which was under a conventional greenhouse. In winter, the
engineer/owner pumped some of the swimming pool water into the house, to heat
the house. This was done years ago, in PA, and it's probably all gone now,
along with the engineer. I wonder how it worked. A water-source heat pump?

Here is one way to heat a pool:

1. Put a 16 x 32' piece of plastic on the ground, and lay some cement blocks
on top. Lay some plywood or steel greenhouse bench mesh on top of the cement
blocks. One could also support the plywood with 55 gallon drums full of water,
standing upright. 

2. Buy a 15 x 30' rectangular steel pool from J C Penny, for about $2000.
Or build one out of 2 x 6s on 4' centers, with ferrocement over 4' wide
chicken wire. Set this up on top of the hollow platform above.

3. Make a 15 x 30' rigid pool cover out of 2" of beadboard and 2 x 4s, with a
layer of Thermo-ply underneath, sprayed with clear urethane, shiny side down,
and another layer of Thermo-ply on top, shiny side up. This would be hinged
along the 32' north edge of the pool, with a few counterweights hanging over
the north edge.

4. Buy a 21' x 36' commercial plastic film greenhouse, for about $1000, from
Stuppy at (800) 877-5025 or E C Geiger at (800) 432-9434, and put it up over
the pool. Or build an A frame over the pool, with Dynaglas for the south side
and ferrocement over kerfed 16' 2 x 4s, every 4', for the curved north side.
Attach a garage door opener to the top, to lift up the pool cover to an angle
of about 45 degrees, when the sun is shining and the pool needs heat. 

5. Make a hollow bench along each 16' edge of the pool, to form an air
distribution plenum, for the space under the pool. At one endwall of the
greenhouse, make a 4 ft^2 vertical airshaft or duct from the peak of the
greenhouse to the middle of the bench. Fit a 2000 cfm exhaust fan (perhaps
solar powered) to the middle of the bench at the other end of the pool. 

In the Philadelphia area, in January, the average amount of sun that comes
into the greenhouse, Ein, is about 12' (height) x 36' x 1000 Btu/ft^2/day, ie
430K Btu/day, over about 6 hours. The average outdoor temperature in this area
in January is about 30F. Say the pool were kept at 80F, in January.

Then the amount of energy that leaves the pool each day, Eout, would be about

    6 hours x 15 x 30 (80-30)/R1 = 135K thru the top, when the sun is shining,
+  18 hours x 15 x 30 (80-30)/R12 = 34K thru the top, at other times,
+  24 hours x 360 ft^2 (80-30)/Rs = 430K/Rs, thru the sides.

So if Ein = Eout,  135K + 34K + 430K/Rs = 430K, so 430K/Rs = 261K, so the side
insulation would have to be at least Rs = 430K/261K, about R2, eg 1/2" of
white beadboard, glued to the sides of the pool.

If the 12,000 gallon pool were closed up, while the sun didn't shine for a few
days, it would initially lose heat at a rate of about 45K Btu/day through the
top, and 130K Btu/day through the R2 sides, so the water temperature in the
pool would initially decrease by about (45K + 130K)/(12,000 x 8lb/gallon) = 2
degrees F per day.

Possible enhancements include a solar pool cover to reduce evaporation losses
when the top is up, a white surface or a reflecting pond on the ground along
the south edge of the greenhouse, a layer of vertical glazing along the 30'
south side of the pool, tomato plants, an indoor herb garden, orange trees,
lights, fountains, insulation for the north greenhouse roof, and another layer
of glazing or UV-transparent Tedlar film for the south side, to allow tanning
in January. 

Nick

PS: This might also make a dandy sewage treatment system.