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Re: Solar heat for swimming pool



James VanBokkelen <jbvb@satisfied.apocalypse.org> wrote:

>Most pools, particularly those that date back more than a few years,
>have absolutely no insulation between the water and the ground.  If
>you can't find out about insulation from whoever built your pool, one
>thing you can try (if you have an existing pool heater) is to bring
>the pool up to 80F or so, cover it, and shut off the heater.  Then see
>how long it takes for the temperature to drop 10F...

It would be interesting to know the results, if anyone tries this. 

The ground itself may be a pretty good insulator, if we can keep it dry.
Chapter 22 of ASHRAE Handbook of Fundamentals has a graph of R-value vs.
moisture content of soils that suggests that artificially dry soil may
have a very high R-value. They say that plants, eg grass, normally keep
soil fairly dry, until the soil gets so dry that they begin to wilt, at
which point (10% moisture content?) they stop wicking water out of the soil
underneath, so it never gets much dryer than that, in meadows. Damp soil
is a good conductor of _upward_ heatflow when heat below evaporates water,
which condenses above.

One old solar greenhouse book I have suggests making perimeter insulation by
digging 2 2' deep trenches to make a ring of undisturbed earth 2' wide, then
covering that ring with a U-shaped piece of poly film, open at the bottom,
which the authors say will make an R-24 insulator. Seems to me it would be
better to just dig one trench around a swimming pool, line it with a U-shaped 
vapor barrier, open at the top, put the soil back in the trench (a day's work
with a backhoe?) , and cover the pool and trench with a commercial plastic
glazed greenhouse to keep it warm and dry.

Nick