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re: concrete thickness
19 dec 1999
paul wrote:
>but, the insulation cost _really_ drives the [cistern] price up..... :)
>> how about a $500 24' 15k gal. circular swimming pool with a cover...
1. the pool has about 300ft^2 of sidewall and 450 ft^2 of top surface.
if it's uninsulated, with a slowly-moving airfilm thermal conductance
of 750x1.5btu/h-f-ft^2 = 1125 btu/h-f, and it's -10 f outdoors, so
(32-(-10))1125 = 47k btu/h of heat flows out of the water while it is
freezing, which requires a total of 15kx8x144 = 17.3 million btu, it
will freeze solid in about 17300k/47k = 367 hours or 15 days at -10 f.
2. the ground underneath can provide some significant heat to keep the
pool from freezing, especially if it is dampish, with some evaporation
in lower layers and condensation above. where i live near philadelphia,
the average (deep) soil temp is 54.3 f.
3. it wouldn't be hard to insulate the sidewall with something like an
earth berm or some bags of leaves under a typar skirt (typar is a darker
stronger cheaper housewrap than tyvek, with an unconditional lifetime
guarantee.) this could also increase the rain-gathering area.
4. the cover (floating or simple geodesic) might be 90% transparent,
collecting about 0.9x620x450ft^2 = 251k btu on an average 30.4 f
january day near phila, enough to melt 1740 pounds of ice per day,
ie a 3/4" layer. at 30.4 f, 24h(32-30.4)1125 = 43k btu/day of heat
flows out of the uninsulated pool, enough to freeze a 1/8" layer.
nick
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