[Greenbuilding] A Solar Hot Tub

[Nick Pine]

Dan writes:

>> You mentioned a storage tank selling for $100. Is that in US dollars? 
>> What brand and where/how can one buy?

Tractor Supply sells their 21-77227 2'x6'x2'-deep 169 gallon oval galvanized 
stock tank for $108.78.

 >... A 3' tall x 5' diameter hot tub used for 2 hours per day in December 
in Sacramento would need 5747+7527 = 13,274 Btu/day...

>... With a pond thermal conductance of about 32ft^2/(R1+1/0.58) = 11.75 
>Btu/h-F and Tw (F) water and 21,061 = 6h(Tw-49)11.75+13,274, Tw = 159 F on 
>an average day. With perfect insulation, the pond could keep the tub 104 F 
>for 2377(159-110)/13274 = 8.9 cloudy days in a row as it cooled from 159 to 
>110 F, with 2 hours per day of off-cover tub use, with no help from the 
>electric heater.

The tank could keep the tub hot for 169x8.33(159-110)/13274 = 5.2 cloudy 
days in a row, or more, if we 1) extend the reflective north wall of the 
A-frame above the glazing to raise the water temp to 170 F (an upper limit 
for the EPDM liner) or 2) add some calcium chloride and a $10 1"x100' HDPE 
black plastic water pipe to the tank as a heat exchanger and lower the 
average-day water temp to 140 F max (an upper limit for the plastic pipe) 
and evaporate water on sunny days and add water back to the tank on cloudy 
days with a solenoid valve (scrounged from a dead washing machine :-) and a 
$10 snap-disk thermostat and Tractor Supply's 21-70322 $8.55 float valve, 
when the tank temp drops below 110 F.

Nick