[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?

Tom Gocze (tom at hotandcold.tv) used to make cylindrical "Softanks" with kerfed polyiso foamboard wrapped with a sheet of high-strength plastic. He suggests getting a local sheet metal shop to make an octagonal box with flanges and putting foamboard pieces inside with a folded EPDM rubber liner inside that. 

> Also, what's an "isolated-store hot tub heater"?

The isolated store could be a separate higher-temp well-insulated water tank that keeps the tub warm for 5 cloudy days in a row. A 3' tall x 5' diameter hot tub used for 2 hours per day in Sacramento would need 5747+7527 = 13,274 Btu/day...

20 PI=4*ATN(1)
30 D=5'tub diameter (feet)
40 H=3'tub depth (feet)
50 ALID=PI*(D/2)^2'lid area (ft^2)
60 ATUB=PI*D*H'tub wall area (ft^2)
70 RV=15'R-value of tub and lid (ft^2-F-h/Btu)...
120 EON=22*(104-45.3)*(ALID+ATUB)/RV'lid-on energy (Btu/day)
130 PW=EXP(17.863-9621/(460+104))'tub vapor pressure ("Hg)
140 PA=29.921/(1+.62198/.0087)'air vapor pressure ("Hg)
150 EOFF=2*((104-45.3)*ATUB/RV+100*ALID*(PW-PA))'lid-off energy (Btu/day)
160 SKWH1=(EON+EOFF)/3412'solar heat (kWh/day)
170 PRINT 1;EON,EOFF,SKWH1,EKWH,DELAY...

    Eon        Eoff          Esun          Elec        
    (Btu/day)  (Btu/day)     (kWh/day)     (kWh/day)     

 1  5747.492   7526.979      3.890525      0  

We could make an oval Softank 3' wide x 7' long with $1/ft^2 2" polyiso foamboard inside 10 cent/ft^2 2"x4" welded wire fence that holds 2377 pounds of water under a 4'x8' foamboard solar pond cover with a 35 cent/ft^2 EPDM layer on top under an equilateral A-frame with a 4'x8' $1.75/ft^2 Therma-Glas Plus twinwall polycarbonate 60-degree sloped south wall and a 4'x8' 90%-reflective 60-degree foil-foamboard north wall. 

With 80% U0.58 polycarbonate solar transmission and an R1 inner 5 cent/ft^2 4'x8' horizontal greenhouse polyethylene film glazing with 90% solar transmission to protect the polycarb from water vapor damage, the draindown pond cover might collect 0.9x0.8x8'(2'x550Btu/ft^2 [from above] +0.9x4'cos(30)x820Btu/ft^2 [from the south]) = 21,061 Btu over 6 hours on an average December day with a 49 F daytime outdoor temperature.

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. If cloudy days were like coin flips, the tub could be 100(1-2^-8.9) = 99.8% solar heated.

Nick