[Greenbuilding] A Solar Hot Tub

[Nick Pine]

> >... 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 cover 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.
 
> 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... 

That requires raising the solar gain to 6h(170-49)11.75+13,274 = 21,805 Btu/day. An extension with a 45 degree elevation would reflect beam sun down into the box at dawn if it's less than 2/cos(45) = 2.8' long...

... 0.9x0.8x8'((2-Lcos(45))x550Btu/ft^2 [from above] + 0.9x(4cos(30)+Lcos(45))x820Btu/ft^2 [from the south]) = 21,805
makes L = 1 foot, approximately, so it might look like this from the west side, viewed in a fixed font:

             t      --- 6.2'
         1'x 
         e  
        n           --- 5.5'
       n s
      n   s
     n     s                     south ->
 4' n       s  4'
   n         s
  n           s 
 n             s
n----------------   --- 2'
|     |    |     |
|     |tank|     | 
 ----------------   --- 0'

|       4'       |

Nick