[Greenbuilding] A cloudy-day DHW heat store

nick pine nick at early.com
Thu Dec 8 14:15:06 PST 2011

>... If all you want is hot water, you might not need a whole
greenhouse, just a long east-west water-filled polyethylene duct
on the ground inside a larger duct that's filled with air during
the day and soap bubbles at night.

We could start by putting 48' of 2"x4'x8' Styrofoam over a $66
12'x50' piece of 4-year 6-mil polyethylene film on flat ground,
then place a $43 24" diameter x 48' long polyethylene film duct
from http://www.greenhousemegastore.com/product/power-tube-tubing/fan-jets-power-tubes
on the Styrofoam, then raise the film edges and seal them together
with 2" horizontal plastic conduit at the top to make a 4'x4'x4'x50'
equilateral Tablerone bar with supports for the conduit every 8' or so,
then fill the duct with about 9400 pounds of water and make an opening
for soap bubble foam near the bottom of one triangular endwall and
another opening for an air return near the top of the opposite endwall,
like this, viewed in a fixed font:

      conduit          conduitconduitconduitconduitconduit
     f f               e                                 e air --> 
    f   film           n                                 n
4' f     f 4'          d                                 d
  f       f            w                                 w
f  duct   f           a   ductductductductductductduct  a
f foamboard f  foam -->l    foamboardfoamboardfoamboard  l 
fffffffffffff          lfffffffffffffffffffffffffffffffffl
      4'                                50'

On an average January day in Phila, the film envelope would be filled
with air, and it would transmit about 0.9x50'(1000x3.46'+620x4') 
= 267K Btu. Over 6 hours with water temp T, it would lose about
6h(T-34)50'x4'/R1 Btu/day. With no useful heat output, T = 257 F :-)

At 140 F, it could supply about (140-80)9400 = 564K Btu to a house.

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