I need some advice on insulation and heat transfer for a solar-hot-
water storage system. Here are the specs:
4500 square feet ICF/SIP (roof) construction
12 4x8 flat panels mounted due south at 40 degrees (our latitude)
DHW and space heat
5554 Degree days heating (Boulder, CO)
Hydronic heating
HRV
Tankless water heater for backup
Heat storage:
2 80-gallon tanks with heat exchangers for DHW
38x22x4 feet sand bed with multiple loops of PEX tubing (heat in
from solar, out to incoming DHW and hydronic)
(We've ruled out the huge-water-tank approach: no room, too
expensive, too afraid of what'll happen when it starts leaking, etc.)
Questions:
Any projections as to whether the components are sized correctly? My
calculations shows that they ballpark are, although I must admit that
the system is designed as much by what'll fit as what's optimal...
We'd like to heat incoming HRV air from the sand using some sort of
heat exchanger. The possibilities are:
1) Bury concrete block in the sand, push air through it with a fan
2) Bury pipe (PVC or ribbed ABS septic drain line) and push air
through it
3) Make another PEX loop through heat storage and use water/air heat
exchanger (requires pump)
I'm thinking 2 would be easiest, 1 would be cheapest (depending on the
source of the blocks) and probably have the best thermal transfer, and
3 would be the most flexible (easier temp control). But are any of
them worth considering or should we just accept the cold incoming air
and rely on the radiant floors to rewarm it? It's all the same heat
source after all...
The sand bed heat storage is under the garage floor (don't want it
directly under living space because we want a crawl space and because
we get 70 degree days here in January). Stem walls (backfilled about
4') and garage walls will be ICF, the ceiling of the garage will be
the floor of the rec/bonus room, and we'll put a vapor barrier above
and below the sand, but what (if any) insulation should be used above
(between it and the slab) and below (between it and the ground) the
sand?
Will the sand transfer heat fast enough to the pex tubing running
through it, or should I wrap the pex in aluminum plates (like a staple-
up hydronic system)?
Do we want to consider putting some sort of wetting system in the
sand? It would greatly increase capacity and heat transfer, but would
there be negative effects too?
Anyone know of an absorption-type water chiller for residential
cooling? Evaporative coolers work well here, but I figure as long as
we got lots of excess heat in the summer, why not run the cooling with
it too?
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