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 muliple 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 absorbtion-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?

PS: I realize post this is somewhat off topic, but inexplicably there's no active solar forum on this site....