The Canadian project mentioned in a previous post is the Drake Landing Solar Community, using BTES,
http://www.dlsc.ca/borehole.htm . This is one version of a seasonal thermal store (see
http://en.wikipedia.org/wiki/Seasonal_thermal_store ), and a similar, lower temperature concept is an annualized geo solar storage,
http://en.wikipedia.org/wiki/Annualized_geothermal_solar ,
http://www.greenershelter.org/index.php?pg=2
From the ideas I seen so far, I think the best solution is to dedicate a basement or sub-basement space to thermal storage using wet sand. The space should be waterproofed from outside water infiltration (possibly Platon,
http://www.systemplaton.com/ and/or Drylok and/or RadonSeal and/or exterior foundation tiling/drains), and insulated as well (polyisocyanurate and a radiant barrier?). In a new house, it would be a sub-basement, possibly made using ICFs, pre-fabricated concrete, or tilt-up construction, with 8 to 12 foot walls. For an existing house, a new 2-3 car garage with a waterproofed, insulated basement under it could work. Appropriately spaced PEX or pex-al-pex (PAP) would probably be the most cost effective solution for heat exchanging. A portion might be dedicated to cold storage, by circulating an antifreeze solution to the outside air in the winter. The reason for wet sand is to increase the thermal performance (better heat conductivity and use of storage volume) (.19 BTU/lb.°F for dry sand at ~ 110 lbs/cu.ft.) (
http://2the4.net/heat1.htm ). Water itself would be 1 BTU/lb.°F and 62.4 lbs/cu.ft., but that would need an actual tank, tank liner and a roof/cover capable of spanning the full width of the basement and supporting everything above, while wet sand could have a poured floor above it. If one did want to make the subbasement not have a poured slab ceiling and instead have a real span, pre-fab/pre-stressed concrete spans could be used, or products like Insul-Deck,
http://www.insul-deck.org/ , or Quad-deck,
http://www.quadlock.com/products/quad-deck.htm .
I do not know whether it makes more sense to simply have wet sand (damp), or saturated sand. The main points of using wet/damp sand are it can't easily all leak away, should need less maintenance than an actual water tank, and could use material already existing at the construction site that might otherwise need to be hauled away.
In areas where the soil is sand or gravel at sub-basement depths (i.e. between 7 to 20 feet below the surface), the area could be excavated, basement floor and walls created, waterproofed, and insulated, and then the sand/gravel backfilled into this space in layers along with PEX/PAP coils for heat exchangers. Areas with deep clay might do better by bringing sand from elsewhere, although bringing in sand might cost around $10/ton (prices I saw online).
In order to control the moisture level in the sand, a water refill/topping off mechanism would be appropriate. This could be achieved by simply running a loop of perforated PEX at the top of the thermal store. The other half of moisture control would be a way to drain excess moisture from the store, near the bottom. I think it would also make sense to leave a corner of the sub-basement, say 6x6 feet not filled in, as an access point to the lowest level of the basement and to a sump pump that might be necessary to drain away any infiltrating ground water from high water table events and to handle water collected by exterior perimeter drains. One would want to make sure that the ceiling of this sub-basement was also well sealed and insulated to prevent heat and water vapor and moisture from escaping into the house above when it wasn't wanted.
Note: while at this stage of construction, one could plan for a rain water cistern that could collect roof-runoff for non-potable uses like yard watering and toilet flushing. Properly placed, this same cistern could also be refilled from the perimeter drains.
Thermal sensors positioned throughout the thermal store would be a asset to knowing how much it has stored. I think waterproofed versions of the DS18S20 sensors discussed at
https://www.hearth.com/econtent/index.php/forums/viewthread/17569/ might be a good choice.
DS18S20 specifications:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2815
DS18S20 with Linux:
http://www.digitemp.com/building.shtml
DS18S20+-ND pricing:
http://www.digikey.com/scripts/us/dksus.dll?Detail?name=DS18S20+-ND
This page has a good description of sub-basement thermal storage, and is one of the sources for some of the ideas above (I had thought about large-scale water storage, however the large-scale solid thermal mass ideas here are also good). While this page primarily focuses on an air tube based system, it would be readily adapted to a fluid circulation system. Fluid circulation avoids potential air contamination problems, but adds complexity by needing additional heat exchangers.
http://mb-soft.com/solar/subbase.html
More information related to the thermal storage - Underground A/C Alternate System -
http://mb-soft.com/solar/alternwa.htm
More information related to the thermal storage:
http://mb-soft.com/solar/intake.html
One recently published book where DIY homeowner Kenneth Clive created a small ~6 foot cube rock storage system for his solar collector is 'Build Your Solar Heating System',
http://www.amazon.com/Build-Your-Solar-Heating-System/dp/0975423622 .
This article published in 1978 by Steve Eckhoff and Martin Okos, Department of Agricultural Engineering, Purdue University also covers thermal storage ideas:
http://www.ces.purdue.edu/extmedia/ae/ae-89.html
