It seems that a large water storage tank would do the same thing. Then you could slowly flow it back into your ground loop water return.
I heard someone talking about this before but I do not know of any actual applications.

Marc

I don't think that will work so well for a number of reasons, the biggest is where do you think the heat from cooling goes? Why don't you post this in the Geothermal forum where the experts are?

Using an un-insulated chunk of ground for seasonal thermal storage will have piss-poor performance. (Especially if that ground sees any groundwater flow!) Running at cold temps the solar will have optimal collection efficiency, but with no insulation to contain the heat it will dissipate. The large area of unglazed collector area (the surrounding ground surface) would continue to be a larger factor in the ground temperature in your wells than any type of fabricated collector you pipe into it locally.

Very large, well insulated underground tanks can work, but tend to be on the pricey side to install. IIRC there have been a few experiemental installations like that in Ohio, using multiple pre-cast septic-tanks insulated with sprayed polyurethane foam.

The Riverdale Net Zero project in Alberta uses a medium-sized low-temp storage tank in combination with a heat pump to improve wintertime collector efficiency by keeping temps low:

http://www.riverdalenetzero.ca/Home.html

http://www.riverdalenetzero.ca/Riverdale_NetZero_house_--_project_profile.pdf

But unless you're already super-insulating (probably not, it takes a 5-ton system to heat your place), you'll need a helluva lot more tank than that (and a correspondingly larger amount of collector area.)

Cost/benefit wise, you'd likely be better off boosting the thermal performance of the building envelope by doubling it's average insulation value & buying 2 tons less compressor than any solar heat-dumping into the wells or low-temp storage tanks. At that point active solar thermal (with higher-temp storage) sufficient to carry the bulk of the shoulder-season heat loads may become the next-most cost effective upgrade.

caja715
The amount of solar radiation you can collect each day is going to be very small compared to the amount of heat your house will will need to stay warm.
On clear sunny days you may be able to collect about 50K BTU per day, 2 panel system, your Geo unit is producing 60K BTU per hour.
Tanks are equally useless unless they are the size of large(1 acre) and deep(10-12 feet). There are just not enough BTUs to collect.
Check out the Geo portion of this forum. Also, see
http://forum.geoexchange.org/geothermal-geoexchange-forums/general-discussions/
Your concern should be if your Geo filed entering water temp is getting to low and is causing a lock out.

IIRC, the multiple insulated septic-tank storage used in the Ohio experiments were good for about 10 days worth for a superinsulated house (this one clearly isn't superinsulated, if it needs a 5 ton compressor on design-day.)  They were for a fully solar heated house not a house heated with a 5 ton load heated with an earth-coupled heat pump .  I like the idea of a subterranean lake as seasonal solar-storage though. :-)

There are plenty of seasonal BTUs to collect, (the peak 60KBTU/h load should only happen for ~3% of the total hours in the heating season if the contractor did the heat load calc right), it's just hard to store enough for long periods with heating loads that high.  To do anything very useful with solar it's more cost-effective to cut the load way back with a high-performance building envelope first, then the required size of the solar technology & storage capacity shrinks to something manageable (as in the Riverdale Net Zero project), and you only need to consider at most a week's worth of storage capacity (with some backup) to smooth it out on the coldest/darkest periods, not a month's worth.

People fall in love with the concept heating their homes with solar from it's low operating costs before doing the math on how much solar capital it takes to actually support the load.  For most houses it's hard to make a case for significant-scale solar heating retrofit on a performance basis.  If you first get the design-day heat load under 15KBTU/h, (under 10K if you can), it becomes more feasable/affordable.

I like some of the engineering of the Riverdale house, such as the notion of using a small heat pump and a decent sized tank to be able to store heat at room temp (or even well-below room temp), boosting the midwinter performance of the solar collectors in a cold climate by probably 30% or more.  This approach was likely cheaper & easier than buying & mounting 30% more collector AREA.  (If I ever retrofit my house you can bet I'll be doing the math on that!)

I have a GT system and am going to add solar. Goto the GT forum and see my thread if you get a chance. I can see a lot of benefit to the solar but in the summer unless you have some way of using the heat storing it is nearly out of the question. I am installing a pool and I will use the heat for the pool in the summer and from fall through spring I'll use the heat to supliment the GT.

That's what this forum is all about. I agree more or less with everyones take on this idea. Isn't this taking the passive system concept and forcing it?

I plan on having an ICF basement with SIP main floor. I think you all basically feel that a storage system using water will need to be to large. I agree.
The original take on this was to feed stored water for night time heat suppliment to a heat pump or GT. The combination of Passive, super insulated home (to included radaint floors) and it sounded feasible until the load is calculated for the storage tank grew to an enormous size.