If the storage tank is anything smaller than a pond, then it is likely to work for only a few days. If it is all rock, then I'd look at drilling (open or closed loop).

Without knowing the specifics of the application it is hard to comment in detail. My instinct is that Jonr is correct. There is an unfortunate tendency to confuse the usefulness of big thermal storage tanks with a properly designed closed or open loop system able to accept or reject heat more or less indefinitely at a design rate.

For a closed circuit storage arrangement to be useful through an entire heating or cooling season, it would have to accept or supply on the order of 100 million Btu. Since most single family residential projects don't have the budget or space for a million gallon tank, this approach fails feasibility analysis.

if the tank is not insulated, advection and convection with the surrounding soil/rock will have a positive effect. I think a million gallons is a little overkill unless the heat loss/gain is over 300KBtu/hr. Like it was said, without knowing the specifics, it is difficult to suggest a solution.

I remember reading about a geo loop application where one of the pro's on here ran a spring in to a large tank, maybe a stormwater storage tank. My apologies, as I don't remember who it was, but he put the loop coils in the tank and the spring constantly flushed the tank. From what I recall, with a bit of tweaking it was working well. Maybe his experience could help you a bit with your situation.

Based on a 3 ton unit for this home in San Antonio TX, can you caclulate:
1. size of pond (or alternative water reservoir) needed
2. total length of pipe needed

For the purpose of argument I ignored conduction from the tank to the surrounding ground to make the point that a very very large tank is needed to source / sink a season's worth of HVAC without ground contact. A closed loop system is essentially a very very spread out tank. At least one geo manufacturer refers to closed loop systems as "earth-coupled".

What I'm getting at is that it is far more efficient and cost effective to construct a tank of closed loop tubes in proximity with thousands of tons of earth rather than a big central tank in contact with far less earth.

I hope that makes sense.

Hopefully this thread isn't too stale to reopen.

I've been playing around with a few idea's along the lines of stormwater storage and wanted to see if there was any merit to trying something like this (Longer term heat storage inspired by this community: http://www.dlsc.ca/).

This would definitely be a DIY project, with cost a major consideration, as well as having the understanding that this probably would not heat the whole house, but be more of a supplemental heat source. I'm not planning on drilling any ground loops as I'm on solid rock and the cost would be prohibitive for us. Other heat sources we'll be using are passive solar design and wood heat (rocket mass heater: http://www.richsoil.com/rocket-stove-mass-heater.jsp). The house will be on the west coast of Canada with very mild winters (our current cabin almost heats itself just with passive solar, although cold mornings can be quite cool without using wood or any method of storing the solar heat).

I'm going to be installing a tank for fire protection of three buildings. So I was thinking of using the thermal mass of this water for heat storage. If I oversize the tank and insulated it, and then place the loops inside this tank I could pull some heat from here. Instead on relying on ground conduction to replenish the heat supply, I would run a solar water heating system to replenish some of the heat in the tank. I also have a source of winter runoff for about 6 months of the year. I plan generating some micro-hydro from this stream, which should yield about 700-1000watts during the heating season. If I augmented the heat in the tank with a resistance heater, and the solar collectors (not as efficient when it's raining, but that's when the creek turns to a torrent, which might increase my resistance heater output if I build multiple generators), I wonder if this system could provide for some of the heating requirements for the house. I also have the option of an open loop system using the creek as another source of geothermal, but as it's flow is a bit inconsistent, I was thinking of using the fire protection tank to augment the open loop system when it's dryer. This way I'd have two loops, one of which should always work during the heating season.

The main design consideration of using the water is for consistency - The creek has a useable flow about 80% of the time for 6 months If I can store a reasonable amount of heat in the water tank (say a weeks worth), and constantly be replenishing it, then I'm hoping that this will make for a reasonably consistent system.

This winter I plan on collecting some useful data to start doing some math (stream temps, flow rate and amount of sunny days in the winter) with a portable weather station/data logger, as well as calculating my total useable head for the microhydro next time I get out to the property.

This is just concept right now, so any ideas are welcome.

Thanks for any responses!

How big a tank?

water source system tonnage?

I could probably fit up to a 5000 gallon tank and build around it to insulate. Tonnage would be approximately 3 Tonns. The solar collector might be around 100sq/ft, more if required.

These a very ballpark numbers to see if this would be even feasable. I was thinking of potentially using wood heat to "recharge" the storage tank as needed.

Thanks

3 tons nominal changes the temp of a 5000 gallon tank just short of 1 degree per hour.

You might be better off burning wood directly in a space heater

Collector area sounds light, but collectors will be more efficient at lower output temperatures.

Sounds like a bigger tank and more btu's to replenish the heat of the water will be in order if I choose to entertain this idea ...
In your experience what would be the minimum usable temperature that the running water should be for an open loop system?

Thanks

45-ish, but know that my experience is non-existent in the area of minimum usable temperatures...never an issue in Florida.

It depends on how you discharge your water. The discharge is the coldest part in the system, if you discharge underground in discharge well, as long as you are above freezing, you will be fine. It is a different ball game if you discharge down a pipe exposed to ambient temperature or into a creek or something similar. You can get lower than 40F EWT if you increase your flowrate so your discharge temp will be warmer.

Customer in the hill country just outside of San Antonio TX is looking for a cost effective way to cool the house is summer months. Considerations: 1 acre sloped lot with typical hill county rocks. Intend to use aerobic septic system for all landscape irrigation would like to use geothermal for cooling. Installing a large (size yet to be determined) storm-water storage tank. Questions: Can the condensing coil be installed in a storm-water storage tank and then run a line set to AIR Handling unit on the 3rd floor? Concrete underground water storage tank not insulated. Home will have 3 floors. 1st floor will be a garage - no AC needed. About 2500 sq ft total for 2nd & 3rd floors. Thanks for the input/suggestions

Seriously, you couldn't have started a new thread, rather than tagging on to a nearly 4 years-stale thread?

Since this thread is already up to 2 pages I'd recommend starting your own, to make it more specific to what you're looking for.

And when you do that, specify whether this is new construction vs. an existing house, the volume and shape of the tank, and where you think your cooling & heating load numbers fall.