Some ideas regarding and call for input on geothermal heat pumps

In Alaska we have a fairly sophisticated solar heat system. So sophisticated it doesn't work at all; not after its first several months - and that was some time ago. But this thread isn't about Alaska.

Our rural Arizona home is set up like many and perhaps most that are not on a municipal water grid. We pump water from the aquifer a zillion feet u/g and store it in an underground cistern; ours is 2,550 gallons (9,600 liters). A second well pump there provides pressurized water for household use but effectively, the cistern always is close to full.

So I was contemplating a geothermal system the other day and now am thinking that a supremely efficient system would be one that has its circ'n lines embedded solely inside that cistern. It seems to me that the heat transfer capacity of an u/g reservoir of that size not only would be superior to a more conventional one by orders of magnitude, but the cost of installing it would be staggeringly lower, and its reliability far greater. The reason is that the heat specificity of water or, rather, its ability to transfer heat, is much, much greater than dry or even moist soil.

Not only that, but consider the uses of domestic water: most of it is for cleaning: dishes, clothes, bodies. For only a very small amount of usage is cool water desired (that glass of water; the ice maker), so a system like this would provide the additional energy effiiciency of lowering one's water heating cost. I don't have a clue as to what the dT would be, but I'm thinking that something like going from 54ºF to 70-75º is probably in the ballpark.

In the wintertime a heat pump runs in the opposite direction, and this set-up under those circumstances would of course be extracting heat from that cistern to warm the house. If my concept works at all it would be one far more suited to AZ, TX, FL and SoCal than for Minnesota....or Alaska. (The way around that conundrum would likely be to use other ways to provide winter heat).

So, a Q for all: does anyone know if this is a Known Possibility, and whether it is used anywhere? Are there pitfalls I haven't considered?

No, you're not understanding it correctly; that's because I left out an important piece. The surface area in question is not between the tank's wall and the ground, rather, it is between an array of tubing - piped from the house - that is installed ​inside the cistern - and the water the cistern holds. There would no transfer of fluids between the pipes and the cistern's water, only a heat transfer. Have I explained myself properly now?

Right Deon....as I explained in my post. This is for Arizona - far more energy expended in the 11.9 summer months they have than those gelid months....

No - not a big water heater. My thought is that the calories extracted from a house's air as the heat pump acts as a room A/C are likely enough to raise the cistern water's temp by some amount - 5º? 10º? - and that there is a nice bonus in that the house's water heater then has to work all that much less. But what is of overweening importance is that the rather cumbersome, expensive and inefficient process of tapping into a deep soil layer - through the means of drill pipes and a transfer of calories down one set of pipes and up another - all is circumvented.

Remember, too, that the fluid bath of the cistern is absolutely not static: that water is drawn out constantly as it is consumed, and is replaced by cool well water. This makes a big difference, and addresses on of adiggs's concerns. And, fortuitously and fortunately, this occurs most vigorously in just those summer months....because for our particular situation, we irrigate a large orchard - so that fluid outflow becomes the heat escapement that is his concern.

Yaah....I should sell slots for interns to assist me and....

oh, right. As a Moderator I'm going to have to report this thread as a non-contributing vendor.


Seriously, I am hoping to hear from some HVAC folks on this one.

That is precisely the system I have envisioned; the nice additional fillip I have emplaced is that, by heating the cistern's water, we would be reducing our water-heating bill. You, on the other hand, would have had warmer swimming pool water earlier and later in the season. And as I'm sure you would not have been using that pool during the winter months, it mattered not that you were cooling its water in order to heat your house. (We don't have a pool - those 'swimming months' we're in Alaska).

Very glad to hear that the system exists; I'll take a look at your link.

By the way - snow today in Wickenburg! Rare enough in Dec or Jan. Super-uncommon in Feb.

Excellent comments and input from your own system, Beeee; I absolutely would be confined to a closed-loop system because I indeed am very much aware of the contamination problem (likelihood = 100%).

I have experiences with four separate heat exchangers, disregarding engine-based radiators which also can be so defined. One set was a spectacular, high-tech, compact and pricey unit (I have a total of five of them). Great specs.....lousy in-field utility. Second one was an off-the-shelf variant of former, with the same drawbacks. A third is a finned compact coil of copper tubing - a rugged workhorse and effectively identical to what is in a water heater jacket in a domestic furnace. I like. The last is a very long copper pipe carefully coiled into a cylinder - least-cost solution and probably most effective for a dropped-in-cistern set up. Any custom set-up also could be some combination of the latter two.

I also do have UV treatment going on as well - but not for our domestic water; rather as part of our koi pond filtration. Were I to be building de novo I almost certainly would include same in our domestic water but my potability tests of our current set-up are perfect. Were I to enact all this I definitely would monitor typical High-Ts in the cistern and consider emplacing UV if they hit some unspecified point.

As in my OP, more or less 54ºF.