TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Some ideas regarding and call for input on geothermal heat pumps

Discussion in 'Energy, Environment, and Policy' started by AudubonB, Feb 1, 2016.

  1. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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?
     
  2. TheTalkingMule

    TheTalkingMule Active Member

    Joined:
    Oct 20, 2012
    Messages:
    1,272
    Location:
    Philadelphia, PA
    Geothermal is about surface area. How much surface area would that tank have relative to traditional tubing?

    Just by a quick google it seems that heat transfer wouldn't be fast enough between the tank and soil to be reliable. Though most of the concerns were about the cistern freezing while trying to heat the home, not an issue in AZ I assume.
     
  3. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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?
     
  4. deonb

    deonb Active Member

    Joined:
    Mar 4, 2013
    Messages:
    3,020
    Location:
    Redmond, WA
    If you extract heat from the cistern, won't that cool the water down? And then you have to heat it back up to be usable.

    I can see in the summer it would be a benefit, since you still want hot water in the summer. But you don't want cold water in the winter.
     
  5. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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....
     
  6. TheTalkingMule

    TheTalkingMule Active Member

    Joined:
    Oct 20, 2012
    Messages:
    1,272
    Location:
    Philadelphia, PA
    Yup, I see what you're saying. It may work if your cistern is huge, but at the end of the day the concern would be the the transfer between the water and the soil, therefore the surface area of the tank(and it's ability to transfer heat) is the main issue. The water would heat up mighty quick, but it could work to a degree if all you're doing for the most part is cooling.
     
  7. adiggs

    adiggs Active Member

    Joined:
    Sep 25, 2012
    Messages:
    1,352
    Location:
    Portland, OR
    Sounds like you're angling for a REALLY BIG hot water heater, and a need to chill your hot water for drinking purposes. Of course, that's entirely reasonable as you don't need much water for drinking - it's all the other stuff.

    As a proponent of long showers, any plan that leads to a 2500 gallon hot water tank sounds like a winner to me!


    Ok - it won't really be a 2500 gallon hot water heater, and I don't have any particular technical knowledge regarding what you're thinking. The specific assumption I would question, is whether the tank will heat to a certain point and achieve something reasonably stable as more heat in the tank disperses into the soil around the tank. If that doesn't happen, then all that heat you're pumping out of your house is going to keep accumulating in the form of hotter and hotter water in the cistern.

    I figure transferring heat in and out of the cistern won't be an issue - drop a big coil of copper tubing if nothing else and run the geothermal heat transfer liquid through that tubing.

    But will enough heat escape from the cistern during the hot months?
     
  8. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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.
     
  9. RubberToe

    RubberToe Supporting the greater good

    Joined:
    Jun 28, 2012
    Messages:
    843
    Location:
    Pasadena, Ca
    AudubonB,
    Sounds like an interesting project. Please keep us posted if you follow through on this. I'll bet wk057 would fly out and do it in a weekend as a side project :smile:

    RT
     
  10. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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.
     
  11. ohmman

    ohmman Maximum Plaid Member

    Joined:
    Feb 13, 2014
    Messages:
    4,859
    Location:
    North Bay, CA
    So I had a system like this in my house in Texas. I didn't use my cistern (though we had one), I used the swimming pool. We had a heat exchanger mounted on our AC pad through which we pumped pool water. Three compressors, one for each zone in the house. In the spring, summer, and fall, we ran AC and dumped heat into the pool. Once the pool hit a preset temperature, we used an evaporative cooling tower instead of the pool water for heat exchange. In the winter, we drew the heat out of the pool. The drop across the exchanger was about 10 degrees, so once the pool hit about 45 degrees, we had to worry about freezing. Our backup heat was hydronic using our hot water heater, so we were in good shape when those rare cold fronts sweep through Texas and keep it below freezing for a few days.

    You could do something like this by pumping your cistern water through a heat exchanger. From a maintenance perspective, it might be more attractive than dropping the lines into the cistern directly.

    "This New House" on DIY did a segment on our system.

    ThermalFlow is the system. I am happy to make an introduction to the founder if you're interested.
     
  12. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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.
     
  13. Tamar

    Tamar Member

    Joined:
    Mar 23, 2015
    Messages:
    166
    Location:
    St. Paul, MN
  14. Cosmacelf

    Cosmacelf Active Member

    Joined:
    Mar 6, 2013
    Messages:
    3,399
    Location:
    San Diego
    Ohmman, all I see on that thermal flow website is their evaporative cooling tower. Do they also make a water to water heat exchanger that you used?
     
  15. ohmman

    ohmman Maximum Plaid Member

    Joined:
    Feb 13, 2014
    Messages:
    4,859
    Location:
    North Bay, CA
    They do (or did when we installed it). That website doesn't appear to have been updated in a very long time - if you're interested, email Mac Word at the email address on the Contact link. If you can't get a hold of him, PM me and I'll put you in touch.
     
  16. beeeerock

    beeeerock Active Member

    Joined:
    Mar 12, 2015
    Messages:
    1,372
    Location:
    Kamloops BC Canada
    My home is heated and cooled with a closed-loop vertical (well) system. I love it and wouldn't move into another house that didn't have it.

    I'm pondering your reservoir and wondering if an open-loop system using the reservoir as your source (and disposal) might be easier to implement than the closed-loop system you're describing.

    In a sense, what you'd have is a pond-based system, but using pristine groundwater. In the case of a 'real' pond, the water is unlikely to be clean, meaning the heat exchanger is going to have issues with build-up of gunk... that's really why a close-loop system is used in that circumstance.

    An open-loop system would typically pull cool groundwater through the heat exchanger, before disposing of it... disposal is the big concern with a well, because you *really* don't want to introduce bacteria into the aquifer. Getting approval for this sort of system can be challenging, as it should be.

    However, you'd be using the clean groundwater and 'disposing' of it to your home, orchard, lawn etc. So pursuing the open-loop idea might be easier. But you might ponder a few things off the top of my head:

    • potential to grow 'stuff' in your tank if the water is warm (UV treatment of potable water perhaps, since groundwater isn't typically treated when used straight from the well.)
    • efficiency of recirculating water - perhaps run irrigation water through the system and off to point of consumption rather than back to tank (smart valving system, based on water demand).
    • how much winter water use is expected and whether you're going to run the reservoir down to the point where things start to freeze. I managed to freeze my closed-loops when the front door was unavoidably open one winter, doing some unexpected work!
    I think a geo-exchange designer could do the calculations based on your home's thermal load (seasonally) and the volume of water consumed (again, seasonally). But so much depends on how well-insulated the house is, what the solar load on windows etc. amounts to... meaning I think the idea is a good one that could likely work, but you'd want to do some specific calculations to make sure it worked reliably and as hoped. It's a very specific area of expertise and your idea is a little outside the box, so I wouldn't rely upon the comments of anyone not specifically in the business - no offense to anyone here!!
     
  17. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    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.
     
  18. beeeerock

    beeeerock Active Member

    Joined:
    Mar 12, 2015
    Messages:
    1,372
    Location:
    Kamloops BC Canada
    I suppose it depends on how much you want to cobble together yourself vs. purchasing a turn-key solution. If you were going closer to 'turn-key', I'd look at the open loop system, because they should be available with parts that would be food-grade. I would at least confirm they aren't before moving to closed-loop.

    I'm thinking about a few things when I say that... efficiency is one thing - I'm trying to visualize the heat transfer points and relative deltas in each scenario... and want to say the closed loop would be less efficient. But it's making my head hurt and I can't back that with any certainty right now!

    I'd probably look at the chemistry of your water before saying 'copper' for sure - HDPE is more inert although less efficient in transferring heat... and I'd probably say 'no' to PVC.

    Without flow in the reservoir, I suspect you could get thermal zones near the pipes - efficiency again. How much, I don't know.

    Pumping water through the exchanger and back to the reservoir would ensure circulation in the reservoir... something that might be more valuable when the water temperature rises!

    You could incorporate a UV filter right into the heat exchanger circuit, thus continually treating the stored/circulated water.

    A closed-loop system would likely mean pumping some sort of glycol/water mixture through the pipe. You wouldn't want that leaking into your reservoir if something went wrong.

    My system was built by Waterfurnace (waterfurnace.ca) about 15 or 16 years ago. They might be worth reaching out to, with the closed vs. open question and specifically, potability. I have no direct experience with the open-loop systems, but like the theory.

    I should also add that my system is forced-air... the box looks like a regular gas-fired furnace. You have to assume that the temperature in the house is to remain about the same through the day and night. The technology doesn't work well when you want to bump up or down - slow and steady is the name of the game.
     
  19. strider

    strider Active Member

    Joined:
    Oct 20, 2010
    Messages:
    2,920
    Location:
    NE Oklahoma
    This is a classic "pond loop" or "lake loop" geothermal system but with the added benefit that the temp of the "pond" would be fairly constant so efficiency would be fairly constant regardless of outside temperature (assuming constant use and replenishing of the water). You could "off the shelf" this easily - you just drop the loops into the cistern instead of a pond. Do you know the temperature of the water in the cistern?
     
  20. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

    Joined:
    Mar 24, 2013
    Messages:
    4,251
    Location:
    Denali Highway, Alaska
    As in my OP, more or less 54ºF.
     

Share This Page