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Rainwater tank ground-source heat-pump synergy

Sean Hembree | Posted in Energy Efficiency and Durability on

I’d like to look into the feasibility of using a open-loop ground source direct-exchange heat pump system in conjunction with a below grade rainwater catchment storage tank.

My company installs onsite water conservation measures.  We are designing a rainwater to irrigation system for a school but they are interested in retrofitting existing structures with a ground-source heat pumps.  Heating and cooling load are about evenly split in this region (climate zone 8-9).

Have you seen this done before?  Any considerations as we look at this design?


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  1. Vlad Shpurik | | #1

    Take a look at this video:
    @ 3:00 they talk about something similar to what you are trying to do. Not a lot of details, but perhaps you would find it useful.

  2. Expert Member
    Peter Engle | | #2

    First I've got a question about your location and climate zone. Climate zone 8-9 is not balanced between heating and cooling: It is a very cold climate. Think interior Alaska and northern Canada. Where are you located?

    Second, while this approach could work, you should run some numbers first. You should look at the heating and cooling loads of the building and compare that to the temperature rise/drop of the full cistern. Also, if you are using it for irrigation, will it remain full? Do you have enough rainfall to continuously refill the cistern, or will it approach empty at times? Lower volume of stored water equals lower thermal mass for heat exchange.

    I think you will find that, to provide enough heat/cool for the building, the cistern has to be quite a bit larger than you expect.

  3. Expert Member
    Dana Dorsett | | #3

    >"Have you seen this done before? Any considerations as we look at this design?"

    These folks have designed many water-water heat up systems using large-ish buried tanks:

    They take advantage of the heat of fusion of water to get a 144x "apparent mass" of the water, so it takes less volume than you might imagine. The efficiency of water to water heat pumps still quite good even with 32F water. As long as water doesn't freeze completely it will stay in an efficient zone with good heat transfer characteristics, but ice has half the specific heat of liquid water and performance falls rapidly below 32F. So it's a matter of sizing it correctly for the loads and designing the internal heat exchangers adequately.

    By not insulating the tank there is additional thermal mass to be used from the surrounding soil, but the surface area of the tank isn't big enough to make that a large performance factor.

    They sometimes take the additional step of adding low temperature solar thermal to Even when it's 0F outside flat panel solar thermal delivers very decent efficiency at making 40-50F water out of 32F water. A schematic of one such system is here:

    The additional expense of the low temp solar makes more sense in a heating dominated climate than a climate split evenly between heating/cooling. In your case if new irrigation water is going into the tank on a regular basis that will also add heat to the tank in winter, cool it off in summer (assuming the heat pump is also used for cooling.)

    If your location is roughly evenly split between heating and cooling you are probably in DOE US climate zone 3:

  4. Sean Hembree | | #4

    Thanks all, To clarify, I was referring to USDA climate zone 8-9 not DOE. Sorry to be confusing. Regarding capacity, the water catchment potential of the buildings in question was about 70,000 gallons/year so I'm not worried about running out of water it would just be a matter of sizing the tanks properly.

    Dana's comment that surface area of the tank isn't big enough to make that a large performance factor probably answers my question about whether or not to have a below or above ground tank, especially with the larger tanks we are talking about. It cost about 2.5 times more per gallon to install water catchment below grade than above grade.

    The open loop system seems like an elegant design since the heating load would correspond the the irrigation season. The rainwater input would be colder in winter but if the systems could remain in the efficient range down to 32 deg. F It should be OK. They rarely get a hard freeze or snow for more than a few days at at time.

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