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Thermal energy storage

JimPancanke | Posted in Green Building Techniques on

~3900 square foot house.

I am thinking of building a water based TES tank using ICF in a crawl space.

The plan would be to use RV antifreeze in an active (pump) circulating heat exchanger that sits at the bottom of a tank of water. Domestic water supply would pass through another heat exchanger that sits at the top (for stratified water column effects) inside this tank and preheat water for radiant hydronic boiler and domestic hot water.

I was thinking 12’x12′ @ 6′ tall for a 864 cubic foot / 6463 gallon tank.

The heated rv antifreeze would come from either wall assemblies on the southern exposure of the house or from a roof top.

I am still doing research and was hoping someone here might have references I could read or comments on the above approach.

More than a few questions that I still have outstanding.

1. Should I divide the tank into multiple tanks such that if one tank is at 95 degrees but the circulating temperature of a heat transfer zone is 77 I could still sink heat.
2. If I make the tank out of ICF, do ICFs typically insulate the bottom of the footing from the ground.
3. Ideas on the lid of the tank (suspended slab covered with foam?)
4. Volume of tank – if the ground temperature is averaging 47 degrees year round and my incoming domestic water supply is at about the same – can my tank be too big… would it save more energy to have the tank at 95 degrees for 8 months or 77 degrees all year (for instance). So can my tank be too big?
5. Heat recovery zones on house ( Volume of zones, size of pipes ). I was thinking 1″ pex behind say black corrugated metal siding. Pex running through concrete pavers tinted black. How many gallons should I aim to have the zones.
6. In Seattle, should I expect that I will recover enough heat over the summer and avoid the loss of the heat to the ground / crawl such that the heat will last for more than a couple months into the winter. Should I expect that I will be able to recover heat in the fall/winter/spring ?
7. For the radiant heat installation I am thinking a low heat with a higher volume / distribution in the slab (any references on good distribution / good target heat to be pushing )
8. Controls for a variable speed pump and sensors.

I was thinking of starting smallish and trying to see how well the system works. I’d use 1/3 of the tank and possibly hook up the most productive heat zones (southern wall). Bare minimum I should be able to pre-heat some of the water supply for hot water showers beyond the average ground temperature.

I’ll continue more research and post more questions. I have probably 6 months before I plan to start building so still have some time.

Thanks for any thoughts / links / recommendations / constructive criticism.


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  1. GBA Editor
    Martin Holladay | | #1

    There are at least two problems with the idea of building a large tank in your basement for thermal storage:

    1. Handling leaks.

    2. The fact that the value of the energy stored by this system is much too low to justify the high cost of the equipment.

    Hundreds of people have done what you proposed. Even 10 years ago, when PV was still expensive, most of these people concluded that adding PV was a more logical approach than building Rube Goldberg devices like the one you propose in your basement. Now that the price of PV has dropped precipitously, these schemes make less sense than ever.

    In September 2008, I wrote a report for Energy Design Update on the Riverdale Net Zero house in Edmonton, Alberta. I wrote, "To store late-summer solar heat as long as possible, the basement of each Riverdale unit is equipped with a 4,500-gallon concrete storage tank. The rectangular tanks were built on site. “We have 4 inches of styrofoam underneath each tank, and we insulated the walls to R-50 on the outside of the concrete,” said [Peter] Amerongen. “We waterproofed the tanks three different ways, because the first two didn’t work. We ended up using a water-stop between the two pours -- between the bottom of the tank and the wall pour -- but the tank leaked at the seam. We used high fly-ash concrete reinforced with rebar on a 100-millimeter grid, so the concrete should have been waterproof, but it wasn’t. Next we sealed the bottom joint with epoxy. I didn’t really trust the epoxy, so we test-filled the tank and concluded that the tank was leaking. We took out the epoxy and carved a keyway in the joint and used a waterproofing agent called Xypex. We ended up treating all of the tank walls with Xypex.”

    "The Riverdale design team assumed that an active solar thermal system was an essential component for a zero-energy house in Edmonton. As the project progressed, however, the solar thermal system became increasingly large and complex. “Even though we made the house envelope as efficient as we could, so that the house could be heated at minus 32 degrees Celsius with the equivalent of six hair dryers, we were still amazed at the size of the solar thermal system we needed,” Howell told EDU. “We needed to extend the solar thermal collectors and the PV array beyond the roof line. We were also surprised at the huge complexity of the solar space heating system. We really did not expect solar space heating to be this complex.”
    Amerongen echoed Howell’s observation. “We spent more on solar thermal than we ever expected,” said Amerongen. “The solar thermal system is not hugely practical. We have a fair amount of technology chasing small amounts of energy, going after the last little bits of thermal load. When it comes to chasing net-zero energy, in some ways it actually makes more sense to go 95% of the way on a number of houses than to go 100% of the way on a single house.”
    As the building neared completion, Howell began to doubt the wisdom of including a solar thermal system. Howell recounted, “I said, ‘Peter, this is ridiculous. Let me go back and see what would happen if we had eliminated the solar thermal equipment.’ After running the numbers, I was dumbstruck. The cheapest house would be have been a house without a solar thermal system, using just PV and geothermal heat. That would have cost $8,000 less than what we did. If we went with a PV-only system -- with no solar thermal and no geothermal, only passive solar heat, electric resistance heat, and a PV system -- it would have cost only $1,000 more than what we are doing now, and it would be hugely less complex. I was stunned with this. The increased area of the PV array would have been identical to the area of the existing solar thermal collectors, and the PV array could have been integrated into a single sloped roof.”"

    As I said, PV has only gotten cheaper since 2008.

    By the way, all of these basement tanks leak. Even the tank at Marc Rosenbaum's award-winning solar thermal house in Hanover, N.H. leaked.

  2. krom | | #2

    Thorsten Chlup uses one in his home in alaska.

    He uses a steel tank with a liner that was designed for replacement if/when needed. There is a presentation floating around about the entire build (it also has info on the alaskan wall)

  3. JimPancanke | | #3


    Thank you for the response!

    It surprises me that all the tanks leak as I would have thought the techniques used to build an insulated leak proof swimming pool could be achieved. Do you know what is being referred to when they mention water stop between the two pours. I'd envisioned a similar pour setup, walls and then floor. I was thinking you could just use something like pond liner - EPDM - with no seams to line the insulated concrete tank.

    For the complexity, do you know what specific things they found complex about the system. It would seem to me you have the tank, a pump and a couple sensor for when to activate the pump. I wonder if as they mention it was their drive for the net zero and the 100% instead of 95% that pushed the complexity too high. So perhaps in the case of wanting a 100% net zero the numbers work in favor of pv... and perhaps the numbers will always favor a pv system.

    I'll do more research on pv systems and run some numbers. My impression is pv panels have a lifetime where as simple pipes that are doing thermal heat transfer should last longer.

    I read recently about the housing development in Alberta that used some ground rock to store heat over the summer and achieved 97% of the their space heating requirements only using the garage roof as the thermal collector surface. I anticipate requiring a lot less heat than a home in Alberta so my thought was that the thermal collector surface could be a lot smaller (or at worse the same size as a garage roof). The heat sink in their case is going to leak heat to the surrounding soil I'd expect more so than an insulated tank that is sitting in your basement which would be leaking heat to your basement/foundation and basement soil (maybe in the summer which might not be ideal) but should at least keep work towards keeping the house warmer in the heating season.

    Thank you for the references and the discussion !


  4. GBA Editor
    Martin Holladay | | #4

    It's hard to store thermal energy for more than a few days.

    Yes, tanks leak. Over the next 30 years, a big water tank in your basement is going to require significantly more maintenance than a PV array.

    If you like math, don't forget to calculate the value of the BTUs you hope to capture with your $10,000 to $20,000 system.

  5. JimPancanke | | #5


    I reviewed the Edmonton project.

    Thank you for the reference.

    I'd argue that the thermal energy collector system that I've described will work better in the Seattle climate for a number of reasons:

    In Seattle in the fall/spring/possibly even winter sunny days the air temperature and sun allow the thermal array to work. My impression is Edmonton will be able to use it in the summer but if the air temperature is lower than ground temperature you can't use it. In Seattle the air temperature goes above ground (I'll have to do some calcs) a much greater percentage of time than Edmonton. So in the heating season if there is a sunny day the system should be able to capture that into the tank with limited heat loss since it will be used immediately for the heating.

    The Edmonton project claims 21% of the heat is achieved from the storage tank - this would seem to mean that they are able to store heat gains for more than a few days with their storage tank.

    The tank design that I propose is fully contained within the buildings foundation so the only heat loss to soil area is the bottom of the tank in my design. The Edmonton tank was 1/2 sitting in soil under the garage. 1/2 of the top of the tank was also under the garage slab. In my case these walls of the tank are inside the house - so the heat loss will be to the inside of the house. The biggest question in my mind is how long it does tank for the heat to be lost from the tank and whether the system should even store energy in the mid of summer if it will be lost relatively quickly to the house. One additional factor is that the tank is in a crawl space and 4' of the tank is planned to be surrounded by drain rock so the heat that is lost to this area should be somewhat sunk into the drain rock (I am not sure if this is a positive or negative though I am thinking positive as it would delay the heat loss to the house pushing closer towards the heating season).

    It is interesting that the Edmonton project chose to go with a forced air heating system that converts from the solar thermal into solar air and I could see that being quite a bit more complex than the system I am proposing where the incoming water that goes to a hot water boiler or hot water tank is preheated by the water in the tank. The Edmonton project also presumably looses the effects of a stratified water column as they circulate the water and it drains back into the tank causing turbulence and de-stratification.

    I agree on your 10k-20k number and I do like math so I'll calculate the BTU and payback costs and if solar pvc is quite a bit more cost effective I'll go that way. I am interested in the building science though so forgive me if I delve in depth into the tank option.

    I am sure you are correct that the tank will eventually develop a leak. The concrete will surely develop cracks and then depending on how long the EPDM last it would require replacement. As long as the tank is accessible it hopefully isn't too large of an issue to replace the EPDM. A larger concern for me is you potentially have this large tank of water sitting inside your house... which is not without risk of moisture issues.

    Thanks again for the reference and discussion. I appreciate the thoughts !


  6. GBA Editor
    Martin Holladay | | #6

    Good luck with your project. It would be great if you kept careful track of your costs, if you included energy monitoring equipment, and if you reported back with a guest blog after one year of system monitoring.

  7. GBA Editor
    Martin Holladay | | #7

    Here is another solar thermal system maintenance report, from an article I wrote for the October 2008 issue of Energy Design Update. The report comes from Larry -- he prefers not to use his last name, to maintain some privacy -- the owner of the award-winning solar house in Hanover, N.H. that was designed by Marc Rosenbaum.

    “The only major failure has been the storage tank for the solar hot water,” Larry told EDU. “The copper tank was built on-site by a guy from the Boston area; he used lapped soldered seams. Anyway, a seam on the bottom of the tank sprang a leak—a very small leak. It didn’t flood the basement, but it was leaking for a while, and it soaked the plywood and insulation installed around the bottom of the tank, so that by the time I noticed the leak there was a lot of slime and mold. This was actually the second time the tank failed—the first time was just a month or two after we moved in—so I elected not to repair the tank. I bought a commercially available 1,200-gallon steel tank lined with a rubber membrane. With shipping and fittings, the tank cost about $2,400. I pulled out the old tank last winter, and I’m still in the process of installing the new tank.” Compared to the tank problem, most of Larry’s other maintenance issues have been minor. “One circulator pump burned out—the main collector pump,” said Larry. “That cost $300. There is one other problem: up on the roof, 30 feet in the air, one stainless-steel batten on the site-built solar collector has lifted a corner. I haven’t repaired it because I don’t know how to get up there.”

  8. JimPancanke | | #8

    Thank you. I am definitely interested in tracking how effective the project ends up being and more than willing to share the results (even if I fail utterly).

    I did a quick calc on degree heating days and it looks like if I can achieve the 21% they achieved that would equate to 38% in my area even ignoring that my system should hopefully be more effective and efficient (though the temperature in Edmonton in summer probably provides more heat for storage so maybe not).

  9. JimPancanke | | #9


    Thanks for the reference.

    I found the youtube video:

    Alaska's First Net Zero Energy Homes Performance Update

  10. user-945061 | | #10

    Everything you need to know about thermal storage, cost effective R80 wall insulation, and GSHP's in homes with peak loss below 5000btu/hr, summed up in one handy video:

  11. JimPancanke | | #11


    Are you trying to say that the first step is a doozy?

    Hard not to like ground hog day.


  12. Airithol | | #12


    Here is some more reading. It includes Thorsten Chluup's build and others.

    One other thing I'd point out is that depending on how you plan to distribute the heat from storage, you might want to think about a vertical tank to take advantage of stratification. I recall Thorsten talking about this in one of the videos, the main point being that lower incoming temperatures are usable in a well stratified tank.

    Finally, if you have not been to, there are many DIY projects with detailed information.


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