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Seasonal thermal energy storage

Aedi | Posted in General Questions on

Hi all,

This is more for a general discussion than a specific question. I noticed that the “Why solar since net metering is over?” (https://www.greenbuildingadvisor.com/question/why-solar-since-net-metering-is-over) thread was generating a lot of discussion, and it got me thinking about alternatives to batteries for storing energy.

In temperate climates, seasonal thermal storage seems to make a lot of sense on paper. After all, there is usually a surplus of solar in the summer, and a shortage in the winter for space heating. But I haven’t heard much about it on the residential scale. Wikipedia has a good overview of the different technologies: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage

Has anyone had any experience with seasonal thermal storage solutions? Are any of these technologies reasonable at the residential scale, or do they have potential to be? Do they have any role at all in the buildings of the future? Or will alternative technologies always be cheaper and more sensible?

For example, aquifer thermal energy storage seems really cool (if situational), but I don’t think it would ever make sense for a home without a geothermal heat pump, which are quite expensive. You’d have to be able to store a lot of energy in order to have a reasonable buyback period. Though I suppose if your house has greenhouse, it could work.

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Replies

  1. Jon R | | #1

    I've run the numbers and while it is possible, it's not something you want to do. On the other hand, short term thermal storage (1-3 days) can be easy, cost effective and increasingly important as we use more unreliable renewable energy.

    1. andyfrog | | #6

      This is really interesting, can you elaborate on short term thermal storage?

      1. Expert Member
        Zephyr7 | | #7

        The easiest way to do it is to use large volumes of water in insulated containers. I have designed cooling systems at work this way. I design datacenter facilities, and we can't loose cooling in those facilities since the computers are all backed up and are constantly producing heat. The chillers that run the chilled water cooling plant can't immediately restart after a power interruption, so even when the generators kick in (about 10 seconds or so after an outage), the chillers won't typically be back up at full power for 10-15 minutes or so. It is inefficient to use battery backup on the chillers, since the UPS systems are only about 94% efficient, and the batteries are a maintenance item. What I do is to use large insulated tanks of water in the chilled water loop, and using the specific heat of water, I can calculate the volume of water required to maintain the datacenter temperature within a specific target range during the time the chillers are offline. The water tanks don't waste any energy (maybe a tiny bit, but much less than the UPS systems would), and they don't need to be periodically replaced, either.

        People using wood fired boilers often used large tanks (500-1,000 gallons or so is common) to improve their system efficiency so that they only have to fire their boiler once a day, then use the energy stored in the big tanks of hot water to heat their home.

        Solid thermal mass can be used also, but it's usually easier to work with water, especially in the case of an existing hydronic system such as the case with a boiler or chiller.

        Bill

  2. Expert Member
    Michael Maines | | #2

    With enough earth and windows, you can build a passive annual heat storage (PAHS) or "umbrella house."

    https://www.amazon.com/Passive-Annual-Heat-Storage-Improving/dp/0615905889

    https://www.greenbuildingadvisor.com/question/new-earth-sheltered-pahs-construction

    There are numerous potential moisture issues but some people have done it successfully. Most of us have found that building above grade, using pretty good levels of insulation and air sealing, with good windows and orientation, costs very little for annual heating and cooling, with fewer downsides. But if you like the idea of living below grade, check out the book above.

  3. Expert Member
    Malcolm Taylor | | #3

    Aedi,

    At present it seems like most new houses can quite easily be made efficient enough that their plug-loads exceed the amount of electricity needed for space heat - and that excess electrical production or storage will become increasingly useful for other purposes, like subsidizing the energy used in transport. So it seems that as the case for electrical generation and storage grows stronger, that for heat storage diminishes.

    One approach commonly used now is to try and balance the cost of increasing of insulation to that of installing PV systems. I'd suggest the same balancing equations would make sense, looking at reducing heating costs through more insulation or instead providing thermal storage.

    1. Jon R | | #4

      Aedi, I encourage you to at least take a stab at comparing environmental damage (vs just cost). And compare to things other than just the house itself. For example, an electric car might do more good/$ than more insulation, rooftop PV or thermal storage. Money is a limited resource - spend it wisely.

    2. Aedi | | #5

      Malcolm,

      This is a sound answer. It is true that it is simple to design a solar system capable of meeting winter loads even in northern climates (if you deal with the snow), but doing so can mean you are generating much more than you need in the summer. Finding a use for that excess energy is important, and using it in transportation or some other sector seems more helpful than trying to store it for winter. It does seem likely that technology will improve to the point where that is a practical solution.And of course, balancing equations are always a practical approach.

  4. GBA Editor
    Martin Holladay | | #8

    Aedi,
    For a deep dive into potential problems with this approach, see "Revisiting the Sunrise House in Fairbanks."

    The basic problem with seasonal thermal storage is that the economic value of the stored energy is too low to justify the very high cost of the hardware required to collect and store it. The hardware investment is questionable -- in terms both of basic economics, and in terms of the environmental impact of the equipment manufacture and installation.

    1. DCContrarian | | #9

      That's a good article. What I get from the story is that the system was never properly engineered. I suspect if it had been engineered it never would have been built.

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