Commercialising individual Seasonal Thermal Energy storage

  • I've become a little obsessed with Seasonal Thermal Energy Storage (STES) and it's practical implementations e.g. Borehole Thermal Energy storage (BTES), Underground Thermal energy storage (UTES).

    The projects I've seen on the internet are fairly poorly documented & communicated and all done on a community-wide scale. I think there's massive potential here we are missing, residential and industrial heating and cooling make up big chunks of global energy use. I'm wondering if there is a way to commercialise UTES on an individual level, so the same way a homeowner chooses to buy a Tesla power wall, they could choose to buy a Seasonal thermal battery.
    I'm thinking of jumping in and trying to bury a thermal energy store underground and heat it up but I'm struggling to make the trade-offs in design due to lack of understanding of the heat loss and gain.
    Anyone know of anybody doing something similar, or anyone who can give me advice on some of the calculations? Any feedback much appreciated!

  • Hey @Simon-Shepard, this sounds like really interesting potential storage tech.

    @Eric-Chaves is working on energy storage as well, more so on the Underground Pumped Hydro Storage (UPHS) side of things. Different tech obviously but he might have some interesting thoughts here.

    Check out this post to learn about his project

  • @Simon-Shepard what is driving the untapped potential that you see for STES technologies? Why do you think they haven't been leveraged yet? Sounds like an interesting idea, I'm just curious for more info

  • Hi, I am not working in this field, but recently I have made the "back-of-the-envelope calculation" of the needed thickness of insulation to keep for example a volume of water at high temperature during a given time... the point is that the "decay time" of the temperature is proportional to the ratio surface over volume, i.e. the size of the thermal storage... so I think it is the reason why large scale storage is preferable, unless vacuum based insulation is used (cryogenic storage dewar), or phase change material ?

    Assuming uniform temperature inside the tank, and only thermal conduction through the insulation, the equation for temperature evolution is:

    rhoCp*V *dT/dt = -k/th * S ( T - T_exterior)


    • rhoCp: density times specific heat capacity of the storage material
    • k: thermal conductivity of the insulation layer
    • th: thickness of the insulation layer
    • V and S are the Volume and Surface of the tank

    the solution is T(t) = T0 exp(-t/tau) where tau is the decay time:

    tau = k/rhoCp * S/( V * th )

    thus the S/V ratio

  • @Pascal-Ramsey I think there are 3 core challenges that prevent STES leverage on an individual property level,

    a) Most ground source heat pumps, involve tearing up a fairly large area of your property to install the ground loop under, people don't want to see their entire lawn shredded and you need to get heavy machinery like a digger onsite. Potentially this is where BTES could come in.

    b) No one has figured out the best way to capture the sun's heat in the summer for pumping underground. Evacuated solar tubes are expensive and may not be needed - you need to trade this off against cost, so you need to figure out how to make a cost-efficient solar thermal collector for the task.

    c) Hooking this up to an existing properties' heating infrastructure, so this is the real challenge if homes are using water-based conductive heating (e.g. underfloor hot water pipes) or hot air you need to build 'integrations' for each of these different methods.

    Overall, one reason that this isn't being leveraged more is that a lot of the world uses natural gas to heat water to heat their home and the cost is so low, there's not a very incentive to switch. The cost of STES has to be really low to make sense for most people.

    I found an interesting video of a private person installation here:

  • @xdze2 The more research I have done, the more I am moving away from the idea of storing the heat in a liquid or a phase change material, it adds a level of extra cost and maintenance to the system that I think just won't pay off in the end. There is another company I found trying to do this :

    I am thinking more and more that the storage of the heat in the soil itself is the simplest and most effective thing to do. Charging the soil up in the beginning and then recharging it every year. Like these guys suggest :

  • @Simon-Shepard I agree with you that direct storage in soil is a simpler solution, nevertheless the reasoning about the ratio surface over volume is similar... I think the next problem is about geology, type of soil, permeability... which I know nothing about

    it make me thinking about another startup I have seen some years ago:

    the idea is to use compressed air to store energy, but because heat is generated during compression, in order to increase efficiency there is also a heat storage system included (I think by circulating the hot air through a rock bed). They plan to use existing cavity in mountain, like cave or old tunnel...

  • @xdze2 That's an interesting link - thanks. I also wondered about compressed air storage, Low tech engineering blog did quite a good article about it a while back :

  • Hey @Simon-Shepard,

    I'm not actively working on this tech, but I've read just a little bit and it sounds like it's very cost-effective.

    Check out what Princeton University is implementing on their campus.
    Ted Borer - Energy Plant Directory at Princeton University. See video at 1:25.

    Thanks for the shout out @Eric-Vanular. My research into UPHS has now led to a company called Terrament. In the near term we're actually focusing on solid-mass underground gravity storage instead of UPHS -- just because it's easier to start with. Here's our latest semi-public slide deck if anyone wants to check it out.

  • Hey @Eric-Chaves,

    thanks for the video link, really interesting to see that Princeton is also following the same line of thought.
    Also good luck with Terrament, I think it's a great technology to pursue and could be a vital part of our renewable energy storage landscape. If only there was a way to create an efficient smaller home version, I'd be up for prototyping it in my backyard - I spent some time researching hydraulic accumulators to see if I could think of a way to use one to easily/losslessly store energy, but I couldn't come up with a way to make it work.

  • @Simon-Shepard have your thoughts or plans progressed on the STES front? I thought this was a really interesting idea and would love to follow any pilot projects that might be happening

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