Domestic Thermal Storage 3: Concrete

Friends, this is the third and final article comparing different types of thermal storage.

In previous articles I looked at the humble domestic hot water (DHW) cylinder and the use of a phase-change materials (PCMs) to store heat.

In this article I will look at the use of ‘very big lumps of something very hot‘ as a thermal store.

This brute force technology is implemented in a so-called Zero Emission Boiler (ZEB) from the Tepeo company, and in a behemoth of a storage device akin to a miniature version of the ‘Sand Battery’ that got me started on this, the Warmstone store from the Caldera company.

Zero Emission Boiler (ZEB)

A ZEB thermal store is a device about the size of washing machine that is conceived as replacement for a gas boiler in settings where a heat pump cannot be installed.

It is generally placed wherever the gas boiler was, but being essentially a large lump of concrete, it is extremely heavy – 370 kg – and must be placed only on the ground floor of a house.

Click on the image for a larger version. A ZEB thermal store in an extraordinarily uncluttered kitchen as pictured on the Tepeo website.

The difference between a ZEB and the other thermal storage devices I have described is in the amount of energy which can be stored – a ZEB can store 40 kWh of thermal energy – around 5 times more than a typical DHW cylinder. This is enough thermal energy to not just provide hot water to a house (typically 5 to 10 kWh/day) but also to heat an entire home via its central heating system.

One can imagine a ZEB as being a centralised version of old-fashioned electrical storage heaters. It stores thermal energy by electrically heating a block of ‘high-density concrete’ to an astonishing 800 °C.

At these high temperatures, heat loss is significant, but my estimates suggest that ~150 mm of insulation around a 40 cm cubical block, should limit heat losses to ~ 5.6 kWh/day or around 14%/day.

Extracting thermal energy from a ZEB at 800 °C into water flowing at (say) 50 °C is tricky. Slightly to my surprise, energy cannot be extracted rapidly enough for this to instantly heat water and so it cannot be used to replace a combination boiler. It must still be used with a DHW cylinder as an intermediate store of hot water. However it seems likely to me that Tepeo will solve this problem in the next few years.

Below is a YouTube video  in which Robert Llewelyn discusses the ZEB he has had installed in his own home.


A Warmstone thermal store is something like a miniature version of the ‘Sand Battery‘ that claimed to store heat inter-seasonally. But instead of storing 8 MWh like the ‘sand battery’ – it stores ‘only’ 0.1 MWh or 100 kWh.

It achieves this large capacity by heating a large mass of material – probably concrete of some description – to 500 °C. But it uses vacuum installation to reduce the heat losses to just 4.8 kWh/day – or 5% per day – half the fractional losses of a DHW cylinder or PCM material.

Unsurprisingly the device is large and heavy, weighing 1.8 tonnes and standing 1.8 metres tall with a diameter of 1 metre, so this too large for it to pretend to be a domestic installation. The company imagine it being used in large homes which have outhouses or large gardens.


In the first article I looked at thermal storage in a DHW cylinder. This is the default thermal storage that many people still have in their homes – holding about 8 kWh of thermal energy.

In the second article I looked at Sunamp’s PCM storage which can operate in practice like a DHW cylinder – but is typically less than half the physical size while storing similar amounts of thermal energy.

In this last article I looked at two companies looking to ‘go large’ and store one to two days use of thermal energy for a home. To achieve this they have used large masses of stored material electrically-heated to high temperatures. Surprisingly, despite the large masses and high temperatures, the rate at which water can be heated by these devices is still (currently) limited and so they must both still be used with a DHW cylinder.

All the technologies beyond the basic DHW cylinder all feature computer technology which allows Apps to control the storage and allow integration with smart home controllers – something which is apparently very important, but is an area in which I have absolutely no interest: sorry.

What I learned in writing these articles is the very simple lesson that there is no magic to thermal storage technology. There are no magic materials and there is no magical insulation. To store more energy one simply needs a large mass of material, heated to a high temperature, and kept as well insulated as possible.

15 Responses to “Domestic Thermal Storage 3: Concrete”

  1. Nicola Terry Says:

    When it comes to thermal store materials, the rate at which you can get heat in and out is important. Water is fine because it flows and mixes but for solids they need reasonable conductivity as well excellent specific heat capacity. Hence it doesn’t surprise me that solid stores struggle to replace a combi boiler where it has to supply a lot of heat quickly. But I did not know that so thank-you.

    • protonsforbreakfast Says:

      Good Morning.

      I didn’t mention the thermal conductivity issue but it is – as you say – a problem for all thermal storage devices.

      For the PCM devices – the pipes become coated in solid material which inhibits convective heat transfer. For the high temperature devices I think the way heat is extracted is covered by patents for the Caldera device. So presumably some clever engineering is used to overcome the basic problem that the heat storage media are typically poor thermal conductors!

      And if they ever do sort out the problem of heating quickly – then in this part of the world they will have to contend with a serious limescale problem.

      Best wishes


      • Peter Says:

        Very interesting article. I am looking to replace my conventional heating and hot water system with the minimal of disruption to my house. I was looking at solar for hot water and wondered if seperate solar water tank together with a mains electricity electric boiler could be used for the heating?

      • protonsforbreakfast Says:


        “I am looking to replace my conventional heating and hot water system with the minimal of disruption to my house. ”

        You and many other people! There are so many options available now that I really can’t think what to recommend except to ask experts. You can use the HeatGeek web site to search for local installers.

        In this house, I chose to go all electrical. With Solar PV, a 13.5 kWh battery, an air source heat pump, and a conventional DHW storage cylinder. I did this in part because was re-wiring (though pretty disruptive) is generally more flexible than re-plumbing. I also considered the technology more reliable.

        But there are other combinations of generation and storage that also work well. It all depends on how much money and time you have available, and the existing state of your home.

        In any case: I wish you good luck!


  2. Matteo Ricci Cipolloni Says:

    What about hydrated salts as storage
    This is a stream of research publications that start from 2017.
    Maybe an article about this could be interesting

    • protonsforbreakfast Says:

      Thank you for the linked article. That looks like a really interesting idea that I had not heard of. I can think of all kinds of difficulties, but I think there could be niches where it is a great solution.

      Do you know if there are any commercial products available yet?

  3. Aaron Reese Says:

    I think you should reach out to Richard Campbell from .Net Rocks. (A US based podcast focussing on programming) they have a regular ‘geek out’ episode where they discuss the current state of technology in various areas (nuclear fusion space travel, alternative fuels etc). In an accessible and grown up way without the hysteria. I am sure you would be able to provide valuable context and commentary on the subjects they cover.
    Aaron Reese

    • protonsforbreakfast Says:

      Aaron, Good Morning, and thank you for your thoughts. I will check out that podcast!

      Best wishes


  4. David Cawkwell Says:

    Thanks for the article.

    This is something I have been thinking of. I have just moved to Portugal to an off grid house and although initially I use propane for heating. I will have surplus solar energy in Spring, Summer and Autumn. Portugal usually has sunny days and colder nights in the winter. I have decided to put together a few either steel drums or second hand hot water tanks insulate well and use the solar to heat them up to power the underfloor heating. I can then use propane to boost this should it be needed in poor weather.

    I have studied the Earth Ship houses that use a large thermal mass to smooth out temperature but I think they make the mistake of putting the thermal mass in the house where it is less controllable. A better approach it to have the thermal mass separate to heat storage.

    I think heat pumps are a mistake in the UK they do not work well in the coldest periods of British weather.

    • protonsforbreakfast Says:

      David, Good morning from the UK

      First of all, thank you for stopping by and good luck with your off-grid adventure!

      I’m guessing you would be using water in the steel drums or hot water cylinders? I don’t know enough about your system to comment in detail, but here are a couple of thoughts.

      1. One system might be to have pipes from the under floor heating circulate through your thermal store. If you got the thermal mass about right, in summer this could cool the house and heat it in winter.

      2. Try to get as numerical as you can. A 200 litre water container has around 10 kWh of stored thermal energy at 60 °C. So if you know your winter demand in kWh/day, that will give a handle on the likely size of your thermal store. My guess is it would need to be quite big – perhaps 20 cubic metres.

      3. It’s a tricky job to both insulate the thermal store AND charge it via solar. Perhaps the store could be buried and you could cover or uncover it in autumn/spring.

      Regarding heat pumps in the UK, I have to tell you Heat Pumps work excellently in the UK. My own air source heat pump kept us warm last winter and even on the coldest days (-3 °C) operated with 250% efficiency. And they will keep working to well below -20 °C. Over the year supplying hot water and heating, it’s seasonally averaged efficiency was 350%. A ground source heat pump operating off your thermal store would likely operate very well in Portugal. Some ground source heat pumps can operate reversibly and heat the ground in summer providing cooling to the UFH – basically using your house as the solar collector.

      Best wishes


  5. Andrew Waugh Says:

    Good Morning, I’ve only recently come across your blog and while I have not yet gone through everything you’ve written I thought I’d drop in a thought of my own while I have a spare moment.

    Regarding heat storage I note all the various systems/methods described. Most seem to be “wet” systems, and all seem to have more or less the same issue – extraction of stored energy.
    You mention heat losses from such systems and yet nowhere do I see any utilisation of the losses – that’s surely a waste in itself?

    I cannot see anywhere that there’s a warm air heating system used.

    Surely such a system could utilise both general heat losses and stored heat inputs much more efficiently?
    It would also simplifies energy extraction since no liquids are involved – a simple air/air heat exchanger is all that’s needed and temperature modulation is a piece of cake.

    I have to declare a couple of interests – I have a warm air heating system at home, I also have solar PV + battery (7 years in now and loads of data) – and for my sins I’m in the industrial heating business, so “heat loss” is pretty much my stock in trade.

    • protonsforbreakfast Says:

      Andrew, Good Afternoon,

      Warm air heat is unusual in the UK, which is a shame, because it is well suited to the requirements of many domestic settings. My guess is that because heat is distributed with ducts rather than pipes it takes up slightly more space than conventional central heating, and space is especially tight in UK homes compared with (say) US homes. But in new builds I think it would be a great idea – using even lower temperatures than underfloor heating.

      Regarding heat losses from heat stores, I think they are in fact considered. Heat losses from a DHW tank, a PSM store, a ZEB boiler or an AGA cooker all leak into the house and – in the heating season – this is considered a benefit. For a DHW tank cupboard is often used for ‘airing’ clothes.

      I think the reason for the predominance of ‘wet storage’ is because of teh predominance of ‘wet heating’.

      That sounds like a very interesting set up and I would be interested to see teh data. Do you have a web page? A friend sent me his solar data to analyse ( and I would be happy to do the same for your data if you were interested.

      In any case, best wishes


  6. Andrew Waugh Says:

    Thanks for the response Michael. My home was built in 79 when warm air was in vogue. The upside is we have summer “air conditioning” which really only blows air around the house but it does help.
    I am thinking of building a substantial heat storage device for myself so looking around for useful hints. I am tending towards a steel core (happen to have plenty of that around) and a concrete outer casing, a little like a nuclear reactor – but without the glow-in-the-dark features.
    I’ll think about the data share, it’s all in a clunky spreadsheet.

  7. protonsforbreakfast Says:

    It sounds fascinating. If you are interested I would be happy to send you my ‘clunky spreadsheet’ that I used for estimating heat loss from storage: but I guess you are probably all over that alll ready. M

  8. John Fors Says:


    I really enjoyed your summary articles on thermal storage.

    Over the last couple of years I have done a electric-only conversion of my house, which sounds very similar to what seem to have done. Although the water based storage is the least energy dense in your review, it sure has numerous important advantages, including – low cost off-the-shelf components, no advanced controls, minimal risk of fire or injury in case of leak of super heated materials, allows modular installation, well suited for DYI work, and so on.

    I am especially interested in the combination of PV-with-thermal storage for day-to-night home heating which uses a very large share of residential energy in US. Presently, one can get a reasonable air-to-water heat pump with 300L tank for around $2-3K + installation. Combined with 1-2 additional passive large storage tanks at $1-2K each this can support a substantial heating capacity.

    However, the crucial missing part of the discussion is that there must be a way to actually use the thermal heat for heating the house. The most obvious approach is to have a hydroponic heating system, but this will be a quite invasive system to install after the fact. Far easier is it to retrofit a home with mini-split air-to-air heat pump system, to handle both heating and cooling. The natural over-night energy storage then needs to be batteries. In the US a DYI 10kWh battery is around $3K. For a modern average sized home heat pumps may need 30-40kWh/day, which thus would require around $10-15K for batteries and inverters. I believe it will be difficult to install a thermal energy storage + hydroponics at lower cost. And it is reasonable to believe battery prices will continue to decline, even if gradually.

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