There is at least one large scale system in this category; the Caldera ‘Warmstone’ I understand this unit weighs in at ~1.5 tonnes and can store ~100 kWh of energy. The mind boggles as to whether many sites could install such units and I wonder if the electrical supply might need to be beefed up a little? Toodles.

Hi @editor . I started a thread about my experience owning a ZEB here: https://renewableheatinghub.co.uk/forums/unusual-heating-alteratives/heat-batteries/

Full disclosure: I work at tepeo, and my ZEB has been my primary heat source for my house over the last 3 heating seasons.

@toodles The tepeo ZEB is a residential heat battery, whereas the Caldera Warmstone is an industrial thermal store.

OK so ZEB is basically a variant on a storage heater, but designed to interface to a water borne central heating system.  It has a SCOP of 1, but because it stores energy can benefit from cheap electricity.  That makes its green credentials an interesting discussion, the carbon footprint (per kWh) of UK electricity is on average fairly close to that of gas, but decreasing - the main carbon advantage of a heat pump comes from its COP not because electricity is (at present) inherently cleaner.  There is some statistics to show that cheap rate electricity is however better than average, but not by a factor of 3-4 which is the typical SCOP of a heat pump system. 

ZEB has a storage capacity of 40kWh and a max electrical consumption of 9kW.  Assuming you get 6 hours of cheap rate electricity and during this time you both heat the house and charge, that makes it suitable for a house with a heat loss of up to about 2.5kW, 3kW at a push, so its for low loss/small houses only unless you are prepared to pay standard rate for s significant proportion of your heating.  This wont cover DHW though and the ZEB website recommends a sunamp or immersion.

So given that what are the use cases?  The obvious ones are

1. where it is impossible to install a heat pump, not even one of the 'through wall' types that dont have an ODU (eg where the freeholder/landlord prohibits drilling though the wall of a leasehold/rented property, or a property with thick outside walls and no or little external space) 

2. very low emission homes where the running cost is anyway small and so the (likely, depending on tarrif) higher running cost relative to a heat pump is insignificant

I can see that a material proportion of homes do fit into this category although freeholder/landlord prohibition on drilling through walls really should be outlawed!  Its unclear how long the currently generous ToU tariffs will be around for so there is a risk associated with that, but at the same time it's unlikely that ToU incentives will disappear altogether, they could even become more common if load management gets more difficult as electrification proceeds.

For me this is a solution of last resort, but one for which there is a sizeable niche (eg modestly sized leasehold flats with an awkward freeholder - probably the majority!) albeit a niche which could, and arguably should, largely be eliminated by legislation!

The bigger problem which occurs is that it's basically aimed at properties with an existing heating system.  What's the incentive to get a much larger unit in a property that likely is quite small and that will almost certainly be more expensive to run to replace than the existing boiler (because ToU rates aren't that cheap and the COP is 1) with a marginal improvement in carbon footprint?

@jamespa a ZEB is a variant of a storage heater in the same way an A2W heat pump is a variant of a fridge: they use the same underlying physics as their counterparts (dry core thermal storage, the vapour compression cycle) and interface that to a hydronic heating system with an advanced control system. But those are pretty unhelpful comparisons for the layperson, as both heat battery and heat pump are step changes in the application and performance of their respective technologies. They are both very effective approaches to decarbonising residential heating, and transitioning homes away from burning gas and oil.

The Government's 2025 conversion factors spreadsheet gives natural gas a gross calorific value of 183gCO2/kWh, which rises to 215gCO2/kWh for heat delivered from an 85% efficient gas boiler. The national average grid carbon intensity during the last heating season was 124gCO2/kWh, with the regional grid average being almost the same at 123gCO2/kWh where I live. That's under 60% of the carbon impact of burning gas. The distribution of carbon intensity isn't completely uniform across the day either, so last winter my ZEB used electricity with an average intensity of 101gCO2/kWh. Overall it consumed 8776kWh for a total carbon impact of 889kgCO2, compared with 1886kgCO2 if I'd burnt the same quantity of gas.

There are numerous studies (BEIS, CCC, Nesta, etc.) showing that all of the UK housing stock is technically capable of being heated with a heat pump. However the same studies also show that 20% are not practicably heat pump-able, including homes without outside space, in conservation areas and heritage sites, upper floor flats, and so on. And as you point out, there's the friction between free- and lease-holders to account for as well. 20% is several million homes, so there needs to be options for those homeowners other than heat pumps, otherwise they'll never make the transition.

While heat batteries are ideal for smaller properties my 1980 chalet bungalow with cavity walls and double glazing has a floor area of about 100m2 (1080ft2) and has a heat transfer coefficient of ~150W/K (a heat loss at -3C of about 3.6kW). It is neither a small house nor is it particularly well insulated, but paired with a dual-rate tariff the ZEB managed 94% of it's charging last winter during the off-peak overnight window. That equated to 7.8p/kWh across the heating season, which is pretty close to what I'd be paying for gas (at the moment!).

I think time-of-use tariffs will be with us for a long time, but I can see dual-rate tariffs eventually running their course and being replaced with multi-rate tariffs that reflect the in-day variation of wholesale costs. It seems reasonable that energy suppliers will use tariffs to influence the consumption of their customers to align with their own costs. But ultimately heat batteries are inherently hugely flexible and are able to contribute to balancing the grid, so that is also an opportunity for homeowners to offset some of their heating costs without any impact on their comfort.

Having heat batteries available as one of the options for a small but sizeable proportion of the UK housing stock seems like the right thing to do, given that they can significantly reduce emissions and aren't super expensive to run. For me the last resort for decarbonising heating is a toss-up between a direct electric flow boiler and a biomass boiler, but even those can have their place if they're transitioning a home away from gas.

There's an article from RHH coming soon which will provide additional information about the tepeo ZEB heat battery boiler. And I'm happy to share my feedback of my own ownership experience with a ZEB.

Posted by: @jamespa

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That makes its green credentials an interesting discussion, the carbon footprint (per kWh) of UK electricity is on average fairly close to that of gas, but decreasing - the main carbon advantage of a heat pump comes from its COP not because electricity is (at present) inherently cleaner. 

I had a read through the link. 
I may have missed the worked out summary…
But since electricity is currently around 50% fossil fuel free and the fossil fuel is basically gas, unless conversion and electricity losses are over 50%, they could not cause more emissions than gas. Which is only max 85% converted into usable heat.. 

Posted by: @wully

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The Government's 2025 conversion factors spreadsheet gives natural gas a gross calorific value of 183gCO2/kWh, which rises to 215gCO2/kWh for heat delivered from an 85% efficient gas boiler. The national average grid carbon intensity during the last heating season was 124gCO2/kWh, with the regional grid average being almost the same at 123gCO2/kWh where I live. That's under 60% of the carbon impact of burning gas. The distribution of carbon intensity isn't completely uniform across the day either, so last winter my ZEB used electricity with an average intensity of 101gCO2/kWh. Overall it consumed 8776kWh for a total carbon impact of 889kgCO2, compared with 1886kgCO2 if I'd burnt the same quantity of gas.

Sorry you are correct, I was remembering figures from the past and electricity really has got a lot greener by quite a bit more than I had thought.  @batpred apologies to you too!

The real-world totals for the year are interesting, presumably from near real time carbon intensity data.

Posted by: @wully

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Having heat batteries available as one of the options for a small but sizeable proportion of the UK housing stock seems like the right thing to do, given that they can significantly reduce emissions and aren't super expensive to run.

Agreed, I did describe it as a 'sizeable niche' in my original post!

Posted by: @wully

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While heat batteries are ideal for smaller properties my 1980 chalet bungalow with cavity walls and double glazing has a floor area of about 100m2 (1080ft2) and has a heat transfer coefficient of ~150W/K (a heat loss at -3C of about 3.6kW). It is neither a small house nor is it particularly well insulated, but paired with a dual-rate tariff the ZEB managed 94% of it's charging last winter during the off-peak overnight window. That equated to 7.8p/kWh across the heating season, which is pretty close to what I'd be paying for gas (at the moment!).

Thats very believable for a 3.6kW property.  The Ofgem typical house has a gas consumption of 11.2MWh/year, so a bit more than your 8.8MWh, which would presumably increase the average cost.  However that would be partially offset by the saving in standing charge if the conversion allowed one to move away from gas completely.