Thermal Storage Batteries (Phase-Change Devices)

Anyone got experience of thermal storage batteries?


  • By directly heating water on demand - subject to heat stored, of course - do such devices get around possible problems of not knowing just how much hot water you have available, which is often a slight unknown for any cylinder of hot water?
  • I suppose the phase-change is from solid to liquid (or, more-or-less liquid), and then back again to solid?
  • Am I right in thinking a large box-shaped item in a corner is a lot easier to accommodate than a very large, bulky and heavy 200ltr water cylinder?

Thanks for any info anyone may have!

Hi Mike - I’ve been looking to get a Tepeo Zeb installed for a while. Your assumptions are correct, more info here;

If they had installers in the North of England I would have got one by now!

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Thanks. Recent discussion initiated by Tim Gilbert (in Home Energy Systems) indicates that a thermal storage battery will only do the required phase-change to store heat if the incoming heat is at about 53degC ; while many heat pumps can do this, I understand 53deg is near the top end of a HP’s range, requiring more electricity to reach such a temp, and therefore dropping the COP noticeably (for the period of the ‘melt’ of the storage material in the battery.) So it seems that, compared to a water cylinder, such a heat battery would be better re flexibility/reliability of on-demand hot water (or, near-on-demand), but at the cost of making the heat pump work at the edge of its ‘comfort zone’ to reach 53degC ; by contrast a cylinder of water can be heated by a more-typical heat pump output of 40degC, albeit over a longer time period, of course.
(You may guess that the rest of the household here would like to hang on to the luxury of the on-demand bath, as afforded by our current gas combi/condensing boiler.)

Yes we have 2no Sunamp duties that together make up our hot water storage. They work great and are very space saving relative to a water cylinder.
Yes when using a heat pump to heat up / phase change the Sunamp unit you will have a somewhat more restricted choice of models to choose from, although the more recent models tend to be able to operate up to ~60C with little problem. There may also (probably) be slight reduction in COP from your heat pump relative to cylinder HW storage, especially if you run your cylinder below 50C a lot of the time.
Also worth noting that Sunamp units have very low heat loss so they will store heat for a long time.Sunamp and other storage devices - #3 by zapaman

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Thanks, have now read that other thread, v useful - we’re in a regular 3bed semi, but no obvious stand- alone space for a bulky insulated cylinder, maybe 2 small ones might work tho.

The two cylinder arrangement suits houses (often larger or bungalows) where the plumbing hasn’t been considered in the layout, so there are ridiculously long pipe runs. The two cylinders can then be installed near their points of use and the long hot water pipe isolated or removed.

The two cylinder solution suits direct electric heating for at least one of the cylinders otherwise you end up with another long pipe run, from heat source to cylinder, and back.

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…I’m actually thinking that thermal storage has an end-result that goes against the grain: after spending some time and resources getting a house to run on low-water-temp heating, a thermal battery makes it necessary to crank the temp up again…

If you are using the thermal store for heating that could be the case but you would very quickly deplete your thermal reserve. You can have low temperature heating from a high temperature source by blending flow and return in the same way that underfloor heating can run at a lower temperature than radiators in the rest of the house.

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As per Tim’s reply I am not sure what you mean here.
A heat battery would most likely be used for Hot water storage.
The flow temp for hot water is set independently from the space heating so that allows rads or underfloor heating to be set for max efficiency.
You would only likely choose a heat battery (for hot water storage) if the space saving requirements were paramount anyway.


Yes quite, think I momentarily forgot the separation of DHW and heating there… so, the higher ASHP working temp to heat the thermal storage battery for DHW would therefore happen for a short-ish period, dropping back to the lower working temp when it reverts to space/radiator heating? (Re DHW ‘draw’, we’re talking max. of one bath per day here, plus a bit of manual dishwashing.)

The heat pump ought to be running at optimum flow temperature at all times that it is needed (so a high hysteresis). The draw off heat is independent of the heat pump temperature. You can simultaneously draw off hot water at, say, 55°C and domestic heating blended down to 35°C if that is what you need.

The theoretical maximum draw off temperature would be a degree or so less than the thermal store, unless you have a very low flow rate, which could potentially bring it up to storage temperature.

I’ve just realised that the above answer may not fit in with what you are planning. The above suits a thermal store with direct resistive heating.

With a heat pump it is probably more efficient, as you said, to run the heating direct from the heat pump and top up the thermal store for hot water (higher temperature) during off peak times.