Thermal batteries, are they worth it

@mike_killian got me thinking about thermal batteries and whether they are worth getting, particularly in modern rabbit hutch properties with no space for a cylinder.

Of course a thermal battery is technically anything that stores heat for later use, including night storage heaters and DHW cylinders. In this post and hopefully in the resulting thread I am looking for owners and users of the more compact phase change thermal batteries used for DHW and/or space heating.

I am particularly interested in their use with solar thermal and heat pump systems, whereas the bog standard system is exclusively electrically heated using resistive heating with a COP of “only” 1.

@mattfranklin I seem to remember that you had a heat pump system installed some time back. How do you find it after a year or more of experience?

Anyone else able to contribute?

My current heat store is a 300l gravity fed cylinder with a coil for solar thermal at the bottom, a second coil above that supplied by my heat pump and the top ⅓ can be heated by an immersion heater.
The cylinder is coated in spray foam from the factory and over that I have fitted a cylinder jacket as additional insulation. I am unlikely to change that arrangement but if my thirst for knowledge leads to a revelation I may well change my mind.

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I seem to recall reading that phase change thermal batteries need a relatively high temperature for the phase change and because of this the CoP of air sourced heat pumps suffers. However, some thermal batteries may be available that are specifically designed (in terms of the temperature needed for phase change) for heat pumps.

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Prompted by your post, I looked on the Sunamp website and found this:


So it seems you would need a high temperature range heat pump.

Edit: for completeness I found this:

The lack of support for solar thermal removes them as a potential for me but I was mainly asking out of interest.

I think other brands of heat battery are available and may be more suitable for either heat pump or solar thermal.

These days PV diversion is a growing area for thermal storage, so more details and practical experience would be good to here.

…so it seems that, compared to a water cylinder, such a heat battery would be better re flexibility/reliability of on-demand (or, near-on-demand) hot water, but at the cost of making the heat pump work at the edge of its ‘comfort zone’ to reach 53degC ; by contrast the cylinder of water can be heated by a more-typical heat pump output of 40degC, but over a longer time period, of course.
(The reader may guess that the rest of the household here would like to hang on to the luxury of the on-demand bath…)

See other thread for some more comments on my experience with Sunamp set up.
This graphic from my OpenEnergyMon dashboard may be of interest.
It shows the internal temperature inside my 2 Sunamp units. The labels should be relatively self explanatory but, Sunamp ASHP is connected to our HP and phase changes at ~50C and Sunamp iPV is (direct) electrically heated using PV diversion (or off-peak top up if required) and phase changes at ~57C.
You can see that because someone has just taken a shower that the Sunamp iPV unit is fully “depleted” but the Sunamp ASHP is fully topped up meaning there is still sufficient HW for the next person.

A few hours later (now 15:35h on 11 June) and the state of the Sunamp temperatures look like this

There are certain high temperature heat pumps on the market with which certain specific thermal batteries can be paired. We are talking about a sub-niche overlap of two niche markets. That equates to high cost. It also creates doubt about long term support for such a small niche

Agreed (mostly), although it is likely that more and more HPs will have the ability to operate up to 60C. Clearly you would not choose a heat battery thermal store unless its characteristics made that desirable in your set-up. That means primarily the space saving characteristics and the ability (for example) to fit your entire HW storage into the space normally occupied by (say) a washing machine.

I did consider getting the smallest Sunamp for our kitchen, to avoid long pipe runs. Unfortunately there was no space. I was hoping for something to fit behind the plinth.
Thinking about it afterwards I realised that would be impossible as there would be no space for the “innards” by the time enough insulation had been fitted. Wall mounting would have been an alternative if they didn’t weigh a metaphorical ton.