Do ASHPs work with heat batteries?

[ReedRichards]

A heat battery uses a phase change material that can absorb and re-transmit a lot of heat at one particular temperature, the temperature at which the phase change takes place.  The standard Sunamp heat battery uses a material where the phase change takes place at about 58 C.  So to charge a Sunamp heat battery with water you need to supply it with water at above 58 C (it could be just above) and the water leaving the battery will be at 58 C.  It's pushing a heat pump to achieve that sort of water temperature and if it can do it then the CoP will not be good.

 

On the other hand if you are heating a tank of water then every degree by which you raise the temperature takes the same amount of energy so if you are heating from cold you start off with a good CoP which diminishes as the tank temperature increases.  In my case the tank temperature is set to 50 C and the leaving water temperature gets to about 55 C to achieve this.

 

So you have the option of a heat battery where almost the entire charging process takes place at a temperature where the CoP will be poor or a tank of water where only the last few degrees of charging will approach a similar poor CoP.  Therefore I was surprised to read that some contributors use heat batteries because I would have thought they were ill-suited to efficient use with a heat pump.

 

Am I wrong?     

[Marvin]

Could it be there using them by heating with PV electricity during the day?

[SteamyTea]

The two main advantages that the Sunamp has is the compact size and the low thermal losses.

Not sure if that is worth the extra money.

 

[TW9]

Sunamp now sell heat batteries with PCM that works at different temperatures. One of ours charges at 45 degrees.

[J1mbo]

I was looking at these, thinking that they would be a better option than buffer cylinders. But I suppose it depends how quickly energy can be transferred to and from? i.e. the kW that a heat battery can sink at dT of 5°. With dual coils it could be buffer cylinder and heat exchanger in one.

Edited December 6, 2021 by J1mbo

[ReedRichards]

58 minutes ago, TW9 said:

Sunamp now sell heat batteries with PCM that works at different temperatures. One of ours charges at 45 degrees.

 

Ah, so that's it;  45 C would be much better suited to a heat pump.  Do you have to pay a premium for a non-standard PCM?

 

2 hours ago, SteamyTea said:

The two main advantages that the Sunamp has is the compact size and the low thermal losses.

Not sure if that is worth the extra money.

 

 

I agree, although if you don't have an airing cupboard then the compact size might be particularly useful.

[TW9]

I didn't pay for mine so I can't tell you if there's a price difference for the lower temperature PCM.

 

I don't think a heat battery is suitable as a replacement for a buffer tank. It's better used as a thermal store. We have a small buffer then the two heat batteries are used as thermal stores. The system pulls heat from them for the central heating and only uses the heat pump when the batteries have been drained.

 

We have separate heat batteries for hot water.

Edited December 6, 2021 by TW9

[joth]

It's a bit surprising SunAmp still aren't on top of this. It would be ~fairly simple to integrate a Willis (immersion) heater inline on the water coil input to the heat battery, and have a tiny bit of control logic that monitors temperature and flow rate on the coil and automatically "tops up" the incoming water temp to whatever is needed to achieve the phase change. This would make it a "drop in" replacement for a UVC on thermal store, and could work out of the box with any hot water source (be it air or ground source heat pump, or gas/oil system boiler, back-boiler or solar thermal). And it could serve as the backup if the primary heat source failed, and the basis for a PV divert solution.

 

Instead they seem to be on a path to developing many different PCM types and making custom controller firmware and in-house  compatibility testing with every heat pump, which is really struggling to scale. Even if over the long-arc they do find a market for different PCMs, in the meantime they really need to establish themselves as reliable and easy to install, and a catch-all solution like this would help them get there.

 

[joth]

I came across this on the Octopus blog the other day: https://tepeo.com/thezeb

 

Startup working on "better than PCM" density for heat storage. (Even if the rest of the product website is hot air, I do like their use #ShowYourStripes poster in their mock up images)

 

[JohnMo]

Sounds like a 40kW storage heater, with a heat exchanger to transfer heated air to water

[TW9]

They mention time of use tariffs as the key to this working, by charging the battery when electricity is cheap. But the only company who have ever offered one is Octopus and that's currently closed due to instability and rising prices in the electricity market.

 

OVO have a platform called Kaluza that can be used to manage devices like this. It's operational but still being trialled in a domestic environment. I can't see how this company are going to compete with OVO on something this complex.

[ReedRichards]

There is still Economy 7, a very basic time-of-use tariff.  

[JohnMo]

Is Sumamp the only people out there, that have a thermal battery that can be charged with water and electricity?