Battery or thermal storage for HP timeshifting

[CharlieB]

Wouldn't just going for a big battery also have the advantage of allowing you to use off peak, cheap, low carbon electricity for all of your electricity needs. I. E. You kill two birds with one larger stone, rather than having to find space for additional thermal store?

Yes there'd clearly be that benefit, but I'm not sure it would add up much. I haven't done the sums but my sense is time-shifting other electric loads won't make much difference. In my case, and plenty of others where HPs might replace oil not gas, space for an outdoor thermal store wouldn't be an issue.

To be honest the heading of this thread is a bit missleading. I want to get a sense of whether thermal storage to significantly timeshift heat pump operation can make sense. It's not really an either or for me.

[Andy]

You can probably plug your own numbers in to what I have done. I have a 12 kW GSHP .
[edit ops. Slipped a digit. Figures corrected]
For my house when it is cold like for instance now it's around -1.7ºC. This is sometimes for large periods of time. My GSHP COP is at 3.5 with an input of 1.4Kw and output of 5.33kW. That's 127kWh for the day. The radiators are running at 42.4 out and 35.4 return for an average of 38.9. So to get the same out put you need your flow to be at least 42.4 to get the same results.

So without tank losses you are looking at 127000/1.16/17 hrs=7470 litres to able to drive the radiators. If you store at double the temperature ie 49 degrees then you will need approx 3235 litres of storage. However, the COP plummets as you go nearer 50 degrees. Looking at my DHW tank the COP is 2.46 at 49 top/45 bottom degrees. The temperature in reality will need to be a lot higher to counter mixing as you circulate the water at some speed. From what I can find it is really hard to counter mixing.

Now, assuming your non peak times are 7 hours then you will need 127000/2.7 (an average cop) 47kWh input or 6.7kW from the grid to your HP to charge the tank let alone service your heating. Single phase wouldn't be able to recharge in the time required.

A 7800 litre tank is not cheap and will need the fittings for a very large coil.

My battery of 38kWh (about the same foot print as 280liter tank) will do the 17hrs* 1.4kW and 23.8kWh and is easily charged in the 7hours. I am looking to increase it soon. There is then another 8 usable kWh for the house. Any sunshine also goes directly to the house/heat pump before charging the batteries. I went the battery route which also gave me power cut resilience and more savings throughout the year. I'd recommend the home brew battery thread which gets the batteries cheap which can then be coupled with a Victron Multiplus/Quattro or similar. It's also got the ability to service the house all year which saves significant sums of money.

https://camelot-forum.co.uk/phpBB3/view ... &start=830

[Marcus]

Ok, if a 3 - 4000l thermal store is viable then in principle it could work, although there can be an additional issue if the thermal store and the heating circuit and the hp are hydraulically separate: if you're going through heat exchangers from hp >> ts >> heating circuits you're going to take an efficiency hit at each exchange.

So if relying on hp i would still think batteries would work better. I have no idea on the cost of buying or making a thermal store that size though.

Looking at your signature i see you have solar thermal and wood for fuel: potentially both of these could play very well with a big thermal store - which changes the battery vs thermal store dynamic.

[robl]

What type of meter are you using, and where is it located in the circuit? - I have the same inverter with PV and batteries, and the import readings from the inverter CT coil are pretty far out - almost always ~2kWh per day higher than the smart meter readings (although the export readings from inverter vs smart meter are much much closer). I then doubt the inverter Load figures too, although I don't have another way to measure that.

We've got this meter, it's in the garage CU, after the 32A MCB that then connects to the Sunsynk:
It's got a tiny little button on it, makes it cycle through different measurements.
https://m.media-amazon.com/images/I/710KKNcOKEL.pdf

[CharlieB]

Thanks Andy,
Very useful indeed. I'm circling around and getting closer to a sense of just how big a thermal store tank would need to be and how much hassle. Big and lots I think. But not yet Really big or really lots!

Marcus, yes absolutely. Thermal store definitely makes sense if you have irregular high temp heat input as well, and I'd been thinking about rigging up a log boiler as we sell logs (and solar thermal). If I go with this plan I'd almost certainly d o that if I can find a cheap second hand batch log boiler, but it's getting less replicable then.

[HML]

Ok, if a 3 - 4000l thermal store is viable then in principle it could work, although there can be an additional issue if the thermal store and the heating circuit and the hp are hydraulically separate: if you're going through heat exchangers from hp >> ts >> heating circuits you're going to take an efficiency hit at each exchange.

So if relying on hp i would still think batteries would work better. I have no idea on the cost of buying or making a thermal store that size though.

Looking at your signature i see you have solar thermal and wood for fuel: potentially both of these could play very well with a big thermal store - which changes the battery vs thermal store dynamic.

In 2008 I installed a 2,500l thermal store to work with a gasifying log boiler and a large solar thermal array.

At the time the basic cost of the store was £1900. By the time you'd added on the solar coil, other fittings, delivery and VAT it was £3,000. On top of that you need a substantial feed and expansion tank or expansion vessel, also expensive in the sizes needed.

It worked fine in that high temperature system, it could store a days worth of energy for the coldest days we had. However, it had to be heated to a high temperature to store a useful amount of heat (bad for a heat pump, as others have noted) and it is very difficult to keep heat in for any length of time. Even if the store is well insulated every connection adds another heat loss.

Overall a bad idea compared with batteries. The battery system here will store just about enough energy for a cold days heating with a heat pump in a badly insulated house. Yesterday it supplied 51kWh (total, including all the other electrical use) and 110kWh of heat energy. It was a cold wet day so the Cop was bad at 3.1.

[CharlieB]

A very quick google shows 5,000 litre thermal stores at £3-4k. That's not installed though. I'm pretty sure we'd be talking £5-6k once installed by a friendly plumber. Which is roughly price of an off-the-shelf 10kWh battery, right?

[CharlieB]

And thanks HML. Yep, it's definitely beginning to feel like just HP producing elatively low temp heat and thermal store doesn't make a lot of sense. Certainly with domestic payments for providing grid flexibility as they are - just simple time-of-day tariffs. A high temp heat input on top of the HP might just make the difference though - whether that's logs or, in future, 'waste' electricity*.

I'm trying to think my way to a solution that lots of people could adopt though, and I have a real concern about millions of domestic batteries.

* ie if someone works out how to aggregate domestic consumers so negative wholesale electricity prices can be seen by homes. That way COP wouldn't matter, or you'd just install a large immersion heater. I'm certain we'll get to that point, and probably sooner than most people think.

[CharlieB]

Now, assuming your non peak times are 7 hours then you will need 127000/2.7 (an average cop) 47kWh input or 6.7kW from the grid to your HP to charge the tank let alone service your heating. Single phase wouldn't be able to recharge in the time required.

Hi Andy. I wouldn't be trying to load ALL the heating into a 7 hour window. It's more about taking it off the worst of the grid peaks (ie 3-4 hours in the morning and 3-4 hours in the evening), and seperately being able to 'charge' opportunistically when the grid is loaded with wind. (I'm coming at this from the Transmission System Operator point of view if you like, not the homeowner.)

A smaller thermal store enabling just that relatively minor timeshift would make a big difference to the grid, even if it might well not be cost effective.

TBH spending the money on insulating and draft-proofing the house would almost certainly be far more sensible - then the thermal mass would probably give me that run-through without too much loss of comfort. We all know, I hope, that insulation, insulation, insulation is the answer but we still spend lots of time thinking through energy generation and storage plans!

[Countrypaul]

How much heat does your house actually need and what heat emitters does it have? This will make a big difference to how you use your thermal store(s).

We have a 11.2kW Ecodan ASHP (which I think it oversized for what we need) this is linked to a 430L thermal store and that in turn runs the UFH in the house (225m2). We have thermostats in each ground floor room which are set to slightly higher temperature overnight so on (E7 electricity) then fall back to keep the rooms at 20C unti late evening when they fall back to 17/18C as there is no real point in triggering the ASHP to use peak time electricity shortly before the E7 period. The ground floor UFH is block and beam with 120mm PUR and a 70mm screed - so about 20 tonnes of screed which is the main "thermal store". When the temperature is above 0 all day then ASHP rarely cuts in, when it stays below 0 all day the ASHP cuts in during the day on occasion. About 200-250L of the TS is used to for the UFH. The top of the TS is boosted by an Eddi (when the sunshines) or timer/manually if required.
The use of thermal mass within the building for the thermal storage should be considered in any plans you eventually adopt. The ASHP normally runs with an output temperature of 40C though I sometimes turn that down to 30 in late spring/early autumn the UFH then runs around 25C rather than the normal winter 30.

Before we installed the ASHP we went one winter using immersion heaters in the TS this showed we could get by on 6kW plus the various background sources such as cooking, computers, TV etc. We do have a MVHR system, tripple glazing on the north windows/door, double elsewhere, and an average of about 150mm of insulation in the walls (mainly PUR), along with a 3.3kWp PV installation.

The thermal store was designed with conection for a WBS, solar thermal as well as the ASHP and immersion heaters (3). Given that our heat usage appears as though it is much lower than originally envisaged, the use of a WBS (with boiler) will proably never be realised, and the solar thermal is probably likely to become more PV if we do anything on that front now.

Next step will (if I get my way) be a garage so I have somewhere to work, but with more PV and a battery system for the house (and garage).