Battery or thermal storage for HP timeshifting

[CharlieB]

Thanks CPaul. Yep, I think you've confirmed my feeling that insulating and draftproofing the hosue would be the best bet as it would enable the thermal mass of the building to be used. And to get on with replacing radiators with underfloor.

Though we're in an exposed stone-walled farmhouse, so I don't think there's any way we could go as far as you have. And realistically the underfloor, if we manage it, won't have a big screed.

For what it's worth heatpump is saying it's been using 30 - 60 kWh a day and outputting 120-180kWh. So usable thermal storage of 60-80 ish kWh would make a big difference. I suspect 'usable' is going to be a big uncertainty. (And a cop of 3-4, which to me feels pretty good at this time of year for ASHP? I always knew we'd need to have a fairly high flow temp to our radiators.)

[sharpener]

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.

A bit late to this thread but here is my three ha'porth:

1. The theory: by dividing the number of seconds in an hour by the specific heat of water you get

1 kWh is equal to 861 litre-degrees

This relationship can be interpreted in various ways e.g.

(i) 1kW will heat an 861 litre tank by 1 deg/hour or more practically a 210 litre tank by 4 deg/hour

(ii) A 1 kW heat pump will maintain a 5 deg delta T with a flow rate of 172 l/hr

(iii) Heating a 210 l tank from 10 to 50 C will take 10kWh


2. The practice:

I am planning to install a 12kW Vaillant Arotherm plus HP. The intention is to the extent possible to run it off E7 at night, solar PV during the day and the battery in the evening.

To supplement this I was originally intending to put in a 210 l thermal store on the primary circuit which will be charged to 55C along with the hot water and discharged through the radiator circuit in the evening to heat the bedrooms.

The economics of this were pretty marginal. The extra cost of the tank, pump and valves is over £1000 and the labour is a bit difficult to reverse-engineer from the installers' quote but at least another £500.

Operationally it would store at most 5kWh thermal (there will be some parasitic losses) and if we assume a CoP of 2.5 while charging to 55C this equates to 2kWh electrial input to the HP. So over the 6 month heating season it would move 360kWh from peak rate to off-peak*.With the difference in Octopus' E7 rates being 21p/unit this is an annual saving of £75 so at current prices a 5% RoI. Clearly if we assume some level of inflation in electricity prices it would get better but cba to do the DCF calc right now.

Octopus Cosy has a 27p difference between peak and off-peak rates but it does not have long enough at night rate to charge my EV. Octopus Go is about the same difference but has six hours cheap 2330 - 0530, however the finish time is rather early for the HP. Either would require a smart meter which I have previously been told by Scottish Power I cannot have because coverage is not good enough (though I see wifi metering may be coming).

3. Cost vs battery storage

As above the cost is £1500 for 5kWh thermal so £300/kWh, and installed alongside an HP is is zero-rated for VAT.

In comparison the current replacement cost of my 10.65 kWh Pylontech system inc BMS is £3264 plus VAT (zero rated if part of a PV system) and if we assume 90% usable capacity, a round-trip efficiency of 75% and a SCoP of 3 then £182/kWh so at first sight a clear winner. Always supposing you already have the inverter.

But the thermal store should last for 20 years at least. Whereas the Pylontech batteries are now only warranted for 7 years (mine were 10 years).

Difficult isn't it!!!


* A more accurate calculation would be the difference in cost between heat generated in real time in peak hours and the available heat from the store plus the losses.

[AlBargey]

To follow on from Sharpeners post, there's an useful blog post by Energy Stats today:

https://energy-stats.uk/hot-water-cylin ... nd-mixing/

With some useful ready reckoner tables for hot water storage and use with a HP.

[Andy]

Another thing to check on your HP specifications is how much power will go to your hot water tank. For instance my NIBE has the tank built in to the unit. I did all my calculations but forgot about the fact that the DHW would charge at maximum of 6kW despite the HP being a 12kW unit. You need to check what output your heat pump will put in to a store if you are trying to do it in a limited time period. Things like the size of the coil will make a big difference to performance.

[sharpener]

Another thing to check on your HP specifications is how much power will go to your hot water tank. For instance my NIBE has the tank built in to the unit. I did all my calculations but forgot about the fact that the DHW would charge at maximum of 6kW despite the HP being a 12kW unit. You need to check what output your heat pump will put in to a store if you are trying to do it in a limited time period. Things like the size of the coil will make a big difference to performance.

Yes, however the Arotherm plus will try to put 12kW into the DHW if it can but we only have a small coil fed with 15mm pipe. Vaillant have agreed it should work OK as modelling shows the HP will turn down progressively to 6kW as the temp rises and safely get to 55C before cutting out completely.

The thermal store will be helpful in this regard. If we configure it as a second logical DHW tank and charge it at the same time as the HW it will share the full 12kW output between the two and the total time the heating is off will thereby be minimised.

Update to my previous post:

With 10% p.a. inflation in electricity prices the payback time for the thermal store is reduced to 13 years. And with 20% it is only 10 years, after which the savings accrue to as much as £15k by Year 20.

Given all this and the better lifespan than adding extra batteries we have decided to go for the biggest thermal store we can accomodate and with thicker insulation to reduce the standing losses.

[Moxi]

Is there a way to link to Gravy Miners epic DIY heat store that he built and the problems encountered with the home brew heat exchanger in the days of the other place ? Mmmmm Gravy!

Moxi