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ASHP with large thermal store (for load shifting)


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I'm investigating the pros and cons of having a large thermal store heated by an ASHP for home space heating. This year we'll have a decent amount of PV installed and will probably move our electricity supply to an overnight EV tariff. We're also planning on installing an ASHP (prob a 13kw unit) for space heating.  
 
We have the space in a utility room for a large thermal store for space heating. DHW is already provided separately by a combined heat-pump cylinder.  
 
I've been looking at simple, relatively-inexpensive  thermal stores without coils or PHEs.  For example:
 
1000l for £895. Which looks attractive for the price. 
 
https://www.ebay.co.uk/itm/114920430255
1500l for £1298
 
2000l for £1598
 
I'm thinking one of these in a  "4 pipe" setup we could prioritise heating the thermal store during cheap off-peak "EV hours" and possibly also during the winter PV generating hours - In the winter we could then avoid using grid electricity for the ASHP during the morning and evening peak. 
 
I was thinking TS would be around 35-40 degrees to get the max COP from the ASHP. Radiators would be sized accordingly. But maybe it should be higher temp to maximise energy storage. 
 
As a very rough calculation:
 
In an off-peak 4 hour window, the ASHP could potentially use 10kWh electricity to generate 30kWh of heat
 
Say 1kWh of off-peak electricity is 20p cheaper than peak rate
 
Ball-park £2 / day savings (not accounting for losses, running the heating circulation pump etc) 
Sounds worthwhile enough to consider further. 
 
Pros:
Not massively more costly than a small buffer tank / volumiser.
Allows shifting of the ASHP run times away from when it might otherwise be used. 
Cheaper than adding additional battery storage for off-peak or PV energy
Heat loss from the store would be indoors, so not really lost. 
 
Cons: 
Not practical to fill the entire thermal store with glycol mix. I've read this isn't necessarily an issue, as the risk of a prolonged power cut during freezing weather is pretty small. Some say not to bother with glycol at all and accept the small risk of the ashp freezing. 
 
Has anyone else done similar?  any considerations I'm missing? 
 
Many thanks
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2 hours ago, apesort said:

In the winter we could then avoid using grid electricity for the ASHP during the morning and evening peak. 

I take it you mean avoiding using peak rate grid electricity for these events, AKA “load-shifting”?

 

I’m a big fan of the idea of utilising the ‘huge’ ( 1000 - 2000L ) TS btw :) I fitted a 2600L TS for one client, to take the brunt of what his log gasification boiler had to offer in a very short period of time. So also ‘load-shifting’ at its best.

 

2 hours ago, apesort said:

and possibly also during the winter PV generating hours

 What is the size of the ‘decent’ array you have? Solar PV drops to between 25% and 0% in winter, so a 10kWp array in the winter becomes a 0-2.5kWp array ( with the 2.5 being what you’d get on a bright winters day, which we don’t get very many of in good ol’ Blighty ) ;) 

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4.2 [kJ.kg-1.K-1] x 1000 [kg] x 1 [K] = 4,200 kJ = 1.167 kWh

 

Now in winter, you may need to run your heating system at a higher flow temperature, say 45 °C and let the store drop no lower than 38 °C.

So 7 K to play with.

 

7 [K] x 1.167 [kWh] = 8.169 kWh.

 

To put that into perspective, my old storage heater, which takes a 3 kW load for up to 7 hours, can store 21 kWh.  It is 1 m by 0.25 m by 0.8 m and weighs about 200 kg.

OK, it only has a CoP of 1, but only cost a few hundred quid.

Easy to wire into a simple control system to take excess PV.

And it cannot leak.

 

What kind of CoP does this 13 kW ASHP offer and what kWs does it deliver at 0° OAT?

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Another minor con is that by load-shifting to the middle of the night you will be running the ASHP when outside temperatures are coldest so you will achieve the minimum efficiency over the 24 hours when you might have run the heat pump.  On a Time-of-Use tariff you should still be getting the electricity much cheaper so it's only a minor con.  But if you had a big battery you might achieve a lower running cost by charging the battery in the middle of the night then using it to power your heat pump to charge the thermal store in the middle of the day.

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14 hours ago, apesort said:
 
I'm thinking one of these in a  "4 pipe" setup we could prioritise heating the thermal store during cheap off-peak "EV hours" and possibly also during the winter PV generating hours - In the winter we could then avoid using grid electricity for the ASHP during the morning and evening peak

What sort of ASHP?  If monoblock, then the circuit going out to the heat pump needs antifreeze.  You don't want to be filling your 1000L thermal store with antifreeze so you need the break of either a heat exchanger or input coil to separate the outside ASHP circuit from the inside store / heating circuit.

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22 minutes ago, ProDave said:

You don't want to be filling your 1000L thermal store with antifreeze so you need the break of either a heat exchanger or input coil to separate the outside ASHP circuit from the inside store / heating circuit.

However you need to make sure a coil will cope, mine didn’t and had to convert it to non coil 🤔

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I using a 160l thermal store as a buffer, standard coil rubbish when using low temperature flows.  I have just added a large 40 plate heat exchanger as well as the coil.

 

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PHE Sized for.

Primary side 6kW input at 27 degrees supply 22 degrees return.  There is an additional return temp drop as flow goes through primary coil on its way to the boiler of about a degree.  Secondary side 25 supply and 21 return.

 

So an approach temp of 2 degrees.

 

Using PHE (Hexonic LA34) with a size of 471x81mm plates x40.

 

 

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6 hours ago, JohnMo said:

PHE Sized for.

Primary side 6kW input at 27 degrees supply 22 degrees return.  There is an additional return temp drop as flow goes through primary coil on its way to the boiler of about a degree.  Secondary side 25 supply and 21 return.

 

So an approach temp of 2 degrees.

 

Using PHE (Hexonic LA34) with a size of 471x81mm plates x40.

 

 

Interesting. I'm about to add 150L buffer tank to my system which will need about £150 of additional glycol.

Alternatively I could get a PHE and a pump for about the same amount. But then it's an additional 40W running an extra pump whenever I need to charge or discharge the buffer. Unless I make some more major (and unusual) schematic changes. 

 Hmmmm. 

 

Where does one buy Hedonic gear? I see lots of Nordic Tec PHE (along with made to measure insulation boxes) on the eBay 

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I bought this from eBay. Which is the same as mentioned above, even comes with insulation.

 

Item number 304182801695.

 

This is it installed but used PIR insulation.

 

 

IMG_20220528_130522-1.jpg

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1 minute ago, SteamyTea said:

Looks very similar to what our Jeremy Harris originally used.  He found the thermal losses very high.

You sure that's not his old one??

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Think it's the same, but not much losses in when it gets to around 30 degs during heating season. Any other heating is by solar which will get consumed during heating season.  But may get toasty in the summer, we will see.

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On 28/05/2022 at 08:22, ReedRichards said:

Another minor con is that by load-shifting to the middle of the night you will be running the ASHP when outside temperatures are coldest so you will achieve the minimum efficiency over the 24 hours when you might have run the heat pump.  On a Time-of-Use tariff you should still be getting the electricity much cheaper so it's only a minor con.  But if you had a big battery you might achieve a lower running cost by charging the battery in the middle of the night then using it to power your heat pump to charge the thermal store in the middle of the day.

 

Good point, thanks. I guess in the ideal world we'd have battery capacity for all our energy needs inc. heating. This is the lower cost alternative for energy storage.   

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On 28/05/2022 at 09:08, ProDave said:

What sort of ASHP?  If monoblock, then the circuit going out to the heat pump needs antifreeze.  You don't want to be filling your 1000L thermal store with antifreeze so you need the break of either a heat exchanger or input coil to separate the outside ASHP circuit from the inside store / heating circuit.

My understanding is that glycol isn't really necessary if you accept the risks. The ASHP will have a antifreeze function which will prevent freezing provided it has power.  It would only freeze if there was a combination of a lengthy power cut combined with freezing weather. Good article about it here: https://renewableheatinghub.co.uk/do-air-source-heat-pumps-really-need-glycol

 

 

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On 27/05/2022 at 22:14, SteamyTea said:

4.2 [kJ.kg-1.K-1] x 1000 [kg] x 1 [K] = 4,200 kJ = 1.167 kWh

 

Now in winter, you may need to run your heating system at a higher flow temperature, say 45 °C and let the store drop no lower than 38 °C.

So 7 K to play with.

 

7 [K] x 1.167 [kWh] = 8.169 kWh.

 

To put that into perspective, my old storage heater, which takes a 3 kW load for up to 7 hours, can store 21 kWh.  It is 1 m by 0.25 m by 0.8 m and weighs about 200 kg.

OK, it only has a CoP of 1, but only cost a few hundred quid.

Easy to wire into a simple control system to take excess PV.

And it cannot leak.

 

What kind of CoP does this 13 kW ASHP offer and what kWs does it deliver at 0° OAT?

Thank you. May I ask why you say "drop no lower than 38 °C."  Rather than say let the store run down to close to 21°C or whatever room temperature is ?

 

It certainly looks like a 2000l tank would be much more worthwhile storage for the amount of energy that we're looking to store.    

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32 minutes ago, apesort said:

Rather than say let the store run down to close to 21°C or whatever room temperature is ?

As the temperatures reach parity, power is reduced in a non linear manner.

So you need a temperature difference to reduce the time it takes to heat.

 

Look up Newton's Law of Cooling (it works for heating as well).

 

image.png.6d3ca29f9508a56e441f93abe46cd925.png

 

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Have you considered heat batteries instead of a tank of water? We have two Sunamp heat batteries that are used for this purpose, storing about 18kWh of heat.

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1 hour ago, SteamyTea said:

And they store energy at a useful temperature.

I don't think so.  Their standard Phase Change Material has a transition temperature of around 56 C which is higher than you would want for a heat pump.  Also unlike a tank of water, which you can heat by gradually raising the temperature (as the efficiency of the heat pump drops), with a PCM you have to charge it at its transition temperature which means you will be operating the heat pump at near-minimum efficiency for much longer.  

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1 minute ago, ReedRichards said:

Their standard Phase Change Material has a transition temperature of around 56 C which is higher than you would want for a heat pump. 

Don't they do the lower transition temperature ones any more?

3 minutes ago, ReedRichards said:

you can heat by gradually raising the temperature (as the efficiency of the heat pump drops)

I keep meaning to model this.

You may be better off, overall, having high temperature input at the begining as the power transfer is better i.e. less time to increase the store by 1 K.

Then increase the temperatures input difference as the store increases. As long as parity is not that close, and the HP has the capacity, and the store does not loose too much energy, it may be cheaper.

Trouble is, reality is messy and and we cannot control the weather in advance of our space and DHW needs.

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3 hours ago, SteamyTea said:

Don't they do the lower transition temperature ones any more?

 

Possibly they do but you have to search their literature quite hard to even find out what the transition temperature of their standard PCM is.  You would think that his page https://sunamp.com/savings-add-up-with-economy-10-ashp-sunamp/ would mention the need for a different PCM with a lower transition temperature, but it doesn't.

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