Thermal Store - viability

[JamesPa]

Continuing the discussion about the viability of thermal stores for DHW delivery which originated in the Vaillant Arotherm Thread, I think I have worked out the equations governing the amount of heated water a TS can deliver.  I will present the argument hear in case anyone is interested, please feel free to critique/correct (or better still for our sanity ignore).  I also attach a model.  If the model is right, you do need a BIG store to make a reasonable amount of DHW if the TS is at 60C or less.
Surprise, surprise, the first two laws of thermodynamics (energy = constant and heat flows from hot to cold) are the governing factor.
Argument:
Consider a storage tank with volume v at temperature T.  Consider also an incoming body of water also of volume v at temperature t1.  At this stage lets assume  complete mixing of the water in the tank, no stratification and that the heat exchanger is 'perfect' in the sense that there is no temperature difference across it
The maximum energy which can be transferred to the incoming water before the two bodies of water reach the same temperature is v c (T-t1)/2, at which point no more energy can transfer (c is the specific heat capacity of water).  This is enough to heat v litres of incoming water by (T-t1)/2.  Turning this round the amount of water that you can heat from t2 to t1 with a perfectly mixed TS of volume v initially at temperature T is:
v(T-t1)/2(t1-t2)  (t2 being the starting temperature of the incoming water).
With ideal stratification you do better:
Consider now the situation where you have incoming water at 15C with a target outgoing temp of 45C.
Pass this through a tank with volume v and a thermal gradient from 30C at the bottom to 60C at the top (same stored energy as a tank all at 45). This tank can cool down to the point where its temperature is 15C at the bottom and 45C at the top, before it will stop heating the incoming water to 45C. Since the tank has cooled on average by 15C it has supplied enough energy to the incoming water only to heat 50l through 30C.
Combining these two I think that the maximum energy you can extract from a tank of volume v initially all at temperature T, into a stream which enters at t1 and exits at t2 (so has an average temperature of (t1+t2)/2 = tavg is:
v c (T-tavg)/2
The volume of water this will heat from t1 to t2 is
v c (T-tavg)/2(c (t2-t1))
= v (T - tavg)/2(t1-t2)
To take into account the 'approach temperatures' ie the smallest temperature difference across the heat exchangers, you simply subtract the sum of the approach temperatures from T (I think) before you substitute it into the equation.
Here is an example:
CW temp	10C (t2)
DHW Temp	45C (t1)
Flow Temp	60V (T)
Flow->TS Approach	2.5C	*
TS->DHW Approach	2.5V	*
Volume	300l (v)
* I have no idea at all if these are reasonable and they make quite a big difference.
Using the equations above this 300l store can deliver 40l at 45C if the TS is fully mixed, and 118l at 45C if the TS is stratified.
Whereas if you store and use water at 45C in a 300l tank, you get 300l of water at 45C
(If anyone is interested still) can anyone see an error in this and are the conclusions reasonable.  It seems a bit harsh on the thermal store, but perhaps thats right?

Thermal Store Model V1.xls

Edited August 16, 2023 by JamesPa

[ProDave]

I have tried in the past to explain this but not everyone accepts it.  TS's only work when heated by something that van heat them very much hotter than an ASHP can.

 

 

[Iceverge]

Haven't gone into your calcs but the assumption is correct. You will get far less useable stored water from a TS compared with an UVC.

 

An UVC can be heated efficiently by an ASHP to 50 Deg with a good COP. You will have almost all of that at your disposal. 

 

However a thermal store is all out of ideas at 50 Deg as the coil won't be able to transfer enough energy fast enough and the DHW will only be luke warm.  (A plate heat exchanger is better but that's a heatbank rather than a TS really) 

 

They really like being as hot as possible. The one I installed ( Maxipod 250l ) for my parents ticks over at 80 Deg on a solid fuel Rayburn and works excellently. 

 

However run an ASHP at that temp and your COP would be awful.

 

 

 

Edited August 16, 2023 by Iceverge

[JohnMo]

51 minutes ago, JamesPa said:

Using the equations above this 300l store can deliver 40l at 45C if the TS is fully mixed, and 118l at 45C if the TS is stratified.          

Whereas if you store and use water at 45C in a 300l tank, you get 300l of water at 45C

Sounds about right, you heat lots of water an get very little out. That's a similar experience as my thermal store, hence why have bought an unvented cylinder.  I heat about 180L of water an get a shower and some general washing up. Not the most economical use of electric.

[Iceverge]

I would say that if the TS drops to 50 Deg you will not get any more useable DHW. Therefore the useable energy is typically the volume X delta T  storage temp - 50) X heat capacity of water.

 

If you run your store at 80 Deg from a gas boiler and have a 200l TS then you have 25.2MJ of energy to play with. 

 

However if you run it at 55 Deg from an ASHP you'll need a 1200l TS to match it,

 

It can be done but often it's not practical. 

 

P.S.

For what it's worth I reckon our UVC is at an average of 30 Deg when it stops providing useable hot water. 

 

So for a storage temp of 80 deg 

 

(80deg-30deg) X Volume X 4.2Kj/KgK = 25.2MJ 

 

The equivalent volume of a high temp UVC is 120l.  About 60% of a high temp TS and 10% of a thermal store with ASHP. 

 

A low temp ASHP would be 240l for the same stored energy. 

 

TLDR. 

 

All the below give the same amount of hot water. 

 

200l TS @ 80deg. 

1200l TS @ 55deg

120l UVC @ 80deg

240l UVC @ 55deg. 

 

 

 

 

 

 

Edited August 16, 2023 by Iceverge

[JamesPa]

5 hours ago, ProDave said:

I have tried in the past to explain this but not everyone accepts it.  TS's only work when heated by something that van heat them very much hotter than an ASHP can.

 

 

I can't speak for others, but for me it wasn't a case of not accepting it, the fact that a TS will yield less dhw is clear and the reason obvious. 

 

I originally did the maths/physics because someone on the openenergymonitor forum challenged me to explain the physical principles, and it piqued my interest because I realised I couldn't quantify the benefit of stratification, which clearly has the potential to make a big difference as many here have pointed out.

 

Having done so it gives a quantitative argument not just a qualitative one, which makes any design choice easier/more certain.

[dpmiller]

Well I did it, and it works. Stores enough heat that the ASHP can start up and start recovering *before* temperature changes are noticeable at the tap. Whilst the COP mightn't be brill, it's a darned sight better than an electric shower.

Recovery to setpoint gives a "burn time" of about 30mins after a -wifey's long hair approved- 15min shower.

Tank sensor is halfway up and the tank is heated to 54C, the outlet tempering valve is around 48c.

CH/UFH flow passes through the coil at all times so the stratification is somewhat reduced, the bottom of the tank tends not to drop below 40C during the heating season

 

Additionally we've two immersions on PV- top to 65C, then bottom to 90C if and when there's plenty of sun, lol

 

And a boiler stove can heat the vessel through a  Laddomat

 

I prefer to look at it as a "summing point" as all energy in or out of the system passes through the TS

[JamesPa]

34 minutes ago, dpmiller said:

Well I did it, and it works. Stores enough heat that the ASHP can start up and start recovering *before* temperature changes are noticeable at the tap. Whilst the COP mightn't be brill, it's a darned sight better than an electric shower.

Recovery to setpoint gives a "burn time" of about 30mins after a -wifey's long hair approved- 15min shower.

Tank sensor is halfway up and the tank is heated to 54C, the outlet tempering valve is around 48c.

CH/UFH flow passes through the coil at all times so the stratification is somewhat reduced, the bottom of the tank tends not to drop below 40C during the heating season

 

Additionally we've two immersions on PV- top to 65C, then bottom to 90C if and when there's plenty of sun, lol

 

And a boiler stove can heat the vessel through a  Laddomat

 

I prefer to look at it as a "summing point" as all energy in or out of the system passes through the TS

Well done! 

 

Can you share the sizes/temperatures, it would be good to calibrate my model to check if I've made a mistake in the logic.

[dpmiller]

300l tank, filled to (IIRC) 270 to the expansion volume. It's a Worldheat cylinder 2050h x 550dia

Both the DHW and recovery coils are 3m2 corrugated tube

ASHP is a CoolEnergy IVT9. I have it configured to control on return temps and deltaT is 7C, and the circulating pump is under PWM control

Heatup and UFH are mainly carried out during the E7 period. First the UFH as necessary, then the TS is boosted to 54C +0.5C (the IVT allows a variable overshoot to stop the compressor from doing a hard stop) just before morning showers . Cylinder hysteresis is -1.5C to cut in.

Daytime recovery temperature for the cylinder is 52C + 0.5C

CH return temp is 42C so flow can be briefly in the 49C region. The works well as we have some rads and towel rails upstairs and these get to the flow temp before the system trims back. the UFH sips from this and is blended to 32C or so.

It's worth noting that as the cylinder sensor is halfway down the cylinder- midway between the coils-  and the flow temp to achieve the required 54C is a bit higher, the tank seems to end up being around 57C at the top, 55C at the sensor, and never lower than about 37C right at the bottom underneath the coil

 

 

[Beelbeebub]

thanks, i currently have a 750l TS which is used as the summing point for the solar, log burner and gas boiler with the UFH taking a blended down temp of about 30C

 

I was wondering if I had to ditch it when i eventually go for a HP.

 

I'd def run the UFH direct, but  I was wondering if i could totally fill the TS with 55C water (it's a direct from boiler store, no coil) and get enough 45C water out.  

 

Currently it stores about half full at 67C top temp, with the middle around 45C and the bottom around 20C, lower if i've just used lots of hot water.

 

Be a massive saving if I could keep the TS

[JamesPa]

13 minutes ago, Beelbeebub said:

thanks, i currently have a 750l TS which is used as the summing point for the solar, log burner and gas boiler with the UFH taking a blended down temp of about 30C

 

I was wondering if I had to ditch it when i eventually go for a HP.

 

I'd def run the UFH direct, but  I was wondering if i could totally fill the TS with 55C water (it's a direct from boiler store, no coil) and get enough 45C water out.  

 

Currently it stores about half full at 67C top temp, with the middle around 45C and the bottom around 20C, lower if i've just used lots of hot water.

 

Be a massive saving if I could keep the TS

Put the numbers into the excel I posted (I'd do it for you but ukpn are changing out their bits of my elec installation, I only requested a fuse upgrade but they took one look and said 'we will just do the lot, it's that old').

[JohnMo]

2 hours ago, Beelbeebub said:

thanks, i currently have a 750l TS which is used as the summing point for the solar, log burner and gas boiler

That's a lot of energy, bet it looses 4 to 5kWh a day. That's what 1 or 2 people person use a day for hot water.

 

£980 for a 210L slimline UVC with a 3m2 coil, that's what I have just paid. Heat that once a day, run UFH on WC without any mixing. Heating 750L of water to 55 is nonsense, to get, not a huge amount of hot water and then blending down for heating? Why would you.

2 hours ago, Beelbeebub said:

it's a direct from boiler store, no coil

 

A lot of heat pumps will not run open vented. Until mine sees above 1.2bar, it doesn't start.

[DanDee]

23 minutes ago, JohnMo said:

A lot of heat pumps will not run open vented. Until mine sees above 1.2bar, it doesn't start.

Make it think it is over 1.2bar

[JohnMo]

The reason they want pressurised systems is to keep air out and corrosion.

[JamesPa]

19 minutes ago, JohnMo said:

The reason they want pressurised systems is to keep air out and corrosion.

Makes sense.  Also with low temp bio contamination of the circulation system must be a risk and sealed eliminates that. Less important though if the ashp does the legionella cycle natively as opposed to just firing up the immersion, which I now learn Vaillants do.

[Beelbeebub]

2 hours ago, JohnMo said:

A lot of heat pumps will not run open vented. Until mine sees above 1.2bar, it doesn't start.

No I mean the heat store water is the CH water. Runs at 1.5bar.

 

2 hours ago, JohnMo said:

That's a lot of energy, bet it looses 4 to 5kWh a day. That's what 1 or 2 people person use a day for hot water.

Yeah, that seems about right from my measurements, though the store isn't held at that all the time. It drifts down to about 48C before boiler kicks in. 

 

Actually, as I monitor the dhw coil temp I might be able to experiment with what temp you need the TS to be at to give a certain output temp of water. 

 

Hummm. 

 

Anyway, if I swap to a HP, ripping that big tank out would be a right sod so I may be stuck with it and live or get around the inefficiency.  On the plus side it does allow me to get a lot of free DHW from the solar in the summer - last year we had weeks with zero gas burn, less so this year!

 

And it also. Allows the integration of the log burner in winter - one IKEA bag of wood is DHW and heating for 24hrs, though ironically we can only run the burner when it's bear or below zero otherwise we get too warm!