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Green Building Bible, Fourth Edition
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    • CommentAuthorBenM
    • CommentTimeOct 19th 2021
     
    Hi,

    With the recent announcement (another leak?) of the potential proposed £5,000 grant for air source heat pumps, I was wondering if anyone had any experience or knowledge of using a heat pump with a thermal store?

    We have a Gledhill Torrent 350 sol thermal store, with inputs from a gas boiler, multi-fuel stove and solar thermal. I've been pondering maybe potentially replacing the boiler with a heat pump and the announcement has just popped it back into my head.

    Be interested to hear from anyone with experience of heat pumps and thermal stores; or even anyone else considering the same thing.

    Cheers

    Ben
    • CommentAuthorrevor
    • CommentTimeOct 19th 2021
     
    We have exactly the same set up apart from not having multi fuel stove input and the thought has crossed my mind also. But having only just installed our system 2years ago and discounted heat pump because of cost size and requiring very little heat requirement went for gas, (and being rural is LPG), so am reluctant to make any moves until I have too. I have the added complication that the UFH pipes are spaced for gas boiler temperatures so not as easy as putting in larger radiators as don't have any.
    It is possible Gledhill will have some info on this.
  1.  
    The heat pump installers I'm speaking to are keen to replace our existing cylinder for one with a much bigger coil in it. I've not yet agreed that is necessary.
    • CommentAuthorBenM
    • CommentTimeOct 19th 2021
     
    Posted By: revorWe have exactly the same set up apart from not having multi fuel stove input and the thought has crossed my mind also. But having only just installed our system 2years ago and discounted heat pump because of cost size and requiring very little heat requirement went for gas, (and being rural is LPG), so am reluctant to make any moves until I have too. I have the added complication that the UFH pipes are spaced for gas boiler temperatures so not as easy as putting in larger radiators as don't have any.
    It is possible Gledhill will have some info on this.


    Spoke to Gledhill this afternoon, after they rapidly responded to my email. They've had loads of enquiries since the press release on the grant. Ironically, they no longer make a TS for a heat pump!

    Unfortunately, the Gledhill torrent doesn't use a coil but instead is direct. As ASHP use glycol, you can't replace the boiler with a heat pump.

    Can't seem to find any heat pumps that don't use glycol, so looks like the gas boiler might be sticking around a while longer!
    • CommentAuthorphiledge
    • CommentTimeOct 19th 2021 edited
     
    Posted By: BenM

    Unfortunately, the Gledhill torrent doesn't use a coil but instead is direct. As ASHP use glycol, you can't replace the boiler with a heat pump.


    Never used one myself but likely youd be able to use a plate heat exchanger to interface to the current boiler tappings? I think Gledhill manufacture heat exchangers so Im surprised they didnt suggest one??
    • CommentAuthorBenM
    • CommentTimeOct 19th 2021
     
    Posted By: philedge
    Posted By: BenM

    Unfortunately, the Gledhill torrent doesn't use a coil but instead is direct. As ASHP use glycol, you can't replace the boiler with a heat pump.


    Never used one myself but likely youd be able to use a plate heat exchanger to interface to the current boiler tappings?


    The DHW water does actually work on an external PHE, so I guess the glycol wouldn't get into the domestic hot water. I suppose it's an interesting prospect to have more than 350l of glycol swimming around the tank and heating system. Not sure I fancy it going through the stove back boiler!
    • CommentAuthorphiledge
    • CommentTimeOct 19th 2021
     
    Posted By: BenMNot sure I fancy it going through the stove back boiler!


    The glycol would only go between the HP and heat exchanger input. Exchanger output would use whatever you have in the TS now.

    If you are drawing DHW through a heat exchanger, Im not sure a HP would get the TS hot enough to give a decent DHW flow??
    • CommentAuthorEd Davies
    • CommentTimeOct 19th 2021
     
    Posted By: BenM…it's an interesting prospect to have more than 350l of glycol swimming around the tank and heating system.
    More expensive than interesting, I'd think.
    • CommentAuthorBenM
    • CommentTimeOct 19th 2021
     
    Posted By: philedge
    Posted By: BenMNot sure I fancy it going through the stove back boiler!


    The glycol would only go between the HP and heat exchanger input. Exchanger output would use whatever you have in the TS now.

    If you are drawing DHW through a heat exchanger, Im not sure a HP would get the TS hot enough to give a decent DHW flow??


    I'm not entirely sure how it works, to be honest. The man from Gledhill seemed pretty adamant you can't do it because of the use of glycol. I do wonder if there are HPs that use water instead
    • CommentAuthorBenM
    • CommentTimeOct 19th 2021
     
    Posted By: Ed Davies
    Posted By: BenM…it's an interesting prospect to have more than 350l of glycol swimming around the tank and heating system.
    More expensive than interesting, I'd think.


    Indeed!
    • CommentAuthorRobL
    • CommentTimeOct 19th 2021
     
    I've seen a monobloc heatpump use water - it had some sort of frost protection automatic bleed valve at the lowest point, to prevent pipes from freezing up. Can't remember where I saw it though and it sounds a bit non standard, most manufacturers want a pressurised system and you can't be loosing a bit of water out of it when it gets cold.
    • CommentAuthorrevor
    • CommentTimeOct 19th 2021
     
    Posted By: philedgeIf you are drawing DHW through a heat exchanger, Im not sure a HP would get the TS hot enough to give a decent DHW flow??


    Would a HP work efficiently with a thermal store. It could work with a suitably sized heat exchanger HP glycol one side TS content on other side and some flow controls. To get the temperature up to an useable domestic hot water temperature the COP would drop quite significantly would it not?
    • CommentAuthorphiledge
    • CommentTimeOct 19th 2021 edited
     
    Posted By: revorTo get the temperature up to an useable domestic hot water temperature the COP would drop quite significantly would it not?


    I beleive the COP would drop particularly as drawing the DHW through a heat exchanger needs the TS to be higher than DHW temp, hence quering the OP switching to HP.
    • CommentAuthorGreenPaddy
    • CommentTimeOct 20th 2021
     
    Hi BenM,

    I've done HP with thermal store a few times, but that's with split units (ie. GS, or AS where the fan unit is separate from the exchanger (being located inside the thermal envelope).

    It's not a perfect solution, and not where you'd start from, but there were other drivers to have a thermal store in those cases, and already having that type of set up is a definite driver, as scrapping function equipment makes my toes curl.

    I don't see why putting a plate HX in lieu of a coil inside the TS is a problem. Probably as good if not better surface area for heat transfer in a PHX. You do need an additional pump of course.

    There is certainly a need for slightly higher heat transfer fluid temp when making DHW via a coil in a TS, as opposed to the traditional hot water cylinder, but we're talking about getting rid of fosil fuels, or a CoP of at least 3 versus direct elect, so I'm not put off by marginal impacts on CoP, due to that required slightly higher temp.

    I've had a number clients who tell me that HP salesmen have told them that a HP can NOT be linked to a TS. Well, I must have done the impossible, or the salesmen are talking out their rear trouser panels.
    • CommentAuthorGareth J
    • CommentTimeOct 20th 2021
     
    In such a scenario, would an in-line electric "booster" invalidate any grants? It's probably only a fairly small amount of DHW that actually needs to be piping hot. Say, for showers and kitchen sink. Maybe reduced COP and losses associated with storing higher temps would mean that there'd be a cut off point where you'd actually use less electric just boosting HW temps at point of use.

    Regarding UFH and spacing, would be easy enough to test it out this winter and work out what your minimum flow temperature is that lets you maintain a habitable temperature. Though I suspect again there might be some grant stipulations that certain levels are met irrelevant of any real world data you have.
  2.  
    There's been a few threads about this - some people are getting very good efficiency using heat pumps to heat DHW.

    The tricks seem to be:
    -use the heat pump to heat the incoming cold feed water up from 10degC, rather than topping up the temperature of a store or cylinder that is already close to 70degC
    -feed the sinks and shower with a lot of warm water at 40degC, rather than a small amount of hot water at 70degC which immediately gets blended down with cold
    -if warmer water is needed for dishwashing etc then heat it electrically at point-of-use
    -choose a heat pump with one of the newer refrigerants that work well at 50+degC (not R410A).

    With a heat pump you're ideally feeding UFH or lower temperature radiators, so could keep the thermal store at a lower temperature, but then it doesn't store so many kWh of heat. Heat pumps can be pretty powerful (fast recovery times) so this might not matter, unless you are trying to use off-peak electricity. Some heat pumps installers recommend a buffer tank of say 100litres to stop the HP short-cycling and for defrost cycles, so maybe a TS is just a bigger one of those?
    • CommentAuthorEd Davies
    • CommentTimeOct 20th 2021
     
    I like the idea of heating DHW in two stages. First stage up to, say 25 to 35°C, could come off a heat pump via a large TS at UFH temperatures. Second stage might come from a small TS or a phase-change store (e.g., https://sunamp.com/residential/ ) heated from the HP running less efficiently but only for the relatively small amount of energy needed for the top-up stage of the heating.

    In other words, you don't need to take the hit on CoP efficiency for all of the DHW heating energy, only for the hotter half or so of it.
    •  
      CommentAuthordjh
    • CommentTimeOct 20th 2021
     
    Posted By: Ed DaviesIn other words, you don't need to take the hit on CoP efficiency for all of the DHW heating energy, only for the hotter half or so of it.

    But as long as the COP remains above 1 it is still preferable to use the HP for heating to the higher temperature. So the key feature is the ability of a HP to change its output temperature at different times, much like combi boilers do.
  3.  
    Two steps are better than one, but it's more efficient if the heating happens in a sequence of very many small steps (this is less 'irreversible' in thermodynamics)

    IE
    Step 1)Fill the tank with water at 10degC
    2)Heat it all up to 11degC - very efficient
    3)heat it all up to 12degC - the HP needs to increase its working temperature as DJH mentioned
    4)heat it all up to 13degC
    -and so on .....
    Step 36) heat it up to 45degC. By now the CoP has probably dropped from 5 down to 3, so stop here.

    Then consume all of the warm water, before reheating the whole tank from properly cold - don't reheat every time a small amount is drawn off.

    Paradoxically, those phase-change stores are not very well matched with heat pumps, because they require all of the latent heat input to be provided at the melting temperature of the PCM, typically 50-60degC. Better with a sensible-heat store which can accept much of its input at lower temperatures.
    •  
      CommentAuthordjh
    • CommentTimeOct 21st 2021
     
    Posted By: WillInAberdeenTwo steps are better than one, but it's more efficient if the heating happens in very many small steps (this is less 'irreversible' in thermodynamics)

    IE
    Step 1)Fill the tank with water at 10degC
    2)Heat it all up to 11degC - very efficient
    3)heat it all up to 12degC
    4)heat it all up to 13degC
    -and so on .....
    Step 36) heat it up to 45degC. By now the CoP has probably dropped from 5 down to 3, so stop here.

    This is typically what happens when heating water anyway, isn't it? Water adjacent to heating element (coil, immersion, PHX etc) heats a bit, convects away and is replaced by fresh cold water, repeat adinfinitum.

    Then consume all of the warm water, before reheating the whole tank from properly cold - don't reheat every time a small amount is drawn off.

    This defeats one of the main purposes of storing hot water in our house. Namely to have a reserve of one or two days hot water so that if the sun doesn't shine we aren't reduced to cold showers. But I expect the purpose could be easily met in principle by the expedient of splitting the hot water into separate day-sized sections. So consume all the hot water in one day-sized tank, then reheat that.

    Of course in practice this is done by stratifying a single tank. Hot water is at the top. As heat is extracted, the cooled water is returned to the tank at the bottom (either direct from cold mains in a cylinder or from e.g PHX return in thermal store) where it can be heated from cold by the heater whilst the remaining stored hot water stays at the top. All complicated by losses and consequent circulation, of course.

    Paradoxically, those phase-change stores are not very well matched with heat pumps, because they require all of the latent heat input to be provided at the melting temperature of the PCM, typically 50-60degC. Better with a sensible-heat store which can accept much of its input at lower temperatures.

    That's nice to hear. :bigsmile:
  4.  
    >>>>This is typically what happens when heating water anyway, isn't it?

    Not in the scheme Ed mentioned, where half the heat comes out of a TS at a single UFH temperature, and the other half from a PCM at another discrete temperature. I reworded to clarify (perhaps..)
    • CommentAuthorEd Davies
    • CommentTimeOct 21st 2021
     
    Posted By: WillInAberdeenParadoxically, those phase-change stores are not very well matched with heat pumps, because they require all of the latent heat input to be provided at the melting temperature of the PCM, typically 50-60degC. Better with a sensible-heat store which can accept much of its input at lower temperatures.
    But if you want hot water to be available at any time then you need your store to be at or above the required hot water temperature all the time. A sensible-heat store needs to either be very large or well above the required temperature if it's to store reasonable amounts of heat. A PCM store can be relatively compact and only just above the required temperature to store useful amounts of heat.

    In other words, a PCM store will need to be heated at the required temperature + a small amount all the time it's being heated whereas a sensible-heat store will be heated at the required temperature + half a much larger amount on average.
  5.  
    A smart PCM store would have a stack of different PCM materials with different melting points, so it could accept heat input at a range of different temperatures, like a TS but with the advantages of compact size and low losses. Haven't found one yet though!
    •  
      CommentAuthorDamonHD
    • CommentTimeOct 23rd 2021
     
    Andrew Bissell of Sunamp asked me to pass on the following:



    This is actually the whole concept in my initial Sunamp patent in 2006.

    A “multi bank” thermal store with each having a different phase change material plus valves to control access to each bank .

    We could easily build that today from our kit - but demand is for simple packaged solutions.

    We do sell a lot of heat batteries to connect to heat pumps for hot water preparation. Models for Samsung gen 6 R32, Daikin 3 R, Daikin 3 H HT and Vaillant arotherm plus ASHP and Kensa shoebox GSHP available. Many more onstages of testing. A few specials on semi-public test eg with NIBE (see @zapaman on Twitter)
  6.  
    Interesting, thanks for sharing this! (though not wanting to see particular brands extolling their wares on GBF). For example, the Daikin 3R that was mentioned:

    at 35 degC: SCoP = 4.4
    at 55 degC: SCoP = 3.2
    (MCS register figures)

    So there is a SCoP benefit of around +1.2 to supplying (part of) the DHW heat at incoming cold water temperatures in a tank, rather than supplying all of the heat at the upper melting temperature of a PCM store.

    If you do this by raising the tank temperature sequentially in time, you don't need any extra valves, you just start with a tankful of cold water and gradually heat it until it's hot.

    Id envisage a PCM store with layers of different PCM, arranged in order of melting points and a single water inlet. To charge it, you start by flowing water through at 20deg, which melts the first layer. Once that has all melted the HP heats the water to 25degC and melts the next layer, and so on until all the layers melt. To withdraw heat, flow cold water through the layers in order from coldest to hottest, so it withdraws some heat from each layer sequentially as its temperature increases. The whole thing in a simple package, functionally equivalent to a TS but smaller and less losses.

    Edit: unless Im looking in the wrong place, the spec for the PCM product that was mentioned, requires all the input from the heatpump to come at 65-80degC. Some heatpumps can do this, but SCOP won't be so good. The twitter user that was mentioned, reported a CoP around 2.0.
    • CommentAuthorEd Davies
    • CommentTimeOct 23rd 2021 edited
     
    But most people want warm-enough water available all the time. They heat it from just about warm enough to whatever temperature is needed to store enough energy.
  7.  
    Indeed - the folks who reported on this, have heated a cylinder full of water to DHW temperature. Then they draw off nice hot water from the top whenever they want, replacing it with cold water at the bottom of the tank, and retaining a layer of hot at the top which is available whenever they want. When the tank is nearly full of cold, with just a small hot layer left at the top, they start the HP and heat it all up again. No need to run out of hot water. Same principle would apply for a layered PCM store.

    Are you perhaps thinking of an unstratified tank where the whole tank gets mixed with cold water, so the tap runs tepid? Wouldn't be good!
  8.  
    See eg https://trystanlea.org.uk/heatpump2020

    "Annual domestic hot water COP: 3.83"

    He explains how he heats the cylinder in two discrete cycles each day, so he has two whole cylinder-fulls per day of water that is just-warm-enough at 40-42degC to run directly out of a shower head without mixing it down with cold.
    • CommentAuthorBowman
    • CommentTimeOct 23rd 2021
     
    FWIW we have a 300L thermal store with ASHP coil (not connected - a bit short of funds) but heated via immersion heaters. The DHW is heated via PHX, we can get a thoroughly decent 10 minute shower when the store is at 50C.
    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2021
     
    Posted By: WillInAberdeenIndeed - the folks who reported on this, have heated a cylinder full of water to DHW temperature. Then they draw off nice hot water from the top whenever they want, replacing it with cold water at the bottom of the tank, and retaining a layer of hot at the top which is available whenever they want.

    It's available in product form from Mixergy. There may be others. The original form of the idea was available in Germany from Solvis just using clever design without all the computers and pumps Mixergy use.
   
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