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Thermal stores for storing heat at off peak times

  • 14-11-2021 10:53am
    Registered Users Posts: 10,328 ✭✭✭✭

    Hi all,

    I've got a bit of a long term plan with my house to make it more energy efficient. It's an A2 rated house with air source heat pump, and all appliances are electric and pretty efficient

    I'm planning to install solar PV at some point plus batteries to reduce my energy bills and make the house less reliant on the grid

    One thing I'm toying with is a thermal store to store up heat energy during off peak times. I can already do this with the hot water, but I'd like to do the same with the heating

    What I'm thinking is of getting a large buffer tank of around 200 litres with an immersion. I can then heat this off of the grid at cheap times or off excess solar (not likely in winter when I need the heating, but I can dream 😁)

    We'll probably be replacing the heat pump around that time, so it might tie in with that reasonably well. Ideally it'd be great to find a heat pump that ties in with solar PV as well. But there doesn't seem to be many options there

    I'm wondering if anyone has tried something like this before and how it worked out?

    Also, I know there a few proprietary systems out there, but I'm slightly wary of them as the support might not be great long term. My theory with a standard tank is that any qualified plumber should be able to maintain it of there are issues down the line

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost


  • Registered Users Posts: 6,191 ✭✭✭10-10-20

    Have you played around with a Specific Heat Capacity calculator?

    200Kg (200l) of water with a delta of 60 degrees C gives you 13.9kWh of thermal capacity. (I took 60oC as a nominal value)

    Divide your expected floor-heating draw per hour into this for a semi-realistic value as to how it will theoretically supplement the heating.

    I suppose in theory it's sweet. The buffer tank would be in-line with the heating system but on the input of the boiler/HP, but part of the difficulty arises when the buffer is cold and there is no solar - do you really want the HP bringing that up to temperature? For that you need a bypass to isolate the tank.

    Post edited by 10-10-20 on

  • Moderators, Home & Garden Moderators Posts: 5,447 Mod ✭✭✭✭graememk

    Not done any of this, nor have a heatpump. But like the thought experiment. and its a Sunday morning!

    Specific heat capacity of water is about 4200J/kg and 1J/s is 1W. water has a density of 1, so 1L = 1KG

    to heat 200L of water 10 degrees C ( 4200*200*10 = 8400000J) that works out about 2.33 kWh.

    Is your house on radiators or UFH? (generally the flow temps for rads on a heat pump house is 45, and 35 for UFH. I think i remember you saying you had rads)

    Say you get the tank heated to 70, and you can pull it down to 40 before the heatpump needs to kick in and supplement it/take over. Thats 30c of useable heat. Thats 7kwh of heat. 300L tank would be 10.5kwh.

    you also fit a mixing valve & required pumps so that your flow temperature is the same as if the heat pump was providing it. (and isolation when your water battery is "empty"

    if that was provided by a heatpump. it would only use 1-2kwh ish of electricity.

    There is also the sunamp heat batteries.

    If you were going the solar route, Normal batteries could be the simpler option, As your in a new house, a grant is currently out of the picture.

    Depending on how hands on you are, Getting solar in with a hybrid inverter and DIY'ing the battery could be a runner.

  • Registered Users Posts: 995 ✭✭✭iColdFusion

    Not an expert in these areas but I would have thought:

    If you want to heat a tank of water put in solar thermal panels.

    You won't be able to use PV panels to heat water via an electric immersion if that energy is going to the batteries

    Is it not just better to store your PV energy in batteries to use for heat pump heating during the day? COP of 3+ of heat pump will maximise return on stored energy especially as it'll be running during the day when air temps are highest but not costing you grid peak rates.

    Overall I'm very surprised you need to even think about this for an A2 house, would have thought that would nearly heat itself from body heat alone!

  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    Thanks for the calculator, I'll definitely check it out

    I'm thinking realistically I'd be heating the buffer at night when electricity is cheap, heating the buffer from solar would be a bonus. So it should rarely be cold

    I know some diverters let you set priority, it would probably make sense to prioritise the hot water over the heating buffer

    I need to get my head around heat loss calculations from a water cylinder. For this to work the buffer tank needs to be well insulated, otherwise it'll just warm up the hot press. I know that the hotter the water the quicker the heat loss, so maybe it'll be better to have the buffer at a lower temperature...

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    Ah you've worked it all out for me 👍

    Batteries for solar would definitely be first choice, much better for storing energy, especially in winter when the sun is being scarce

    I'd probably use the night rate to heat the buffer. Realistically I won't have enough sunlight in winter to provide energy to the heating. So best case is to try and generate and store heat when it's cheap and use it later

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

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  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    Batteries are definitely best, but I can't help looking for other ways to save energy where possible 😁

    The insulation is pretty good, so heating is quite efficient. It does come with some annoyances though, like being tied to a single fuel price which is going up. Although to be fair gas isn't exactly cheap these days either

    My other motivation is to ensure the house isn't totally useless in a power cut. I know I need extra isolation systems with PV for this otherwise the ESB won't be happy with me. Having a store of hot water would also stop the batteries running down too quickly

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    Hmm, some quick math tells me that using the immersion to supply 8kWh of heat at the night rate would probably be the same as 2.7kWh of heat pump electricity.

    I assumed a COP of 3 given we'd be in winter and the heat pump has a bit of a tougher life then

    They both work out at around €0.48 using the energia EV plan. So not exactly a saving

    Not exactly working out as the greatest money making scheme ever is it? 😬

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Registered Users Posts: 2,326 ✭✭✭bullit_dodger

    I really like the idea in principle, but in reality I think adding 5-10Kwp of panels and 20-30KwHr of a DIY battery is probably the way to go. With the battery you could use that energy you've stored from night rate via the heat pump to generate the hot water with a COP of 2-3. Storing the energy via hot water directly would be subjected to too much losses I think, and you don't get the "free" energy that the heat pump gives you. It's a shame really, as I was also thinking along the same lines raisin on how can I get the panels to "do more".

    Trick of course is for most people where do you put 10Kwp+ of panels. The batteries most of us can find a space in the attic, or a shed etc, but 10Kwp of panels has a pretty big surface area. Damm velux windows ruins one of my roofs, so I can only get another 2x panels up there by reconfiguring the layout - which would bring me up to ~6.0Kwp. But to really "go at it" I reckon 10Kwp and 25KwHr battery.

    The one thing that I did come up to help the with heating is oddly crypto mining. The mining rig consumes 300-400 Watts and most of that comes off in heat. So when I'm working from home, the mining rig in the office has meant that I don't haven't (so far this year) turned on the heating. Not exactly easy to maintain a 22C room, as you have to open the window/door when it goes above as there's no temperature control..... but it uses night rate/solar power most of the time and I get a few coins out of it, and no Gas burnt in the boiler.

  • Moderators, Home & Garden Moderators Posts: 5,447 Mod ✭✭✭✭graememk

  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    I'm starting to think it might be better econimics to just have a big hot water tank with normal buffer, and build a bigger battery

    I was planning to use 300Ah cells to make a 14.4kWh battery, maybe I'll use 400Ah cells instead and get a 19.8kWh battery

    The handy thing with hot water is that I'll use it even in summer

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

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  • Moderators, Home & Garden Moderators Posts: 5,447 Mod ✭✭✭✭graememk

    or 32 200ah cells! or even 48... them new 5g hybrids can do 5kw.

  • Registered Users Posts: 2,326 ✭✭✭bullit_dodger

    Course bigger batteries only really makes sense if you have the panels to fill them. Sure filling them with night rate is useful, but to really payback quick, it's storing the PV generated energy and then being able to cycle the battery in 12-24 hrs so you'll need to have the consumption too.

    In an ideal word (as a thought experiment) if battery storage was dirt, and I mean dirt cheap we'd all have 100Kwhr of storage. Fill that during the sunny days and with whatever production we grab over rainy days we'd have enough in the batteries to ride through bad days. Sadly we're not there yet in terms of pricing.

  • Registered Users Posts: 10,328 ✭✭✭✭the_amazing_raisin

    Well I can definitely get 18 panels on the current south facing roof, so that's 6.66kWp to start

    But I'd like to build an extension out the back and one of those log cabins in the back garden to use as an office/shed/workshop for crazy projects

    I reckon I could fit a dozen panels on each so that's a grand total of 40 panels if it goes to plan giving 14.8kWp

    I think the ESB might get annoyed with me though 😂

    "The internet never fails to misremember" - Sebastian Ruiz, aka Frost

  • Moderators, Home & Garden Moderators Posts: 5,447 Mod ✭✭✭✭graememk

    Well if you can shift the majority of the heat pump use to night rate at the least. Or away from the upcoming peak times.

    @the_amazing_raisin any idea of how much your heat pump uses per day on average? - bonus points for day rate usage 😂