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Solar PV to heat water for central heating

2

Comments

  • 70sbudgie
    70sbudgie Posts: 785 Forumite
    500 Posts Second Anniversary Photogenic Name Dropper
    QrizB said:
    70sbudgie said:
    But I am also aware that heat pumps are also significantly less efficient at these times (probably only about 150% efficient), so a I believe it is worth exploring all possibilities.
    I think you';ve got some duff info there. Yes, there will be circumstances where heat pumps are only 150% efficient but for a typical domestic heat pump these will be few and far between.
    All major heat puimp manufacturers publish data books which show the performance of their heat pumps under various conditions. For example here is a recent Mitsubishi data book; this link will take you to the page for a range of air-to-water heat pumps:
    Looking at the top-left table, you'll see that with an ambient temperature of -10C and a water flow temperature of 50C you're still getting a COP of 2.06. The number of times a year when the ambient temperature is -10C but there's also a surplus of electricity from a solar PV system is pretty low, even in the frozen North of the UK. Here is the subtropical South I don't recall it ever happening.
    At more common temperatures, as you might expect in the shoulder months where you have surplus PV, COPs are much higher.
    I don't agree that I've had duff information - I accept that my figures aren't completely correct. But from the tables you shared, the "nominal" line, for 2°C ambient temperature and an outlet temperature of 60° (or even 55°) the CoP is less than 2. 

    I do live in the north of England and 2° ambient is not unusual through December and January. The difference between the price of electricity and that of gas is far too high for a CoP of 2 to be cost effective for 2 out of 12 months. 
    4.3kW PV, 3.6kW inverter. Octopus Agile import, gas Tracker. Zoe. Ripple x 3. Cheshire
  • QrizB
    QrizB Posts: 15,314 Forumite
    10,000 Posts Third Anniversary Photogenic Name Dropper
    70sbudgie said:
    I don't agree that I've had duff information - I accept that my figures aren't completely correct. But from the tables you shared, the "nominal" line, for 2°C ambient temperature and an outlet temperature of 60° (or even 55°) the CoP is less than 2.
    ... but you shouldn't be running a gas boiler with a flow temperature of 60C, let alone a heat pump.
    My gas boiler (like many others on the forum) runs a 50C flow temp. My house is warm and my hot water is plentiful.

    N. Hampshire, he/him. Octopus Intelligent Go elec & Tracker gas / Shell (now TT) BB / Lebara mobi. Ripple Kirk Hill member.
    2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 33MWh generated, long-term average 2.6 Os.
    Not exactly back from my break, but dipping in and out of the forum.
    Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs, Best kettle!
  • 70sbudgie
    70sbudgie Posts: 785 Forumite
    500 Posts Second Anniversary Photogenic Name Dropper
    QrizB said:
    70sbudgie said:
    I don't agree that I've had duff information - I accept that my figures aren't completely correct. But from the tables you shared, the "nominal" line, for 2°C ambient temperature and an outlet temperature of 60° (or even 55°) the CoP is less than 2.
    ... but you shouldn't be running a gas boiler with a flow temperature of 60C, let alone a heat pump.
    My gas boiler (like many others on the forum) runs a 50C flow temp. My house is warm and my hot water is plentiful.

    I don't run my boiler at 60°, but that tends to be the temperature that most people expect their radiators to be. I think the members on this forum are almost certainly not in the "most people" category.

    I also have a large amount of wet underfloor heating, so a heat pump would be a lot more efficient.

    But my inlaws don't and they are more likely to be in the category that would be encouraged to replace their gas boiler with a heat pump. And consequently see their energy bills increase. 

    My concern is that heat pumps are being marketed as the saviours of decarbonising domestic heat (at the expense of alternative technologies imho) and unless you are sufficiently technically minded to understand the seasonal variations and constraints, it won't operate as expected - when it is most needed. 
    4.3kW PV, 3.6kW inverter. Octopus Agile import, gas Tracker. Zoe. Ripple x 3. Cheshire
  • QrizB
    QrizB Posts: 15,314 Forumite
    10,000 Posts Third Anniversary Photogenic Name Dropper
    70sbudgie said:
    My concern is that heat pumps are being marketed as the saviours of decarbonising domestic heat (at the expense of alternative technologies imho) and unless you are sufficiently technically minded to understand the seasonal variations and constraints, it won't operate as expected - when it is most needed. 
    In the specific case of this thread, I mentioned them as an effective way of making use of surplus PV to provice space heating during the shoulder months. There are several forum users, including Martyn1981, who do exactly this and they have been very happy with them for that purpose.
    Looking at your wider point, we've also got several forum members, including Reed_Richards (thread) and danrv (thread), who have replaced fossil fuel boilers with heat pumps and are satisfied by those, too. Two very different systems in quite different properties.
    N. Hampshire, he/him. Octopus Intelligent Go elec & Tracker gas / Shell (now TT) BB / Lebara mobi. Ripple Kirk Hill member.
    2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 33MWh generated, long-term average 2.6 Os.
    Not exactly back from my break, but dipping in and out of the forum.
    Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs, Best kettle!
  • NoobSolar
    NoobSolar Posts: 117 Forumite
    Second Anniversary 10 Posts
    Simplified version of your idea: buy a small (maybe just 1kw) heater - or even an aircon unit that can dual-function as a heater - and on those rare Winter days where your (presumably significantly larger than average) PV system has some spare energy, set the heater to heat the coldest room in the house for a couple of hours. This is a good use of limited spare energy. Just insulate the bejeezus out of that colder external corner bedroom. If your solar provides readings over an app on your smartphone, perhaps an Internet - operated heater would be a good idea.
    London. 6.4kwh system, South facing. 16 Hyundai 400kw all black panels w/ optimisers, 6kw Solaredge HD Wave inverter, Solar Iboost with two immersion heaters on one 240l hw tank. Octopus Flux. Ivar 5 Wood Burning Stove. Leaf 62kwh plus Zappi. Two chickens: 1 Light Sussex, 1 Speckled Rock. Omlet walk-in run. Approx 1.5 eggs per day egg generation rate using Marriage's organic layers pellets.
  • Freepost
    Freepost Posts: 219 Forumite
    Part of the Furniture 100 Posts Name Dropper Combo Breaker
    Can it be done, sure.
    Would it be worth it, well due to there only being maybe 4 months of the year that are both bright and cool is the only times it will likely help.
    Also the water in the second tank may have to be temperature controlled if its for floor heating (I don't have floor heating by water, but note most sources suggest a lower temperature than radiators)

    Hi, thank you for your reply, most certainly in the dark depths of winter we wouldn't generate enough power from the Solar PV to boost the central heating but October/November and March/April/May could be interesting. With regard to the temperature, since the ground floor has underfloor heating, I understand that the temperature is handled by the manifold mixing the input/output of the system and the first floor central heating runs as normal in the range 50~60°.

    So, whereas we have an Indirect HotWater Cylinder for the DHW, this tank I believe has a heat exchanger contained within the storage tank, basically a coil of pipe work that the hot water from the system boiler passes through and this coil of pipe work radiates the heat from the hot water from the system boiler into the cold water within the tank.

    Consequently, if I want to also use the energy from the Solar PV to assist the system boiler in heating up the water for central heating system, the hot water from the system boiler would pass through a Direct Hotwater Cylinder, to be further heated by the immersion heater powered by the solar PV before passing onto the central heating system.

    This is along the lines of what I am thinking, are there any problems with it apart from the uncertainty of the Solar PV supplying sufficient energy?

    Regards

    F







  • Martyn1981
    Martyn1981 Posts: 15,123 Forumite
    Part of the Furniture 10,000 Posts Name Dropper Photogenic
    QrizB said:
    70sbudgie said:
    My concern is that heat pumps are being marketed as the saviours of decarbonising domestic heat (at the expense of alternative technologies imho) and unless you are sufficiently technically minded to understand the seasonal variations and constraints, it won't operate as expected - when it is most needed. 
    In the specific case of this thread, I mentioned them as an effective way of making use of surplus PV to provice space heating during the shoulder months. There are several forum users, including Martyn1981, who do exactly this and they have been very happy with them for that purpose.
    Looking at your wider point, we've also got several forum members, including Reed_Richards (thread) and danrv (thread), who have replaced fossil fuel boilers with heat pumps and are satisfied by those, too. Two very different systems in quite different properties.
    Hiya QrizB, I know very little about heat pumps for water, but everything I've seen posted, be it yourself, or the excellent advice from Reed Richards & Shinytop, is that you'll literally kill the COP with excessively high output temps, such as 60C. So whilst I'm no expert, just an avid observer/absorber of useful info on here (and elsewhere), I agree entirely with your suggestion about COP's.

    Moving to A2A as a supplemental heating source for the shoulder months, I would note two things, firstly that using excess PV (without a battery), means that HP use will be during the day, which on average will have higher air temps than the night, so I expect/assume a better COP, than that for the average daily temp over 24hrs due to time of usage weighting.

    Secondly, and this is more to do with the occasional boost we get in the winter months, the sunnier days, with limited excess PV from an E/W system, are often colder days, because the skies are clear, whereas the clouds can block sun, but also trap heat IYSWIM, so perhaps less COP, but depending on the value of your export, still worth it. But again, daytime use (v's night) should have an impact on the COP.

    I suppose as a caveat I should mention that living in Cardiff we are subject to what is locally described as the Cardiff Bowl. The S. Wales hills partially surrounding the city help to trap temps, as the sea (well, Bristol Channel) has a stabilising effect on local temps, preventing excessive fluctuations.
    Mart. Cardiff. 8.72 kWp PV systems (2.12 SSW 4.6 ESE & 2.0 WNW). 20kWh battery storage. Two A2A units for cleaner heating. Two BEV's for cleaner driving.

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • Solarchaser
    Solarchaser Posts: 1,730 Forumite
    Fifth Anniversary 1,000 Posts Name Dropper
    Freepost said:
    Can it be done, sure.
    Would it be worth it, well due to there only being maybe 4 months of the year that are both bright and cool is the only times it will likely help.
    Also the water in the second tank may have to be temperature controlled if its for floor heating (I don't have floor heating by water, but note most sources suggest a lower temperature than radiators)

    Hi, thank you for your reply, most certainly in the dark depths of winter we wouldn't generate enough power from the Solar PV to boost the central heating but October/November and March/April/May could be interesting. With regard to the temperature, since the ground floor has underfloor heating, I understand that the temperature is handled by the manifold mixing the input/output of the system and the first floor central heating runs as normal in the range 50~60°.

    So, whereas we have an Indirect HotWater Cylinder for the DHW, this tank I believe has a heat exchanger contained within the storage tank, basically a coil of pipe work that the hot water from the system boiler passes through and this coil of pipe work radiates the heat from the hot water from the system boiler into the cold water within the tank.

    Consequently, if I want to also use the energy from the Solar PV to assist the system boiler in heating up the water for central heating system, the hot water from the system boiler would pass through a Direct Hotwater Cylinder, to be further heated by the immersion heater powered by the solar PV before passing onto the central heating system.

    This is along the lines of what I am thinking, are there any problems with it apart from the uncertainty of the Solar PV supplying sufficient energy?

    Regards

    F







    If your manifold already mixes, then it seems that part is sorted 👍

    The only problem I forsee is when that tank is not being heated, it will be cold, and that could thermosyphon heat from your system tank, and so you would pay more to heat both tanks in winter.

    Cards on the table I'm in the process of doing something not totally different from what you propose. 
    I have two large water tanks that I am heating with solar at the moment and will heat with off peak electric in winter, they are dual purpose, being used for both hot water and central heating.

    I had my gas supply removed 2 months ago.

    I am using the tanks in reverse,  using the internal coils (2 separate coils) to heat the cold water to hot, and the other to heat radiators. 
    The water in my tanks will not leave those tanks.

    It's still new, I'm still working out the kinks, and thermosyphon is a larger issue than I thought it would be using the internal coils as heat exchangers, but I'm hoping to get round it by installing check valves/one way valves
    West central Scotland
    4kw sse since 2014 and 6.6kw wsw / ene split since 2019
    24kwh leaf, 75Kwh Tesla and Lux 3600 with 60Kwh storage
  • Freepost
    Freepost Posts: 219 Forumite
    Part of the Furniture 100 Posts Name Dropper Combo Breaker

    The only problem I forsee is when that tank is not being heated, it will be cold, and that could thermosyphon heat from your system tank, and so you would pay more to heat both tanks in winter.

    I didn't think of that but would assume that problem could be overcome with a motorised diverter valve?


    Cards on the table I'm in the process of doing something not totally different from what you propose. 
    I have two large water tanks that I am heating with solar at the moment and will heat with off peak electric in winter, they are dual purpose, being used for both hot water and central heating.

    I had my gas supply removed 2 months ago.

    I am using the tanks in reverse,  using the internal coils (2 separate coils) to heat the cold water to hot, and the other to heat radiators. 
    The water in my tanks will not leave those tanks.

    It's still new, I'm still working out the kinks, and thermosyphon is a larger issue than I thought it would be using the internal coils as heat exchangers, but I'm hoping to get round it by installing check valves/one way valves

    If I am understanding this correctly, the hot water in the tank is heating the water in the coil for onward transmission to the central heating circuit and/or the DHW. I am assuming one coil for CH and the other for DHW. Considering this will the speed of transference of heat from the hot water in the tank to the cold water in the coil be sufficient to provide DHW as it is being run? I imagine that there is a lot of maths to be worked out with this but it looks very interesting;-)

    F.

  • Solarchaser
    Solarchaser Posts: 1,730 Forumite
    Fifth Anniversary 1,000 Posts Name Dropper
    Motorised diverter would definitely solve one side of it, so you would probably want to go with two.
    Though the motorised valve I have has the body of the valve getting pretty warm, and so heat is lost through it. 
    So far I'd say check valves are better because they are alot cheaper, don't need electronics to control them, and can be easily covered with pipe insulation.

    Yes you have it right, that's exactly how it works, and yeah it works, it's provided hot water for the last 2 months including quite a few baths.
    It also works well at central heating.

    My issues are around thermosyphon and controlling the heat.
    For hot water, it's no issue, it goes through a thermostatic mixer valve before taps to stop scalding, but the central heating is a loop of the same water, so can't really be blended, so I've added in an adjustable bypass valve to stop the radiators getting too hot, and there's some optimisation of that to be done yet.

    I'll maybe do a wee thread on the tanks
    West central Scotland
    4kw sse since 2014 and 6.6kw wsw / ene split since 2019
    24kwh leaf, 75Kwh Tesla and Lux 3600 with 60Kwh storage
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