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Hot water heat pump vs solar thermal

  • 28-06-2018 8:55pm
    #1
    H.E. Pennypacker
    Closed Accounts Posts: 844 ✭✭✭


    I've been looking at putting in a renewable hot water source, considered PV but have been advised against it. Solar thermal is the one that most people seem to like but a heat pump has been suggested and has got me thinking.


    At a rough guess, hot water currently costs us around €350 a year. We use mostly night rate electricity in summer and an oil fired boiler in winter to heat the cylinder. We have two toddlers who are increasing our hot water demand as time goes on so the hot water bill is likely to move towards €400 and beyond annually as things stand.


    I've been quoted €4,200 and €4,600 for solar thermal (including grant). I've been quoted €3,800 for a heat pump hot water tank. Payback is slightly less than ten years for the heat pump tank and slightly more than ten years for solar thermal at current energy prices.



    I'm leaning towards the heat pump solution as it is programmable and so can be run on night rate electricity. It will also provide hot water all year round, unlike solar thermal which will need additional energy during the winter months. Solar thermal has some potential expense in terms of pump replacement and servicing costs. I don't know what the lifespan of the heat pump will be.


    In my case, I can't draw warm air from a wet room for the heat pump so it'll use outside air as a source which may affect the COP.


    Is there any reason as to why a heat pump water tank isn't as good a solution as solar thermal apart from its energy consumption over the summer months? is there anything else about the heat pump that I'd need to consider?


Comments

  • #2
    Calahonda52
    Registered Users Posts: 12,163 ✭✭✭✭Calahonda52


    Why the no no on PV?
    Once FIT are brought in PV will go up in price.
    HP will not give you HW , i.e. 60C water, all year round: that is a lie.

    The design delta T is usually 50 (60-10) so once the 10 goes to 5 the evaporator will start to freeze up, requiring elec to melt it.
    ST, IMO, is over rated especially in weather like today when you would be dumping massive amounts of heat, where as with PV it would just keep generating....

    “I can’t pay my staff or mortgage with instagram likes”.



  • #3
    H.E. Pennypacker
    Closed Accounts Posts: 844 ✭✭✭H.E. Pennypacker


    Calahonda52 wrote: »
    Why the no no on PV?
    Once FIT are brought in PV will go up in price.
    HP will not give you HW , i.e. 60C water, all year round: that is a lie.

    The design delta T is usually 50 (60-10) so once the 10 goes to 5 the evaporator will start to freeze up, requiring elec to melt it.
    ST, IMO, is over rated especially in weather like today when you would be dumping massive amounts of heat, where as with PV it would just keep generating....

    Non-specialist suppliers (and posters on here) were of the opinion that solar pv is inefficient (roughly 20% yield) in terms of panel area and that hot water diversion is expensive in terms of diversion - basically that its a 'last resort' before feeding the grid as payback on the diverter is low. The option that I was offered was a dedicated hot water heat pump rather than a central heating HP. but interestingly when I look at the manufacturer's website, they talk of 70% environmental heat and 30% electrical so it seems that it too needs a boost.

    I agree with you that ST is either oversized or undersized and my preference would be to leave my existing plumbing undisturbed as its a bit messy. I've already had one installer here talking about fitting a one way valve to a solid fuel stove (to prevent heating the stove boiler via its coil when the ST coil is below it in the cylinder) which makes me uncomfortable. I like the idea of pv but most people that I've spoken with tend to dislike it as a hot water source unless its a last resort). PV would be my neatest option in terms of minimising the disturbance to my current setup.

    Price-wise a 2.4kw pv array and diverter works out at around €4,600 which is reasonably competitive compared with ST. No guarantee of a FIT either although there is mention of an EU directive to make it mandatory.

    I've been told of a €500 per kw grant for pv on the way (possibly retrospective too :-) but that might just be sales talk.


  • #4
    quentingargan
    Registered Users Posts: 1,627 ✭✭✭quentingargan


    H.E. Pennypacker wrote: »
    I've been told of a €500 per kw grant for pv on the way (possibly retrospective too :-) but that might just be sales talk.
    Definitely speculative. No figures have been given, and retrospective has never been done before.

    Efficiency of a panel might be <20% for PV and much higher for thermal, but it is the return on the system price that really counts unless roof space is very limited. Maintenance costs need to be factored in. On solar thermal you need to change glycol every two years or so, which takes the fun out of it... PV is pretty much maintenance free.


  • #5
    freddyuk
    Registered Users Posts: 1,110 ✭✭✭freddyuk


    quentingargan wrote: »
    Definitely speculative. No figures have been given, and retrospective has never been done before.

    Efficiency of a panel might be <20% for PV and much higher for thermal, but it is the return on the system price that really counts unless roof space is very limited. Maintenance costs need to be factored in. On solar thermal you need to change glycol every two years or so, which takes the fun out of it... PV is pretty much maintenance free.


    Q, this is not true if quality solar fluid is used. Trouble is your average plumber will use the cheapest option and this will lead to issues especially now the systems are being stretched to limits. There are alternative fluids that do not need changing and will withstand higher temperatures but they cost more.

    A PV system may need a new inverter every 8 -10years so this needs to be factored in too and these cost a lot more than glycol replacement. No installer will ever tell you to budget for an inverter replacement.


  • #6
    quentingargan
    Registered Users Posts: 1,627 ✭✭✭quentingargan


    freddyuk wrote: »
    A PV system may need a new inverter every 8 -10years so this needs to be factored in too and these cost a lot more than glycol replacement. No installer will ever tell you to budget for an inverter replacement.
    New 2kw inverters are about €350. That would be €35 to €40 per year. The design life of most solar inverters is over 20 years. They may fail earlier.

    I agree that a properly designed solar array with overheat diversion and high-temp glycol should have much lower maintenance, but that hasn't been happening much.

    Either way, I would argue that a PV system using 1/3 or more of its power as electricity at 18c per Kwhr and the rest displacing water heating at 9c from oil has a better payback than solar thermal. Neither option is a "get rich quick" product as is often claimed by over-zealous sales people.

    But to say PV is a no-no is not justifiable, and anyone wanting to reduce their CO2 emissions with a system that will cover its costs over time could consider any of the options, including heat pump running off-peak. It depends on your water usage, electricity daytime usage trends, and ease of roof access / immersion access.


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  • #7
    unkel
    Registered Users Posts: 64,679 ✭✭✭✭unkel


    quentingargan wrote: »
    On solar thermal you need to change glycol every two years or so

    On a system like Kingspan Thermomax the coolant doesn't get very hot and doesn't need replacing anywhere near 2 years. I'd like to think I will get the guts of a decade out of the original coolant

    As for the OP - if DHW is costing you €350 per year and a new system will cost you over €4k to install (after the generous subsidy) plus some average maintenance of say €50-€100 per year, your pay back period will be long

    I have a 40 tube system. We use a lot of hot water (family of 5 with 4 women liking their big baths / long showers). And I had to upgrade my standard Irish 120l cylinder to a 360l one anyway. And I got a very good price for the install. For me the payback is <10 years

    I like renewables, but be careful and do your sums. Most renewables are very expensive and have extremely long pay back times. Solar PV is terrible in that regard (zero subsidies and zero payback for the electricity you generate). And I have a small DIY solar PV system myself. Which has been providing the grid (the tax payer) with almost all my production for the past month or so for free. You're welcome :D


  • #8
    H.E. Pennypacker
    Closed Accounts Posts: 844 ✭✭✭H.E. Pennypacker


    unkel wrote: »


    As for the OP - if DHW is costing you €350 per year and a new system will cost you over €4k to install (after the generous subsidy) plus some average maintenance of say €50-€100 per year, your pay back period will be long

    I have a 40 tube system. We use a lot of hot water (family of 5 with 4 women liking their big baths / long showers). And I had to upgrade my standard Irish 120l cylinder to a 360l one anyway. And I got a very good price for the install. For me the payback is <10 years

    I like renewables, but be careful and do your sums. Most renewables are very expensive and have extremely long pay back times. Solar PV is terrible in that regard (zero subsidies and zero payback for the electricity you generate). And I have a small DIY solar PV system myself. Which has been providing the grid (the tax payer) with almost all my production for the past month or so for free. You're welcome :D


    For me there's the added advantage of more plentiful hot water so its about convenience and the idea of not having to run the immersion during the day to heat up more water once the water heated overnight is used up. I can live with a longer return on investment



    We could do with a new cylinder in any event, ours is 120 litre with only 20m factory insulation.


    Having given it some thought, I can't see the heat pump cylinder working for me. I have heat recovery ventilation fitted so I don't have a warm air source and it'd be too complicated to tap into the exhaust air from the ventilation unit. That leaves me using external air which'd really hit the COP (calculated at 20.5C).


    As a matter of interest, what size is your PV array? If you didn't have solar thermal and had a diverter, how much would you have lost to the grid?


  • #9
    unkel
    Registered Users Posts: 64,679 ✭✭✭✭unkel


    H.E. Pennypacker wrote: »
    For me there's the added advantage of more plentiful hot water so its about convenience and the idea of not having to run the immersion during the day to heat up more water once the water heated overnight is used up. I can live with a longer return on investment

    There's that. And if you are going to stay in your house for decades it will pay for itself and make you money eventually and of course you are being green.
    H.E. Pennypacker wrote: »
    We could do with a new cylinder in any event, ours is 120 litre with only 20m factory insulation.

    Yeah, my new cylinder apparently only loses 1C per 24h in heat. Very well insulated. My "hot press" is no longer a hot press :)
    H.E. Pennypacker wrote: »
    As a matter of interest, what size is your PV array? If you didn't have solar thermal and had a diverter, how much would you have lost to the grid?

    It's only a mickey mouse DIY 3 panel system. At the moment I'm losing about 80% of it to the grid, but I don't mind.


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