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COP - diminishing returns on investment?

QrizB
QrizB Posts: 15,328 Forumite
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edited 19 January am31 10:52AM in Heat pumps
A bit of a thought experiment, so bear with me.
We can all hopefully agree that a COP of 5 is great, a COP of 3 is kinda OK and a COP of 2 is poor. There are plenty of threads where people give hints and tips on how to improve your COP. But what's it worth spending to get that better COP?
Imagine, if you will, a household that needs 10MWh per year of heat (this is close to the Ofgem average dual fuel property). Imagine also that electricity is 25p/kWh, just to make the maths easier. And I'm going to look at costs over ten years in present-day money, as I'm writing this on my phone and don't want to be flipping between browser tabs to calculate discount rates and whatnot.
COP 1 (direct electric heaters): annual fuel cost £2500, 10 year cost £25k.
COP 2: 10yr £12.5k
COP 3: 10yr £8.3k
COP 4: 10yr £6k
COP 5: 10yr £5k
So if our example 10MWh/yr household has a rubbish heatpump system with a COP of 2, it's probably worth spending £4k to get to a COP of 3. That could potentially buy you a replacement heat pump, if everything else is OK. Or a whole heap of radiators, and/or an improved control system.
With a COP of 3, you're looking at £2k to get to a COP or 4. It's unlikely to be worthwhile replacing the heat pump itself, unless it's already end-of-life or you can get a cheap used one. You could still swap out radiators and improve your controls.
With a COP of 4, you've only got a £1k budget to get to a COP of 5. You've got options for DIY improvements but you're really only going to be tinkering with the details.
The last conclusion I'm going to reach is that (for my example property) if it's going to cost more than £20k to fit, you may as well stick with direct electric heating. (A neighbour for example has recently had a heat pump fitted and after the £7.5k grant it still cost him £12k. The economics of that seem a bit marginal.)
Thoughts and discussion, folks?
I know there's at least one accountant who's a regular here and I'm hoping he'll share his thoughts on this!

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!
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Comments

  • FreeBear
    FreeBear Posts: 17,335 Forumite
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    Tweaking the heat pump parameters is a low cost way to boost performance. I suspect some installers will leave everything on factory defaults which may not be the best settings for performance.
    Improving insulation levels and cutting cold draughts will reduce heat losses, and some measures are relatively cheap. Killing those nasty cold draughts can improve quality of life which is difficult to put a value on - I've spent <mumble><mumble> thousands on replacement doors & windows as well as insulating & draught proofing. It has helped to cut heating costs, but probably only saving ~£250 a year (based on current rates). But no more cold draughts makes it worthwhile.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
  • Reed_Richards
    Reed_Richards Posts: 5,095 Forumite
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    It's an intriguing question but I'm not sure about the basic premise that you can spend more money to get a better SCOP (and it is the Seasonal average figure you mean).  You tend to get the heat pump that your installers want to install; they all claim to have fairly similar SCOPs, I'm not convinced that you can completely trust the figures given and what you actually achieve will depend quite a bit on how the full system is configured and operated.

    My bungalow uses about 5 MWh per year for heating and hot water so I find the Ofgem average figure quoted quite surprising.  But that must mean that I have even less of an incentive to spend money to improve my SCOP, if I could work out how to do that. 

    The heat pump itself might only last 10 years but the rest of the infrastructure should have a much longer life; presumably your neighbour wasn't just getting a heat pump.

      
    Reed
  • matt_drummer
    matt_drummer Posts: 1,954 Forumite
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    edited 19 January pm31 12:33PM
    QrizB said:
    A bit of a thought experiment, so bear with me.
    We can all hopefully agree that a COP of 5 is great, a COP of 3 is kinda OK and a COP of 2 is poor. There are plenty of threads where people give hints and tips on how to improve your COP. But what's it worth spending to get that better COP?
    Imagine, if you will, a household that needs 10MWh per year of heat (this is close to the Ofgem average dual fuel property). Imagine also that electricity is 25p/kWh, just to make the maths easier. And I'm going to look at costs over ten years in present-day money, as I'm writing this on my phone and don't want to be flipping between browser tabs to calculate discount rates and whatnot.
    COP 1 (direct electric heaters): annual fuel cost £2500, 10 year cost £25k.
    COP 2: 10yr £12.5k
    COP 3: 10yr £8.3k
    COP 4: 10yr £6k
    COP 5: 10yr £5k
    So if our example 10MWh/yr household has a rubbish heatpump system with a COP of 2, it's probably worth spending £4k to get to a COP of 3. That could potentially buy you a replacement heat pump, if everything else is OK. Or a whole heap of radiators, and/or an improved control system.
    With a COP of 3, you're looking at £2k to get to a COP or 4. It's unlikely to be worthwhile replacing the heat pump itself, unless it's already end-of-life or you can get a cheap used one. You could still swap out radiators and improve your controls.
    With a COP of 4, you've only got a £1k budget to get to a COP of 5. You've got options for DIY improvements but you're really only going to be tinkering with the details.
    The last conclusion I'm going to reach is that (for my example property) if it's going to cost more than £20k to fit, you may as well stick with direct electric heating. (A neighbour for example has recently had a heat pump fitted and after the £7.5k grant it still cost him £12k. The economics of that seem a bit marginal.)
    Thoughts and discussion, folks?
    I know there's at least one accountant who's a regular here and I'm hoping he'll share his thoughts on this!

    There's a lot there to think about.

    Basically it is about right and direct electric heating is really cheap to buy so that may make almost as much sense financially.

    However, for many people, it is not just about the ultimate cost, it is more than that and just being as efficient as possible has a reward that justifies the cost. The same way we don't all just buy the cheapest cars available even though that makes most financial sense.

    Looking at my own situation.

    A higher COP/SCOP means I need less electricity, and as I try to get most of my electricity from battery storage, if I stick within the constraints of my electricity tariff and inverter capability then I buy electricity at 7p instead of 28p.

    Keeping it simple (ignoring battery losses for example)

    If my heat pump uses 20kWh a day at a COP of 5.0 then all my electricity comes from the batteries at 7p per kWh, so £1.40 a day

    If my COP was only 2.50 then I would need 40kWh to produce the same amount of heat


    That would cost me ((20 x £0.07) + (20 @ £0.28)) = £7.00

    That is effectively a COP of 1.0 compared to my actual COP of 5.0

    I could have bought 100kWh of electricity @ 7p per kWh for my £7.00, the same amount of heat that I actually produced for 20kWh of electricity at a COP of 5.0

    A COP of 2.50 for me wouldn't actually be twice as expensive compared to my COP of 5.0 it would actually be five times more expensive.

    That is why COP is important for me, not just because I want to do it, but because the cost implication is actually much bigger than it first appears.


    Direct electric heating would be even worse, assuming 100kWh of heat in a day delivered in the same way as my heat pump would cost £25.00 on the SVT, maybe £23.00 on IOG, and if it was NSH, maybe £15.00

    So in my case 11 to 18 times less efficient than my heat pump at SCOP of 5.0 in terms of running costs.

  • matt_drummer
    matt_drummer Posts: 1,954 Forumite
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    Thinking about this in a wider way rather than on a personal level.

    Greater efficiency means that less energy is consumed and therefore, less energy needs to be generated.

    That is the goal here.

    I am sure that for many people the personal sacrificies needed to achieve this (spending money) are not attractive.

    However, there are people who do value the environment more than their own wealth and that is why some of us strive for the best efficiency.

    Of course, this is only possible if you have the means to do it and I am not suggesting that anybody should get themselves into financial difficulty just to get a better SCOP from their heat pump!
  • NedS
    NedS Posts: 4,083 Forumite
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    edited 19 January pm31 2:05PM
    The time to get things right is at install time. I appreciate this is not always easy (or cheap) in a retrofit. If you end up with a system with a SCOP of 2 then you have a bad install or major issues, simple as that. The key to a decent SCOP is low flow temps, and that means large emitters (large radiators and/or underfloor heating). COPs are also dependant on ambient temps, so if you live in a very cold part of the country you are likely to see lower COPs than somewhere that is predominantly mild.
    Ultimately all heat pumps are similar - the tech (and even the parts) are largely the same. What varies is the rest of the system - what the heat pump is connected to - pipework, buffer tanks, heat exchangers, radiators etc. The other main difference between heat pumps is their control panels and how easy they are to use.
    I believe if you get the rest of the system right, then the heat pump should perform to it's manufacturers specs. Keep is simple, no zoning, no TRVs, no buffers (but you may need a volumiser), large diameter pipe runs, good system volume and radiators as large as you can accommodate to keep the design temp as low as possible. Do not accept a design temp of 50C when you can change out some radiators and get that down to 40C. Looking at the specs for my system at 0C outside, my system would give a cop of around 2 with a 50C flow temp, or a COP of 3 if I can get that flow temp down to 35C - that's a huge difference in running costs on those cold days.
    With regard to the financials, I think that's difficult especially with electricity pricing where it is at the moment. Unless you can access significantly cheaper electricity through cheaper tariffs and battery storage as @matt_drummer does, I think the best you can realistically hope for is that it will cost in the same ball park to run as gas or oil. The issue is then the higher installation costs, as a heat pump is less likely to be a drop in replacement for a gas or oil boiler as pipework and radiator replacements are likely needed pushing up costs significantly. I do worry about the running costs where they are dropping in HT heat pumps as direct replacements for boilers without addressing the pipework or radiators that could be upgraded to improve the system.
    We were lucky in that we had ours fitted on the ECO4 grant scheme so didn't have to consider installation costs, only running costs vs the oil boiler it was replacing.

  • QrizB
    QrizB Posts: 15,328 Forumite
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    edited 19 January pm31 1:31PM
    My bungalow uses about 5 MWh per year for heating and hot water so I find the Ofgem average figure quoted quite surprising ... 
    Is that electrical input (so 15MWh of heat at a COP of 3), or heat output? From your previous posts I suspect the former? Ofgem's figure is for the latter (it's 11500kWh of gas, which allowing for boiler efficiency works out as ~10MWh of useable heat).
    The heat pump itself might only last 10 years but the rest of the infrastructure should have a much longer life; presumably your neighbour wasn't just getting a heat pump.
    10 years was picked as an accounting boundary rather than a life figure, but I accept your point. Yes my neighbour was getting quite a lot more. His property was plumbed for GCH but had been extended and modified to include a wood stove and the system was a bit higgledy-piggledy. From his description is was almost (but not quite) a full re-plumb.
    Keeping it simple (ignoring battery losses for example)
    If my heat pump uses 20kWh a day at a COP of 5.0 then all my electricity comes from the batteries at 7p per kWh, so £1.40 a day
    If my COP was only 2.50 then I would need 40kWh to produce the same amount of heat
    That would cost me ((20 x £0.07) + (20 @ £0.28)) = £7.00
    You're assuming you couldn't just buy twice as much battery, so you'd still paying 7p/kWh. 20kWh of battery is about £2.5k at present.
    Direct electric heating would be even worse, assuming 100kWh of heat in a day delivered in the same way as my heat pump would cost £25.00 on the SVT, maybe £23.00 on IOG, and if it was NSH, maybe £15.00
    Or 100kWh of battery! I realise this might sound silly but @Solarchaser does it with a thermal store, so why not? It would be cheaper than my neighbour's heat pump ...
    However, there are people who do value the environment more than their own wealth and that is why some of us strive for the best efficiency.
    Of course, this is only possible if you have the means to do it and I am not suggesting that anybody should get themselves into financial difficulty just to get a better SCOP from their heat pump!
    I agree with both those points, but at the same time I do occasionally hear accounts (not necessarily involving heat pumps) of people spending relatively large sums for marginal savings of money/resources/whatever. The thought that prompted this thread was something along the line of "people compare COPs but what does that actually mean in cash terms?" and it's not meant to be judgemental, just trying to give some perspective.
    NedS said:
    Do not accept a design temp of 50C when you can change out some radiators and get that down to 40C. Looking at the specs for my system at 0C outside, my system would give a cop of around 2 with a 50C flow temp, or a COP of 3 if I can get that flow temp down to 35C - that's a huge difference in running costs on those cold days.
    Yes, exactly. But if you're given a system with a design flow temp of 50C (perhaps because you've gone with a cheap installer who meets the minimum specs for the BUS grant, naming no cephalopods here) then it's up to you to decide whether to stick wirh the system as-is or whether to eg. fit larger emitters. And big radiators aren't cheap. So how much is it worth spending to get that flow temperature down? That's where I was going with my thoughts.
    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!
  • matt_drummer
    matt_drummer Posts: 1,954 Forumite
    1,000 Posts Second Anniversary Name Dropper
    I have enough batteries.

    I just can't charge them enough without any sunshine.

    I have a GivEnergy AC coupled inverter and until they get their act together you can only use one at a time.

    That limits by charging and discharging ability. 

    As long as my heat pump is efficient, I can last a week to ten days with very little sun, the batteries start at 100% but I just get less and less each day as I consume slightly more than I can put back into them.

    If my heat pump was less efficient, I couldn't do it and I would need to buy electricity at 28p rather than 7p, effectively reducing my efficiency by a factor of 4.

    In regards to the thermal store using 100kWh of battery, maybe cheaper but it's not really the point.

    It is only 100% efficient so does not contribute to cutting energy consumption in the same way that a 500% efficient heat pump does.

    I know your point was about cost efficiency and that this is a site dedicated to saving money.

    But, I am never going to stop because I believe in this as being best for all of us. Heat pumps and evs are not about being cheaper, they can be, but that is not the reason for their existence, they are here to cut down on energy use and burning fossil fuels.

    I know it's not popular because money matters most to the majority, but no amount of money is of any value if we make the planet uninhabitable.
  • Reed_Richards
    Reed_Richards Posts: 5,095 Forumite
    Part of the Furniture 1,000 Posts Photogenic Name Dropper
    QrizB said:
    My bungalow uses about 5 MWh per year for heating and hot water so I find the Ofgem average figure quoted quite surprising ... 
    Is that electrical input (so 15MWh of heat at a COP of 3), or heat output? From your previous posts I suspect the former? Ofgem's figure is for the latter (it's 11500kWh of gas, which allowing for boiler efficiency works out as ~10MWh of useable heat).
    Your suspicions are correct.  I have no way of measuring the heat output I achieve.

    QrizB said:

    Or 100kWh of battery! I realise this might sound silly but @Solarchaser does it with a thermal store, so why not? It would be cheaper than my neighbour's heat pump ...
    The reason why not is that you don't need it.  Assuming you have 5 to 7 hours of cheap electricity to charge your battery you only need enough charge to last you through the remaining 17 to 19 hours of the day.  The worst rate of consumption I have observed was about 60 kWh per day so a 50 kWh battery should cover that for me.  But times of such high consumption are rare so if you made an analysis of battery cost vs. potential saving then I'm sure the break even point would require a much smaller battery than whatever is needed to cover those few very cold days.     
    Reed
  • matt_drummer
    matt_drummer Posts: 1,954 Forumite
    1,000 Posts Second Anniversary Name Dropper
    QrizB said:

    Yes, exactly. But if you're given a system with a design flow temp of 50C (perhaps because you've gone with a cheap installer who meets the minimum specs for the BUS grant, naming no cephalopods here) then it's up to you to decide whether to stick wirh the system as-is or whether to eg. fit larger emitters. And big radiators aren't cheap. So how much is it worth spending to get that flow temperature down? That's where I was going with my thoughts.
    Probably not much if the heating works and your main concern is money.

    That is not judgemental either, just factual.

    There comes a point when you get very little return on the cost, it's just not worth it.

    The MCS do set minimum efficiency levels as part of the grant process.

    Although, radiators should last quite a long time in a low flow temperature system, I believe that it is air and higher flow temperatures that promote the most corrosion?

    You can go another way if you have space, adding more radiators is just as good as increasing sizes, maybe not so good for asthetics though.

    Last time I looked, a 600 x 1200 K2 Stelrad radiator was about £80.

    Octopus buy them even cheaper and with no VAT at installation.

    It shouldn't be that expensive to get to a lower flow temperature.
  • FreeBear
    FreeBear Posts: 17,335 Forumite
    Ninth Anniversary 10,000 Posts Name Dropper Photogenic
    QrizB said:
    My bungalow uses about 5 MWh per year for heating and hot water so I find the Ofgem average figure quoted quite surprising ... 
    Is that electrical input (so 15MWh of heat at a COP of 3), or heat output? From your previous posts I suspect the former? Ofgem's figure is for the latter (it's 11500kWh of gas, which allowing for boiler efficiency works out as ~10MWh of useable heat).
    Your suspicions are correct.  I have no way of measuring the heat output I achieve.
    You measure the water flow along with the flow & return temperatures. With those three bits of information, it is trivial to calculate the amount of energy being used.
    Or fit a heat meter and let it do the monitoring & calculations for you - I'm using a L&G T230 to monitor the output from my boiler. Numbers are updated every 5 minutes, which is good enough for most.
    Her courage will change the world.

    Treasure the moments that you have. Savour them for as long as you can for they will never come back again.
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