Octopus Heat Pumps

Martyn1981

orbit500 said:

If they have achieved an SCOP of 3 with the capability to flow at over 70C when required, then that's a significant achievement imo.

I will believe that when I see it. I’m sure many here already know this but there is a limit to heat pump efficiency given by:
COP=T1/deltaT where T1=flow temp in kelvin and deltaT=flow-outdoor temp.
so at 70c and 0c outdoor (which is when you’d want 70c flow):
CoP=343/(70-0) = 4.9
in real life more likely 2, I’d say.

This is all very interesting to me, as I have microbore, so not 100% sure that a 'normal' HP would manage for the coldest one or two weeks per year.

But, my reading of Nick's comment was an overall SCOP of 3, rather than a COP of 3 at 70C.

Not being pedantic, but the "70C when required", fits with my concerns and wants. If it's not needed for any significant length of time in the heating season, perhaps 2/26 weeks, then even allowing for the extra weighting of that period (more hours per day, and colder outside temps), then a lower, but acceptable SCOP may be achievable?

However, totally agree/appreciate that expecting or hoping for a SCOP of 3, if run at 70C all year, isn't achievable.

shinytop

Martyn1981 said:

orbit500 said:

If they have achieved an SCOP of 3 with the capability to flow at over 70C when required, then that's a significant achievement imo.

I will believe that when I see it. I’m sure many here already know this but there is a limit to heat pump efficiency given by:
COP=T1/deltaT where T1=flow temp in kelvin and deltaT=flow-outdoor temp.
so at 70c and 0c outdoor (which is when you’d want 70c flow):
CoP=343/(70-0) = 4.9
in real life more likely 2, I’d say.

This is all very interesting to me, as I have microbore, so not 100% sure that a 'normal' HP would manage for the coldest one or two weeks per year.

But, my reading of Nick's comment was an overall SCOP of 3, rather than a COP of 3 at 70C.

Not being pedantic, but the "70C when required", fits with my concerns and wants. If it's not needed for any significant length of time in the heating season, perhaps 2/26 weeks, then even allowing for the extra weighting of that period (more hours per day, and colder outside temps), then a lower, but acceptable SCOP may be achievable?

However, totally agree/appreciate that expecting or hoping for a SCOP of 3, if run at 70C all year, isn't achievable.

70 degrees is a bit of a meaningless headline.  A 'normal' ASHP has a maximum output of 60 deg C, give or take. No sensible design would ever have it reaching that high a temperature for heating, even at its design air temperature, which is normally around -3 deg C in the UK.  The maximum is normally 50 deg but is usually more like 45, sometimes 40.  Weather compensation means it'll be a lot less most of the time. So even if you add on 10 to a 'normal' ASHP it's going to be running at 40-60 and never 70. COP will be lower but not disastrously so. 

The other point to note is that the maximum output of an ASHP decreases as ambient temperature decreases and flow temperature increases. So a, say, nominally rated 8kW HT ASHP may not be able to deliver 8kW @ -5 deg C and 70 deg flow. 

I thought 70 deg was on the hot side even for ff heating?  

Martyn1981

Yep, 70C very high. Been running our GCH rads at the lowest temp for a couple of years, just to test out the impact of lower temps, with HP in mind. Only struggled a few times, those approx 2 weeks pa, I've suggested. But, big but, there would still be lots of spare, as the GCH isn't on 24/7. I suspect a 'normal' ASHP would work for us, but not sure how the install rules, assessment, even the subsidy, work. So given the negativity I've received in the past, regarding microbore, the high temp solution seems to cover all angles.

Interesting to see how little heating is needed when averaged out. For the last few years we've consumed ~3,600kWh of gas across Dec, Jan and Feb. That includes gas oven, but doubt that's significant. So at a gas boiler efficiency of 90% that's ~3,250kWh. In reality, our 24yr old Worcester Combi won't be 90%.

So, over 90days, that's only 36kWh per day. So at a COP of 2, on bad days, perhaps 18kWh. I appreciate that the bad days will be a lot different on average to the good days, within that 90day period, but it all suggests to me that it won't be a problem.

Screwdriva

shinytop said:

So even if you add on 10 to a 'normal' ASHP it's going to be running at 40-60 and never 70. COP will be lower but not disastrously so. 

Don't mean to disagree but this goes against my understanding of heat pumps. The difference between running a heat pump at 40 degrees vs. 60 degrees flow has historically been disastrous from a running cost perspective.

This is why heat pump installers have advocated wall insulation, underfloor heating etc. so that they can reduce flow temperature as much as possible. The sensitivity of heat pump SCOP to flow temperature is incredibly high.

Unless Octopus have some miraculous new technology or heavily discounted tariffs, I can't see this ending well. 

Netexporter

Unless Octopus have some miraculous new technology

I've assumed they're using propane as the refrigerant, as you can get a higher COP at higher flow temperatures, but other makers use propane already, and don't make particularly extravagant claims.

In theory, CO2 refrigerant heat pumps can achieve boiler-like performance but, as I understand it, it only seems to work at larger, district heating, scale. Or have Octopus cracked that nut?

QrizB

Screwdriva said:

shinytop said:

So even if you add on 10 to a 'normal' ASHP it's going to be running at 40-60 and never 70. COP will be lower but not disastrously so. 

Don't mean to disagree but this goes against my understanding of heat pumps. The difference between running a heat pump at 40 degrees vs. 60 degrees flow has historically been disastrous from a running cost perspective.

Your'e looking at a COP change of about 0.9 for those Daikins we were discussing a couple of pages back. SCOP at 35C flow temp is 4.51, at 55C flow is 3.58.

With 30p/kWh electricity (on the high side vs. the current SVT) that's the difference between paying 6.65p/kWh for heat, and 8.38p/kWh. £173 a year for a property with an 10MWh/yr heat demand.

Whether that's "disastrous" or not depends on your personal financial situation, but it's 0.5% of the average UK full-time salary.

Netexporter said:

Unless Octopus have some miraculous new technology

I've assumed they're using propane as the refrigerant, as you can get a higher COP at higher flow temperatures, but other makers use propane already, and don't make particularly extravagant claims.

Those Daikins (again) use R32.

Netexporter said:

In theory, CO2 refrigerant heat pumps can achieve boiler-like performance but, as I understand it, it only seems to work at larger, district heating, scale. Or have Octopus cracked that nut?

There's a small Mitsubishi - the QUHZ - that uses CO2 as refrigerant.

https://les.mitsubishielectric.co.uk/products/residential-heating/outdoor/ecodan-quhz-monobloc-air-source-heat-pump

shinytop

Screwdriva said:

shinytop said:

So even if you add on 10 to a 'normal' ASHP it's going to be running at 40-60 and never 70. COP will be lower but not disastrously so. 

Don't mean to disagree but this goes against my understanding of heat pumps. The difference between running a heat pump at 40 degrees vs. 60 degrees flow has historically been disastrous from a running cost perspective.

This is why heat pump installers have advocated wall insulation, underfloor heating etc. so that they can reduce flow temperature as much as possible. The sensitivity of heat pump SCOP to flow temperature is incredibly high.

Unless Octopus have some miraculous new technology or heavily discounted tariffs, I can't see this ending well. 

I agree, but that's not what I said.  The difference I'm talking about is 10 degrees, not 20.  I said that most normal ASHPs run  at 45-50 maximum; my HT example one would be 55-60 maximum. An Ecodan like mine would lose about 25% of its COP going from 50 to 60 deg at -7 deg C ambient.  Not ideal but definitely not disastrous. And it would be doing that only at the very very coldest time. (And not in my house btw).

Reed_Richards

This is the energy (kWh) used by my heat pump over a 3 week period at the start of last year.  You will see that there is over a factor of two difference between the worst day, 6th January, and the best day, 19th January.  I didn't change my heating regime over that time period, it's all down to the effect of the outside temperature.  I'm using weather compensation if the outside temperature is higher than - 4 C, below that temperature the water temperature for heating is 50 C.  The amount of variation I see due to outside temperature makes me think that I would pay a huge penalty in power consumption if I had to boost the water temperature to 70 C to cope with cold days.  It looks to me as if the SCOP is likely to be significantly influenced by the COP achieved on cold days because cold days require so much more energy  

Screwdriva

QrizB said:

Your'e looking at a COP change of about 0.9 for those Daikins we were discussing a couple of pages back. SCOP at 35C flow temp is 4.51, at 55C flow is 3.58.

With 30p/kWh electricity (on the high side vs. the current SVT) that's the difference between paying 6.65p/kWh for heat, and 8.38p/kWh. £173 a year for a property with an 10MWh/yr heat demand.

Whether that's "disastrous" or not depends on your personal financial situation, but it's 0.5% of the average UK full-time salary.

If a Daikin ASHP achieved a SCOP of 3.6 @ 55C flow in a real world installation, I'd personally be over the moon with achieving cost parity with a gas boiler.

That said, I suspect (and have heard) that won't be the case with the Octopus installations. More than happy to be proven incorrect with actual numbers. In my view, installer skill is probably the single most important variable in ASHP installations, far more than a spec sheet. 

1961Nick

Martyn1981 said:

orbit500 said:

If they have achieved an SCOP of 3 with the capability to flow at over 70C when required, then that's a significant achievement imo.

I will believe that when I see it. I’m sure many here already know this but there is a limit to heat pump efficiency given by:
COP=T1/deltaT where T1=flow temp in kelvin and deltaT=flow-outdoor temp.
so at 70c and 0c outdoor (which is when you’d want 70c flow):
CoP=343/(70-0) = 4.9
in real life more likely 2, I’d say.

This is all very interesting to me, as I have microbore, so not 100% sure that a 'normal' HP would manage for the coldest one or two weeks per year.

But, my reading of Nick's comment was an overall SCOP of 3, rather than a COP of 3 at 70C.

Not being pedantic, but the "70C when required", fits with my concerns and wants. If it's not needed for any significant length of time in the heating season, perhaps 2/26 weeks, then even allowing for the extra weighting of that period (more hours per day, and colder outside temps), then a lower, but acceptable SCOP may be achievable?

However, totally agree/appreciate that expecting or hoping for a SCOP of 3, if run at 70C all year, isn't achievable.

Another factor to take account of Mart, is what type of microbore plumbing you have. Those with a central manifold & long microbore pipe runs are the ones that will struggle with the higher flow rates required by a heat pump. If your heating circuit is mainly 28mm/22mm/15mm with the microbore tee'd off for each rad then it won't make much difference from being piped entirely in 15mm - the number of bends in microbore is a key restriction.