ASHP efficiency in hot weather

[joe90]

Just been stood in front of my ASHP getting cool whilst it tops up the DHW as it’s scorching down here and wondered if they were more efficient in hot weather?.there is more heat in the air during this hot weather, if I run it on E7 it will run at night when the air is cooler so less air heat is available. Am I barking up the wrong tree?

[ProDave]

I think Jeremy did the calculations and the difference is only a few % in the heat capacity of air over the range we are talking about.  so I am sure the HP will run at a better COP just now, but the saving by running it instead on the off peak rate will be greater.

 

Mine topped up the tank today powered by the PV with no import.  Mine only comes on after 11AM to maximise the chance of enough PV generation to run it.

[Jeremy Harris]

Yes, the difference is roughly proportional to the absolute temperature, in K, rather than the relative temperature in °C.   There's a bit more to it, as at lower temperatures the properties of the refrigerant used have an impact, but the difference in the heat contained in a given volume of air is proportional to the absolute temperature. 

 

So, 20°C is 293.15 K and 30°C is 303.15 K.

 

Although 30°C is apparently 50% hotter than 20°C, in absolute terms it's only ~3.4% hotter, so there's only ~3.4% more heat in air at 30°C than there is in air at 20°C

[joe90]

Thanks guys, makes sense, so will continue to think of E7 for the future. 

[H F]

Our installers told us that in temperatures above 20C, DHW was negligible in terms of electricity requirements because the air that's being stripped is so warm which means that the heat pump doesn't have to work as hard.

 

Having said that, we have a solar PV array with an iBoost that uses electricity that would have gone to the grid to power the immersion heater, so our ASHP hasn't even come on in the past 10 or so days.

Edited July 25, 2019 by Home Farm
added text

[Jeremy Harris]

45 minutes ago, Home Farm said:

Our installers told us that in temperatures above 20C, DHW was negligible in terms of electricity requirements because the air that's being stripped is so warm which means that the heat pump doesn't have to work as hard.

 

Having said that, we have a solar PV array with an iBoost that uses electricity that would have gone to the grid to power the immersion heater, so our ASHP hasn't even come on in the past 10 or so days.

 

Not at all sure about the "negligible" bit, that doesn't really stand up to scrutiny at all well, and smacks of BS.   There is certainly an efficiency difference between running an ASHP in warm weather versus cool weather, but not by a massive amount, and it certainly never gets anywhere near a state where the energy usage becomes negligible.

 

Air at 30°C only contains about 3.4% more heat than air at 20°C, for example, and all heat pumps have a characteristic operating curve that gives the Coefficient of Performance (COP) versus the operating temperature differential (difference between the ambient air input temperature and the water flow temperature for an ASHP) for dry air (humidity has a couple of incidental effects on performance).

 

This is the characteristic curve for our ASHP when operating with dry air, by way of example:

 

There is clearly a reduction in the amount of electrical energy needed to heat a given volume of water as the COP increases, but it isn't as big a difference as to ever result in the amount of energy used being negligible.  If you were heating a full tank of 300 litres of hot water to 55°C with an ASHP with the above efficiency curve and an incoming mains water temperature of 10°C, then the electrical energy usage for different ambient temperatures would be:

 

At 10°C ambient = 5.2 kWh

 

At 15°C ambient = 4.7 kWh

 

At 20°C ambient = 4.2 kWh

 

At 25°C ambient = 3.8 kWh

 

It annoys me that suppliers think they can bamboozle customers like this, and it was exactly this sort of misrepresentation that resulted in there being so many unhappy ASHP owners in the early days of their introduction here.  They were mis-sold on a fairly wide scale, enough for the Energy Saving Trust to recognise the problem when they conducted their first survey into the efficiently of heat pumps in the UK domestic sector.

[jack]

1 minute ago, JSHarris said:

It annoys me that suppliers think they can bamboozle customers like this, and it was exactly this sort of misrepresentation that resulted in there being so many unhappy ASHP owners in the early days of their introduction here.  They were mis-sold on a fairly wide scale, enough for the Energy Saving Trust to recognise the problem when they conducted their first survey into the efficiently of heat pumps in the UK domestic sector.

 

Don't attribute to malice that which is adequately explained by ignorance. They may well believe what they said!

[Jeremy Harris]

1 minute ago, jack said:

 

Don't attribute to malice that which is adequately explained by ignorance. They may well believe what they said!

 

Good point, it is easy to overlook ignorance as a factor.  Sometimes it's easy to forget that heat pumps are relatively new to the UK and the knowledge of the way they work isn't that widely understood yet.

[jack]

2 minutes ago, JSHarris said:

Good point, it is easy to overlook ignorance as a factor.  Sometimes it's easy to forget that heat pumps are relatively new to the UK and the knowledge of the way they work isn't that widely understood yet.

 

I think that with ASHPs, there are some non-intuitive things about the way they operate that makes this sort of error easy to propagate. It seems obvious that they'd struggle waaaay more to heat water when it's cold (to us!) outside compared to when it's warm.

[H F]

Incredibly insightful comments as always JS. I've started my own study to determine the performance of our heat pump to see if there is an increase at all at spring/summer temperatures versus the winter. The pump's only been in operation since March so my data is limited. But your insights are invaluable.

[joe90]

3 minutes ago, jack said:

 

I think that with ASHPs, there are some non-intuitive things about the way they operate that makes this sort of error easy to propagate. It seems obvious that they'd struggle waaaay more to heat water when it's cold (to us!) outside compared to when it's warm.

 

The very reason I asked the question. I knew I would get a “correct” answer on this forum rather than BS from a retailer ?

[Jeremy Harris]

1 minute ago, Home Farm said:

Incredibly insightful comments as always JS. I've started my own study to determine the performance of our heat pump to see if there is an increase at all at spring/summer temperatures versus the winter. The pump's only been in operation since March so my data is limited. But your insights are invaluable.

 

There will be a marked difference between winter and summer, but a fair bit of this will be due to relative humidity, rather than air temperature.  The very worst operating conditions for an ASHP are when it's cool and wet, as the evaporator will tend to ice up and the ASHP will run reverse cycles to defrost it.  For ours this happens most often when it's about 5°C ambient and wet, although I found (by experiment) that reducing the ∆_T stopped it defrosting completely, which resulted in a significant improvement in real-world COP.

[jack]

15 minutes ago, JSHarris said:

There will be a marked difference between winter and summer, but a fair bit of this will be due to relative humidity, rather than air temperature.  The very worst operating conditions for an ASHP are when it's cool and wet, as the evaporator will tend to ice up and the ASHP will run reverse cycles to defrost it.  For ours this happens most often when it's about 5°C ambient and wet, although I found (by experiment) that reducing the ∆_T stopped it defrosting completely, which resulted in a significant improvement in real-world COP.

 

From memory some of the efficiency loss at lower (non-freezing) temps can be made up by increased energy from condensation in damp weather.

[ragg987]

44 minutes ago, JSHarris said:

At 10°C ambient = 5.2 kWh

...

At 25°C ambient = 3.8 kWh 

If we take a summer night temperature to be 10 to 15C (about 5kWh) and summer daytime to be 25C, then this represents a 25% saving. Not insignificant.

 

Depends on your objective. E7 may well be cheaper but you are burning more fuel, and potentially emitting more CO2.

[Jeremy Harris]

16 minutes ago, jack said:

 

From memory some of the efficiency loss at lower (non-freezing) temps can be made up by increased energy from condensation in damp weather.

 

Yes, there will be a bit of additional heat available from the water vapour content, due to enthalpy.  Might do a few sums later to see how big an effect it has.

[Jeremy Harris]

1 minute ago, ragg987 said:

If we take a summer night temperature to be 10 to 15C (about 5kWh) and summer daytime to be 25C, then this represents a 25% saving. Not insignificant.

 

Depends on your objective. E7 may well be cheaper but you are burning more fuel, and potentially emitting more CO2.

 

No, it's not insignificant, but then reducing energy use in hot weather to 75% of that in cold weather doesn't make the energy use negligible in hot weather, either.

[H F]

15 hours ago, JSHarris said:

∆_T

What does that mean JS?

[H F]

In our ASHP the evaporator is placed at a higher level to prevent cold air recirculation, which according to the manufacture results in better performance levels. Don’t know if there’s any truth to this.

 

As I’m looking into this by way of comparison, can I ask what brand and size ASHPs everyone has, with a rough annual COP?

[le-cerveau]

15 hours ago, JSHarris said:

that reducing the ∆_T stopped it defrosting completely,

 

I don't have a defrosting issue and have a ΔT of 5^oC, details in my CoP post:

 

9 minutes ago, Home Farm said:

What does that mean JS?

It is teh difference between flow and return temperature in an ASHP, the system will modify the compressor and/or pump to the differential flow temperature, my system alters pump speed.

[Jeremy Harris]

21 minutes ago, Home Farm said:

In our ASHP the evaporator is placed at a higher level to prevent cold air recirculation, which according to the manufacture results in better performance levels. Don’t know if there’s any truth to this.

 

As I’m looking into this by way of comparison, can I ask what brand and size ASHPs everyone has, with a rough annual COP?

 

 

Doesn't make a significant difference, as the throw from the fan pushes the cold exhaust air a fair way away.  Might make a tiny fraction of a percent difference on a very still day with zero wind, but I doubt if anyone could measure it.

 

11 minutes ago, le-cerveau said:

 

I don't have a defrosting issue and have a ΔT of 5^oC, details in my CoP post:

 

It is teh difference between flow and return temperature in an ASHP, the system will modify the compressor and/or pump to the differential flow temperature, my system alters pump speed.

 

 

No, ∆_T in this context refers to the differential temperature between the ambient air from which heat is being extracted and the flow temperature of the water that is being heated.  This is the main factor that determines how hard the heat pump is working.   

 

For our, pretty typical, (re-badged Carrier) inverter controlled ASHP as  ∆_T gets greater the COP reduces, as shown in the dry air curve earlier.

 

Altering the power output, by adjusting the water flow rate, is an effective way of reducing the load on the heat pump, and so maintaining a higher COP, although there's no "free lunch" by doing this, as the power output is being reduced when it is most needed, so the running time will then increase for a given energy output.  It does reduce the possibility of icing, though, by reducing the heat pump load for the prevailing outdoor conditions.  I would guess that it also takes account of outdoor RH when doing this.  Our's measure RH at the input to the evaporator, and seems to use that as a part of the fan speed control and defrosting system, as well as displaying RH on the indoor display.
 

[H F]

Random question: what would be considered an acceptable COP for ASHPs in the UK?

[PeterW]

5 minutes ago, Home Farm said:

Random question: what would be considered an acceptable COP for ASHPs in the UK?

 

For one not going above 47c (ie not doing high temperature water) then I'd like to see the 2.5-3 range

[ragg987]

5 hours ago, Home Farm said:

Random question: what would be considered an acceptable COP for ASHPs in the UK?

We get 4 on average, mixed DHW (to 50C) and space heating (auto-adapts between 23C and 30C using built-in weather compensation).

[Jeremy Harris]

On 26/07/2019 at 15:30, ragg987 said:

We get 4 on average, mixed DHW (to 50C) and space heating (auto-adapts between 23C and 30C using built-in weather compensation).

 

Out of interest, how are you measuring COP?

 

I spent a fair bit of time measuring the water flow rate through our ASHP (not as easy as I thought it would be) and then used this, together with the differential temperature between flow and return to calculate the output power.  With an energy meter on the electricity supply I was able to obtain the COP at any instant by comparing the input power and output power.

 

It was a bit of a PITA to do, especially as the output power turned out to vary a fair bit at times, and those variations lagged behind the variations in input power.  I'd be really interested if you have come up with an easier way to measure it, especially if it's capable of logging COP over a reasonably long period.

[SteamyTea]

1 minute ago, JSHarris said:

I'd be really interested if you have come up with an easier way to measure it, especially if it's capable of logging COP over a reasonably long period.

I would like to know as I can then cobble something together and monitor @joe90

Do you actually need to know the flow rates, can you just make assumptions based on the input and output temperatures?  Bit like working out the efficiency of a heat exchanger.