Dummies guide to COP?

I only really have a GCSE understanding of physics, along with the odd thing I've learned here and there

Does anyone have a dummies guide of how COP works and how certain materials are able to achieve above 100% efficiency?

And why a GSHP can achieve a higher COP than an ASHP, as well as any limitations ie: is the COP of 3.5 on average for ASHP set in stone? Or could they get even better? If so, why so and if not, why not?

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  • Exiled_Tyke
    Exiled_Tyke Posts: 1,182
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    Wikipedia has a decent explanation of this.  

    https://en.wikipedia.org/wiki/Coefficient_of_performance

    If my understanding is correct (and it seems to fit with what Wiki says) the issue is the temperature of the medium which is being drawn from.   So and ASHP performs less well at lower outside temperatures - bringing the COP down.   Therefore GSHPs achieve a higher COP simply because the temperature deep underground is higher. 

    There are others here who know far more than me so I'm happy to be corrected if I've got this wrong! 
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  • matelodave
    matelodave Posts: 8,569
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  • QrizB
    QrizB Posts: 13,630
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    COP is an acronym for Coefficient of Performance. It's the ratio between the useful energy you get out of a system vs. the energy you put into it.
    Alternatively, if you multiply the COP by 100 you get the efficiency of the system, in %.
    A direct electric heater gives out exactly the same amount of heat as the electrical energy you put in. It has a COP of 1 and an efficiency of 100%.
    A gas boiler will lose some energy in the flue gases. You might get 0.9kWh of useful heat from each 1kWh of gas you burn. This would be a COP of 0.9 and an efficiency of 90%.
    A heat pump doesn't directly generate heat. What it does is move heat from one place to another. For an ASHP, it is chilling the air and then using that energy to heat water; for a GSHP it chills brine, which then is pumped through underground pipes and is warmed up again by heat stored in of the soil. Typically you get more heat transferred into the water than the electrical energy you put into the system and so the COP is larger than one (and the efficiency greater than 100%).
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  • matelodave
    matelodave Posts: 8,569
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  • Martyn1981
    Martyn1981 Posts: 14,668
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    edited 9 April 2022 at 6:08PM
    GS v's AS, is simply down to the temperature difference of the medium you are stealing (borrowing) the heat from. The  average 'ground' temperature is equal to the average air temperature of the locality. But if you do vertical GSHP it's possible to get deep enough to start to get a bit of geothermal heat, but to keep it simple, typically all the heat from GSHP or ASHP is solar.

    Now, the average air temp will vary a lot across the year, from hot in the summer to very cold in the winter, whereas the ground temp will vary less as it takes a long time to heat up or cool down. So in the winter the ground temp will (on average) be higher than the air temp, and hey presto, you get a better COP as the medium you are borrowing the heat from is warmer.

    I assume that ASHP is more efficient in the summer, but since demand for DHW is much smaller than space heating + DHW (in the winter), GSHP still wins out, and will still of course have a good COP in the summer, just not as good as ASHP.

    Addendum - Also need to consider the ground/soil. If it's extremely stable then you could cool it down with the GSHP and lose efficiency over the winter (whereas air circulates). If the soil is damp/wet, then the 'cold' can be transmitted away better. I think this is taken into account when the amount of pipework/coils/area is calculated.
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  • Reed_Richards
    Reed_Richards Posts: 3,979
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    Also note the difference between the instantaneous Coefficient of Performance and the Seasonal Coefficient of Performance.   The latter is what has been calculated to be the figure you should achieve over a full year.  Particularly in the case of an ASHP, the instantaneous COP is highly dependent on the outside air temperature.    
    Reed
  • waqasahmed
    waqasahmed Posts: 1,926
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    Also note the difference between the instantaneous Coefficient of Performance and the Seasonal Coefficient of Performance.   The latter is what has been calculated to be the figure you should achieve over a full year.  Particularly in the case of an ASHP, the instantaneous COP is highly dependent on the outside air temperature.    
    Including being less effective in winter :(
  • shinytop
    shinytop Posts: 2,088
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    edited 10 April 2022 at 7:41AM
    Also note the difference between the instantaneous Coefficient of Performance and the Seasonal Coefficient of Performance.   The latter is what has been calculated to be the figure you should achieve over a full year.  Particularly in the case of an ASHP, the instantaneous COP is highly dependent on the outside air temperature.    
    Including being less effective in winter :(
    It is less effective in winter just when you need it most but it's not too bad.  My ASHP's COP at 5am when it started up was just over 3 at 1 deg C or so. It's not a lot colder than that very often in the UK.  As @Reed_Richards says it's the full year number that's important.

    I'd have liked a GSHP but they are a lot more expensive and disruptive to install. 
  • Reed_Richards
    Reed_Richards Posts: 3,979
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    shinytop said:

    I'd have liked a GSHP but they are a lot more expensive and disruptive to install. 
    Although GSHPs achieve better SCoPs that ASHPs, there must be times when the air temperature is higher than the ground temperature and the ASHP is the more efficient of the two.  What I would like is a system that combines both and selects whichever is the more efficient to run.  AFAIK that's something you cannot buy!
    Reed
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