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Air Source Heat Pump limit of Coefficient Of Performance being less than one?

CliveS

COP is a measure of efficiency a ratio of  Output energy/Input energy.  Thus if 2 kW electric input can give 4 kW of energy output then the COP=2

Google this:-   Can coefficient of performance be less than 1?  Google answer is

"Thus, we can say that the COP of the refrigerator and air-conditioner can be less than one or greater than one. The above formula also shows that the COP of the heat pump can never be less than one; it is always more than one."

I am not sure this final statement is correct because if you are driving the fridge compressor outside the heated room then all the friction power is lost into the atmosphere and not into the refrigerant.  So  if the heat source is more than say 10 degree.C less than the room temperature inside you could be loosing energy rather than saving it?

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  • in reply to CliveS
    CliveS said:
    To get high COP needs the heat source to be warmer than the output temperature like geothermal which is ideal.

    That does not sound right to me. The heat source needs to be hotter than the evaporator. Don't forget that a standard domestic freezer uses a heat pump. The advantage of even a cool stream is that it contains a lot more heat than the atmosphere.

  • in reply to CliveS

    If the heat source is warmer than the output, then you don't need a heat pump at all.  If my house sat on top of a thermal spring, I could just pump the hot water round the radiators.  That's a few watts to run a small pump.

    A well set-up heat pump system should achieve a COP of 3 to 4 under ideal operating conditions.  That's the magic of heat pumps.  By "ideal", I mean warm (but not hot) radiators or underfloor heating, and moderately chilly weather outside.

    It only really goes wrong when the outside temperature goes below freezing all day, and the outside unit ices up.  Then you need to apply heat to defrost it before the heat pump can start again.  Even then, they should still deliver a COP better than 1.

  • in reply to Simon Barker

    But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

    On another point, I drove through a town today that was severely disrupted by the installation of mains gas pipes . . .

  • in reply to Grumpy
    Grumpy said:
    But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

    True.  Until someone works out how to get the price of electricity back down to something sensible, nobody (and that includes me) have ant real incentive to scrap our old gas boilers.

    I really want to be greener, but not if it comes at a massive cost.

    This givernment is great on targets, and mostly clueless on how to achieve them.

  • in reply to Simon Barker
    Simon Barker said:
    Grumpy said:
    But a COP>1 isn't good enough if you are using an energy source, electricity, that is at least 3 times more expensive than readily available oil or gas.

    True.  Until someone works out how to get the price of electricity back down to something sensible, nobody (and that includes me) have ant real incentive to scrap our old gas boilers.

    Leccy is about 3 times the price of gas (or coal) because the conversion process is only about 30% efficient. So nothing gained unless you can get the COP above 3.

  • in reply to Chris Pearson

    and even if it wasn't, because in the UK we are generating electricity by burning gas in a process that is a lot less efficiant than your boiler, it is the wrong thing to do to switch off the boiler, as we burn more gas using electric heating than we do burning the gas where the heat is actually needed.

    Once more than 2/3 of our electricity is generated by another means we can come back and re-consider what is best for the planet.

    Mike.

  • in reply to mapj1

    Yes, I agree that electricity is the most expensive option for heating water even with an ASHP; unless COP > 2.

    Electric hot water in a house is certainly not the greenest option either.  Wood burners, oil fired boilers and even gas are more efficient and cheaper options and depending on power station chimney scrubbers will probably emit less CO2 overall anyway.

  • Has anyone out there a swimming pool ASHP as I would like them to do a little experiment for me as follows:-

    1. Take the temperature of the input pool water Tin

    2.Take the temperature of the water coming out of the ASHP Tout

    3. Take the temperature of the air entering the ASHP Tair

    4. Monitor the input voltage and current to the ASHP and average power used for a one hour period.=kWh input

    5. Measure the flow rate in litres per minute and multiply by 60 to get an hour rate and volume in litres/hr.

    6. Calculate the exact amount of energy produced per hour  by multiplying litres/hr times temperature difference Tout - Tin times 4.18 cal/Watt

    7 Calculate COP kWh output/kWh input.

    8 Repeat process with different air temperatureTair.  Say on warm day 20deg.C, average day15 deg.C, cool day10 deg.C and cold day 5 deg.C

    ( Graph the result COP against Air temperature Tair and let everyone see the results please.

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