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Heat Pump - basic physics/energy transfer query

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  • 02-04-2006 2:12pm
    #1
    iplogger1
    Registered Users Posts: 277 ✭✭


    In some ways for my info and in other ways to settle a little argument ;)

    A relative of mine and I got bogged down in a technical argument over the electricity power requirements of a GSHP/geothermal heat pump (assuming the usual horizontal ground collector installation).

    He couldn't see why a GSHP pump would require any appreciably more electricity units per hr to run compared to your standard domestic refrigerator given that the principles of physics used to run each were the same. I tried to argue that the energy requirements (output) for the GSHP versus the small contained fridge were not comparable and that you require a lot more electricity power input for running the GSHP. He actually assumed that in the case of a power outage that a small wind turbine (eg, like those microturbines being offered by surfacepower.com which offer at best 1kw/H equiv peak) would be sufficient to run a good deal of your domestic electricity *including* the heat pump in the event of a power outage for 6-24hours !! I wasn't winning the argument because I couldn't convince him of the additional work needed to be provided by the heat pump in order to heat a large house versus the (inverted) heat exchange ratios needed to run a domestic fridge.

    Can someone with some basic knowledge of the heat pump explain from first principles why the geothermal heat pumps eat so much more electricity juice compared to your fridge ! I assume the majority of the energy input requirements of the heat pump are required for the heat exchange process rather than the pumping of the water to heat the UFH system or the circulation of the water/glycol or saline,etc in the collector loops.

    Can someone identify the physical processes and why the extra cost compared to fridge !

    thanks!

    ~ipl


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    #2
    pencil
    Closed Accounts Posts: 606 ✭✭✭pencil


    iplogger1 wrote:

    Can someone identify the physical processes and why the extra cost compared to fridge !

    thanks!

    ~ipl

    Maybe I'm completely missing the point but would it not be a difference of scale?

    The fridges pump would only be moving a small amount of fluid around a short distance (say one litre around 2 meters). Where as the GSHP would be moving gallons of fluid over a large distance (Kms). A lot more energy required!

    Also the heat exchange would be a passive process.

    Not an expert on this by any means!


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    #3
    patrido
    Closed Accounts Posts: 299 ✭✭patrido


    glad to see i'm not the only one with friends like that :) Where do they get their logic?! :D

    it's absolutely a matter of scale. a typical fridge takes some heat out of a few kgs of food and "pumps" it into the room. the motor on a fridge is usually between 75 and 200 Watts. it will use between 0.5 and 2 kwh per 24 hours.

    a GSHP is pumping heat on a vastly bigger scale - the collector area is huge, the motor is likely to be 1KW for the smallest model, and I have seen models up to 9kW requiring 3 phase leccy. The power usage per 24 hours could run into the hundreds.

    Tell your friend that I wish my home was that easy to heat :)


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