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Variable speed heat pump

  1. Aug 6, 2015 #1

    The problem is in connection with variable speed heat pumps used for heating purposes, where the source/reservoir is ground/rock. The ground has temperatures around 5-7 celsius.

    Is it correct that these systems only can deliver one temperature at one load? E.g.:
    • at 30 % of full power, it delivers 35 celsius
    • at 50 % of full power, it delivers 42 celsius
    • at 80 % of full power, it delivers 50 celsius
    Or can these systems be set up such that the deliver e.g.:
    • 50 celsius at 50 % of full power
    • 40 celsius at 90 % of full power
    • etc.?
    The reason for my question is this: In projects I have encountered, the needed temperature for building heating system is 40 celsius at one particular day. The way the variable speed heat pump delivers this heat is by e.g. running at 60 %, producing heated water et 50 celsius, which is circulated and mixed with the return of the heating system - e.g. holding 30 celsius - before achieving at 40 celsius outgoing temperature. This is illustrated in the image:

    Attached Files:

  2. jcsd
  3. Aug 6, 2015 #2


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    Staff: Mentor

    Do you have a make and model for the heat pump?

    What you are saying is a bit surprising varying the speed typically does not change the compression ratio, so it shouldn't change the thermodynamics at all (only the mass flow). There may be a limitation specific to that heat pump, but other heat pumps (or chillers) can have a set-point dialed-in and hold it under varying loads. In fact, it is more common with a constant speed/capacity heating or cooling unit to have a reservoir because of their lack of ability to vary their output.
  4. Aug 6, 2015 #3
    It is a Bitzer semi-hermetic 18.4 kW, controlled by a frequency controller.

    I thought that one could vary the pressure on the condenser side, thus achieving a different temperature. By varying the speed of the compressor one would be able to decrease or increase the flow of the refrigerant. But is this incorrect?

    BTW! In the illustration in my first post it should say L/hour, not L/s
    Last edited: Aug 6, 2015
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