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# Determine COP performance curves in DesignBuilder for an ASHP

I’m working on an nZEB house with an Air Sourced Heat Pump and a heated floor. I’m using DesignBuilder for the energy modelling and I’m struggling with the COP performance curves. I’ve determined the COP performance curves based on the manufacturers technical sheet and in function of the ASHP output water temperature and exterior temperature.

Excerpt from my excel file: link text

The problem is when I introduce the data in to DesignBuilder I obtained 4 efficiency curves with a high COP at -15°C and with a low COP at 20°C. Excerpt from DesignBuilder link text

I’ve also used the EnergyPlus CurveFitTool to determine the EIR function of temperature for a Heat Pump. From what I understand the X axis in DesignBuilder is the Y axis in EnegyPlus. Is this just a representation problem in DesignBuilder graphic or I’m missing something?!

Exerpt from the CurveFitTool:

! 0.9923 R Squared

HPHeatingEIRFTemp !- Name

0.0415922426 !- Coefficient1 Constant

0.0015509775 !- Coefficient2 x

0.0000097144 !- Coefficient3 x**2

-0.0008778336 !- Coefficient4 y

0.0000187025 !- Coefficient5 y**2

-0.0000288599 !- Coefficient6 x*y

35.00 !- Minimum Value of x

55 !- Maximum Value of x

-10 !- Minimum Value of y

40 !- Maximum Value of y

0.0632 !- Minimum Curve Output

0.1828 !- Maximum Curve Output

Temperature !- Input Unit Type for X

Temperature !- Input Unit Type for Y

Dimensionless !- Output Unit Type

By dividing the annual electricity consumption of an electrical boiler with a COP of 1 to the ASHP with the calculated COPFT equation I’ve obtained an annual COP of little over 3. This seems OK to me for and air sourced heat pump, considering that I heat the water (heating + domestic hot water) to 55°C and the exterior temperature reaches -15°C during winter.

Has anyone encountered this problem before?

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After a closer look in the EnergyPlus IDF file I’ve found the following:

For an ASHP the x axis = exterior air temperature and y axis = condenser inlet water temperature.

For a Chiller is the other way around: x axis = chilled water temperature and y axis = exterior air temperature.

My mistake was that I used an excel created for chiller performance curves. After I switched the axes between them I’ve obtained the correct performance curves.

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Hello. I have entered the coefficients also. I did not face any problem. But all my temperatures were positive. I guess may be due to negative low temperature you are facing this problem.

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