Realistic temperature-dependent COP modeling equation for ccASHP (space heating)

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With variable-speed heat pumps, the EnergyPlus COP calculations are more complex than a COP vs. outdoor temperature equation. The COP is also a function of compressor speed (and thus the heating load of the space and how the heat pump was sized), outdoor humidity, and indoor temperature as well. There are also non-linear effects like defrost and backup heat. All these effects can cause real world COP to vary +/- 30%. See Fig 32 in https://www.nrel.gov/docs/fy16osti/65... for an example for ductless heat pumps using field data from the northwest US.

You can use that same figure to try to derive a COP vs. temperature equation, but keep in mind you will not be capturing all the aforementioned effects. You could at least try to represent the range of real-world COP values seen in that figure.

Good luck!