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

asked 2021-10-21 19:32:39 -0500

rm2222's avatar

updated 2022-02-22 13:23:10 -0500

I looked at ResStock's documentation and source code and it is difficult for me to understand how they have modeled the COP equations in their code. Does anyone have any explanation/understanding of it or any better source to get an equation to model the temperature-dependent coefficient of performance for cold climate air source heat pumps?

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answered 2021-10-25 16:18:35 -0500

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 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!

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Thank you for the help, Eric! I had a follow-up question regarding the curve shown in Figure 32-- is there a physical reason behind why the COP goes down after 50 F (10 C)? It is not commonly seen that the COP drops at higher temperatures, right? There are only a few data points that indicate a downward trend in the figure as well, so what is the reason behind generalizing a downward trend?

rm2222's avatar rm2222  ( 2021-11-15 15:48:29 -0500 )edit

The downward trend is observed in field data (e.g., Fig 32) and that is consistent with manufacturer provided data as well:

Eric Wilson's avatar Eric Wilson  ( 2021-11-15 17:47:58 -0500 )edit

Thank you, Eric!

rm2222's avatar rm2222  ( 2021-11-15 20:48:26 -0500 )edit

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Asked: 2021-10-21 19:27:04 -0500

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Last updated: Oct 25 '21