Design Builder ASHP Model - COP Remains Constant

asked 2021-08-18 06:08:05 -0500

tom_g's avatar

updated 2021-08-18 10:34:47 -0500

Hello, I am a beginner on Design Builder and modelling an ASHP for my university project and after a bit of help. I have essentially used the DB template for an ASHP but have entered my own COPRFT and CAPRFT performance curves to model the DX coil. For my project I would like to show the variations in the ASHP COP throughout the year, however the COP remains pretty constant all the way though. Based on the performance curves I have entered there should be a clear degradation in COP when the outside temperatures drop in the winter months. Any ideas?

EDIT: See screenshots below (I am unsure how to plot a COP graph with design builder so I divided the coil heating rate by the coil electricity rate), actually the COP varies very slightly by about 0.1 and drops in the Summer.


1: 1.0835128015 2: -0.0102478525 3: 0.0000044789 4: 0.0384748935 5: -0.0003111287 6: -0.0004052715

X range: 20 to 45 Y range: -10 to 20


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@tom_g can you add your curve coefficients to your post, either in text form or as a screenshot from DesignBuilder? That will help others know how to fix this. Also, can you add a plot of COP as an output to validate that it is constant?

Aaron Boranian's avatar Aaron Boranian  ( 2021-08-18 08:41:13 -0500 )edit

@tom_g from the DB screenshots, the name of the coil is "Aux HW Loop Water Heater Heat Pump Water Heating Coil". When you say "ASHP" you mean rejecting heat to air on the condenser side paired with water heating on the evaporator side? Some times "ASHP" is assumed to be heating/cooling air on the evaporator side.

If so, are the performance curves you're trying to apply for a Sanden heat pump within a heat pump water heater (HPWH) system?

Aaron Boranian's avatar Aaron Boranian  ( 2021-08-18 10:26:57 -0500 )edit

@Aaron Boranian it is an air source heat pump used for water heating purposes, the evaporator absorbs heat from the outdoor air and the condenser rejects heat to a water loop. The water loop indirectly heats up a hot water tank which in turn supplies the space heating system. I have added a screenshot of the system schematic if this helps.

tom_g's avatar tom_g  ( 2021-08-18 10:38:43 -0500 )edit

In conclusion, is the best way to determine the COP is dividing the coil heating rate by the coil electricity rate? I would appreciate the clarification :)

sajithwjay's avatar sajithwjay  ( 2021-08-30 16:53:36 -0500 )edit

@tom_g How were you able to connect the water outlet group to the AUX HW Loop demand side, I have tried to model something similar however I noticed that the water outlet is only able to be connected to a DHW loop. When I try to connect components between the heat pump demand splitter/mixer, the water outlet connection are greyed out indicating that it is not something that you can hook up to. Was wondering if you added anything else or editted some blocks?

MaxG2000's avatar MaxG2000  ( 2024-03-07 06:43:38 -0500 )edit