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How to model a R744 (CO2) transcritical HP for DHW in EnergyPlus?

asked 2023-11-29 06:09:41 -0600

joelndl's avatar

Nowadays, most of the main Heat Pump manufacturers for domestic and commercial/industrial sector offer R744 (CO2) heat pumps for the production of DHW, specially for large consumptions where high temperatures are compulsory to prevent the legionella infections. These HP work better when the DHW storage is stratified in multiple tanks but they have to be sized accordingly. Do you know if it is possible to model the HP somehow in EnergyPlus? There are multiple papers available but I haven't found out how to implement it on EnergyPlus... Thank you in advance!

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answered 2023-12-10 21:36:23 -0600

Keigo's avatar

You can use WaterHeater:HeatPump:PumpedCondenser or WaterHeater:HeatPump:WrappedCondenser to model heat pump water heaters. There is no input field for the refrigerant type. You can customise performance curves of the DX coil in the water heater.

Please refer to ExampleFiles WaterHeaterHeatPumpStratifiedTank.idf, WaterHeaterHeatPumpWrappedCondenser.idf, HeatPumpWaterHeater.idf and HeatPumpWaterHeaterStratified.idf.

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Since CO2 needs 2 pairs of curves (1 for subcritical and 1 for transcritical operation), he probably has to do 2 simulations and combine the results according to the outside temperature

Petros Dalavouras's avatar Petros Dalavouras  ( 2023-12-13 04:08:52 -0600 )edit
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I don't think so. The curves are Heating Capacity Function of Temperature, Heating Capacity Function of Air Flow Fraction, Heating Capacity Function of Water Flow Fraction, Heating COP Function of Temperature, Heating COP Function of Air Flow Fraction, Heating COP Function of Water Flow Fraction and Part Load Fraction Correlation Curves. CO2 is transcritical at the compressor outlet even if the outside temp is low. Even if CO2 becomes subcritical at the compressor outlet with low outside temp, I think such changes as a function of temp can be approximated by one regression curve.

Keigo's avatar Keigo  ( 2023-12-13 08:15:26 -0600 )edit
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You are correct, it's for DHW so the discharge will always be above the critical point (31.1oC) thus the operation will be transcritical. I got carried away from refrigeration

Petros Dalavouras's avatar Petros Dalavouras  ( 2023-12-13 14:11:17 -0600 )edit

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Asked: 2023-11-29 06:09:41 -0600

Seen: 97 times

Last updated: Dec 10 '23