Modelica-based Heat Pump Model for Transient and Steady-State Simulation Using Low-GWP Refrigerants

Due to the relatively high global warming potential (GWP) value of R410A, much effort has been devoted to the exploration of potential refrigerants to replace R410A in the heat pump applications. Those studies involving natural refrigerants, which are of zero or single-digit GWP values, have not yet demonstrated the readiness for the substitution because of various reasons such as low cycle efficiency, toxicity and flammability. Henceforward, some synthetic refrigerants whose GWP values are significantly lower than R410A such as R32 and some R32-based blends such as D2Y60, are getting more attention. To evaluate the transient performance of those low-GWP refrigerants, a Modelica-based heat pump model is developed to simulate a heat pump cycle during both steady state operations and transient operations. The model includes an efficiency-based compressor model, two segmented heat exchanger models, a control volume-based valve model and segmented pipe models. The heat exchanger model is capable of simulating multi-bank multi-circuit tube-fin heat exchangers with an arbitrary number of segments. The pipe model is developed based on the segmented heat exchanger model, and its implementation provides for better charge prediction compared to single lump model. In order to speed up the simulations, accelerated refrigerant property routines were developed for R32 based on REFPROP. System-level steady-state and transient simulations for several alternative low-GWP working fluids, R32 and D2Y60, are conducted. The simulation results are compared with the published experimental data obtained from the Alternative Refrigerant Evaluation Program (AREP). The data includes steady-state operation data based on ASHRAE A, B and C tests, and transient data from ASHRAE D cyclic operation test. The validation of R32 and D2Y60 steady-state data shows a maximum deviation of 7.5% and an average deviation of 4%. The transient simulation well captures the dynamic performance of vapor compression cycle during start-up and shut-down.