Modelica, Ecosimpro, Dynamic Simulations, Heat pump, Object oriented modeling
Object-oriented physical-modelling platforms greatly facilitate the task of the modelling engineer by abstracting away a lot of the complexity associated with sorting the governing equations and also the nuances of the numerical methods used for solving the differential-algebraic equations (DAEs). For this reason, they have been steadily gaining in popularity in the field of thermofluid simulations. In this study, we compare two platforms of this type: Dymola and EcosimPro. Dymola is a physical modelling environment originally developed at Lund University and now being developed by Dassault Systèmes, and is a commercially available implementation of the open-source physical modelling language Modelica. EcosimPro is a proprietary tool developed by Empresarios Agrupados A.I.E originally for the European Space Agency and now sold to the general public. Both platforms utilise object-oriented modelling paradigms such as multiple inheritance, encapsulation (of behaviour within classes), abstraction (hiding model complexity from the user) and acausal equation handling (equations may be written in any order). We use these platforms to conduct a realistic exercise of modelling and simulating a relatively complex residential heat pump system in both heating and cooling modes and comparing the results against measured data. Component libraries have been prepared in both the platforms for modelling system components. Two-phase flow has been accounted for using slip-ratio based void fraction correlations. In general, the component models have been kept as similar as possible between the two platforms. The heat pump under investigation is a residential, 3-ton unit with a scroll compressor. The cooling mode uses a thermostatic expansion valve (TXV) as the expansion device while the heating mode uses a short-tube orifice. A reversing valve controls the flow direction. The heat pump has been tested under both heating and cooling modes as per ASHRAE’s Standard 116-2010 cyclic test conditions. The measured values have been compared against simulations results from both platforms. The refrigerant pressures and temperatures and the heat exchanger air outlet temperatures are compared. The indoor unit air-side capacity and the compressor power consumption integrated over the on-period are also compared. Additionally, the Seasonal Energy Efficiency Ratio (SEER), the Cooling Load Factor (CLF) and the Cyclic Degradation Coefficient (Cd) are compared which help quantify the performance of the heat pump. Finally, qualitative comparisons of the transients associated with the refrigerant charge migration after shutdown have been made, as this migration is responsible for cycling losses associated with dynamic heat pump operation. The two platforms prove to be similarly capable at simulating an advanced cycle. Both platforms can predict the pressure and temperature transients during the on-off cycling of the heat pump, as well as the performance parameters such as accumulated capacities and the SEER rating. Finally, both models predict the simulated charge to be within 80% of the actual charge, which enables a more realistic depiction of system transients.