Comparison of moving boundary and finite-volume heat exchangers models in the Modelica language

When modelling low capacity energy systems such as a small (5–150 kWel) organic Rankine cycle unit, the governing dynamics are mainly concentrated in the heat exchangers. As a consequence, accuracy and simulation speed of the higher level system model mainly depend on the heat exchanger model formulation. In particular, the modelling of thermodynamic systems characterized by evaporation or condensation, requires heat exchanger models capable of handling phase transitions. To this aim, the finite volume (FV) and the moving boundary (MB) approaches are the most widely used. The two models are developed and included in the open-source ThermoCycle Modelica library. In this contribution a comparison between the two approaches is performed. Their performance is tested in terms of model integrity and accuracy during transient conditions. Furthermore the models are used to simulate the evaporator of an ORC system and their responses are validated against experimental data collected on an 11 kWel ORC power unit.

[1]  Vincent Lemort,et al.  Experimental study on an open-drive scroll expander integrated into an ORC (Organic Rankine Cycle) system with R245fa as working fluid , 2013 .

[2]  L. Petzold.  A description of dassl: a differential/algebraic system solver , 1982 .

[3]  Vincent Lemort,et al.  Methods to Increase the Robustness of Finite-Volume Flow Models in Thermodynamic Systems , 2014 .

[4]  Vincent Lemort,et al.  Experimental comparison of organic fluids for low temperature ORC (organic Rankine cycle) systems for waste heat recovery applications , 2016 .

[5]  Vincent Lemort,et al.  Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp , 2014, Industrial & engineering chemistry research.

[6]  James E. Braun,et al.  A comparison of moving-boundary and finite-volume formulations for transients in centrifugal chillers , 2008 .

[7]  Johannes F. Broenink,et al.  Modelica: An International Effort to Design the Next Generation Modelling Language , 1997 .

[8]  Baligh El Hefni,et al.  Dynamic Modeling of Concentrated Solar Power Plants with the ThermoSysPro Library (Parabolic Trough Collectors, Fresnel Reflector and Solar-Hybrid) , 2014 .

[9]  Sebastián Dormido,et al.  Switching moving boundary models for two-phase flow evaporators and condensers , 2015, Commun. Nonlinear Sci. Numer. Simul..

[10]  Sylvain Quoilin,et al.  Sustainable energy conversion through the use of Organic Rankine Cycles for waste heat recovery and solar applications , 2011 .

[11]  E. W. Grald,et al.  Prediction of cyclic heat pump performance with a fully distributed model and a comparison with experimental data , 1987 .

[12]  V. Mehrmann,et al.  Hybrid systems of differential-algebraic equations – Analysis and numerical solution , 2009 .

[13]  Jonas Eborn,et al.  Development of a Modelica Base Library for Modeling of Thermo-Hydraulic Systems , 2000 .

[14]  Vincent Lemort,et al.  Experimental campaign and modeling of a low-capacity waste heat recovery system based on a single screw expander , 2014 .

[15]  Michael Wetter,et al.  Modelica Library for Building Heating, Ventilation and Air-Conditioning Systems , 2009 .

[16]  F. Casella,et al.  Object-Oriented Modelling & Simulation of Power Plants with Modelica , 2005, Proceedings of the 44th IEEE Conference on Decision and Control.

[17]  Eric W. Grald,et al.  A moving-boundary formulation for modeling time-dependent two-phase flows , 1992 .

[18]  Francesco Casella,et al.  Modelica open library for power plant simulation: design and experimental validation , 2003 .

[19]  Piero Colonna,et al.  Dynamic modeling of steam power cycles. Part I-Modeling paradigm and validation , 2007 .

[20]  Vincent Lemort,et al.  ThermoCycle: A Modelica library for the simulation of thermodynamic systems , 2014 .