COMPARISON OF MOVING BOUNDARY AND FINITE-VOLUME HEAT EXCHANGERS MODELS IN THE MODELICA LANGUAGE

asme-orc2015 Tracking Number 27

Dynamic modeling has assumed an important role in energy system design, in particular when control issues are considered. In the last years, new methodologies for dynamic modeling have been developed and implemented in various dynamic modeling languages, such as the open-access Modelica language. When considering low capacity energy systems such as an ORC system, the governing dynamics is mainly concentrated in the heat exchangers. As a consequence, accuracy and simulation speed of a "high-level" model mainly depend on the heat exchanger models. In particular, heat exchangers models capable of handling phase changes are required for thermodynamic cycles presenting evaporation or condensation. To that aim, the two usual approaches are the finite-volume (FV) and moving boundary (MB) methods. This paper presents a MB model developed in the framework of the open-source ThermoCycle Modelica library. A comparison of this model with the more traditional FV approach is performed in terms of simulation speed, robustness and accuracy. The stability and integrity of the models are demonstrated as single components and within a complete ORC system model. The moving boundary model has been formulated in a way that allows switching between different configurations, i.e. general, flooded and dry evaporators and condensers, and it results to be very effective in terms of simulation speed and accuracy compared with the finite volume model.