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
