Comparison of Moving Boundary and Finite-Volume Heat Exchanger Models in the Modelica Language

When modeling low capacity energy systems, such as a small size (5–150kWel) organic
Rankine cycle unit, the governing dynamics are mainly concentrated in the heat exchangers. As a
consequence, the accuracy and simulation speed of the higher level system model mainly depend
on the heat exchanger model formulation. In particular, the modeling of thermo-flow 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 presented.
An integrity and accuracy test is designed to evaluate the performance of the FV and MB models
during transient conditions. In order to analyze how the two modeling approaches perform when
integrated at a system level, two organic Rankine cycle (ORC) system models are built using the
FV and the MB evaporator model, and their responses are compared against experimental data
collected on an 11kWel ORC power unit. Additionally, the effect of the void fraction value in the
MB evaporator model and of the number of control volumes (CVs) in the FV one is investigated. The
results allow drawing general guidelines for the development of heat exchanger dynamic models
involving two-phase flows.