Semi-empirical model of a multi-diaphragm pump in an organic Rankine cycle test rig

ORC (Organic Rankine Cycle) power systems represent an effective option to exploit low grade heat fluxes. Several papers have recently been published on ORCs systems, most of them dealing with the maximization of overall system performance and selection of the working fluid. Recently, the attention has moved also to system components with particular focus on the expander. Less attention was paid to the pump which, however, may contribute significantly to the system consumption, especially in small size systems where pump efficiency is considerably low. This paper presents a semi-empirical thermodynamic model of a displacement pump integrated into an ORC experimental unit. The pump behaviour at design and off-design conditions is modelled as a set of thermodynamic processes the main geometrical parameters of which are calibrated using experimental data. Calibration is performed by setting an optimization procedure which minimizes the error between operation parameters calculated by the model and measured in the test rig. Maximum errors were in the order of 7% for the mass flow rate, 5.5% for the shaft power and 0.06% for the outlet temperature of the working fluid. The model was used to calculate the characteristic curves of the pump and identify conditions of incipient cavitation. Good agreement with experimental data was found in both cases.