The scroll expander model has been previously proposed by Lemort et al. [5] and partly validated by tests with steam. In this model, the evolution of the fluid through the expander is decomposed into the following steps (as shown in Fig. 3):

• Supply pressure drop (su^ su,1,1)

• Cooling-down in the supply port of the expander (su1,1 ^ su,1);

• Isentropic expansion from the supply pressure down to the adapted pressure imposed by the internal expansion volume ratio of the expander (su,1 ^ ad);

• Expansion at a fixed volume from the adapted pressure to the exhaust pressure (ad ^ ex,2);

• Mixing between suction flow and leakage flow (ex,2 ^ ex,1) and

•

Cooling-down or heating-up in the exhaust port (ex,1 ^ ex).

The model requires only nine parameters (heat transfer coefficients, friction torque, leakage area, pressure drop equivalent diameter). Those nine parameters, defined for a specific type of expander and for a specific working fluid, are determined on the basis of experimental data.