In the last decade, the organic Rankine cycle (ORC) has become a mature technology to convert

low temperature heat to electricity. However these ORCs are all of the subcritical type. Yet, studies

in scientific literature and our own research clearly shows the potential for increased performance

with alternative cycle architectures. The trilateral cycle (TLC) is one of the most promising

modifications and can boost the power output up to 35%. The main challenge however is the

development of high efficiency expanders which can cope with liquid-vapour mixtures (i.e twophase

mixtures) at the inlet and during expansion. Volumetric expanders are considered suitable,

yet few results are presented up to now. Experimental results are scarce and there is no model

that can predict the performance of the expansion process. This is mainly due to the knowledge

gap on the fundamental aspects of two-phase expansion and the non-equilibrium effects. In this

research proposal, the expansion process in a variable volume chamber will be experimentally

measured with the help of state of the art capacitive void fraction sensors. A model of the twophase

expansion is proposed, specifically focussing on the interfacial effects between the liquidvapour

phase. Different working fluids and velocity profiles of the expander are considered. The

new model will be implemented in full cycle simulations to better assess the performance and

optimization potential.