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 fluid-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 of TLCs.