Organic rankine cycle (ORC) heat-to-power systems can improve efficiency and reduce greenhouse gas emissions from power generation and from a wide range of industrial processes. Research into the application of transcritical and supercritical cycles, as well as the role of new working fluids and blends, has identified the expander as a critical component in such systems. For many researchers, today’s commercially available small expander systems do provide a good basis for lab development: Turbine expanders do not tolerate condensing mixtures that can be generated during the expansion, especially at part load. Screw and scroll expanders can suffer from high bypass losses, especially at higher pressure ratios. Piston expanders are seen by many as a good solution for small scale waste heat recovery systems.

Working with the University of Brighton, Libertine developed a generic free piston expander module that can be adapted for many different lab based ORC applications. Initially configured as a two cylinder expander rated for 20kWe output power, this prototype was installed in the ORC test facility located within the University of Brighton’s Sir Harry Ricardo labs in 2015. Libertine developed a new passive inlet valve operated by the ‘bounce’ event at the end of each stroke and validated the performance of this valve against an AMESim multi-physics model. Four of these valves were integrated into the two cylinder expander, together with Libertine’s efficient flux switching linear machine and active exhaust valves giving full dynamic control over the expansion event. Single cylinder variants of this expander can be configured for a range of working fluids, inlet conditions and nominal output powers in the range 2-10kWe.

PERFORMANCE