Gas expander development

Libertine’s Linear Power Systems have been used to create a flexible piston expander product for lab-based Organic Rankine Cycles and other thermodynamic systems R&D.

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

Application

Lab-based ORC expander

Working fluid

Ethanol

Mover diameter

48 mm

Maximum stroke

400 mm

Inlet pressure

Up to 30 bar

Inlet temperature

Up to 250 oC

Expansion ratio

20:1

Specific force

>0.2 kN/kg

“The expander is one of the most important components in the system. We needed an expander that has a high expansion ratio so that it can operate with a higher inlet pressure, and also is tolerant to condensing fluid. The free piston expander has the right characteristics to do that. We don’t design and make mechanical devices, that’s a distraction for us away from the areas we’re most intererested in. The collaboration with Libertine has opened up a new type of hardware device we can use to further our research.”

Dr Rob Morgan
Reader, Advanced Engineering Centre, University of Brighton