Modelling, simulation, and optimisation of a novel liquid piston system for energy recovery

A simple, scalable, and efficient method is proposed to extract energy from a pressurised gas flow to generate electrical power. The proposed liquid piston system is fed continuously with air into the piston chamber, which is filled with water. The water is driven by the gas's expansion, and their interaction enables an isothermal expansion. A dynamic model was developed in gPROMS and validated against 300 W and 4 kW experimental data. The accuracy of the power output predictions has a maximum discrepancy of less than 6.4% from the experimental values. Subsequently, a sensitivity analysis and optimisation using the height to diameter piston geometry ratio was performed to study the variation in the thermal efficiency. The simulations were performed for energy systems from 300 kW to 100 MW and for several intake gas temperatures. The study shows that lower piston geometry ratios of 4 are preferred for 300 kW and 1 MW case studies. Because of reduced heat transfer between water and air, these systems reach 92.5% thermal efficiency. Moreover, the predicted thermal efficiencies for high power ratings were as high as 82%. The power output values obtained were constant in time. Finally, an energy and exergy assessment and worst-case scenario with decreased turbo machinery efficiency are presented. The proposed method matches the specific power of traditional power cycles and has potential uses for energy recovery in cryogenic plants.