Modeling and Cycle Optimization of a Near-Isothermal Liquid Piston Compressor/Expander

Abstract

A novel liquid piston compressor/expander for Compressed Air Energy Storage is proposed in this thesis. The compressor/expander has been developed to be coupled with intermittent renewable energy sources such as wind energy. Energy is captured and stored in the form of high pressure compressed air in an open accumulator when supply exceeds demand and is regenerated during peak demand. The porous media inserts within the compression chamber augment heat transfer and maintain a near-isothermal compression/expansion trajectory. The near isothermal behavior of the compressor/expander in addition to the open accumulator architecture improve both efficiency and power density. The design, schematic layout, operation and implementation of a functional prototype has been presented with the porous media inserts, an integrated flow intensifier which aids in increased compression/expansion rates at lower pressures and custom low-pressure and high-pressure air valves with functionality to operate passively as well as actively. The operation of the prototype has been simulated through a 1-D system level simulation which represents all subsystems of the prototype developed. The simulation models the water pump, the pressure dynamics of the water nodes, the actuation of the water/air valves, movement of the liquid piston, mass transfer between the chamber, well tank and the accumulator. The results of the simulation help understand the operation of the cycle, critical parameters and how the state of the system changes with time for both storage and regeneration operation modes. The simulation results are used to perform an exergy, work and loss analysis to validate the simulation and determine cycle metrics such as efficiency and power density. Several case studies are presented, discussing the impact of controller’s decisions on the performance of the system. The results of the case studies discuss the significance of the precise switching of various valves on performance characteristics such as mass of air ejected during the storage cycle, mass of air available for expansion during the regeneration cycle and valve switching conditions that affect energy losses. The results of the case studies will be used towards optimizing the operation of the functional prototype in future.