Design, Modeling, and Energy Analysis of a Liquid Piston Compressor-Expander with Integrated Flow Intensifier

Abstract

In this thesis, a novel air compressor-expander for application to a compressed air energy storage (CAES) system is designed and investigated with simulation. CAES technology can be combined with intermittent renewable energy sources to capture energy that exceeds demand. The proposed method of CAES uses a liquid piston to compress air to a high pressure for storage and regenerate energy later. The liquid piston system pumps a liquid (water) into a compression chamber, decreasing the volume of the air mass and increasing its pressure. The use of a liquid piston allows for minimal air leakage at high pressures and allows for heat transfer enhancement. As the gas is compressed, the temperature will rise; if the gas is moved to storage in this heated state, the liquid piston will need to apply more flow work to move the gas and the thermal energy will dissipate in storage. Therefore, it is important to compress the gas in a near-isothermal state. The shape, heat transfer characteristics, and compression rate of the liquid piston system can be optimized to maximize the efficiency and power density of the system while minimizing the cost associated with manufacturing the system. A functional liquid piston compressor-expander (LPC-E) system is designed and manufactured. Previous optimizations have shown that the compression and expansion rates should be greatest when the air pressure is low. A “flow intensifier”, or linear hydraulic transformer, is implemented to amplify the air volumetric change rate at the expense of higher pressure supplied by the water pump. This flow intensifier is integrated with an LPC-E system to form a functioning, cost-effective CAES prototype. In addition to a custom flow intensifier, an air valve assembly is designed and manufactured. The air valves are designed to accommodate the high air pressure, high air flow rates, and to be water compatible. These custom assemblies are combined with other components to complete a functional prototype. The operation of the integrated flow intensifier/LPC-E system is simulated to determine how the system functions. The simulation considers water pump behavior, controller action, flow intensifier movement, and air thermodynamics. The results of this simulation can be used to examine any abnormalities in the system, detect the impact of controller switching parameters on performance, and observe how the system changes during repeated storage and regeneration operation. Simulation results can be used to determine the energy storage ability, or “exergy”, of the system. An analysis is performed to determine the exergy, applied work, and energy losses of the system to determine how system parameters impact performance. These results can be used to determine the efficiency, power density, and other performance characteristics of the LPC-E CAES system when the system is set to store or regenerate energy. Several studies are performed to examine the impact of varying controller switching parameters on system performance. Coupled with the exergy-work-loss analysis, these studies show that controller selection has a significant effect on a variety of performance characteristics, ranging from valve throttling losses to work input to the system to quantity of air mass moved to storage.