DEVELOPMENT OF A NEAR-ISOTHERMAL COMPRESSION PROCESS UTILIZING LIQUID PISTON TECHNOLOGY FOR TRANSCRITICAL CO2 CYCLE

Abstract

Compressors are critical components in various industries and are commonly used in numerous applications. With the increasing concerns about global warming, there has been a significant focus on improving compressor efficiency, a subject of extensive research and innovation. This is particularly true for the heating, ventilation, air conditioning, and refrigeration industries since almost every household relies on air conditioning and refrigeration systems. Despite numerous proposed and investigated technologies for efficiency improvements, there remains substantial potential for further advancements. The compression process is often modeled as an isentropic process in which a significant portion of the work provided during compression is converted to heat, increasing the overall input power required. This inefficiency highlights the ongoing need for innovative solutions to reduce the input power of compressors. This dissertation primarily focuses on the experimental development and theoretical investigation of a near-isothermal liquid piston compressor in a transcritical CO2 cycle. The liquid piston compressor employs a liquid piston instead of traditional mechanical pistons to compress gas. This design offers high volumetric efficiency and allows for flexible compression chamber geometries. This innovative compressor design discharges compression heat and internal energy as a form of heat through a compressor-integrated gas cooler during compression. Three successively improved prototypes were constructed to validate the concept and enhance the system. A proof-of-concept test facility was fabricated to demonstrate the feasibility of this design. Furthermore, a complete refrigeration cycle system incorporating the liquid piston compressor was developed. Based on the experimental results, an improved second prototype was built and sent to the Helix Innovation Center of Copeland for field testing. The results show that the isothermal efficiency achieved is 93.5% in the proof-of-concept tests with a self-manufactured copper bare tube heat exchanger as the compression chamber. 90% isothermal efficiency was observed in the first system prototype with a microchannel heat exchanger, and 89 % isothermal efficiency in the second system prototype. The highest compressor coefficient of performance (COP) achieved was 1.82 in the second system prototype. This performance was observed under an average suction pressure of 3,800 kPa and a gas cooler pressure of 10,000 kPa under 35°C ambient temperature. Simulations revealed that the near-isothermal liquid piston compressor could achieve high isothermal efficiency by using heat transfer through the compression chamber and the chamber's thermal mass. This technology's potential applications extend beyond refrigeration, including compressed air energy storage, hydrogen storage, and compressed natural gas systems. These applications were investigated and discussed, highlighting this innovative compressor design's versatility and potential impact. The liquid piston compressor developed in this study exhibits substantial potential for reducing compression work, as supported by both experimental data and simulation modeling. Theintegrated gas cooler in the liquid piston compressor facilitates near-isothermal compression by effectively dissipating both compression heat and internal energy as a form of heat. This heat discharge enhances compression efficiency and improves overall system performance. Future work will prioritize selecting a hydraulic fluid with minimal solubility for CO2 to mitigate degassing issues during compression. Additionally, current market-available pumps do not adequately meet the requirements of the transcritical CO2 cycle. Therefore, developing a semi-hermetic pump will be crucial for the next generation of transcritical CO2 liquid piston compressors. Finally, integrating this pump with an optimized gas cooler and achieving a size comparable to traditional compressors will be essential to making the developed device commercially competitive.