Integration a Rotary Pressure Exchanger in Transcritical CO2 Refrigeration System: Modeling and Annual Energy Saving Estimation

Abstract

Abstract This paper presents the use of a rotary gas pressure exchanger (PX) in enhancing the efficiency of transcritical CO2 refrigeration systems. Pressure Exchanger based system utilizes the high-pressure discharge from the condenser/gas cooler as a driving fluid, effectively compressing the low-pressure gas (either from flash gas in receiver or from a bypass source) with minimal external energy. This process significantly lowers the power consumption of the main compressor. Concurrently, the PX expands the high-pressure supercritical/liquid CO2 into a two-phase state, preparing it for heat absorption and thereby mirroring the role of a standard high-pressure expansion valve. The study delves deeper into the architecture of CO2 refrigeration systems that incorporate the PX as a mechanical subcooler. The source of high-pressure fluid is derived from the gas cooler, while the low-pressure gas originates from a bypass line connected to the main gas cooler exit flow. This bypassed flow undergoes expansion and is then utilized to lower the temperature of the flow exiting the main gas cooler, achieved through a counter-flow heat exchanger. The superheated flow from the bypass serves as the low-pressure inlet to the PX. Such integration markedly diminishes the flash gas presence in the receiver, leading to a reduction in the overall workload of the compressor and a consequent boost in energy efficiency. The paper presents a detailed modeling approach that addresses the simultaneous energy and mass balance equations integral to the cycle when integrated with PX G. It also establishes mass flow constraints, which are contingent on the internal physics of PX G and the density ratio of the low and high-pressure streams as they enter PX G. The analysis encompasses various performance metrics such as the overall system Coefficient of Performance (COP), the work done by medium-temperature (MT) and low-temperature (LT) compressors, liquid content post-expansion through PX G, flash gas production, and superheat control. These factors are evaluated across a broad spectrum of conditions, including diverse ambient temperatures, evaporator duties, and load splits between MT and LT evaporators. Additionally, the paper discusses multiple techno-economic scenarios considering different system parameters and architectures. These architectures include an adiabatic gas cooler, a parallel compressor, and an internal heat exchanger. An analysis of annualized work savings across various geographical locations in North America and Europe is presented, highlighting the variance in temperature patterns. The findings demonstrate that PX presence enables the system to operate more frequently under extreme temperatures and reduces the likelihood of operational shutdowns during heatwaves, particularly on hot summer days. A single PX unit can approximately add about 5.4 m3/hr or a 20% capacity increase to a standard 150 kW store.