Development and validation of CO2 cooling systems with expansion work recovery

Abstract

The 2023 revised strategy of the International Maritime Organization (IMO) emphasises achieving net-zero greenhouse gas emissions within the shipping sector. Notably, nearly 40% of the energy demand in passenger ships is attributed to cooling and heating systems. Currently, these systems predominantly utilize R134a, a synthetic PFAS (per- and polyfluoroalkyl substances) chemical with a high global warming potential. In 2020, the European Union observed an approximate emission of 75,000 tonnes of PFAS into the environment. Subsequently, in 2023, a joint proposal was initiated by several member states of the EU aimed to prohibit the utilization of PFAS. If ratified, this proposition would lead to the ban of PFAS usage by either 2025 or 2026 [1, 2]. To align with existing and forthcoming environmental regulations, there is a critical need to transition towards zero-emission fuels and adopt natural refrigerants. Within the realm of natural refrigerants, carbon dioxide (CO2) stands out due to its non-toxic and non-flammable nature, addressing safety concerns onboard passenger ships. CO2 refrigerants have gained widespread acceptance in sectors such as supermarkets, the hotel industry, process industries, and fishing vessels. Given its suitability and environmental advantages, adopting CO2-based cooling and heating systems is anticipated to witness an upward trend in passenger ships, aligning with the evolving regulatory landscape and the IMO’s overarching goal of achieving net-zero emissions. Operating a CO2 transcritical system in high ambient temperatures presents challenges, particularly expansion losses. Over the past decade, ejectors have been widely employed to recover expansion work, but with certain limitations. The pressure exchanger (PX) is a recent advancement gaining prominence, specifically designed for expansion work recovery. The PX device is distinct in its capability to recover expansion work from the gas cooler to the receiver pressure and employ it to compress the flash gas. With its four ports and internal rotor, the PX seamlessly expands and compresses without requiring physical separation. A pressure lift is essential for the PX to propel the flash gas into its system, which is subsequently compressed to a pressure slightly lower than the gas cooler’s. This underscores the PX’s need for two low-pressure lift devices to effectively compress the flash gas from the receiver to the gas cooler pressure. The current practice involves employing two small booster compressors to fulfil this essential role in the CO2 transcritical system. This study investigates a novel integration concept incorporating a pressure exchanger (PX) with two innovative low-lift ejectors. Instead of relying on two booster compressors, the proposed approach leverages two ejectors to achieve the same objective. To enable the ejectors to compress flash gas from the PX compression outlet to the gas cooler pressure, a prerequisite is to maintain a compressor discharge pressure higher than the gas cooler pressure. A numerical analysis of this innovative PX integration concept is conducted, and its performance is compared with that of standard booster, parallel, and ejector configurations. The investigation findings indicate the viability of the PX integration concept, estimating a potential performance enhancement ranging from 2% to 6% when compared to the ejector configuration. Following the theoretical investigation, an experimental setup was constructed in the Varmeteknisk NTNU laboratory to validate the conceptual findings. For this endeavour, a retrofit was performed on the SuperSmart CO2 transcritical facility, integrating the pressure exchanger (PX), two newly introduced ejectors, and associated fittings and measuring instruments. A tailored control strategy was developed, and two PID controllers were incorporated into the existing control software to facilitate the operation of the PX mode. Experiments were conducted to assess the cooling capacity of 70 kW with an evaporation temperature of 0 ℃ and gas cooler outlet temperatures of 33 ℃, 35 ℃, 37 ℃, and 38 ℃. The system maintained steady-state conditions for a duration of 10 minutes while recording data. The experimental outcomes have successfully validated the proof of concept, establishing a solid foundation for further exploration of the pressure exchanger (PX) and applying low-lift ejectors. Discrepancies observed between numerical predictions and experimental results are thoroughly discussed, with potential solutions highlighted for addressing these disparities. The completion of the PhD thesis signifies the successful achievement of its objectives, encompassing the development of the pressure exchanger (PX) integration concept, numerical investigation, the establishment of an experimental setup, and the subsequent verification of the conceptual framework.