Parametric analysis of hybrid elastocaloric – CO2 cooling system

Abstract

This study examines the potential application of elastocaloric refrigeration (eC) technology for the subcooling of CO2 in transcritical single-stage refrigeration cycles. Elastocaloric refrigeration, a solid-state refrigeration technology, possesses significant untapped potential due to its environmentally friendly characteristics, primarily its lack of harmful operational fluids. However, its direct stand-alone application has been limited due to the relatively small temperature spans produced by current proof-of-concept elastocaloric devices. Efforts of the scientific community have focused on extending the temperature difference that eC systems can provide, while this work offers an alternative to that challenge, offering an application solution for the current state of technology. The study proposes for the first-time a unique integration of eC technology and CO2 cooling systems, aiming to capitalise on the respective weaknesses of these technologies and transform them into strengths. In the proposed solution, an eC device operates as an external agent to subcool CO2, with the objective of enhancing the energy performance of the refrigeration system. This concept is motivated by the recent advancements in CO2 cooling systems and the growing recognition of subcooling as a promising method to boost the performance of such systems. The hybrid system’s performance was evaluated across various ambient temperatures, ranging from 20 °C to 35 °C, and at an evaporating level of −15 °C. It is evaluated by means of a calculation model based on the data obtained experimentally from an elastocaloric regenerator and an experimental CO2 plant. The system‘s energy efficiency was analysed in comparison to a non-subcooled CO2 cycle, and a third cycle comprising both eC subcooling and an expander for energy recovery from the expansion process. The results demonstrated a considerable increase in the coefficient of performance (COP) with the use of eC subcooling: increments of 2.7 % at 20 °C, 4.5 % at 25 °C, 9.6 % at 30 °C and 13.1 % at 35 °C, where more significant increments were observed at higher ambient temperatures. The eC Subcooler with expander reaches increments of 7.5 %, 11.1 %, 18.1 % and 22.2 % respectively. Additionally, the eC subcooler allowed a reduction in the optimum gas-cooler pressure by up to 5 bar at the highest environment temperature. Despite these promising results, the study underscores the necessity for further optimisation and improvement of elastocaloric devices. Additionally, it emphasizes the importance of energy recovery strategies from the expansion process in transcritical CO2 cooling system.