Bibliometric analysis of CO2 heat pump research on applications system configurations and performance enhancement

Abstract

The global transition toward low-carbon energy systems has positioned CO 2 (R744) heat pumps as a promising technology for sustainable heating and cooling applications. This study presents a comprehensive bibliometric analysis combined with a structured technical interpretation of the scientific literature on CO 2 heat pumps, aiming to identify major research trends, technological developments, and persisting knowledge gaps. Four thematic domains were explored through Scopus-based bibliometric mapping using VOSviewer, focusing on applications of CO 2 heat pumps, system configurations and architectures, performance and efficiency enhancement, and integration with thermal energy storage. The keyword co-occurrence networks reveal a clear evolution of the field from early investigations cantered on the transcritical CO 2 cycle and domestic hot water production, toward more complex, integrated systems combining renewable energy sources, waste heat recovery, and intelligent control strategies. Despite significant progress, the analysis highlights key research gaps, including the limited experimental validation of large-scale systems, insufficient coupling between CO 2 heat pumps and thermal energy storage under dynamic operating conditions, and a lack of standardized methodologies for performance modelling and control optimization. Additionally, studies on dynamic operation, life-cycle assessment, and cost-effectiveness remain underrepresented. The integration of thermal energy storage is emerging as a pivotal research frontier, offering pathways for load flexibility and higher system efficiency. This bibliometric investigation provides a structured overview of the intellectual landscape and evolution of CO 2 heat pump research, providing decision-oriented insights into the main challenges and opportunities guiding future advancements in sustainable heating and cooling technologies.