Lei Chai, S.A. Tassou
• PTES plays a crucial role in large-scale deployment of renewable energy. • Working principle and employed thermodynamic cycles of PTES based on CO 2 cycles. • Technical and operational issues related to the main components and control systems. • Current research and development status and future research trends. Pumped thermal energy storage (PTES) is an innovative physical energy storage technology that converts surplus electricity into heat, stores it in thermal reservoirs, and reconverts it into electricity when needed. When CO 2 is employed as the working fluid, PTES can offer advantages of potentially high round-trip efficiency, compact system equipment, and good operational flexibility without significant geographical limitations, which make it superior for applications in large-scale energy storage scenarios. However, there remain technical challenges to be addressed before the technology can be widely adopted: Selection of most appropriate thermodynamic configuration; design and operation of the main components, such as expanders, compressors, heat exchangers, thermal energy storage materials and storage tanks; and design of control systems to maximise control system efficiency and reliability. This review paper provides the first comprehensive technology review focused exclusively on PTES systems based on CO 2 cycles. Unlike previous studies that have addressed PTES more generally or considered CO 2 only as one working fluid among many, this review aims to provide insightful information on recent advancements in PTES systems based on CO 2 cycles, and the technical and operational issues related to the main components and control systems. Following description of working principles and characteristics of thermodynamic cycles, this review discusses the current research and development status, technical challenges and risks, and future research trends of the main components and control approaches. By identifying technical challenges and research gaps, this work provides a foundation for advancing PTES technologies toward commercial readiness.