Recent progress in organic Rankine cycle targeting utilisation of ultra-low-temperature heat towards carbon neutrality

Organic Rankine cycle (ORC) has been widely utilised in electricity generation due to its attractive thermodynamic performance, compact system structure, and flexibility in system scale-down and operating condition. Plenty of studies have been conducted on ORCs with significant progress. Recently, the ORC targeting ultra-lowtemperature heat sources has raised considerable interest as it is one of the soundest technologies that can maximise the conversion of renewable and sustainable energy sources into useful power supply for accelerating towards the carbon neutrality target. However, there is a lack of reviews on the characteristics and applications of ultra-low-temperature ORC systems. Therefore, the present comprehensive review focuses on the recent development of ultra-low-temperature ORCs for utilisation of heat at or below 150 degrees C. Firstly, the characteristics of ultra-low-temperature ORCs are presented in the introduction. Subsequently, the significant research progress and technological development are discussed in separate categories, i.e., working fluids, cycle modification methods, integration systems, and applications. Finally, engineering features, challenges, and promising research directions are presented. This study aims to help researchers and engineers get acquainted with the progress, issues, and challenges in the field of ultra-low-temperature ORC, and promote awareness of the technology to achieve broader commercial and industrial applications.