Investigation on organic Rankine cycle technology for utilizing low-temperature waste heat sources from fusion reactors

Fusion energy is recognized as one of the cleanest base-load power sources with abundant fuel reserves, inherent safety and near-zero carbon emissions, which is expected to fundamentally address global fossil energy depletion and climate change. Fusion reactors generate multi-grade waste heat covering wide temperature intervals: high-temperature heat from blanket modules can be matched with steam Rankine or supercritical CO 2 Brayton cycles, while massive low-temperature cooling waste heat below 200 °C is produced from divertor cooling loops and vacuum vessel heat dissipation. Such low-grade heat cannot be efficiently recovered by conventional high-temperature power cycles, resulting in substantial energy loss and constraining the overall energy utilization efficiency of fusion facilities. To address this bottleneck, this paper establishes an Organic Rankine Cycle (ORC) thermodynamic model coupled with the REFPROP fluid property database for recovering fusion low-temperature waste heat, and systematically performs thermal performance evaluation and component-level exergy analysis on ten typical organic working fluids including alkanes, dimethyl ether and fluorinated refrigerants within the heat source range of 80-180 °C. The simulation results indicate that evaporating temperature serves as the dominant parameter governing cycle thermal efficiency, exergy efficiency and circulating mass flow rate. With the rise of evaporator outlet temperature, thermal efficiency and exergy efficiency increase rapidly, whereas the required working fluid mass flow rate decreases monotonically. Under identical operating conditions, R1234ze(E) requires the maximum circulation flow rate of 10.349 kg/s, while pentane and isopentane exhibit the lowest flow rates of 3.359 kg/s and 3.505 kg/s repectively. For full-range heat sources (80-180 °C), pentane, isopentane and R123 deliver balanced superior thermodynamic performance; R123 exhibits the maximum thermal efficiency of 16.86 % at at an evaporation temperature of 170 °C.R1234ze(E), R134a, R152a and dimethyl ether are alternative candidates exclusive to heat sources below 100 °C. R245fa owns mature industrial supporting technology but is limited by its high Global Warming Potential (GWP) and narrow applicable temperature range. Sensitivity analysis reveals that the variation of expander isentropic efficiency imposes far more significant impacts on system performance than pump efficiency. Second-law exergy decomposition indicates that evaporator and condenser are the two dominant exergy destruction sources of the whole system, while pump exergy loss remains negligible. Comprehensive comparison covering thermodynamic characteristics, environmental friendliness and operation adaptability suggests pentane and isopentane are the optimal matching working fluids for the proposed fusion waste heat ORC system, featuring low total exergy dissipation, minimal circulation flow and mild environmental hazards. This work provides quantitative reference for working fluid screening and heat exchanger structural optimization of fusion low-grade heat recovery ORC systems, and offers clear optimization priorities targeted at core irreversible loss components.