Innovative design and performance evaluation of a compact 1 MW radial inflow turbine with non-azeotropic fluids for ocean thermal energy conversion applications

The large-scale utilization of ocean thermal energy necessitates cost reduction, which is achievable primarily through system scaling. In this context, this study explores megawatt-scale turbine technology as a key enabler for the commercialization of ocean thermal energy conversion (OTEC) systems. A compact 1 MW radial inflow turbine was designed and modeled, with particular attention to enhancing performance and minimizing system footprint. To meet these requirements, a novel non-azeotropic working fluid mixture—ammonia/R1123 at a mass ratio of 0.76/0.24—was proposed to optimize thermodynamic efficiency. The turbine design process involved the preliminary optimization of key geometric parameters using a particle swarm optimization (PSO) algorithm, followed by detailed three-dimensional computational fluid dynamics (CFD) simulations. Spontaneous nucleation theory was employed instead of conventional equilibrium-based models to capture the nonequilibrium condensation more accurately during ammonia wet expansion, representing a novel approach not previously reported. Under optimal operating conditions, the turbine achieved an isentropic efficiency of 91.74 % and an output power of 1049.91 kW, thereby fulfilling the design objectives. The maximum supersaturation ratio was calculated to be 1.008, below the threshold for spontaneous droplet nucleation, indicating that wet expansion was successfully avoided. Furthermore, off-design performance analyses confirmed the turbine's operational robustness across varying conditions, demonstrating its practical feasibility for integration into large-scale OTEC systems.