Ocean thermal energy conversion efficiency: a systematic review of thermodynamic limits, system designs, and performance drivers

Ocean Thermal Energy Conversion (OTEC) offers continuous and predictable renewable power by exploiting the temperature difference between warm surface seawater and cold deep seawater; however, its widespread deployment is constrained by inherently low conversion efficiency. This systematic review critically synthesizes recent advances in OTEC efficiency, drawing on experimental, numerical, and optimization-based studies published between 2023 and 2026. Standalone closed-cycle OTEC systems are shown to typically achieve thermal efficiencies of 1.5–3.0% at pilot scales (1–50 kW), with performance strongly limited by heat-exchange losses and parasitic pumping power. Significant efficiency improvements are reported through advanced thermodynamic cycles (Kalina, Uehara, temperature-lifting, and secondary heat recovery), zeotropic and binary working-fluid mixtures, and component-level innovations such as magnetic levitation expanders and optimized heat exchangers. Hybrid configurations, including solar-assisted and waste-heat-assisted OTEC, demonstrate the largest relative gains, with reported increases in net power output exceeding 70–150% and substantial reductions in levelized cost of energy under favorable site conditions. Nevertheless, cross-study comparison remains challenging due to inconsistent efficiency definitions, incomplete reporting of parasitic losses, and limited consideration of long-term ocean thermal variability. This review highlights the need for standardized performance metrics, system-level efficiency assessment, and large-scale demonstration projects. Overall, the findings indicate that future OTEC efficiency gains will be driven primarily by integrated, hybrid, and site-specific system designs rather than incremental improvements to standalone cycles.