Experimental study and operational optimization of a dual-stage ocean thermal energy conversion

The dual-stage organic Rankine cycle offers an efficient and scalable approach to convert ocean renewable thermal energy into electricity. However, facilities based on this system in ocean thermal energy conversion still face obstacles, including a lack of theoretical design guidance and insufficient understanding of their operational characteristics. To address these issues, this work developed a thermodynamic model to guide the optimal design of the system and built a 3 kW-scale proof-of-concept prototype to evaluate its operational characteristics. Thermodynamic analysis under rated conditions was conducted to verify its superiority. The system's response to warm seawater temperature variation was experimentally investigated. Furthermore, a data-driven modeling-based annual operational strategy maximizing daily net power output was proposed. Results show that equal thermal energy distribution between stages is optimal. The prototype exhibited a maximum cycle thermal efficiency of 2.39%, which is 13% superior to conventional single-stage systems under a 25 °C ocean temperature difference. Techno-economic analysis indicates that, under favorable design conditions, a megawatt-scale dual-stage ORC OTEC can achieve significantly higher thermal efficiency and less than half the LCOE of a single-stage counterpart. Additionally, the optimal warm seawater flow rate for maximum power output depends on the water temperature. By dynamically optimizing the seawater flow rate based on annual daily temperature variations of seawater, the net power output can be increased by 25.2% compared to fixed-flow operation. This research provides a new perspective for high-efficient operation of ocean thermal energy conversion.