Numerical modelling and performance analysis of closed ocean thermal energy conversion cycles in the South China sea

The performance of ocean thermal energy conversion (OTEC) systems in the South China Sea differs significantly due to its relatively low surface seawater temperature, and the influence mechanisms remain unclear. In the present study, numerical models for OTEC systems were developed for Rankine, Kalina, and Uehara OTEC cycles, and a distributed parameter model and a mass flow rate distribution model for parallel PHEs were utilized to accurately evaluate the effects of heat transfer and pressure drop characteristics on overall cycle performance. The influences of operation conditions on performance of various OTEC systems in the South China Sea were analyzed. Results indicate that, under equivalent operating conditions, the low surface temperature in the South China Sea reduces thermal efficiency by 13.29 % compared to other regions. To achieve comparable efficiency levels, the condensation temperature must decrease by at least 3 °C. The Rankine cycle achieves maximally 64 % higher thermal efficiency than the Kalina and Uehara cycles, while the Uehara cycle exhibits 8.38 % and 22.02 % higher net power generation efficiency compared to the Kalina and Rankine cycles, respectively. Decreasing the temperature differences between the seawater inlet and outlet for both the evaporator and condenser improves thermal efficiency, but overly small temperature differences cause significant pressure drops of the heat exchangers. The minimum pressure drop is observed when the temperature difference is between 3 and 5 °C. To maximize net power generation efficiency, a balance between heat transfer efficiency and pressure drop must be maintained. The optimal temperature changes for power generation are 4.5 °C for the Rankine cycle and 5.5 °C for the Kalina and Uehara cycles. To ensure positive annual power generation efficiency under seasonal variations in the South China Sea, the Uehara cycle combined with cold seawater temperature of 3–4 °C is recommended. The findings provide valuable insights into the design and operation of OTEC systems, leading to potential energy and environmental advantages.