Due to the recent energy shortage, global warming, and environment pollution, the importance of energy saving and environment regulation is rapidly increasing. One of the methods to resolve these problems is using new renewable energy. Ocean thermal energy conversion, which is one way of using new renewable energy, is a power cycle utilizing the temperature difference between surface water and deep water. As ocean thermal energy conversion uses a heat source at low temperature, it is essential to use an organic Rankine cycle. Thus, this study examined the characteristics of an organic Rankine cycle for ocean thermal energy conversion according to pinch point analysis and a transcritical cycle.
First, thermal efficiency analysis on an organic Rankine cycle depending on various types of working fluid and cycle was conducted. A classic simple Rankine cycle, regenerative Rankine cycles, and a Kalina cycle were considered in the analysis. In addition, nine types of single working fluid and three types of mixed working fluid were selected. For cycle analysis methods, pinch point analysis was conducted. As for single working fluid, thermal efficiency was the highest in RE245fa2 in a simple Rankine cycle and regenerative Rankine cycles. As for mixed working fluid, thermal efficiency was the highest when the composition ratio of NH3 to H2O was 0.9:0.1 in a Kalina cycle. Compared to a simple Rankine cycle, a Rankine cycle with open feedliquid heater, a Rankine cycle with integrated regenerator, and a Kalina cycle showed thermal efficiency increase rates of approx. 2.0%, 1.0%, and 10%, respectively.
Second, exergy analysis on the cycles at each heat exchanger was conducted considering the influence of pinch point temperature difference and that of outlet temperatures of a heat source and a heat sink. Thermodynamic performance was analyzed by applying seven types of working fluid to the cycles designed according to pinch point analysis. As a result of performance analysis, as pinch point temperature difference and the temperature difference between inlet and outlet of a heat source or a heat sink were low at each heat exchanger, second law efficiency increased but cycle irreversibility and exergy destruction factor decreased. In addition, cycle irreversibility largely changed where thermodynamic change occurred. Of the selected types of working fluid, RE245fa2 showed the most excellent thermodynamic performance.
Lastly, recent research related to a transcritical cycle of an organic Rankine cycle using a heat source at low temperature was reviewed. A transcritical cycle was made up of an solar-boosted ocean thermal energy conversion system using R744, economical and stable working fluid, and then thermodynamic performance analysis was conducted according to the state of turbine inlet. As a result, a transcritical cycle showed better thermodynamic performance as turbine inlet temperature was high. On the other hand, turbine inlet pressure of a transcritical cycle showed better thermodynamic performance than a subcritical cycle only in the optimized state. Compared to an optimized transcritical simple Rankine cycle, an optimized transcritical Rankine cycle with open feedliquid heater showed increased second law efficiency and reduced cycle irreversibility.