Abstract
Ocean Thermal Energy Conversion (OTEC) using non-azeotropic mixtures such as ammonia/water as working fluid and the multistage cycle has been investigated in order to improve the thermal efficiency of the cycle because of small ocean temperature differences. The performance and effectiveness of the multistage cycle are barely understood. In addition, previous evaluation methods of heat exchange process cannot clearly indicate the influence of the thermophysical characteristics of the working fluid on the power output. Consequently, this study investigated the influence of reduction of the irreversible losses in the heat exchange process on the system performance in double-stage Rankine cycle using pure working fluid. Single Rankine, double-stage Rankine and Kalina cycles were analyzed to ascertain the system characteristics. The simple evaluation method of the temperature difference between the working fluid and the seawater is applied to this analysis. From the results of the parametric performance analysis it can be considered that double-stage Rankine cycle using pure working fluid can reduce the irreversible losses in the heat exchange process as with the Kalina cycle using an ammonia/water mixture. Considering the maximum power efficiency obtained in the study, double-stage Rankine and Kalina cycles can improve the power output by reducing the irreversible losses in the cycle.
Highlights
At the 2015 United Nations Climate Change Conference (COP21), the Paris Agreement was adopted as a global warming countermeasure to be implemented after 2020 in both developed and developing countries
The chain line refers to the double-stage Rankine cycle using ammonia, the two-dot chain line to HFC134a, the dashed line to the single Rankine cycle using ammonia, the dotted line to HFC134a, the solid line to the Kalina cycle, and the circle mark to the maximum value of the power output
The double-stage Rankine cycle allows for the improvement of Ocean Thermal Energy Conversion (OTEC) system
Summary
At the 2015 United Nations Climate Change Conference (COP21), the Paris Agreement was adopted as a global warming countermeasure to be implemented after 2020 in both developed and developing countries. The use of non-azeotropic mixtures such as ammonia/water as working fluid in the power generation system was proposed by Kalina [9]. The authors [14] indicated the influence of the evaluation method of heat exchange process between heat source and working fluid on the power generation system using a low-temperature heat source. This study investigated the influence of reduction of the irreversible losses in the heat exchange process on the system performance in the double-stage Rankine cycle. The heat transfer performance is affected differently by various types of working fluids and composition of mixture, three cycles are analyzed and evaluated under the condition of the same number of heat transfer unit without the effect of the characteristics of working fluid in order to confirm the system characteristics of the double-stage Rankine cycle
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