Second law analysis and optimization of a combined triple power cycle

In this investigation, a combined triple (Brayton/Rankine/Rankine)/(gas/steam/ammonia) power cycle is analyzed. In the triple cycle, the exhaust of the Brayton gas topping cycle is used in a heat recovery steam generator (HRSG) to produce steam for a Rankine steam middle cycle followed by an ammonia Rankine bottoming cycle. The ammonia bottoming cycle provides a practical and more efficient hot and cold streams thermal matching for the triple cycle HRSG as compared to the HRSG of a conventional combined power cycle. Through exergy analysis of the cycle, the exergy of the exhaust streams and the irreversibility of each component in the cycle are determined, using reasonably practical constraints for the system components. These constraints are mainly due to the size of components and are conveniently parameterized and analyzed. The triple cycle was analyzed and optimized with respect to important system parameters, such as the gas topping cycle pressure ratio, gas turbine inlet temperature, HRSG pinch point, gas/steam approach temperature difference, rate of steam injection into combustion chamber and the effectiveness of the heat exchangers. One goal of the study is to find what configuration will achieve a thermal efficiency of 60% when reasonably practical constraints for system parameters are used. (C) 2001 Elsevier Science Ltd. All rights reserved.