First- and Second-Law Analysis of Steam-Turbine Cogeneration Systems

Abstract

The paper presents an analysis of a cogeneration plant. The performance of the plant is compared to a conventional plant with separate production of process heat and power. The analysis is first- and second-law based and, therefore, quantifies the irreversibilities of the different components of each plant. In the cogeneration plant, the heat required in the boiler can be obtained either from fuel firing (condensing or back-pressure turbine plant) or from exhaust gases of a simple gas turbine (gas turbine cogeneration plant). The present study compares the two methods. The influence of the heat-to-power ratio and the process pressure on the thermal efficiency, utilization factor, and irreversibilities of the different components of each plant is presented. The results show that the total irreversibility of the cogeneration plant is lower by 38 percent compared to the conventional plant. This reduction in the irreversibility is accompanied by an increase in the thermal efficiency and utilization factor by 25 and 24 percent, respectively. The results show that most irreversible losses are due to boiler.