                     Amir Vosoogh, Ebrahim Hajidavalloo Mechanical Engineering Department Mahshahr Azad university, Shahid chamran university Mahshahr, Ahvaz, Khuzestan IRAN Vosoogh_amir@yahoo.com Abstract:  Energy and exergy efficiencies of a supercritical power plant under different ambient conditions have been studied in this paper. Irreversibility rate of the plant components as well as the whole plant were examined at different weather conditions. The effect of ambient condition variation on the condenser pressure change was considered in this analysis to present a better approach which is close to the real condition. It was found that ambient temperature increase has more adverse effect on the exergy efficiency than the energy efficiency. When ambient temperature increases from 5 to 50°C, the energy and exergy efficiencies of the power plant reduce 2.3% and 8.2%, respectively. The exergy analysis shows that the boiler is the main source of exergy destruction and the share of heat transfer irreversibility in the boiler is higher than the combustion irreversibility. As ambient temperature increases, the exergy efficiency of the boiler, the condenser, the heaters and the feed water pump decrease, while the exergy efficiency of the turbine improves slightly. It was also found that when ambient relative humidity increases, the energy and exergy efficiencies of the power plant decrease. The rate of decrease in exergy efficiency is higher than energy efficiency. KeyWords:  Supercritical power plant, irreversibility, thermal efficiency, exergy efficiency, condenser pressure, ambient temperature   Improving the performance of power plant is a never ending subject. Exergy analysis is a method that uses the second law of thermodynamics to evaluate the performance of the thermal systems. It is a useful tool for furthering the goal of more efficient energy3resource use, for it enables the locations, types and magnitudes of wastes and losses to be accurately identified and meaningful efficiencies to be determined [1]. Summaries of the evolution of exergy analysis through the late 1980s are provided by Kotas [2], Moran and Sciubba [3], Bejan et al. [4], Rosen [5], and Dincer [6]. In recent years, many researches have used exergy analysis in the industrial processes. Dincer studied the way that the energy and exergy efficiencies of a thermal power plant are affected by parameters such as the boiler pressure and temperature and the ratio of mass flow rate of the steam leaving the turbine for feed water heaters [7]. Kopac and Hilalci studied the effect of ambient temperature on the exergy efficiency of Catalagzi power plant in Turkey [8]. They found that the highest exergy losses take place in the boiler, while the highest energy losses occur in the condenser. They also found that an increase in ambient temperature decrease the exergy efficiency of all power plant components except the condenser. In their analysis they assumed a constant condenser pressure at different ambient temperatures which is not consistent with practical situation. Kanoglu et al. discussed various definitions of energy and exergy efficiencies in the most conventional power cycles and showed that these terms are frequently misused or misunderstood [9]. They suggested using a careful definition of energy and exergy efficiencies before using them for designing, analyzing and optimizing any thermal systems. Aljundi studied energy and exergy analysis of AL3Hossien power plant in Jordan and showed that maximum exergy destruction occurs in the boiler (77%) followed by the turbine (13%) [10]. He also discussed the effect of varying the reference environment state on the exergy analysis and found that for moderate change in the reference state, no drastic change in the performance of the major components are realized. Despite many publications on the exergy analysis of the power plant, most of them have used simplifying assumptions which may not be consistent with real conditions. For example, most researchers [8,10] used a constant condenser pressure assumption even though ambient temperature varied considerably, which is not consistent with real situation. Moreover, no report has been mentioned on the exergy analysis of a supercritical power plant yet. In this paper, an attempt was taken to analyze the effect of ambient temperature and relative humidity on the irreversibility rate and NEW ASPECTS of FLUID MECHANICS, HEAT TRANSFER and ENVIRONMENT ISSN: 1792-4596 199 ISBN: 978-960-474-215-8