SUMMARYHydrocarbons (HCs) are excellent refrigerants in many ways such as energy efficiency, critical point, solubility, transport and heat transfer properties, but they are also flammable, which causes the need for changes in standards, production and product. There are increasing number of scientists and engineers who believe that an alternative solution, which has been overlooked, may be provided by using HCs. The main objective of this study is to perform energy and exergy analyses for a vapor compression refrigeration system with an internal heat exchanger using a HC, isobutene (R600a). For a refrigeration capacity of 1 kW and cold chamber temperature of 08C, energy and exergy balances are taken into account to determine the performance of the refrigeration system. Energy and exergy fluxes are determined, and irreversibility rates are calculated for every component of the system. It is seen that the compressor has the highest irreversibility rate, and the heat exchanger has the lowest. Also from the result of the analysis, it is found that condenser and evaporator temperatures have strong effects on energetic and exergetic performances of the system such as coefficient of performance (COP), efficiency ratio (t), exergetic efficiency (x) and irreversibility rate.

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SUMMARYThe goal of this study is to carry out exergy analyses for an experimental variable-speed refrigeration system working with R404a in order to determine irreversibility rates and exergetic efficiencies of system components and the overall system. For this aim, an experimental refrigeration system was designed with a frequency inverter mounted on compressor electric motor. Controlling the rotational speed of the compressor with a frequency inverter is one of the best methods to vary the capacity of the refrigeration system. The experiments were made for different compressor electric motor frequencies. The results showed that at low-frequency values, irreversibility rates of the system decreased and exergetic efficiencies were increased. In addition, the major irreversibility occurs in the compressor by 61.47-61.83% followed by condenser by 17.00-16.52%, evaporator by 12.39-13.73% and expansion valve by 6.24-6.76% for different compressor frequencies.

The natural gas combined cycle power plant is one of the best options for generating electricity due to its use of low carbon fuels, high efficiency, and operational flexibility. These plants consist of a combination of Brayton and Rankine cycles, and investigation of these cycles is performed in this paper. Here the parameters of pressure and temperature used in the calculations are taken from a combined cycle power plant. The net power output of the system at 25°C ambient temperature and 101.325 kPa pressure was calculated as 45 and 12 MW for Brayton and Rankine cycles, respectively. In addition, Brayton, Rankine, and combined cycle efficiencies were calculated as 37.5%, 27%, and 47.5%, while the exergy efficiencies were determined as 36%, 44%, and 46%, respectively. In the system elements, the most energy was lost in the combustion chamber and the highest exergy efficiency was achieved in the compressor at 95% level.Impacts of an increase in ambient temperature, compressor pressure ratio, and change on turbine inlet temperature were further investigated. Energy (Sankey) and exergy (Grossman) flow diagrams were further drawn based on the analyses obtained from the combined cycle power plants.

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