Thermodynamics

588 | ThermodynamicsT2 3BoilerMercurypumpMERCURYCYCLEMercuryturbineSaturation dome(mercury)31Heat exchanger4MERCURYCYCLEQ247Steampump6STEAMCYCLE7SteamturbineSuperheater61STEAMCYCLE5 8Saturationdome(steam)5Condenser8sFIGURE 10–26Mercury–water binary vapor cycle.237°C, which is too high as the minimum temperature in the cycle. Therefore,the use of mercury as a working fluid is limited to the high-temperaturecycles. Other disadvantages of mercury are its toxicity and high cost. Themass flow rate of mercury in binary vapor cycles is several times that of waterbecause of its low enthalpy of vaporization.It is evident from the T-s diagram in Fig. 10–26 that the binary vapor cycleapproximates the Carnot cycle more closely than the steam cycle for thesame temperature limits. Therefore, the thermal efficiency of a power plantcan be increased by switching to binary cycles. The use of mercury–waterbinary cycles in the United States dates back to 1928. Several such plantshave been built since then in the New England area, where fuel costs are typicallyhigher. A small (40-MW) mercury–steam power plant that was in servicein New Hampshire in 1950 had a higher thermal efficiency than most ofthe large modern power plants in use at that time.Studies show that thermal efficiencies of 50 percent or higher are possiblewith binary vapor cycles. However, binary vapor cycles are not economicallyattractive because of their high initial cost and the competition offered by thecombined gas–steam power plants.