CONCEPT:Desirable characteristics of working fluid in a power-producing cycle:The fluid should have a high critical temperature so that the saturation pressure at the maximum permissible temperature is relatively low.The saturation pressure at the temperature of heat rejection should be above atmospheric pressure.The specific heat of the liquid should be small.The saturation vapour line of T-s diagram should be steep, very close to the turbine so that excessive moisture does not appear during expansion.The freezing point of the fluid should be below room temperature so that it does not get solidified while FLOWING through the pipelines.The fluid should be CHEMICALLY stable.The fluid should be non-toxic, non-corrosive, and low in cost.No single fluid can meet all the requirements as mentioned, therefore a binary cycle is used. Binary cycle:A binary vapour cycle is a combination of two cycles, one in a high-temperature region and the other in a lower temperature regionIn the binary cycle (two-fluid), two cycles with DIFFERENT working fluids are COUPLED in seriesThe heat rejected by one being utilized by another cycle; So, the overall efficiency of the plant increases.The overall efficiency of two-cycle coupled in series is η = η1 + η2 - η1η2.Explanation:Overall efficiency of binary vapour cycle is:η = 1 – (1 – η1) (1 – η2)η = 1 – (1 – η2 – η1 + η1η2)η = η1 + η2 – η1η2