A power plant operating on heat from a geothermal source uses isobutane as the w

Question

A power plant operating on heat from a geothermal source uses isobutane as the working medium in a Rankine cycle (Fig. 8.3) Isobutane is heated at 3.100 kPa (a pressure just a little below its critical pressure) to a temperature of 140degreeC at which conditions it enters the turbine. Isentropic expansion in the turbine prouuces vapor at 450 kPa, which is cooled and condensed to saturated liquid and pumpecI to the heaterboiler. If the flowrate of isobutane is 75 kg 8"1, what ts the power output of the Rankine cycle and what are the heat-transfer rates in the heater/botler and cooler condenser? What is the thermal efficiency of the cycle? The vapor pressure of isobutane is given in Table B.2 of App. B.

Explanation / Answer

Here, Power output = (3400/450) * 75

                                 = 566.67 W

Heat transfer rate in heater/boiler = 75 * 450

                                                        = 33750 J/sec

Heat transfer rate in cooler    = 75 * 450/52

                                                 = 649.038 J/sec

Thermal efficiency of cycle = 0.80 * (649.038/33750) * 12

                                           = 0.1846              ( or   18.46 %)

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