Chapter 10.9, Problem 28P

A binary geothermal power plant uses geothermal water at 160°C as the heat source. The plant operates on the simple Rankine cycle with isobutane as the working fluid. Heat is transferred to the cycle by a heat exchanger in which geothermal liquid water enters at 160°C at a rate of 555.9 kg/s and leaves at 90°C. Isobutane enters the turbine at 3.25 MPa and 147°C and leaves at 79.5°C and 410 kPa. Isobutane is condensed in an air-cooled condenser and pumped to the heat exchanger pressure. Assuming the pump to have an isentropic efficiency of 90 percent, determine (a) the isentropic efficiency of the turbine, (b) the net power output of the plant, and (c) the thermal efficiency of the plant.

The properties of isobutane are h₁ = 273.01 kJ/kg, v₁ = 0.001842 m³/kg, h₃ = 761.54 kJ/kg, h₄ = 689.74 kJ/kg, h_4s = 670.40 kJ/kg. Take the specific heat of geothermal water to be c_p = 4.258 kJ/kg·°C.

FIGURE P10–28