8-26 A geothermal power plant uses geothermal liquid water at 160C at a rate of 440 kg/s as the heat source, and produces 14 MW of net power in an environment at 25C. If 18.5 MW of exergy entering the plant with the geothermal water is destructed within the plant, determine (a) the exergy of the geothermal water entering the plant, (b) the second-law effi- ciency, and (c) the exergy of the heat rejected from the plant. Exergy Analysis of Closed Systems 8-27C Is a process during which no entropy is generated (Sgen 0) necessarily reversible? 8-28C Can a system have a higher second-law efficiency than the first-law efficiency during a process? Give examples. 8-29 A piston-cylinder device initially contains 2 L of air at 100 kPa and 25C. Air is now compressed to a final state of 600 kPa and 150C. The useful work input is 1.2 kJ. Assuming AIR V = 2L P = 100 kPa T = 25C FIGURE P8-29 the surroundings are at 100 kPa and 25C, determine (a) the exergy of the air at the initial and the final states, (b) the mini- mum work that must be supplied to accomplish this compres- sion process, and (c) the second-law efficiency of this process. Answers: (a) 0, 0.171 kJ, (b) 0.171 kJ, (c) 14.3 percent 8-30 A piston-cylinder device contains 5 kg of refrigerant- 134a at 0.7 MPa and 60C. The refrigerant is now cooled at constant pressure until it exists as a liquid at 24C. If the sur- roundings are at 100 kPa and 24C, determine (a) the exergy of the refrigerant at the initial and the final states and (b) the exergy destroyed during this process.