Thermodynamics

8–114 A passive solar house that is losing heat to the outdoorsat 5°C at an average rate of 50,000 kJ/h is maintainedat 22°C at all times during a winter night for 10 h. Thehouse is to be heated by 50 glass containers, each containing20 L of water that is heated to 80°C during the day byabsorbing solar energy. A thermostat-controlled 15-kWback-up electric resistance heater turns on whenever necessaryto keep the house at 22°C. Determine (a) how long theelectric heating system was on that night, (b) the exergydestruction, and (c) the minimum work input required forthat night, in kJ.8–115 Steam at 9 MPa and 500°C enters a two-stage adiabaticturbine at a rate of 15 kg/s. Ten percent of the steam isextracted at the end of the first stage at a pressure of 1.4 MPafor other use. The remainder of the steam is further expandedin the second stage and leaves the turbine at 50 kPa. If theturbine has an isentropic efficiency of 88 percent, determinethe wasted power potential during this process as a result ofirreversibilities. Assume the surroundings to be at 25°C.8–116 Steam enters a two-stage adiabatic turbine at 8 MPaand 500°C. It expands in the first stage to a state of 2 MPaand 350°C. Steam is then reheated at constant pressure to atemperature of 500°C before it is routed to the second stage,where it exits at 30 kPa and a quality of 97 percent. The workoutput of the turbine is 5 MW. Assuming the surroundings tobe at 25°C, determine the reversible power output and therate of exergy destruction within this turbine.Answers: 5457 kW, 457 kWStage IHeat2 MPa350°C 2 MPa8 MPa500°C500°CStage IIChapter 8 | 4818–118 Consider a well-insulated horizontal rigid cylinderthat is divided into two compartments by a piston that is freeto move but does not allow either gas to leak into the otherside. Initially, one side of the piston contains 1 m 3 of N 2 gasat 500 kPa and 80°C while the other side contains 1 m 3 of Hegas at 500 kPa and 25°C. Now thermal equilibrium is establishedin the cylinder as a result of heat transfer through thepiston. Using constant specific heats at room temperature,determine (a) the final equilibrium temperature in the cylinderand (b) the wasted work potential during this process.What would your answer be if the piston were not free tomove? Take T 0 25°C.N 21 m 3500 kPa80°CHe1 m 3500 kPa25°CFIGURE P8–1188–119 Repeat Prob. 8–118 by assuming the piston is madeof 5 kg of copper initially at the average temperature of thetwo gases on both sides.8–120E Argon gas enters an adiabatic turbine at 1500°Fand 200 psia at a rate of 40 lbm/min and exhausts at 30 psia.If the power output of the turbine is 95 hp, determine (a) theisentropic efficiency and (b) the second-law efficiency of theturbine. Assume the surroundings to be at 77°F.8–121 In large steam power plants, the feedwater isfrequently heated in closed feedwater heaters,which are basically heat exchangers, by steam extracted fromthe turbine at some stage. Steam enters the feedwater heaterat 1 MPa and 200°C and leaves as saturated liquid at thesame pressure. Feedwater enters the heater at 2.5 MPa and50°C and leaves 10°C below the exit temperature of the5 MWSteamfromturbine1 MPa200°CFIGURE P8–11630 kPax = 97%Feedwater2.5 MPa50°C8–117 One ton of liquid water at 80°C is brought into awell-insulated and well-sealed 4-m 5-m 6-m room initiallyat 22°C and 100 kPa. Assuming constant specific heatsfor both the air and water at room temperature, determine(a) the final equilibrium temperature in the room, (b) theexergy destruction, (c) the maximum amount of work thatcan be produced during this process, in kJ. Take T 0 10°C.Sat. liquidFIGURE P8–121