Thermodynamics

294 | ThermodynamicsSystem boundary·W net,outBOILERResistance heaterPUMPTURBINEGENERATORFIGURE 6–28A perpetual-motion machine thatviolates the first law ofthermodynamics (PMM1).CONDENSER·Q outConsider the steam power plant shown in Fig. 6–28. It is proposed to heatthe steam by resistance heaters placed inside the boiler, instead of by theenergy supplied from fossil or nuclear fuels. Part of the electricity generatedby the plant is to be used to power the resistors as well as the pump. Therest of the electric energy is to be supplied to the electric network as the network output. The inventor claims that once the system is started, this powerplant will produce electricity indefinitely without requiring any energy inputfrom the outside.Well, here is an invention that could solve the world’s energy problem—ifit works, of course. A careful examination of this invention reveals that thesystem enclosed by the shaded area is continuously supplying energy to theoutside at a rate of Q . out Ẇ net,out without receiving any energy. That is, thissystem is creating energy at a rate of Q . out Ẇ net,out , which is clearly a violationof the first law. Therefore, this wonderful device is nothing more thana PMM1 and does not warrant any further consideration.Now let us consider another novel idea by the same inventor. Convincedthat energy cannot be created, the inventor suggests the following modificationthat will greatly improve the thermal efficiency of that power plantwithout violating the first law. Aware that more than one-half of the heattransferred to the steam in the furnace is discarded in the condenser to theenvironment, the inventor suggests getting rid of this wasteful componentand sending the steam to the pump as soon as it leaves the turbine, as shownin Fig. 6–29. This way, all the heat transferred to the steam in the boiler willbe converted to work, and thus the power plant will have a theoretical efficiencyof 100 percent. The inventor realizes that some heat losses and frictionbetween the moving components are unavoidable and that these effectswill hurt the efficiency somewhat, but still expects the efficiency to be noless than 80 percent (as opposed to 40 percent in most actual power plants)for a carefully designed system.Well, the possibility of doubling the efficiency would certainly be verytempting to plant managers and, if not properly trained, they would probablygive this idea a chance, since intuitively they see nothing wrong withit. A student of thermodynamics, however, will immediately label this