Thermodynamics

80 | ThermodynamicsTABLE 2–1The efficacy of different lightingsystemsEfficacy,Type of lighting lumens/WCombustionCandle 0.2IncandescentOrdinary 6–20Halogen 16–25FluorescentOrdinary 40–60High output 70–90Compact 50–80High-intensity dischargeMercury vapor 50–60Metal halide 56–125High-pressure sodium 100–150Low-pressure sodium up to 20015 W 60 WFIGURE 2–55A 15-W compact fluorescent lampprovides as much light as a 60-Wincandescent lamp.AFUE of some new high-efficiency furnaces exceeds 96 percent, but thehigh cost of such furnaces cannot be justified for locations with mild tomoderate winters. Such high efficiencies are achieved by reclaiming most ofthe heat in the flue gases, condensing the water vapor, and discharging theflue gases at temperatures as low as 38°C (or 100°F) instead of about 200°C(or 400°F) for the conventional models.For car engines, the work output is understood to be the power deliveredby the crankshaft. But for power plants, the work output can be the mechanicalpower at the turbine exit, or the electrical power output of the generator.A generator is a device that converts mechanical energy to electricalenergy, and the effectiveness of a generator is characterized by the generatorefficiency, which is the ratio of the electrical power output to the mechanicalpower input. The thermal efficiency of a power plant, which is of primaryinterest in thermodynamics, is usually defined as the ratio of the net shaftwork output of the turbine to the heat input to the working fluid. The effectsof other factors are incorporated by defining an overall efficiency for thepower plant as the ratio of the net electrical power output to the rate of fuelenergy input. That is,W# net,electrich overall h combustion h thermal h generator HHV m # net(2–43)The overall efficiencies are about 26–30 percent for gasoline automotiveengines, 34–40 percent for diesel engines, and 40–60 percent for largepower plants.We are all familiar with the conversion of electrical energy to light byincandescent lightbulbs, fluorescent tubes, and high-intensity dischargelamps. The efficiency for the conversion of electricity to light can bedefined as the ratio of the energy converted to light to the electrical energyconsumed. For example, common incandescent lightbulbs convert about 10percent of the electrical energy they consume to light; the rest of the energyconsumed is dissipated as heat, which adds to the cooling load of the airconditioner in summer. However, it is more common to express the effectivenessof this conversion process by lighting efficacy, which is defined asthe amount of light output in lumens per W of electricity consumed.The efficacy of different lighting systems is given in Table 2–1. Note thata compact fluorescent lightbulb produces about four times as much light asan incandescent lightbulb per W, and thus a 15-W fluorescent bulb canreplace a 60-W incandescent lightbulb (Fig. 2–55). Also, a compact fluorescentbulb lasts about 10,000 h, which is 10 times as long as an incandescentbulb, and it plugs directly into the socket of an incandescent lamp.Therefore, despite their higher initial cost, compact fluorescents reducethe lighting costs considerably through reduced electricity consumption.Sodium-filled high-intensity discharge lamps provide the most efficientlighting, but their use is limited to outdoor use because of their yellowishlight.We can also define efficiency for cooking appliances since they convertelectrical or chemical energy to heat for cooking. The efficiency of a cookingappliance can be defined as the ratio of the useful energy transferred to