Thermodynamics

284 | ThermodynamicsorIt can also be expressed ash th W net,outQ in(6–4)h th 1 Q outQ in(6–5)High-temperature reservoirat T HHEQ LQ HW net,outsince W net,out Q in Q out .Cyclic devices of practical interest such as heat engines, refrigerators, andheat pumps operate between a high-temperature medium (or reservoir) attemperature T H and a low-temperature medium (or reservoir) at temperatureT L . To bring uniformity to the treatment of heat engines, refrigerators, andheat pumps, we define these two quantities:Q H magnitude of heat transfer between the cyclic device and the hightemperaturemedium at temperature T HQ L magnitude of heat transfer between the cyclic device and the lowtemperaturemedium at temperature T LNotice that both Q L and Q H are defined as magnitudes and therefore arepositive quantities. The direction of Q H and Q L is easily determined byinspection. Then the net work output and thermal efficiency relations forany heat engine (shown in Fig. 6–13) can also be expressed asW net,out Q H Q LLow-temperature reservoirat T LFIGURE 6–13Schematic of a heat engine.andorh th W net,outQ HFurnaceh th 1 Q LQ H(6–6)HEThe atmosphereQ H = 100 MJW net,out = 55 MJQ L = 45 MJFIGURE 6–14Even the most efficient heat enginesreject almost one-half of the energythey receive as waste heat.The thermal efficiency of a heat engine is always less than unity since bothQ L and Q H are defined as positive quantities.Thermal efficiency is a measure of how efficiently a heat engine convertsthe heat that it receives to work. Heat engines are built for the purpose ofconverting heat to work, and engineers are constantly trying to improve theefficiencies of these devices since increased efficiency means less fuel consumptionand thus lower fuel bills and less pollution.The thermal efficiencies of work-producing devices are relatively low.Ordinary spark-ignition automobile engines have a thermal efficiency ofabout 25 percent. That is, an automobile engine converts about 25 percentof the chemical energy of the gasoline to mechanical work. This number isas high as 40 percent for diesel engines and large gas-turbine plants and ashigh as 60 percent for large combined gas-steam power plants. Thus, evenwith the most efficient heat engines available today, almost one-half of theenergy supplied ends up in the rivers, lakes, or the atmosphere as waste oruseless energy (Fig. 6–14).