Thermodynamics

1COP R Q H >Q L 1 andCOP 1HP 1 Q L >Q HChapter 6 | 309It is tempting to judge things on the basis of their quantity instead of theirquality since assessing quality is much more difficult than assessing quantity.However, assessments made on the basis of quantity only (the first law)may be grossly inadequate and misleading.6–11 THE CARNOT REFRIGERATOR■ INTERACTIVEAND HEAT PUMPTUTORIALA refrigerator or a heat pump that operates on the reversed Carnot cycle is SEE TUTORIAL CH. 6, SEC. 11 ON THE DVD.called a Carnot refrigerator, or a Carnot heat pump. The coefficient ofperformance of any refrigerator or heat pump, reversible or irreversible, isgiven by Eqs. 6–9 and 6–11 aswhere Q L is the amount of heat absorbed from the low-temperature mediumand Q H is the amount of heat rejected to the high-temperature medium. TheCOPs of all reversible refrigerators or heat pumps can be determined byreplacing the heat transfer ratios in the above relations by the ratios of theabsolute temperatures of the high- and low-temperature reservoirs, asexpressed by Eq. 6–16. Then the COP relations for reversible refrigeratorsand heat pumps becomeandCOP R,rev 1T H >T L 1(6–20)1COP HP,rev (6–21)1 T L >T HThese are the highest coefficients of performance that a refrigerator or aheat pump operating between the temperature limits of T L and T H can have.All actual refrigerators or heat pumps operating between these temperaturelimits (T L and T H ) have lower coefficients of performance (Fig. 6–51).Warm environmentat T H = 300 KReversiblerefrigeratorCOP R = 11IrreversiblerefrigeratorCOP R = 7ImpossiblerefrigeratorCOP R = 13Cool refrigerated spaceat T L = 275 KFIGURE 6–51No refrigerator can have a higher COPthan a reversible refrigerator operatingbetween the same temperature limits.