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S E C T I O N 2 2 . 4 • The Carnot Engine 677 Hence, the thermal efficiency of a Carnot engine is (22.6) This result indicates that all Carnot engines operating between the same two tem- peratures have the same efficiency. 5 Equation 22.6 can be applied to any working substance operating in a Carnot cycle between two energy reservoirs. According to this equation, the efficiency is zero if c ! T h , as one would expect. The efficiency increases as T c is lowered and as T h is raised. However, the efficiency can be unity (100%) only if T c ! 0 K. Such reservoirs are not available; thus, the maximum efficiency is always less than 100%. In most prac- c is near room temperature, which is about 300 K. Therefore, one usually strives to increase the efficiency by raising T h . Theoretically, a Carnot-cycle heat engine run in reverse constitutes the most effective heat pump possible, and it determines the The Carnot COP for a heat pump in the cooling mode is As the difference between the temperatures of the two reservoirs approaches zero in COP C (cooling mode) ! T c T h " T c ! ! Q
h ! ! Q
h ! " ! Q
c ! ! 1 1 " ! Q
c ! ! Q
h ! ! 1 1 " T c T h ! T h T h " T c COP C (heating mode) ! !Q
h
! W e
C ! 1 " T c T h 5 In order for the processes in the Carnot cycle to be reversible, they must be carried out infinitesimally slowly. Thus, although the Carnot engine is the most efficient engine possible, it has c and T h , and is given by e C - A ! 1 " (T c /T h ) 1/2 . The Curzon–Ahlborn efficiency e C-A provides a closer approximation to the efficiencies of real engines than does the Carnot efficiency. Efficiency of a Carnot engine Quick Quiz 22.4 Three engines operate between reservoirs separated in temperature by 300 K. The reservoir temperatures are as follows: Engine A: h ! 1 000 K, T c ! 700 K; Engine B: T h ! 800 K, T c ! 500 K ; Engine C: T h ! 600 K, T c ! 300 K. Rank the engines in order of theoretically possible efficiency, from highest to lowest. |