Chapt. 21 Thermodynamics III: Second Law and Entropy 171 form the cold bath as the reversible frig. Prove that the net system violates the 2nd law of thermodynamics and hence is impossible. 021 qfull 00300 2 3 0 moderate math: eFciency and waste heat 3. A power plant extracts energy from steam at 250 ◦ C and delivers 800 MW (megawatts) of electric power. It rejects waste heat to a river at 30 ◦ C. The overall eFciency is 0 . 28. a) What is the ideal Carnot eFciency for a heat engine operating between the two heat baths, steam and river? HINT: Remember to use Kelvin temperatures. b) Using the ACTUAL eFciency and the general expression ε = W/Q hot bath , what is the total power extracted from the steam: i.e., delivered power plus waste-heat power? c) What is the actual waste-heat power discharged to the river? d) How many houses requiring 10 4 W each to heat could be heated by the waste heat if it were somehow accessible. 021 qfull 00400 3 5 0 tough thinking: entropy always increases
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