Heat Engines forward and backward § 18.6–18.7

Heat Engines Divert heat flow

Heat Engine Heat flow can be diverted to useful work As long as entropy still increases ThTh QhQh QcQc W TcTc Q h = W + Q c Q c /T c ≥ Q h /T h

Heat Engine Efficiency One type of performance ThTh QhQh QcQc W TcTc e = W /Q h

Limits to Efficiency e = W /Q h = (Q h –Q c )/Q h = 1 – Q c /Q h now Q c /T c ≥ Q h /T h, so Q c /Q h ≥ T c /T h thus e ≤ 1 – T c /T h

Refrigerators and Heat Pumps push heat upstream

Heat Pump or Refrigerator Heat flows uphill! Still, entropy must increase ThTh TcTc Q h = W + Q c Q h /T h ≥ Q c /T c QhQh QcQc W

Performance of a Heat Pump We want Q h COP = Q h /W ThTh TcTc QhQh QcQc W

Performance of a Refrigerator We want Q c COP = Q c /W ThTh TcTc QhQh QcQc W