Heat Engines forward and backward § 15.8–15.10

Heat Engines Divert heat flow

Heat Engine Qh = W + Qc Qc/Tc ≥ Qh/Th Heat flow can be diverted to useful work As long as entropy still increases

Heat Engine e = W /Qh Efficiency = e One type of performance Th Tc Qh Qc W Tc e = W /Qh Efficiency = e One type of performance

Refrigerators and Heat Pumps push heat upstream

Devices Refrigerator Heat Pump Removes heat from interior, lowering the temperature Goal: keep interior colder than exterior Heat Pump Transfers heat from cold surrounding to warm interior Goal: keep interior warmer than exterior

Heat Pump or Refrigerator Th Qh = W + Qc Qh/Th ≥ Qc/Tc Qh W Qc Tc Heat flows uphill! Still, entropy must increase

Performance of a Refrigerator Th Qh W Qc Tc We want Qc, must provide W COP = Qc/W

Performance of a Heat Pump Th Qh W Qc Tc We want Qh, must provide W COP = Qh/W