1. Heat Engines
forward and backward
§ 15.8–15.10

2. Heat Engines
Divert heat flow

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

4. Heat Engine e = W /Qh Efficiency = e One type of performance Th Tc QhQc W Tc
e = W /Qh
Efficiency = e
One type of performance

5. Refrigerators and Heat Pumps
push heat upstream

6. 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

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

8. Performance of a RefrigeratorTh Qh W Qc Tc
We want Qc, must provide W
COP = Qc/W

9. Performance of a Heat PumpTh Qh W Qc Tc
We want Qh, must provide W
COP = Qh/W