Heat Engine vs Refrigerator — Energy Flow in Opposite Directions
Heat Engine
A device that converts heat flowing from hot to cold into useful work — car engines, steam turbines, power plants. The second law caps efficiency below 100%: some heat must always be dumped.
Unit: dimensionless (often %)
Example: A car engine burns fuel (Qh), turns ~25–30% into motion (W), and dumps the rest as hot exhaust and radiator heat (Qc).
Refrigerator / Heat Pump
A device that uses work to move heat from cold to hot — against its natural direction. A fridge pumps heat out of the food compartment into your kitchen; a heat pump warms a house using outdoor air.
Unit: dimensionless
Example: A fridge with COP 4 moves 4 joules of heat out of the food for every 1 joule of electricity — that's why heat pumps beat resistive heaters.
Key differences
| Aspect | Heat Engine | Refrigerator / Heat Pump |
|---|---|---|
| Energy conversion | Heat → work | Work → heat moved cold to hot |
| Purpose | Produce work from fuel heat | Cool one side / heat the other |
| Performance metric | Efficiency η = W/Qh (always < 1) | COP = Qc/W (can exceed 1 — even 4–5) |
| Second-law limit | Carnot cap: η ≤ 1 − Tc/Th | Carnot cap on COP too, but COP > 1 is allowed |
| Heat flow direction | With the natural hot→cold flow | Against it — work pays the price |
A COP above 1 looks like free energy, but nothing is created — the pump is moving existing heat, not making it. Efficiency and COP measure different jobs, so comparing '30% vs 400%' across the two is meaningless.