- 2075 Baisakh · 6 marks
Heat engine, heat pump, refrigerator and performance
Short answer
A heat engine receives heat from a hot source, produces net work, and rejects the rest; its performance is thermal efficiency η = W/Q_H. A refrigerator removes heat from a cold space using work input, with COP_R = Q_L/W. A heat pump delivers heat to a warm space using work, with COP_HP = Q_H/W.
Define heat engine, heat pump and refrigerator. Also define factors used to measure their performance.
All three are cyclic devices operating between a high-temperature reservoir (T_H) and a low-temperature reservoir (T_L). They differ in purpose and in the direction of energy flow.
Heat engine
A device that operates in a cycle, receives heat Q_H from a high-temperature source, converts part of it to net work W, and rejects the rest Q_L to a low-temperature sink. Examples: steam power plant, petrol engine.
Hot source T_H
| Q_H
v
[ Heat engine ] ---> W (out)
| Q_L
v
Cold sink T_L
Performance: thermal efficiency η = W_net/Q_H = 1 − Q_L/Q_H. For a reversible engine, η = 1 − T_L/T_H.
Refrigerator
A cyclic device that removes heat Q_L from a low-temperature space (the cold body), using work input W, and rejects Q_H to the surroundings. Its purpose is to keep the cold space cold. Examples: domestic refrigerator, freezer, air conditioner in cooling mode.
Performance: coefficient of performance COP_R = Q_L/W = Q_L/(Q_H − Q_L). For a reversible refrigerator, COP_R = T_L/(T_H − T_L).
Heat pump
A cyclic device that supplies heat Q_H to a warm space (e.g. a room in winter) by absorbing Q_L from a colder source (outside air or ground), using work input W. Its purpose is to keep the warm space warm.
Performance COP_HP = Q_H/W = Q_H/(Q_H − Q_L). For a reversible heat pump, COP_HP = T_H/(T_H − T_L).
Relation
COP_HP = COP_R + 1
| Device | Purpose | Performance measure |
|---|---|---|
| Heat engine | Produce work | Thermal efficiency η = W/Q_H (always < 1) |
| Refrigerator | Remove Q_L from cold space | COP_R = Q_L/W (can exceed 1) |
| Heat pump | Deliver Q_H to warm space | COP_HP = Q_H/W (always > 1) |
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Heat pump and COP; second law analysis · Fundamental of Thermodynamics & Heat Transfer, chapter 5
- Heat pump at 50% of ideal: minimum surrounding temperature · Fundamental of Thermodynamics & Heat Transfer, chapter 5
- Refrigerator COP 4: power input and cost · Fundamental of Thermodynamics & Heat Transfer, chapter 5
- Three-process gas cycle: net work, heat, type · Fundamental of Thermodynamics & Heat Transfer, chapter 4 · asked 2×
- Ideal diesel engine CR 20: efficiency and MEP · Fundamental of Thermodynamics & Heat Transfer, chapter 6
- Spark ignition vs compression ignition engine · Fundamental of Thermodynamics & Heat Transfer, chapter 6
- Heat balance sheet of a diesel engine · Internal Combustion Engines, chapter 8
- Engine performance parameters and efficiencies · Internal Combustion Engines, chapter 8
- Driving factors for performance design · Computer Organization and Architecture, chapter 1
- Factors affecting thermal performance · Building Technology, chapter 1
All Fundamental of Thermodynamics & Heat Transfer chapter 5 questions →