A heat pump is a heating and cooling system that moves thermal energy from one place to another instead of generating heat directly. In winter, it pulls heat from outdoor air, ground, or water and delivers it indoors. In summer, the same machine can reverse direction and carry heat out of the building.
This matters because moving heat can use much less electrical energy than making heat with resistance coils.
The system works by circulating a refrigerant through coils, a compressor, an expansion device, and a reversing valve. The refrigerant evaporates at low pressure to absorb heat, then is compressed to a higher pressure and temperature so it can release heat elsewhere. Fans blow air across the indoor and outdoor coils to speed up heat transfer.
The reversing valve changes which coil acts as the evaporator and which acts as the condenser.
Key Facts
- A heat pump transfers heat from a cold region to a warm region using work input.
- Coefficient of performance for heating: COP_heating = Q_hot / W_in.
- Coefficient of performance for cooling: COP_cooling = Q_cold / W_in.
- Energy balance for a heat pump: Q_hot = Q_cold + W_in.
- The compressor raises refrigerant pressure and temperature: higher pressure usually means higher boiling temperature.
- The expansion valve lowers refrigerant pressure and temperature before the evaporator.
Vocabulary
- Refrigerant
- A working fluid that absorbs and releases heat as it changes pressure and phase inside the heat pump.
- Evaporator coil
- The coil where the refrigerant boils and absorbs heat from the surrounding air or source.
- Condenser coil
- The coil where the refrigerant condenses and releases heat to the surrounding air or space.
- Compressor
- A motor-driven device that raises the pressure and temperature of the refrigerant vapor.
- Reversing valve
- A valve that switches the direction of refrigerant flow so the heat pump can heat or cool a building.
Common Mistakes to Avoid
- Thinking a heat pump creates heat like a furnace, which is wrong because it mainly moves existing thermal energy using electrical work.
- Labeling the outdoor coil as always the condenser, which is wrong because in heating mode the outdoor coil usually acts as the evaporator.
- Assuming cold outdoor air contains no heat, which is wrong because air above absolute zero still has thermal energy that can be extracted.
- Confusing efficiency with COP, which is wrong because a heat pump COP can be greater than 1 since it moves heat rather than converting work entirely into heat.
Practice Questions
- 1 A heat pump delivers 9000 J of heat to a room while using 3000 J of electrical work. What is its heating COP?
- 2 A heat pump has COP_heating = 3.5 and uses 2.0 kWh of electrical energy. How much heat energy does it deliver indoors in kWh?
- 3 In heating mode, explain why the outdoor coil can collect heat even when the outside air feels cold.