Heat pumps (refrigerators): The reverse of heat engines
A heat pump is a device that uses mechanical energy to pump heat from a cold to a hot reservoir.
If the heat engine is run in reverse, one gets a heat pump.
In an analogous way, the heat pump must carry some working substance (usually a liquefiable gas) through a cyclic process, during which
- mechanical work is done on the working substance (Win),
- the working substance absorbs heat (Qc) from a cold reservoir, and
- discharges heat (Qh) to a hot reservoir to return to the initial state, and the cycle repeats.
Examples of heat pumps: Refrigerators, air-conditioners.
Schematic of a refrigerator
Refrigerators and aircons use fluorinated hydrocarbons such as Freon-12, Freon-22, etc, which can be compressed to a liquid by mechanical work. The liquid is pumped into the cold reservoir to be kept cool and allowed to evaporate to the gas state. The liquid thus absorbs heat of vaporization in the process. This heat of vaporization is then dumped into the hot reservoir outside the refrigerator where the gas is compressed back into the liquid state.
A few mechanical details of a heat pump
- The compressor delivers a gas, typically CCl2F2 or another member of the “Freon” family, at high temperature and pressure to condenser coil B.
- Heat is removed from the gas by water or air cooling, resulting in condensation to a liquid under high pressure.
- The liquid passes through the throttling valve to emerge as a mixture of liquid and vapor at a low temperature.
- In evaporator coil D, the liquid absorbs heat and evaporates into the vapor, which enters the compressor and repeats the cycle.
The cooling function is exploited in refrigerators and air-cons, while the heating function is exploited in heat pumps.
Schematic of a heat pump in cooling mode (aircon)
Schematic of a heat pump in heating mode
Schematic of a home air-con (combined unit)
Why use heat pumps?
Heat pumps make better use of electrical (and other forms of) energy because they can use one unit of electrical energy to transfer more than one unit of heat from a cold to a hot region.
- If you use an electric resistance heater, you can only heat up your house (or your water) with 1 kWh of heat for every kWh of electrical energy.
- If you use a heat pump instead, you can use the same 1 kWh of electrical energy to “pump” typically 3 – 4 kWh of heat (given by the coefficient of heating) from the cooler outside surroundings into your house (or your hot water) for heating. So, with 1 kWh of electrical power, you can get 4 – 5 kWh of heat into your house.
Next: Heat Engines – Thermal Efficiency
Thanks for the clear explanation! It’s amazing how heat pumps can provide so much more efficiency compared to traditional heating methods.
Heat is removed from the gas by water or air cooling, resulting in condensation to a liquid under high pressure.
This was such a clear and engaging explanation—thank you! I especially liked how you broke down the heat pump cycle with diagrams and real-world examples like refrigerators and air-cons. It made the concept so much easier to grasp. Keep up the great work—you’re making science accessible and fun!
This was such a well-written and informative post! 🌟 You’ve explained the concept of heat pumps so clearly, especially the comparison to reversed heat engines—it really helped me understand the underlying mechanics. The diagrams and practical examples like refrigerators and air conditioners made it all come together perfectly. Thank you for taking the time to break it down so thoughtfully. Please keep writing more content like this—you’re truly gifted at making complex topics feel approachable and interesting!
This article gives a great explanation of how heat pumps work and why they’re more efficient than traditional heating methods. The diagrams and breakdowns make it easy to follow, even for non-engineers. Really appreciate the clear insights into how energy can be used more wisely—thank you for sharing this!
Heat pumps, like refrigerators, operate as the reverse of heat engines by transferring heat from a colder region to a warmer one using external work. Instead of converting heat into work, they use work to move heat, making them essential for cooling and heating applications in homes and industries.
Great article! Heat pumps are an excellent energy-efficient solution for both heating and cooling. Their ability to transfer heat rather than generate it makes them a cost-effective and environmentally friendly option. You did a great job explaining how they work—very informative
This article provides a clear and concise explanation of how heat pumps function as the reverse of heat engines. The breakdown of working principles, refrigerant cycles, and efficiency benefits is highly informative. The comparison with electric resistance heaters highlights the advantages of heat pumps effectively. Thanks for sharing this valuable knowledge!
Heat pumps, like refrigerators, operate as the reverse of heat engines. They transfer heat from a colder area to a warmer one using mechanical work. While heat engines convert heat into work, heat pumps use work to move heat against its natural direction, cooling spaces effectively.
This article offers an insightful explanation of how heat pumps work and highlights their efficiency in utilizing electrical energy. The breakdown of both the cooling and heating modes, as well as the comparison with electric resistance heaters, is particularly helpful for understanding their energy-saving benefits. Thank you for making this complex topic easy to grasp!