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.