The same refrigeration cycle can cool a cold store, heat a building, or do both. What matters is where heat is absorbed and where it is released.
The same refrigeration cycle can cool a cold store, heat a building, or do both. What matters is where heat is absorbed and where it is released.
A heat pump does not create heat from nothing. It transfers thermal energy from a lower temperature level to a higher one by means of a refrigerant.
The refrigerant evaporates at low pressure and absorbs heat from its surroundings. The compressor raises the refrigerant pressure and temperature. At the higher pressure, the refrigerant can condense and release the heat it carries, for example to heat a building.
The refrigerant is the heat-transfer medium that circulates through a refrigeration or heat-pump cycle.
A refrigerant is a substance that circulates in a closed refrigeration or heat-pump system and carries heat energy from one place to another. Examples include carbon dioxide (R744), propane (R290) and various fluorinated refrigerants. Different refrigerants have different pressure, temperature and other properties.
In the refrigeration cycle, the refrigerant changes state between liquid and vapour. As it evaporates it absorbs heat energy, and as it condenses it releases heat. This phase change is fundamental to the refrigeration process.
Learn more: phase changes and the energy associated with them are covered in more detail in the dedicated Phase changes section.
The refrigerant acts as the heat-transfer medium.
Low pressure gives the refrigerant a low boiling temperature. The refrigerant boils and absorbs heat from the ground, outdoor air or the space being cooled.
The vapour is compressed to a higher pressure, raising its temperature. The compressor uses electrical energy to do this.
At high pressure, the refrigerant condenses at a higher temperature and releases thermal energy to a building, water or outdoor air.
An expansion device, such as an expansion valve or capillary tube, reduces the refrigerant pressure. As the pressure drops, part of the liquid flashes into vapour and the refrigerant cools before entering the evaporator.
Fluid circulating in the ground loop collects heat from the ground. The heat-pump refrigerant receives this heat in the evaporator, and the compressor raises the temperature level to one suitable for heating the building.
The ground is the heat source – the building is the heat sink.
In a cold store, the evaporator absorbs heat from inside the store. The condenser or gas cooler rejects this heat to the surroundings.
From a thermodynamic point of view, it is still a heat pump: it pumps heat from a colder space to warmer surroundings.
Even sub-zero outdoor air contains thermal energy. The refrigerant evaporates in the outdoor unit at a sufficiently low temperature for heat to flow from the outdoor air into the refrigerant.
After compression, the higher pressure allows heat to be released in the indoor unit at a temperature higher than the room temperature.
A refrigerator uses the same refrigeration cycle as large heat pumps. The evaporator inside the cabinet operates at low pressure: the refrigerant boils and absorbs heat from the air and food inside the refrigerator.
The refrigerant is the circulating fluid that transports heat. It absorbs heat as it evaporates and releases heat as it condenses. Refrigerants used in domestic refrigerators have changed over the decades: R12 was once common, followed by widespread use of R134a, while many modern domestic refrigerators use R600a, or isobutane.
The black compressor in the image raises the refrigerant pressure and temperature. The condenser, shown in red at the back or sides of the cabinet, then releases heat to the room.
This is why the back or side of a refrigerator feels warm. The room receives the heat transferred from inside the refrigerator + the electrical energy used by the compressor.
The refrigerant phase-change temperature depends on pressure. Lower evaporating pressure makes the refrigerant boil at a lower temperature. When the compressor raises the pressure, the refrigerant can condense at a higher temperature.
The condenser releases more heat than the evaporator absorbed from the surroundings because the compressor’s electrical input is added to the energy flow.
That is why the back or side of a refrigerator feels warm: it releases to the room the heat transferred from inside the refrigerator + the energy used by the compressor.
The heat released by the condenser consists of the thermal energy transferred through the evaporator plus the work done by the compressor.