Domestic heat pumps are devices that can be used for both heating (in the colder months), and cooling (in the warmer months), for residential use. Heat pumps have been used for some time now at industrial scale, in the refrigeration industry, to provide the cooling necessary for the storage of food products (refrigerators, freezers, cold rooms), in fish factories, chicken factories, and so on. Heat pumps are also widely use for their heating purposes, in almost all industries, saving energy costs by reducing the electricity consumption of the facilities.
Since heat pumps have such a great turnover, they became a popular choice for residential buildings, where both heating and cooling (air conditioning) can be provided at a lower cost. Because of the higher COP (coefficient of performance), heat pumps have become a good candidate for heating domestic houses too. Although people have been using heat pumps in their houses for a long time (through their refrigerators and freezers, which are heat pumps in miniature), they still needed to be educated on the working principle and the advantages of heat pumps, when used for heating purposes.
How Does a Heat Pump Work?
The working principle of a domestic heat pump is the same as for a refrigerator, freezer, or air conditioning unit. The heat pump system can be divided in 4 parts:
- The EVAPORATOR – a device where heat is extracted from a room (or environment), via a transfer fluid, which is called a refrigerant. The refrigerant enters this evaporator (which technically is a heat exchanger), as liquid and leaves as vapor. It is very important that the refrigerant responsible for cooling the space leaves as vapor, for the next stage.
- The COMPRESSOR area – here, the refrigerant is compressed, increasing its pressure, and as a result, increasing its temperature. Now, when entering the compressor, the refrigerant needs to be in vapor phase, otherwise it will destroy the compressor. Also, here a temperature lift occurs, which will be used in the next stage.
- The CONDENSER/SUBCOOLER – a device where heat is rejected to the environment or to the place that needs to be warmed up. All the heat that has been gained through compression, in the earlier stage, is now used for heating. Here, the heat can be used to heat up a space (air), or water (water that can be later used for heating a house, or just for daily use). The device it’s called a condenser, because, normally, here the refrigerant condenses, and becomes liquid again (still at higher pressure).
- The EXPANSION area- after leaving the condenser, the refrigerant enters an expansion valve/or a throttling valve where its pressure is reduced. From here, the refrigerant will enter the evaporator part, and a new cycle restarts.
What Is a Refrigerant?
A heat pump needs a working fluid that is able to transfer heat to and from the environment. The refrigerant changes phases inside the heat pump cycle, going from liquid to vapor in the evaporator (producing the “cooling”), and from vapor back to liquid in the condenser part of the system (producing the “heating”). There are many types of refrigerants that can be used, and the main characteristic that needs to be taken into consideration when designing a heat pump, is, of course, the temperature at which it changes phases. For freezers for example, a refrigerant with a lower evaporation temperature is needed in order to ensure the -20 degrees centigrade (Celsius) or lower required. The toxicity of the refrigerant, the flammability risk, and its availability are very important as well. As a home user, you can decide on which type of heat pump to buy, but not on the type of refrigerant that specific heat pump or air conditioner unit comes with. That is because, each heat pump unit is specifically designed for a certain type of refrigerant.
Some of the most commonly used refrigerants are presented in the following table. Due to so many recent climate changes, engineers and scientists have started to consider the Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) when designing new heat pump and air conditioning units. Carbon dioxide (CO2), is a great candidate nowadays, as it has an ODP of 0, and a GWP of 1.
Heat pumps have started to get more and more traction in the past years, as people are more and more aware of the benefits of heating their homes with alternative heating solutions. The sustainability of new technologies developed plays an important role when it comes to the integration and their acceptance for the end users. Natural refrigerant based heat pumps are part of these sustainable heating solutions, that will become more and more popular as the technology is perfect.