A variety of refrigerants are available for usage in mechanical heat pumps. With each of them having their own advantages and disadvantages the choice of refrigerant depends on several criteria.
In general the following criteria must be taken into account:
Pressure: At a given temperature the condensation pressure is different for different refrigerants. For certain refrigerants at high temperatures, pressure will become too high and normal heat pump components can no longer be applied. Low pressure is another risk: for low pressures the volume that needs to be swept increases. This requires larger components and thus an increase in investments. The figure below shows the temperature of evaporation as a function of pressure for several commonly used refrigerants.
At the download-page fysical properties of several commonly used refrigerants can be downloaded.
Critical temperature: Above a certain temperature a refrigerant reaches its supercritical area. Within the supercritical range the fluid and gaseous phase of the refrigerant can no longer be distinquished.
Energy efficiency: The efficiency of a heat pump depends on the choice of refrigerant.
Natural versus synthetic refrigerants: Most synthetic refrigerants (HFC's) contribute strongly to the greenhouse effect in case of leakage. This impact can be 3000 times higher as compared to CO2.
Besides the criteria mentioned above, several other factors are also involved in decision making. Investment costs, required size of the installation and safety and permits have to be taken into account when deciding what refrigerant to use.
All refrigerants are denoted by a code. The code is started by a letter 'R' (Refrigerant) and followed by a number. From this number the following properties can be deduced:
R000-R399: Chemical refrigerants of which the composition is determined by their code number. General code: Rxyz= R(number of C atoms-1)(number of H atoms -1)(number of F atoms). R134, for example, consists of two C atoms, 4 H atoms and 4 F atoms: C2H2F4.
R400-series: Zeotrope mixtures of refrigerants that do not have a evaporation point but rather a evaporation range. Evaporation takes place not on a fixed temperature but during the increase of temperature with a few degrees.
R500-series: Azeotrope mixtures with a fixed evaporation point.
R600-series: Other organic refrigerants.
R700-series: Inorganic refrigerants.
Refrigerants for heat pumps
Below, several frequently used refrigerants in heat pumps are described.
R134a is used as a refrigerant for medium sized or large heat pump systems. As compared to refrigerants R407c and R410a its efficiency is much higher. However, as compared to refrigerant NH3 its efficiency is lower. The pressure in R134 is fairly low. Due to this the volume that needs to be swept by the compressor is rather high and therefore higher investments are needed for installation.
R407c and R410a are frequently used in small to medium sized heat pump sytems. These refrigerants are regularly used in installations that have the dual function of both air conditioning and heating. R410a is supercritical above a temperature of 71 °C. Above this temperature a difference betweeen gaseous and liquid phase can no longer be made. R410a can be applied in low temperature heat pump systems. As compared to R134a the volume that can be swept by the compressor is smaller; implying lower investement costs. On the other hand: its efficiency as compared to R134a is lower as well.
R600 (butane) and R600a (isobutane) are used for refrigeration installations. These refrigerants are also suitable for use in a heat pump, especially for temperatures higher than 80 °C. Many refrigerants give high pressures at these temperature levels. For R600 and R600a the increase of pressure with temperature is much lower. Because of a fire and explosion hazard, installations with R600 and R600a should meet safety requirements of NPR-7600.
R717 (Ammonia) is the most suitable refrigerant for usage in a heat pump in industrial environments. Ammonia has a high efficiency and can easiliy be applied below temperatures of 80 °C. Expectations are that this range will soon be widened up to temperatures of 90 °C. Furhtermore, Ammmonia is a natural refrigerant that does not contribute to the greenhouse effect. Ammonia has been frequently used as a refrigerant in refrigeration systems. The experience and knowledge that has been built up due to this, can be used when installing heat pumps with Ammonia as a refrigerant. It is inflammable and toxic but due to its strong odor, leakages can be detected fast. An Ammonia installation should meet the requierements of PGS-13.
R744 (CO2) is another natural rerfigerant. It is used in refrigeration installations, often in combination with Ammonia compressors in order to decrease the amount of Ammonia in the system. For low temperature freezer applications a system can be built up of CO2 compressors combined with Ammonia. For CO2 the transcritical temperature is 31 °C. Above this temperature condensation does not take place at a constant temperature, instead it occurs over a temperature range (temperature glide). Therefore the heat pump can only be applied in situations where a potential user allows for heating at non-constant temperature. Water is a suitable user. Some heat pumps do function above the supercritical temperature: CO2 supercritical heat pumps.
R718 (Water) can also be used as a refrigerant. Water has the advantages that it is easily obtained and does no damage to the environment. It can only be applied at high temperatures (> 100 °C). At these temperatures pressures in other refrigerants can become too high. A disadvantage of the use of water is its low density in the gaseous state. A relatively high compressor capacity is nescessary because to this.