Refrigerant types

In the present study, refrigerants from different chemical groups are included. The LCA comprises hydrofluorocarbons, hydrocarbons, hydrofluoroethers, dichloromethane, and carbon dioxide. The METEOR study includes additionally mixtures of hydrofluorocarbons and ammonium. In the following paragraphs, the different groups will be shortly introduced.

Chlorofluorocarbons and hydrochlorofluorocarbons

Chlorofluorocarbons (CFCs) are hydrocarbons whose hydrogen atoms are completely replaced by chlorine or fluorine atoms. They have a high ODP and GWP due to their very long atmospheric lifetimes (Destatis 2006b).

Hydrofluorochlorocarbons (HCFCs) are hydrocarbons whose hydrogen atoms are only partially replaced by chlorine or fluorine atoms. In general, their ODPs are much lower than those of CFCs. GWPs of HCFCs are also considerably lower but can reach still relatively high values in some cases. The main advantage of HCFCs is the fact that they are decomposed in the troposphere and only a small part of their emissions enters the stratosphere (Destatis 2006b).

Fluorocarbons and hydrofluorocarbons

Perfluorocarbons (PFCs) are entirely fluorinated carbohydrates which contain no chlorine. Hydrofluorocarbons (HFCs) are not completely fluorinated, they contain hydrogen atoms. Both substance classes are relevant to global warming. GWPs of the various substances may differ greatly (Table 31). In contrast to CFCs and HCFCs, fluorocarbons have an ODP of zero (Destatis 2006b).

Hydrocarbons

Hydrocarbons (HCs) are organic compounds that consist solely of hydrogen and carbon. They all contain a carbon backbone with hydrogen atoms attached to it. The structure may be cyclic or aliphatic. The molecular structure of hydrocarbons varies from the simple methane (CH₄) to very heavy and complex molecules. Albeit this class consists of stable and unreactive molecules, the flammability and the ability to react with halons must be regarded (Destatis 2006b).

Blends

Blends are mixtures from two or more compounds, which contain at least one compound that has either an ODP or GWP₁₀₀ greater zero. They gradually replace the restricted CFCs. The values of ODP and GWP₁₀₀ are calculated on the basis of the values of the individual substances of each mixture (Destatis 2006b). For example the GWP₁₀₀ for R410A which consist to 50 % of R32 and R125 is calculated as follows:

GWP100(R410A) = 0.5*GWP100(R32) + 0.5*GWP100(R125) = Equation 1

= 0.5*670 kg CO₂ eq. + 0.5*3450 kg CO₂ eq. =

= 2060 kg CO₂ eq.

Hydrofluoroethers

Hydrofluoroethers (HFEs) are the third generation of CFC-substitutes. They are partly fluorinated ethers. Their physical and chemical properties are similar to those of CFCs and HCFCs, but their hydrogen content results in lower atmospheric lifetimes and lower GWP₁₀₀. They have a zero value of ODP, are low in toxicity, and are non-flammable.

(IPCC 2006, Tsai 2005)

Natural refrigerants

Air and water are natural refrigerants with zero ODP and GWP₁₀₀ values.

Conveniently, HCs, ammonia, and carbon dioxide are considered as natural refrigerants (Devotta et al. 2001, eurammon 2002, eurammon 2005a, eurammon 2005b).

Nomenclature of refrigerants

The chemical names of halogenated refrigerants are often very long. Therefore, a nomenclature system was developed to identify the different substances. It is described in DIN 8960 (1998). The number assigned to each refrigerant is related to its chemical composition and consists of four digits (Table 1). Each digit describes a characteristic of the molecule:

- 1st digit: number of carbon to carbon double bonds (if 0, not given) - 2nd digit: number of carbon atoms minus one

- 3rd digit: number of hydrogen atoms plus one - 4th digit: number of fluorine atoms

Any spare atoms are assumed to be chlorine unless otherwise noted.

For blends of refrigerants another nomenclature system is used. Usually, non-azeotropic mixtures are assigned numbers in the 400 series and non-azeotropic mixtures in the 500 series in order of their commercial introduction. Blends containing the same compounds but different mass proportions of them are distinguished by subsequent capital letters.

Other organic refrigerants, which cannot be identified by the regular numbering system because they contain nine or more hydrogen atoms, are assigned arbitrary numbers in the 600 series. Inorganic refrigerants are allocated to the 700 series. The molecular weight is used prefixed by the number 7. In Table 1 are for some refrigerants the numbers listed which arise from DIN 8960 (1998).

Table 1: Example of the nomenclature of selected refrigerants

Refrigerant type Refrigerant number Formula Refrigerant name

CFC R11 CCl3F Trichlorofluoromethane

HCFC R22 CHClF2 Chlorodifluoromethane

HFC R32 CH₂F₄ Difluoromethane

HFC R125 C2HF5 Pentafluoroethane

HC R290 C3H8 Propane

HC R600 C4H10 Butane

R717 NH3 Ammonia

Blend R407C R32 (50 %), R125 (50 %)

Properties of good refrigerants

The requirements that a substance must meet in order to be considered as a good refrigerant in vapour compression systems include its ability to absorb high amounts of energy by expanding its volume slightly, so that the work done by the compressor remains relatively low ensuring an energy efficient process and a comparable small compressor. This is influenced by the parameters latent heat of vaporisation and specific volume. A good refrigerant should have a high latent heat of vaporisation, which means that the substance can absorb high amounts of energy while changing from its liquid to its vapour phase. Additional, it should have a low vapour specific volume meaning that a determined mass of refrigerant is occupying a reduced space.

Furthermore, a refrigerant should have a low specific heat in its liquid phase but high values in its vapour phase. The former guarantees that the refrigerants needs low amounts of energy while increasing its temperature. This is supporting its vaporisation. The latter means that the vapour can take up large amounts of energy but changes its temperature to a lesser degree. This makes the vapour condensation before approaching the compressor

and the condenser more unlikely.

All the above mentioned properties influence the efficiency of the refrigeration process in thermodynamic terms. Besides those thermodynamic aspects, toxicological and safety aspects should also be considered. Consequently, a refrigerant should favour low toxicity and low flammability as substance intrinsic attributes. Also, cost aspects are of relevance in economical terms.