*3.2.4 Environmental criteria*

In general, there are three parameters that we need to consider in the aspect of environmental criteria: ozone depletion potential (ODP), global warming potential (GWP), and atmospheric lifetime (ALT).

ODP represents the degradation effect on the ozone layer in the atmosphere of each fluid, then GWP is a measure of the total energy that fluid gases can trap in the atmosphere relative to the heat absorbed by the same amount of carbon dioxide emission during a given time, whereas ALT represents the potential for the atmospheric accumulation of each chemical. The Montreal Protocol in 1987 proposed the phase out of ozonedepleting substances such as CFCs and HCFC, and the Kyoto Protocol proposed the reduction of greenhouse gases emissions, such as carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Generally, working fluids with zero ODP values, minimal GWP values, and low ALT values are recommended during the selection of working fluids [14].

### *3.2.5 Safety criteria*

In choosing suitable working fluids, the potential operating dangerousness needs to be considered. According to the ASHRAE 34 safety classification standard, the fluids are classified into two toxicity groups A and B and into three flammability groups 1, 2, and 3, as shown in **Table 1** [14]. In general, fluids with nontoxic, nonflammable, and noncorrosive characteristics are preferable; however, in practice, it is hard to meet all the criteria so a relatively satisfactory fluid is sufficient during the selection.


**Table 1.**

*Classification of fluids according to toxicity and flammability.*

All in all, working fluids with high critical temperature, high density, high latent heat, low triple point temperature, low condensation temperature, condensation pressure slightly higher than the atmospheric pressure, zero ODP, low GWP, low ATL, and low risk of toxicity and flammability are expected in terms of system efficiency, economy, environmental protection, and safety.
