1. Thermodynamic fluids

All thermal machines have at least one fluid flowing through them, which undergoes changes (heating, cooling, compression, expansion, etc.) and exchanges energy with the outside world. For example, in a steam power plant, this is water; in a gas turbine, it is air and burnt gases; in a refrigerator, it is a refrigerant. This fluid is generically referred to as a thermodynamic or working fluid.

Since the end of the 19th century, thermodynamic fluids have been used both to convert heat into mechanical energy (motor cycles) and to produce refrigeration or heat pump systems (receiver cycles).

Over the past thirty years, two environmental concerns have called into question the use of thermodynamic fluids, mainly those used until recently in refrigeration machines: the depletion of the ozone layer and the increase in the greenhouse effect....