The organic working fluid is the main characteristic of ORCs. The selection of working fluids is an important research content for the studies of ORCs. The general requirements for ORC working fluids include the following [17-20].
(1) Desirable thermodynamic properties. The working fluids can lead to good cycle thermal efficiency and power output.
(2) Good thermal stability and material compatibility. The working fluids cannot decompose in ORC systems and do not react with system materials.
(3) Good environmental properties. The working fluids should have 0 ozone depleting potential (ODP) and small global warming potential (GWP), and meet other environmental requirements.
(4) No safety problems. No or low flammability and toxicity is better for working fluid selection.
(5) Low cost.
There are many studies about the working fluid selection because of its significance for ORCs, and various organics are considered to be suitable ORC working fluids. At the beginning of ORC research, chlorofluorocarbons (CFCs) and aromatic hydrocarbons were the frequently-used working fluids. Because of increasingly stringent environmental and safety requirements, CFCs have been eliminated for ORC working fluids. In recent studies, suitable working fluids include hydrofluorocarbons (HFCs), hydrocarbons (HCs), hydrofluoroolefins (HFOs), and siloxanes, which have different characteristics and apply to different cycle conditions [21-28].
(1) HFCs. HFCs have been widely used as ORC working fluids in some commercial units because they have been well studied in the fields of refrigeration and heat pumps. HFCs have good thermodynamic properties as ORC working fluids. However, some HFCs with high GWP values will be eliminated in the future because of stringent environmental requirements.
(2) HCs. HCs usually have lower GWP than HFCs, which are good environmental properties for ORC working fluids. HCs also have relatively high critical temperatures and can be suitable in a wide heat source temperature range. Thus, HCs, such as butanes and pentanes, have been widely used in some commercial ORC systems. The main disadvantage of HCs is the flammability problem.
(3) HFOs. HFOs are the potential substitute working fluids for HFCs with similar thermodynamic properties and considerably lower GWP. Even though the new HFOs have high costs at present, their development merits more attention.
(4) Siloxanes. Siloxanes are considered to be suitable choices for high temperature ORCs because of their high critical temperatures, low flammability and toxicity, and good thermal stability.
All organics will decompose at a sufficiently high temperature to different extents. If the working fluids decompose in the ORC systems, the operational parameters will change from the design values to affect the system efficiency and power output. The decomposition products may affect the system components, and may even cause safety problems. Thus, the thermal stability of working fluids is the key property for ORC fluid selection.
The importance of working fluid thermal stability has been recognized in initial ORC studies. For example, Curran et al.  and Badr et al.  both mentioned that the thermal stability of working fluids should be considered primarily because of the possible decomposition. Curran et al.  summarized the approximate decomposition temperatures of 20 fluids, including R11, R21, R113, and toluene. Badr et al.  also summarized the approximate decomposition temperatures of some CFCs and HCs. However, these thermal stability results were collected mainly in different research fields with different experimental systems, conditions, and procedures. The decomposition temperatures were called “approximate” because of the large differences among the results for the same fluid. For example, the decomposition temperatures of R12 were 120℃, 204℃, and 300℃ in different literatures by the statistics of Badr et al. The thermal stability studies in other research fields focused on different aims and temperature extents, therefore, the results could not meet the requirements of ORC research.
Even though the thermal stability of working fluids are important for ORCs, there are few specialized thermal stability experiments for ORCs in the initial studies because of the low cycle temperatures during this period. Because of the rising ORC heat source temperatures and more suitable working fluids, studies about the thermal stability of working fluids have become urgent and specialized results have been increasing in recent years.