For the identification of the safety level of a working fluid, the ASHRAE 34 safety classification that applies for refrigerants can be used. This classification is based on two key parameters: the toxicity and the flammability. Toxicity can be identified by some numbers such as TLV (Threshold Limit Value). Flammability is generally identified by the LFL (Lower flammability limit) and the HOC (Heat of Combustion). The ASHRAE standard 34 is made of two classes of toxicity (A: non-toxic B: toxic.) and three groups of flammability characteristics (1: no flame propagation, 2: low flammability limit and 3: high flammability limit). The matrix shown in Fig. 2 was adopted for an easy identification of the safety level of any substance. From this classification, the most desirable class for an ORC working fluid is

A1. In addition to the aforementioned desirable safety properties, the working fluid should be non-explosive and non-radio-active.

Fig. 2: ASHRAE 34 Refrigerant Safety Classification