**1. Introduction**

According to data analysis, the energy utilization rate of developing countries is only 33%, which is 10–20% lower than that of developed countries [1]; and 60–65% of industrial energy consumption [2] is finally converted into low-grade waste heat with temperatures in the range 50–280°C, which has great potential for energy recovery. Utilizing the considerable number of undeveloped renewable waste heat is an effective means to alleviate energy shortages and achieve carbon neutrality.

The supercritical Rankine cycle, Kalina cycle, and trilateral flash cycle, and Organic Rankine Cycles (ORCs) are used for low-and medium-temperature waste heat recovery. Among them, ORCs are one of the most promising technologies for utilizing lowgrade waste heat. The working fluid is the carrier for energy transfer and conversion in the ORC system. Under the same working conditions, the performance of ORC systems with different working fluids varies widely, thus, working fluid screening has always been an important direction and hotspot in ORC research. There is also the possibility, if the level of heat is low, to combine the production of electricity and heat (cogeneration); depending on the request, we can produce high quality heat using a heat exchanger (coupled with heat pump alimented by ORC for example).

In recent years, an increased scrutiny has been placed on the environmental aspect of working fluids, and regulatory pressures are driving global awareness of their impact on the environment. In 2016, the Kigali Amendment to the Montreal Protocol was adopted [3], whereby committed states must reduce the production of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) according to a set schedule. The ozone depletion potential (ODP) and low global warming potential (GWP) are of particular interest, and emphasis has been placed on choosing a working fluid that demonstrates an ability to meet these climate protection initiatives. The currently used working fluids HFCs, HCFCs, and PFCs will not be evaluated because of their ODP and high GWP concerns, respectively [4]. The HVAC&R (heating, ventilation, air conditioning, and refrigeration) industry has to focus on the search for working fluids with zero (or extremely low) ODP and low GWP [5].

To find suitable low-GWP working fluids, reasonable selection criteria and corresponding knowledge for screening working fluids of ORC system should be provided for industrial applications. This chapter will introduce the working fluids selection criterion and screening methods for eco-friendly working fluids.
