## 1 Introduction
As described in the above paragraph, for the low-GWP fluids, although they have an interesting environmental aspect and good performances, these ORC performances in comparison to classical fluids are not always similar or better. This could be an issue in the case of fluid replacement in an existing ORC installation with a necessity to maintain a certain level of performance in electrical production. The intensification of working fluids performances in existing systems is investigated so as to implement low-GWP fluids in existing ORC plants using banned fluids and thus enabling these installations to continue to operate at equivalent levels of performances [2; 19; 62; 63; 64]. One expected and investigated way to improve an existing ORC is to implement a zeotropic mixture as the working fluid in the existing system [6; 19; 62; 63; 65]. A zeotropic mixture is a blend of two or more pure fluids, with the specificity to have a non-isothermal phase-change at constant pressure, an evaporation step, and a condensation step in the ORC; this in contrast to a pure fluid and an azeotropic mixture, whose change from a liquid state to a gaseous state is, reciprocally, performed at constant temperature and constant pressure. This non-isothermal phase-change, also called the temperature glide, could decrease the temperature difference between the working fluids and the heat source and heat sink and thus decrease the irreversibility in ORC exchangers [62]. The recent interest in zeotropic mixtures for ORC systems has produced a very low number of experimental investigations in comparison to refrigeration and heat pump systems, where mixtures have been widely studied. To the best of the authors' knowledge, less than 20 experimental studies are available in the current ORC literature, and, consequently, each of them has been implemented in the available database to be clearly compared. Theoretical and simulation analyses of zeotropic mixtures' impacts in the ORC have provided some trends and assumptions in terms of ORC behaviors and performances. [PERSON] et al. [66] and [PERSON] et al. [67] studied the reduction of irreversibility in hot and cold ORC exchangers. They pointed out that the use of zeotropic mixtures could lead to an optimization of the heat and sink sources according to specific operations of an ORC system and improve the exchanger and global installation performances. [PERSON] et al. [67] and [PERSON] et al. [68] studied the optimal heat source temperature for a zeotropic mixture. [PERSON] et al. [67] developed a statistical approach to validate the proper operation of a zeotropic mixture in an ORC with its critical temperature. This expression depends only on the heat source temperature and the evaporation pinch temperature of the optimal pure fluid designed for the same ORC and which is one of the fluids in the mixture. [PERSON] et al. [68]showed that a zeotropic mixture has an optimal composition according to the temperature heat source, which could be variable if the heat source temperature changes. [PERSON] et al. [69] developed a Figure Of Merit (FOM), arising from [PERSON] et al. [70], which allows for the same ORC performance predictions as the thermal efficiency for an ORC system with only six parameters, while the thermal efficiency in its entire theoretical expression needs 14 parameters. However, the use of a zeotropic mixture needs to be carefully considered due to the possibility of shift [71; 72] and fractionating [73], which lead to a variation in the mixture's composition inside the ORC and could change and decrease the ORC's performance. Another possible drawback with the use of zeotropic mixtures is the variation of the heat transfer coefficients. This variation could lead to an increase in the required exchangers' heat transfer area to keep the ORC's performances comparable to those of pure fluids [62]. The available ORC zeotropic mixture literature deals with blends with only classical fluids or classical and low-GWP fluids to the best of the authors' knowledge.