## 2 Materials and Methods
### Working Fluid Selection  
The choice of the working fluid is an important parameter in the design of an ORC installation. Several studies have examined the determining criteria for selecting the most suitable fluid for an ORC installation [2; 19]. Theoretical efficiency, price, availability, and human and environmental constraints are recurring parameters that are taken into account to select the most appropriate ORC fluid.  
In this study, the described system uses two \"different\" media as the working fluid: a pure fluid and a zeotropic mixture. The pure fluid is the NovecTM649 and the zeotropic mixture is a blend of the previous pure fluid and the pure fluid HFE7000, with a mass proportion of 80% and 20%, respectively, for the mixture.  
The two pure fluids are suggested as possible substitutes for HCFC and HFC [75; 76], and considered as interesting ORC working fluids [20; 56]. The pure fluid NovecTM649 and the mixture are non-toxic, non-flammable, and have a low environmental impact with a null Ozone Depletion Potential (ODP = 0) and a low Global Warming Potential (GWP\({}_{\text{Novec}}\)TM\({}_{649}=1\), GWP\({}_{\text{Novec}}\)TM\({}_{649/\text{HFE/000}}=107\)) as a greenhouse gas. The pure fluid and the obtained mixture are dry fluids, which provide two significant advantages for the installation: it is not necessary to overheat the steam to avoid the formation of droplets within the turbine, the working fluids being in the dry steam state throughout the expansion stage; this results in a lower minimum hot source temperature compared to the case requiring overheating. Due to the low operating pressures (less than 6 bars), the addition of specific and expensive high-pressure protections is not necessary.  
The studied zeotropic mixture has been selected as a blend of NovecTM649 and HFE7000. Different aspects led to the selection of these two pure fluids for the mixture's composition. Some environmental, thermodynamic, and operating interests have already been discussed. Other important criteria guided the fluids' selection. The first criterion was to keep the designed ORC pure fluid, the NovecTM649, as a part of the mixture due to the very good environmental aspect and the good performance suggested in the literature and obtained with preliminary simulations. The second criterion was to implement only low-GWP fluids inside the ORC to search for a solution regarding environmental and replacement fluids. The third criterion was that a zeotropic mixture is not possible between all existing fluids and the blend of NovecTM649 and HFE7000 allows for the creation of this type of specific mixture. The last criterion, but not the least, is the materials' chemical compatibility between the fluid properties and the ORC component materials. The specific mass composition (80%/20%) of the mixture is one of the test compositions of the zeotropic mixture that is currently implemented and investigated inside the ORC. This composition is analyzed in this paper because of the interesting results obtained with it and also to study the ORC's behavior with gradual implementation of HFE7000 in the working fluid's composition. The main properties of NovecTM649, HFE7000, and NovecTM649/HFE7000 are listed in Table 1.