## 6 Organic Rankine Cycle Fluid Selection and System Optimization  
The selection of the appropriate organic fluid for each respective application is a demanding challenge for engineers since the performance and efficiency of the organic Rankine cycle system is seriously affected by the working fluid. However, the properties of the fluid are not the only criteria for selection, as the cost and the environmental impact of each fluid may limit the list of available fluids. As far as the cost is concerned, the engineer should decide which fluid would decrease the payback period and offer the maximum output and thermal efficiency at the same time. The properties of the organic fluids can be divided into four categories, each one of them being equally important for the efficient and safe operation of the ORC system. Thermodynamic properties of the organic fluids vary in several aspects, as do the density, viscosity, boiling point temperature, pressure, and the latent heat of vaporization. Each one of these parameters affect not only the thermal efficiency of the system, but also the design and construction of the respective internal combustion engine configuration. Critical and maximum operating conditions constitute the process-related properties and are linked with the efficiency of the organic Rankine cycle system. As far as the safety and environmental aspects are concerned, the toxicity and flammability of the fluid concerns engineers, while the global warming potential and ozone depletion danger are the major dangers for the environment. The working fluids should compromise among several criteria specified below [10; 11; 20; 21; 25; 30; 31; 32; 33; 34]:  
* Low condensation temperature
* Very low freezing point
* No need to superheat (dry fluid)
* Eco-friendly (low global warming potential and ozone depletion potential)
* Low flammability and toxicity  
Considering all of aforementioned requirements, R245 fa was selected as the working fluid in this study entirely considering prior experience and potential for widespread use. This organic fluid has no ozone impact (ODP), low global warming impact (GWP), it is non-flammable, and its thermodynamic properties fulfill the above criteria. After the selection of the working fluid, the final model was created. In GT-Power software some assumptions were considered and are shown below [2; 12; 13; 18; 22]:  
* The evaporator exhaust gas initial pressure and temperature are equal with the exhaust gas outlet pressure and temperature, respectively.
* The condenser coolant initial pressure and temperature are equal with the coolant outlet pressure and temperature, respectively.
* No heat is lost to the surroundings.  
Figure 3: Ford 2.0 L Zetec-SE DOHC Engine modeled in GT-Power software (7.3b2, Gamma Technologies LLC, Westmont, IL, USA, 2015): (**a**) integrated conventional engine’s components; (**b**) dynamic Hybrid Electric Vehicle (HEV) subsystem integrated with the base engine components.  
The input values for the ORC model, such as exhaust temperature and exhaust mass flow rate, were obtained from a table regarding thirty different cases from 2000 cc engines at several engine speeds and BMEP ranges. Figure 4a illustrates the schematic view of the ORC system modeled in GT-Power. Data obtained included parameters such as the evaporator energy, turbine power, pump efficiency, turbine efficiency, and the pressure rise in the pump. The design parameters of the ORC system used in the simulation are presented in Table 1.