## 3 First Plant Modeling and Choice of the Working Fluid
### Choice of Working Fluid
The working fluids flow rates have been determined by the standard Evaporator model of Aspen Plus(r)[29], starting from a tentative value (3 kg/s). The main components of the plant (Figure 3) have been modeled on the basis of the hypothesis below:  
* Turbine (3-4): discharge pressure (minimum of the cycle) and isentropic efficiency are set constant. * Condenser (4-1): it allows the condensation of the full amount of the working fluid, with no limits on the maximum temperature reached by the cold fluid. * Circulation pump (1-2): output pressure is set constant (maximum of the cycle). * Evaporator (2-3): the outlet temperature of the working fluid has been fixed (maximum temperature of the cycle); taking the maximum pressure into account, the full amount of the organic fluid may reach the state of dry saturated or superheated vapor. Figure 3: Flowchart of simple ORC circuit. A comparison of the results obtained for the different working fluids is shown in Table 1.  
This preliminary comparison shows that the most promising fluids appear to be benzene and toluene. Toluene in particular allows to raise the evaporation temperature of the cycle at 290 \({}^{\circ}\)C and the maximum cycle pressure to 28 bar. Benzene and cyclohexane have a critical temperature below 290 \({}^{\circ}\)C, whilst MDM has a slightly higher critical temperature (291 \({}^{\circ}\)C) but its adoption is convenient in internal recuperated ORCs, due to the shape of its saturation curves in the T-s plane [41]. For toluene in addition, it is possible to further reduce the condensing pressure to 0.1 bar, thanks to the temperature of cold fluid source (see water), and a good resistance to degradation at high temperature (above 300 \({}^{\circ}\)C) is also experimentally demonstrated for toluene [42]; this fluid is also less toxic than benzene. Then, toluene has been identified as the best working fluid for the considered application. The results obtained in these new conditions are presented in Table 2. The optimization of the mass flow rate of the chosen fluid allows an even higher maximum power of 611 kW to be obtained from the bottoming cycle, corresponding to a toluene mass flow rate of 3.5 kg/s. As previously described, the engine releases also a flow of hot water (the HT cooling circuit) that is an additional opportunity of energy recovery. This heat source is at low temperature (about 90 \({}^{\circ}\)C), so that the fluids considered in Table 1 are not suitable for energy recovery in a Rankine cycle, because they are characterized by high boiling temperatures. In this case it would be appropriate using a low temperature boiling fluid, like isobutene, which has an evaporation temperature equal to \(-\)12 \({}^{\circ}\)C at 1 bar, a T\({}_{\text{cr}}\) = 135 \({}^{\circ}\)C and shows good performance in comparison with other hydrocarbons and refrigerants at maximum cycle temperature of about 90 \({}^{\circ}\)C [43]. Therefore, a simple Rankine cycle with isobutene as the working fluid has been simulated, always according to the scheme in Figure 3. It has been coupled to the HT water circuit of the ICE, replacing the heat exchanger \"HT WHR (optional)\" in Figure 2 with the evaporator of the ORC. The possible output of the latter has been evaluated using a set of thermodynamic parameters consistent with the operating conditions of the engine at maximum load [5]:  
* temperature of HT water circuit (hot source): 91 \({}^{\circ}\)C;
* mass flow rate of HT water circuit: 37.5 kg/s;
* evaporating temperature of the working fluid: 81 \({}^{\circ}\)C;
* maximum pressure of the working fluid: 13 bar;
* condensing pressure of the working fluid: 6.5 bar;
* tentative working fluid flow rate: 7 kg/s;
* temperature of condenser cooling flow: 38 \({}^{\circ}\)C.