## 3 First Plant Modeling and Choice of the Working Fluid
### Choice of Working Fluid  
As it is well known, the convenience of using systems based on ORC cycles lies primarily on favorable characteristics of some organic liquids in comparison with water, i.e., larger molecular mass, lower critical temperature, lower critical pressure, lower condensation entropy and lower solidification temperature [30; 31; 32]. In fact, the feature that makes water unsuitable to use in applications of small size and low maximum temperature of the cycle, is its large entropy of evaporation, compared with that of organic fluids. By using organic fluids, it is possible to operate with higher mass flow rates and lower expansion pressures, achieving a power output higher than that of a steam cycle working with the same conditions. In the present study four different pure substances are compared: benzene, cyclohexane, octa-methylsiloxane (MDM) and toluene. Cyclohexane, benzene and toluene are selected as working fluids because they possess efficient thermal performance at high temperature, favorable environmental characteristics and high decomposition temperatures [33]. MDM and toluene have already found industrial application in ORC cycles [34; 35; 36] with temperature of the heat source in the range 300-400 \({}^{\circ}\)C and electric power output greater than 100 kW. Different kinds of fluids are currently studied and used for lower temperatures of the heat source and small size of the system. In particular some studies consider the option of using refrigerant for waste heat recovery from small size ICEs [37; 38; 39]. It is worth noting that, for the typical hot source temperature of this application, the fluids considered in the present study (in particular toluene and MDM) always obtain efficiencies close to the maximum achievable value [34], with small gaps with respect to the best performing fluid, if any. Obviously, different fluids are better suited for lower hot source temperature, like the isobutene introduced in the Two-pressure-levels recovery system, presented further on in this paper. First, a simple Rankine cycle, according to the scheme shown in Figure 3, has been simulated with the Aspen Plus(r) code considering each one of the four organic fluids. In order to assure safe operation on board of the ships, thermal oil is used as intermediate media between the exhaust gasses and the organic fluid. The exhaust gasses flow rate and temperature are 9.2 kg/s and 400 \({}^{\circ}\)C, respectively at 100% of the nominal load and they change with load up to 5.3 kg/s and 442 \({}^{\circ}\)C, respectively at 50% of the nominal load [5]. The thermal oil is heated in the heat exchanger at the left hand side in Figure 3, which is also shown as \"Exhaust Gas Recuperator (optional)\" at the top of Figure 2. In this preliminary evaluation, nominal conditions only have been considered and the temperature and mass flow rate of the thermal oil have been fixed, jointly with the maximum and minimum pressures regarded as acceptable for the cycle, in order of obtaining a homogeneous comparison of the four fluids. In the detailed design of the heat exchangers (Section 5), as well as in the off-design analysis of the ORC regenerated cycle Section 6), these limitations will be relaxed. The following operating conditions and mass flow rates are set for all the organic working fluids, consistently with the hypothesis of recovering energy only from the exhaust gas of the engine Wartsila 6L50 DF:  
* temperature of hot source (thermal oil Dowtherm-G(r)): 350 \({}^{\circ}\)C;
* mass flow rate of hot source (thermal oil Dowtherm-G(r)[40]): 5 kg/s;
* maximum evaporating temperature of the organic fluid: 250 \({}^{\circ}\)C;
* maximum pressure of the organic fluid: 15 bar;
* minimum condensing pressure of the organic fluid: 0.3 bar;  
The final cold sink is the sea water. Considering a possible intermediate circuit, a precautionary value of 38 \({}^{\circ}\)C is assumed for the cooling fluid of the condenser of the energy recovery system.