source with higher temperature is the EGR gases, which is cooled from 509 ○C to 222 ○C, and the thermal source with lower temperature is the cooling water, which is typically cooled in the car radiator from 85 ○C to 80 ○C, although this source gives more power (205 kW), due to high water flow and its specific heat。 Due to these peculiarities of the considered heat sources, the optimal working fluid obtained does not correspond to an organic fluid, as initially could be expected。

However, due to the low temperature difference between the vaporizer and condenser, imposed by the initial restrictions, the achieved work output is not significant compared to the total waste heat given in Table 2。 Fig。 4 shows how the cycle using R245fa has better efficiency and output power than water in the low evapo- ration temperature zone。 On the contrary, using water as a working fluid permits complete energy recovery at higher superheater temperature and also gives higher efficiency and power output。 The main drawback is the low evaporation and condensation pressures when the water cycle operates with a  temperature  lower than 100 ○C。

Fig。 5 shows that the Rankine cycle has a significant effect on total power compared with the Reference engine system (311 kW of the engine plus 31 kW provided by the cycle)。 This increment is equivalent to a 10% of power increment over the reference configuration。 One of the biggest drawbacks of this configuration would be the important increase in the total heat transfer processes (868 kW vs 357 kW in the reference engine configuration) and consequently in the size of the heat  exchangers。

Fig。 9。 (Top) Temperature heat sources vs transferred heat in the configuration with high temperature heat sources。 (Bottom) Energy scheme cycle configuration。

5。Configuration with all heat sources。 Binary cycle

In the previous section, R245fa and water have been initially selected as the best solution for low and high evaporation tempera- tures respectively。 For this reason, the use of two coupled cycles (binary cycle) tries to obtain the maximum power from all the considered heat sources。 The iterative-parametric study is performed in order to accomplish these criteria。 The fluid tables are used in order to obtain the optimal combination of: maximum temperature cycle, evaporation and condensation temperature and mass flow。

Fig。 6 shows the results for the studied water cycles considering the maximum temperature and the evaporation temperature of the

cycle as the independent variables。 The condensation conditions have been fixed to 100 ○C and 1 bar due to technological reasons。

The optimal solution needs an agreement between maximum temperature and the working fluid mass flow to obtain the greatest power output。 The top left graph in Fig。 7 represents the different heat transfer processes taking place in the high temperature cycle (Water cycle)。 Heat sources are represented by the thin lines。 The black bold line represents the working fluid evaporation process。 The graph shows that the water is heated from 137 ○C to 220 ○C by the exhaust gas heat source。 At this point, the water evaporates at constant pressure receiving heat from the exhaust gases and the EGR cooler。 The rest of the EGR cooler heat is used for heating  the

steam up to 470 ○C。 The superheated steam will then be expanded through the turbine。

The working fluid used in the low temperature cycle must be an organic fluid。 In the previous section, the R245fa has been selected as the best option for this kind of cycles and it is used in this case。 In

Table 4

Summary results。

Configurations Output power (kW)

Power increment (%)

Total heat transfer process (kW)