Evaporation Temperature (ºC)

Fig。 6。 2-D plot of output power, working fluid mass flow and cycle efficiency varying evaporator and maximum cycle temperature (condensation temperature 100 ○ C)。

sources, the following criteria have been considered for the selec- tion of the bottoming cycle working   fluid:

-Pressure ratio in the expansion machine must be lower than 25 to achieve an efficient expansion process。

-The studied cycles must keep a minimum temperature differ- ence between the working fluid and the engine heat sources to guarantee minimum irreversibilities in the heat transfer process。 This difference is usually fixed to 10 ○C [31]。 This is called Pinch- Point and it can be presented along with the fluid evaporation process。 This point is usually denoted as “PP” as shown in Fig。 3。

Fig。 4 shows the results (Output Power and Cycle Efficiency) of the  working  fluid  parametric  study。  This  figure  shows  only  the

results obtained for R245fa fluid and water since the other studied organic fluids provide similar results to those of R245fa。 Since the energy used as a  heat  source  in  this  case  is  typically  wasted, the efficiency of the Rankine cycle is a secondary criterion。 Thus, the main goal of the study is to maximize the power   output。

The striped area on the right of the plot illustrates the working points that do not fulfil the Pinch-Point restriction。 In addition, the top striped area shows the zone where the mass flow necessary to maintain the cycle is so low that it does not allow a complete energy recovery for each of the considered heat sources。 Finally, the bottom striped area corresponds to the Rankine cycles in which the expansion process  crosses  below  the  saturated  steam  curve (this area is only represented in the case of water because it has a saturated vapour curve with a negative  slope)。

Fig。 7。 (Top) Temperature heat sources vs transferred heat in Top and Bottom cycle。 (Bottom) Energy scheme of binary cycle configuration。

Steam Cycler Output Power (kW)

Working Fluid mass flow (kg/s)

90    100  110  120  130  140  150  160  170       180

Evaporation Temperature (ºC)

90    100  110  120  130  140  150  160  170       180

Evaporation Temperature (ºC)

90    100  110  120  130  140  150  160  170       180

Evaporation Temperature (ºC)

Fig。 8。 2-D plot of output power, working fluid mass flow and cycle efficiency varying evaporator and maximum cycle temperature (condensation temperature 50 ○ C)。

Table 3 shows the optimum points for each studied working fluid。 A similar work output (around 27 kW) and cycle efficiency (around 6%) are obtained at similar working conditions for almost all investigated organic fluids。 Therefore, the R245fa was selected as the working fluid in ORC with low temperature heat sources, due to its reasonable cycle output work and mild condensation pressure at 50 ○C [32]。

This study becomes an unusual study。 Generally, in conventional ORC studies, the researcher seeks a single heat source with a certain temperature and mass flow to analyze the best Rankine cycle, in order to maximize the obtained power。 But in this study, from different heat sources selected previously (Table 2), which corre- spond to the wasted energy sources of the IC engine, the main objective is to find the Rankine cycle that best fits to these sources。 Considering that some of these sources have the input and output temperatures fixed。 This parametric study was made with a cycle which has a condensing temperature of 40 ○C。 If 70 ○C is considered as the evaporating temperature and the water is superheated to temperatures around 170 ○C (Fig。 4), the water provides better results than the R245fa。 This result is because the R245fa in these conditions cannot be superheated above 82 ○C。 The reason is the different pinch-point restrictions that exist in the heat exchange between the different waste heat sources。 Sometimes, these restrictions do not allow the energy recovering from all heat sources considered in the study (striped zones on top of the graphs in  Fig。  4)。  These  considered  sources  have   been   included in Fig。  5(Top  graphs)  and  they  are  very  different  i。e。:  The thermal