FCS Characterization Energy Losses in Steady State Conditions

3 FCS Characterization Energy Losses in Steady-State Conditions 

The Fig. 6.4 shows the characteristic curves of the stack, in terms of voltage and power versus stack current. The stack output voltage decreases from 32 V, at low load, to about 22 V, at the highest load tested (115 A), when the stack power reaches the peak value of 2.5 kW. The polarization curve presents a linear relationship between voltage and current in the working range of 10-90 A, with a voltage decrease from 27 to 24 V. 

The Fig. 6.5 shows the relation between R and the stack power, which is used for all the tests reported in this chapter. This figure evidences that the selected air management strategy determines R values decrease from R = 9 at open circuit to about R — 2 for a stack power between 500 and 2000 kW. These values of R are specifically selected to minimize the air compressor energy losses (see Sect. 4.3). 

The voltage data of Fig. 6.4 are presented also in terms of stack efficiency curve in Fig. 6.6, where t/stack is plotted against the electric power produced by the FCS and measured at the DC-DC converter inlet. In the same hgure, the actual and theoretical curves of FCS efficiency (j/fcs) are also reported, they refer to the real consumptions of ancillary components or to the ideal losses expected for that same system after optimization of all individual components (maximum FCS efficiency in Fig. 6.6). The stack efficiency varies from 0.7 at 0.1 kW to 0.56 at 

8 kW of FCS power, while actual FCS efficiency does not result higher than 50%, due to the different sources of energy loss inside the system. 

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