# Selected Aspects of Designing Modular PEMFC Stacks as Power Sources for Unmanned Aerial Vehicles
## 3 Results
### Electrical Tests of the Designed Two-Module 2-kW PEMFC Stack
In this configuration, modules (1) and (2) were characterized by the same cooling conditions and were cooled more evenly than in Figure 10a; accordingly, this cooling configuration was the preferable solution. Figure 11 presents the U-I and P-I curves recorded for the designed 2-kW PEMFC stack, assembled from two modules, where cooling air flowed through modules (1) and (2) according to situation from Figure 10a. Direct comparison of U-I and P-I curves recorded for modules operated as independent components of the 2-kW PEMFC stack (Figure 6) with those recorded for the same modules where cooling air flows through the two modules (1) and (2) arranged in series indicated that in the situation presented in Figure 10a, the level of electrical performance (Figure 11a,b) is slightly lower. A lower value of maximum electrical power P was reached for both modules compared to the original experimental data. In this situation the total power recorded for the two-module PEMFC stack was less than 2000 W.  
Figure 10: Cooling air flow through modules (1) and (2) arranged in (**a**) series or (**b**) parallel. Figure 12a,b presents the electrical performance and temperature distribution T\({}_{1}\) and T\({}_{2}\) recorded under an applied electrical load of I = 48 A for PEMFC stack modules (1) and (2) connected in parallel, where the cooling air flows through modules (1) and (2) arranged in series (situation from Figure 10a). Figure 12a shows the current waveforms recorded for PEMFC modules (1) and (2), electrically connected in parallel, operating under an electric load of I = 48 A. Based on Figure 12a, waveforms of different currents can be observed in modules (1) and (2), resulting from their different operating conditions. As shown by results from previous analyses of cooling conditions (Figure 10a), a lower operating temperature was recorded for module (1)--T\({}_{1}\) than for module (2)--T\({}_{2}\). After switching on SCU\({}_{1}\) for module (1) and SCU\({}_{2}\) for module (2), momentary interferences can be observed during the measurement of current I and temperature \(t\)-values during self-moistening of the membranes in modules 1 and 2. This phenomenon is caused by changes in internal resistance. After a few seconds, both modules returned to the set operating conditions. Figure 11: (**a**) Voltage (U)-current (I\({}_{\text{m}}\)) and power (P)-current (I\({}_{\text{m}}\)) curves recorded for individual fuel cell modules (1) and (2) which are electrically connected in series (situation from Figure 10a); (**b**) Voltage (U)-current (I) and power (P)-current (I) curves recorded for power source 2-kW PEMFC stack consisting of two fuel cell modules (1) and (2) which are electrically connected in series (situation from Figure 10a). Figure 9.0 _Cont._  
Figure 12b presents an increase in voltage U from 31 to 35 V for modules (1) and (2) connected electrically in parallel, caused by switching on SCU\({}_{1}\) and SCU\({}_{2}\), which is the result of improved self-moistening of the electrolyte. The increase in voltage at a constant load of I = 48 A also resulted in a corresponding increase in total output P from 1480 to 1670 W.  
1.1 Comparison of Electrical Performance of the Designed Two-Module PEMFC Fuel Cell Stack with Other Commercial PEMFC Fuel Cell Stacks  
To evaluate the performance of our designed fuel cell stack, we compared our results with those from other PEMFC fuel cell stacks with the same electrical power. Figure 13 presents the U-I and P-I curves recorded for two different 2-kW PEMFC stacks, one constructed from two modules, (1) and (2), 2 \(\times\) 1 kW 2M (green points) connected in parallel; other commercial fuel cell stacks, HG (blue points) and BCH (red lines), characterized by power of 2 kW in one module, have been added for reference. Based on an analysis of Figure 13, it can be concluded that the expected increase in current due to parallel electrical connections was observed in a comparison of current for single modules (1) and (2).