# 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
The operation of the PEMFC module under a higher electrical load caused an increase in temperature from \(\sim\)24 to \(\sim\)50 \({}^{\circ}\)C. The temperature distribution measured via thermographic camera (in accordance with the experimental setup presented in Figure 3) in a 1-kW PEMFC module operating under an electrical load of I = 25 A is presented in Figure 9.  
Figure 8: (**a**,**b**) The variation of resistances: \(\mathrm{R_{\Omega}}\), (\(\mathrm{R_{ct}}+\mathrm{R_{m}}\)) or \(\mathrm{R_{t}}\) vs. applied electrical load recorded for PEMFC modules (1) and (2). Figure 7: (**a**,**b**) Impedance spectra recorded for 1-kW PEMFC module (1) or (2) under applied electrical loads: I = 1, 10, 16, and 25 A, and (**c**) example of equivalent electrical circuit used to analyze the impedance spectra, L is the pseudo-inductance, \(\mathrm{R_{\Omega}}\)—ohmic resistance, \(\mathrm{R_{ct}}\)—charge transfer resistance, \(\mathrm{R_{m}}\)—mass transport resistance, \(\mathrm{CPE_{1}}\) and \(\mathrm{CPE_{2}}\)—constant phase elements connected in parallel with \(\mathrm{R_{ct}}\) and \(\mathrm{R_{m}}\). Cooling air was supplied to modules (1) and (2) by air axial fans. Four fans were installed in each 1-kW PEMFC module. The maximum air flow capacity for modules (1) and (2) was 8 * 120 m\({}^{3}\)/h = 960 m\({}^{3}\)/h, which, given the difference in the temperature of the flowing cooling air of 10 \({}^{\circ}\)C, enabled the discharge of 2650 W of heat. This value is sufficient to cool a 2-kW fuel cell stack. \[Q=\dot{V}\cdotp\cdotp C_{pp}\cdot\Delta T \tag{1}\]  
\[Q=8\cdot 120\frac{m^{3}}{h}\cdot 1\frac{h}{3600s}\cdot 1.3\frac{kg}{m^{3}}\cdot 10 20\frac{J}{kg\cdot K}\cdot 10K=2650W \tag{2}\]  
where: \(Q\) is the heat picked up by the cooling air; \(\dot{V}\) is the stream of air; \(\rho\) is the air density. \(C_{pp}\) is the specific heat of the air; \(\Delta T\) is the temperature difference between the inlet and outlet of the cooling air. The concept of parallel vs. series air flow through electrically connected modules (1) and (2) is presented in Figure 10a,b. Figure 10: _Cont._  
Figure 9: The temperature distribution in 1-kW PEMFC module (1) operating under an electrical load of 1 = 25 A, measured via thermographic camera. Figure 10a presents a situation in which cooling air flows through modules (1) and (2) arranged in series. Air at ambient temperature was drawn from outside into cool module (1), where it received heat and moisture (H\({}_{2}\)O) released by the cathode. This preheated air, which flowed through module (2) only, was discharged outside after it had cooled down and taken the moisture from module (2). In this cooling configuration, modules (1) and (2) were characterized by different cooling conditions, as module (1) was directly cooled with ambient air from the surroundings, while module (2) was cooled with air that had already been preheated in module (1) and thus was at a higher temperature. In this unfavourable situation, the control system of the cooling fans must force the greater air flow necessary to cool module (2). Increased cooling air flow can lead to a temperature gradient in the module, which may be directly caused by increased moisture loss through selected MEA cells or by the presence of MEA cells at much higher temperatures than the others in the module. This phenomenon causes a higher level of energy consumption for cooling module (2) than for module (1). In addition, it can lead to more rapid degradation of individual cells in module (2). Figure 10b, on the other hand, presents cooling air flow through modules (1) and (2) arranged in parallel. The situation was different in this arrangement of modules (1) and (2), where air was drawn into the fans from outside, from the space between modules (1) and (2), which were arranged in parallel at a distance of 3 cm. This air, after taking on the excess heat and moisture released during operation, was discharged outside to the environment.