# 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
According to Figure 13, nominal power output P\({}_{\rm nom}\) was obtained for a current-voltage pair close to I = -75 A and U = -28 V for power source (I), a 2-kW PEMFC stack consisting of two fuel cell modules (1) and (2) electrically connected in parallel. These data agree closely with the voltage values within the range 26-38 V assumed to be required for operation in a drone. Direct comparison of the performance of our two-module 2-kW PEMFC stack with that of commercial 2-kW PEMFC stacks enables us to state that higher values of current (I) were obtained in the case of two-module electrical parallel connections of this kind. Moreover, as a result of direct comparison of the voltage (U)-current (I) curve, a lesser decrease in voltage (U) with increasing current (I) was observed. These lesser decreases may be advantageous in the case of the integration of a PEMFC fuel cell stack with the U-I characteristics of a Li-Po battery. These factors are important for the development of an energy management system in terms of increasing  
Figure 9.1 (**a**) Waveforms of current originating from I\({}_{\rm m1}\)—module (1), I\({}_{\rm m2}\)—module (2) and temperature T during performance of the PEMFC stack with cooling air flow through modules (1) and (2) arranged in series (situation from Figure 10a). The electrical load applied was I = 48 Å. (**b**) Waveforms of power originating from P\({}_{\rm m1}\)—module (1), P\({}_{\rm m2}\)—module (2) and total power P = P\({}_{\rm m1}\) + P\({}_{\rm m2}\) from the PEMFC stack vs. time with cooling air flow through modules (1) and (2) arranged in parallel (situation from Figure 10b). The electrical load applied was I = 48 Å.  
battery life (through preventing rapid discharge) and increasing flight distance due to the potential for recharging the battery in flight. An additional advantage of this kind of two-module construction for a 2-kW PEMFC stack is that the modules are easier to position side by side such that the outlet of the cooling air being drawn between the two modules of the stack is directed evenly between the two sides. In the case of one module, the flow of the cooling air stream moves from one side to the other, which, for an airframe in which the stack of fuel cells is placed along the fuselage, would result in the formation of an additional unfavourable twisting moment that would hinder the drone from flying straight ahead. Direction of the cooling air outlet from two stacks from the fuselage evenly to both sides results in reciprocal cancellation of the torsional moment and, as a result, no forces hinder straight flight. [PERSON] et al. [45] analyzed the strategies of development of cooling technologies for a LT-PEMFC stack with electrical power ranging from 2 to 5 kW. According to these authors, air-cooled proton exchange membrane fuel cells are usually preferable below 1 kW, due to potentially limited development capacity. The main advantage of the application of an air-cooling system is its simpler construction and a lower consumption of power by the accessory system compared to a liquid cooling system. The main drawback of air-cooling systems in PEMFC fuel cells is that the maximum temperature difference throughout the stack can reach 9-14 \({}^{\circ}\)C. Nonuniform temperature distributions result in nonuniform performance distribution due to the ohmic and charge transfer resistance of individual MEA cells in the PEMFC stack along with temperature. The variation of temperature (T\({}_{1}\)) and (T\({}_{2}\)) in both modules (1) and (2) vs. power is presented in Figure 9.2  
The maximum temperatures (Tm1) and (Tm2) in both modules were measured in the centre of the PEMFC stack. An increase in temperature Tm1 or Tm2 vs. electrical power was observed. The difference \(\Delta\)T measured in both PEMFC modules was no higher than 1-2 \({}^{\circ}\)C. These data also indicated that, due to limited dimensions and space, applications in drone construction may also constitute a prospective solution for installation of a cooling system.