# 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 rational utilisation of hydrogen is also important for the application of PEMFC stacks in a hybrid propulsion unit. Firstly, the predicted duration of operation of a PEMFC stack during the flight of a drone depends on the amount of hydrogen stored in a composite bottle. The consumption of hydrogen vs. time varies with the electrical power and dynamic performance of the fuel cells. Figure 9.3 U-I and P-I curves recorded for three different 2-kW PEMFC stacks, one constructed from modules (1) and (2)—2 \(\times\) 1 kW—2M (green points) connected in parallel, and selected commercial fuel cell stacks, HG (blue points) BCH (red points) with power of 2 kW in a single module. Figure 15a presents the dependence of total hydrogen consumption when electrical power was produced by the two-module PEMFC stack. Figure 9.4 Variation in temperature Tm1 and Tm2 vs. electrical power (P) recorded for a 2-kW modular PEMFC stack. Figure 9.5 _Cont._  
The data recorded for the reference commercial PEMFC stacks with the same electrical power output of 2 kW is also presented. As can be seen, total hydrogen consumption increases with an increase in electrical power within the range 0-2000 W. A lower value of total hydrogen utilisation vs. electrical power was recorded for the PEMFC stack constructed from two 1-kW PEMFC modules (2M) compared to the PEMFC stack (BCH). In the case of comparison to the HG PEMFC stack, slightly lower consumption was also recorded. As can be seen from Figure 15b, the longest duration of PEMFC fuel cell operation with nominal power of 2 kW is expected for the two-module PEMFC stack (2M). This duration is slightly greater than that recorded for the two analyzed reference 2-kW PEMFC stacks (HG, BCH) assembled as a single module. The main reason for the observed difference may be connected with the process of hydrogen purge operation according to the algorithm applied in PEMFC construction. Figure 16 presents the electrical power required to supply the BOP devices (mainly cooling fans) vs. power produced by the PEMFC stack. In further development research, a new geometry of metallic bipolar plates for a 2-module PEMFC stack may lead to a significant reduction in the overall mass of the PEMFC stack. It is expected that by replacing the graphite bipolar plates with metallic plates, the total mass can be reduced by about 50%; an increase in specific power density in the range of 500-550 W can be achieved, which is representative of a lightweight PEMFC stack with metallic bipolar plates. On the other hand, the achieved power density W/kg for a 2-kW PEMFC stack is \(\sim\)270 W/kg, and the specific power density of whole hydrogen system (calculated as the sum of the mass of a complete 2-module PEMFC stack and the lightweight composite cylinders with the entire hydrogen gas infrastructure) is \(\sim\)133 W/kg. This value fits well within the range of a commercial PEMFC stack with graphite bipolar plates [19; 46]. Figure 9.6 (**a**) Comparison of hydrogen consumption F\({}_{\text{H2}}\) vs. power performance P of our designed 2-module fuel cell stack (2M) with other PEMFC fuel cell stacks, BCH and HG, with the same power of 2 kW. (**b**) Duration of 2-kW PEMFC stack operation for the same hydrogen storage. The hydrogen is compressed under 220, 300, and 350 bar of pressure in composite bottles (V = 13.6 dm\({}^{3}\)). 3.1.2. Test of the Electrical Performance of the Designed Two-Module PEMFC Fuel Cell Stack with a Propeller Connected to a BLDC Motor  
The PEMFC stack was integrated into a propulsion system involving a BLDC motor with a 30 \(\times\) 12 propeller, as depicted in Figure 9.7  
Figure 17a,b present the U-I. P-I, curves recorded for the investigated propulsion unit (a) and dependence of the rotational speed of a 30 \(\times\) 12 propeller connected to a BLDC motor vs. power (b) respectively. In Figure 17a the dependence of total hydrogen consumption F\({}_{\rm H}\) (Ndm\({}^{3}\)/min) vs. current (I) was also added.