# Selected Aspects of Designing Modular PEMFC Stacks as Power Sources for Unmanned Aerial Vehicles
## 1 Introduction  
In recent years, unmanned aerial vehicles (UAVs) have become more and more widely used for civil, rescue, and military applications. Drones are flying machines, included in the category of UAVs, which fly autonomously or are remotely piloted from the ground by an operator. Advances in design, the construction of flying robots, and progress in navigation, remote control capabilities, and power storage systems have enabled the design and development of different types of fully equipped UAVs, which can find new potential applications in challenging environments [1, 2]. In such missions, human involvement may be difficult, impossible, or dangerous, usually leading to risks to life or health. Reductions in operating costs and minimization of infrastructure requirements may also be among the benefits of the use of drone technology [3, 4]. The category of drone applications depends on the drone's mission (military, civil), types of flight zones (outdoor, indoor), and types of environments (underwater, water, ground, air, space). UAVs used for military applications can accomplish special tasks such as firing missiles, dropping bombs, making camouflaged flights, disrupting communications, transporting medical supplies to battlefields, operating as invisible spies, maintaining surveillance, conducting planetary exploration, and providing search and rescue services. Depending on the flight mission of UAVs, their construction and size, and the type of equipment installed on board, power sources are required for both primary propulsion and auxiliary systems [5, 6, 7, 8, 9, 10]. Electrochemical battery-powered electrical motors are the most popular choices for small and mid-sized drones. Another option for electrical engines is represented by application of hydrogen-oxygen fuel cells,electrochemical devices which directly convert the chemical energy of fuel (mainly hydrogen) into electricity. These devices can be operated for as long as reagents (especially fuel) can be supplied. The operating time of power sources involving fuel cells depends on their level of electric power consumption and the amount of hydrogen stored [11; 12; 13]. For UAV applications based on quiet operation, improved safety, and precise power management and control, which involve the coverage of longer distances and superior performance in harsh conditions, and also considering the reduction of pollutants and greenhouse gas emissions, the application of hydrogen in fuel cells for electrical propulsion appears to be preferred [14; 15; 16; 17; 18]. In drone applications, fuel cells are limited in their ability to provide the required power for maneuvers: take off, climbing or flight under variable atmospheric conditions. When the power load profile varies greatly, some unpleasant phenomena may occur in hydrogen-oxygen fuel cells: (i) local dehydration of Nafion-based membrane electrode assembly (MEA), (ii) membrane flooding, or (iii) reactant starvation. These lead to a reduction in the durability and lifetime of the fuel cells. The polymer membrane fuel cells (PEMFC) should be hybridized with high-discharge lithium batteries. The battery can also be charged to keep SOC above the prescribed limit. Hybrid electric control system in such power sources can be active or passive. Active systems use energy control and management strategies based on DC/DC converters, while passive ones adjust the voltages of the different power sources at the main DC bus either by direct connection or by using power diodes [19; 20; 21]. The commercial AeroStack hybrid power source involving a 200 W PEMFC stack was the most frequently studied hybrid power source unit. It was discovered that the battery has considerable importance on the hybrid power system during dynamic variable electrical loads. The battery protected the PEMFC fuel cells during operation under considerable increase of electrical load, which could lead to faster damage or operation in the low efficiency region [22; 23; 24]. [PERSON], et al.