Fuel Cell/Battery Hybrid System Structures for Unmanned Air Vehicle Applications

Unmanned aerial vehicles (UAVs), as remote-controlled or autonomous flying devices characterized by high flexibility and mobility, were usually employed to conduct remote sensing and surveillance missions. Recent developments in fuel cell technologies show great potential to increase flight duration of UAVs with satisfactory fuel economy. For UAVs powered by fuel cell propulsion systems, the maneuverability and the power performance may be considerably affected, since fuel cell has some drawbacks such as the long start-up time, delayed response and weak power performance. The integration of fuel cells with other power sources can significantly improve the dynamic load-response, the power performance, and the energy storage capacity of UAV propulsion systems. Fuel cell/battery hybrid power system is a well-known and popular scheme. The fuel cell/battery hybrid propulsion system has the benefits of both fuel cell and battery. The battery can supply the UAV dynamic load requirements, thereby protecting fuel cell from dynamic load changes that might cause fuel cell damage. Applying batteries to the fuel cell UAV power system leads to operating stability of both fuel cell and overall propulsion. In this paper, the fuel cell/battery hybrid system structures developed for UAVs are presented.UAVs have been widely applied in practice as one of the best applicable candidates to conduct remote sensing and surveillance missions. Typically, UAVs are powered by conventional internal combustion engines (ICEs) and electric motors [۱, ۲]. Compared to the pure ICE-based UAVs, the UAVs powered by electric motors are more light, reliable, and efficient [۳]. Furthermore, electric motors are superior to ICEs in terms of dynamic response. In terms of working performance and environmental concerns associated with hydrocarbon and other harmful emissions, electric propulsion systems would become the mainstream for these small commercial UAVs [۴]. In the recent years, using fuel cells as the major power source has become very popular for electric propulsion systems in aircrafts or UAVs [۵]. Although fuel cells used as power sources in UAVs have many advantages, they have some defects including their operating efficiency at the low/high power output mode and low dynamic response [۶]. To overcome these shortcomings of fuel cell, a practical approach is to hybrid fuel cell with other power sources. Batteries and super capacitors are two of these power sources. Battery is an essential energy storage device to achieve the stable output for hybrid propulsion systems, while the super capacitor can be used as an uninterruptable power supply to deal with the emergency power supply requirements. The fuel cell/battery hybrid system structures developed for UAVs are presented and categorized in the next section.In general, hybrid propulsion systems for UAV applications can be classified into two types: direct and indirect hybrid structures. As the name suggests, the direct hybrid structure directly connects the power sources to the DC-bus or load without any DC-DC power conditioning systems to regulate the load voltage. The structure is shown in figure ۱-a.The indirect hybrid schemes include the fully active and semi-active structures. For the fully active structure as shown in figure ۱-b, all power sources can be actively controlled by DC-DC converters. Although the fully active structure can completely control each power source, it also causes an increase in the weight and cost of the power conditioning systems. In contrast, for the semi-active structures, which are shown in figure ۱-c and ۱-d, at least one of the power sources is passively controlled without the DC-DC converter. In the structure shown in figure ۱-c, fuel cell power is conditioned by the DC-DC converter to meet the battery and loads electrical requirements. In the other semi-active structure shown in figure ۱-d, the battery power is conditioned by the DC-DC converter to meet the fuel cell and load electrical requirements. Indeed, the balance of cost and weight and also the control scheme selection are important challenges in the design process of the fuel cell hybrid propulsion systems in UAVs.Figure ۱: Fuel cell/battery hybrid system structures for UAV applications [۷].Available power sources, lightweight design requirements and required levels of the system reliability and durability are among the important factors that can determine the appropriate structure of the fuel cell/battery hybrid UAV propulsion systems