Hydrogen fuel cells are demonstrated as the propulsion system for long-endurance, small, electric unmanned air vehicles (UAVs). Flight times of >24 hours were demonstrated for the 35-lb Ion Tiger fuel cell UAV while carrying a 5-lb payload. This paper describes the design criteria and development process used to meet these performance goals, including setting the specifications for the vehicle, fuel cell, cooling, and fueling systems.
Small electric unmanned air vehicles are often considered limited by the low endurance of several hours, mainly set by the energy of their battery energy storage system. The Naval Research Laboratory has been extending the duration of electric UAVs through the use of hydrogen fuel cells, which take advantage of both the high energy of H 2 fuel in combination with the high efficiency (~50%) of polymer fuel cells. In this paper, we describe a project to demonstrate the 3-day flight of NRL's 35-lb Ion Tiger UAV on liquid hydrogen (LH2) fuel. The use of LH2 on a small UAV is complicated because of the limited weight and volume budget, plus the extensive insulation required to keep the LH2 in its cryogenic state (20K) for multiple days, plus match the variable H 2 demand of the fuel cell under flight conditions. A compact, lightweight LH2 flight vessel was successfully designed and built with the aid of extensive thermal modeling. The system performs well in flight and is on track to deliver a 3-day flight. We also discuss issues related to LH2 handling and transportation. While LH2 systems are complex, they can provide unprecedented flight duration for small UAVs.
The Naval Research Laboratory (NRL) has developed a proton exchange membrane fuel cell (PEMFC) powered unmanned air vehicle (UAV) called the Ion Tiger. The Ion Tiger fuel cell produces 600 W electric (gross) and a comparable amount of waste heat that must be rejected by a cooling system. This study was undertaken to design a cooling system for the Ion Tiger and investigate cooling approaches that may be suitable for future PEMFC powered air vehicles. The performance of several flat plate radiators and compact heat exchangers (CHEs) were evaluated, both in a wind tunnel and in the laboratory. The flat plate radiators were too heavy and occupied too large an area to be practical; a CHE was the lightest and smallest option. A simple thermal model for the flat plate radiators was developed and then used to explain the experimental results. An energy balance model of the fuel cell system and a UA model of the selected CHE were used to size the cooling system. The energy balance and UA models were validated using test flight data. The models were also used to explore the performance envelop of the cooling system which is lightweight, consumes little parasitic power and enables the Ion Tiger to fly continuously at full power in ambient air temperatures up to 55 °C. The combined fuel cell/CHE balance model is applicable (with minor modifications) to other PEMFC systems utilizing a CHE for cooling.
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