The possibility of implementing fuel cell technology in Unmanned Aerial Vehicle (UAV) propulsion systems is considered. Potential advantages of the Proton Exchange Membrane or Polymer Electrolyte Membrane (PEMFC) and Direct Methanol Fuel Cells (DMFC), their fuels (hydrogen and methanol), and their storage systems are revised from technical and environmental standpoints. Some operating commercial applications are described. Main constraints for these kinds of fuel cells are analyzed in order to elucidate the viability of future developments. Since the low power density is the main problem of fuel cells, hybridization with electric batteries, necessary in most cases, is also explored.
Unmanned Aerial Vehicles (UAVs), encouraged by recent technological developments, have seen a dramatic interest boost in recent years and are already considered as an integral and indispensable part of modern armed forces [
Fossil fuels pose a serious environmental and economic problem. They are the main cause for the increase of CO2 presence in the atmosphere, declared to be one of the most important culprits of global warming and the atmospheric emissions of other pollutants [
Historical world oil prices in the period 1861–2012.
Of course, the world of aviation is not an exception to the above considerations. UAVs are in their nascent stage of development (in fact, their regulations are in process of being written), although the implementation of fuel cell propulsion systems is more advanced than in conventional aircrafts. The main reason is the fact that UAVs are unmanned, thus, weighting comparatively less and not needing the life support systems for the crew and passengers, making them ideal for this new technology. Also, fuel cells are still too heavy to propel any large aircraft; they have a lower power density when compared with conventional turbines [
The propulsion system of a UAV consists of the following elements: energy source: chemical fuels (fossil fuels, biofuels, and chemicals), electricity, solar energy (in conjunction with photovoltaic cells), hydrogen, methanol, and energy mechanics; storage media: tanks, batteries, capacitors, metal hydrides, and so forth; mechanical energy converter: internal combustion engine, and fuel cell + electric motor; lift/thrust converter: rotor, fan, propeller, jet engine, and so forth.
Lift/thrust conversion systems are closely linked to the type of aircraft (fixed wing, rotary, lighter than air, etc). In addition, propulsion systems usually include power control, rpm control, heat management system, and an auxiliary electrical power generator.
An example is the propulsion system shown in Figure
Diagram of the elements of a propulsion system with a mechanical energy converter based on a fuel cell + electric motor combination [
Despite the recent boom in greener propulsion systems (electric, solar, hybrids, hydrogen internal combustion engine, etc.), the vast majority of current UAV engines are still driven by conventional internal combustion engines, normally fed with fossil fuels [ alternative thermal systems: where different thermodynamic cycles, fuel, or engine types can be used (e.g., spark-ignition reciprocating engines fuelled by gasoline); electrical systems: where the power required is obtained through an electric motor and power is generated or stored in different ways; hybrid systems: combining any of the systems listed above, even the same type (e.g., a combination of fuel cell and battery or Regenerative Fuel Cell Systems, RFC, which combine fuel cell, battery, and photovoltaic cells).
Hybridization can be achieved by combining a heat engine/electric motor with batteries [
Diagram examples of serial (a) and parallel (b) hybrid propulsion systems [
UAV propulsion systems can be classified by their type of mechanical energy conversion. Since internal combustion is the usual propulsion method, it is separated into jet engines and reciprocating engines according to the fuel used (petrol or diesel). Jet engines: they produce thrust and can be classified into jet engines and turbofans. Jet engines (jet-turbine or turbojet, ca. 30 g/(kN·s) [ Reciprocating engines: there is a great variety attending to the combustion process (spark-ignition engines, compression ignition engines, etc.), cycle (two-stroke and four-stroke engines), intake manifold pressure (engine naturally aspirated and supercharged engines), air or water cooled, and so forth. In this case, the main classification is determined by the type of working cycle and its corresponding fuel, for example, aviation gasoline (piston-avgas, otto cycle, ca. 0.3 kg/kWh) or diesel (piston-diesel, Diesel cycle, ca. 0.24 kg/kWh) [ Electric motors: they convert electricity into mechanical energy by moving a propeller, fan, or rotor. Electrical energy is supplied by a battery, photovoltaic or fuel cell. They have the advantage of being the quietest and having one of the lowest thermal signatures. Currently, mainly Micro- and Mini-UAVs are powered by batteries and electric motors [ Other types of engines: The Wankel rotary engine, which operates simply and smoothly, is gaining acceptance thanks to their sealing and torsional vibration problems being solved. Currently, Israeli Elbit Hermes 180 and Hermes 450 UAV with 28 and 38 kW, respectively, use these types of engines; they have high durability and a specific consumption of 0.35 kg/kWh [
In order to compare different types of mechanical energy conversion components used in UAVs, in terms of power densities, technical data have been collected and represented in Table
Examples of power densities of different UAV powerplant components.
Specifications from manufacturers’ websites | |||||
---|---|---|---|---|---|
Type | Manufacturer/model | Weight |
Peak power |
Power density |
Application |
Reciprocating engine (two-stroke) | Rotax 503 [ |
33.2 | 37 | 1.11 | INTA SIVA UAV (SR) |
Turboprop | Honeywell TPE 331-10 [ |
153 | 671 | 4.38 | PREDATOR B (MALE) |
Turbofan | Pratt & Whitney Canada PW545B [ |
347 | 18.32 kN | 10.86 | PREDATOR C |
Electric motor | ElectriFly GPMG4805 Brushless DC [ |
1.48 | 8.4 | 5.68 | Radio-Controlled Aircraft |
Lithium ion battery | ANR 26650 Cylindrical [ |
— | — | 2.60 | Portable high power |
H2 fuel cell | UTRC Gen 1 [ |
1.78 | 1.2 | 0.675 | Helicopter mini-UAV |
H2 fuel cell | Protonex Ion Tiger UAV (NRL) [ |
1 | 0.550 | 0.550 | Fixed-wing CR UAV |
Solar array | Several Manufacturers [ |
— | — | 0.06 | Spacecraft Applications |
Wankel | O.S. Engines 49-PI Type II 4.97 cc [ |
0.333 | 0.934 | 2.8 | UAV Wankel engine |
In 2008 there were 85 electric-powered UAVs whilst in 2012, there were around 232 [
2008–2012 evolution with % electric UAVs trend.
2008–2012 evolution with % fuel cell UAVs trend.
After observing the evolution in the use of electric motors and fuel cells in UAVs, it is important to check the breakdown (according to [
UAVs histogram according to sizes and powerplants. “Electric” includes batteries, fuel cells, and solar. “Other” includes rocket propelled, Wankel, laser powered, and hydrogen combustion.
In any case, the engine is only one component of the propulsion system. It must be mounted on the UAV and it must be provided with ignition or starting means, fuel supply, its required cooling control, and exhaust gas management if required. All these facts will influence the final choice. When comparing power and energy densities not only must the weight of the engine be taken into account, but also the weight of the energy storage system and the engine’s auxiliary systems. In large UAVs, the engine will therefore have a great size and will probably come with its large associated subsystems. However, in a small UAV there will probably be a need for space saving and, consequently, a subsequent selection of those subsystems.
With regard to electric propulsion systems, batteries are currently limited to 150–200 Wh/kg and are expected to show an increase of up to 300 Wh/kg within the next few years; one order of magnitude lower than the specific energy density expected in hydrogen fuel cells [
Recent advances in weight saving of electric motors and batteries have allowed for electric propulsion to be more competitive. The main drawback is the low specific energy density of batteries, resulting in large volumes (around four times the equivalent volume of fossil fuels for a given energy). Thus, the problem relies on the endurance of these vehicles.
The breakthroughs in internal combustion engines are focused on downsizing and HCCI technology (Homogeneous Charge Compression Ignition). This technology is based on producing autoignition of a lean and homogeneous mixture (air/fuel ratio >25) at multiple points in the combustion chamber [
UAVs are not manned and they typically carry light payloads (e.g., surveillance and communications), so the sum of the propulsion system and the fuel usually exceeds one-third of the total UAV weight, a higher proportion than in conventional aviation [
Among the most interesting alternative propulsion systems for UAVs are those based on fuel cells. It is still an immature but growing technology, with room for improvement in weight, volume, and cost reduction. Fuel cells are electrochemical systems that convert the chemical energy contained in fuels directly into electric energy. When fed with hydrogen, they produce no greenhouse gases, the only products being water and heat, and the level of noise generation by the engine is low. Water, as well as heat and low oxygen-containing exhaust air, is side product of the fuel cell that could have other applications to compensate the weight disadvantages (particularly in large UAVs), such as water supply for other subsystems, deicing, or inerting of a fossil fuel tank [
The main advantages of fuel cells are their low emissions, high efficiency, modularity, reversibility (this property is exploited in RFC Systems) [
Different types of fuel cells exist, differing in the operational temperature range and the electrolyte used. Combining these types of fuel cells with other elements, new concepts emerge, such as, RFC Systems, where the stack uses its reversibility mode as an electrolyzer or as a fuel cell (e.g., Helios UAV/NASA solar power combined with day/night cycles), or hybrid systems, which rely on batteries when more maneuvering power is required [
In addition to the UAVs, general aviation and commercial transport aircraft are possible applications for fuel cells. Several projects like Antares DLR-H2, developed in 2009 by the German Aerospace Center, or the first flight of a manned fuel cell aircraft performed by Boeing’s Madrid branch in 2008 are examples of general aviation applications. On the other hand, A320 ATRA and ENFICA-FC [
SOFCs are high temperature fuel cells (above 800°C). The development state of SOFCs is inferior to that of PEMFCs, apart from being heavier. Therefore this paper will focus on PEMFC. However, reforming and cleaning of kerosene for SOFCs are relatively simple [
PEM fuel cells operate at low temperature (typically 50–70°C); the use of polymeric electrolyte provides high current densities. They have a quick start-up, a high specific energy density, and a low specific power density. Therefore, they offer potential advantages for low maneuverability and high endurance UAVs, which is the type of UAV most developed today (mostly used in surveillance applications). The reason is that these properties allow for the development of relatively light, low cost, and reduced volume systems compared to other types of fuel cells [
Direct Methanol Fuel Cell (DMFC) stacks, a class of PEM fuel cells directly fuelled with methanol, have about half the efficiency (higher heat losses) and power density than PEM fuel cells, but higher energy density (greater endurance) and simplicity [
For very small UAVs, compressed hydrogen systems are not practical, since they are not downwardly scalable. In this case, it is preferable to use noncryogenic liquid fuels such as methanol or chemical hydrides, where fuel storage and delivery are considered more critical than the battery performance itself.
Developments in which a fuel cell delivers power to an electric motor, which in turn drives a propeller or a rotor, already exist, such as a helicopter UAV powered by a PEM fuel cell fed with compressed hydrogen. Recent advances in compressed hydrogen powered PEM fuel cells have achieved power densities of up to 1.4 kW/kg in the 100 kW range, although for the 1 kW range, only 250 W/kg is commercially available [
Possibilities of implementing fuel cells propulsion systems in UAVs.
Auxiliary systems of a fuel cell powered UAV include heat management, humidification system, tanks, and controller. The weight of these systems is very variable and can reach from 14.83% (UTRC UAV helicopter demonstrator) [
Airships are also a potential new application for fuel cells. There is a rebirth of airships resulting in various studies and prototypes, especially in the field of monitoring and surveillance, where the latest technological advances in materials and navigation are being incorporated [
Thus, the advantages of a fuel cell powered electric motor can be summed up as follows: more efficient than fossil fuel technologies, high energy density, which means greater endurance, reliability: few moving parts and easy automation, flexibility of operation: in that they are reversible, can work at high performance without interruption for a wide range of power demands, and can also rapidly change their power output; anyway, the latest point is not enough for demanding maneuvers like takeoff, thus the support, with batteries is often necessary, modular and easy to implement, direct energy conversion (no combustion), negligible noise and vibration, low or zero emissions, a variety of applications: in addition to propulsion systems in UAVs, they might be used in APUs, auxiliary power systems, ground control stations, and so forth, water, as well as heat and low oxygen-containing exhaust air, is side product of the fuel cell that could have other applications to compensate the weight disadvantages (particularly in large UAVs), such as water supply for other subsystems, deicing, or inerting of a fossil fuel tank.
Main disadvantages are high cost: it is not yet a mature technology and uses expensive materials like platinum, used as a catalyst, sensitivity to fuel contamination, requiring expensive filtering systems, the need for qualified maintenance personnel, low power density compared to other systems, especially in DMFC stacks, unavailability of hydrogen: H2, one of the fuels used in fuel cells, is not naturally abundant; it must be obtained through water electrolysis or hydrocarbons reforming, defining it as an energy carrier rather than an energy source; there is also currently no distribution infrastructure, unproven reliability for commercial use: there is little “real” commercial fuel cell UAVs and where they exist, their implementation is very recent, and their technology is therefore still far from mature, safety issues regarding H2 handling (hydrogen gas forms explosive mixtures with air) and methanol toxicity, this fact has influence, for a military setting, on the logistics of the fuel supply in the battlefield.
With the reasons given above, the advantages and disadvantages of a UAV propulsion system based on fuel cells are practically the same as those of the fuel cell stack itself, slightly worsened by the need to add the electric motor and its accompanying weight.
Power versus weight of PEM and DMFC commercial fuel cells.
The main problem of fuel cells is their low power density, meaning the UAV performance would have restrictions and would be very dependent on its aerodynamic design, weight, and fuel cell performance [
Relevant technical data of PEM fuel cells for UAV applications have been collected and summarized and are shown in Table
Examples of PEM fuel cells for UAV applications.
Specifications from manufacturers’ websites and [ | |||||
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Fuel type | Manufacturer/model | FC weight |
FC power |
FC power density |
Application/remarks |
Chemical hydride cartridge | Horizon Energy Systems/AEROPAK | 3.5 | 200 | 57.14 | IAI Bird Eye 650 LE UAV |
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Sodium borohydride | Protonex/UAV C-250 | 1.2 | 250 | 208.33 | 500 W peak power with batteries. Fuel 833 Wh/kg hydrated. Cartridge 1.8 kg, 1.5 l. |
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Compressed H2 | Protonex/Spider Lion UAV (NRL) | 1.77 | 95 | 53.67 | Spider Lion Micro-UAV 2005, 3-hour flight |
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Compressed H2 | Protonex/Ion Tiger UAV (NRL) | 1 | 550 | 550 | Ion Tiger UAV. 550 W FC (1 kg + 3.6 kg tank 0.5 kg H2), 26 h 1 m flight record in 2009. |
|
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Compressed H2 | EnergyOr/EO-310-XLE | 3.95 | 310 | 78.48 | Radiant Coral Technologies demonstrator UAV 1st flight February 25, 2013. |
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Compressed H2 | EnergyOr/EO-210-XLE | 3.65 | 250 | 68.49 | The weight includes auxiliary systems |
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Compressed H2 | DLR/HyFish UAV | 3 | 1000 | 333.33 | HyFish UAV 2007. 0.5 hour flight |
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Compressed H2 | UTRC/Gen1 | 1.78 | 1200 | 674.16 | Helicopter UAV (October 11, 2009). FC (675 W/kg). Powerplant (500 W/kg). Minicopter Maxi Joker. 20 m flight |
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Compressed H2 | BCS/BCS500 | 6.35 | 500 | 78.74 | Georgia Tech University UAV 2006. Powerplant weight 12 kg |
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Compressed H2 | Horizon Energy Systems/H-100 | 1.36 | 100 | 73.53 | Johannesburg University Piper Cub UAV |
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Sodium borohydride | Protonex/ProCore VI | 0.408 | 800 | 1960.78 | Aerovironment Puma UAV 2008. Endurance 9 h |
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Compressed H2 | Horizon Fuel Cell Technologies | 5 | 650 | 130 | Pterosoar micro-UAV 2008. Oklahoma State and California State Universities. 15.5 h endurance. FC 480 Wh/kg |
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Liquid H2 | NASA/Sensor Technology/Aerovironment | — | — | — | Aerovironment Global Observer (GO-1). 65,000 ft alt., 7-day endurance. PL 180 kg |
Nowadays, the specific energy density of a PEM fuel cell system, which implies higher endurance in UAVs, is about 700–1000 Wh/kg, but it is to be increased up to 10 kWh/kg within the next 10–15 years and to 20 kWh/kg within 20–30 years, which, if achieved, will enable the all-electric flight of a large commercial aircraft [
Attention should also be paid to the potential performance loss in fuel cells depending on the environmental conditions of the aircraft flight (i.e., changes in pressure/altitude, temperature [
Could the feasibility to implement a fuel cell in a UAV be asserted? The short answer is yes although, obviously, not for all combinations of fuel cells and UAVs. From the study carried out in this work, the following conclusions can be drawn. Fuel cell technology is still immature but improving, with clear room for improvement in weight, volume, and costs reductions. Compared with conventional systems, fuel cells offer higher energy density and lower specific power density. Fuel cells offer potential advantages in low maneuverability UAVs, which are currently the most manufactured type (e.g., surveillance applications), and in high endurance operations. PEM fuel cells have reached a more mature market. The facts of being of low temperature and having a fast start-up time are features consistent with the requirements of most UAVs. Reforming other fuels (e.g., natural gas, gasoline, etc.) remains an uninteresting option because the weight and volume of the reformer would need to be added. A jet fuel reformer could only be assessed in the case of large UAVs [ There are different fuel storage systems for each type of fuel cell. Therefore, in the case of a UAV, it is essential to minimize the weight of the overall propulsion system (fuel cell/storage system) without forgetting to consider the weight of auxiliary systems, such as thermal control or water management systems. Several commercial UAV applications of PEM hydrogen fuel cells already exist. There is even an example of a fuel cell powered UAV rotorcraft (demonstrator, [ Compared to hydrogen and despite its toxicity which is not an important problem following the proper protocols, methanol has advantages in terms of its storage systems, especially with logistics, in energy density and cost, and even in safety issues. DMFC fuel cell systems have less power density in comparison to other types of fuel cells, but higher energy density. The possibility of implementing DMFC stacks in UAVs is considered an interesting alternative and left for future work. Airship propulsion requirements are relatively modest, being the greatest for takeoff and landing. Electric motors offer good prospects with respect to the currently used internal combustion engines, and consequently fuel cells can play an important role. Specifically, DMFC would be suitable for airships because of its long-range and low power density. Since the low power density is the main problem of fuel cells, hybridization with electric batteries would be necessary in most cases.
Auxiliary Power Unit
Close Range
Defense Advanced Research Projects Agency
Direct Current
Direct Methanol Fuel Cell
Environmentally Friendly Intercity Aircraft
Fuel cell
High Altitude Long Endurance
Homogeneous Charge Compression Ignition
Heavier-Than-Air
Low Altitude Deep Penetration
Low Altitude Long Endurance
Medium Altitude Long Endurance
Medium Range
Medium Range Endurance
Maximum Takeoff Weight
National Aeronautics and Space Administration
Naval Research Laboratory
Proton Exchange Membrane, Polymer Electrolyte Membrane
Proton Exchange Membrane Fuel Cell, Polymer Electrolyte Membrane Fuel Cell
Payload
Ram Air Turbine
Regenerative Fuel Cell Systems
Solid Oxide Fuel Cell
Short Range
Stratospheric
Unmanned Aerial System
Unmanned Aerial Vehicle
Unmanned Combat Aerial Vehicle
United States
United Technologies Research Center
Unmanned Vehicle Systems.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work has been partially supported by the Spanish Ministerio de Ciencia y Tecnología in the framework of Project codes ENE2007-67584-C03-03ALT and ENE2011-28735-C02-02. The authors would also like to thank Mr. Hugo Gee for his careful reading of the paper.