1. Introduction
Depending on their structure, drones can be divided into three types: fixed-wing drones, single-rotor drones, and multi-rotor drones [
1,
2]. Compared to the other two types of drones, multi-rotor drones can take off and land vertically, hover in the same place for a long time [
3], and exhibit a simple structure and strong maneuverability [
4], making them very suitable for aerial photography [
5,
6], patrol inspection [
7,
8], pesticide spraying [
9,
10,
11,
12], and other types of missions in various fields, including commercial consumption or engineering applications. Accordingly, multi-rotor drones are currently the mainstream products in the drone market.
Despite their promising advantages, the short flight time of multi-rotor drones is one of the key factors limiting their further development [
13]. Lithium batteries are used as the power source in most of the existing mature multi-rotor drones. However, the energy density of lithium batteries is 130–200 Wh/kg, whereas the power loading (the weight that can be lifted by unit power) of multi-rotor drones is typically approximately 10 g/W, which limits the battery weight that can be handled by the drone [
14], making the flight time of multi-rotor drones powered by lithium battery very short (typically within 40 min) [
15]. Therefore, battery replacement is required during the frequent start and stop operations of these drones. In contrast, the energy density of fuel cell systems is 250–540 Wh/kg [
16], indicating that fuel cells can power at least twice the flight time that can be powered by lithium batteries at the same weight, making them very suitable for long-time flight [
17]. This is of great significance for improving the endurance of multi-rotor drones, improving charging efficiency, and reducing the labor intensity of operators.
Figure 1 shows a Ragone plot illustrating the energy density vs. power density of various power sources, and the plot indicates the significantly higher energy density of fuel cells compared to other power sources. In addition to the previously mentioned limitations of lithium batteries as the power source of drones, there are some other disadvantages [
18]: 1) lithium batteries easily short circuit or overcharge, making them unsuitable for long-term use; 2) lithium batteries experience a large temperature increase during operation, which may result in the burning of the drone if the temperature increases to a critical point; 3) easy detection by infrared detectors owing to the heat generated by the lithium battery during the flight of the drone, thus limiting the application of drones in the military field; 4) negative impact on the environment during the recycling process. In contrast, the service life of fuel cells is approximately three times that of lithium batteries [
19] and fuel cells only discharge water during operation, making them superior to lithium batteries in terms of environmental impact. Consequently, drones powered by fuel cells have emerged as a hot technology being researched in various countries.
The types of fuel cells commonly used in drones are proton exchange membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), and solid oxide fuel cells (SOFC). Among these, PEMFCs exhibit a light weight, high energy density, low operating temperature, and long service life, making them the most widely used fuel cells in drones [
21].
As shown in
Figure 2, a typical PEMFC structure is divided into a cathode flow channel, anode flow channel, gas diffusion layer, catalytic layer, and proton exchange membrane [
22]. Hydrogen diffuses into the gas diffusion layer after passing through the anode flow channel, loses electrons under the action of the catalytic layer, and transfers protons to the cathode side through the proton exchange membrane. Oxygen diffuses into the gas diffusion layer after passing through the cathode channel, and combines with the protons passing through the proton exchange membrane and the electrons from the external circuit to form water under the influence of the catalyst. Through these reactions, a fuel cell can form a continuous current between the cathode and anode. Typically, the cathode and anode flow channels are designed on the front and back of the same conductive plate to form a bipolar plate [
23], and the gas diffusion layer, the catalytic layer, and the proton exchange membrane are combined to form a membrane electrode [
24]. This design enables the stacking and combination of the fuel cell stack in a “bipolar plate-membrane electrode-bipolar plate” configuration.
During operation, PEMFCs generate tremendous heat, which can result in the drying out of the membrane electrode and the subsequent deterioration of the performance of the cell if the heat is not discharged in time [
25]. Consequently, a cooling system is an important part of fuel cells. Depending on the cooling methods, PEMFCs can be divided into water-cooled type and air-cooled type [
26], Compared to water-cooled PEMFCs, air-cooled PEMFCs cool the stack via air purging without complicated auxiliary systems, making the entire system simpler and lighter. This is particularly advantageous in low-power devices, such as drones, which require less heat dissipation. Depending on their structure, air-cooled PEMFCs can be further subdivided into three types: area air-cooled [
27], edge air-cooled [
28,
29], and open-cathode type [
30]. In the open-cathode type, the cathode channel is exposed to the atmosphere, and the flowing air simultaneously provides the oxygen required for the cathode reaction and cools the stack through fan suction. This type of fuel cell combines the advantages of simple structure and large power range, making it the most widely used and advanced fuel cell among the three types of air-cooled PEMFCs.
In summary, the open-cathode PEMFC has the advantages of a high energy density, low noise, and no pollution, and is an ideal power source for multi-rotor drones. This paper presents the development history and technical status of hydrogen fuel cell multi-rotor drones, and analyzes the key technologies that need further research in the field of hydrogen fuel cell multi-rotor drones.
2. Development and Application of of hydrogen fuel cell multi-rotor drones
Air-cooled fuel cells are often used in small mobile devices, such as bicycles, forklifts, and drones. Among these devices, drones are not only the most technically difficult devices for fuel cell applications but also the devices that best demonstrate the superiority of fuel cells.
Research on hydrogen fuel cell multi-rotor drones just started in recent years. In 2015, EnergyOr, located in Montreal, Canada, developed H2Quad series drones powered by its EPOD fuel cell [
31]. This drone can fly for 2 h with a load of 1 kg, and its effective flight radius is three times that of battery-powered multi-rotor drones [
32]. Similarly, a British company, Intelligent Energy, tested DJI Matrice 100 drone equipped with its fuel cells in 2015, and observed that the flight time of the drone can reach up to 1 h. Additionally, in 2015, Singapore's Horizon Energy System (HES) launched Hycopter fuel cell drone for large-scale industrial maintenance and inspection [
33]. This drone can fly for 3 h with a 12-L (3.5 kg) gas cylinder, and they implemented a highly lightweight design for each key component of its power system. Wuhan Zhongyu Power Co., Ltd. released a six-rotor drone "Ranger" equipped with its fuel cell system, HyLite1200, in 2015 [
34]. The drone uses a 9 L/30 MPa high-pressure gas cylinder, and achieved a flight time of 3 h 30 min during the field test, setting a record for the flight time of drones at the time.
The world's first manufactured hydrogen fuel cell multi-rotor drone is HYDrone-1800 [
35,
36], which was released by MicroMultiCopter Aero Technology (MMC) in 2016. The fuselage of HYDrone-1800 is composed of carbon fiber materials and adopts a 6-axis design. It has a wheelbase of approximately 1.8 m, a maximum load of 25 kg, and a maximum flight radius of 100 km. The drone can fly continuously for 270 min with a 14-L gas cylinder and is suitable for inspection operations in various outdoor environments. In 2017, FlightWave developed a multi-rotor drone named Jupiter-H2 powered by Intelligent Energy's 650W fuel cell [
37]. The Jupiter-H2 uses a narrow profile 70 cm fuselage and is equipped with 8 high thrust engines, and achieved a flight time of above 2 h. Some early hydrogen fuel cell multi-rotor drones are shown in
Figure 3.
Owing to its long flight time and good environmental protection, the application of fuel cell drones has been gradually promoted. In 2018, ISS Aerospace in the UK launched a drone named Sensus4 powered by Intelligent Energy's air-cooled fuel cell platform IE-Soar 800W [
38], and also launched Sensus6 [
39] powered by IE-Soar 2400W in 2019. These two drones are equipped with light gas cylinders produced by AMS, with payloads of 1.5 and 8 kg, respectively. The main application fields of these drones include the energy, environment, and military industries. In 2018, HES commercially released Hycopter [
40], and the fuel cell system of this drone has an energy density of 700 Wh/kg and a power density of more than 1 W/g. Additionally, Hycopter uses a hydrogen regulator with a weight of only 140g, has a flight time of 3.5 h, and can carry high-speed precision cameras and various sensors for longer periods. In the same year, Skycorp, an Estonian drone manufacturing company, released a fuel cell quadcopter drone with AI functions——e-Drone Zero [
41]. The drone was equipped with an Intelligent Energy's IE-Soar 800 W fuel cell and an AI operating system that can perform complex operations and provide security measures, such as obstacle avoidance based on machine vision. In 2019, the drone photography company, BATCAM, applied the fuel cells of Intelligent Energy to a multi-rotor drone [
42], making the drone’s flight time reach 70 min with a load of 5 kg, whereas the flight time of multi-rotor drones using lithium batteries from the same company is only 12 min. In 2020, Norway’s Nordic Unmanned installed HES’s 2000W air-cooled fuel cell system [
43] on its Staaker BG-200 drone, and after a successful test flight, the company planned to further apply the drone to logistics, search, rescue, and inspection. Some fuel cell multi-rotor drones used for inspection are shown in
Figure 4.
To further improve the flight time of fuel cell multi-rotor drones, researchers are attempting to increase the hydrogen carrying capacity of drones by changing the hydrogen storage method. Based on Intelligent Energy's IE-Soar 800W fuel cell in 2019, Meta Vista from South Korea equipped a drone with a 6 L liquid hydrogen tank as shown in
Figure 5, increasing the energy density of the power source to 1865 Wh/kg, increasing the flight time of the drone beyond 12 h [
44]. However, owing to the high cost of using liquid hydrogen, the difficulty of storage and the imperfection of related technologies, other drone companies have not attempted this technical route, and compressed gaseous hydrogen storage method is still the most widely used hydrogen storage method in multi-rotor drones.
With the advancement of technology, fuel cells for drones are gradually being commercialized and serialized. Currently, the companies with the most in-depth and advanced research on fuel cell drones include the Intelligent Energy from the United Kingdom and Doosan Mobility Innovation (DMI) from South Korea, and both companies can independently develop air-cooled fuel cell stacks and power systems for drones. The common features of drones manufactured by these companies include high integration and modularization. Intelligent Energy's IE-Soar series of fuel cells for drones are currently one of the lightest fuel cell modules in the world, and this company has achieved a high stack power density exceeding 800 W/kg, including IE-Soar 650W [
45], IE-Soar 800W [
46] and IE-Soar 2400W [
47]. These modules can increase the weight and space that the auxiliary equipment can occupy, thereby prolonging the flight time of the drone.
Figure 6 shows an IE-Soar 650W fuel cell module and a drone powered by the IE-Soar 650W. To enhance the series connection of fuel cell stacks, Intelligent Energy has developed a power path module (PPM) [
48,
49] adapted to IE-Soar 650W and IE-Soar 800W. This module can simultaneously distribute hydrogen and transfer energy, and combine various fuel cell power modules (FCPM) in different ways, and this plays a very important role in expanding the power range of drones. Based on the PPM module, Zepher from the United States used two IE-Soar 800W fuel cells on the vertical take-off and landing drone of the US Army [
50].
The most significant feature of the products of DMI is the high integration. The components of the entire fuel cell systems manufactured by this company, including the gas cylinder and the auxiliary power source, are integrated in the same pack, allowing users to easily match the drones they need. Representative fuel cell packs of DMI include DP20 [
51], DP30 [
52] and DM30 [
53]. The DP30 has a power of 2600 W and is considered the world’s largest power fuel cell power pack module. The DM30 is the module removing housing from DP30. Additionally, DMI has developed ultra-light bipolar plates and a special stack structure for drones with a highly lightweight fuel cell pack, while ensuring the durability of the fuel cell and the uniformity of the output performance of the battery. As shown in
Figure 7, the overall weight of DP30, consisting of a 10.8-L cylinder, is only 12 kg. Based on its own fuel cell pack, DMI has successively developed a series of drones, such as DT20, DS30 and DT30 [
54]. Two DP30 modules have been combined to produce a 5.2 kW hydrogen fuel cell system, which was used to power a 39 kg medium-sized hexacopter, and the flight test results proved the feasibility of the 5.2 kW fuel cell system medium-sized hexacopter to perform stable flights. Currently, DMI’s innovative drones have been applied in many fields, such as sea rescue, wind power inspection, road surface inspection, pipeline inspection, and logistics distribution [
55].
Ballard launched FCair-600 and FCair-1200, two fuel cell systems for drones (
Figure 8) [
56]. These FCair series fuel cell systems utilize water-cooled fuel cell stacks, which significantly increases the overall weight of the system compared to that of air-cooled fuel cell systems of the same power, therefore, water-cooled fuel cell stacks have not been widely used in multi-rotor drones.
China is the largest producer and consumer of hydrogen fuel cell drones in the world at present. As shown in
Figure 9, in 2022, a total of 659 hydrogen fuel cell drones were produced globally, of which China accounted for 27.2%, and a total of 639 hydrogen fuel cell drones were sold, of which China accounted for 27.6%. Many Chinese companies have advanced manufacturing technology for hydrogen fuel cell drones. Beijing Xinyan Chuangneng Technology Co, Ltd. from China launched a six-rotor hydrogen fuel cell drone. The drone can fly continuously for 331 min with a 19 L/35 MPa light gas cylinder, setting a record for the flight time of a fuel cell drone based on a compressed gaseous hydrogen storage method [
57]. Additionally, the drone also demonstrated that China is at the forefront of hydrogen fuel cell fabrication in the world. Zhejiang Hydrogen Aviation Technology Co., Ltd., another hydrogen drone company from China, developed Hercules ACFC-48-1700 and Hercules ACFC-48-2700 air-cooled stacks specially for drones. The stacks utilize carbon nano-microporous stacking technology, and exhibits a power density of approximately 700 W/kg. This same company manufactured Hydrocopter-04 drone, which is based on two 1250 W air-cooled stacks. This drone can fly for up to 4.5 h without load, and has been tested in various fields. With the continuous advancement in hydrogen energy drone technology, China launched the national standard "Hydrogen Fuel Cell Power System for Unmanned Aerial Vehicles" in June 2020 [
58], which is the world's first national hydrogen fuel cell standard for drones.
4. Summary and Future Scope
With the advancement of technology, hydrogen fuel cell multi-rotor drones have gradually moved towards industrialization and modularization. Many fuel cell companies have developed their own series of multi-rotor drones; however, hydrogen fuel cell multi-rotor drones are still in the exploratory stage and have not yet been fully recognized by the market. Moreover, their high cost problem is yet to be resolved. To further improve the performance of multi-rotor drones and expand their application range, in-depth research in basic science, engineering design, and top-level planning is still required. This paper summarizes some technical directions that fuel cell multi-rotor drone should focus on in the future:
- 1
Optimization of hydrogen storage methods;
Long flight time is the biggest advantage of hydrogen fuel cells as the power source of multi-rotor drones. To further exploit this advantage, future research should focus on lightweight design, hydrogen storage methods, and energy management strategies. Among these, optimizing hydrogen storage methods is the technical route that can most significantly improve the drone's flight time. Particularly, if efficient solid-state hydrogen storage can be achieved at a low cost, hydrogen fuel cell multi-rotor drones can truly achieve long-term and long-distance flight, and the drone industry will also usher in a revolutionary change.
- 2
Cathode gas filtration system;
As the cathode of the open-cathode air-cooled fuel cell is directly connected to the atmosphere, if the working environment is heavily polluted, the pollutants in the air will directly damage the membrane electrode. This will result in a decrease in the life of the fuel cell, hindering the use of hydrogen fuel cell drones in the polluted environments, such as coal mines and chemical plants. To expand the application scenarios of hydrogen fuel cell drones, further research on the cathode gas filtration system is needed.
- 3
Auxiliary equipment.
To achieve the large-scale application of hydrogen fuel cell drones, in addition to the drone equipment itself, it is necessary to systematically plan and design a complete set of technologies for hydrogen storage, hydrogen transportation, and hydrogenation. Compared to hydrogen fuel cell vehicles, multi-rotor drones utilize very little hydrogen, so a portable mobile hydrogen refueling process can be designed for them to meet the frequent use.
The research and development of hydrogen fuel cell multi-rotor drones is a systematic project that integrates new energy, robotics, energy management, and many other technologies. Its development is closely related to the progress of basic disciplines such as materials, chemistry, and thermodynamics, indicating that if hydrogen fuel cell multi-rotor drones are properly applied in the future, their research and development will become increasingly subdivided and specialized, and top-level design will become increasingly significant.