Lightweight hydrogen storage cylinder for fuel cell propulsion systems to be applied in drones

https://doi.org/10.1016/j.ijpvp.2021.104428Get rights and content

Highlights

  • Lightweight type 4 cylinder is fabricated using polyethylene terephthalate liner.

  • Filling test using hydrogen gas is conducted.

  • Parametric study on internal phenomena of the developed cylinder is conducted.

  • Storage density among different cylinders are compared.

  • Storage density of developed type-4 cylinder has the highest value of 4.8 %.

Abstract

A lightweight type-4 cylinder is developed using a polyethylene terephthalate (PET) liner for application in drones. We conducted a parametric study on the internal phenomena of the developed cylinder during the filling process to examine the applicability of the cylinder to drones. First, we fabricated a type-4 cylinder with a volume of 6.8 L. Then, variations in pressure and temperature inside the cylinder were examined during the filling test. At the end of the filling test, the internal pressure reached 450 bar, and the temperature reached 58 °C. The expansion of the cylinder at the center of the cylinder body was larger than that at the dome knuckle. Subsequently, the effect of the inlet gas temperature on the internal phenomena of the developed cylinder was examined. As the inlet gas temperature decreased, both the temperature of the stored gas and the expansion rate decreased. Finally, the storage density of the developed cylinder was compared with that of other types of vessels. The storage density of the developed cylinder was equal to 4.8 %, which higher than that of other types of cylinders. Through this study, the applicability of PET as a liner for a lightweight hydrogen storage cylinder was examined.

Introduction

The applicability of drones has expanded from search, rescue, security, and surveillance to science, research, and unmanned cargo systems. In fact, industry analysts have reported that the commercial drone market is expected to grow to 6.30 billion USD by 2026 [1,2]. Small drones typically use electrical energy supplied by a battery system as the power source because of the low noise and weight provided by the battery. However, battery systems have been found to be unsatisfactory as a power source for drones because they have a long charging time, low energy density, and severe limitations with regard to endurance. These flaws can be addressed by using the fuel-cell–battery hybrid system as a power source [3]. Since hydrogen is the lightest element, supplying power using hydrogen gas can increase the energy density of the system. Unfortunately, the relatively heavy weight of the battery–fuel-cell hybrid system can reduce the flying time of the drone. Therefore, a system containing only a fuel cell is preferred as the power source for drones, necessitating the development of a lightweight fuel cell system.

Research on reducing the weight of fuel cell propulsion systems for application in drones has mainly focused on increasing the mass storage density of hydrogen. Among the various storage techniques, chemical hydrides and compressed hydrogen are primarily used; this is because the storage densities exhibited these two methods are substantially higher than those exhibited by other hydrogen storage methods [4,5]. Furthermore, chemical hydrides are chemically bonded to hydrogen. Hydrogen has been employed in drones through an NaBH4 alkaline solution with a hydrogen weight percent of 10.8 wt%. In this application, Co/Al2O3 was used as a catalyst, yielding a flying time of 2.5 h. However, this system required large auxiliaries, such as a catalyst reactor, liquid pump, and fuel cylinder; thus, the system weight was heavy. Furthermore, a by-product was produced [6,7]. Although various previous studies have attempted to identify a useful material that has a high weight percent [8,9], the systems developed in those studies required several sub-systems; thus, the overall system was substantially heavier. Therefore, the use of chemical hydrides seems inappropriate for applications involving drones; particularly is a lighter system is necessary.

Hydrogen also can be stored as a compressed gas without any significant physical or chemical bonding to other materials. In mobility applications, compressed hydrogen is the most popular fuel, owing to its high energy density. For similar reasons, compression is an appropriate choice as a method for storing fuel for drones. To use compressed hydrogen in drones, lightweight cylinder technology is necessary. Cylinders are classified into four types, listed in Table 1, depending on the structure and materials.

Although type-3 and -4 vessels both show good pressure capacity and weight performance, the type-4 vessel is the most promising type of cylinder for drones; this is because it has a lighter weight, owing to the use of a polymeric liner, as well as good endurance. Because the material of the type-4 cylinder easily melts at high temperatures, the temperature changes in internal gas during the filling process has been of substantial research interest to satisfy safety standards [11]. The temperature rise inside the gas cylinder during fast hydrogen filling has been analyzed. In particular, the temperature and pressure inside a cylinder were monitored as a 90.5 L cylinder was filled from 50 to 650 bar. At a filling rate of 1.7 bar/s, the maximum temperature in the cylinder reached 75 °C. Therefore, the fueling time should at least be 7 min [12,13]. In addition, the level of turbulence in the cylinder during the refueling process was examined. Using a suitable turbulence model, the flow of the high velocity inlet jet penetrating the stagnant gas in the cylinder can be expressed. The combination of a realistic k–ε model and Reynolds stress model can reduce the calculation time by 86 % compared with that of other models; furthermore, it can increase the accuracy by up to 90 % [14,15]. In addition, the temperature distributions corresponding to various burst types in a type-4 cylinder were studied. Through experimental and numerical methods, burst simulations in a pressurized hydrogen storage vessel were examined, demonstrating that the burst type was not related to the temperature distribution but rather to fiber breakage [16,17]. A recent study on the development of a type-4 vessel with novel liner materials has been reported [18]. This study involved the development of a type-4 vessel using a nylon liner as a self-contained breathing apparatus. This cylinder can store air at pressures of up to 1100 bar; furthermore, it exhibits a gas penetration that is 160 times lower than that of a high-density polyethylene cylinder. Nevertheless, in terms of significance, studies on type-4 vessels seem insufficient. This is due to the “commercial immaturity” of the type-4 vessel. The cylinder is difficult to purchase because the technology is still immature, as is the market of type-4 cylinders. This complication has led to insufficient studies regarding the development of type-4 cylinders. Despite the difficulties involved in the development of type-4 cylinders, a substantial number of studies on type-4 vessels are needed to accelerate the commercialization of type-4 cylinders in the fuel cell propulsion market.

The objective of this study is to develop a lightweight type-4 cylinder with a polyethylene terephthalate (PET) liner for application in drones. PET is easier to process than Nylons, so that it is possible to produce thinner liner with it. Another advantage is that PET has gas permeability lower than those of other materials. Permeation of hydrogen is one of the important issues that have been raised for type-4 cylinders. We fabricated a type-4 cylinder using a PET liner and conducted a parametric study on the thermal behavior of the developed cylinder during a filling test using hydrogen to examine the safety of the system. Safety factor is 3, which is based on KGS AC 418 and EN12245 and is defined by the ratio of burst pressure to service pressure. Noted that the same regulation applies to the cylinder installed in DT30 of Doosan Mobility. First, we fabricated a type-4 cylinder using a PET liner. Then, the variation of pressure and temperature inside the cylinder during the filling test was examined; furthermore, a leakage test was also conducted. Through a parametric study, we examined whether the developed cylinder satisfies the safety standard for hydrogen cylinders. Moreover, the effect of the inlet gas temperature on changes in the final temperature and pressure at the end of filling process was analyzed to reveal the internal conditions in the type-4 cylinder. Lastly, a comparison of the charge capacity per unit mass for different types of cylinders was conducted to confirm the advantages of the developed cylinder. The results of this study can facilitate the development of an efficient and economic drone technology, resulting in increased flight time; this will have a substantial impact on fuel cell propulsion.

Section snippets

Experiments

In this section, the fabrication process of the cylinder and the experimental setup of the filling test are described. A type-4 composite cylinder was developed using PET as a liner, and the thermal behavior of the vessel during the hydrogen gas filling test was examined.

Results and discussion

In this section, the changes in the pressure, temperature, and expansion of the cylinder during the filling test are elucidated. Ultimately, the advantages of the developed type-4 composite cylinder with PET serving as a liner were confirmed through comparison with other types of cylinders.

Conclusion

In this study, a lightweight type-4 cylinder was developed using PET as a liner for application in drones. We examined the safety of the developed cylinder during the filling test. First, the type-4 cylinder was fabricated. The PET liner was obtained through the blow molding process; then, it was wound with carbon fiber during the filament winding process and hardened by heating. The volume of the developed vessel was 6.8 L, and the target pressure was 300 bar. Next, a parametric study on the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT [grant numbers NRF-2019R1F1A1058036, NRF-2019M3E6A1064705], as well as by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Korean government (MOTIE) (No 20007892, Development of tube skid using cylinders (700 bar class 1400 L)

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