Charging strategy through analysis of charging influence factors of ultra-light hydrogen storage with polyethylene terephthalate liner
Introduction
The continued use of fossil fuels not only leads to fuel depletion, but also causes environmental problems, such as CO2 pollution and global warming [[1], [2], [3], [4]]. Among the various alternative energies being developed, hydrogen energy is currently in the spotlight as a new renewable energy source [5]. Hydrogen has advantages such as eco-friendly, high energy efficiency and reproducibility [6]. Therefore, it can be used in a variety of fields, and its use in drones, whose military and civilian applications are advancing, has been considered. Most small drones are electrically powered because they must be lightweight and easy to control. In view of this, the interest in developing hydrogen-powered drones instead of using heavy power systems has recently been increasing [7].
Among various storage methods, the storage of hydrogen in the form of gas is the most widely used approach commercially [8]. To store hydrogen in a gaseous state, it must be maintained under high pressures because of its low energy density per unit volume [9,10]. Accordingly, it is necessary to develop a vessel that can safely store hydrogen under high pressure [11]. In addition, in the case of hydrogen-powered drones, the storage vessel should be lightweight because the flight time decreases as the weight increases.
For the storage of hydrogen in gas form, a type 4 vessel reinforced with a carbon fiber composite material on a non-metallic liner is widely used [12]. This vessel can store hydrogen at higher pressures than other types of containers and is considerably light and durable [13,14]. However, the liner used inside it is extremely heat-sensitive, and to ensure safety, the gas temperature inside the vessel should not exceed 85 °C [15,16].
Studies on the effects of temperature on hydrogen storage vessels have been conducted. First, existing studies on type 3 were performed as follows. Zheng et al. [17] tested a 74.3-L type 3 vessel with a target pressure of 70 MPa and found that the pre-cooling system can effectively reduce the maximum gas temperature during the refueling process. Dicken et al. [18] investigated a 74-L type 3 vessel and found that the highest rate of temperature increase was achieved at the onset of filling while the ratio of the current mass to the initial mass of gas at that point was the lowest. J. Xiao et al. [19] modeled a 150.8-L hydrogen tank and found that during the charging and discharging processes, the characteristic times were dominated by the flow rates. In the two dormancy processes, they were dominated by a pseudo mass flow rate, defined as the ratio of heat transfer ability to the specific heat capacity of the system. Next, there are existing studies about type 4 vessels. R. Ortiz Cebolla et al. [20] performed experiments on a 29-L type 4 vessel with a target pressure of 70 MPa. The experiments showed that the maximum temperature while filling a type 4 tank for 2–5 min can exceed the maximum temperature of 85 °C. Moreover, T. Bourgeois et al. [21] performed experiments and zero-dimensional modeling on a 90.5-L type 4 vessel with a target pressure of 700 bar. The effective heat transfer coefficient between the gas inside the vessel and wall at the specific ranges of Reynolds number and Rayleigh number was investigated. In previous studies, mainly type 3 was studied, and type 4 studies were insufficient. In addition, in the case of type 4, there is a lack of prior research on small vessels. The heat transfer coefficient varies depending on the flow and liner material. Then, there is necessary to lower the temperature in small vessels for ensuring the safety. Therefore, the present study is conducted.
In this work, the temperature characteristics of a lightweight type 4 vessel are analyzed during charging, and an optimized charging strategy, which lowers the temperature inside the vessel after charging and reduces the charging time, is developed. The vessel employed in this study is a 6.8-L type 4 vessel with a polyethylene terephthalate (PET) liner for weight reduction. Firstly, the appropriate heat transfer coefficient between the gas inside the vessel and liner is calculated using experimental results. Then, using the derived heat transfer coefficient, a parametric study is conducted on four variables: temperature of injected hydrogen, ambient temperature, mass flow rate, and initial pressure inside the vessel. Finally, a charging strategy that can control the temperature inside the vessel and charging time during charging is formulated. In this study, the temperature characteristics of a type 4 vessel with PET liner, and the effect of each of the above-mentioned four variables during charging are determined. In addition, an optimized charging strategy, which affords important information on the development and temperature control of vessel to ensure safety, is proposed in the paper.
Section snippets
Thermodynamic model
A model of a hydrogen storage vessel is developed in this work. The heat transfer in the hydrogen storage vessel and its walls is illustrated in Fig. 1. The hydrogen entering the vessel is heated by the Joule-Thompson effect and compression. The generated heat is then reduced by convection between the gas inside the vessel and inner wall and conduction to the PET liner and carbon fiber composite; thereafter, the heat is exchanged with ambient air outside the vessel.
It is assumed that the
Heat transfer coefficient
Several studies on the heat transfer coefficient of type 4 vessels have been conducted [20,21]. However, unlike conventional vessels, the model in this study uses a PET liner whose properties differ from those of existing liners, and its heat transfer coefficient varies according to the flow as well as properties. Hence, it is difficult to find a suitable heat transfer coefficient for this liner from previous studies. To overcome this issue, the results of experiments conducted by KGS are
Conclusion
In this paper, the temperature characteristics of a lightweight type 4 vessel are analyzed. Then, an optimized charging strategy that lowers the temperature inside the vessel after charging and reduces the charging time is formulated. Based on experimental results, the heat transfer coefficient between the gas inside the vessel and liner is derived. A parametric study is subsequently performed using the newly defined heat transfer coefficient.
- (1)
The heat transfer coefficient between the gas inside
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 research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2019R1F1A105803612). This work was supported by the “Development Project for Automotive Industry Core Technology (Green Car)” program funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea)/Project Title: Development of tube skid using pressure vessels (700 bar class 1400L) made of towpreg with a single shipment for hydrogen of
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