Aenert. Research Laboratory news
Hydrogen is considered to be one of the pillars of the imminent energy transition. The gas, however, brings along several challenges concerning its safe transportation as it is highly explosive and must therefore be transported in high-pressure tanks. These tanks have a pressure of 700 bar and are made of fiber-reinforced composites (FRC). Compared to metal tanks, these are ideal for use in the mobility and transport sector due to their low mass.
To ensure the highest security, the H2 pressure tanks undergo extensive testing before they are used for the first time. It is also important that the tanks maintain their integrity in the face of recurring stresses caused by refueling and withdrawal of hydrogen or in the event of damage (e.g., rear-end collision). To prevent this from occurring, regular maintenance of the high-pressure storage systems is also of paramount importance. However, the tank inspection which is carried out every two years consists merely of an external visual inspection. Damage inside the tank using this conventional inspection method cannot be detected. Alternatively, damage can be averted by continuously monitoring the pressure vessel — a process known as structural health monitoring or SHM for short.
Now (2023), in the course of a joint research project HyMon, researchers from the Fraunhofer Institute for Structural Durability and System Reliability LBF are developing a sensor-based on-board structural monitoring system enabling continuous surveillance of the H2 pressure tanks, which is aimed at providing a high level of safety for hydrogen vehicles.
One of the main aims of this on-board structural monitoring system which features suitable sensors and evaluation electronics is to provide data for service and repair. There, acoustic emission sensors play a critical role. The working principle is based on sensors detecting high-frequency sound waves. If a single carbon fiber tears in the pressure tank, a sound wave is generated that travels through the fibers. The sensors then determine the number of broken fibers. Special load cases, such as rear-end collisions, can damage local areas of the tanks, causing a lot of fibers to break in a very short space of time. The measurement signals are processed by evaluation electronics to provide information about the health status of the tank. The requisite algorithms and methods for detecting fiber breaks are being developed at Fraunhofer LBF. These include, for example, sound wave frequency analyses.
In addition to the acoustic emission sensors, fiber-optic strain sensors are also integrated into the tanks. They consist of light-conducting glass fibers with fiber Bragg grating sensors integrated into them. The glass fibers are enveloped into the FRC layer of the tank during manufacturing or applied to the surface afterwards to enable continuous or periodic automated monitoring of strains at the hydrogen tank. Unlike conventional strain sensors, these glass fibers are particularly suitable for monitoring carbon fiber-reinforced plastics due to their resilience to high material strains and load cycles. The measurement data from the strain sensors is used firstly to verify the calculation models of the pressure tanks and secondly to gain insight into how the material behaviour changes throughout the tank’s service life in order to draw conclusions about the fatigue state of the material.
The first stage of the testing process will include producing various types of damage such as fiber breaks, matrix breaks or delaminations in the test rig at Fraunhofer LBF with the help of sensor-equipped carbon fiber flat specimen. The damage signals will be recorded with the sensors. Next, the sensors will be assessed as to whether they are capable of recording the signals in sufficient quality and whether the algorithms can classify the damage mechanisms correctly on the basis of the signals. In the next step, the entire sensor system will be tested inside thin-walled tank models and then on high-pressure hydrogen tanks, which are subjected to cyclical stresses under internal pressure until failure occurs. The research teams want to find out how many sensors are needed for structural monitoring, where they need to be positioned, and which adhesives are most suitable for attaching them to the hydrogen tank. Eventually, a test vehicle will be equipped with sensors and on-board structural monitoring and validated by combining a virtual crash with a real-life test setup.
In view of the current energy crisis, efficient energy storage has become a top priority of many industries. In 2020, scientists made a model and carried out an analysis of hydrogen storage vessels together with complete structural and thermal analysis. They completed the basic structural design of the airborne cryogenic liquid hydrogen tank in this research. The problem of excessive heat leakage of the traditional support structure was addressed by creating a new insulating support structure. Then, they evaluated the thermal performance of the designed tank. The structure of the tank was analysed by the combination of the film container theory and finite element numerical simulation method. The structure of the adiabatic support was analyzed by using the Hertz contact theory and numerical simulation method. A simple structure analysis method to design a similar container structure and point-contact support structure was created. Bases for further structural optimisation design of hydrogen tank will be provided also. The analysis was conducted using materials such as titanium, nickel alloy and some coated powders like alumina, titania and zirconium oxide.
Image: (from left to right) Titanium, nickel, zirconium
Source: Siddharth Senthil Kumar, Bibin Chidambaranathan, Ramachandran Manickam/ Design and Analysis of Hydrogen Storage Tank with Different Materials by Ansys/ IOP Conference Series Materials Science and Engineering 810(1):012016, May 2020/ DOI:10.1088/1757-899X/810/1/012016/ Open Access This is an Open Access article is distributed under the terms of the CC BY 3.0 Deed Attribution 3.0 Unported
In 2020, the behaviour of compressed hydrogen gas in a hydrogen tank was analysed for its discharge. The effects of the gas models were examined to account for hydrogen gas behavior at the discharge temperature and pressure conditions. Turbulence model effects were analyzed to consider the accuracy of each model and compared in terms of the turbulence intensity. From the study of gas model effect, the Redlich–Kwong equation (an empirical, algebraic equation that relates temperature, pressure and volume of gases) was found to be one of the realistic gas models of the discharging gas flow. In this area, the shear stress transport model and Reynolds stress model could analyse the compressed gas behavior more accurately, showing a lower turbulence intensity than those of the realizable and renormalization group models.
Image: The cross-section of a 74-L Type III hydrogen tank
Source: Moo-Sun Kim, Joon-Hyoung Ryu, Seung-Jun Oh, Jeong-Hyeon Yang/ Numerical Investigation on Influence of Gas and Turbulence Model for Type III Hydrogen Tank under Discharge Condition/ Energies 13(23):6432, December 2020/ DOI:10.3390/en13236432/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)
There are several advantages to using sensors on a tank: They can catch the high-frequency sound waves when a fiber breaks, which enables the algorithms to detect the broken fibers and count them. A hydrogen tank is at the end of its useful life if the rate of fiber breakage suddenly increases. Also, constant on-board structural monitoring ensures an increased level of safety for hydrogen vehicles, as potential damage can be assessed even in the case of minor impacts, such as hitting a bollard, and the remaining service life of the tank can be estimated. Moreover, a comprehensive quality assurance has the benefit of avoiding unnecessary replacement of hydrogen tanks.
The project partners’ objective is to develop the complete system into a standard status monitoring system for future applications. The research team is positive that their system will make a valuable contribution towards a sustainable hydrogen economy.
By the Editorial Board
Hydrogen is considered to be one of the pillars of the imminent energy transition. The gas, however, brings along several challenges concerning its safe transportation as it is highly explosive and must therefore be transported in high-pressure tanks. These tanks have a pressure of 700 bar and are made of fiber-reinforced composites (FRC). Compared to metal tanks, these are ideal for use in the mobility and transport sector due to their low mass.
To ensure the highest security, the H2 pressure tanks undergo extensive testing before they are used for the first time. It is also important that the tanks maintain their integrity in the face of recurring stresses caused by refueling and withdrawal of hydrogen or in the event of damage (e.g., rear-end collision). To prevent this from occurring, regular maintenance of the high-pressure storage systems is also of paramount importance. However, the tank inspection which is carried out every two years consists merely of an external visual inspection. Damage inside the tank using this conventional inspection method cannot be detected. Alternatively, damage can be averted by continuously monitoring the pressure vessel — a process known as structural health monitoring or SHM for short.
Now (2023), in the course of a joint research project HyMon, researchers from the Fraunhofer Institute for Structural Durability and System Reliability LBF are developing a sensor-based on-board structural monitoring system enabling continuous surveillance of the H2 pressure tanks, which is aimed at providing a high level of safety for hydrogen vehicles.
One of the main aims of this on-board structural monitoring system which features suitable sensors and evaluation electronics is to provide data for service and repair. There, acoustic emission sensors play a critical role. The working principle is based on sensors detecting high-frequency sound waves. If a single carbon fiber tears in the pressure tank, a sound wave is generated that travels through the fibers. The sensors then determine the number of broken fibers. Special load cases, such as rear-end collisions, can damage local areas of the tanks, causing a lot of fibers to break in a very short space of time. The measurement signals are processed by evaluation electronics to provide information about the health status of the tank. The requisite algorithms and methods for detecting fiber breaks are being developed at Fraunhofer LBF. These include, for example, sound wave frequency analyses.
In addition to the acoustic emission sensors, fiber-optic strain sensors are also integrated into the tanks. They consist of light-conducting glass fibers with fiber Bragg grating sensors integrated into them. The glass fibers are enveloped into the FRC layer of the tank during manufacturing or applied to the surface afterwards to enable continuous or periodic automated monitoring of strains at the hydrogen tank. Unlike conventional strain sensors, these glass fibers are particularly suitable for monitoring carbon fiber-reinforced plastics due to their resilience to high material strains and load cycles. The measurement data from the strain sensors is used firstly to verify the calculation models of the pressure tanks and secondly to gain insight into how the material behaviour changes throughout the tank’s service life in order to draw conclusions about the fatigue state of the material.
The first stage of the testing process will include producing various types of damage such as fiber breaks, matrix breaks or delaminations in the test rig at Fraunhofer LBF with the help of sensor-equipped carbon fiber flat specimen. The damage signals will be recorded with the sensors. Next, the sensors will be assessed as to whether they are capable of recording the signals in sufficient quality and whether the algorithms can classify the damage mechanisms correctly on the basis of the signals. In the next step, the entire sensor system will be tested inside thin-walled tank models and then on high-pressure hydrogen tanks, which are subjected to cyclical stresses under internal pressure until failure occurs. The research teams want to find out how many sensors are needed for structural monitoring, where they need to be positioned, and which adhesives are most suitable for attaching them to the hydrogen tank. Eventually, a test vehicle will be equipped with sensors and on-board structural monitoring and validated by combining a virtual crash with a real-life test setup.
In view of the current energy crisis, efficient energy storage has become a top priority of many industries. In 2020, scientists made a model and carried out an analysis of hydrogen storage vessels together with complete structural and thermal analysis. They completed the basic structural design of the airborne cryogenic liquid hydrogen tank in this research. The problem of excessive heat leakage of the traditional support structure was addressed by creating a new insulating support structure. Then, they evaluated the thermal performance of the designed tank. The structure of the tank was analysed by the combination of the film container theory and finite element numerical simulation method. The structure of the adiabatic support was analyzed by using the Hertz contact theory and numerical simulation method. A simple structure analysis method to design a similar container structure and point-contact support structure was created. Bases for further structural optimisation design of hydrogen tank will be provided also. The analysis was conducted using materials such as titanium, nickel alloy and some coated powders like alumina, titania and zirconium oxide.
Image: (from left to right) Titanium, nickel, zirconium
Source: Siddharth Senthil Kumar, Bibin Chidambaranathan, Ramachandran Manickam/ Design and Analysis of Hydrogen Storage Tank with Different Materials by Ansys/ IOP Conference Series Materials Science and Engineering 810(1):012016, May 2020/ DOI:10.1088/1757-899X/810/1/012016/ Open Access This is an Open Access article is distributed under the terms of the CC BY 3.0 Deed Attribution 3.0 Unported
In 2020, the behaviour of compressed hydrogen gas in a hydrogen tank was analysed for its discharge. The effects of the gas models were examined to account for hydrogen gas behavior at the discharge temperature and pressure conditions. Turbulence model effects were analyzed to consider the accuracy of each model and compared in terms of the turbulence intensity. From the study of gas model effect, the Redlich–Kwong equation (an empirical, algebraic equation that relates temperature, pressure and volume of gases) was found to be one of the realistic gas models of the discharging gas flow. In this area, the shear stress transport model and Reynolds stress model could analyse the compressed gas behavior more accurately, showing a lower turbulence intensity than those of the realizable and renormalization group models.
Image: The cross-section of a 74-L Type III hydrogen tank
Source: Moo-Sun Kim, Joon-Hyoung Ryu, Seung-Jun Oh, Jeong-Hyeon Yang/ Numerical Investigation on Influence of Gas and Turbulence Model for Type III Hydrogen Tank under Discharge Condition/ Energies 13(23):6432, December 2020/ DOI:10.3390/en13236432/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)
There are several advantages to using sensors on a tank: They can catch the high-frequency sound waves when a fiber breaks, which enables the algorithms to detect the broken fibers and count them. A hydrogen tank is at the end of its useful life if the rate of fiber breakage suddenly increases. Also, constant on-board structural monitoring ensures an increased level of safety for hydrogen vehicles, as potential damage can be assessed even in the case of minor impacts, such as hitting a bollard, and the remaining service life of the tank can be estimated. Moreover, a comprehensive quality assurance has the benefit of avoiding unnecessary replacement of hydrogen tanks.
The project partners’ objective is to develop the complete system into a standard status monitoring system for future applications. The research team is positive that their system will make a valuable contribution towards a sustainable hydrogen economy.
By the Editorial Board
