Hydrogen fuel tank safety is tested and true
Fear of the unknown or unfamiliar is a completely natural and human response. The FCEV is relatively new to consumers, and can be unfamiliar to drivers, but are they really dangerous as some describe? Has the fear of unknown caused us to forget about the highly efficient an eco-friendly properties of hydrogen cells, frightened by the word-association with the hydrogen-bomb? We sat down with the researchers at the Hyundai and Kia Motors Mabuk Environment Technology Center, the cradle of Korea’s FCEV technology, and asked industry-leading experts. Our first question was about the safety of hydrogen fuel cells.
We met with the Hyundai-Ki Motors Mabuk Environment Technology Center Hydrogen Cell System Test Team part leader Hwang Ki-ho(left), Cell system Test Team lead researcher Chung Myeong-ju (right)
Structure of hydrogen
Hydrogen energy is still misunderstood by many. In fact, hydrogen energy is confused with nuclear power. How are the two different?
The hydrogen bomb has very little to do with FCEV technology, and the only real connection is the word hydrogen. The connection is comparable to how the world around us being consisted of atoms, and the atom bomb having the word atom in it. A better understanding of the technology behind the FCEV and the hydrogen bomb can clear this misunderstanding.
Hydrogen utilized by FCEVs are in the form of hydrogen molecules. Hydrogen gas is pressurized at about 700bar of pressure and stored in a cryogenic tank. To generate torque, gas is fed to a lower-pressure cell and put through an electro-chemical reaction to generate electricity. The safety and reliability of hydrogen fuel tanks concerning their high-pressure storage have been thoroughly engineered and tested, achieving a level of safety similar to that of standard CNG engines, which is the traditional means of engine locomotion.
In the case of the hydrogen bomb, materials are not pressurized hydrogen gas, but deuterium and tritium. Both deuterium and tritium are extremely in their natural states and require tremendous energy to subsist. Even if deuterium and tritium were present, getting the explosive power of a hydrogen bomb will require extreme heat and pressure. That is, over a hundred million degrees at thousands of bars of pressure. Such heat and pressure necessary to detonate a hydrogen bomb actually requires the detonation of a smaller nuclear bomb to “kindle” a fusion reaction. Unlike the hydrogen bomb, FCEVs simply combine oxygen and hydrogen to generate electric power.
General knowledge about hydrogen is that it is easily oxidized, or combustible. In the case that a vehicle fire occurs inside or outside the NEXO, or if the vehicle leaks hydrogen gas, is the hydrogen fuel tank safe?
First, there has not been a single case of an FCEV accident as a result of leaking hydrogen. Hydrogen is a gas 14 times lighter than the atmosphere, so it immediately vaporizes into the air if there is a leak. Even in the case of a vehicle fire, the chances of leaked hydrogen combusting is virtually nonexistent.
In addition, there are multiple real-time sensors that detect any leakage on the fuel tank, feed system, and the fuel cell stacks. If a hydrogen leak is detected, either during standard operation or in an external impact that damages the feed system, the driver’s dashboard display puts up a warning. The safety system may even choke the hydrogen fuel tank valve, preventing mass ejection of hydrogen from the tank, which may lead to other hazards.
If a fire causes the temperature around the hydrogen fuel tank to elevate to excessive levels, the detection system force-expels the hydrogen from the tank into the atmosphere. Due to these redundant safety measures, the tanks will not explode; even if the vehicle undergoes complete incineration. The tank itself is covered in a fire-proof coating. In terms of vehicle fires, leaks, and explosions, HCEVs are measurably safer than gasoline combustion vehicles.
What about hydrogen leaks in confined spaces, for example in the garage or in a tunnel?
Under very specific circumstances, a confined environment rich in hydrogen may lead to a fire or an explosion. The hydrogen containment system in the NEXO is designed to suppress any leakage to 1/60 of government policy standards for hydrogen leakage, and its safety has been verified many times over.
Furthermore, garages or tunnels are actually not completely enclosed spaces. There are building laws that dictate the facility of airflow through such structures. Unless those laws are intentionally disobeyed and any chance for airflow hermetically sealed, hydrogen leakage resulting in an explosion is extremely unlikely.
What materials are used for the hydrogen fuel tanks, how is it structured, and what is the strength of each material?
The inner surface of the hydrogen fuel tanks are made of a thin polyamide liner (nylon) that minimizes hydrogen permeation. The outer surface is covered in reinforced plastic (carbon fiber + epoxy) o 20-25mm thickness capable of maintaining a pressure of 700 bar.
In relation to the safety of the hydrogen fuel tanks, strength is key for rupture safety, while stiffness is important for durability. Strength is resistance to breakage when subjected to force, while stiffness is resistance to deformation when subjected to force. Carbon fiber reinforced plastics have many benefits over steel by weight. Two of such benefits are 6-times greater strength and 4-times greater stiffness. In other words reinforced plastics are lighter while being more dependable.
That sounds like it will be quite resistant to any damage caused by impact.
The hydrogen fuel tanks have withstood drop-tests from 1.8 meters without any damage to their functions. A bullet shot into a hydrogen fuel tank pressurized at 700bar resulted only in hydrogen gas being emitted into the atmosphere through the puncture, with no explosion. Even in a rear-impact test performed under conditions identical to standard internal combustion engines, the hydrogen fuel tanks had ejected all hydrogen, and not even trace amounts were detected. Even if there is a vehicle fire, redundant fire-resistant materials and safety mechanisms prevent escalation while rapidly expelling hydrogen contents, thereby preventing any possibility of an explosion. Such comprehensive safety systems earned NEXO the first maximum five-star overall rating by Euro NCAP.
Hydrogen fuel tanks are required by law to be tested for safety on 14 different items
What items are tested at the Mabuk Environment Technology Center to ensure the safety, durability, and other metrics of the hydrogen fuel tanks?
the Korean Ministry of Land, Infrastructure and Transport (MOLIT)’s administrative notice on the Regulation for Safety of CNG Pressure Vessels sets forth 14 items to be tested for safety, including rupture test, bonfire test, and penetration test. Furthermore, other safety tests are undertaken, including front and rear crash tests, extreme cold and weather condition tests.
Tests for NEXO’s hydrogen fuel tanks have nearly doubled since early development. Safety standards were raised for mass production. The technology needed not only meet regulations at home and abroad, but also satisfy Hyundai’s own standards designated for more than 200 separate tests. That is to say, we set much stricter standards for safety and durability than those set upon us by governing bodies.
For example, the drop-damage test to simulate damage during transportation, and the durability test against acute damage to the composite material have each been performed 12,000 times. Furthermore, more than 200 NEXOs were used to thoroughly vet the overall safety of the vehicle.
The internal pressure of the hydrogen fuel tanks is very high at 700bar. How is that pressure controlled until it is fed into the stack to generate electricity? And more importantly, how is that high pressure controlled to avoid harm to people?
FCEVs have a fuel cell stack inside that generates electricity using the hydrogen fuel and the oxygen drawn in from the atmosphere outside. The fuel cell stack requires pressure, and the minimum required pressure increases with the desired electrical output of the stack. The standard pressure inside the hydrogen fuel tank was about 350bar, but advancements in technology have allowed for greater pressure with safety and stability. Now the standard is about 700bar, and specifically in the case of the NEXO, maximum burst pressure reaches up to 1,575bar.
Instantly lowering the high pressure within the hydrogen fuel tanks is a challenge, so through the hydrogen fuel feed system connecting into the fuel cell stack, the pressure is curtailed over a two-stage process. First, the pressure of 700bar is lowered to 16bar through the first decompression device, then at the inlet of the fuel cell stack where hydrogen reacts with oxygen to generate electricity, the pressure is dropped even further to about 1.0-1.5bar.
Hydrogen is stored under high pressure in the fuel tank at 700bar, and as it is supplied to the fuel cell module, it undergoes decompression at 16bar, and the process involves certain risks due to high pressure and flammability of the gas. As such, the engineers applied a variety of safety mechanisms and logics. From the more basic leak-detection sensors to the more sophisticated high-pressure and medium-pressure sensors, the logic sensors detect excess pressure and micro leakages. Safety mechanism also includes logics that allow recharging only when the ignition is off, and logics that can detect hydrogen leakage which may occur during recharging.
The goal of our team is to ensure the safety of the vehicle operator
Is the durability or safety of the hydrogen fuel tanks affected by long-term operation of FCEV?
The goal of our team is to ensure the safety of the vehicle operator. The carbon-fiber composites coating the outer shell of the hydrogen fuel tanks are layered dozens upon dozens in patterns. This means that any damage to the carbon-fiber coating is limited locally to only a few strands, and does not impact the performance of the tank’s safety or durability.
Hydrogen fuel tank components meet the highest safety standards, and they certification conditions are one of the strictest. In fact, the tanks are capable of withstanding external pressure equivalent to 15km or deeper under the ocean surface, durable for daily charges for 123 years, and safe after left attended after charging at 700bar pressure. Despite such engineered safety margins, the legal service life is maximum 15 years (20 in Europe) or 4,000 (5,000 in Europe), whichever comes first. As such, we have engineered our hydrogen fuel tanks to far surpass safety and durability standards set forth by law.
We tested the tank-charging process about 45,000 times and then another 15,000 times for post-drop or extreme weather conditions. There is no need to feel any more anxious than when you drive a fossil-fuel-based internal combustion engine, which is to say not at all. There is virtually no chance of hydrogen fuel tanks exploding.
Last year, NEXO turned heads by earning the first maximum five-star overall rating from Euro NCAP the European car safety performance assessment programme.
What challenges did the team overcome to achieve that rating?
The NEXO has three hydrogen fuel tanks, each the same size. Of the three tanks, tank no.2 and tank no.3 are on either side of the rear suspension. In order to improve ride quality, we swapped out the torsion-beam suspension for a multi-link system. This required a bolt fixture to hinge the support and the lower arm, a protruding component near the tanks. During a crash test, we found that the protruding bolt could become a hazard to the composite surface of the hydrogen fuel tanks, and housed the bolt and the vulnerable portion of the tank with a metal plate. We tested it again with the safety precautions, and found that even in an impact situation, the protrusion did not impact the hydrogen fuel tanks.
The NEXO has been a leading herald of the FCEV. There has already been outstanding reviews of the NEXO from around the world.
The team is following-up on the NEXO with yet another next-generation FCEV. What goals has the team set forth in terms of safety and performance?
Dispelling ungrounded fear of hydrogen fuel tanks is going to be a major goal. We are developing a smart tank for hydrogen fuel tanks, that provides the vehicle operator with real-time notifications of the tank’s status. Considering the safety of FCEVs and their hydrogen fuel tanks, we are already in a pretty good place. Seeing it from a more consumer-oriented perspective, we understand there is some uneasiness about the hydrogen fuel tanks, and we are trying to address that from an engineering point of view.
A key factor underlying all of this is cost reduction. FCEV popularization will require lower pricing, somewhere comparable to conventional internal-combustion automobiles. The high cost of hydrogen fuel tank is the main obstacle at the moment. Our research aims to achieve all three: safer vehicles made with less costly hydrogen fuel tanks and improved range. For example, we are researching alternative means of hydrogen storage; rather than high-pressure tanks, we are looking into liquid or solid-state storage, and various other means to maximize storage efficiency. Storage efficiency in hydrogen fuel tanks is measured in terms of the weight of total stored hydrogen gas divided by the weight of the hydrogen fuel tank. NEXO fuel tanks are among the world’s most efficient at 5.7%.
Current hydrogen fuel tanks are bulky and not the most space-efficient. The team is looking for ways to improve this aspect of efficiency with different tank shapes and form factors. As hydrogen fuel tanks become more lightweight and cheaper to manufacture, the HCEV will become increasingly mainstream.