Automobiles can be categorized into 4 major types, depending on their power systems: the internal combustion engine (gasoline, diesel, LPG), internal combustion+electric motor (hybrid, plug-in hybrid), electric motor, and fuel-cell electric vehicles. The majority of experts in the automobile industry see electric vehicles and fuel-cell electric vehicles to soon take over the automotive industry. So how are electric vehicles (EV) different from fuel-cell electric vehicles (FCEV)? EVs have large batteries that power the drive train motors, and can be charged by connecting power cables to the battery outlet.
FCEVs also have large batteries, but are capable of generating its own electrcity using a fuel cell stack system that uses chemical reaction between stored hydrogen and atmospheric oxygen. Refueling with hydrogen rather than electrical outlets, using atmospheric oxygen, using filtered dust-free air to generate a byproduct of only pure water, are some of the key differences between FCEVs and EVs. Is there any risk for people in any portion of that process? To find answers for those questions, we met with the research and development team members who worked on the safety and durability aspects of the fuel cell system of FCEVs.
Safer and more durable fuel cell system
The key to any FCEV’s green performance is the fuel cell system. How is safety and durability ensured for the fuel cell system?
Unlike combustion engines, a fuel cell system has several hundred volts flowing through it during operation. High voltage electricity can be harmful if it is not controlled, so electric safety is a key priority. From the most basic measures such as insulation resistance to prevent current leakage to securing appropriate electrical conductivity between components and grounding in the case of leakage, there are multiple factors that need to be considered and accounted for. Another important aspect is ensuring that hydrogen does not leak. In other words, electrical safety and hydrogen safety are two key safety considerations.
At the Technology Center, tests were performed to ensure safety and durability of not only the basic functions, but also electrical insulation, airtightness, durability, resistance to vibration, shock, water, dust, corrosion, and exposure to extreme water conditions, (high, low, and thermal cycling). This included repeated tests in hot and humid environments ranging from 35 to 95°C, as well as meticulous durability tests. The fuel cell system in the NEXO comes with a 10-year 160,000km durability guarantee, but the team at the Technology Center set engineering standards far exceeding that guarantee.
What kind of tests is the team conducting for the safety of fuel cell systems?
In addition to the tests described above, we have also engineered the system to meet standards in airtightness, insulation, and various performance standards. In fact, our goal is to engineer a system capable of protecting and controlling the fuel cell system, even under the direst contingencies. There can be hundreds of different ways that the control system can identify and diagnose malfunctions. Sometimes, a sensor may malfunction or there may be a serious hydrogen leakage, or high-voltage current might leak due to insulation failure. Each of those situations require minute response, care, and control.
The vast number of variables and combinations demand close attention, but also crucial part of raising public awareness of how safe the FCEV is. We are overshooting policy standards for mass produced FCEV, aiming for perfection.
Can you go into greater detail about the control systems for hydrogen leakage or voltage issues?
The dashboard warns the operator of a hydrogen leakage when leaked hydrogen is detected by a sensor. If the detection mechanism identifies a worsening situation, it chokes off the hydrogen feed, among other electronic control measures. Voltage issues are controlled in a similar way. Fuel cell stacks have multiple high-voltage components, and when any abnormality is detected, a warning on the dashboard warns the vehicle operator of the abnormal voltage situation. Once the system identifies a serious issue, which demands operator attention, it turns off the ignition so the automobile can be parked and the operator can dismount and make a clear assessment of the high-voltage components for issues. This was an intentional and strategic decision for operator safety in terms of high-voltage situations.
FCEVs seem to share certain aspects with high-tech electronic appliances, rather than 4-wheeled means of transportation. What other issues could occur, besides hydrogen leakage or high-voltage leakage? Also, how is each notified to the driver?
There are quite a few possible developments. For example, the hydrogen fuel cell stack requires unhindered supply of hydrogen and oxygen, but sometimes one of the two can be insufficient. It is not a critical safety issue, but it does require immediate technician attention. When the safety system identifies such an issue, the system limits output or induces an emergency operation situation.
What operator safety measures and fuel cell system mechanisms are implemented for collisions? Are there separate structures installed, for this purpose?
In a collision, the most important thing is to protect the vehicle operator. The same goes for pedestrians who may be affected by such a situation. We performed various tests to minimize injury to vehicle operators and pedestrians, which resulted in the NEXO being given the maximum five-star rating for collision safety by the Euro NCAP, the first FCEV to receive such a rating.
Further measures were taken to meet safety standards and regulations concerning electric and hydrogen hazards in the most catastrophic situations where the fuel cell system is damaged. The operator, passenger, first responders and even bystanders were considered. For example, a clause in high-voltage system related law states that “a fuel stack system should not become separate from the automobile in a collision”. The intention for the clause is to minimize hazard to passengers and rescue personnel. We ensured that our system meets that collision requirement of not having the fuel cell system become unfixed and separate from the chassis. We opted for an even stronger material for the supporting bracket to fix the fuel cell to the chassis, and a more lightweight mount that combines plastic and metal while also meeting safety laws.
We added an impact beam within the fuel cell stack to prevent deformation and electrical shortage, improving collision safety. We also added a rapid discharge mechanism to dispel all high voltage current from the fuel cell stack as a safety feature against when the fuel cell stack becomes uncontrollable due to a catastrophic collision. A similar safety mechanism was added for hydrogen, shutting off all valves when there is an accident. These are just examples of safety mechanisms that are easier to describe. There are far more with multiple redundancies too.
Is there a universal safety standard among FCEV manufacturers?
Yes there is, and it’s called the Global Technical Regulation (GTR) that is regularly revised to ensure FCEV safety internationally. Every market with FCEVs either has or is developing localized safety regulations and laws based on international standards. They generally include specifications to ensure electrical safety and hydrogen safety. That brings us to our case in Korea, and of course we are in full observance of standards and regulations through very robust testing and evaluation. There is some divergence among manufacturers because of varying system configurations, but the intention is always to maximize safety for people: there is no doubt about that.
Cooling is a major consideration in electric vehicles. The fuel cell stack generates energy, and that also generates heat. How does that impact durability?
Fuel cell systems require greater cooling than internal combustion engines. This is to prevent the electrolyte membrane within the fuel cell stack from being exposed to high heat, as that would curb optimal performance. The NEXO’s fuel cell stack is designed to keep the internal temperature at 85°C and under, using cooling controls.
To meet the specifications that come with higher voltage usage, we implemented a larger-radius fan, and tested performance in extreme conditions such as the Mojave Desert in the United States, where temperatures easily surpass 40°C. As it stands, the NEXO’s in-class cooling performance exceeds the Toyota Mira and the Honda Clarity. Fuel cell systems have a safety feature that throttles output when the stack’s internal temperature surpasses 8 °C, and the NEXO has significantly less throttling in comparison to other FCEVs.
What about extreme low temperature conditions?
Sub-freezing conditions can cause resistance within the fuel cell stack, on the periphery of the electrolyte membrane. Electric power generation suffers under such conditions. Another issue under sub-freezing conditions is that water, the byproduct of the hydrogen-water reaction, can freeze and cause a blockage in the fuel cell system, causing suboptimal airflow. When the internal temperature passes boiling point, the lack of moisture in the electrolyte membrane can also be cause of suboptimal performance. Optimized performance requires optimal temperature control. We engineered the NEXO to work optimally between extreme cold conditions of 30°C below freezing, to extreme hot conditions of 45-50°C such as in the Mojave Desert, without any trouble with ignition to driving.
Initially, the lowest ignition temperature for NEXO was 25°C below freezing, but the team managed to get it down to 30. How was that possible?
We chose a strategic control system that raises the stack temperature rapidly while maintains overall system efficiency. We used a separate bypass line over the fuel cell stack, and a separate heater to avoid cooling water going through the stack. This allowed us to also use the heat from the stack to charge a high-voltage battery. This allows the NEXO to turn the ignition quickly even under extreme conditions, and successfully boosts efficiency by capturing otherwise lost energy. That is the main engineering difference that allows the NEXO to turn its ignition on within 30-40 seconds, even after 24 hours in the natural elements at 30°C below freezing.
The NEXO features a new technology called MEA(Membrane Electrode Assembly) to lower costs and increasing durability, raising the fuel cell stack performance by 12.5%. How was that possible?
Fuel cell stacks consist of hundreds of cells, and each cell consists of an electrolyte membrane and catalyst, a fuel electrode (hydrogen), and an air electrode (oxygen). The performance of this cell was improved by 12.5%. The primary reason to this improvement is higher quality material use in the catalyst and electrolyte membranes, as well as a redesigned geometry of the hydrogen-oxygen passage (between the separating-plate and the gas-diffusion-layer).
We also chose a separate strategy for controlling the fuel cell stack. Previous designs supplied 1-bar gas into the cell, but the NEXO is capable of supplying gas at dynamic pressures. Durability issues were addressed in a similar way. We implemented a more robust cell design, as well as improved durability of the fuel cell stack by bolstering the internal components durable against high-voltage currents. In other words, we improved the NEXO in terms of materials used and control systems implemented.
Lowering costs is a major challenge for FCEVs becoming more accessible. Platinum used as catalyst in the fuel cell stack is a major reason for high cost. How far has research come for replacement catalyst?
We can reduce production costs and increase efficiency as new technologies become available. For example, fuel cell stacks were first used in the Apollo Program in 1967. The amount of platinum used in the fuel cells then and in 2007 varies by a factor of ten, approximately. Just between the last the NEXO and the current model the NEXO, platinum use was lowered by more than a quarter, and the goal for the next FCEV model is to cut that figure by half.
Recycling platinum catalysts is one possible option, but disposal is the economical opiton in Korea. As FCEV demand grows, recycling platinum catalysts may become a more economical option to disposal.