"Uncle Tang explains" Which is safer, hydrogen fuel cells or ternary lithium batteries?

This year can be said to be the first year of electric vehicle safety. In the past, both manufacturers and users who purchased electric vehicles only cared about performance and range, and no one cared about battery safety. It was not until the Tesla spontaneous combustion incident that people began to pay more and more attention to the safety of electric vehicles. This also made many performance defects of ternary lithium batteries that were previously unknown and avoided by manufacturers gradually surfaced, making people gradually realize the benefits of lithium iron phosphate.

If we compare ternary lithium with lithium iron phosphate, I believe many people will soon know that lithium iron phosphate is better than ternary lithium batteries in terms of safety. But if we compare ternary lithium with hydrogen fuel cells, which one is safer? In the past, people always talked about hydrogen with fear, thinking that hydrogen is a much more dangerous way of energy storage than gasoline. Is this really the case? How does a hydrogen fuel cell work? How is its safety designed? Compared with ternary lithium, which one is safer? This issue will explain to you

■ Core technology of hydrogen fuel cell power system: hydrogen storage tank + fuel cell reactor

Just like the battery pack in an electric car, the hydrogen storage tank is a device used to store hydrogen, and the fuel cell reactor is a device used to convert hydrogen and oxygen into electricity and water. Together, the two form a battery pack equivalent to an electric car, which can continuously provide power for the motor and the entire vehicle's electrical equipment.

The benefits of hydrogen fuel cells are obvious. The power conversion efficiency is extremely high, and the process is very quiet, and does not require a lot of heat dissipation. This is why the Toyota Mirai can travel more than 700 kilometers with only 5kg of hydrogen. The great energy density and high energy conversion efficiency are the biggest features of hydrogen fuel cells. The core technology of fuel cell reactors is proton exchange membranes and catalysts. These technologies are currently mainly monopolized by large European and American companies, and have not formed a large-scale industrialization in China. So it is easy to understand why the reactor is listed as a core technology, so why is the gas tank also a core technology?

▲Hydrogen storage tank made of carbon fiber + composite materials

■ The quality of sealing and durability determines the performance of the gas tank

Just like the equipment manufacturing level of a country, the performance of sealing materials in the industrial and civilian fields is also an important basis for measuring the manufacturing level of a country. Many European and American countries are absolutely confidential about the formula and manufacturing process of high-performance sealing materials. This is why in the traditional automotive field, those top-performance brake systems and fuel common rail systems for gasoline and diesel direct injection engines all come from European and American suppliers, because their core technology is sealing technology.

▲The structure of the hydrogen fuel cell gas tank

The same is true for hydrogen fuel cell vehicles. If you want to increase the mileage, you need to carry more hydrogen. The pressure resistance of the pressure container that can store hydrogen is the key to how much hydrogen can be carried. Because the density of hydrogen is very small, it must be compressed or even cryogenically liquefied before being stored in a pressure container to obtain sufficient energy density. The greater the pressure the container can withstand, the more hydrogen it can carry per unit volume. The greater the pressure the container can withstand, the higher the requirements for sealing and pressure resistance. Toyota Mirai's performance in hydrogen storage is world-leading, and the model has been mass-produced. Even so, carrying 5kg of hydrogen still requires two large gas tanks, which takes up a lot of space in the rear of the car, making the entire vehicle different from traditional electric vehicles in shape, and the height of the rear is relatively high.

▲Such a huge hydrogen storage tank takes up a lot of space and can only store 5kg of hydrogen

■ What are the safety designs of hydrogen storage tanks? Compared with ternary lithium batteries, which one is safer?

During the operation of the fuel cell vehicle, the fuel cell reactor is a place for chemical reactions. It only needs a small amount of hydrogen to continue to work. If a hydrogen leak or safety accident occurs, the hydrogen supply path can be quickly cut off to prevent the combustion of hydrogen. As everyone can imagine, the gas tank is the most likely factor to affect the safety of fuel cell vehicles.

Therefore, when designing a gas tank, the safety collision problem must be fully considered: on the one hand, the shell material of the gas tank needs to be light enough, and on the other hand, it needs to be strong enough to withstand the impact. Therefore, hydrogen storage tanks are generally made of expensive carbon fiber and other composite materials. However, we know that no matter how strong the material used to make the gas tank shell is, it can only guarantee collision safety under certain speed conditions. However, the speed of actual road driving is definitely much higher than the speed of safe collision in the laboratory. So once the hydrogen tank ruptures and causes hydrogen leakage and mixing with air, will it explode immediately?

In fact, this point is taken into consideration in the design of hydrogen storage. Because the hydrogen pressure inside the hydrogen storage tank is very high, an emergency pressure relief valve is designed on the gas tank. Once a hydrogen leak or fire occurs, the emergency pressure relief valve will automatically open and quickly release the hydrogen in the gas tank. We know that the density of hydrogen is much smaller than that of air, and it must be combined with oxygen to produce combustion or explosion. The hydrogen released by high pressure has little chance of contact with oxygen in the part close to the gas tank due to the high pressure and extremely fast flow rate. The hydrogen far away from the gas tank will rise rapidly in the air due to its low density. Even if there is an open flame in the air at this time to meet the ignition conditions, it will only form a fireball above the air and will not cause casualties. This high-speed hydrogen release technology is also used in large gas tanks at hydrogen refueling stations.

▲Core technology of hydrogen fuel cell vehicles

Compared with ternary lithium batteries, hydrogen storage should be safer because dangerous hydrogen can be quickly released from the crowd by isolating it from oxygen or releasing it quickly. In a sense, it is even safer than liquid gasoline. As time goes by, ternary lithium batteries age, and short circuits may occur internally, or they may spontaneously combust due to external impacts. The most terrifying thing is that whether it is spontaneous combustion caused from the inside out or combustion caused from the outside in, a large amount of oxygen will be released during the process, which means that even if it is isolated from the air, the battery will burn more and more vigorously.

■ Development prospects of hydrogen energy and ternary lithium batteries

In China, the biggest advantage of ternary lithium batteries is that their production capacity has been greatly developed under the encouragement of national policies in recent years. It can be said that Chinese companies have mastered their core technologies and have become the world's largest lithium battery producer. The cost has been greatly reduced and a large-scale industry has been fully formed. As for hydrogen fuel cell technology, China has not formed a large-scale industry, whether it is the process formula of reactor catalysts or the core technology of gas storage tanks, so the cost is extremely high. However, China has very good industrial conditions for the development of hydrogen energy. The hydrogen byproducts produced by the domestic petrochemical industry each year are sufficient for vehicle use. The cost of obtaining hydrogen is relatively low, and there is no need to rely on high-energy-consuming methods such as electrolysis of water to obtain hydrogen. This has laid a good foundation for the large-scale development of fuel cell vehicles.