Practically speaking, whether it is a fuel cell system or a lithium-ion battery, the consequences of a safety accident are extremely serious. However, from the perspective of system control, the author personally believes that fuel cells are easier to control in terms of the controllability of safety factors than lithium-ion power batteries.
2.1.2 Safety analysis of fuel cellsThe safety evaluation of the fuel cell system (PEMFC system) is quite different from that of the lithium ion battery module. The safety evaluation of PEMFC is mainly for the two parts of PEMFC stack and hydrogen storage system, and they are all directly related to hydrogen.
Safety of PEMFC stack: PEMFC stack is a lot of single cells assembled according to the filter press. The stack is only the place where the hydrogen and oxygen react electrochemically. It does not store energy itself. This is compared with the conventional secondary battery. It is very different. There are two main aspects of the safety control of the PEMFC stack. One is the protection of the battery pack. After detecting the abnormal voltage and temperature, the supply of hydrogen and air can be cut off in a very short time to avoid accidents.
On the other hand, hydrogen monitoring is the main safety hazard. The comprehensive test results of Toyota and Daimler-Benz on its FC-EV show that even if the reactor is short-circuited under the working condition, it will not cause the fire and explosion of the stack, mainly because of the amount of hydrogen inside the stack. Not big, and hydrogen/air can be cut off quickly. For the stack itself, there are two main points for the leakage of hydrogen, one at the hydrogen supply interface and the other at the MEA stacking gap. The current hydrogen sensor technology is very mature in terms of sensitivity and reliability, and can ensure that the control system cuts off the hydrogen gas path in a very short time, thereby avoiding the accumulation of hydrogen in the power cabin.
Safety of hydrogen storage system: The biggest safety hazard of PEMFC system lies in hydrogen storage tank. At present, FC-EV generally uses a glass fiber/carbon fiber reinforced ultra-high pressure aluminum bottle for hydrogen storage, and the pressure can be as high as 700 bar. The storage of hydrogen depends on the volume and quantity of aluminum bottles. Currently, the FC-EVs of several major automobile companies are generally loaded with 5-10Kg of hydrogen, which can meet the cruising range of 450-700Km. In general, the explosion volume of hydrogen ranges from 13-59%. Then you need to analyze under what circumstances the hydrogen will leak and the explosion problem may be caused after the leak.
For the hydrogen storage tank, the biggest safety hazard is the hydrogen leak caused by the damage of the cylinder under the action of external force. Collision friction between the stack itself or with the body metal parts may create a spark that detonates the leaking hydrogen. Therefore, how to avoid the hydrogen storage tank from being damaged by external force and how to avoid hydrogen explosion after damage is the most critical safety assessment factor of FC-EV.
The 700 bar high-pressure aluminum bottle, which is widely used at present, has thousands of pressure/decompression test records in the world. It should be said that it is effective in stress fatigue resistance. The hydrogen storage bottle has been subjected to rifle shooting under full load conditions. experiment. In order to avoid external damage, several major international automobile companies generally choose to place the hydrogen storage tank in the relatively safe part of the car under the rear seat or the back.
A hydrogen sensor is installed on the side of the gas tank, the cab and the power cabin to detect the hydrogen concentration on-line. The hydrogen storage tank is also equipped with an emergency drain valve to reduce the accumulation of hydrogen after damage. In general, fuel cell vehicles are only likely to cause major safety problems such as explosions in the event of a major traffic accident or stress fatigue that causes the hydrogen storage bottle to leak hydrogen. Usually, it takes a few seconds for the hydrogen leak to accumulate to the lower explosive concentration, and the passenger has a certain escape time under the alarm of the hydrogen sensor. Hydrogen is characterized by a very light leak and then rises rapidly, as long as it is well ventilated, there is generally no risk of explosion on open roads.
What I want to point out here is that people have some misunderstandings about the safety of hydrogen. The results of Japanese research experiments show that under the conditions of fuel leakage and ignition in gasoline and hydrogen fuel cell vehicles respectively, the oil leakage under the gasoline car caught fire in 3 seconds, while the hydrogen gas quickly rushed to the top of the car. After a minute and a half, the open flame of the fuel cell vehicle has been extinguished, and the gasoline car is booming and eventually burned out of the frame (as shown in the figure above).
Many domestic and foreign research institutes such as BMW, Daimler-Benz and China Automotive Research Center have also done collisions, soaking and dropping experiments of hydrogen fuel cells, collision and burning tests of hydrogen storage tanks, and collisions of fuel cell vehicles. There were no major safety issues in the tests. However, the author still emphasizes here that whether it is a lithium-ion pure electric vehicle or a fuel-cell vehicle, the comprehensive evaluation of safety issues requires large-scale testing and data collection on the basis of mass production, and it is possible to have a deeper understanding.
The BMS safety monitoring of large-scale lithium-ion power batteries is mainly based on the changes of cell temperature and voltage/current. As we can see from the above discussion, the thermal runaway inside the lithium battery is a chain exothermic gas-generating chemical reaction, that is, It is extremely short to say that the control time left for the BMS. The safety hazard of the fuel cell system comes from hydrogen. Essentially, the safety of PEMFC stacks is primarily physical processes (hydrogen leaks and controls), while lithium-ion batteries are chemical processes (chain reactions).
Practically speaking, whether it is a fuel cell system or a lithium-ion battery, the consequences of a safety accident are extremely serious. However, from the perspective of system control, the author personally believes that fuel cells are easier to control in terms of the controllability of safety factors than lithium-ion power batteries.
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