Factors such as rarefied air and low temperatures in the plateau environment will bring challenges to the water management, temperature control, and performance of fuel cell systems. Therefore, fuel cell vehicles need to be specially tested and debugged in the plateau environment to ensure their safe, reliable, economical and efficient operation.
The performance of international advanced models in the plateau environment
Figure 1 Comparison of different altitude starts
First of all, compare the difference between the plain and the plateau when moving. Compared with the plain environment, the air intake pressure is 20% lower and the air compressor motor speed is 51% higher when Plateau 1 starts, and the air intake pressure is 26% lower and the air compressor motor speed is 116% higher when Plateau 2 is started. It can be seen that the plateau environment has a great influence on the air intake pressure and the speed of the air compressor motor, and the international advanced models do not control the low-pressure setting value of hydrogen and the speed of the hydrogen circulation pump on the plateau compared with the strategy of the plain.
Figure 2 Comparison of idle speeds at different altitudes
Figure 3 Comparison of power ratings at different altitudes
Compared with the rated power performance of the international advanced models in the Plains and Plateaus 2, the power of the fuel cell is limited. Based on our previous testing experience, there are 5 scenarios that can lead to fuel cell system power limitations. They are stack temperature too high, single cell voltage too low, hydrogen leakage, hydrogen pressure too low, and air pressure too low. After analyzing the relevant signals, we ruled out the first four causes. Focusing on the performance of the air compressor, the peak speed of the air compressor in the plain is 15540rpm, and the speed in the plateau reaches 16170rpm, which is 96 of the rated speed8%。The air compressor is working at close to full capacity, but it still cannot provide the inlet pressure and flow required by the stack. As a result, the output power of the fuel cell stack in the plateau area will be reduced.
The diffusion effect is the phenomenon of spontaneous transport of substances from an area of high concentration to an area of low concentration. In plateau areas, the partial pressure of oxygen in the cathode flow field is lower due to the thin air, and the concentration difference between the cathode flow field and the reaction area is smaller, and the diffusion effect of oxygen is reduced. Especially at high power output, the performance of fuel cell stacks is greatly affected by the diffusion effect, and the output power will be significantly reduced.
The strategy of international advanced models in the plateau environment
Figure 4 Air compressor map
In order to compare the path difference between the operating conditions of the plateau and the plain air compressor, the path of the plateau and the plain operating condition points was drawn on the basis of the map data obtained from the air compressor bench test, and the operating characteristics of the air compressor were intuitively displayed. The higher the altitude, the slower the air intake pressure increases with the increase of the speed of the air compressor, and does not increase after reaching a certain speed, because the speed of the air compressor has reached the limit at this time, in order to increase the output power of the stack as much as possible, the excess coefficient must be increased and the air pressure cannot be selected, so the opening of the back pressure valve gradually increases, the air pressure also decreases immediately, and the air flow gradually increases at the same time to ensure the power demand of the stack.
It can be seen from the figure that the air compressor is in the area of low flow and high pressure ratio when running in Plateau 2, and the risk of surge increases. Compared with the plain environment, the performance of other major components such as hydrogen circulation pump and cooling water pump in the plateau environment has not changed much. The performance of the compressor hardware itself (maximum speed, surge area) determines the performance of the fuel cell system in the plateau environment. Therefore, if you want to make fuel cell vehicles achieve better performance on the plateau, improving the performance of the air compressor itself is an important direction for the development of the industry in the future.
For more information on the performance and control strategies of fuel cell vehicles in the plateau environment, welcome everyone to come to the Tehi Xi class in December.
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Tehi Hydrogen Testing will hold the "Hydrogen Energy Vehicle Prospect Information to Current Product Development Enlightenment Xi Workshop" from December 13th to December 14th. The seminar will invite the first batch of senior engineers, registered safety engineers and practical experts engaged in hydrogen energy testing and evaluation in China to conduct a two-day professional training, answer and exchange on the issues that everyone is concerned about.
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