At present, the cold start problem of Proton Exchange Membrane Fuel Cell (PEMFC) is a major constraint to the further development of fuel cells, and in order to improve the cold start capability of fuel cells, the common methods mainly include structure optimization, auxiliary start and load control optimization. Among them, structural optimization and auxiliary start provide the basis for hardware guarantee for the improvement of cold start performance, and operation strategies such as load control can further improve the cold start performance of fuel cells conveniently and efficiently.
PEMFC's cold-start load control strategy can be divided into current, voltage, and power according to its main control objects. The three control strategies have their own advantages and disadvantages, and it is necessary to choose a more suitable load control strategy according to the comprehensive comparison between the start-up process and its own conditions.
Galvanic start-up is the most common load control strategy in many experiments and model studies. The current is relatively easy to control during cold start, so the constant current mode is beneficial for stabilizing the flow rate in the experiment [1]. For this strategy, there are currently two directions of current optimization.
The first is to take full advantage of the electrochemical heat production of the fuel cell before it is completely clogged with ice [2], so this strategy usually uses a lower current density, because a lower current density means a lower water production rate, which gives the ionomer in the stack time to recombine to absorb water to form modal water, thereby delaying the accumulation of ice. Although its heat production efficiency is reduced relative to high current density, the total heat generated is comparatively more.
The second strategy prefers to allow the stack to heat up quickly, successfully starting the stack before the ice is plugged [3]. This strategy assumes that the higher the final temperature after a failed cold start, the higher the likelihood of a successful cold start [4]. Therefore, this strategy chooses a higher start-up current density because it increases the rate of heat generation and reduces heat loss during start-up.
In addition, linear converter start-up combines the characteristics of both and is a better current control strategy. From the above explanation, it can be seen that the lower current density slows down the formation of ice and increases the total amount of heat generated. The higher current density facilitates rapid temperature rise, and linear converter initiation combines the advantages of both [5]. At the initial low current, ice is prevented from forming; As the temperature increases and the operating current increases, the rate of ice formation remains low even when more water is produced, as the water capacity has increased. However, there are also problems with linear converter start-up, and the current rise rate and rise time will greatly affect the performance of cold start, which requires many experiments and ** to finalize.
Constant voltage start-up is also a load control strategy that can be adopted, which is more difficult to control than the constant-current start-up mode, and often requires the cooperation of DC DC converters to ensure voltage stability [6]. However, at low voltage, the stack can always maintain a high heat production rate during cold start, which can generate more waste heat than constant current start, but this also reduces the output power and efficiency of the stack [7].
When fuel cells are used as part of a vehicle's hybrid system, they tend to operate at constant power for optimal performance and efficiency. The initial current density of constant power start-up is large, and the redistribution of membrane water in MEA is caused by the electroosmotic resistance effect of high current, the water content in the anode catalytic layer decreases sharply, and the total resistance of the battery increases, and the current density also decreases. Therefore, from the perspective of cold start, the constant power has a limited effect on the improvement of cold start ability.
Maximum power start is also an important control strategy for cold start, which can quickly enable the fuel cell to reach full load. The contrast between heat production and constant pressure and constant current start is shown in Figure 1, and it can be found that unlike the decrease in the heat production rate in the later stage in the constant pressure mode, the heat production rate increases after the first decline stage in the maximum power cold start mode. This advantage is essential for a quick start-up [8]. However, the limitation of the maximum power strategy is also obvious, for the cold start process, the maximum power point of the fuel cell is changed in real time, and it is difficult to determine the maximum power point in actual operation.
At present, the PEMFC cold-start load control strategy can be divided into three categories: current, voltage and power, and the advantages and disadvantages of the three types of control strategies are shown in Table 1. In the actual and experimental process, it is necessary to comprehensively consider the operating status and environmental conditions of the stack, and select the best cold-start load control strategy in combination with the cold-start requirements.
Advantages and disadvantages of control strategy: low current, low current density, easy current control; The low water yield rate can delay ice accumulation; The total heat production is high, and the heat production rate is low; Long start-up time, high current density, easy current control; High rate of heat production; Reduce the heat loss during start-up, and the icing rate is faster; It is easy to cause the degradation of the stack structure, and the linear converter combines the advantages of the above two advantages, and the rise rate is difficult to determine. It is difficult to formulate a strategy to maintain a high heat production rate of the stack at a constant voltage. Generating more waste heat requires the cooperation of DC DC converters; The output power and efficiency of the stack are reduced, and the constant power of the stack can make the stack obtain the best performance and efficiency, and it is difficult to improve the cold start capacity of the stack, and the maximum power makes the battery quickly reach the working state of full load; The heat production capacity is strong, and the determination of the maximum power point is complex and difficult to control.
*Note: Jimei Power].
1] luo y, jiao k. cold start of proton exchange membrane fuel cell[j]. progress in energy and combustion science, 2018, 64: 29-61.
2] tajiri k, tabuchi y, wang c y. isothermal cold start of polymer electrolyte fuel cells[j]. journal of the electrochemical society, 2006, 154(2): b147.
3] lei l, he p, he p, et al. a comparative study: the effect of current loading modes on the cold start-up process of pemfc stack[j]. energy conversion and management, 2022, 251: 114991.
4] hu k, chu t, li f, et al. effect of different control strategies on rapid cold start-up of a 30-cell proton exchange membrane fuel cell stack[j]. international journal of hydrogen energy, 2021, 46(62): 31788-31797.
5] yang x, sun j, sun s, et al. an efficient cold start strategy for proton exchange membrane fuel cell stacks[j]. journal of power sources, 2022, 542: 231492.
6] zenith f, skogestad s. control of fuel cell power output[j]. journal of process control, 2007, 17(4): 333-347.
7] yang y, ma t, du b, et al. investigation on the operating conditions of proton exchange membrane fuel cell based on constant voltage cold start mode[j]. energies, 2021, 14(3): 660.
8] pan m, li d, pan c, et al. maximum power tracking-based adaptive cold start strategy for proton exchange membrane fuel cell[j]. energy conversion and management, 2022, 273: 116387.
9] du q, jia b, luo y, et al. maximum power cold start mode of proton exchange membrane fuel cell[j]. international journal of hydrogen energy, 2014, 39(16): 8390-8400.