## 4 Cold Start Control Strategy for PEMFC-MH Coupling System
### Analysis of PEMFC-MH System Cold Start Results
The heating power of the MHT in the self-heating is significantly reduced compared with that in the preheating because the increase in temperature and SOC in the MHT leads to a decrease in the hydrogen absorption rate. When the rate of heat production from hydrogen absorption is lower than the rate of temperature increase for the circulating coolant, the coolant in turn heats the MHT. As shown in Figure 14, the heating power of the MHT at 127 s is 0, and the MHT becomes a burden in self-heating, and at this time, the bypass valve can be closed to reduce the cold start time. The temperature and output power variation for the stack during the cold start stage are shown in Figure 15, from which it can be seen that the temperature of the stack in the preheating period is significantly lower than that of the self-starting, and the stack is heated from a temperature of \(-20\)\({}^{\circ}\)C to an operating temperature of 65 \({}^{\circ}\)C in a total of 145 s, and the end of the cold start only takes 101 s.  
The percentage of heat provided by MHTs at different SOCs during cold start is shown in Figure 16. The preheating is longer compared to the self-heating, and there is a lower output power for the fuel cell at the beginning of the start-up; the proportion of heat generated by the self-heating of the fuel cell is only about 20%. When the initial SOC of the  
Figure 14: Comparison of heating powers of different heat sources during cold start. Figure 15: Temperature and output power variation for the stack during the cold start. MHT is low, the proportion of heat provided by the MHT is about 55%, which means it is the main heat source in the cold start stage. The system energy consumption during the cold start includes the air compressor, water pump, and radiator, and the system hydrogen consumption includes the hydrogen absorbed by the MHT and the hydrogen consumed by the self-starting of the stack. A comparison of the cold start performance of the PEMFC-MH system with and without MHT heating when the initial SOC of the MH is 0 is shown in Table 4.  
MHT generates a large amount of heat when absorbing hydrogen and does not require additional energy consumption. The reasonable use of this part of the heat can effectively reduce the cold start time and energy consumption of the PEMFC-MH system; compared with the normal cold start, the use of MHT heating can reduce the cold start time by 59.3% and the parasitic energy consumption by 62.4%. It is worth noting that the use of MHT heating will increase the cold start hydrogen consumption of the system, although this part of the hydrogen consumption can be re-supplied to the fuel cell through the heat-coupled system without wasting hydrogen, but it still has a high requirement for the external hydrogen source. The PEMFC-MH system in this paper is designed to ensure the hydrogen charging pressure of the MHT by adding external gaseous hydrogen tanks during the cold start. Since the pressure in the buffer tank is always kept at a low pressure of 4 bar during the cold start and normal operation, if the buffer tank is designed to be large enough to hold enough hydrogen to last until the end of the cold start, the buffer tank alone is sufficient for charging the MHT and supplying hydrogen to the fuel cell without the need for an additional gaseous hydrogen tank.