100 W proton membrane fuel cell emergency power supply system

China has a vast territory and various natural disasters occur frequently. The gasoline generators commonly used in emergency power supply equipment for disaster relief and emergency response are relatively bulky, noisy and release harmful gases. Lithium batteries, nickel-metal hydride batteries, lead-acid batteries, etc. have short continuous power supply time and cannot provide charging recovery in emergency situations. This paper proposes an emergency power supply system that uses PEM fuel cells and lithium batteries for co-supply. Continuous power supply can also be guaranteed during the replacement of hydrogen storage containers. The control system uses fuzzy algorithms to dynamically distribute and manage energy according to the lithium battery SOC, the optimal working state of the fuel cell and the load conditions. A prototype for use in emergency situations has been developed. The system has a long continuous power supply time (2 to 3 times that of the current commonly used equipment), is noiseless and has zero emissions. It can achieve good results and is an ideal emergency power supply equipment for disaster relief and emergency response.
1 System composition
The composition of the fuel cell emergency power supply system is shown in Figure 1.

The system consists of a 120 W proton membrane fuel cell, a fuel cell controller, a lithium battery and management system, and an energy management unit. The lithium battery index is 13.2 V/10 Ah, to ensure the tactical requirement of emergency full power support for 1 hour when the fuel cell fails or the fuel is exhausted and replaced in time. Fuel cell stack index: power is 120 W, output voltage is 15 V~28 V. The fuel cell controller mainly completes the control and monitoring of the stack temperature, input hydrogen and air pressure, flow, and abnormal conditions of the stack, and transmits information to the system controller through the CAN bus. The system controller mainly completes the real-time detection of the load size, lithium battery SOC and fuel cell stack working condition, and dynamically manages energy according to the fuzzy algorithm, so that each component of the emergency power supply system works in the best state, so as to improve the efficiency of the whole machine and the service life of key components.
2 Circuit Design
2.1 Charging and Battery Management Circuit The
lithium battery charging circuit is shown in Figure 2. The DC voltage is added to the 15th pin of MAX1873 through the isolation diode D5. Ql is the charging drive signal output switch tube. R4 is the charging current detection resistor, which is used to detect the size of the output current. R2 is the detection resistor of the system current. R5 and R6 are the output charging voltage adjustment resistors.

The 15 V to 28 V voltage output by the fuel cell passes through the isolation diode D5 and the total current detection circuit, one way passes through R2 and the DC/DC circuit to the output end, and the other way passes through Q1, inductor L1, D6 and R4 to charge the lithium battery. The voltage on R4 is proportional to the charging current, which is amplified by the voltage error amplifier and converted into a DC component and input into the microprocessor. The microprocessor will output a reverse control voltage from the 14th pin of MAX1873 to reduce the conduction current of Ql. If the current flowing through R4 is too small, the control voltage output by the 14th pin of MAX1873 will increase the current of Ql accordingly, which will make the battery pack have a constant current value. When the current is very small and reaches the minimum charging current or 0, MAX1873 outputs a low-level pulse control signal from the 14th pin to turn off BGl and stop charging the battery. When the control input is low, BG2 is turned on, the charging control pin 6 (ICHG/EN) is low, the 14th pin outputs a low level, BG1 is turned off, and charging stops. At this time, the charging current is only 1 μA, and it is in the off state (charging is prohibited).
2.2 DC conversion and control circuit The
DC/DC conversion circuit uses the XL4012 integrated converter, with an input voltage of 3.6 V~36 V, a switching frequency of 2 800 kHz, and an output voltage that can be adjusted from 0.8 V~28 V. The conversion efficiency is as high as 95%, the maximum output current is 12 A, and the peripheral circuit is simple.
There are many parameters that need to be detected in the emergency power supply system: the output voltage and output current of the fuel cell; the charging current, battery voltage and battery SOC of the charging and BMS; the output current and output voltage of the output end. Therefore, it is necessary to expand the A/D interface. The system control adopts 89S51CPU, and the A/D adopts TLV2543 chip. The chip has 10 analog voltage inputs and uses a serial interface with the single-chip microcomputer. It occupies less port resources and has a faster conversion speed. The display adopts LCD1602 liquid crystal display. When the backlight is not used, the dynamic current of the liquid crystal is not more than 5 mA. It mainly displays the working status of the fuel cell, the SOC and charging and discharging status of the lithium battery, the output voltage, output current information, the efficiency of the whole machine and other power supply information.
3 Fuzzy control algorithm
Let the fuel cell be in the best state, and at the same time let the lithium battery charge state be above SOCmin. The output power of the lithium battery is adjusted with the power share allocated to the fuel cell as the constraint condition. For lithium batteries, when the minimum limit value of the battery SOC (SOCmin) is less than or equal to 30%, the lithium battery must be charged; when the SOC is between 50% and 70%, it can be charged or discharged depending on the load demand power; when the SOC is greater than 90%, it is not charged. The load power Pg and the lithium battery state of charge SOC are used as the input variables of the fuzzy control, and the fuel cell output power Pfc and the lithium battery output power Pb are used as the output variables of the fuzzy controller. The basic domains of the fuzzy input variables Pg and SOC are [0, 100] W and [30, 90]%, respectively. The input variables are fuzzified, and the fuzzy subsets are {ZO (zero), PS (positive small), PM (positive middle), PB (positive large)}; the domain of the fuzzy output variable Pb is [-100, 110] kW, and the fuzzy subsets are also {NB (negative large), NM (negative middle), NS (negative small), ZO (zero), PS (positive small), PM (positive middle), PB (positive large)}, and the domain of the fuzzy output variable Pfc is [0, 110] kW, and the fuzzy subsets are also {ZO (zero), PS (positive small), PM (positive middle), PB (positive large)}.
The membership function of the fuzzy controller that selects input and output fuzzy variables based on the load power Pg and the charge state of the lithium battery is a triangle as shown in Figures 3, 4, 5 and 6.

5 System Simulation
A fuzzy controller is established in the Matlab simulation system. The domains of the input variables of the fuzzy control, the target power Pg and the state of charge SOC of the lithium battery, are taken as [-100, 110] W and [30, 90]%, respectively. The domains of the output variables of the fuzzy controller, the fuel cell distribution output power Pfc and the lithium battery distribution output power Pb, are taken as [0, 110] kW and [-100, 110] W, respectively. The lithium battery is 10 Ah/13.2 V, and the initial state of charge SOC of the battery is 60%. At the same time, the time is taken in Matlab/Simulink as 0 to 15 min, and the simulation waveform is shown in Figure 8.

6 Prototype test and evaluation
According to the battery SOC and load size, the fuzzy algorithm is used to dynamically allocate and manage the energy of PEM fuel cells and lithium batteries. A prototype was developed. The actual test shows that the power supply efficiency of the whole machine is above 90%, and the specific power is 120 W/500 g. When the initial SOC of the lithium battery is 80%, it can continuously supply power for a 600-liter metal hydrogen storage tank for about 16 hours. The continuous working time and maintenance are greatly improved compared with the performance of traditional emergency power supply equipment. It is currently being industrialized and has great promotion value.
References
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[2] Wang Wenbo. The development direction of fuel cells [J]. High Technology and Industrialization, 2010(01): 118.
[3] Jin Keruan, Xinbo. Composite fuel cell power supply system [J]. Transactions of China Electrotechnical Society, 2008, 23(3): 92-98.