Basic knowledge of fuel cell system

The fuel cell system is mainly composed of battery stack, hydrogen and oxygen supply device, humidifier and deionized water supply device, cooling device, exhaust gas and product discharge device. Among them, the hydrogen is provided by a high-pressure gas cylinder, and the oxygen (air) is provided by a blower. After the hydrogen and oxygen are decompressed by a pressure reducing valve and then humidified by a humidifier, they enter the fuel electrode and oxygen electrode of the battery stack for reactive power generation. The reactants are discharged with the tail gas, and the water is collected and discharged. The battery temperature is adjusted by the circulating water volume.

The performance of the battery stack depends on the performance of a single battery. The output voltage of the battery stack is the sum of the voltages of the individual cells that make up the battery stack. The life of the battery stack depends on the life of the single cell that is damaged first. Therefore, the uniformity of the performance of a single battery has a great impact on the battery stack, and a detection device should be set to detect the output voltage of each single battery online to ensure that the battery performance is intact.

Low-temperature proton exchange membrane fuel cells are classified into two types: normal pressure type and pressurized type according to the pressure of the input air. Just as the internal combustion engine system uses exhaust turbocharging to increase the power density of the engine, the fuel cell system can also increase its power density by increasing the reaction gas pressure. This fuel cell system is called a supercharged fuel cell system. The fuel cell system whose reaction gas pressure is about one atmosphere (1.013×105Pa) is called an atmospheric fuel cell system.

① Supercharged fuel cell system.
The proton exchange membrane fuel cell stack has two inlets and two outlets, which are respectively connected to the hydrogen circuit (fuel electrode) and the air circuit (oxygen electrode). The hydrogen from the hydrogen storage tank enters the fuel electrode inlet In1 through the pressure regulating valve and the jet pump. The system takes an excessive supply of hydrogen, and the hydrogen discharged from the fuel electrode outlet Outl returns to the jet pump to realize the recycling of hydrogen.

The compressor and the expander are installed on the same drive shaft. When the fuel cell is started, the compressor is driven by the motor 1 (powered by the auxiliary battery), and air is pressed into the oxygen electrode inlet In2 through the surge tank. After the fuel cell is started, the compressor is converted to a motor 2 powered by the fuel cell. The motor 2 has a higher power and a higher voltage than the motor 1. The air flow is adjusted by controlling the speed of the motor 2 to meet the excess air ratio and power demand.
The larger the excess air coefficient is, the larger the compressor power consumption, the smaller the net power output by the fuel cell, and the lower the efficiency. In order to improve the efficiency of the fuel cell system, in addition to reasonably optimizing the excess air coefficient value according to the fuel cell operating conditions, the gas discharged from the oxygen electrode outlet Out2 enters the expander inlet, and the remaining energy in the discharged gas is used to drive the compression by the expander. Machine to realize energy recovery. The gas discharged from the expander is discharged into the atmosphere through the gas-water separator.

In a supercharged fuel cell system, the compressor is a very critical component. There are many types that can be selected, including twin-screw type, Roots rotor type and vane type.

In order to improve the efficiency of the system, the compressor and expander are used in combination to recover the energy in the exhaust gas while completing the compressed air, reducing power consumption. The compression-expander system has fast dynamic response time, compact size, light weight and low cost.
Air pressurization requires power consumption, and this power is called parasitic power. Although people install an expander on the side of the air outlet to recover the expansion power, even if the system developed by Ballard with excellent technology, when working at a pressure of 0.3 MPa, there is still about 20% of the total power consumption in the auxiliary In the system, the main consumption is the air compressor. Due to the addition of the compression-expander system, the entire fuel cell system is large and complex, and the price is high. The compressor is used to minimize the parasitic power, but it also limits the total amount of air that can enter the oxygen electrode.

② Atmospheric fuel cell system.
Although the power density is increased by supercharging, the overall efficiency of the system is reduced. In response to this shortcoming, the American UTC company has developed an atmospheric fuel cell system. The parasitic power of the system is only about 5% of the power emitted by the fuel cell.

The atmospheric fuel cell system has the following characteristics.
a. Directly humidify the electrolyte membrane with liquid water at the fuel pole to ensure that the electrolyte membrane is fully hydrated.
b. The oxygen electrode supplies approximately atmospheric air, with low parasitic power loss and high system efficiency.
c. The oxygen electrode is supplied with a non-humidified air flow. The system does not need a humidification module, and the amount of liquid water in the flow channel is small, so the pressure drops to a very low level.
d. In order to remove the water generated at the oxygen electrode, the air flow rate supplied to the oxygen electrode is large, so a large amount of water will not accumulate.
e. The fuel cell stack is cooled by directly using the water entering the reaction gas, which greatly simplifies the cooling system.
f. Because it is a low-pressure system, the sealing, pipe joints, and piping between the battery stack and the system are easy to handle.

The diaphragm-type water pump in the system sends water to the water channel of the fuel electrode in order to directly humidify the electrolyte membrane with liquid water. Theoretically, the supply water flow only needs to be equal to the flow required for evaporation, but in order to maintain a continuous flow and remove bubbles , Can make the water circulate slightly. In order to maintain the pressure in the water channel on the bipolar plate at the fuel pole slightly higher than the pressure in the hydrogen channel and prevent water from being replaced by hydrogen, a back pressure valve is installed on the water outlet pipe to make the water pressure in the stack greater than that in the hydrogen pipe. In the pressure. The atmospheric fuel cell system has a hydrogen circulation channel. After the hydrogen comes out of the cell stack, it first passes through the water tank, and then returns to the fuel electrode inlet via the diaphragm water pump (hydrogen pump) installed in the channel. The diaphragm water pump is used to flush the water condensate in the hydrogen circulation channel, otherwise there may be a lack of hydrogen to participate in the reaction in some parts of the battery stack. If the system is large, the method proposed by Ballard can also be used to circulate the hydrogen using a jet pump. This method can utilize the energy contained in the compressed hydrogen itself.