What are the components of a fuel cell vehicle? The basic principles of a hydrogen fuel cell electric vehicle

Composition of fuel cell vehicles

Fuel cell vehicles are mainly composed of fuel cells, power batteries, high-pressure gas tanks, fuel cell boosters, motors, power control devices (including DC/AC converters), etc.

Fuel cells do not directly burn fuel to generate power, but use electrochemistry to convert the chemical energy of fuel into electrical energy. In other words, a fuel cell vehicle is equivalent to an electric vehicle with its own electrochemical generator. In theory, fuel cells can operate at a thermal efficiency close to 100%, which is very economical. Due to the limitations of various technical factors, the total conversion efficiency of various fuel cells currently in operation is mostly in the range of 45%-60%, and if the heat exhaust utilization is considered, it can reach more than 80%. According to the different types of electrolytes, current fuel cells are mainly divided into the following six types: proton exchange membrane fuel cells (PEMFC), methanol fuel cells (DMFC), phosphate fuel cells (PAFC), alkaline fuel cells (AFC), molten salt fuel cells (MCFC) and solid oxide fuel cells (SOFC). From the actual application situation, proton exchange membrane is considered to be an ideal technical solution for automotive fuel cells with its outstanding advantages such as high power density, light weight, small size, long life, mature technology, and rapid start-up at low temperature. It is also a widely used and relatively mature technical path. In the past three years, the proportion of global installed capacity in the total installed capacity has remained stable at more than 75%.

The power battery is mainly used to store the excess power of the fuel cell and the electric energy recovered during the vehicle's driving process so that it can be used when the vehicle accelerates suddenly and for use by on-board electrical appliances.

High-pressure gas storage tanks are mainly used to store hydrogen. When storing hydrogen in high-pressure gas cylinders, a low-temperature insulation device must be used to maintain the low temperature. The low-temperature insulation device is a complex system.

In a fuel cell system, the generated electrical energy needs to be boosted by a fuel cell booster before it can be supplied to the motor. The final output voltage must reach a certain value to meet the maximum output requirements of the motor.

The power control device is mainly composed of the fuel cell engine management system (FCE-ECU), the battery management system (BMS), the power control system (PCU) and the vehicle control system (VMS).

1. Engine management system. The fuel cell engine management system controls the power output of the fuel cell engine according to the power setting value of the vehicle controller, monitors the working status of the engine, ensures stable and reliable operation of the engine, and performs fault diagnosis and management. The specific components of the system include hydrogen supply system, oxygen supply system, water circulation and cooling system.

2. Battery management system. The battery management system is divided into two levels. The lower level LECU is responsible for measuring the physical parameters of the battery pack, such as voltage and temperature, and performing overcharge and overdischarge protection and intra-group and inter-group balancing; the upper level CECU is responsible for current detection and remaining capacity (SOC) estimation of the power battery pack, as well as related fault diagnosis, and runs the high-voltage leakage protection strategy at the same time.

3. Power control system. The power control system includes DC/DC converter, DC/AC converter, DCL, air conditioning controller, air conditioning compressor inverter, and motor cooling system controller. The functions of DC/DC converter and DC/AC converter are as mentioned above. DCL is responsible for converting high-voltage power into 12V/24V low-voltage power required by system components, and the motor cooling system controller is responsible for the water cooling system control of the motor and PCU.

4. Vehicle control system. The core of the vehicle control system is the multi-energy control system (including the brake energy feedback function). On the one hand, it receives the demand information from the driver (such as ignition switch, accelerator pedal, brake pedal, gear information, etc.) to realize the whole vehicle working condition control; on the other hand, based on the actual working condition feedback (such as vehicle speed, braking, motor speed, etc.), as well as the status of the power system (fuel cell and battery voltage, current, etc.), it performs energy distribution adjustment and control according to the pre-matched multi-energy control system.

How hydrogen fuel cell vehicles work

Hydrogen fuel cells are power generation devices that directly convert the chemical energy of hydrogen and oxygen into electrical energy. Hydrogen fuel cell vehicles have significant performance advantages over existing energy vehicle models.

Figure 1 Performance of hydrogen fuel cell vehicles and other models

The basic principle of hydrogen fuel cells is the reverse reaction of water electrolysis, supplying hydrogen and oxygen to the positive and negative electrodes respectively (anode reaction: 2H2+2O2-→2H2O+4e-; cathode reaction: O2+4e-→2O2-; overall battery reaction: 2H2+O2→2H2O). After hydrogen diffuses outward through the anode and reacts with the electrolyte, it releases electrons through the external load to reach the cathode. Therefore, hydrogen fuel cell vehicles rely on hydrogen to generate electricity, and then drive the car through the electric motor inside the car. Hydrogen energy has the characteristics of energy saving, environmental protection, low carbon and high efficiency. Compared with existing energy models, hydrogen fuel cell vehicles have certain advantages in safety, energy replenishment efficiency, cruising range, low-temperature start-up, energy conversion efficiency, recovery and emissions, and are expected to usher in large-scale market promotion in the future.

Figure 2 Working principle of hydrogen fuel cell vehicle

Why can't hydrogen fuel cell vehicles be promoted on a large scale at present?

The core factor that currently limits the promotion of hydrogen fuel cell vehicles is the high cost. (1) On the one hand, the cost comes from the high vehicle purchase cost. Take Changan Automobile as an example. Its new model Deep Blue SL03 offers three models: pure electric, extended-range and hydrogen fuel. The prices are 200,000 yuan, 170,000 yuan and 700,000 yuan respectively. It can be seen that for the same model, the price of the hydrogen fuel version is much higher than that of other models. According to calculations, the main reason for the high vehicle purchase cost is the high manufacturing cost of the core component fuel cell stack. The cost of the fuel cell stack accounts for more than 60% of the cost of the whole vehicle. The localization process of core raw materials such as proton exchange membranes, catalysts and gas diffusion layers in the stack is slow, and most products still rely on foreign imports. With the technological breakthroughs of domestic manufacturers driving domestic substitution, the cost is expected to gradually drop in the future;

(2) Costs also come from the operating costs during the use of vehicles. In terms of hydrogen production, traditional coal-based hydrogen production currently has high environmental pollution, and industrial byproduct hydrogen and more advanced water electrolysis hydrogen production have high costs. In terms of hydrogen storage and transportation, the traditional 20MPa gas hydrogen truck consignment mode is difficult to meet large demand and long-distance transportation. The international advanced level usually adopts 50MPa high-pressure hydrogen storage. In the future, liquid hydrogen transportation and pipeline transportation will be more likely to meet long-distance and large-capacity transportation needs. In terms of hydrogen refueling, the current construction cost of a single hydrogen refueling station is 2-3 times that of a gas station of the same specifications. In summary, the current lower-priced cases include the water electrolysis hydrogen production integrated station in Zhangjiakou Wangshan, with a hydrogen refueling agreement price of 30 yuan/kg, and the higher-priced cases include the external hydrogen supply station in Shanghai, with a price of 60-70 yuan/kg. Compared with the current fuel and charging prices, there is still a lot of room for decline in the future.

Figure 3 Left: Fuel cell vehicle cost structure; Right: Fuel cell stack cost structure

Future market prospects

Favorable policies and technological progress promote product parity, and the market is expected to grow gradually. Just as the electric vehicle industry relied on subsidy policies in the early stages of development, the state's policy support for hydrogen fuel cell vehicles is also of great significance in the early stages of the industry's development. In the "New Energy Vehicle Industry Development Plan (2021-2035)", the state clearly pointed out that it is necessary to orderly promote the construction of a hydrogen fuel cell vehicle supply system, including improving the economy of hydrogen fuel production, storage and transportation and promoting the construction of hydrogen refueling infrastructure. In addition, the Ministry of Finance, the Ministry of Industry and Information Technology, the Ministry of Science and Technology and the National Development and Reform Commission jointly issued the "Notice on Improving the Financial Subsidy Policy for the Promotion and Application of New Energy Vehicles" in 2020, pointing out that the current purchase subsidies for fuel cell vehicles will be adjusted to select cities or regions with foundations, enthusiasm and characteristics, focusing on the technical research and industrial application of key components. Demonstration, the central government will adopt the "award instead of subsidy" method to reward demonstration cities. At present, the core components in fuel cell stacks have gradually been replaced by domestic products. With the cost reduction effect brought about by technological breakthroughs by domestic enterprises and the gradual scale-up of product volume, the cost is expected to gradually drop in the future, promoting market-oriented applications.