The use of fuel cells in the power drive system of vehicles can provide an effective solution to the problems of global energy shortage and environmental pollution. With the continuous development of fuel cell technology, how to apply fuel cells to vehicle systems and solve the problems of matching with many complex subsystems in vehicles becomes more important.
Fuel cell vehicles can be divided into pure fuel cell drive and hybrid drive according to the driving form. According to the energy source, it can be divided into two methods: vehicle-mounted pure hydrogen and fuel reforming. Since fuel cell electric vehicles are still in the early stage of research and development, various technical solutions have their own advantages and disadvantages.
The pure fuel cell vehicle has only one power source, the fuel cell, and all the power loads of the vehicle are borne by the fuel cell. Its main disadvantages include the following aspects:
① The power of the fuel cell is large and the cost is expensive;
② It puts forward high requirements for the dynamic performance and reliability of the fuel cell system;
③Recovery of braking energy cannot be performed.
Based on the above unfavorable factors, the current fuel cell vehicle mainly adopts the form of hybrid drive, that is, on the basis of the fuel cell, a group of batteries or super capacitors are added as another power source. The fuel cell vehicle using the "fuel cell + battery (FC + B)" hybrid drive form reduces the requirements for the power and dynamic characteristics of the fuel cell and the cost of the fuel cell system, but the disadvantage is that it increases the complexity of the drive system, its weight and volume are increased, and the maintenance or replacement cost of the battery.
According to the proportion of the power provided by the fuel cell in the total required power of the vehicle, fuel cell hybrid vehicles can be divided into two categories: energy hybrid and power hybrid.
1. Energy Hybrid
In the early stage of the development of fuel cell vehicles, due to the limitation of technical level, the power of fuel cells is small, and it is difficult to meet all the power requirements of the vehicle. Therefore, the fuel cell can only provide a part of the power demand of the whole vehicle during the driving process of the vehicle, and the insufficient part needs to be provided by other power sources (such as batteries). The fuel cell vehicle using this hybrid drive form is an energy hybrid fuel cell vehicle. In order to meet the required performance indicators, energy-hybrid fuel cell vehicles often need to be equipped with large-capacity battery packs, resulting in increased vehicle weight, poor power performance, and tight layout space. The fuel cell of the energy hybrid fuel cell vehicle can often work in the rated power region with higher system efficiency, but after each operation of the vehicle, in addition to refueling with hydrogen fuel, it also needs to use ground power to charge the battery pack.
2. Power Hybrid
With the continuous maturity of fuel cell technology and the gradual improvement of performance, the proportion of power provided by fuel cells is increasing, so that the capacity of the battery can be reduced, thereby reducing the weight of the vehicle and improving the power performance. In order to recover braking energy, a certain number of batteries are also required, but the battery pack only provides a small part of the power required by the whole vehicle. The fuel cell is the main power source, and the battery is the auxiliary power source. The power required by the vehicle is mainly provided by the fuel cell. The battery only provides power when the fuel cell starts, the car climbs and accelerates, and the braking energy is recovered when the car brakes. A fuel cell vehicle using this hybrid drive form is a power hybrid fuel cell vehicle.
Due to the high specific energy and specific power of nickel-metal hydride batteries or ion batteries, the volume and weight of the battery pack can be reduced, and they are now more and more used in auxiliary batteries for fuel cell hybrid vehicles. However, because the price of these batteries is still very expensive at present, and the changes of the working voltage, current, temperature, etc. of the batteries during use are closely related to their safety, they often need to be equipped with a special battery management system.
At present, fuel cell hybrid vehicles have various driving forms. In addition to the "FC+B" introduced above, in recent years, the "FC+C" drive form has appeared in the power hybrid fuel cell vehicle, that is, the combination of fuel cells and super capacitors completely abandons the batteries with short life, high cost and complex use requirements. The outstanding advantages of using supercapacitors are long life and high efficiency, and can effectively reduce the cost of use, which is conducive to the commercialization and application of fuel cell vehicles. In addition, some vehicles adopt the driving form of "fuel cell + battery + super capacitor" (FC+B+C). In this form, a group of supercapacitors are connected in parallel on the power bus to provide/absorb the peak current during acceleration and emergency braking, thereby reducing the burden on the battery and prolonging the service life of the battery.