1. Energy management of plug-in hybrid electric vehicles
Given the driving range and driving cycle conditions, for a plug-in hybrid electric vehicles PHEV, since there is no pure electric range, the goal of its energy management is to minimize fuel consumption under the given driving cycle and total battery energy conditions. This is closely related to the characteristics of the energy source (battery and engine).
Figure 1 shows a model of an idealized hybrid PHEV for energy management research. In this model, the power losses of the mechanical coupling and transmission are factored into the vehicle power demand, which is obtained by adding the engine output power and the electric motor output power.
Figure 1 - Idealized hybrid PHEV model for energy management research
The lower case p represents lost power and the upper case P represents total power or output power.
The vehicle power demand can be calculated using the driving cycle profile. Figure 2 shows the distribution of the power required for a passenger car to run under the UDDS urban cycle condition of the US EPA. Figure 3 shows the distribution of rated demand power, where f(P0)/s refers to the number of times the vehicle is in a certain demand power P0.
Figure 3 - Distribution of rated power demand
2. Energy management control of fuel cell vehicles
The drive system of a fuel cell electric vehicle can be divided into two forms according to the different working modes: a pure fuel cell (FC) drive system and a hybrid drive system consisting of a fuel cell and an auxiliary power source. The pure fuel cell drive system 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 are: the required power of the fuel cell is large, which leads to an increase in cost; it puts forward high requirements for the dynamic performance and reliability of the fuel cell system; it cannot recover braking energy. Therefore, in order to effectively solve the above problems, it is necessary to use another set of energy storage system as the auxiliary power source of the fuel cell system to work in conjunction with the fuel cell to form a hybrid drive system to jointly provide power for the vehicle.
In the fuel cell vehicle using the hybrid drive system, the main power source is the fuel cell (FC), and the applicable auxiliary power sources are the battery (B), the ultra-capacitor (UC), and the ultra-high-speed flywheel (FW). The hybrid drive system composed of fuel cell and auxiliary power source mainly has several forms, such as FC+UC, FC+FW, FC+B and FC+B+UC. At present, major research institutions generally study the FC+B hybrid drive form.
The traditional internal combustion engine hybrid vehicle uses the combination of the internal combustion engine and the electric motor to drive the vehicle (that is, the power distribution between the two power sources), while in the fuel cell hybrid electric vehicle (FCHEV), it is the combination of electricity and electricity, That is, what is to be done is power distribution. Figure 4 shows the energy flow of a fuel cell hybrid electric vehicle, showing the power input or output relationship of the motor, battery, and fuel cell. According to the different control strategies of electric motor, battery and fuel cell power distribution, fuel cell hybrid electric vehicles mainly have two control modes: power follower mode (power follower) and switch mode (thermostat).
The basic idea of the switch mode is: optimal control of the fuel cell, that is, adjust the fuel cell with the minimum hydrogen consumption as the goal, make it work at the best efficiency point, make the fuel cell always work in the relatively low hydrogen consumption area, and use the battery as a power balance device to meet the specific vehicle driving power requirements.
The basic idea of the power follow mode is: when the state of charge (SOC) of the battery is between the lowest setting value (SOClow) and the highest setting value (SOChigh), the fuel cell outputs power within a certain set range, and the output power must not only meet the requirements of driving the vehicle, but also charge the battery pack. This power is called balanced power (that is, the battery is supplemented with energy to make it in the best SOC range).