# Research on an Energy Recovery Strategy for Fuel Cell Commercial Vehicles Based on Slope Estimation
## 5 Experimental Simulation and Conclusions
### Construction of the Electric Power following System  
The fuel cell and power cell work together to supply energy for the fuel cell commercial vehicles' electrical system. Together with the fuel cell, the power cell is a crucial auxiliary energy source in the fuel cell power system, supplying the necessary power for the vehicle during load beginning, acceleration, and climbing. It also recovers some of the brake energy used during vehicle deceleration and braking. Fuel cell vehicles obtain their power mostly from fuel cells, which are devices that directly transform chemical energy into electrical energy. To balance the two types of batteries, the Matlab/Simulink platform was utilized to construct an exquisite power tracking system, which effectively regulates the output power of both fuel cells and power cells. The core idea of the power following system is to maintain the battery working in the optimal range and accurate output power, The power following system includes the following types [28]:  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Curb weight/kg & 5000 \\ Full load mass/kg & 9000 \\ Front area of the vehicle/m\({}^{2}\) & 6.6454 \\ Drag coefficient & 0.563 \\ Rolling radius/m & 0.413 \\ Transmission resistance & 0.015 \\ Transmission efficiency & 0.98 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Vehicle parameters.  
Figure 8: Joint simulation process.  
Firstly, the power of fuel cells and power cells should be evaluated, express as:  
\[P_{b}\leq P_{bmax} \tag{47}\]  
\[P_{f}\leq P_{fmax} \tag{48}\]  
Step1: The fuel cell and power batteries are inoperable when the vehicle is not started, expressed as:  
\[P_{b}=P_{f}=0 \tag{49}\]  
Step2: When the vehicle is in braking condition, the State of Charge (SOC) of the power battery is lower than the SOC\({}_{\max}\) (the highest critical point of power batteries), with the fuel cell output mode off, and the regenerative braking mode activated, expressed as:  
\[P_{f}=0 \tag{50}\]  
\[P_{b}=\beta P_{load} \tag{51}\]  
In the formula, \(\beta\) is the braking force distribution coefficient and \(P_{load}\) is the vehicle power demand.  
Step3: The power cell uses all of its energy when the vehicle is first started since the fuel cell's immediate power is insufficient. It is decided whether to turn on the fuel cell once the necessary circumstances are met.  
Step4: The analysis of the working conditions of the power battery and fuel cell during normal vehicle operation is as follows:  
1. The power battery operates independently  
When the power demand of the entire vehicle is low and the output power of the power battery is greater than the demand power, and the SOC of the power battery is greater than SOC\({}_{\max}\), the fuel cell is closed.  
2. Fuel cells work independently  
When the fuel cell starts normally and the maximum output power of the power battery cannot meet the normal demand power, and the SOC of the power battery is less  
Figure 9: Vehicle transmission system.  
than the set expected value, the charging mode is activated to restore the SOC of the power battery to the expected value. The power relationship is as follows:  
\[P_{f}=P_{load}+P_{b} \tag{52}\]  
\[SOC^{\star}=\frac{SOC_{max}+SOC_{min}}{2} \tag{53}\]  
In the formula, \(SOC^{\star}\) is the set SOC expectation value, \(SOC_{max}\) and \(SOC_{min}\) are the maximum and minimum value of SOC for power batteries.  
3. Joint output of power cells and fuel cells  
When the vehicle is in the acceleration or climbing stage, the output power of the fuel cell is not enough to meet the power demand of the vehicle, and the SOC of the fuel cell is greater than the set expected value. At this time, the output power of the power cell will be controlled to ensure that the fuel cell operates efficiently. The power relationship is as follows:  
\[P_{f}=P_{load}-P_{b} \tag{54}\]