1. Introduction
Nowadays, environmental pollution becomes an urgent issue that undoubtedly influences the health of humans and other creatures living in the world. This situation has spurred more studies to come up with renewable energy sources as a long-term solution for human survival. According to research by the International Energy Agency (IEA), from 1990 to 2018, renewable electricity generation rose rapidly with wind, solar photovoltaic (PV), and hydrogen resources. In which, the growth of hydrogen energy increased 97.3% and was forecast to remain the world’s largest source of green energy [
1]. It can be seen that hydrogen is one of the essential elements in the energy structure as well as has great potential to be widely used in the 21st century.
As the most proper and effective system using hydrogen, the PEMFC has been attracted as a potential candidate to combine with conventional energy devices such as a battery (BAT) or a supercapacitor (SC) [
2] in hybrid systems. The lower operating temperatures and higher energy conversion are the leading properties of the PEMFC in comparison with other types of fuel cells [
3]. However, slow dynamic response and incapability of recovering exceeded energy have become the key obstacles for widespread applications that use the PEMFC as a standalone energy source because the variation of load demand may result in fuel starvation, membrane drying issues, blooding; thus, causing the degradation of PEMFC lifetime [
4,
5,
6]. Hence, it is necessary to develop an integration of the PEMFC with other interconnected electrical storage devices such as BAT and/or the SC. For example, the integration of PEMFC with both BATs and SCs was investigated in many applications such as hybrid electric vehicles [
7,
8], construction machinery [
9,
10], and hybrid tramway powertrain [
11,
12], and some other fields of automation or power systems. This hybrid configuration can exhibit better performance, reduce the system size, solve the problem of fuel economy, and prolong the lifespan of energy devices. However, the problem of effective management should be considered for complex hybrid power systems. Therefore, the development of an EMS is required for appropriate energy assignment between the PEMFC and the energy storage system to match the required power of the powertrain.
Following the historical literature for the hybrid tramway’s EMS design, optimization-based methods have been successfully applied in several research works; however, the local optimal solutions of real-time optimizations or inconvenience of off-line computation in global optimizations are existing issues to address when implemented on hybrid systems in practice. Besides, most studies on optimization for the hybrid tramway have been implemented in two-device configurations such as BAT-SC, FC-BAT, and FC-SC because it is a challenge to achieve optimal objects for three-device configurations under complicated power-sharing strategies [
13,
14,
15]. Thus, rule-based EMS can be regarded as a simplified and effective selection to design efficient power distribution strategies for the hybrid system, especially the high-power system such as tramways, excavators, and so on. In [
16], a power flow control strategy was developed for a switcher locomotive-powered PEMFC-BAT-SC hybrid system to maintain levels of power demand while keeping the appropriate state of charge (SOC) on the energy storage devices (ESDs). In [
17], Garcia et al. presented an adaptive EMS based on states of the PEMFC and BAT to distribute the load power demand for each energy source. This strategy could ensure the power performance and satisfy the driving cycle of the hybrid tramway system under different working conditions. Developed from the work [
17], an operational mode control and cascade control loop were carried out in [
18]. The simulation results showed that the proposed EMS was able to produce an appropriate power for the traction load while keeping the SOC of ESDs and the DC bus voltage at the desired level. To achieve better efficiency of a hybrid LF-LRV system, Qi Li et al. [
19,
20] proposed a state machine strategy based on droop control to coordinate multiple power sources following the states of load change. The obtained results confirmed that the proposed algorithm could satisfy the rapid variations of power demand, ensure the steady operation during the most of diving cycle, and enhance overall tramway efficiency. Despite achieving good performance of power distribution, this method remains a drawback of not flexible operation because of switching modes that are usually based on the on/off mechanism to adapt to the particular working conditions. This may cause instability and delay to the system if the order of charge and discharge for ESDs is not appropriate such as power shortage for the hybrid system in the case of a sudden load change.
Considered a powerful tool, the fuzzy logic method has been applied to solve the complex issues of the logical process, especially in the power allocation for the hybrid system. Unlike the classical logic algorithm that requires clear knowledge, accurate equations, and exact numeric data of a system, fuzzy logic combines a different way of thinking, which allows complicated systems to be modeled using a higher degree of flexibility based on human knowledge and experience. In the fuel cell hybrid vehicular system, the fuzzy logic-based approach has been applied to develop the power management strategy in many works. Truong et al. [
21] employed the fuzzy logic control (FLC)-based EMS, developed based on the rule-based method from the previous work [
22], to satisfy the power demand, reduce fuel consumption and maintain storage devices’ SOC for excavators. For an electric vehicle application, Qi Li et al. [
23] used FLC to build EMSs for hybrid FC-BAT and FC-BAT-SC configurations to improve the fuel economy of the car and extend the mileage of the journey. In [
24], Ameur et al. exploited the master-slave model-based FLC strategy for an EMS design to improve the system efficiency and prolong the component lifespan for a renewable hybrid system. Ahmadi et al. constructed a fuzzy-based EMS with a genetic algorithm for a PEMFC integrated with BATs-SCs to improve the hybrid vehicle behaviors [
25]. The feasibility of the FLC methods for the PEMFC hybrid system-based transportation applications was investigated in some papers [
26,
27,
28,
29,
30,
31,
32]. For the hybrid tramway systems, the use of FLC was first recommended by Qi Li et al. [
33]. In this work, the multi-level Haar-Wavelet transform was incorporated into the structure of the 2-FLC approach to separate the high- and low-frequency features of the power demand. The results indicated that the proposed strategy achieved high efficiency without compromising the stack efficiency of the PEMFC and coordinated the power demand to each power source appropriately. In [
34], Zhang et al. designed a fuzzy controller for a locomotive system with input variables of load power demand and BAT SOC and output variable of the PEMFC required power. The study indicated that using this controller could not only keep the dynamic response of the PEMFC in the optimal region but also satisfy the dynamic requirements of the hybrid system. Besides, it also maintained the BAT SOC between 0.6 and 0.8 during the various operating conditions or quick changes of load. To guarantee an ideal BAT power in an FC-BAT-SC tramway system, Piraino et al. [
35] applied the FLC to find out the suitable factor of the BAT. In this strategy, the BAT power, BAT SOC, and SC SOC were input variables while the BAT corrective factor was the output variable. The simulation was implemented with a real driving cycle and the results proved that the suggested strategy not only made a sufficient BAT power to meet sudden and unexpected demand variations but also avoided critical SOC levels of the BAT and SC. Based on the advantages of FLC, Fragiacomo et al. [
36] combined this technique and equivalent consumption minimization strategy (ECMS) to design an EMS for the hybrid locomotive. Based on detailed evaluations of stochastic uncertainties in tramway operation, a suboptimal real-time power-sharing technique was proposed to deal with operation uncertainties, enhance fuel efficiency, and guarantee system durability. In this work, the FLC was conducted to coordinate the power flows of energy sources that could adapt to the requirement of load power. In [
37], Peng et al. established a fuzzy logic-based differential power compensation module to balance the performance deterioration of the PEMFC and BAT for a hybrid tramway system. In the aforementioned studies, the requirements of power distribution between the PEMFC and ESDs are effectively implemented by using fuzzy logic techniques. Nevertheless, the voltage control scheme of the DC bus was not considered to design, so the output voltage could not be maintained at the desired value. This thing can cause instability and decrease the working performance of the hybrid system.
Based on the literature, although there are more challenges in the design procedure to achieve high efficiency, the fuzzy logic approach is still a good solution to construct an EMS for a hybrid PEMFC-BAT-SC system. This technique can handle most situations of operating behavior and mutual impacts of the charging and discharging process of ESDs to keep the high performance of all energy sources. Motivated from the above analyses and building on our previous work in [
38], this paper proposes an EMS to improve the energy efficiency and fuel economy of a hybrid tramway system by using two levels of control. High-level control includes fuzzy logic rules and a bus voltage control loop to define the reference power of PEMFC and BAT. Then, low-level control is utilized to determine the appropriate control signal for converters of PEMFC and BAT. Accordingly, the main contributions of this research can be summarized as follows: Firstly, a new fuzzy logic strategy is designed to determine the sufficient power of the PEMFC based on the status of the BAT SOC and the required power of load under different working scenarios. Then, an adaptive proportional-integral-derivative (PID) control is employed to maintain the DC bus voltage at a steady state by using the BAT. Furthermore, the SOC regulator is constructed to maintain the SOC of BAT within the desired range to protect the BAT from the depth of charge or discharge and prolong the lifespan of the component. Finally, the proposed strategy is conducted and compared to the other completed strategies to verify the effectiveness.
The rest of this paper is organized as follows. The configuration of the hybrid tramway is dedicatedly described in
Section 2, and then
Section 3 analyzes the proposed energy management strategy comprehensively with high- and low-level control to guarantee the system qualification. Based on the presented strategies, comparative simulations with other approaches are given in
Section 4 to validate the effectiveness of the proposed strategy. Finally,
Section 5 presents conclusions as well as merits for further developments.
4. Simulation Results
In this section, a comparative simulation between the proposed strategy and previous methods is conducted to evaluate the strategy’s effectiveness for the hybrid PEMFC-BAT-SC tramway system under different operating conditions. In detail, the proposed fuzzy EMS (F-EMS) was implemented and compared with the two other strategies: an RB-EMS in [
18], and an ECMS in [
44].
To comprehensively investigate EMS approaches with various operating situations, the load profile as in
Figure 5 was considered with several load levels such as acceleration, deceleration, and regeneration in practical working conditions. In addition, the modeling of the hybrid tramway was deployed in Matlab/Simulink 2019b environment with a sampling time for displaying simulation results at 0.1 ms. By reasonably selecting components, characteristics of energy sources and parameters of the proposed EMS are listed in
Table 2,
Table 3,
Table 4 and
Table 5.
The system qualification is described in
Figure 9,
Figure 10,
Figure 11,
Figure 12 and
Figure 13. Firstly, simulation results of load power adaptation by using three EMSs are shown in
Figure 9 in which a continuous black line represents the required power of load, a continuous red line depicts the output power of the proposed F-EMS, the power of RB-EMS is shown by a dashed-dot blue line, and the power of ECMS is described as a dashed-dot green line. As can be seen in
Figure 9a, the required power of the proposed F-EMS satisfies load requirements better than the RB-EMS and ECMS at each time of transient peak power. Although the PEMFC, with the lowest dynamics, cannot instantly react to the load change, the load tracking effort can still be ensured due to the compensation from the BAT and SC during various working modes. In
Figure 9b, the ECMS takes an insufficient power in the range of
kW, while the RB-EMS obtains a smaller error approximated
kW, and the proposed F-EMS achieves the highest distributed accuracy within
kW. Furthermore, the proposed F-EMS has the lowest average inadequate power on the driving cycle of the hybrid system. This reveals that the suggested methodology is able to guarantee the load power demand under different operating conditions.
Power distributions of the PEMFC, BAT, and SC under three comparative EMSs are presented in
Figure 10. As expressed in
Figure 10a, the PEMFC powers of the RB-EMS and ECMS have a high power level that can diminish the aging and performance of the PEMFC system. Meanwhile, the proposed F-EMS provides a better performance indicator with a suitable change rate of PEMFC power and smaller power fluctuation, which can improve the fuel economy and durability of the PEMFC system. The power capability of the BAT under three EMSs is depicted in
Figure 10b. It can be seen that the BAT power of the proposed strategy is higher than those of both the RB-EMS and ECMS during the period of acceleration. This is because the output power of the PEMFC when using the proposed F-EMS is lower than the other methods, thus the BAT power must be more discharged to compensate for the lacking power from the fuel cell source to match the load power demand. However, in the regenerative mode, the proposed F-EMS can provide a smooth power response to charge the redundant power from the DC bus to the BAT. The comparative result of the SC power distribution is illustrated in
Figure 10c. Due to the fast power response, the SC is employed to supplement the slow power response of the PEMFC and BAT to power the load. As a result, the proposed algorithm regulates the SC power in a suitable range that can accommodate abrupt power of load and reduce power fluctuation of the PEMFC and BAT despite the rapid load change.
The comparison of DC bus voltage corresponding to the load power demand is shown in
Figure 11. By using the proposed F-EMS, the DC bus voltage is steadily maintained at around 750 V with a smaller fluctuation than using the RB-EMS and ECMS. In the change interval of the load, the DC bus peak voltage is in the range of
V, approximated by a 1% voltage ripple, by using the proposed F-EMS. This result is better than the ones under the RB-EMS with the range of
V and ECMS with the range of
V.
Simulation results of both BAT and SC SOC are shown in
Figure 12, which describes the charge and discharge status at each timeline when the load changes. In
Figure 12a, the suggested F-EMS can maintain the increasing range of BAT SOC lower than the RB-EMS and ECMS. For the SC,
Figure 12b shows that the SOC level varies around 85%. Herein, the proposed approach achieves a SOC varying range within
% that is lower than the ECMS with the range of
%, while RB-EMS has a large fluctuation in
%.
The hydrogen consumption and the PEMFC stack efficiency of three EMSs are described in
Figure 13. As a result, the proposed F-EMS consumes lower hydrogen fuel than the RB-EMS and ECMS as presented in
Figure 13a. In the case of the proposed strategy, the average amount of hydrogen consumption is 0.275 kg during the time of the driving cycle, whereas the total fuel consumption of the RB-EMS and ECMS reaches 0.35 kg in the same working conditions. It proves that the proposed approach gives better fuel economy with the hydrogen consuming less than 21.4% in comparison with other approaches. Furthermore, with the same driving cycle, the PEMFC stack efficiency of three EMSs is evaluated as well. As shown in
Figure 13b, the PEMFC stack achieves an efficiency from 53% to nearly 60% by conducting with proposed F-EMS, while this efficiency is in the range of
% if using the RB-EMS and ECMS. The aforementioned results show that the proposed F-EMS is a better effective strategy than other strategies for saving fuel consumption and enhancing efficiency for the PEMFC stack.
5. Conclusions
In this paper, a comprehensive power-sharing strategy was proposed to properly coordinate the energy from the load power demand to the PEMFC, BAT, and SC. The proposed methodology was constructed based on hierarchical techniques with high- and low-level control systems. In the high-level control, the FLRs were applied to determine the reference PEMFC power based on the status of the BAT SOC and the required power of load under different operating scenarios. To regulate the energy sources to match these references, low-level control with DC/DC converters dynamics and regulators were systematically analyzed based on each device’s characteristics as the necessary criterion. To guarantee the stability of the DC bus voltage, and thus for the whole system qualification, the adaptive PID controller was employed to maintain the bus voltage around the desired value regardless of the load change. Simulation results displayed that the proposed method could match the load requirements, keep the stability of the DC bus with a smaller voltage ripple, and achieve higher working performance for the hybrid tramway system rather than the other approaches. Moreover, the achievements in this study not only guarantee the power supply efficiency but also reduce hydrogen consumption and prolong the lifespan of energy sources. However, the challenges of optimal fuel economy and improving the PEMFC efficiency were not comprehensively addressed in this study, which should be explored in depth. Furthermore, the advanced configuration with a bidirectional converter should be installed to control the power flow of SC, which can enhance the system performance, rapidly compensate for the high peak power, and prolong the lifetime of energy devices. Consequently, this study serves as a premise to develop advanced EMSs for hybrid PEMFC applications in the future.