Experimental investigation on the dynamic performance of a hybrid PEM fuel cell/battery system for lightweight electric vehicle application
Introduction
With the development of the automotive industry and the increase of the transportation requirement, the number of vehicles is experiencing a rapid growth all over the world, especially in developing countries. On the contrary, the conventional unrenewable energy resources such as petroleum and natural gas are decreasing because of increasing energy consumption, so that many countries have to depend highly on oil import and suffer higher oil prices. Moreover, the environmental problems concerning the air pollution and greenhouse gas emission become more and more serious, which have turned into global challenges. A latest example was the UN Climate Change Conference opened in Copenhagen, Denmark at the end of last year. The delegates, experts and activists from nearly 200 countries gathered to find common ground including reduction of greenhouse gas emissions, promotion and transfer of new more eco-friendly technology and necessary funding. Therefore, it is urgent for the governments and automobile manufacturers to develop a new generation of vehicles based on environmentally–friendly technologies of energy utilization [1].
During the recent two decades, considerable R&D activities have concentrated on the hydrogen fuel cell which seems to be an effective solution to mitigate the aforementioned difficulties. The hydrogen fuel cell plays a key role in the hydrogen economy which provides clean and sustainable energy for the people’s future power. It is worth noting that the proton exchange membrane fuel cell (PEMFC) has attracted more attention than other types due to its outstanding advantages such as low-temperature operation, quick start-up, high efficiency, zero emission, low noise and vibration. Thus, it is acknowledged to be the most promising candidate for vehicular applications as the replacement of conventional internal combustion engine (ICE) [2], [3]. It is also likely to be used for portable and stationary power generation in a wide range of power levels [4], [5]. However, the PEMFC has to face some intractable technical and economic troubles before real commercialization in mainstream consumer markets. For example, the PEMFC exhibits somewhat “soft” characteristics because of its slow responses to the external load, especially during the peak power periods or transient events [6]. Furthermore, the power supply based on a sole PEMFC system requires higher construction cost and fuel consumption. In this regard, an additional energy storage device seems to be a good choice used as an assistant power source due to its advantages in transient discharging ability and energy storage efficiency. When the PEMFC is unable to fully satisfy the external load demand, a supplemented power source can produce a transient power output during the accelerating and climbing processes so as to alleviate the sluggish response of the pure fuel cell. On the other hand, if the PEMFC is able to provide the demanded power by itself, its redundant electricity can be used to recharge the storage device. Therefore, a hybrid system combining the PEMFC and back-up power source appears more competent for electric vehicles. For most of today’s hybrid application, the secondary batteries are usually treated as the auxiliary energy storage systems [7], [8], [9], [10]. Additionally, the ultra-capacitor is also regarded as a popular partner to complement the fuel cells [6]. Other attractive alternatives for energy storage include photovoltaic arrays [11], flywheels [12] and so forth.
In previous research work, plenty of achievements were reported concerning various PEMFC-based transportation applications. Some of them were successfully put into real-life demonstration rather than the laboratory practices or short-term road tests. For example, in China, both the government and research institutes have taken great efforts to develop PEMFC electric vehicles (PEM-FCEVs) in the past ten years and successfully demonstrated so-called green buses and cars based on PEMFC technology for the public services during 2008 Beijing Olympic Games. And a more extensive PEM-FCEV demonstration can be expected to be implemented in course of the coming 2010 World Exposition held in Shanghai.
Generally speaking, the power level of the fuel cell system depends on the load requirement and energy management strategies. As for the vehicular application, the fuel cell system above 10 kW usually involves complex system design and a high cost. Such applications involve various transportation tools such as large-scale locomotives [13], [14], city buses [15], [16] and cars [17]. On the other hand, in recent years, lightweight FCEVs (i.e. several-kilowatt class) have shown great prospects due to simpler system integration and lower investment risk, thereby making it more likely to realize large-scale production and commercialization earlier. Corbo and co-workers [8], [9], [10] conducted a series of work on the basis of commercial PEMFC systems for vehicular application. Their studies mainly focused on engineering problems and covered different power levels for transportation applications. They previously published a hybrid system based on a 20 kW PEMFC stack and lead–acid batteries for minibus propulsion [8]. At the same time, they also carried out an investigation of a low-power PEMFC stack around 2.4 kW in a hybrid system for scooter application [9]. Recently they reported the performance of a 2 kW PEMFC stack coupled with lithium ion batteries combined as an electric energy storage system [10]. Jossen et al. [7] also explored the feasibility of a hybrid system with combination of lead–acid batteries and PEMFCs for possible application in an electric scooter. The results suggested that the high efficiency of fuel cells at partial load operation resulted in a good fuel economy for recharging the batteries. Tang et al. [18] developed a PEMFC boat with a battery set hybridized. The 2 kW fuel cell stack provided the main power to run the trolling motor, while the battery functioned for system starting and acceleration. They concluded that the PEMFC system showed good adaptation to drive a boat. Based on an early project of a 500 W fuel cell electric bicycle [19], Hwang et al. [20] carried out a preliminary study on a 5 kW PEMFC system for a light-duty cruising vehicle named MHV. They claimed that the MHV performed satisfactorily over a hundred-kilometer driving test and would be optimized on a hybrid system aiming at further improvement in configuration and performance.
As reviewed above, most of the published literatures deal with the PEMFC system operating thoroughly in a “close” mode. This inevitably employs more external balance-of-plant (BOP) equipments and requires more complicated system integration to create necessary operational conditions. The aim of this work is to validate the feasibility of a 2 kW PEMFC stack in an “open” mode to be the main power in a hybrid system for a lightweight cruising vehicle. In particular, the dynamic performance of the fuel cell/hybrid system is emphatically studied by conducting a series of laboratory and road tests.
Section snippets
Configuration of the hybrid system
In this work, a lightweight electric vehicle (YVK-J08A, Shenzhen Yaveike Tech. Co. Ltd.) for campus cruising application was used as the experimental target, which was modified by replacing the original pure-battery power system with our self-designed hybrid system based on the PEMFC technology (see Fig. 1). The framework of the back seat was redesigned to make enough space for the system installation. The key components of this hybrid system included a 2 kW air-blowing PEMFC stack, a lead–acid
Results and discussion
For transient analysis of the hybrid system under dynamic conditions, the output values and involved operating parameters were treated as functions of the time. The developing trend of each variable and their interrelations could be clearly observed. Fig. 4 presents the performance curves (i.e. current vs. voltage and current vs. power) of the fuel cell stack, from which the power output at different load currents can be identified. However, in the real dynamic operation, the tested values may
Conclusions
A hybrid power system combining a 2 kW air-blowing PEMFC stack and a lead–acid battery pack is developed to drive a lightweight electric vehicle for campus cruising. The feasibility of such an “open” PEMFC system as the main power for vehicular propulsion has been proved. This contribution mainly focuses on the dynamic performances of both the single fuel cell system and the hybrid system in the laboratory and road tests. A series of system parameters is investigated, such as the stack current
Acknowledgements
The research was supported by the key program of NSFC-Guangdong Joint Funds of China (No. U0834002) and the program of Natural Science Foundation of Guangdong Province (No. 07118064). The project (No. 2009ZM0168) financially supported by the Fundamental Research Funds for the Central Universities, is also acknowledged. The authors also would like to acknowledge the Joint-training Program (No. 2009615064) sponsored by China Scholarship Council and the Doctorate Dissertation Innovation Funds
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