Hybrid electric vehicles and their challenges: A review
Introduction
An emphasis on green technology is greatly demanded of modern cities. The significant growth of today's cities has led to an increased use of transportation, resulting in increased pollution and other serious environmental problems. Gases produced by vehicle should be controlled and proactive measures should be taken to minimize these emissions. The automotive industry has introduced hybrid cars, such as the Honda Insight and the Toyota Prius that minimize the use of combustion engines by integrating them with electric motors [1]. Such technology has a positive effect on the environment by reducing gas emission. The greatest challenge in research activities today is developing near zero-emission powered vehicles. Electric vehicles powered by renewable energies offer a possible solution because they only emit natural byproducts and not exhaust fumes, which improve the air quality in cities and, thus the health of their populations [2].
One potential renewable energy device to power vehicles is the FC. A FC is an electrochemical device that produces DC electrical energy through a chemical reaction [3]. It consists of an anode, an anode catalyst layer, an electrolyte, a cathode and a cathode catalyst layer. Multiple FCs are arranged in series or parallel in a stack to produce the desired voltage and current [4]. FCs can be used for transportation applications from scooters to tramways, for combined heat and power (CHP) systems and in portable power supplies. In fact, the applications of FCs start at the small scale requiring 200 W and can reach the level of small power plants requiring 500 kW [5], [6], [7]. FC technology uses hydrogen as the main source of energy that produces the electricity needed to drive an electric vehicle. In comparison to an internal combustion engine (ICE) that emits gases such as NOx and CO2, FC emits water as byproduct [8], [9]. However, the downside to FCs is their slow dynamic properties, and therefore, they require auxiliary sources, such as batteries and SCs [10]. Batteries, which have high power density but low energy density have problem in longer charging time which can take from 1 h to several hours for full charge. On the positive side, batteries supply voltage more consistently than FCs. Batteries that are typically used with FCs, which are lead–acid, Li-ion and Ni–MH batteries [11]. In the energy management system for hybrid vehicles, batteries can be charged during regenerative braking and from the residual energy of FCs in low and no load power systems. In this case, batteries are implemented for energy storage and can supply energy continuously depending on the charge and discharge time cycle. Unfortunately, batteries have a limited life cycle that depends on the operating temperature (approximately 20 °C) and on the depth of discharge and the number of discharge cycles. Typically, lead-acid batteries can sustain1000 cycles while Li-ion batteries are limited to 2000 cycles [11]. In addition, Li-ion and Ni–MH batteries have a higher energy density and are lighter compared with lead–acid batteries. However, lead–acid batteries have an advantage over other batteries in their cost and fast response to current changes [12]. SCs also have the potential for power enhancement in vehicle applications.
SCs are electrochemical capacitors that offer higher power density in comparison with other storage device. They contain an electrical double layer and a separator that separates and holds the electrical charges. The separated charges provide a small amount of potential energy, as low as 2–3 V [13]. The double layer is made of a nano-porous material such as activated carbon that can improve storage density. The capacitance values of SCs can reach 3000 F. Super capacitors or ultra capacitors have a few advantages over batteries such as a longer lifecycle (500,000 cycles), a very high rate of charge/discharge and low internal resistance, which means minimum heat loss and good reversibility [14]. Furthermore, SCs have an efficiency cycle of approximately 90% whereas the efficiency cycle of a battery is approximately 80%. However, SCs are not a source of high energy density. The amount of energy stored per unit weight of SCs is between 3 and 5 W h kg−1, whereas that of a Li-ion battery is approximately 130–140 W h kg−1 [15]. Therefore, the combination of SCs with FCs, which have low power density but high energy density, is a practical alternative to improve the efficiency and performance of HEVs. In addition, SCs have a high charging rate, which allows regenerative braking to be used more efficiently. As SCs have the potential to function as an energy storage device in the future, many industries are interested in fabricating SCs with new technology and material design. The lab experiment shows that the energy density of SCs can be reach up to 300–400 W h kg−1, however, future lithium based batteries are projected to achieve densities around 400–600 W h kg−1 [13]. The Fig. 1 shows comparison between various energy sources and storage in terms of power and energy density.
Another important source of renewable energy for the future is solar cells, also known as photovoltaic (PVs). Solar cells are electronic devices that convert sunlight into electricity [16]. The clear advantage of solar cells over conventional fuels is their ability to convert free solar energy from the sun into electricity without generating significant pollution that might impact the ecology of the planet [17]. Solar cells fall into three main categories: single-crystal silicon, which have the highest efficiency of approximately 25%, polycrystalline silicon with 20% efficiency, and amorphous silicon with approximately 10% efficiency [18]. Single-crystal silicon is the most expensive to produce followed by polycrystalline silicon and then amorphous silicon. New solar cell technologies emerging in the market are thin-film cells, gallium–arsenide cells and tandem PV cells. These technologies hold the promise of improving the efficiency and versatility of solar cells while keeping production costs low [18].
Hybridization in using renewable energy is necessary because no single source currently matches the capability of fossil fuels in terms of both energy and power density. Simulation and modeling of HEVs has been extensively reported in the literature. Garcia et al. [19] and Barret [20] have discussed a FC-battery integrated with two dc/dc converters for a tramway. The active control system, which was the novelty of this paper, enabled both the FC and the battery to be coupled in the case of acceleration and regenerative braking. Another research paper on heavy load vehicles analyzed FC hybrid locomotives, which provided a reduction in capital cost [21]. The combination of batteries and FCs for FC hybrid vehicles was studied by Burnett and Borle [22], who indicated that hybridization minimizes the vehicle weight and fuel necessary as compared with FCs alone. The structure of a hybrid system using a SC and a battery was studied by Camara et al. [23], who linked a SC, to a boost converter with simple parallel topology. The parallel-structured hybrid system yielded a reduction in the weight of the vehicle and required less smoothing inductances of the SC current. The application of a SC in FC hybrid power sources was found to be significant as it can assist the FC during its time response to instantaneous power demands, fuel starvations and voltage drops through aging effects [24]. The behavior of HEV systems and internal combustion vehicles under a reference driving cycle has also been studied by Mierlo et al. [25]. An innovative simulation model of FCs, batteries, SCs, flywheels and engine-generators was developed to describe their functionality and characteristics in a vehicle system. The Vehicle Simulation Program (VSP) software, which is undergoing development, shows high simulation accuracy and allows the evaluation of electric vehicles with complex power management strategies or with a hybrid drive train.
A hybridization system using a battery and SC with a PEMFC power source was found to provide improvements in both energy and power density. This work was verified by Thounthong et al. [26] using the following component parameters: PEMFC (500W, 50A), SC (292F, 30V) and battery (68AH, 24V). These researchers proved that the SC manages to balance the energy demand during the load transition period, and this additional storage of energy enhances the quality and efficiency of the power system distribution. Other researchers are interested in the implementation of solar energy, which is usually combined with a battery. Countries including Australia organize the Darwin-Adelaide World Solar Challenge to provide a challenging platform for developers of solar vehicles to showcase their most recent advances [27]. These solar car races serve as an impetus to researchers to develop high-efficiency solar–electric power sources coupled to aerodynamic bodies that minimize mechanical and electrical losses during operation. The difficulty of using a solar-generated source is that it has non-linear I–V characteristics and, as a consequence, the maximum power delivery to the load needs to be controlled [28], [29]. This need for maximum power point tracking (MPPT) has encouraged the involvement of many researchers in this area. A new design for a boost converter to improve the efficiency of MPPT has been studied by Khatib and Mohamed [30] and Subiyanto in [31]. Further innovative research involving solar-assisted electric auto rickshaw three-wheelers has also been performed [32] using various hybrid drive trains of plug-in, solar, battery and conventional engines. Studies of renewable energy relating to power electronics [33] and controlling PV applications [34] have also been conducted.
One crucial component in developing a HEV is its EMS, whose main tasks are to maximize, control and utilize generated energy to fulfill the demanded loads. Thounthong et al. [26], [35] have reported on the energy management and control system of FCs, solar cells and SCs. A study of series and parallel plug-in hybrid electric vehicles (PHEVs) with dual clutch transmission was performed by Song et al. [36] and Salisa et al. [37] performed modeling and simulation of an EMS for a PHEV. The vehicle performance was compared with a standard U.S. EPA (Environmental Protection agency) drive cycle for highway driving. Bedir and Alouani [38] studied a simple power control strategy for HEVs that controlled the electric motor to provide power in different test situations. The vehicle model was implemented in the ADVISOR vehicle simulator, and preliminary tests indicated a 50% improvement in gas mileage. A study of the EMS of a PEMFC and battery in unmanned aerial vehicle (UAV) electric propulsion was conducted by Karunarathne et al. [39]. The EMS evaluates feedback from the battery, load power and FC parameters and passes this information to the power management system to control the power electronic interface. A fuzzy-based control strategy for hybrid vehicles was developed by Bahar et al. [40] and the EMS for the virtual vehicle design and application was investigated by Ustun et al. [41].
Section snippets
Hybrid vehicle energy states
The three categories for state of energy for HEV implementation are energy sources, energy storage and energy conversion device. In the next section, reviews on some of the latest and past technologies implemented are discussed in detail.
Hybrid vehicle dynamic model
In this section, a detail review on dynamic model of HEV technology is discussed comprehensively.
Characteristic and types of hybrid vehicle
The characteristics and classification of a hybrid vehicle alongside with three examples is presented in this section. The three HEV examples are auto-rickshaw, plug-in HEV and REVS-based HEV. At the end of the chapter, the Adaptive Neuro-Fuzzy Interference System will be described.
Control and component system of HEV
The content of following sections will focus to discuss about applicable control system and energy management system for both HEV as well as multi-sources energy model.
Current challenges and problems
Hybrid electric vehicles are the promising future transport option for the next generation. As the price of crude oil has increased substantially over the past decades, consumers have been forced to seek alternatives energy sources for transportation [108]. In contrast to hybrid vehicle with ICE, a BEV and PHEV are more energy efficient and emit near to zero hazardous emissions. Large group of researchers have contributed to improve efficiency and performance of PHEV [109]. From existing
Conclusion
Hybrid vehicle systems powered by renewable energies are very important research interest of the researchers. Currently, few projects in the world involved in developing in this technology. The purpose of this review paper is to explain detail about hybrid vehicle technologies and their short comes. At the same time, attract researchers involved in this field for finding new solution. Some related studies that have been discussed are renewable energies technology, energy management system and
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