Energy sources and multi-input DC-DC converters used in hybrid electric vehicle applications – A review

With the drastic inclination towards reduction of atmospheric issues, hybrid electric vehicles are becoming the major alternative for internal combustion engine vehicles. Compared to internal combustion engine vehicles, hybrid electric vehicles are remarkable in terms of efficiency, durability and acceleration capability. However, the major drawback of hybrid electric vehicle is energy storage capability. An electric vehicle requires the energy sources with high specific power (W/kg) and high specific energy (Wh/kg) to reduce the charging time. Generally, fuel cells, batteries, ultracapacitors, flywheels and regenerative braking systems are used in hybrid electric vehicles as energy sources and energy storage devices. All these energy storage devices are connected to the different DC-DC converter topologies to increase the input source voltage. From the recent past, most of the hybrid electric vehicles are using multi-input converters to connect more than one energy source in order to improve the efficiency and reliability of the vehicle. This survey presents an assessment of present and future trend of energy storage devices and different multi-input DC-DC converter topologies that are being used in hybrid electric vehicles. In addition, different electric vehicle architectures are also discussed.     

A comparison of high-speed flywheels, batteries, and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle

Fuel cells aboard hybrid electric vehicles (HEVs) are often hybridized with an energy storage system (ESS). Batteries and ultracapacitors are the most common technologies used in ESSs aboard HEVs. High-speed flywheels are an emerging technology with traits that have the potential to make them competitive with more established battery and ultracapacitor technologies in certain vehicular applications. This study compares high-speed flywheels, ultracapacitors, and batteries functioning as the ESS in a fuel cell based HEV on the bases of cost and fuel economy. In this study, computer models were built to simulate the powertrain of a fuel cell based HEV where high-speed flywheels, batteries, and ultracapacitors of a range of sizes were used as the ESS. A simulated vehicle with a powertrain using each of these technologies was run over two different drive cycles in order to see how the different ESSs performed under different driving patterns. The results showed that when cost and fuel economy were both considered, high-speed flywheels were competitive with batteries and ultracapacitors.     

Novel power management for high performance and cost reduction in an electric vehicle

This paper presents a novel power management scheme to achieve high performance and cost reduction in an electric vehicle for short profile fleet application. The measured drive cycle of an internal combustion engine meter-reading vehicle has been analyzed for the optimization of the EV system design and power management. Zinc Bromine batteries will be employed to provide the continuous power for normal driving while ultracapacitors will be employed to provide peak power demand during acceleration and to take regenerative braking energy efficiently during deceleration. The EV motor operates in constant torque mode at motor speeds below the base speed and in constant power mode at motor speeds over the base speed for high efficiency and low cost.     

Evaluation of energy requirements for all-electric range of plug-in hybrid electric two-wheeler

Recently plug-in hybrid electric vehicles (PHEVs) are emerging as one of the promising alternative to improve the sustainability of transportation energy and air quality especially in urban areas. The all-electric range in PHEV design plays a significant role in sizing of battery pack and cost. This paper presents the evaluation of battery energy and power requirements for a plug-in hybrid electric two-wheeler for different all-electric ranges. An analytical vehicle model and MATLAB simulation analysis has been discussed. The MATLAB simulation results estimate the impact of driving cycle and all-electric range on energy capacity, additional mass and initial cost of lead-acid, nickel-metal hydride and lithium-ion batteries. This paper also focuses on influence of cycle life on annual cost of battery pack and recommended suitable battery pack for implementing in plug-in hybrid electric two-wheelers.     

Models of energy sources for EV and HEV: fuel cells,batteries, ultracapacitors,flywheels and engine-generators

Resulting from a Ph.D. research a Vehicle Simulation Programme (VSP) is proposed and continuously developed. It allows simulating the behaviour of electric, hybrid, fuel cell and internal combustion vehicles while driving any reference cycle [Simulation software for comparison and design of electric, hybrid electric and internal combustion vehicles with respect to energy, emissions and performances, Ph.D. Thesis, Department Electrical Engineering, Vrije Universiteit Brussel, Belgium, April 2000]. The goal of the simulation programme is to study power flows in vehicle drive trains and the corresponding component losses, as well as to compare different drive train topologies. This comparison can be realised for energy consumption and emissions as well as for performances (acceleration, range, maximum slope, etc.).The software package and its validation are described in [J. Automot. Eng., SAE IEE 215 (9) (2001) 1043L]. Different hybrid and electric drive trains are implemented in the software [Views on hybrid drive train power management strategies, in: Proceedings of the EVS-17, Montreal, Canada, October 2000]. The models used for the energy sources like fuel cells, batteries, ultracapacitors, flywheels and engine-generator units will be discussed in this paper in three stages: first their functionality and characteristics are described, next the way these characteristics can be implemented in a simulation model will be explained and finally some calculation results will illustrate the approach.This paper is aimed to give an overview of simulation models of energy sources for battery, hybrid and fuel cell electric vehicles. Innovative is the extreme modularity and exchangeability of different components functioning as energy sources. The unique iteration algorithm of the simulation programme allows to accurately simulate drive train maximum performances as well as all kind of power management strategies in different types of hybrid drive trains [IEEE Trans. Veh. Technol., submitted for publication].     

Preliminary experimental evaluation of a four wheel motors,batteries plus ultracapacitors and series hybrid powertrain

This paper reports the preliminary experimental evaluation of a four wheel motors series hybrid prototype equipped with an internal combustion engine coupled to a generator and an energy recovery system (batteries plus ultracapacitors). The paper analyses global efficiency (energy dissipated to overcome the dissipative forces on energy dissipated in fuel), autonomy in electric configuration, and the efficiency of the regenerative braking system. The tests were carried out in a test cell equipped with a chassis dynamometer. The tests were performed according to the current regulated procedures. A constant speed test was performed in order to evaluate the autonomy of the vehicle in the electric configuration. The results show that the real tank to wheels efficiency is about 30% for HOST as a series hybrid and 79% for HOST as an electric vehicle.     

Improvement of ultracapacitors-energy usage in fuel cell based hybrid electric vehicle

Hybrid electric power systems based on fuel cell stack and energy storage sources like batteries and ultracapacitors are a plausible solution to vehicle electrification due to their balance between acceleration performance and range. Having a high degree of hybridization can be advantageous, considering the different characteristics of the power sources. Some parameters to be considered are: specific power and energy, energy and power density, lifetime, cost among others. Ultracapacitors (UC) are of particular interest in electric vehicle applications due to its high-power capability, which is commonly required during acceleration. UCs are commonly used without a power electronics interface due to the high-power processing requirement. Although connecting UCs directly to the DC bus, without using a power converter, presents considerable advantages, the main disadvantage is related to the UC energy-usage capability, which is limited by constant DC bus control. This paper proposes a novel energy-management strategy based on a fuzzy inference system, for fuel-cell/battery/ultracapacitor hybrid electric vehicles. The proposed strategy is able to control the charge and discharge of the UC bank in order to take advantage of its energy storage capability. Experimental results show that the proposed strategy reduces the waste of energy due to dynamic brake in 14%. This represents a reduction in energy consumption from 218 Wh/km to 192 Wh/km for the same driving conditions. By using the proposed energy management strategy, the estimated fuel efficiency in miles per gallon equivalent was also increase from 96 mpge to 109 mpge.     

Research and development of on-board hydrogen-producing fuel cell vehicles

Combining with the characteristics of different types of electric vehicles, the on-board hydrogen-producing fuel cell vehicle design is adopted, which eliminates the problems about the high-pressure hydrogen storage and the hydrogenation process. The fuel cell is used as the main power source to drive the motor, and the lithium battery is used as the auxiliary power source to accelerate and recycle energy in order to meet the special requirements, like energy recovery, power and dynamic characteristics, of fuel cell vehicles. On the ADVISOR simulation platform based on MATLAB/Simulink environment, a hybrid drive model and a pure fuel cell drive model are built, and simulation and comparative analysis are performed. In the hybrid drive model, fuel cells and lithium batteries work in the highly efficient and safe operating areas respectively, and the output power of fuel cell has small fluctuations, improving energy utilization efficiency and extending the service life of the fuel cell. At the same time, the charge and discharge of the lithium battery can be effectively managed to ensure the safety of charging and prolong the service life of the lithium battery.     

Tradeoffs between battery energy capacity and stochastic optimal power management in plug-in hybrid electric vehicles

Recent results in plug-in hybrid electric vehicle (PHEV) power management research suggest that battery energy capacity requirements may be reduced through proper power management algorithm design. Specifically, algorithms which blend fuel and electricity during the charge depletion phase using smaller batteries may perform equally to algorithms that apply electric-only operation during charge depletion using larger batteries. The implication of this result is that “blended” power management algorithms may reduce battery energy capacity requirements, thereby lowering the acquisition costs of PHEVs. This article seeks to quantify the tradeoffs between power management algorithm design and battery energy capacity, in a systematic and rigorous manner. Namely, we (1) construct dynamic PHEV models with scalable battery energy capacities, (2) optimize power management using stochastic control theory, and (3) develop simulation methods to statistically quantify the performance tradeoffs. The degree to which blending enables smaller battery energy capacities is evaluated as a function of both daily driving distance and energy (fuel and electricity) pricing.     

Energy analysis of electric vehicles using batteries or fuel cells through well-to-wheel driving cycle simulations

This work presents a study of the energy and environmental balances for electric vehicles using batteries or fuel cells, through the methodology of the well to wheel (WTW) analysis, applied to ECE-EUDC driving cycle simulations.     

Modelling and simulation of a zero-emission hybrid power plant for a domestic ferry

This paper presents a simulation tool for marine hybrid power-plants equipped with polymer exchange membrane fuel cells and batteries. The virtual model, through the combination of operational data and dynamically modelled subsystems, can simulate power-plants of different sizes and configurations, in order to analyze the response of different energy management strategies. The model aims to replicate the realistic behavior of the components included in the vessel's grid, to asses if the hardware selected by the user is capable of delivering the power set-point requested by the energy management system. The model can then be used to optimize key factors such as hydrogen consumption. The case study presented in the paper demonstrates how the model can be used for the evaluation of a retrofitting operation, replacing a diesel electric power-plant with fuel cells and batteries. The vessel taken into consideration is a domestic ferry, operating car and passenger transport in Denmark. The vessel is outfitted with a diesel electric plant and an alternative hybrid power-plant is proposed. The hybrid configuration is tested using the model in a discrete time-domain.     

A comparative review on power conversion topologies and energy storage system for electric vehicles

Fossil fuel depletion and its adverse impact on global warming is a major driving force for a recent upsurge in the development of hybrid electric vehicles technologies. This paper is a conglomeration of the recent literature in the usages of an energy storage system and power conversion topologies in electric vehicles (EVs). An EV requires sources that have high power and energy density to decrease the charging time. Commonly used energy storage devices in EVs are fuel cells, batteries, ultracapacitors, flywheel, and photovoltaic arrays. The power output from energy storage sources is conditioned to match load characteristics with the source for maximum power delivery. A DC-DC converter topology performs this task by way of transforming voltage under the condition of power invariance. In addition, power electronics is also required to power DC/AC motors efficiently with precise control as these motors provide tractive efforts and acts as prime movers. This paper therefore brings out a critical review of the literature on EV's power conversion topologies and energy storage systems with challenges, opportunities and future directions by systematic classification of EVs and energy storage.     

High-energy electrode investigation for plug-in hybrid electric vehicles

In addition to the development of high-energy density electrode materials for lithium-ion (Li-ion) batteries, other engineering approaches, such as electrode optimization, should be considered in order to meet the energy requirements of plug-in hybrid electric vehicles (PHEV). This work investigates the impact of the electrode thickness on the energy density of (Li-ion) batteries. The impedance results from the hybrid pulse power characterization (HPPC) test indicate that the electrode resistance is inversely proportional to the electrode thickness. This feature makes it possible to use thicker electrodes in (Li-ion) batteries to meet PHEV power requirements. The practical electrode thickness is determined to be around 100 μm, if considering the electrode mechanical integrity when using conventional PVDF binders. Furthermore, cycle performance shows that cells with a higher loading density have a similar capacity retention to cells with a lower loading density.     

Characterization methods and modelling of ultracapacitors for use as peak power sources

This paper suggests both a methodology to characterize ultracapacitors and to model their electrical behaviour. Current levels, frequency intervals, and voltage ranges are adapted to ultracapacitors testing. Experimental data results in the determination of the ultracapacitors performances in terms of energy and power densities, the quantification of the capacitance dependence on voltage, and the modelling of the dynamic behaviour of the device. Then, an electric model is proposed taking into account the ultracapacitors characteristics and their future use as peak power source for hybrid and electric vehicles. After, the parameters identification procedure is explained. Finally, the model validation, both in frequency and time domains, proves the validity of this methodology and the performances of the proposed model.     

High performance nickel–metal hydride battery in bipolar stack design

The consumption of fuel in cars can be reduced by using hybrid concepts. Even for fuel cell vehicles, a high power battery may cut costs and allow the recovery of energy during retarding. Alkaline batteries, such as nickel–metal hydride batteries, have displayed long cycle life combined with high power ability. In order to improve the power/energy ratio of Ni/MH to even higher values, the cells may be arranged in a bipolar stack design.     

The power capability of ultracapacitors and lithium batteries for electric and hybrid vehicle applications

There is much confusion and uncertainty in the literature concerning the useable power capability of batteries and ultracapacitors (electrochemical capacitors) for various applications. Clarification of this confusion is one of the primary objectives of this paper. The three approaches most often applied to determine the power capability of devices are (1) matched impedance power, (2) the min/max method of the USABC, and (3) the pulse energy efficiency approach used at UC Davis. It has been found that widely different power capability for batteries and ultracapacitors can be inferred using these approaches even when the resistance and open-circuit voltage are accurately known. In general, the values obtained using the energy efficiency method for EF = 90-95% are much lower than the other two methods which yield values corresponding to efficiencies of 70-75%. For plug-in hybrid and battery electric vehicle applications, the maximum useable power density for a lithium-ion battery can be higher than that corresponding to 95% efficiency because the peak power of the driveline is used less frequently and consequently charge/discharge efficiently is less important. For these applications, the useable power density of the batteries can be closer to the useable power density of ultracapacitors. In all cases, it is essential that a careful and appropriate measurement is made of the resistance of the devices and the comparisons of the useable power capability be made in a way appropriate for the application for which the devices are to be used.     

Sizing of a fuel cell electric vehicle: A pinch analysis-based approach

Polymer electrolyte fuel cells are considered as a promising alternative to mitigate the CO₂ emission in the transport sector. To achieve an efficient and cost-effective system, hybridisation of the energy storage system with a fuel cell is important. Efficient management of energy is the key in order to achieve an efficient and cost-effective configuration for fuel cell electric vehicle. Optimum sizing of the power source and energy storage system, which is capable of meeting the load requirement of the driving cycle is the key challenge for achieving efficient and cost-effective system. In this work, an alternative methodology based on the principles of pinch analysis is proposed, for sizing the energy storage system and the fuel cell for fuel cell-based electric vehicle, and validated for the Worldwide Harmonized Light Vehicle Test Cycle (WLTC) class-3 driving cycle.     

Investigation of battery end-of-life conditions for plug-in hybrid electric vehicles

Plug-in hybrid electric vehicles (PHEVs) capable of drawing tractive energy from the electric grid represent an energy efficient alternative to conventional vehicles. After several thousand charge depleting cycles, PHEV traction batteries can be subject to energy and power degradation which has the potential to affect vehicle performance and efficiency. This study seeks to understand the effect of battery degradation and the need for battery replacement in PHEVs through the experimental measurement of lithium ion battery lifetime under PHEV-type driving and charging conditions. The dynamic characteristics of the battery performance over its lifetime are then input into a vehicle performance and fuel consumption simulation to understand these effects as a function of battery degradation state, and as a function of vehicle control strategy. The results of this study show that active management of PHEV battery degradation by the vehicle control system can improve PHEV performance and fuel consumption relative to a more passive baseline. Simulation of the performance of the PHEV throughout its battery lifetime shows that battery replacement will be neither economically incentivized nor necessary to maintain performance in PHEVs. These results have important implications for techno-economic evaluations of PHEVs which have treated battery replacement and its costs with inconsistency.     

Hydraulic/electric synergy system (HESS) design for heavy hybrid vehicles

Energy storage source is one of the key factors constraining the development of hybrid drive technology. Single energy storage source is difficult to satisfy the hybrid vehicle’s requirements for both energy density and power density. This paper presents a hydraulic/electric synergy system (HESS) for heavy hybrid vehicles to overcome the existing drawbacks of single energy storage source. The key components in the synergy system are sized to improve the fuel economy potential while satisfying the vehicle performance constraints. In order to achieve optimal fuel economy, energy control strategy tailored specially to the synergy system is designed to manage the power distribution between multiple energy sources based on theirs characteristics. The experiments and simulations demonstrate that the proposed synergy system can provide good fuel economy and overall system efficiency.     

Peak power bi-directional transfer from high speed flywheel toelectrical regulated bus voltage system: a practical proposal forvehicular technology

This paper provides a design outline and implementation procedure for a secondary energy storage unit (SESU) that can be used to meet the peak energy requirements of an electric vehicle during both acceleration and regenerative braking. The life cycle of the electric vehicle's batteries can be extended considerably by supplying peak energy requirements from a secondary source. A simulation study was conducted to determine the peak power and energy requirements over the SAE recommended electric vehicle test procedure. A scaled prototype SESU was built using flywheel energy storage, and tests were performed to determine the energy transfer capabilities of a flywheel coupled high speed permanent magnet synchronous machine through the proposed system's energy storage tank. Results are presented that indicate the necessity of the energy storage tank. An evaluation of the proposed system is also included which indicates the practicality of the system when compared to conventional regenerative control techniques