In the last two decades the pollution problems and the increase of the cost of fossil energy (oil, gas) have become planetary problems. The automobile manufacturers started to react to the urban pollution problems in the nineties by commercializing the electric vehicle. But the battery weight and cost problems were not solved. The batteries must provide energy and peak power during the transient states. These conditions are severe for the batteries. To decrease these severe conditions, the supercapacitors and batteries associated with a good power management present a promising solution. Supercapacitors are storage devices that enable to supply the peaks of power to hybrid vehicles during the transient states. During the steady states, batteries will provide the energy requested. This methodology enables the decrease of the weight and increase of the lifespan of the batteries. Hybridization using batteries and supercapacitors for transport applications is needed when energy and power management are requested during the transient states and steady states.
Top1. Introduction
The use of a hybrid energy storage system is therefore particularly convenient for a complete satisfaction of the electric vehicle demands, in terms of both energy density and power density. An interesting hybrid architecture for electric vehicles is the one based on the battery/supercapacitors combination as shown in Figure.1. The battery should provide to the vehicle the average power level, while the supercapacitors should react to the peak power demands. A hybrid energy storage system requires the combination between an optimal sizing and a proper power management strategy, to precisely determine the size of the different energy sources and to control the power sharing between them according to various possible scenarios of mobility (Amari et al., 2015; Ates et al., 2010; Marzougui et al., 2016).
Figure 1. Block diagram of battery/supercapacitor hybrid energy storage system
A new strategy of energy management between battery and supercapacitors for an urban electric vehicle is suggested. The battery/supercapacitors integration is able to provide notable advantages in the power management of an electric vehicle, in terms of capability of both high energy storage and readiness to deal with fast load variations. In order to manage the charge or discharge of the supercapacitors from or towards a DC voltage bus, a proper bi-directional converter is required. These two sources are connected in parallel to the DC bus through two bidirectional DC–DC converters enabling separate control over the power flow of each source.
A bi-directional DC-DC converter for the supercapacitors application is proposed. In Figure. 2 a possible architecture of the battery/supercapacitors system is shown. Two bi-directional DC-DC converters connect the battery and the supercapacitors to a common bus. The bus voltage is generated by means of the battery-connected DC-DC converter, which also regulates the average current required by the load; the supercapacitors-connected DC-DC converter deals with the peak power load demands: if the load requires a power peak, the supercapacitors are discharged; if the Supercapacitors require to be recharged, the energy is recovered from the bus (Cultura & Salameh, 2015; Payman et al., 2008; Stefan, 2009; Zhang et al., 2015).
Figure 2. Configuration of battery/supercapacitor hybrid energy management system.
Two bi-directional DC–DC converters are linked to the same DC-Link, one manages the battery power ñow and the other manages the supercapacitors power ñow, as shown in Figure. 2. Energy transfer is enabled between sources and load in the mentioned structure as well as from one source to another independently of energy ñow direction. DC-link current, IDC, can be positive or negative whereas voltage across the DC bus is always positive. The two parallel DC–DC converters that interface the supercapacitors and the battery with the DC-Link give a good ñexibility power management either when energy ñow to DC-Link rising the voltage level which is boost behaviour, or when energy ñows from DC-Link to sources showing a typical buck behaviour (Ban et al., 2013; Shi & Crow, 2008; Zhang, Zhenpo, Sun et al, 2014).
Top2. Hybrid Energy Management System:
Vehicle dynamics with load torque applied on the shaft motor is to be considered. This strategy of energy management permits dividing energy between the two sources depending on the state of charge of each source as well as on the vehicle displacement state such as stopping, acceleration, cruising down and uphill, and deceleration. The aim of the proposed strategy is the best use of energy through maximizing the use of Supercapacitors by transferring energy from batteries to Supercapacitors during the standstill phase or when the load applied to the vehicle is small supercapacitors will then be ready in critical situations such as rapid acceleration or in high hills climbing. In order to validate the control design and evaluate our energy management strategy performance, a simulation of an urban hybrid electric vehicle movement with the Matlab/Simulink software is implemented as shown in Figure.3 & 4.
Figure 3. Matlab/Simulink hybrid energy management system.