08-10-2012, 11:49 AM
Alternative Energy Storage System for Hybrid Electric Vehicles
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Abstract
In this paper an alternative energy storage system in the drive train of a hybrid electric vehicle is investigated. In particular, it concentrates on the potential reduction of the stresses of the battery when electrochemical capacitors, a.k.a supercapacitors, are added as a high power energy storage. Different control strategies are evaluated and an estimation of the performance is given. In order to verify the simulation results, a downscaled HEV drive train consisting of NiMH batteries, electrochemical capacitors, a DC/DC converter and an external load, is built and tested. The results show a significantly reduction in battery stresses and a good agreement between the models used in simulations and the laboratory system. Finally, a weight optimisation of such a system is briefly discussed.
INTRODUCTION
The motorised vehicle is more than a hundred years old and has been continuously developed. Today, politicians and consumers are more and more considering the environmental effects of vehicular traffic and accordingly there is an interest in exchanging the conventional mechanical drive train with an electrical one and making the vehicle to either an all-electric vehicle (EV), or a hybrid electric vehicle (HEV). An EV has no internal combustion engine (ICE), but instead a large battery, charged from an external source when the vehicle is at rest. The HEV has a smaller battery, charged from either a generator driven by an on board ICE, or a fuel cell (FC).
Unfortunately, energy density [Wh/kg] and power density [W/kg] of conventional batteries are often dependent on each other. Batteries with high energy density have poor power density and vice versa. In addition, large charge and discharge currents cause losses and heating of the battery, which significantly decreases the battery lifetime. For these reasons, batteries must be oversized in terms of energy capacity to meet the power requirements of an HEV.
DRIVE TRAIN OF A HYBRID ELECTRIC VEHICLE
Drive train overview
As a first step in the investigation of the battery-EC system, the drive train of an HEV is simplified in order to find appropriate models of each component. In general, there are two main design topologies of an HEV, the series design and parallel design. The parallel hybrid vehicle has an ICE and an electric motor arranged in parallel. The vehicle can be directly driven from the ICE or the electric motor, or both at the same time. When not used as a traction motor, the ICE can charge the battery via a generator. In the series hybrid vehicle, the traction motor is electric, with its electrical energy supplied from both a battery and an ICE driven generator or an FC. Using the series hybrid design, the ICE or FC can be kept at their optimum driving conditions and only operating when the battery needs to be charged. Moreover, when an ICE is used it can be made much smaller than a conventional traction ICE, since the battery supplies the peek power needed during acceleration. On the other hand, containing several energy conversions, the overall efficiency of the series HEV might be low compared to that of a parallel design HEV.
CONCLUSIONS
In this paper it was shown theoretically as well as experimentally that a battery-EC system can be used instead of a pure battery system with the same performance but with benefits such as reduced weight and reduced battery stresses. The reduced battery stresses are important in order to increase the lifetime of the battery.
The cost of this system is higher today then a pure battery system but it should be pointed out that EC’s for vehicle application are relatively new, not properly tested in production yet. Consequently, this leads most likely to a high initial total cost for the system, until production volumes have increased.