Models of energy sources for EV and HEV: fuel cells, batteries, ultracapacitors, flywheels and engine-generators
Introduction
The pollution caused by transport is a heavy burden, especially in urban areas [5]. The introduction of clean vehicles, like electrically driven vehicles, would be an interesting move in the direction of a significant reduction in harmful exhaust gases, with a view to a sustainable transport policy [6]. Since several years the family of the battery electric vehicles is extended with the hybrid electric and recently also with the fuel cell electric vehicles. These vehicle technologies use different energy sources, which will be discussed in this paper.
The energy sources that will be considered in this manuscript are mainly used in “series” hybrid vehicles. In series hybrid drives there are no mechanical connections between the internal combustion engine and the wheels [7]. All thermal energy is converted first into mechanical energy in a thermal engine and subsequently into electrical energy by a generator driven by the thermal engine. Additionally, there is an electric traction motor to drive the wheels. Hence a decoupling of energy source operation from the required traction power is possible. In most cases the energy source, also called auxiliary power unit (APU), will act as base power unit delivering power to the battery or directly to the electric traction motor. While driving the battery acts as peak power unit or energy buffer. The series hybrid has the advantage of operating a thermal engine in a selected optimal operating field, for instance, with low specific fuel consumption in the torque-speed operating area [8].
The fuel cell hybrid structure is a series structure in which the engine and the generator are replaced by a fuel cell system producing electric energy starting from stored hydrogen or from a fuel tank feeding a reformer to produce hydrogen [9]. The excess of electricity produced by the fuel cell can be stored in a buffer battery. When the battery is left out one has no longer a hybrid vehicle but a fuel cell electric vehicle. In this case, the fuel cell has to demonstrate enough dynamics to meet variable power demand.
Fig. 1 illustrates a simulation model of a series hybrid with peak power unit, where the generator group provides a constant average power, the battery works as an energy buffer and the flywheel (or ultracapacitor) caters for the brake and acceleration power peaks. In this configuration the battery can be smaller or it can possibly be omitted. The efficiency and reliability of the flywheel or ultracapacitor are an important factor for the development of these drive trains.
This model is part of a vehicle simulation programme (VSP) [2], [3], [4] whose front panel is illustrated by Fig. 2 and for which it is necessary to introduce models for all components or subsystems of each simulated vehicle and particularly of the energy and power sources. The basic modelling strategy used in VSP is the well-tried and trusted method of dividing the drive cycle into a number of time steps and calculating the characteristics of the vehicle at the end of each time interval, which is called the longitudinal dynamics simulation [10], [11].
This paper will focus on the description of different energy or power sources as well as on their simulation models and calculation results.
Section snippets
Description of the fuel cell
The fuel cell electrochemically combines the oxygen of air with a hydrogenated fuel and converts them into water and other elements with a production of electricity. The conversion is similar to that of a conventional battery, except that the reductant and oxidant are continuously supplied to the cell instead of being contained in the cell. In addition, fuel cells are ‘recharged’ by filling up the fuel supply.
When the fuel cell uses hydrogen as a fuel, this hydrogen can either be stored as such
Battery description
Batteries are characterised by their life cycle, energy and power density and energy efficiency. The life cycle represents the number of charging and discharging cycles possible before it loses its ability to hold a useful charge (typically when the available capacity drops under 80% of the initial capacity). Life cycle typically depends on the depth of discharge. When charging and discharging a battery not all energy, delivered to the battery, will be available due to battery losses, which are
Ultracapacitor description
Super or ultracapacitors (800–1500 F) behave, like very high-power, low-capacity batteries but store electric energy by accumulating and separating opposite charges physically, as opposed to batteries, which store energy chemically in reversible chemical reactions. One key aspect of super-capacitors is that they demonstrate excellent life cycle. When fully developed for vehicles, they could be expected to last as long as the car. This is because it is possible to cycle ultracapacitors very
Flywheel description
Flywheels store energy mechanically in the form of kinetic energy. One can distinguish mechanical and electrical flywheels. The mechanical flywheel is connected to the drive system via an axle. The electrical flywheel takes an electrical input to accelerate its rotor by using a built-in motor, and returns the energy by using this same motor as a generator.
The most significant factor affecting flywheel design is the material used to construct the flywheel rim. A flywheel rim needs to be made of
Engine
The engine model described in this chapter can be used in a classical thermal vehicle as main motor or in a hybrid vehicle as part of the APU. It is mainly based on look-up tables of engine maps and calculates the emissions and fuel consumption.
The total engine torque, Tice, is function of engine inertia, J, vehicle acceleration, Ta, resistive torque, Tr, and auxiliary alternator power, Palt, as shown by the following equation:Internal combustion engines require a minimum
Conclusions
Since years ago the automotive industry and several research institutes have developed simulation models to evaluate vehicle performance, fuel consumption and emissions. Most models are oriented towards conventional vehicles with internal combustion engines.
The VSP developed by the Vrije Universiteit Brussel also allows the simulation of these drive trains, however, it is especially develop to simulate battery, hybrid and fuel cell electric vehicles. A great attention is devoted to the
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