There are many architectures used in hybrid systems. We will not attempt to cover them extensively; rather, our objective is to review some typical architectures and to provide a broad overview of terminology. In subsequent sections, we will explore in more detail electrochemical devices for energy storage for specific hybrid architectures. Because there are many different applications and usage patterns, our treatment in this chapter provides just a small sample of the possibilities.
The simplest form of a hybrid is the so-called start–stop 1 hybrid (Figure 12.7). Here, the IC engine is turned off when the vehicle is stopped. While the engine is off, electrical power is provided by the energy storage system to handle the accessory load (e.g., air-conditioning). The same RESS is subsequently used to restart the engine. The RESS is then recharged by the engine. For the start–stop hybrid, all traction power is provided by the engine. Nonetheless, improvement in the range of 3–8% in fuel economy is possible with this simple system. This increase in efficiency comes about because of the low efficiency of the ICE during idling. The start–stop hybrid has the added advantage of reducing pollution by avoiding idling of the engine.
Figure 12.7 Start–stop hybrid.
A block diagram for a conventional vehicle would look the same as that shown in Figure 12.7. The only modifications made for a start–stop hybrid is to increase the size of the starter/alternator and the RESS. These modifications allow for very rapid starting of the engine and provide sufficient energy to run the accessories while the vehicle is stopped briefly.
Next, we consider series- and parallel-hybrid architectures, which are both full-hybrid architectures. Two features distinguish these architectures from that of the start–stop hybrid. First, energy from braking can be recovered. Second, energy from the RESS can be used to provide traction to the wheels. Energy storage is a major component of these hybrid systems, and the large amount of energy needed makes a battery the only practical electrochemical energy storage system. For most hybrid designs, all of the power ultimately comes from the conversion of the fuel, although some of the energy from the fuel is temporarily stored in the RESS for later use in order to optimize total system performance.
The series-hybrid design is shown in Figure 12.8. Solid, black lines depict the flow of electrical energy; the gray lines show transfer of mechanical energy. In contrast to the stop–start and microhybrids, the RESS is used for vehicle propulsion. In the series design, the electric motor is the only means of providing torque to the wheels. The engine, typically an ICE, powers an electrical generator, which converts all of the power generated from the engine into electrical power. This electrical power is either used to charge the battery or supplied to the motor for propulsion. Since all power to the wheels must go through the motor, the motor is sized to meet the maximum power demand. Because the combustion engine is not directly connected to the drive train, it can be run at optimal conditions, which increases efficiency and decreases emissions. The advantages of this architecture are most significant for driving that involves frequent starts and stops. The result is a significant reduction in fuel consumption relative to that of a conventional vehicle. The advantages of the series-hybrid are less significant for long periods of continuous driving, on the highway for instance, because the power from the engine must be converted to electrical energy and then back to mechanical energy since there is no direct coupling of the ICE to the wheels.
Figure 12.8 Series hybrid system.
You were already introduced to a parallel design, shown in Figure 12.2. This design is an alternative architecture that addresses some of the challenges with the series configuration. Power to the wheels can be delivered either by the internal combustion engine through mechanical coupling or from the battery through an electrical motor. The battery is charged from the engine and during braking. A motor-generator is required to convert between mechanical and electrical energy.
Compared to the internal combustion engine, which uses a hydrocarbon fuel with a very high energy density, battery and fuel-cell systems have low specific energy. For a fixed vehicle mass, low energy density or low specific energy translates to reduced range. The key to the success of hybrid vehicles is that these electrochemical devices are able to store and use energy efficiently; that is, the round-trip efficiency can be more than 80%.
Now that we have briefly examined hybrid architectures, we will consider the RESS for several specific cases, beginning with the start–stop hybrid.
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