A key feature of systems that include energy storage is that excess energy can be accumulated for later use. Our focus will be on vehicles, but applications for hybrid power systems are common. For example, a renewable energy system based on wind power alone will struggle to match electrical power production and demand. What do you do when electrical demand is less than the amount being produced by the wind turbine, or when the wind isn’t blowing at all? One option is to store energy when the supply is greater than the demand. This storage could be accomplished with a battery or similar rechargeable energy storage system (RESS). The stored energy can then be used later when the demand is greater than the supply. Similarly, with conventional base-load power generation devices, a nuclear power plant for instance, it is desirable to keep electrical power production constant. Since demand for electricity varies over the day, a method of efficiently storing energy during off-peak times is valuable. Electrolysis of water to produce hydrogen has been proposed to be paired with a nuclear power plant as one means to deal with this issue. The hydrogen would then be recombined with oxygen in a fuel cell (FC) when the electrical demand was high.
The situation in a vehicle is not completely analogous, but there is a clear benefit of having the ability to store energy. Onboard vehicle energy storage will be the primary emphasis of this chapter. The main incentive for electric and hybrid-electric vehicles is to improve their efficiency, which leads to reduced use of fossil fuels and lower emissions of pollutants. Let’s explore this motivation in more detail. The starting point is a conventional vehicle powered by an internal combustion engine.
The motivation and goals for electric and hybrid-electric vehicles are as follows:
Reduced petroleum use
Lower releases of greenhouse gases
Decreased emissions of criteria pollutants
Increased energy efficiency
Figure 12.1 provides an example of how the chemical energy from the fuel is used in an ICE vehicle. Chemical energy is released by combustion and converted into mechanical energy in the engine. This process is a thermal one and limited by the Carnot efficiency. In this example, which combines city and highway driving, only 26% of the available chemical energy reaches the driveline. A further 6% is lost there. As a result about 20% of the chemical energy in the fuel applies torque to the wheels. Of this, 6% is used to accelerate the vehicle after braking. Eight percent is needed to overcome aerodynamic drag on the vehicle, and another 6% to overcome rolling resistance of the tires. Clearly, the situation depicted in Figure 12.1 is not an efficient use of the energy from the fuel.
Figure 12.1 Representation of where the energy of the fuel goes for a typical vehicle driving a combination of city and highway.Source: Data taken from http://www.fueleconomy.gov/feg/atv.shtml
Understanding a little about ICEs and reflecting on how a typical vehicle is driven can help to explain these results. First, the fuel efficiency of an ICE is not constant with power. Typical of thermal devices, there is a point where the efficiency is maximized, but as the power level decreases from this value, the efficiency also decreases. Therefore, a key consideration is where does this maximum occur and at what point is the engine being operated most frequently. As you know, most driving involves frequent starts and stops; on average, the power demanded by the driver is well below the rated power. In fact, this maximum-rated power is needed infrequently. If the engine is sized to provide high power, but operates almost all the time at low power, its efficiency will be low. Consequently, the largest loss in Figure 12.1 (69%) is attributed to engine losses. Ideally, we would operate the engine near the point of maximum efficiency. How can we do that? One answer is to use a hybrid propulsion system. The ICE could provide the average traction power required and be sized so that its efficiency was high at this average power. A second power source would supplement the ICE when high power is needed. Another opportunity to improve efficiency is to recover energy during braking. The kinetic energy of the vehicle is reduced by friction of the brakes, generating heat. This energy is lost and additional fuel must be burned to accelerate the vehicle back up to speed. Lost energy due to braking accounts for about 6% of the total as shown in Figure 12.1. Efficiency would be improved if we could store this kinetic energy and reuse it later. There are other important approaches to improve use of the chemical energy of the fuel: lowering the aerodynamic drag, reducing the weight of the vehicle, and minimizing rolling resistance, for instance. These topics are beyond the scope of this chapter, which explores the use of energy storage and hybridization to increase efficiency.
An onboard RESS coupled with a means of proving torque to the wheels separate from the ICE addresses both of the two ideas described above. In addition to batteries and electrochemical capacitors, there are mechanical ways to store energy: pumped hydroelectric, compressed air systems, hydraulic energy storage, and flywheels. Some of these are used in vehicles, but our focus will be on those systems that use electrochemical energy storage. Let’s briefly consider a few aspects of a hybrid-electric vehicle as illustrated in Figure 12.2—These will be examined in more detail later in this chapter. First, both electrical energy from the RESS and the engine can be used to propel the vehicle. This parallel architecture decouples the torque to the wheels from the torque provided by the engine. Because of this broken link, the engine can be operated selectively under conditions of higher efficiency, which is an important advantage of a hybrid system. Furthermore, electric-only operation is also possible if the energy storage system is large enough. In addition, the hybrid architecture enables the capture and storage of kinetic energy when stopping the vehicle. This energy would otherwise have been lost (dissipated) as heat in friction brakes. To recover this energy, the electric motor also functions as a generator, and the energy is stored in a rechargeable storage system for later use.
Figure 12.2 Parallel-hybrid architecture, one implementation of a hybrid vehicle.
It is important to note that there is a price to be paid for these improvements in efficiency. The added flexibility with hybrid systems is achieved by adding components. The RESS, electric motor/generator, and voltage converter increase the mass of the vehicle, and add cost and complexity. The additional complexity requires more elaborate controls. It is the task of the design engineer to evaluate the trade-offs between efficiency, complexity, and cost in order to provide an optimal vehicle system.
To summarize, the key concepts used in hybrid systems to increase the fuel efficiency of vehicles are (i) to operate the engine at speeds where it is most efficient, and (ii) to recover the kinetic energy wasted during friction braking. Efficient operation of the engine may include turning it off to lower standby/idle losses when the vehicle is stopped for short periods, or to permit all-electric operation if the RESS is of sufficient size. Each of these improvements requires a means of energy storage onboard the vehicle, and hence motivates our interest in the topic. Batteries (Chapters 7 and 8) and electrochemical double-layer capacitors (Chapter 11) are energy storage devices, and their role in hybrid vehicles will be highlighted. Finally, we also consider hybrids where electrochemical energy storage is combined with a fuel cell (Chapters 9 and 10) operating on hydrogen rather than with a petroleum-based ICE.
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