Electrification of the powertrain is a potentially powerful method to reduce fuel consumption (FC) and hence greenhouse gases (GHGs). Electrification comes in a variety of forms, from the simplest stop-start systems with only an augmented alternator, to more complex hybrid systems that supplement the engine with an electric drive, to purely battery electric vehicles and finally, to fuel cell systems. This chapter starts with a brief review of the history of electrified powertrains in vehicles. Next, the various electrification architectures and technologies are discussed, including those in use today and those likely to be implemented to 2030. Finally, the implications of these technologies for fuel economy and cost are evaluated.
Electrically-propelled vehicles (EVs)1 with lead acid batteries had a large share of the market in the early twentieth century, representing about equal numbers with both steam and internal combustion engines (ICEs). EVs lost out to ICEs because gasoline has a much higher energy density than batteries, enabling longer distance travel. Continuous development of the ICE resulted in low-cost, high-performance engines, while EV development, by contrast, stalled. Several experimental vehicles were developed from the mid-1960s to the early 2000s with little or no success in the market.
With increasing concern for reduced fuel consumption, investment in EV research was spurred by the Partnership for a Next Generation of Vehicles (PNGV) program starting in 1993, funded by the U.S. government and the domestic automakers (GM, Ford, and Chrysler). This public-private partnership resulted in prototype, midsize passenger vehicles getting about 80 mpg that were too expensive to make and sell. The market competitiveness of these vehicles was further hindered by a particularly low incentive for fuel economy in the late 1990s, when oil prices dropped to as low as $12/barrel in 1998. The PNGV program was replaced by the FreedomCar program under President Bush, and the focus shifted to fuel cell vehicles using hydrogen as the fuel (NRC 2010).
Greater market success for vehicle electrification came with the combination of ICEs and electric motors in hybrid vehicles (HEVs). The first commercially successful hybrid vehicle—the Prius—was available for sale in Japan in 1997 and was introduced worldwide in 2000. In addition to representing a significant engineering advance in fuel economy compared to conventional vehicles, the Prius’s success was helped by two external factors. From 2000 to 2008, oil prices increased, hitting a high of $145/barrel. Energy security concerns in the United States increased as domestic production decreased and oil imports grew to account for over 50 percent of consumption in the mid-1990s (EIA 2014). In addition, concern increased globally about the effect of GHGs on climate change.
Recently, a new type of electrified powertrain called a plug-in hybrid electric vehicle (PHEV) has been introduced. PHEVs have both a large battery that can be charged from the grid and an internal combustion engine, making it possible to drive a larger percentage of miles, if not all miles, fueled by electricity rather than petroleum. More recently, stop-start technology is being incorporated into ICE vehicles. Currently, electrification technologies have achieved only low penetration volumes: 2.75 percent of the market for HEVs,
1 The following terminology and abbreviations for electrified vehicles will be used in this chapter:
EVs (or xEVs): all vehicles where an electric motor provides all or part of the propulsion.
HEVs: hybrid vehicles, which have two sources of power: an internal combustion engine and an electric drive.
BEVs: battery electric vehicles, where the battery and an electric drivetrain is the source of motive power.
PHEVs: “Plug-in hybrid electric vehicles” are hybrid vehicles with a larger battery that can sustain drive for several miles with the ICE off and can be charged from the grid.
PEVs: “Plug-in electric vehicles” are EVs that derive at least some of their energy by plugging in to the electric grid (BEVs and PHEVs).
FCEVs: fuel cell electric vehicles, which are also known as fuel cell hybrid vehicles.
0.39 percent of the market for PHEVs, and 0.34 percent of the market for BEVs in 2014 (Cobb 2015). Automaker incentive to produce BEVs and FCEVs in the future will be greatly driven by the California zero emission vehicle (ZEV) mandate. The ZEV mandate will be discussed further in this chapter as well as in Chapter 10. The variety of vehicles with some degree of electrification is described in the next section.
Several different electrified vehicle powertrains have been developed and produced with varying commercial success. Typical electrified architectures are defined below and described in greater depth in terms of the relevant engineering principles, implementation of electrified components, and control system requirements. Each architecture is illustrated with example vehicles that are currently in production. In general, hybrid and plug-in electric technologies have been applied to smaller vehicles due to the torque requirements for larger vehicles, which are more difficult to satisfy with these technologies. In Annex Table 4A.1, the committee lists all EVs on sale in the United States in 2014.
Electric Vehicle Categories Defined
Hybrid (HEV) Architectures
- Stop-Start (SS). The engine is turned off when the vehicle pauses in traffic, restarting quickly when the vehicle needs to move again.
- Mild Hybrid (MHEV). A small electric motor and battery combined with an internal combustion engine (ICE) allows for assisted acceleration and regenerative braking.
- Strong Hybrid. A larger electric motor and battery combined with a downsized ICE allow for better regenerative braking as well as periods of electric motor drive.
- — P2 Strong Hybrid. A parallel hybrid with a clutch connecting the single electrical motor and the engine crankshaft. The vehicle uses a conventional transmission.
- — PS Strong Hybrid. A power-split hybrid with a planetary gear set that connects the engine, battery, and two electric motor/generators.
Plug-in Electric Vehicle (PEV) Architectures
- Plug-in Hybrid (PHEV). Strong hybrid with a downsized ICE, often a larger generator and battery for extended electric range, and the necessary electronics to charge the battery and therefore power the vehicle from the electric grid.
- — Series PHEV. The ICE, generator, battery, motor, and transmission are all in series, so all drive to the wheels is ultimately provided by the electric motor, powered by either the battery or the ICE.
- — PS PHEV. Power split similar to the PS strong hybrid but with a larger battery and the ability to drive with the engine off.
- Battery Electric Vehicle (BEV) Architecture. The only source of power is a large battery, charged from the grid, which drives the wheels via a motor connected in series.
Fuel Cell Electric Vehicle (FCEV) Architecture
- The source of power is a fuel cell that generates electricity from a fuel such as hydrogen, either to charge a battery or to drive a motor to power the wheels.
Hybrid Vehicle Fuel Efficiency Fundamentals
HEVs derive all of their energy from petroleum fuel, but compared to conventional vehicles, they use that fuel more efficiently to power the vehicle. Most hybrid vehicles use an internal combustion engine, a battery, and one or more electrical machines. External combustion engines are excluded, although one of the earliest recent hybrids had a Stirling engine (Agarwal et al. 1969). Fuel cell hybrid electric vehicles do not have a combustion engine and will be discussed later in the chapter.
HEVs reduce fuel consumption relative to conventional vehicles in three ways: by implementing regenerative braking, reducing idle, and enabling engine downsizing.
During braking, the kinetic energy of a conventional vehicle is converted into heat in the brakes and is thus lost. An electric motor/generator connected to the drivetrain can act as a generator and return a portion of the braking energy to the battery for reuse. This is called regenerative braking. Regenerative braking is most effective in urban driving and in the urban dynamometer driving schedule (UDDS) cycle, in which about 50 percent of the propulsion energy ends up in the brakes (NRC 2011, 18). Different architectures have different options for regenerative braking, but in the ideal case and with 100 percent efficiency, fuel consumption can be reduced by half for urban driving.
Enabling Engine Downsizing and Efficient Operation
Hybrid powertrains can enable engine downsizing, more efficient use of engine power for motoring, and electrification of accessories. Downsizing the engine and operating closer to its maximum power improves its efficiency. The maximum engine power is many times that required for moving the vehicle along a level road at constant speed. As is discussed in Chapters 1, 2 and 5, the engine is relatively inefficient at
light loads. The electrical machine in hybrids augments the engine power to maintain performance, allowing the use of a smaller engine that operates closer to its best efficiency. Additionally, when motoring, if not all the engine power is needed for propulsion, some of it can be used to recharge the battery.
Hybridization also allows for lower power, more efficient operation, and the use of more efficient engines such as the Atkinson cycle engine. Hybridization and the associated move from mechanical, belt-driven accessories to electricity-driven accessories can increase or decrease efficiency. Powering accessories when needed, rather than whenever the engine is on, reduces energy consumption, as illustrated in Table 2.19 of Chapter 2. However, converting mechanical energy to electrical and back to mechanical incurs losses, for example in the case of the air conditioning compressor.
Engine downsizing can lead to a situation on long steep grades where the vehicle does not have the acceleration the driver expects. If the battery is used to provide additional power to pass several slow moving vehicles, it is conceivable that the battery state of charge (SOC) will drop below the minimum and drivers may not have the passing power they expect. A similar situation exists for PHEVs and BEVs toward the end of their electric range. Insufficient acceleration on steep grades is common in some conventional vehicles with a small engine, for instance vehicles with small three-cylinder engines. Vehicle manufacturers make judgments on how much power to provide to balance performance and cost for both hybrid and conventional vehicles.
Hybrid and Electric Vehicle Architectures
Hybridization of the drivetrain has been implemented to varying degrees, increasing from stop-start and mild hybrids, through strong hybrids to plug-in hybrid electric vehicles, each with increasing degrees of electrification. The varying hybrid architectures, the technology used to implement them, and considerations of cost and consumer acceptance related to that technology are described below.
Stop-Start and Mild Hybrids
Stop-start and mild hybrid systems have minimal electrification and therefore exhibit both the smallest costs and the least fuel consumption reductions. Stop-start systems in vehicles have an augmented starter motor for a quick start and a standard alternator that can accept some of the braking energy. In its simplest form, stopping the engine stops fuel consumption. The reduction in fuel consumption is minimal for these simple stop-start systems, estimated to be 2.1 percent. Mild hybrids also incorporate a motor/generator, either bolted to the crankshaft or connected via a belt. It is used as a generator when the driver applies the brake, and it acts as a motor assisting the engine during acceleration. The SOC is monitored so that the electric motor can start the engine reliably. As the size of the motor/generator is increased, progressively more regenerative braking can be used and then the motor can provide assist during acceleration, thus permitting the engine to be downsized.
The control system for stop-start and mild hybrids is constrained by the battery state and typically involves anticipation of the driver intent. For stop-start systems, the engine should stop when the vehicle stops and quickly restart using the more powerful starter motor. In vehicles with manual transmissions with a neutral gear, the driver’s intent to idle or to launch is clear. In the case of powertrains with automatic transmissions, both the driver intent and the power flow path through the converter complicate the control of the start-stop system where the motor is connected to the crank-shaft. Control strategies for electrified powertrains are further described later in the chapter.
A potential problem with stop-start systems is customer acceptance relative to fuel consumption savings that are on the order of 5 percent for on-road fuel economy (2.1 percent in compliance fuel economy). Restarting the engine creates noise and vibration, which may not be acceptable in the U.S. market, accustomed to automatic transmissions and smooth accelerations from a stop. Another problem of stop-start is that in order to provide cooling during stops, an electrically driven air-conditioning compressor may have to be added, increasing cost. Stop-start systems are finding wide acceptance in Europe, but, with the possible exception of GM in some models, not in the U.S. Stop-start systems are credited with off-cycle benefits that are discussed in Chapters 6 and 8.
One of the more interesting mild hybrid systems is the GM eAssist used in a Buick LaCrosse (Hall 2012; Hawkins et al. 2012). It has a 15 kW induction motor/generator that has three functions: (a) a starter with enough power for an instant start; (b) a generator to keep the battery charged; and (c) a motor to augment the engine during accelerations. It is connected to the engine through an augmented belt. The lithium ion battery, which has 500 Wh capacity, provides 18 kW during acceleration. For a mild hybrid it has remarkable performance: an improvement in fuel economy from 27.8 to 38.1 mpg (37 percent) or a 27 percent reduction in fuel consumption compared to the standard LaCrosse. This dramatic performance improvement is not only due to hybridization, since the hybrid has a smaller engine, downsized from a 3.6L V6 to a 2.4L I4, and other features to reduce fuel consumption. In addition to the regenerative braking and smaller engine, this improvement is accomplished by aggressive fuel cutoff when the driver’s foot is lifted off the accelerator, underbody panels to reduce aerodynamic drag, and a smaller fuel tank to reduce mass. GM claims that the hybrid LaCrosse has performance similar to the conventional LaCrosse, since the electric motor augments the engine in providing acceleration; however, the 0-60 time for the hybrid is slower than that of the conventional, indicating they do not have the same performance. Results were quite
different when eAssist was applied to the Chevrolet Malibu. For the 2013 model year (MY), combined fuel economy for the eAssist Malibu was 38.7 mpg vs 34.8 mpg for the conventional, an 11 percent improvement in mpg and a 10 percent reduction in fuel consumption. For the 2014 MY, the fuel economy of the eAssist Malibu with a 2.4L I4 engine was the same as that of the Malibu with a more efficient 2.5L I4 engine with stop-start.
Battery life is a key element in the design of electrified powertrains and is affected by the conditions of battery use, including the SOC swing. Among other considerations, the allowed swing in the SOC depends on the mode of operation. In hybrids, battery current reverses many times during driving since the electric drive augments the engine during accelerations and recovers energy during braking. DOE specifications call for the battery to survive 300,000 cycles (U.S. DRIVE 2013, 5). To achieve that lifetime, automakers use a narrow swing. GM used a big enough battery so that normally the swing in the SOC is 20 percent of that of the full state of charge. The Prius has had an excellent record of battery life using a NiMH battery with a SOC swing of 20 percent (The Clean Green Car Co. n.d.; Ransom 2011).2 (See also Ingram 2013 for evidence of real-world battery life validation.) In contrast, the Agencies concluded that in the 2017-2025 time frame, mild hybrids would be able to use 40 percent of the SOC and thus in their calculations they assume a battery that has half the cost and size of the LaCrosse’s (EPA/NHTSA 2012a). As described in the section on battery technology, there is no high-power vehicle battery technology targeted for use before 2025 such as could accommodate a 40 percent SOC swing. Vehicle manufacturers acknowledged that, as extended in-use experience is obtained, the battery SOC swing may be increased for all electrified powertrains.
Strong hybrid electric vehicles use a larger motor/generator and battery than mild hybrids, allowing for recapture of energy through regenerative braking as well as greater engine downsizing. The systems are much more costly to implement than stop-start or mild hybrid systems, but they generally exhibit much better fuel economy. The two primary types of strong hybrids on the market, the parallel (P2) and the power split (PS), are described below.
The P2 architecture (see Figure 4.1) has a clutch between the engine and the motor/generator, which provides two advantages: (1) the engine friction does not reduce regenerative braking, and (2) using the transmission, the motor/generator can spin at higher speed to recover more energy. This architecture provides the option of a PHEV if a bigger battery and motor/generator are installed. Installing a larger motor/generator may be a problem in transversely mounted engines. This architecture has the advantage over the power split in that there is no double energy conversion during certain operating conditions. However, the motor/generator operates at a low speed since it is normally coupled to the engine and is therefore larger for the same power. A challenge for P2 configurations is being able to maintain a good drive quality because the clutch connects and disconnects the engine during operation. Some current implementations of the P2 include an augmented cranking motor for smoother engine start.
The P2 control system is more complex than for MHEV since the motor/generator provides regenerative braking as well as acceleration, and so the clutch needs to be disengaged to maximize regeneration at certain speeds and depending on the force applied to the brake pedal. Also, there are more variables to be controlled, including engine speed and torque and transmission gear as well as power flows to and from the battery. The SOC must be monitored so that the battery is not overcharged going down a long hill. Coordination of the motor/generator and the service brakes is necessary since using the service brakes extensively minimizes the energy recovered, but the motor/generator cannot provide emergency braking alone. Coordination becomes particularly important on icy surfaces so that the wheels do not lock and cause skidding.
One example of a P2 hybrid is the Hyundai Sonata (Hyundai Motor Company 2014) that gets 51.5 mpg combined vs. 36.6 mpg for the conventional version (DOE 2014a), a 28.9 percent reduction in fuel consumption (Table 4.5). The standard Sonata has a 2.4L engine rated at 177 hp, with a compression ratio of 11.3:1. The Sonata hybrid has a 2.4L engine with a lower rating of 159 hp, with a compression ratio of 13:1, indicating that this is an Atkinson cycle version of the base engine, with lower power output but improved efficiency. The Sonata hybrid also has a 47 hp permanent magnet electric motor to provide more total power than the standard Sonata (Hyundai 2014).
Power Split Hybrids
Power split hybrids rely on a planetary gear set whose three inputs are the engine, the motor, and the generator (see Figure 4.2). The generator is used to charge the battery and the motor is connected to the wheels to provide additional torque during acceleration and to recover energy from the wheels to recharge the battery. This architecture has several advantages:
- Some of the power goes to the wheels in the most efficient manner: through gears.
2 In exceptional circumstances, such as going down a long hill, the swing may exceed 20 percent, but the battery size is determined for normal operation.
- When engine power is not needed to move the vehicle, some of the power goes to the generator to charge the battery. This raises the engine output and thus improves engine efficiency.
- The motor, battery, and generator can be sized to handle only a fraction of the peak engine power and thus minimize costs.
To some extent, the energy flow is controlled by controlling the engine and generator speeds and the power electronics that control the generator output. However, the kinematics of a planetary three-input gear set require that separately the sum of the power and the torque of the three inputs add up to zero, ignoring the losses in the gears (Meisel 2009, 2011). As a result, some of the engine power has to go through the generator-battery-motor loop, which is less efficient than the direct mechanical connection. For a detailed discussion of this, see Meisel (2009, 2011).
The control system must meet requirements similar to those for the P2 hybrid’s control system for motoring and especially antilock (ABS) braking. In this case there are even more variables since engine speed and torque as well as speed and power flows to the generator and motor are all subject to the requirement that torque and power at the planetary gear need to sum to zero.
Both Toyota and Ford have had great success with the PS architecture. In 2014, Toyota had 66 percent of hybrid sales and Ford 12 percent (annex Table 4A.1). The Toyota Camry LE hybrid has 57.4 mpg as the EPA certification fuel economy, a 50.3 percent increase in fuel economy, equivalent to a 33.5 percent decrease in fuel consumption compared to the regular Toyota Camry, which registers 38.2 mpg. Similarly,
the Ford Fusion Hybrid SE gets 66.1 mpg, a 91.2 percent higher mpg (47.7 percent lower fuel consumption) compared to the 2.5 L Fusion conventional vehicle. It could be that the Ford hybrid has additional fuel saving features: By carefully choosing the gear ratios and the generator control, the system can be tuned to give best results for a specified test cycle, leading to larger gaps between test-cycle and real-world fuel economy. Ford’s and Toyota’s test-cycle fuel economies for urban and highway driving, as a result, do not comport well with the fuel economy experienced by a typical driver. This can be seen by comparing the certification fuel economy values with those on the vehicle’s label, which is adjusted to better reflect real-world fuel economy. For the PS hybrid Ford Fusion, for example, the ratio of label to compliance value is about 0.71 while for the conventional Fusion, the ratio is about 0.75 (EPA 2014a). Separately, Ford recently reduced the amount of fuel economy claimed for the C-Max hybrid, due to aerodynamic differences between it and the Fusion as well as problems with the coastdown procedure and resulting dynamometer settings (Woodyard 2013; EPA 2014b).
A disadvantage of the power split architecture is that when towing or driving under other real-world conditions, performance is not optimum. GM, in collaboration with Chrysler and BMW, tried to correct this by adding another planetary gear set to produce what they called a two-mode drive. Meisel (2009, 2011) goes into great depth comparing the two drives. A more recent paper also discusses this two-mode drive (Arata et al. 2011). The EPA website shows a reduction in fuel consumption of 26.3 percent for the 2013 two-mode model, which is less than other hybrids. The lower efficiency gain as recorded in the test cycle may reflect the fact that the drive was optimized for off-cycle driving or towing and not just for the certification cycles. In any case, this drive was poorly received in the market and was abandoned by Chrysler, BMW, and GM.
PHEVs are the next step from hybrids to full BEVs. They differ from hybrids because they can obtain electricity by charging the battery from the electrical grid, allowing for some portion of their drive to be powered without petroleum. Similar to conventional hybrids, they still have an engine, but they generally have a larger battery and an electric drive capable of propelling the vehicle with the engine turned off. There are various architectures, some of which have been carried over from HEVs. The most popular have batteries that can provide all-electric ranges of about 12, 20, or 40 miles. The advantages of PHEVs over HEVs include these:
- Reduced petroleum consumption because some energy comes from the grid.
- Significantly lower cost per mile driven electrically than with gasoline and an ICE (NRC 2015).
- Reduced GHG emissions depending on the fraction of miles driven electrically and on where and when the PHEV is charged, as the emissions of electric power generators vary by place and time (Anair and Mahmassani 2012). The DOE offers a calculation of these emissions. For example, the Chevrolet Volt emits 300 g CO2/mi, including upstream emissions, for the grid region encompassing much of Michigan. The emissions for much of California, in the CAMX region, result in 200 g CO2/mi (DOE 2014b).
- Reduced tailpipe emissions (NOx, CO, and HC).
- Convenience of operating without liquid fuel much of the time, depending on the driving distance between charges (for instance when commuting).
- In contrast to BEVs, PHEVs are similar to HEVs in their ability to fuel with petroleum upon exhaustion of the battery, to provide range similar to conventional vehicles with easy refueling.
The main disadvantage of PHEVs is cost. To obtain full performance in an all-electric mode they require not only a larger battery but in most cases larger motors and power electronics. The exception is the Toyota Prius PHEV, which appears to use an otherwise identical electrical system as the hybrid, with the addition of a bigger battery. The Prius PHEV has limited capability in the all-electric mode with limited acceleration (0 to 60 in 11.3 s), an all-electric range of 6 miles (MotorTrend n.d.) and a top speed of 62 mph (Siler 2010). PHEV performance is monitored and vehicle controls switch between electric-only, mixed and engine-only drive depending on driving demands and battery charge status. PHEVs with smaller batteries, motors or power electronics will switch more often into mixed or engine-only mode than PHEVs with more robust electric powertrains, though in all cases, controls will switch on the engine to provide power to meet driving demands.
Simple Series PHEV
As shown in Figure 4.3, the engine in a series PHEV drives a generator that charges the battery by means of relatively simple electronics. The battery powers the motor through the main electronics and during regenerative braking returns power to the battery. This system has several advantages:
- The engine can run at constant speed and at full load when charging the battery. Thus it can operate at its maximum efficiency and with simpler emission controls.
- The motor is designed to provide full power and full vehicle performance in all-electric mode.
- The large motor can act as a generator to recover the maximum amount of regenerative braking.
The control system is relatively simple for series PHEVs. The battery provides all the power until the SOC reaches a low level and the engine starts, providing power to charge the battery. Care must be taken that the battery, even in its depleted state, can provide the necessary acceleration. Also, charging the battery during a long downhill drive is monitored to avoid overcharge. As in other hybrids, antilock braking needs to coordinate service brakes and motor torque.
The prime example of a vehicle with series architecture is the GM Volt. It has a battery with a nominal 16 kWh energy rating, but GM chose to operate in most conditions between 20 percent and 80 percent SOC to ensure performance after 100,000 miles of service and an 8-year warranty (GM-Volt. com 2010). The battery cells are made by LG Chem in Korea. EPA and DOE certify the all-electric range as 38 miles. The vehicle has a relatively small engine, 63 kW, since GM required it to maintain performance with the battery depleted. When the engine is turned on to provide charging, it can be connected to the wheels mechanically at high speeds. This avoids the double energy conversion (mechanical to electrical to mechanical) to improve efficiency. Acceleration is provided by the motor, rated at 111 kW. The generator is matched to the 63 kW engine and is rated at 54 kW. Additionally, GM had Goodyear design special low-rolling-resistance tires for improved fuel economy on the Volt.
The main disadvantage of the Volt is high cost. Since it has the largest battery of PHEVs, a large motor and a generator, as well as full power electronics, the Volt costs more than other PHEVs. EPA rates the Volt running on engine-only at 48.4 mpg combined compared to a similarly sized Cruze at 35.1 mpg, showing 27.5 percent lower fuel consumption (DOE n.d.a). However its great advantage is that it is able to carry out a large fraction of all trips in an all-electric mode using no petroleum-based fuel (Gordon-Bloomfield 2012a).
Honda Two-Motor Series Hybrid (2M)
Figure 4.4 (Higuchi et al. 2013) illustrates the new 2M architecture which Honda calls Intelligent Multi-Mode Drive (iMMD). In the EV mode, a clutch connects the drive motor to the wheels. If a charge sustaining mode is used, the engine can provide energy to the battery. In hybrid mode, electric power can come from the engine/generator and the battery. An interesting feature of this architecture is that the size of the various components, engine, motor, and generator can be optimized separately. Honda chose to eliminate a multispeed transmission, or CVT, and instead drive the wheels through a fixed gear ratio in the engine mode. As Figure 4.5 shows, at highway speeds the engine alone drives the vehicle.
By varying the size of the battery, Honda offers both a conventional HEV and a PHEV version of the Accord (Honda n.d.a, n.d.b). The hybrid achieves fuel economy of 69.9 mpg with a 2.0L I4 engine, while a regular Accord has 40.2 mpg with a 2.4L I4 engine. If we assume that the additional performance in the hybrid is provided by the electric motor, then the iMMD provides a 57.6 percent reduction in fuel consumption, or a 73.6 percent improvement in mpg vs. the regular Accord. The PHEV contains a 6.7 kWh battery and has an electric range of 13 miles. Presumably the limited electric range is a result of the limited size of the battery, therefore reducing the cost of the battery. Unlike the PS architecture, however, the motor is larger since it has to provide full power at low speeds. Both the hybrid and the PHEV have the same engine (141 hp at 6,200 rpm) and electric motor (124 kW).
Both the hybrid and the PHEV Accords are new in 2014. Early reviews indicate a concern that the fuel economy in a test drive “failed to achieve numbers even close to its EPA ratings” (Thomas 2014) and that while the vehicle delivers good speed and styling, there are some NVH issues (Csere 2013; Edmunds.com). As Figure 4.4 shows, engaging and
FIGURE 4.4 Honda two-motor series architecture, showing the clutch-modulated connection between the battery, engine, motor and generator, and the wheels.
SOURCE: Higuchi et al. (2013) Reprinted with permission from SAE paper 2013-01-1476. Copyright © 2013 SAE International.
FIGURE 4.5 Honda two-motor series showing modes of operation at various vehicle speeds and driving forces.
SOURCE: Higuchi et al. (2013) Reprinted with permission from SAE paper 2013-01-1476. Copyright © 2013 SAE International.
disengaging several clutches is necessary, which may lead to such NVH issues (see Honda n.d.a, n.d.b).
Power Split PHEVs
These are essentially the same as the power split HEVs with a bigger battery and the ability to drive with the engine turned off. The vehicles in the market are made by Toyota and Ford. One problem is that the HEV drive motor is usually too small to provide full acceleration. Toyota’s solution is to have the engine come on when more power is needed, while Ford has installed a larger motor in its PHEV. The net result is that the Toyota Prius PHEV can operate in the UDDS cycle in all-electric mode for only 6 miles but must use “blended operation” in the more demanding cycles. In contrast, the GM Volt can meet both UDDS and the Highway Fuel Economy Test (HWFET) cycles in all-electric mode. Since the greatest cost in PHEVs is the battery, it will be interesting to see whether the Toyota succeeds better in the market than the GM Volt, which has higher performance but much higher cost.
Battery Electric Vehicles
Unlike hybrids, BEVs derive all their propulsion from energy stored in the battery, so their range is limited by battery size. Battery energy density and cost improvements are therefore central to improved performance of BEVs. Originally, lead acid batteries were used in BEVs. The invention of the nickel metal hydride battery (NiMH) offered a roughly twofold improvement in energy density over the lead acid battery at increased cost and the battery continues to be used for HEVs. The NiMH battery range was still not sufficient for a BEV, as demonstrated by the EV1 vehicle produced by GM. A few hundred of these were produced but were recalled and scrapped by GM. Although the lithium ion battery was invented at Exxon in the 1970s (Whittingham 1973, 1976) and used in small electronic devices and computers in the 1990s, it was not used for vehicle propulsion until 2006 in a limited production Tesla Roadster. Tesla implemented lithium ion batteries more seriously in the Model S in 2012. Other early users of lithium ion batteries were the GM PHEV Volt, described above, and the Nissan Leaf BEV.
The structure of a BEV drivetrain is quite simple in comparison to that of the HEV drivetrain. A schematic of a BEV is shown in Figure 4.6. It requires a large, full perfor-
mance electric motor and power electronics similar to those described for the Volt. It also requires a much larger battery. The 2014 Leaf has an 80 kW motor and a 24 kWh battery (Nissan n.d.) and according to EPA has a range of 84 miles (DOE n.d.b). The Tesla model S has a 265 mile range with an 85 kWh battery.
The Agencies calculate battery cost from the battery size and the cost/kWh, both of which vary by the degree of electrification of the vehicle. Battery size is scaled based on the assumed vehicle range, vehicle weight, SOC swing, and power/distance requirements of the vehicle (Tesla n.d.). The Agencies’ allowed SOC swing is 80 percent (10-90 percent) for BEVs and 70 percent (15-85 percent) for PHEVs (EPA/NHTSA 2012a, 3-147), which is appropriate for those architectures. Once the size of the battery was determined, the cost was evaluated via the BatPaC model. The model included varying cost/kWh for BEV, PHEV, and HEV batteries, primarily due to their focus on energy in the former and power in the latter. The BatPaC model assumes that energy-optimized batteries will require fewer, thicker components, while power-optimized batteries require more, thinner components, increasing the relative cost of the power-optimized battery. The resulting difference in cost/kWh between a PHEV battery and EV battery is exaggerated, however. For example, the Leaf uses 21.5 kWh and the Volt, 10 kWh. This should lead to a greater than 2 to 1 ratio of cost for the battery direct manufacturing cost (DMC). The Agencies reported battery DMCs for standard-size passenger vehicles with 15 percent applied weight reduction for the EV75 similar to the Leaf: $11,174, and for the PHEV40 similar to the Volt, $8,642. This leads to a ratio of 1.29 to 1, making the Leaf appear much less expensive relative to the Volt.
Fuel Cell Hybrid Electric Vehicles
Fuel cell hybrids have an architecture similar to series hybrids, as shown in Figure 4.7, with the engine and generator replaced by a fuel cell. For automotive applications the
fuel cell is powered by hydrogen stored on board. Although they are currently not in mass production, FCEVs offer the possibility of high-efficiency, petroleum-free transportation just like BEVs but without the range limitations of the battery. A significant hurdle to fuel cell vehicle deployment is the extensive infrastructure required. A more detailed discussion of the technology prospects for FCEVs is given later in this chapter.
Enabling Technologies for Vehicle Electrification
Internal Combustion Engines for Hybrid Vehicles
The majority of hybrid vehicles use an SI engine with what is known as the Atkinson cycle. In its modern version, this uses a conventional SI engine with the intake valves held open beyond bottom dead center. This allows flow from the combustion chamber to the inlet manifold and effectively reduces the compression ratio while providing a higher expansion ratio after the mechanical compression ratio is increased to approximately 13:1. The net effect is better engine efficiency (brake specific fuel consumption, BSFC) but lower power output. In hybrids this is acceptable since acceleration is helped by the motor, and efficiency is of paramount importance.
With PHEVs that use the power split architecture, the Atkinson cycle is maintained. In the GM Volt with a series configuration, the engine essentially operates at wide open throttle and there is no need to change the valve timing. Another new idea for hybridization is the use of a very small engine as an emergency backup. The BMW i3, for example, is basically a BEV but has the option of having a small motorcycle engine to extend the range.
Diesel engines can be used in hybrid vehicles; however, none are currently sold in the United States. As stated by some automakers, because diesel hybrids do offer the ultimate technology to reduce petroleum fuel consumption in an ICE, they will perhaps one day be offered for sale in the U.S.
Supervisory Control Strategies in Hybrid Electric Vehicles
HEVs balance multiple power flows in the powertrain. A supervisory controller in HEVs manages power flows among powertrain components such as the engine (or the fuel cell), a motor, and a battery (or capacitors) to minimize cost functions such as fuel consumption, emissions, battery life, and drivability. Many supervisory control strategies for hybrid vehicles have been proposed to fully exploit hardware potential and optimize or negotiate various objectives. Most supervisory control strategies can be classified into rule-based and optimal control approaches.
The rule-based strategies are simple heuristics based on regenerative braking and load leveling. Load leveling is attempting to maintain the engine operation within predetermined regions where fuel efficiency is relatively high. A rule-based control is advantageous for ease of implementation and for the effectiveness of SOC regulation, and it can influence the fuel economy indirectly by tuning the engine and battery operating regions (Hochgraf et al. 1996; Jalil et al. 1997). However, the rule-based strategies do not directly minimize cost functions such as fuel consumption, emissions, and battery life.
On the other hand, optimal strategies are based on optimal control theory and can produce implementable power management by directly manipulating a cost function that weighs the various high-level objectives to be accomplished. Optimal power management relies on models of the torque and efficiency characteristics of all the components that participate in the power flow. These high-level models of the engine (speed, torque, efficiency) and the battery (SOC, voltage, efficiency) make it possible to cascade the cost function objectives to an implementable optimal power split sequence. Additional models are used to cascade the high-level power split commands to the low-level actuator settings such as engine fueling and battery current. For an effective strategy, the supervisory power management needs to be informed of and account for the engine and battery constraints for a reasonable time horizon. Thus the supervisory controller receives signals from the engine control unit (ECU), as discussed in Chapter 2, and the battery management system (BMS), discussed later in this chapter.
To minimize fuel consumption, a truly optimal decision depends on the future decisions where storing some power from the battery now to increase its SOC will pay off later, at a time when an acceleration might be achieved with electric power instead of forcing the engine to operate in a less-efficient region. In real-time implementation on-board a vehicle, information about the future acceleration commands from the driver (driver intent over the entire drive cycle) is obviously very limited. Automated and connected vehicles could provide this necessary future information to the supervisory controllers from vehicle-to-grid (V2G) and vehicle-to-vehicle (V2V) communication along with traffic and terrain information. Currently, however, optimal supervisory methodologies address the inherent dependency on the future information in various ways. Important optimal supervisory methods that have been implemented are discussed below.
The Equivalent Consumption Minimization Strategy (ECMS) and Dynamic Programming (DP) are the most widely studied methods for guaranteeing optimality (Pisu and Rizzoni 2007; Serrao et al. 2011; Lin et al. 2003; Liu and Peng 2008). In an ECMS approach, the total energy to be minimized is considered to be the sum of fuel consumption and battery energy over a driving cycle (Pisu and Rizzoni 2007; Serrao et al. 2011). Typically, an equivalent factor is introduced to convert the battery energy to equivalent fuel energy. In Serrao et al. (2011), adaptive ECMS (A-ECMS) has been proposed to adjust this equivalent factor in real time.
While deterministic DP is used to solve an optimal control problem when the entire driving cycle is given, stochastic DP solves the optimal control when a driving cycle is undetermined (Lin et al. 2003; Liu and Peng 2008). Apart from the causality issue, the use of DP on-board the vehicle is not simple since the computational effort in calculating the optimal decision increases exponentially with the number of state and control variables, also known as “curse of dimensionality.” This strategy requires vast computing resources that are not easily accessible, although secure cloud computing might provide these resources in the future. Nevertheless, thorough off-line analysis of DP results for various drive cycles provides good insight into the nature of optimal solutions (Serrao et al. 2011; Liu and Peng 2008). It should be noted that solutions from ECMS and DP are almost identical when fuel consumption is considered as a primary objective to be minimized. Recently, Model Predictive Control (MPC)-based algorithms have been developed for a supervisory controller in HEVs (Di Cairano et al. 2013; Kim et al. 2015) utilizing a prediction of acceleration demands for a short horizon in the future using driver learning methods (Sun et al. 2014).
The performance of the aforementioned strategies in terms of fuel consumption is reported in Pisu and Rizzoni (2007), Liu and Peng (2008), and Di Cairano (2013). Careful assessments are required because the hybrid architectures considered in the studies differ one from the other (i.e., parallel in Pisu and Rizzoni 2007, series in Di Cairano et al. 2013, and power split in Liu and Peng 2008). Nonetheless, optimal model-based strategies always outperform rule-based strategies over various driving cycles such as UDDS, the HWFET cycle, and US06. Table 4.1 shows that the effectiveness of optimal control-based strategy is demonstrated not only by simulations but also by experiments. It is noted that the reported improvements in fuel economy are significantly affected by driving patterns. As expected, the benefits of vehicle electrification are greater when driving in the city, with frequent stop-and-go operation, than when driving on highways.
Although the field of developing supervisory controllers for HEVs seems to be mature, it is still evolving as other objectives such as drivability and emissions are introduced. For instance, in Opila et al. (2012, 2013) authors investigated the influence of the supervisory controller on the frequency of engine on/off and gear shifting. In Kum et al. (2013), authors studied optimal clutch and motor control strategies that resolve drivability concerns during engine starts. The committee found that well-tuned controls were critical in consumer acceptance of new fuel economy technologies such as stop-start. Furthermore, much effort has been devoted to develop supervisory controllers for diesel-powered hybrid vehicles that can substantially decrease emissions such as generated smoke during transients. In Kim et al. (2015) and Nüesch et al. (2014), authors have formulated optimal control problems by applying MPC and DP, respectively, and shown significant reduction in emissions, pointing to possible simplification and cost reduction of the diesel exhaust after-treatment.
Motors and Power Electronics
Electric motors have been used in vehicle accessories for over a century, but their use expanded rapidly when high-energy magnets were invented in the 1980s. With the emphasis on reduced weight, they are now in use in applications such as electric power steering and engine cooling fans, among others. Increasingly, motors are also used to power vehicle motion. Electric vehicles in the 1970s and 1980s used brush-type traction dc motors that were replaced by induction motors starting with the EV1 in the mid-1990s. With the availability of new high-energy magnets, induction motors were replaced by higher efficiency permanent magnet motors starting with the Toyota Prius. Practically all xEVs use permanent magnet motors. Neodymium, one of the rare earth materials essential in high energy magnets, is mostly mined in China and in 2011 experienced a temporary almost tenfold spike in price (Piggott 2011). This forced a reconsideration of induction motors, which Tesla, GM, and Toyota have used in vehicles. Companies restarted mines for rare earth materials in countries other than China, and permanent magnet motors will presumably regain the market.
Motor cost is one component of the cost of vehicle electrification. The Agencies’ analysis of hybridization cost was based on an FEV teardown study of the Ford Fusion and the
TABLE 4.1 Relative Fuel Economy Improvements Obtained Between Optimal Control Strategies and Rule-Based Strategies in Simulations and Experiments of Various Fuel Economy Drive Cycles (percent). The Optimal Control Strategies MPC and DP Consistently Outperform the Rule-Based Strategies
|Controller||UDDS Simulation||US06 Simulation||UDDS Test|
SOURCE: Di Cairano (2013).
Fusion HEV, a PS hybrid (EPA 2011, 12). In their analysis, the Agencies used the power of the machine to scale cost data obtained from the Fusion to other vehicle classes as well as to the P2 hybrid system (EPA 2011, 125). Use of power to scale motor cost is incorrect. Materials costs scale with motor volume, and volume scales with torque, not power. Fundamentally, the rotor diameter squared multiplied by the rotor length is proportional to torque (Alger 1970; Pyrhönen et al. 2009). Scaling by power rather than torque is important when comparing motors designed to operate at different speeds. The P2 motor is inline with the engine and transmission and has the same revolutions per minute (rpm) as the engine. This constraint is not present in the PS hybrid, thereby allowing the use of a higher speed and smaller motor. Power is equal to torque times speed, so a slow motor, such as is used in a P2 architecture, will have a higher torque and be much heavier than a PS motor that is designed to operate at much higher speed and lower torque. For example, a PS motor that operates at 6,000 rpm will weigh half as much as a P2 motor operating at 3,000 rpm. Also, the PS architecture appears to be more effective in reducing fuel consumption, as illustrated in Table 4.6.
Power electronics are needed to perform two functions in electrified powertrains: (1) they convert the direct current (dc) provided by the battery into an alternating current (ac) of controlled amplitude and frequency to power the electric motors, and (2) they convert the grid power (120/240 V ac) to dc to charge the battery. The technology for powering the motor from the battery has been developed for industrial use over the last 60 years; the main problems are improving efficiency and reducing size while providing adequate cooling. Research is ongoing in the use of wide band gap (WBG) materials in place of silicon and in the development of high-temperature, high-frequency capacitors. Presumably to meet the needs of xEVs, the development of power electronics devices using WBG materials has increased, and power electronics may very well find limited application in vehicles by 2020 (Nikkei 2014). Since devices using WBG materials operate at higher temperatures, their advantage will be reduced package size, easier cooling, and, possibly, higher efficiency. Cost will continue to be an issue.
Choice of voltage for the vehicle electrical system has been based on safety, electric loads, efficiency, and available technologies. The electric system powering accessories and the starter operates at 12 V, powered by a lead acid battery. Attempts made in the past to replace the 12 V system with a higher voltage, specifically with 48 V, did not succeed because of safety concerns. At voltages higher than about 24 V, a break in the wire could create a sustainable arc that could ignite the insulation, causing a fire. xEVs have, in addition, a high-voltage system for the battery and electric drive. Typically this starts at 48 V for stop-start systems and goes up to 500 V. The 12 V system is still used to power all low-power accessories, lights, and the like. To prevent fires, all voltages higher than 12 V use special color-coded wires, heavier insulation, and special connectors. Many xEVs use an electrically driven air-conditioning compressor to provide cooling when the engine is stopped. This is normally driven from the high-voltage battery (Green Car Congress 2014, 2015). Note that these vehicles have a 12 V system for accessories, and often they have a dc to dc converter to make sure the 12 V battery is charged since it provides essential functions.
Power electronics for charging the battery are in development. The simplest way is to use a controlled rectifier, perhaps with a dc to dc voltage booster. In this way the devices used for driving the motor can be reused for charging since the two functions are not performed at the same time. This does not provide galvanic isolation between the battery and the plug, however, and most automakers have used a more complex circuit with high-frequency conversion that allows transformer coupling. For PEVs, an interesting development is to charge the battery without plugging in to an outlet. This can be done by inductive coupling between two coils: one in the ground and the second on the vehicle. The two coils can be separated by as much as 12 inches and the radiated field can be controlled so that it does not exceed harmful levels (Miller and Onar 2013). It does not seem likely that such a feature will have much effect on adoption of PEVs and thus will have minimal impact on fuel economy.
Cooling is critical for batteries, power electronics, and motors. Eventually, all heat generated has to be rejected to the ambient, but how it is done affects both effectiveness and cost. The heat can be rejected to a liquid or directly to the air. Some automakers, like GM, have used the vehicle’s refrigeration systems for cooling. For electrified powertrains, battery cooling is most critical since battery life depends essentially on two factors: deterioration due to temperature during shelf life and the number of charge and discharge cycles (Steffke et al. 2013; Lohse-Busch et al. 2013). Time will tell how critical cooling is for battery life, although there are indications that air cooling for the Leaf battery may be inadequate (Gordon-Bloomfield 2012b). Ideally one would like to have one liquid cooling circuit for the whole powertrain, but the lower temperature required for batteries than for either power electronics or motors makes a single system difficult to optimize. WBG materials offer the possibility of using engine coolant since they can handle higher temperatures.
Other Nonbattery Components
Vehicle electrification requires more than the addition of motors and batteries and, for BEVs, removal of the ICE and transmission. The preceding sections of this chapter describe the potential for engine technology changes, as well as required motors, power electronics, and cooling systems to enable electrification. The chapter describes more sophisticated
software and algorithms for the controls hybrids may require, though these technologies are becoming ever more important in SI engine vehicles as well. Less obvious changes may also be required, especially for PEVs, where the battery is larger and the ICE is either absent or likely to be turned off for large portions of the duty cycle. For example, the ICE provides much of the climate control within a vehicle, so in its absence, systems must be added to heat, cool, and defrost the vehicle.
To evaluate the technologies required for hybridization as well as their costs, the Agencies used an FEV teardown of a Ford Fusion Hybrid, which has a PS architecture. The results were used to estimate the costs for a P2 architecture. Apart from the size of the P2 motor discussed above, the committee agrees with the teardown costs of the PS hybrid as applied to that architecture and the P2 architecture.
The Agencies proceeded to use the hybrid Fusion teardown results to estimate the costs of PHEVs and BEVs. In the opinion of the committee, the Agencies did not fully identify and evaluate the costs of changes inherent to PHEVs and BEVs; these include the body system, brake system, climate controls, a complex dc/dc converter to maintain headlight intensity for the 12 V battery during engine stops, power distribution and control, on-vehicle charger, supplemental heating, high-voltage wiring, battery discharge systems, purchase and installation of a home charger in the case of PEVs and removal of the ICE and transmission in the case of BEVs (EPA/NHTSA 2012a). The scaling of the nonbattery costs from the PS teardown does not adequately estimate the costs for these and other components required for PHEVs and BEVs. Discussions with several automakers and one supplier knowledgeable in the complexities of wiring and other subsystems identified added costs for more extensive wiring, power cables with RFI suppression, and sealed fuel tanks, among other unestimated or underestimated costs. Overall, the committee finds that the range of nonbattery costs includes a low estimate equal to the Agencies’ estimates and a high estimate up to approximately $1,300 and $500 above the Agencies’ estimates for a midsize PHEV 40 and an EV75 in 2025, respectively. These cost increments were developed by multiplying the Agencies’ nonbattery costs, exclusive of the charger, by a factor of 1.5 for both the PHEV 40 and the EV75. The committee’s estimate of the charger cost was the same as that of the Agencies.
Batteries are required in conventional vehicles for electric starting of the vehicle. For the improved fuel economy provided by regenerative braking and electrically powered drive, larger batteries are required. Current vehicles on the market typically use nickel metal hydride (NiMH) or lithium ion (Li-ion) batteries for hybrid vehicles and Li-ion batteries for battery electric vehicles. These high-power, high-energy, large-volume, heavy batteries are the most significant incremental cost for vehicle electrification (Whittingham 2004). Improvements in battery chemistry and engineering are thus critical to reducing the cost of HEVs and BEVs. In this section, current and near-term (to 2025) battery technologies will be discussed. The focus will be on Li-ion batteries since NiMH batteries are not expected to see significant development. Longer-term battery technologies will be discussed in the next section.
The rechargeable, lithium-ion battery was first introduced as a commercially viable product by the Sony Corporation in the early 1990s following more than two decades of research in the field (Whittingham 2004). Since that time, Li-ion technology has matured to the point of dominating the consumer electronics market. State-of-the-art Li-ion batteries now enable portable electronic devices, which have changed the way we live and communicate. The success of Li-ion batteries has prompted a surge in research and development aimed at harnessing the energy-storage capabilities of Li-ion chemistries for more advanced applications such as PEV transportation. However, despite the success of Li-ion with respect to consumer electronics, transportation applications are considerably more demanding, particularly in terms of battery life, safety, and cost (USABC 2013a, 2013b). As such, several major challenges must be addressed and overcome if Li-ion is to power the next fleet of light-duty vehicles. While there are currently many novel Li-ion-related technologies under investigation (Yang et al. 2011), this section will present an overview of current research on some of the most promising near-term technologies related to rechargeable Li-ion systems, including next-generation cathodes, anodes, and electrolytes.
Basic Operating Principles
Figure 4.8 shows a schematic of a typical Li-ion battery consisting of two electrodes (cathode and anode), a separator, and a liquid electrolyte that permeates the system. The cathode, or positive electrode, is lithiated on discharge, while the anode, or negative electrode, is lithiated on charge. As in Figure 4.8, when electrical current is applied to charge the cell, lithium ions move out of the cathode (Li1-xCoO2) and become trapped inside the anode storage medium, which is usually graphitized carbon (LixC6). When the battery is discharged, the lithium ions travel back to the cathode and produce an external electrical current.
Table 4.2 includes a list of current commercial cathodes and the relevant battery metrics for comparison. Layered LiCoO2 (shown in Figure 4.8) has been the standard Li-ion chemistry for almost 30 years, largely because of its high volumetric energy density. During cell operation at 3.0-4.2 V, the practical capacity of the LiCoO2 electrodes is approximately 150 mAh/g, ~50 percent of its theoretical value
FIGURE 4.8 Working Li-ion battery utilizing a LiCoO2 cathode and a graphite anode having aluminum and copper current collectors, respectively. The electrolyte permeates the entire system and, together with the separator, allows for the diffusion of positively charged Li+ ions but not negatively charged electrons. Electrons must travel through the external circuit, constituting an electric current that powers the attached device (load) on discharge.
SOURCE: Amine et al. (2014). Reproduced with permission.
|Material||Voltage (Ave. vs Li/Li+)||Capacity (mAh/g)||Crystal Density (g/cm3)||Tap Density (g/cm3)||Specific Energy (Wh/kg)||Volumetric Energy (Wh/L)|
a Not yet commercialized.
NOTE: Crystal densities are theoretical values, while tap densities represent typical, practical values, determined experimentally as the actual weight per unit volume occupied by a given material. Volumetric energy densities are calculated using the crystal densities for comparison because the optimum, final electrode densities will vary among materials. M = Mn, Ni, or Co in LiMO2. Capacity and voltage targets for 0.5Li2MnO3•0.5LiMO2 are based on DOE’s end-of-life goals for composite materials; crystal density is calculated as an average of Li2MnO3 and LiMn0.5Ni0.5O2.
(273 mAh/g), due to the surface reactivity and instability of the delithiated Li1-x CoO2 structure (Jeong et al. 2011; Lee et al. 2012). This instability, the high possibility of thermal runaway in inadequately controlled batteries, and the relatively high cost of cobalt have led to efforts to find alternative cathode materials to LiCoO2 that provide Li-ion cells with superior energy density, rate capability, safety, and cycle life.
Several alternative cathode materials to LiCoO2 have been exploited by the Li-ion battery industry over the past decade. They include compositional variations of the layered LiCoO2 structure, such as LiNi0.8Co0.15Al0.05O2 (NCA) (Bang et al. 2006; Lee, S.H. et al. 2013); spinel electrodes derived from LiMn2O4, such as lithium-rich compounds in the Li1+xMn2-xO4 system (Park et al. 2008; Gu et al. 2013); and LiFePO4, which has an olivine-type structure (Padhi et al. 1997a, 1997b; Yuan et al. 2011).
Although NCA provides a slightly higher practical capacity (160-185 mAh/g) than LiCoO2, its thermal instability on delithiation, which is due to the presence of the high valance Ni, compromises the safety of Li-ion cells. On the
other hand, spinel LiMn2O4 and olivine LiFePO4 electrodes are significantly more stable to lithium extraction than the layered Co- and Ni-based electrodes (both structurally and thermally), but they deliver relatively low practical capacities in a lithium cell above 3 V, typically 110-160 mAh/g at moderate current rates. It became clear by the end of the 1990s that alternative structurally stable, high-potential cathode materials (>3 V) with rate capabilities and capacities superior to those achievable with standard LiCoO2-, LiMn2O4-, and LiFePO4-type electrodes were required. As an alternative to increase energy, LiNixMnyCozO2 (x < 1, y < 1, and z < 1) (NMC) was developed for potential use in automotive applications and offers capacities similar to NCA (Zonghai et al. 2013; Amine et al. 2011). As a practical reference, Chevy’s PHEV Volt, with a ~40 mile all-electric range, uses a physically blended NMC-based/LiMn2O4 cathode material. To drive the cost down and/or the driving range up, significant advancements in cathode technology are needed beyond NMC. Recently, Argonne National Laboratory has developed a family of high-energy-density lithium- and manganese-rich xLi2MnO3•(1 − x)LiMO2 (M = Mn, Ni, Co) composite cathodes by structurally integrating a Li2MnO3 stabilizing component into an electrochemically active LiMO2 (M = Mn, Ni, Co) electrode (Thackeray et al. 2007; Sun et al. 2005). The relatively high Mn content in these high-energy cathode materials lowers material costs, while the excess lithium boosts specific capacity to 250 mAh/g between 4.6 and 2.5 V and therefore significantly improves the energy density of the battery cell to 900 Wh/kg. However, in practical cells, when these high-energy NMC oxides are cycled against graphite, deliverable capacity decreases dramatically with cycle number along with a significant decay of cell discharge voltage (Li, Y. et al. 2013; Bettge et al. 2013) and a severe loss of energy density, which hinders its practical application in electric vehicles. Table 4.2 summarizes the battery specification of various lithium ion cells, including a high-energy NMC cathode, and Table 4.3 summarizes performance of all battery systems used in electric vehicles currently on the market.
Anode Materials for Rechargeable Li-Ion Batteries
As will be discussed below, lithium batteries with metallic lithium anodes offer the highest theoretical capacity of almost all conventional battery types and in principle should provide the highest energy density of all lithium batteries, primary or secondary, since lithium metal has an extremely high specific capacity (3,860 mAh/g) and lower negative redox potential (-3.04 V vs. standard hydrogen electrode (SHE)) (Aurbach and Cohen 1996).
To avoid the technical hurdles posed by lithium metal as the anode material, lower specific capacity carbon-based materials, such as graphite (372 mAh/g), are most commonly used. In order to overcome the capacity limit of current technology, materials such as Sn and Si (Hou et al. 2013; Deng et al. 2013; Menkin et al. 2014), which form alloys with lithium, are potentially more attractive anode candidates since they can incorporate larger amounts of lithium (Figure 4.9). Among these metals, silicon-based anodes are particularly attractive because of their higher theoretical specific capacity of approximately 4,200 mAh/g (ca. Li4.4Si), which is far larger than that of graphite and oxide materials (Ge et al. 2013). However, the application of bulk silicon anode faces one major problem: During the reaction that forms the silicon–lithium alloy (corresponding to the insertion of lithium in the negative electrode during the charging process), the volume expansion from the delithiated phase to the lithiated phase may reach 380 percent (Figure 4.10). This high expansion, followed by a contraction of the same amplitude upon discharging rapidly leads to irreversible mechanical damage to the electrode and eventually leads to a loss of contact between the negative electrode and the underlying current collector, which causes a rapid capacity fade during cycling. Furthermore, silicon usually possesses low electrical conductivity, which has the effect of kinetically limiting the use of the battery. A significant effort is under way to enable this system by designing conductive binders that can minimize any particle isolation or by incorporating Si in graphene sheet to keep good conductivity at the electrode level during cycling (Wang et al. 2013; Wu et al. 2013; Ye et al. 2014).
|EV||Cathode||Anode||Battery Supplier||Type of Cella||Number of Cells||Electric Energy (kWh)||Power (kW)||Specific Energy Density (Wh/kg)||Electric Range (mi)|
|Tesla Model S||NCA (layered)||Carbon (layered)||Panasonic||C||>7,000||85||270||116 (pack)||265|
|Chevy Volt||LMO (spinel)||Carbon (layered)||LG Chem||P||>200||16||111||88 (pack)||40|
|Nissan Leaf||LMO (spinel)||Carbon (layered)||Nissan NEC JV||P||192||24||90||140 (pack)||84|
|Honda Fit||NCM (layered)||LTO (layered)||Toshiba Corp.||P||432||20||92||100 (cell)||82|
|BYD E6||LFP (olivine)||Carbon (layered)||BYD||P||96||48||75||—||186|
a C, cylindrical; P, prismatic.
FIGURE 4.9 Specific capacities of graphite, LixAl, LixSn, Li and LixSi anodes (mAh/g).
SOURCE: Amine et al. (2014). Reproduced with permission.
FIGURE 4.10 Volume expansion of different Li-metal alloys, including Li4Si.
SOURCE: Committee-generated from data reported in patent WO 2005076389 A2.
Battery Management Systems
A battery management system (BMS) for Li-ion battery packs is responsible for ensuring that all the battery cells operate within prescribed intervals of voltage, temperature, and Li-ion concentration, as shown in Figure 4.11. To this end, the predominant role of the BMS is real-time estimation and prediction of the battery states and their proximity to the limits—SOC, state of power (SOP), and state of health (SOH).
Battery SOC describes the remaining energy of a battery, which is equivalent to the ubiquitous fuel gauge of a conventional vehicle. Information on the battery SOC is very important for supervisory controllers in electrified vehicles to determine power flows to maximize system efficiency. Many studies have been conducted to develop methods for accurate SOC estimation. These methods can be divided into three categories: coulomb counting, voltage-inversion, and model-based estimation.
Coulomb counting relies on the integration of the current drawn from and supplied to a battery over operation time (Ng et al. 2009). This method is advantageous owing to its simple structure and ease of implementation. However, sensor accuracy, temperature-dependent capacity, and calibration of the initial battery SOC make it difficult to accurately estimate subsequent battery SOC. On the other hand, the voltage-inversion method utilizes the one-to-one relationship between voltage and battery SOC (Pop et al. 2005; Dubarry et al. 2013). That is, the available capacity is determined by measuring terminal voltage during battery discharge operations. However, it is not easy to provide constant discharge current during battery operation. Corrections due to current
FIGURE 4.11 The battery management system protects each cell from a variety of detrimental conditions as described by Ilan Gur, ARPA-E program manager of the AMPED program, in his opening remarks at the 2014 AMPED review meeting.
and temperature-dependent SOC make this method more complicated than traditional coulomb counting.
Various model-based methods with current and voltage measurement (closed-loop estimators) have been developed for battery SOC estimation in an effort to overcome the drawback and merge the benefits of the coulomb counting and voltage-inversion methods (Plett 2004a; Lee et al. 2008; Di Domenico et al. 2010; Smith et al. 2010; Kim 2010; Rahimian et al. 2012; Xiong et al. 2013). The closed-loop SOC estimator relies on coulomb counting that is modified by an error between the estimated voltage and measured cell voltages. Clearly, a voltage prediction is necessary for forming the voltage error, and many recent efforts have targeted computationally efficient and physics-based models that emulate the electrochemical cell behavior. The estimation gain can be computed using various techniques such as pole placement, sliding mode observer, and Kalman filter, including extended Kalman filter and unscented Kalman filter.
Battery SOP refers to the constant power that can be safely drawn from or provided to a battery over a certain period of time. Estimating the battery SOP is of vital importance in protecting Li-ion batteries from overheating as well as overcharging/discharging. Much effort has been devoted to developing model-based methods to estimate battery SOP in real time (Plett 2004a; Smith et al. 2010; Xiong et al. 2013; Moura et al. 2013; Kim et al. 2013). Battery SOP estimation is also important for battery thermal management for applications with limited cooling for Li-ion batteries.
Battery SOH as an indicator of battery degradation defines the present performance of a battery relative to its fresh condition. The performance degradation of a battery may be the result not of a single mechanism but of several complicated mechanisms. Nonetheless, degradation mechanisms can lead to either a decrease in total available capacity or an increase in internal resistance. Thus, various model-based estimation techniques have been developed to identify those parameters with voltage and temperature measurement (Kim 2010; Verbrugge and Tate 2004; Goebel et al. 2008; Plett 2004b; Kim and Cho 2011; Lin et al. 2013).
For electrified vehicles, which require high voltage levels, large banks of series-connected cells are used to satisfy the power demand. Generally, a battery pack consists of hundreds of individual cells. Since aging, use, and calendar life lead to cell-to-cell variability, BMS should be able to equalize cells, referring to cell balancing or cell equalizing, in order to prevent individual cell overcharge or overdischarge. Cell balancing methods can be divided into two categories: dissipative and nondissipative. Dissipating methods equalize the cells by extracting energy from the higher charged ones and dissipating it on shunts or resistors (Asumadu et al. 2005) or selectively disconnecting imbalanced cells from the battery pack (Shibata et al. 2001). Nondissipating methods, on the other hand, can be divided into discharge equalizing systems, like multioutput transformers (Kutkut et al. 1999), charge-equalizing systems, like the distributed Cuk converter (Chen et al. 2009; Park et al. 2009), and bidirectional equalizing systems, like a switched capacitor or an inductor circuit (Moo et al. 2003; Speltino et al. 2010). It should be noted that each approach, regardless of its advantages and drawbacks, relies on an estimated SOC to perform cell balancing.
In summary, the BMS is critical for the performance (SOP estimation), utilization (SOC estimation), degradation (SOH estimation), and, finally, the safety of the battery pack. The processor, the voltage and current sensors, wiring harness, and switching network for the cell-balancing add to the cost of a battery. The accuracy of the BMS and confidence in its functionality are also responsible for defining the battery SOC range, hence influencing the battery size and thus the vehicle cost. Finally, the BMS influences the vehicle fuel consumption indirectly, by informing the hybrid electric vehicle supervisory controller about the battery status and availability (SOC, SOP) and hence defining the operating window for the internal combustion engine.
High-Power vs. High-Energy Batteries
Batteries are designed to store energy and deliver it at needed rates, producing the power required to move the vehicle. There are trade-offs in choice of battery chemistry and battery component design to maximize energy or power. For example, batteries that are designed to contain as much charge as possible, as used in BEVs, are designed for higher energy. In contrast, batteries that must survive more charge and discharge cycles, such as those used in HEVs, are designed with higher power in mind. As discussed previously in reference to mild hybrids in particular, HEV batteries are currently designed to be oversized in terms of energy in order to limit the SOC swing to 20 percent for a battery’s lifetime. Designing battery materials that can provide more power would enable a larger SOC swing for HEVs, enabling the necessary acceleration and regenerative braking with use of a smaller battery, which could lower the battery cost. There are attempts to design such batteries, and one technology that can allow a 40 percent swing in SOC is a spinel LMO coupled with LTO. That system is low voltage (2.5 V), however, which means that cells must be added to get the voltage required for the pack. This will lead to higher costs. In addition, the spinel has a dissolution issue that requires overdesigning the battery pack, leading to significant cost increases. As of now, there is no system under development that is targeting high power that can be used in 2025. Most of the materials under development are seeking to provide high energy to enable lowering the cost of BEVs and PHEVs.
Modeling Estimates of Future Battery Costs
The recent penetration of lithium-ion (Li-ion) batteries into the vehicle market has prompted interest in projecting and understanding the costs of this family of chemistries. The Battery Performance and Cost (BatPaC) model is a
calculation method that was developed at Argonne National Laboratory for estimating the manufacturing cost and performance of Li-ion batteries for electric-drive vehicles including HEVs, PHEVs, and BEVs. The BatPaC model is a publicly available bottom-up design and cost model developed for the Li-ion chemistries with support from the U.S. Department of Energy Vehicle Technologies Office. BatPaC has gone through multiple public and private peer reviews sponsored by EPA (ICF 2011) and has been used in the analysis of the 2017-2025 CAFE/GHG rule (EPA/NHTSA 2012a). A detailed description of the BatPaC model is available in Nelson et al. (2011 and 2012).
To date, a number of cost models for various levels of detail have been published in different forms (Anderman et al. 2000; Barnett et al. 2009, 2010; Dinger et al. 2010). The cost of a battery will change depending on the materials chemistry, battery design, and manufacturing process (Gallagher et al. 2011; Nelson et al. 2009; Santini et al. 2010). Therefore, it is necessary to account for all three areas with a bottom-up cost model for Li-ion battery packs used in automotive transportation. The cost of the designed battery is calculated by accounting for every step in the Li-ion battery manufacturing process. The assumed annual production level directly affects each process step. The total cost to the original equipment manufacturer (OEM) calculated by the model includes the materials, manufacturing, and warranty costs for a battery produced in the year 2020 (in 2010 dollars). A user of the model will be able to recreate the calculations and, perhaps more important, understand the driving forces for the results. Almost every variable in the calculation may be changed by the user to represent a system different from the default values pre-entered into the program.
The distinct advantage of using a bottom-up cost and design model is that the entire power-to-energy space may be traversed to examine the correlation between performance and cost. The BatPaC model accounts for the physical limitations of the electrochemical processes within the battery. Thus, unrealistic designs are penalized in energy density and cost, unlike cost models based on linear extrapolations. Additionally, the consequences for cost and energy density from changes in cell capacity, parallel cell groups, and manufacturing capabilities are easily assessed with the model. New proposed materials may also be examined to translate bench-scale values to the design of full-scale battery packs providing realistic energy densities and prices to the OEMs.
Enabling Technologies for Vehicle Electrification to 2030
The committee’s statement of task seeks advice on the fuel economy technologies expected to be available between 2020 and 2030. In order for electric vehicles to become truly mainstream, significant breakthroughs are required in their energy storage systems. As compared to today’s extant Li-ion technologies, vehicular batteries must achieve lower cost, improved safety, longer driving ranges, less refueling time, and less environmental impact. Fuel cell vehicles face challenges in increasing durability and decreasing cost as well as in the hydrogen supply infrastructure. Both systems, and hydrogen fuel cell vehicles in particular, require deployment of an infrastructure for refilling with electricity or hydrogen. The following section describes the battery and fuel cell technologies likely to be in use in some portion of the fleet by 2030.
From 2020 to 2025, the existing cathode chemistries, including NMC cathodes rich in nickel, will likely be refined and the trade-offs between safety, cost, energy density, and power will be better understood. On the anode side, the industry is predicting a blend of mostly graphite with 5 to 10 percent Si-based anode. As a result, the cell energy density will likely increase by 30-50 percent compared to today’s energy performance. By ~2025-2030, the industry is predicting that the use of stabilized high-energy-density lithium-and manganese-rich xLi2MnO3•(1 − x)LiMO2 (M = Mn, Ni, Co) composite cathodes and a blend of graphite and up to 20 percent silicon anode will double the energy density of today’s lithium ion. As a result, the battery will cost significantly less.
Beyond Li-Ion Systems
As discussed above, the rechargeable Li-ion batteries have transformed portable electronic devices and likely will play a key role in the electrification of transport in the near- to midterm. However, the inherent energy density of the current Li-ion technology is not sufficient for the long-term needs of extended-range BEVs. In this section, the committee provides a brief overview of three systems beyond Li-ion—rechargeable Li-S, Li-air, and magnesium batteries— and addresses some of the key challenges for each individual system.
Two major technical bottlenecks prevent the realization of a successful rechargeable Li metal battery (Wu et al. 2014). One is the growth of lithium dendrites during the repeated charge/discharge cycles, which severely compromises the rechargeability of each lithium cell and could also pose a serious safety hazard because of the potential internal short circuit if these dendrites penetrate the separators and contact the cathode directly. The other is the low coulombic efficiency during the repeated cycles, although this can be partially compensated for by an excess amount of lithium. Overcoming these hurdles presents an enormous challenge to the lithium battery industry. Recently, researchers demonstrated that the growth of the lithium dendrites can be partially prevented through either a physical blocking mechanism using solid-state poly(ethylene oxide) copolymer electrolytes (Balsara et al. 2009) or a self-healing mecha-
nism that uses novel electrolyte additives (Ding et al. 2013). However, these mechanisms are effective only under very limited conditions—that is, at high temperature or low current density. Therefore, work is needed to look for a more reliable way to prevent dendrite growth in order to push the lithium anode for broad applications. Despite these obstacles, significant efforts are still being made to capitalize on and exploit the advantages of the metallic lithium systems such as Li-S and Li-air batteries, with a big assumption that these obstacles can be overcome eventually.
The rechargeable Li-S cell operates by reduction of S at the cathode upon discharge to form a series of soluble polysulfide species (Li2S8, Li2S6, Li2S4) that combine with Li to ultimately produce solid Li2S2 and Li2S at the end of discharge, as illustrated in Figure 4.12. Upon charging, Li2S2/Li2S are converted back to S via similar soluble polysulfide intermediates formed in the discharge process and lithium plates to the nominal anode, making the cell reversible. This contrasts with conventional Li-ion cells, where the lithium ions are intercalated in the anode and cathodes, and consequently the Li-S system allows for a much higher lithium storage density (Barghamadi et al. 2013; Xiulei and Nazar 2010; Yang et al. 2013a).
The Li-S battery, if based on the reaction S8 + 16 Li = 8 Li2S, operates at an average voltage of 2.15 V and has a theoretical specific capacity of 1,675 mAh/g-S. This leads to an energy density of 2,600 Wh/kg (2,800 Wh/L) that is five times higher than that of the conventional Li-ion intercalation battery. Sulfur is an abundant material available on a large scale and at low cost as a side product of petroleum and mineral refining, which makes it attractive for low-cost and high-energy rechargeable lithium batteries. Furthermore, the unique feature of the Li-S chemistry provides inherent chemical overcharge protection, enhancing safety, particularly for high-capacity multi-cell battery packs (Yang et al. 2013a).
Although sulfur-based electrochemical cells had already been reported in 1962, the electronically insulating nature of sulfur, the solubility of intermediately formed polysulfides in common liquid organic electrolytes, and the use of metallic lithium as a negative electrode have still not been solved satisfactorily. In addition, the formed polysulfides in the electrolyte migrate to a lithium metal anode and are electrochemically reduced (Yan et al. 2013), resulting in low coulombic efficiency and rapid capacity fade in Li-S batteries.
Recently, the interest in Li-S-based secondary batteries has been steadily increasing thanks to the design of new nanostructure materials that may be able to overcome issues related to the conductivity of bulk materials (Xiulei et al. 2009; Yang et al. 2013b; Zheng et al. 2011). Moreover, the development of new electrolytes, binder materials, and cell design concepts in general has led to significant advances in the field of Li-S-based secondary batteries within the last few years (Barghamadi et al. 2013; Xiulei and Nazar 2010). There is no doubt that Li-S batteries remain attractive for the longer term because of their inherently high energy content, high power capability, and potential for low cost, although they are still in the development stage.
Li-air batteries could theoretically provide the needed order of magnitude improvement in energy density because
FIGURE 4.12 Scheme of a Li-S cell and its electrochemical reactions.
SOURCE: Amine et al. (2014). Reproduced with permission.
they do not need to store their oxidant (Bruce et al. 2011; Bruce et al. 2012). Whereas state-of-the-art Li-ion batteries have achieved 150-200 Wh/kg (of the 900 Wh/kg theoretically possible) at the cell level, Li-air batteries have the potential to achieve 3,620 Wh/kg (when discharged to Li2O2 at 3.1 V) or 5,200 Wh/kg (when discharged to Li2O at 3.1 V). When the “free” oxygen supplied during discharge and released during charge is not included in the calculation, Li-air cells offer ~11,000 Wh/kg. This is basically identical to the lower heating value for gasoline which is ~13,000 Wh/kg when oxygen is supplied externally. Unlike any other battery technology, Li-air energy density is competitive with that of liquid fuels.
During discharge of the Li-air cell, Li is oxidized to Li+ as a metallic Li anode, conducts through an electrolyte made up of a non-aqueous solvent and a Li salt, and reacts with O2 from the air on a cathode made of carbon, a catalyst, and a binder deposited on a carbon paper substrate, as shown in Figure 4.13. The Li-air technology has the potential to significantly reduce the cost well below that of Li-ion technology due to the higher specific energy densities and the lower cost of the proposed cell components, in particular of the carbon-based cathode materials versus the nickel, manganese, and cobalt oxides used in Li-ion battery cathodes. The non-aqueous electrolyte is preferred, as it has been shown to have higher theoretical energy densities than aqueous electrolyte designs (Zheng et al. 2008).
Current Li-air batteries are still in the experimental stage, and the realization of the high theoretical energy densities and practical application of this technology have been limited by the low power output (i.e., low current density), poor cyclability, and low energy efficiency of the cell. These limitations are caused by the materials and system design:
- (1) Unstable electrolytes. The current non-aqueous carbonate electrolytes are volatile, unstable at high potentials, easily oxidized, and reduced at the lithium anode in the presence of crossover oxygen. This seriously limits cycle life (Freunberger et al. 2011; McCloskey et al. 2011a).
- (2) Lithium electrode poisoning due to oxygen crossover and reaction with the electrolyte destroys the integrity and functioning of the cell and shortens its cycle life (Assary et al. 2013).
- (3) Li2O2 and/or Li2O deposition on the carbon cathode surface or within the pores. This creates clogging and restricts the oxygen flow, lowering capacity (Lu et al. 2011; Lu and Shao-Horn 2013).
- (4) Inefficient cathode structure and catalysis. Commonly used carbons and cathode catalysts do not access the full capacity of the oxygen electrode and cause significant charge overpotentials. This lowers rates (Li, F. et al. 2013; Shao et al. 2012; Shao et al. 2013).
FIGURE 4.13 Diagram of a non-aqueous Li-air battery.
SOURCE: Amine et al. (2014). Reproduced with permission.
It has recently become apparent that the electrolyte plays a key role in Li-air cell performance (McCloskey et al. 2011b; Black et al. 2012; Jung et al. 2012). The oxygen anion radical O2– intermediate or other reduction species that may be formed during the discharge process can be highly reactive and may cause the electrochemical response to be dominated by electrolyte decomposition rather than the expected lithium peroxide formation. The overall result is the consumption of the alkyl carbonate electrolyte.
Although electrolyte stability is of paramount importance, cathode materials also represent a major technology challenge in Li-air cell development (Li, F. et al. 2013; Lu and Amine 2013; Shao et al. 2012, 2013). The ultimate goal is to determine how to effectively increase the specific capacity and power capability of Li-air cells yet still achieve long cycle life. Attaining that goal strongly depends on the mate-
rials and their microstructures in the O2-breathing cathode (Lu et al. 2010; Oh et al. 2012).
Though it offers a high theoretical energy density, in practice a Li-air battery may reach an energy density only twice that of a Li-ion battery. Decreasing the lithium metal content may be limited by the difficulty of manufacturing thin lithium metal electrodes, resulting in about a four times excess lithium used. More significantly, if Li-air batteries must use pure O2 rather than ambient air, then the size and weight of an oxygen tank must be taken into account in the energy density calculation. Li-air technology is likely to take more than 30 years before a real practical prototype can be developed and used to power an electric vehicle.
Rechargeable Magnesium Batteries
Magnesium-based batteries are, in principle, a very attractive alternative to other batteries, including Li batteries. Mg is much less expensive than Li because Mg is abundant in the Earth’s crust. Mg and its compounds are usually less toxic and safer than Li compounds because Mg is stable when exposed to the atmosphere. Mg is also lightweight, which, in theory, could enhance the volumetric energy density of the cell. A rechargeable magnesium battery has been regarded as highly promising technology for energy storage and conversion since its first working prototype was ready for demonstration about a decade ago, and it could compete with lead-acid or Ni-Cd batteries in terms of energy density and self-discharge rate (Yoo et al. 2013). Since Mg provides two electrons per atom with electrochemical characteristics similar to Li, Mg batteries can offer a theoretical specific capacity of 2,205 mAh/g. Proper design and architecture should lead to Mg-based batteries with energy densities of 400-1,100 Wh/kg for an open circuit voltage in the range of 0.8-2.1 V, which would make it an attractive candidate for electric vehicles, electrical grid energy storage, and stationary back-up energy.
Possible future directions to achieve the goal of the high-energy-density Mg batteries include (1) developing high-capacity/low-voltage Mg-S (or other equivalent high-capacity redox couples) cathodes and (2) employing moderate-capacity/high-voltage Mg ion intercalation cathodes. To become practical, Mg batteries are still required to attain a specific energy comparable to that of state-of-the-art Li-ion batteries. Additionally, because of the low rate of Mg2+ diffusion, this system very likely will not ultimately provide enough power capability to power an electric vehicle but would remain an attractive candidate for electrical grid energy storage and stationary back-up energy.
All Solid-State Batteries
Solid-state lithium battery designs have the potential to deliver at least two times the volumetric energy density of conventional Li-ion batteries at less than half the cost per kilowatt-hour. This approach eliminates binders, separators, and liquid electrolytes. By eliminating these components, one can get around 95 percent of the theoretical energy density of the active materials. Solid-state batteries could herald a breakthrough in electrified driving because they are more compact and offer higher energy density than state-of-the-art Li-ion batteries (see Figure 4.14). In the absence of a thermally sensitive solid-electrolyte interphase, solid-state batteries intrinsically have a higher tolerance to thermal abuse and are much safer than Li-ion batteries using a flammable electrolyte. In addition, the solid-state electrolyte is mechanically strong enough to efficiently suppress the growth of lithium dendrites, which might cause an internal short inside a lithium battery using a liquid electrolyte, so it can enable the use of lithium metal as the anode for high energy-density batteries.
Solid-state batteries generally have a low power density, primarily because of two physical limitations associated with solid-state electrolytes: (1) low Li-ion conductivity
inside the electrolyte and (2) low ionic conductivity across the solid-solid interface. In principle, solid-state electrolytes are a class of single-ion conductor, in which the Li-ion can diffuse inside the solid while anions are immobilized. The disadvantage of a single-ion conductor is that the anion cannot establish a concentration gradient to assist the transfer of Li-ions in the electrolyte and the electrolyte-electrode material interface. Moreover, the engineering design of a solid-state battery has to be balanced between energy density and power density. A thicker cathode film is ideal to maximize the loading of active components for a high energy density. On the other hand, a thick electrode extends the diffusion path of both Li-ions and electrons during the normal charge/discharge operation, leading to a decrease in the power density. Currently, the design of an efficient electrolyte/cathode interface holds the greatest promise to boost the power density of solid-state batteries without sacrificing their energy density (Ohta et al. 2012).
Toyota is leading all-solid-state battery development and is planning to use solid-state lithium batteries as early as the 2020s. Since 2012, Toyota has managed to achieve fivefold increases in the power output of its experimental solid-state batteries (Kotani 2013). Although the current technology is still in the laboratory stage, Toyota expects it to be ready for cars in the early 2020s. If technology development is successful, the batteries could give BEVs a range of more than 300 miles on a single charge. Their current solid-state battery’s energy density is around 400 Wh/L, compared with a maximum of around 300 Wh/L for Li-ion batteries. Toyota aims to increase the density to between 600 and 700 Wh/L by 2025.
The committee believes that fuel cell technology will be part of the vehicle mix in 2030. From the Final CAFE Rule in the Federal Register it is noted that
- Fuel cell electric vehicles were considered, but deemed not ready in the 2017-2025 timeframe (EPA/NHTSA 2012b, 62706)
- EPA is providing incentive multipliers for fuel cell electric vehicles for CO2 compliance purposes, similar to the multipliers for EVs and PHEVs, in the 2017-2021 MY time frame to promote the increased application of these technologies in the program’s (i.e., the CAFE Rule’s) early model years (EPA/NHTSA 2012b, 62628). Incentives for AFVs within the CAFE and GHG programs are discussed further in Chapter 10.
The proton exchange membrane (PEM) fuel cell is the selected fuel cell technology for the automotive sector as it can be applied to all vehicle classes and platforms. The major automakers (Daimler, Toyota, Honda, Hyundai, GM, Ford, BMW, and Nissan) are working on solutions and vehicle applications. The following teams have emerged as these automakers move toward commercialization: Daimler/Ford/Nissan, Toyota/BMW, and GM/Honda. Hyundai is pursuing FCEVs independently.
This section presents an evaluation of today’s status of PEM fuel cell technology and the plans communicated by the major automakers for future deployment worldwide. The hydrogen infrastructure plans under development to support FCEVs will be discussed in the next section. While the committee does not expect a significant impact on CAFE in the 2025 time period, it will be valuable to understand the development of both the technology and the hydrogen infrastructure to achieve the future prospects of this technology. Additionally, the increasingly stringent ZEV mandate may drive deployment of FCEVs.
The FCEV consists of a fuel cell system, hydrogen storage, power electronics, an electric drive motor/generator, and, typically, a small battery pack to collect regenerative braking and provide additional energy during cold start. The fuel cell system consists of an anode supply system for hydrogen, a cathode supply system for air (oxygen), a thermal management system, other supporting hardware known as balance of plant (BOP), as well as the controls to integrate the electrical power generation into electric vehicle type architectures. The power electronics, electric drive motor/generator, and battery pack have been strongly influenced by much of the work done to date on both HEVs and BEVs. These PEM fuel cell systems produce DC electricity electrochemically (as do battery-type vehicles) and have operating temperatures of 60 to100oC. The basic PEM technology concept and corresponding system architectures lend themselves very well to the transportation sector.
It is very difficult to give cost numbers for a technology still under development. According to an October 16, 2013, DOE Report (Record #13012), the cost of an 80 kWnet automotive polymer electrolyte membrane (PEM) fuel cell system based on 2013 technology and operating on direct hydrogen is projected to be $67/kW when manufactured at a volume of 100,000 units/year and $55/kW at 500,000 units/year (Spendelow and Marcinkoski 2014). Automakers that are part of the U.S. DRIVE partnership participated in the vetting of the report. Key assumptions from this cost analysis report are shown in Table 4.4.
Current costs at low volume (fewer than 10,000 units) appear to be closer to $300-$500/kW leading to systems costs for 100 kW systems of $30,000 to $50,000. These costs clearly need to be driven down through improved materials, better system integration, and greater volumes. An indication of the progress that has been made is that in 2014 Toyota announced that the price of its vehicle to be produced in 2015 would be around $69,000 (Reuters 2014), but price, particularly for a newly introduced technology, may not be indicative of costs to the manufacturer trying to develop a market.
The automakers take different approaches to the fuel cell system. An automotive system shown in schematic form in
|Cell power density||mWgross/cm2||583||715||833||833||1110||984||692|
|Peak stack temperature||°C||70-90||80||80||90||95||87||97|
|PGM total content (gross)||g/kWgross||0.6||0.35||0.18||0.18||0.17||0.20||0.23|
|PGM total content (net)||g/kWnet||0.68||0.39||0.20||0.20||0.19||0.22||0.25|
|Pt cost||$/tr oz.||1100||1100||1100||1100||1100||1100||1500|
|Balance of plant cost||$/kWnet||42||37||33||25||26||26||27|
|System assembly and testing||$/kWnet||2||2||1||1||1||1||1|
SOURCE: Spendelow and Marcinkoski (2014).
Figure 4.15 includes the hydrogen storage and vehicle cooling systems (Mathias 2014).
Some systems use compressor/expanders and others use compressors only to improve overall efficiencies, but at a cost impact. Some use humidifiers to ensure good proton conductivity of the membrane, while others are driving material developments for self-humidifying membranes. In an automotive application, as described, gaseous hydrogen is supplied to the anode side via onboard storage tank(s), and air (oxygen) is supplied to the cathode side under pressure through an electrically driven compressor to improve the operating performance of the system. In order to achieve the high power densities required for packaging in a vehicle, the stacks are liquid cooled and are configured in a series fashion of 200-400 cells depending on the application and system requirements, as shown in Figure 4.16.
Stack power densities today are on the order of 2-3 kW/L. More important from a vehicle perspective are the whole sys-
FIGURE 4.15 Diagram of a fuel cell vehicle, including hydrogen storage, the fuel cell stack, power electronics, and batteries.
SOURCE: M. Mathias, Honda/GM Fuel Cell Partnership – Moving from Technical to Commercial Viability, SAE 2014 Hybrid and Electric Vehicle Technologies Symposium.
FIGURE 4.16 Schematic of a hydrogen fuel cell. Hydrogen (H2) is oxidized at the anode, separating electrons and protons. Electrons pass through the external circuit, doing electrical work before reaching the cathode. In parallel, protons move through the electrolyte to the cathode, where protons and electrons reduce oxygen to water to complete the electrochemical circuit.
SOURCE: Battery University (2003). Sponsor: Cadex Electronics Inc.
tem power and gravimetric power densities, which directly impact the ability to package these systems in the various vehicle platforms.
System efficiencies can approach the DOE target of 60 percent at the 25 percent load points in the typical driving ranges. Figure 4.17 shows actual system efficiencies from four learning demonstration teams participating in the National Renewable Energy Laboratory (NREL) vehicle study program (Wipke 2012). These efficiency numbers include hydrogen as the fuel, electrical power required to run the air compressor and other ancillary power requiring devices, termed the balance of plant (BOP), and the delivery of DC electric power to the inverter. Fuel cell systems can be very efficient from off idle to approximately 25 percent load, where the majority of normal passenger car operation occurs.
Automotive companies have been developing the PEM technology for more than 20 years. Major accomplishments in the areas of durability, cost, and packaging have allowed the construction of prototype fleets of 10s and 100s of vehicles. Much work has been done with the materials supply base for the membranes, catalysts, metal plate materials, and gas diffusion layers, and suppliers are working on the aforementioned BOP components.
An example of cost reduction in materials is in the catalyst area. The last several years have seen a focus on reduction of platinum group metal (PGM) content, Pt alloys, novel support structures, and non-PGM catalysts. Catalyst cost is projected to be the largest contributor to overall system level costs at high volume. Demonstrated small-scale performance at overall catalyst levels of 0.16 gPGM/kW has been demonstrated with the 3M NSTF membrane electrode assembly, which would yield approximately 14.4 gr of PGM catalyst for an 89 kW stack as referenced in the 2013 DOE Annual Merit Review (Satyapal 2013). Current prototype vehicle fleets generally use 50-70 grams of Pt for a 100 kW stack, so much work needs to be done in the scale-up and engineering of full automotive size systems to get to levels currently achievable in the lab.
FIGURE 4.17 Fuel cell system efficiency at various vehicle power loads.
SOURCE: Wipke (2012).
The automakers have announced commercial vehicle sales in the 2015-2017 timeframe. To support their expectations, in 2013 they announced the following partnerships, mentioned earlier: (1) Toyota and BMW, (2) Daimler, Ford, and Nissan, and (3) GM and Honda, all of which were to assist in the product commercialization phase. Additionally, Hyundai has a significant internal program and publicly announced on November 20, 2013, plans to offer its next-generation Tucson fuel cell vehicle for the U.S. market for just $499 per month, including unlimited free hydrogen refueling and At Your Service Valet Maintenance at no extra cost. The first vehicles were delivered to lessees in June 2014 at several Southern California Hyundai dealers (Voelcker 2014). This first commercial implementation of FCEVs shows EPA compliance fuel consumption of 71.1 miles per gallon gas equivalent and a range of 265 miles. These programs and partnerships between the major players are being put in place to help reduce the engineering costs associated with new technology developments and more closely focus suppliers of both the fuel-cell-specific materials and the BOP components and subsystems such as compressors, sensors, and ancillary equipment. These partnerships also give credibility to the technology and its status. Further proof that these partnerships are accelerating commercialization plans is Toyota’s July 2014 announcement of a 2015 vehicle priced at $69,000 in Japan (Gremeil 2014). Additionally, Honda is expected to release a retail FCEV in 2016, supplanting its lease program of the small scale production Honda FCX Clarity.
The automakers are being forthright regarding their perspectives on fuel cells and the challenges yet to be overcome. Toyota has gone on record (Ohnsman 2013) that the automotive fuel cell propulsion system is the system of the future from its perspective and has significant advantages over battery vehicles in the matters of range and refill (or recharge time in the case of batteries). Other automakers have made similar public announcements. Honda, for example, recently announced improvements to the hydrogen vehicle filling process to shorten times and make it more customer-friendly (Honda 2014).
To meet commercial high-volume product targets there are still several hurdles to overcome. Validated durability is a key at both the catalyst and the membrane level. The automakers, national labs, and the supplier base have done a tremendous amount of work to understand fundamental failure mechanisms and then address them through materials development, design improvements, and system-level controls refinements. Material developments in the catalyst area include improvements in the actual catalyst support, alloys of various materials, core/shell type technologies to improve effectiveness and reduce total Pt loading, and several alternatives with non-precious metal catalyst materials and concepts. Thinner supported-type membranes improve voltage performance and efficiencies as well as water management issues. Materials that can conduct and operate at lower levels of relative humidity are the ultimate goal that the supply community continues to pursue. These developments are continuing and occurring globally, with key suppliers and development programs in the United States, Japan, and Europe as the OEM engineering and commercialization programs move forward.
Enabling Infrastructure for PEVs and FCEVs
Gasoline- and diesel-powered vehicles, including conventional HEVs, use the extensive existing petroleum fueling infrastructure. Similarly, PEVs and FCEVs require an infrastructure for fueling on electricity or hydrogen, but this infrastructure may or may not resemble the existing petroleum infrastructure. Much electric infrastructure exists in private and public buildings and may be co-opted for electric vehicle charging; however, public electric fueling infrastructure is still in development. Hydrogen fueling infrastructure is in an even earlier stage of development and cannot rely on existing infrastructure; this represents a higher barrier to FCEV deployment than does the electricity infrastructure for PEV deployment. The infrastructure needs for PEVs and FCEVs are discussed further below.
PEV Charging Infrastructure
Electric fueling infrastructure is as important for PEVs as petroleum fueling infrastructure is for ICEs. The infrastructure that develops to fuel PEVs may not resemble the gas station model that has developed for ICEs, however. Because PEVs currently refuel more slowly than gasoline vehicles, and because an existing infrastructure of power lines and outlets reaches nearly every building, many PEV drivers have found it convenient to refuel at home or other locations where their vehicle remains parked for long periods. Some workplaces have chosen to implement charging at their parking facilities (NRC 2015). In some areas of high PEV deployment, public charging infrastructure is developing. While all PEVs can use 120 and 240 V charging infrastructure, some BEVs such as the Nissan Leaf and Tesla Model S vehicles can also use DC fast charging stations. Both Nissan and Tesla are building national networks of charging stations. Among current commercial models, only the Model S is practical for cross-country travel, as its charging time is much shorter than its range (NRC 2015).
FCEV Hydrogen Fueling Infrastructure
Several parts of the world are preparing for fuel cell vehicles by developing a hydrogen infrastructure for refueling. The hydrogen fuel for FCEVs is required to be very high purity to ensure optimum performance. Work needs to be done to determine trade-offs with performance and life with lower grades of industrially-produced hydrogen. Most notable infrastructure developments are occurring in California, Germany, Japan,
South Korea, and the U.K. Global deployments of hydrogen refueling are shown in Figure 4.18.
The implementation of a hydrogen infrastructure is required for FCEVs to reach a high volume of adoption. There has been much discussion and debate over when and if such an infrastructure should be realized. In the United States, this has become politicized, but in other areas of the world FCEVs and hydrogen are being considered in a more long-term context to reduce CO2 footprints and enable other applications of the technology. As of late 2013, there were 10 public hydrogen fueling stations in the U.S., nine of them in California (DOE 2014c). The California Fuel Cell Partnership reports that if all currently planned and funded stations are built as expected, there will be 37 in the state by 2015 (Elrick 2013). In estimating fueling stations needed to support upcoming FCEVs, the Partnership has identified “68 strategically placed stations required to be operational by the beginning of 2016,” as shown in Figure 4.18.
While South Carolina is the only other state with a public fueling station at present, a group of entities were recently awarded $500,000 from the Texas Emission Reduction Program (TERP) to partially fund the building of the first public hydrogen fueling station in that state at the Port of Houston (Curtin and Gangi 2013). Several additional states have expressed a commitment to provide infrastructure to support forthcoming fuel cell vehicles. On October 24, 2013, eight U.S. governors signed an agreement to support ZEVs and the necessary infrastructure developments (Carroll 2013), setting “a collective target of having at least 3.3 million zero emission vehicles on the road in our states by 2025 and to work together to establish a fueling infrastructure that will adequately support this number of vehicles.” A federal tax credit of up to 30 percent of the cost, not to exceed $30,000, is available for the installation of hydrogen fueling equipment. The credit expired December 31, 2014 (DOE 2005). There is also a tax credit in place of $0.50 per gallon of liquefied hydrogen sold for the purpose of fueling vehicles.
Hybrid Fuel Consumption Estimates
The Agencies estimate the effectiveness of various electrification technologies as described above. For hybrids, a large amount of certification data exists for comparison to the Agencies’ estimates; however, it is impossible to directly validate the EPA/NHTSA estimates of fuel consumption by comparison to vehicles in the market for two reasons:
- (1) Performance of the conventional and hybrid vehicles is not always the same. For example, hybrids generally have faster acceleration 0 to 30 mph than conventional vehicles and less for 0 to 60. Detailed data are not listed by the EPA website and are not reliably available to compare hybrid and conventional vehicle performance.
- (2) A single vehicle comprises a package of technologies, so isolating the effect of hybridization alone can be difficult as manufacturers may modify hybrids with fuel-saving features beyond the electrified powertrain. For example, in the Buick LaCrosse, GM covered the underbody to reduce drag and implemented aggressive regenerative braking (Hawkins et al. 2012).
Additionally, while certification data exist for current and past model years, the standards are binding to 2021 for fuel economy and 2025 for GHGs. Technologies will
FIGURE 4.18 Projected worldwide locations of hydrogen stations.
SOURCE: Toyota (2014).
be improved, developed, and even abandoned in this time frame. Table 4.5 compares the versions of 2014 MY hybrids with their conventional equivalents. The table lists separately examples of MHEV, P2, and PS architectures. The left-hand column shows the reduction in fuel consumption assumed by the Agencies (EPA/NHTSA 2012a, 3-112).
The data for the GM eAssist used as an example in the technical support document (TSD) for an MHEV illustrate the difficulty in using conventional comparator vehicles to derive the benefit of hybridization alone. The Malibu and the LaCrosse are similar vehicles and yet they show different fuel consumption improvements upon hybridization. The 2014 LaCrosse shows an increase of 37 percent in fuel economy between the conventional and hybrid versions, while the 2013 Malibu shows an 11 percent increase in fuel economy. The Malibu comparison was used to determine the fuel consumption reduction of MHEVs because the conventional and hybrid models have the same size engine and are thus more comparable than the LaCrosse models, which do not have the same engine size. In the case of the LaCrosse, the hybrid’s smaller engine as well as greater aerodynamic improvements introduces new fuel saving technologies beyond the MHEV alone. GM claims that because of the boost that the 15 kW motor provides, the LaCrosse eAssist has acceleration similar to that of the conventional LaCrosse despite a smaller engine (Hall 2012; Hawkins et al. 2012). In 0-60 mph performance, however, the 8.6 sec performance of the Buick LaCrosse eAssist with the 2.4L I4 engine differs significantly from the 6.4 sec 0-60 mph performance of the Buick LaCrosse with the 3.6L V6 engine, so a comparison of fuel economy at equal performance cannot be obtained from these two vehicles (Gale 2012). For similar reasons, the Jetta and the Fusion models shown in Table 4.5 were not used directly to determine the fuel consumption reduction benefits of the P2 and PS architectures, respectively.
Using these estimates, the committee concludes that the effect of hybridization is a 10 percent reduction in fuel consumption for the mild hybrid. Note that the Agencies assumed an incremental fuel consumption reduction effectiveness of 6.5 percent for mild hybridization of midsize passenger vehicles. The addition of stop-start incremental effectiveness of 2.1 percent leads to a comparative total effectiveness for a mild hybrid vehicle of 8.6 percent. The TSD reports vehicle simulation resulting in a total fuel consumption effectiveness of 11.6 percent for mild hybrid midsize passenger vehicles relative to the baseline vehicle (EPA/NHTSA 2012a, 3-75, table 3-19; NHTSA 2012).
The Agencies base their high reduction in fuel consumption for the P2 on a Ricardo study (2012) and claim the effectiveness of the P2 hybrid used in this final rulemaking is 48.6 percent for a midsize passenger car (EPA/NHTSA 2012a, 3-124). This total effectiveness estimate includes the transmission effectiveness of 18.7 percent (for the decision tree pathway, including improved controls/externals, six-speed automatic transmission with improved internals, eight-speed dual clutch transmission, high-efficiency gearbox, and shift optimizer). Removing the transmission effectiveness, leaving out stop-start effectiveness of 2.1 percent and the effectiveness of ISG of 6.5 percent, the total incremental effectiveness of P2 alone is 33.6 percent, as can be seen in the NHTSA decision tree. This is larger than indicated by examples in Table 4.5. Part of their reasoning seems to be that by 2017, automakers will find ways to improve the P2 system. Although this may be possible and is used for the committee’s high estimate of fuel consumption reduction, it does not seem to be the case in 2014. The Sonata hybrid shows an increase in mpg of only 40.7 to 40.0 percent, resulting in the low estimated fuel consumption reduction of 28 percent for P2 hybrids. The Jetta, like the LaCrosse, has a downsized, turbocharged engine, so it combines two technologies and is not indicative of what can be achieved with hybridization alone. Further comparisons of P2 vehicles with their conventional analogs are available in Annex Table 4A.2. For the next rulemaking, EPA is developing a full system simulation tool, ALPHA, that will be able to simulate SI and hybrid vehicles and will be publicly available, while NHTSA will be relying upon full system simulation from Argonne National Laboratory using the Autonomie simulation model. In early studies, the ALPHA model had been used to estimate the effectiveness of P2 and PS hybrids and was successfully validated against current examples of these architectures to within 5 percent of the test fuel economy (Lee, S.D. et al. 2013; Lee, B. et al. 2013). ALPHA may improve estimation of fuel consumption reduction for strong hybrids.
In developing the effectiveness of the PS architecture, the TSD states that “In MYs 2012-2016 final rule, EPA and NHTSA used a combination of manufacturer-supplied information and a comparison of vehicles available with and without a hybrid system from EPA’s fuel economy test data to estimate that the effectiveness is 19 to 36 percent for the classes to which it is applied. The estimate would depend on whether engine downsizing is also assumed. In the CAFE incremental model, the range of effectiveness used was 23 to 33 percent as engine downsizing is not assumed (and accounted for elsewhere)” (EPA/NHTSA 2012a). As the table shows for the hybrid Ford Fusion and Toyota Camry, both lie at the upper end of the Agency estimates without any downsizing. The NRC estimate of PS effectiveness is based off the Agency estimate at 33 percent on the low end, and the Camry hybrid-conventional comparison of a fuel consumption reduction of 33.5 percent at the high end.
TABLE 4.5 Comparison of Effectiveness Estimated by the Agencies with EPA Certification Fuel Consumption and Fuel Economy Data of Actual Vehicles. Further Examples of P2 Hybrid Vehicles Are Available in Annex Table 4A.2.
|Agencies’ FC reduction projection for midsize vehicle||Example Models||Engine Size (L) and Type||Transmission||Certification City FE (mpg)||Certification Hwy FE (mpg)||Certification Combined FE (mpg)||Certification Combined FC (g/100 mi)||Fuel Consumption Change (%)||Fuel Economy Improvement (% mpg)|
|MHEV 6.6%||2013 Malibu regulara||2.5||S6||28.34||48.18||34.79||2.87||Baseline||Baseline|
|2013 Malibu eAssista||2.4||S6||31.98||51.88||38.65||2.59||−10.0||11.1|
|2014 LaCrosse regular||3.6 V6||S6||22.46||39.25||27.81||3.60||Baseline||Baseline|
|2014 LaCrosse eAssist||2.4 I4||S6||31.50||51.20||38.10||2.62||−27.0||37.0|
|P2 33.6%||2014 Sonata regular||2.4 I4||AM6||30.42||48.79||36.63||2.73||Baseline||Baseline|
|2014 Sonata hybrid||2.4 I4||AM6||48.00||56.60||51.52||1.94||−28.9||40.7|
|2014 Sonata hyb ltd||2.4 I4||AM6||47.70||56.40||51.26||1.95||−28.5||40.0|
|2014 Jetta regular||2.0||S6||28.10||41.50||32.88||3.04||Baseline||Baseline|
|2014 Jetta hybrid||1.4 T||AM 7||57.50||65.30||60.77||1.65||−45.9||84.8|
|PS 33%||2014 Camry||2.5 I4||S6||32.00||50.00||38.19||2.62||Baseline||Baseline|
|2014 Camry hyb LE||2.5 I4||Var||58.50||56.10||57.40||1.74||−33.5||50.3|
|2014 Camry hyb XLE/SE||2.5 I4||Var||55.07||54.56||54.84||1.82||−30.4||43.6|
|2014 Fusion||2.0 I4 T||S6||28.50||46.60||34.54||2.90||Baseline||Baseline|
|1.5 I4 T||S6||29.63||50.38||36.37||2.75||Alternative baseline||Alternative baseline|
|2.5 I4||S6||28.30||47.38||34.56||2.89||Alternative baseline||Alternative baseline|
|2014 Fusion hybrid||2.0 I4||Var||65.07||67.34||66.07||1.51||−47.7||91.3|
a The 2013 Malibu was used because the 2014 model added stop-start to the conventional vehicle.
Fuel Consumption Measurement
Hybridization of the ICE drivetrain with the addition of an electrical system improves fuel economy and reduces GHG emissions. For compliance purposes, this can be recorded in a straightforward way by measuring the fuel consumed in the test cycles, as for a typical ICE vehicle, and described in the J1711 SAE standard (Hybrid-EV Committee 2010).3 Calculating GHG emissions and fuel economy is more complex for vehicles that receive some of their energy from the electric grid (PHEVs and BEVs), that do not use a liquid fuel (BEVs and FCEVs), or that utilize a fuel that does not produce CO2 at the tailpipe (hydrogen FCEVs). Including the energy used and carbon emitted to generate electricity or hydrogen adds to the complication, as discussed in Chapter 10 (DOE 2014d). The recently completed NRC report Overcoming Barriers to Deployment of Plug-in Electric Vehicles (NRC 2015) discusses some of the complexities of estimating carbon emissions from electricity generation used to fuel BEVs and PHEVs.
The preceding sections describe the technologies used for vehicle electrification, their likely penetrations and estimates of their effectiveness when implemented to 2025, as well as technologies likely to be used to 2030. Additionally, the committee described its estimates of costs, especially noting where they differed from the Agencies’ cost estimates. Particular areas that the Agencies should reexamine in the midterm evaluation include the cost of technologies required for consumer acceptance of stop-start, the cost of motors for strong hybrids, in particular the P2 system, and the nonbattery component costs for PEVs. The committee’s range of most likely costs and effectiveness values are collected in Table 4.6 and used in Chapter 8.
In addition to the costs of individual technologies, the CAFE/GHG standards make assumptions about production volume in order to estimate costs. For the years 2020 and 2025 the Agencies assumed a North American volume of 450,000 and a corresponding degree of learning to estimate costs (EPA/NHTSA 2012a, 3-111). Although this figure may be relevant for some conventional powertrain technologies, in the opinion of the committee this is optimistic for electrified powertrains. Using Ford as an example, 450,000 units sold in 2017 would constitute approximately 15 percent of its 2013 sales. Even if all xEV sales are combined, it is highly unlikely that the xEV share of the market will be that high for Ford. Since unit costs are higher for low volumes, this assumption leads to an underestimate of the cost of hybridization.
The Agencies’ analysis is contradictory in assuming a high volume for the purpose of calculating costs and a low volume for technology penetration to 2025. Only 2 percent of the fleet is projected to be hybrids in the Agencies’ compliance demonstration path. Despite the low projected production volume and the high volume used when calculating the component costs, the battery costs for 2012 seem reasonable. This could be due to one of two things:
- (1) Factories are not utilized to capacity and suppliers are selling at low prices or
- (2) As noted in the TIAX study presented on August 13, 2013 to the NRC Committee on Overcoming Barriers to Electric Vehicle Deployment (Sriramulu and Barnett 2013), the economics of scale kick in at 60,000 units per supplier rather than at 450,000 units for the market as a whole, as assumed by the Agencies.
Table 4.6 collects the committee’s range of most likely fuel consumption reduction measurements and direct manufacturing costs for a midsize car with an I4 engine in 2014. The fuel consumption effectiveness values were generally equal to those estimated by NHTSA. For the P2 and PS, different lower bounds were estimated for the fuel consumption reduction based on comparisons between 2014 hybrids and their conventional counterparts.
The committee’s cost estimates include the Agencies’ costs, which the committee judged to be valid lower estimates of costs for electrification technologies in 2025, reflecting an optimistic scenario of technology development and implementation. The committee’s range of most likely costs also included higher values, reflecting committee expert judgment of the costs of technologies required to implement electrified powertrains. For MY2025, these higher costs were due to +$50 additional nonbattery technologies needed for integration of stop-start (+100 in MY 2017 and +75 in MY 2020), 1.5 × nonbattery technologies costs for BEV and PHEV powertrains, 1.3 × battery costs for the MHEV and P2 that reflect a more conservative SOC swing, and 1.4 × costs for properly sizing the P2 motor by torque rather than power. Justification for these cost increases is described earlier in the chapter.
Finding 4.1 In hybrids, electric current reverses direction many times during driving. To ensure long battery life, DOE specifications call for 300,000 “shallow” cycles, and mild hybrids such as the Buick eAssist use a state of charge swing of 20 percent. In projecting mild hybrid costs, the Agencies sized the battery based on an assumed 40 percent SOC swing, thus making the Agencies estimate of the battery of the mild hybrid half the size and half the cost of current implementations.
3 See 40 CFR § 1066.501I U 1066 F – Electric Vehicles and Hybrid Electric Vehicles at http://www.ecfr.gov/cgi-bin/retrieveECFR?gp=1&SID=99734ec227a2cf053c111fd96e3b22c2&ty=HTML&h=L&mc=true&n=pt40.33.1066&r=PART#sp40.33.1066.f.
|Electrification Technology||NRC Most Likely Fuel Consumption Reductiona (%)||NHTSA Estimated Fuel Consumption Reductiona (%)||NRC Most Likely 2025 MY DMC Costs (2010$)||NHTSA Estimated 2025 MY DMC Costs (2010$)|
|SS||2.1b||2.1b||225 - 275||225|
|MHEV||6.5b||6.5b||888 - 1018||888|
|P2||28.9 - 33.6||33.6||2,041 - 2,588||2,041|
|PS||33 - 33.5||33||2,671||2,671|
|PHEV40||N/A||65.1||8,325 - 9,672||8,325|
|EV75||N/A||87.2||8,451 - 8,963||8,451|
a Relative to baseline unless otherwise noted.
b Relative to previous technology.
Recommendation 4.1 For the midterm review, the Agencies should consult with battery manufacturers and automakers to determine the appropriate size of the battery for hybrids. Battery life is a key element in electrified powertrains, and premature failure should be avoided.
Finding 4.2 Battery cost is the dominant cost for PHEVs and BEVs. It is a function of energy and power requirements, battery chemistry, and required battery life. Battery life depends on the number of cycles required, the stability of the chemistry to cycling at the required state of charge swing, the thermal and stress evolution it undergoes, and its shelf life. Due to rapid development of battery technology, there are no real-world data to validate battery life beyond those from simulations and accelerated aging tests, so the appropriate state of charge swing to meet the conventional powertrain warranty of 8 years and 100,000 miles is unknown. GM is sizing the battery more conservatively than Nissan by using a smaller swing in SOC and, accordingly, a larger battery. The Agencies accepted the state of charge swings of the two automakers and assumed a higher cost per kilowatt hour for the power-optimized battery of the PHEV.
Recommendation 4.2 Proper sizing of the battery is essential for appropriate assessment of both cost and lifetime, parameters particularly critical for the extensive battery requirements of PHEVs and BEVs. For their midterm review, the Agencies should examine auto manufacturers’ experiences of battery life to determine the appropriate state of charge swing for PHEVs and BEVs so that they can assign costs appropriately.
Finding 4.3 The Agencies determine that the P2 architecture is likely to be the dominant strong hybrid technology, based on projected cost and effectiveness of P2 vs. PS hybrids. The cost estimate is partly based on the assumption that electric motors scale as power. In fact, the rotor volume and cost depend entirely on torque and hence the cost of electric motors scale with torque. The P2 motor is inline with the engine and transmission and has the same rotational speed as the engine. This constraint is not present with the PS hybrid, thereby allowing the use of a higher speed, smaller motor. Also, to minimize NVH, it appears that some automakers are not using the crankshaft-mounted electric motor for starting but are augmenting the conventional cranking motor and 12 V battery. The effectiveness of the PS hybrid models now available in the market as compared to the effectiveness of their conventional analogs with the same engine show that the PS architecture provides hybrids with significantly greater reduction in fuel consumption than similar P2 hybrids and their conventional analogs.
Recommendation 4.3 The cost of a P2 hybrid may possibly be higher than predicted by the Agencies and comparable to that of the PS hybrid for comparable performance. For the midterm review, the Agencies should undertake a teardown of the next generation PS and P2 architectures to update cost. Full system simulation of P2 and PS architectures should be undertaken to estimate effectiveness for the midterm review.
Finding 4.4 The committee is not in a position to precisely determine the cost increases for electrified powertrains. Based on inputs from automakers, battery suppliers, and independent consultants, it is the opinion of the committee that the battery cost estimates used by the Agencies are broadly accurate, while the cost of the nonbattery elements is too low, perhaps by a factor of as much as 2. To conservatively estimate these unaccounted-for costs, the committee used a high cost estimate of 1.5 times the Agencies’ estimates for the nonbattery components for BEVs and PHEVs.
Recommendation 4.4 At the time of the midterm review there will be several vehicles with electrified powertrains in the market. The Agencies should commission teardown studies of the most successful examples of (1) stop-start, (2) strong hybrids (PS, P2, and two motor architectures),
(3) PHEV20 and PHEV40, and (4) BEV100. At that time there will be better estimates of volumes for each type in the 2020 to 2025 time frame so that a better estimate of cost can be calculated.
Finding 4.5 Lithium-sulfur and lithium-air batteries will not be used in vehicles in sufficient numbers by 2030 to affect fuel consumption, and it may take more than 20 years before they are in the mass market. These technologies are still in the development stage and have many challenges related to poor efficiency, poor cycle life, and serious safety concerns due to the use of very reactive lithium metal.
Finding 4.6 Limited volumes of FCEVs were introduced in California in 2014 by Hyundai and are being introduced by Toyota in 2015. FCEVs will have minimal impact, if any, on 2025 CAFE compliance based on current automaker plans for market introduction but may become more important by 2030. A coordinated national plan for H2 infrastructure deployment will be required if successful, high-volume FCEV deployment is to be realized.
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|Manufacturer||Model||Sales CY 2014||U.S. Hybrid Share 2014||Architecture|
|Lexus||RX 400 /450 h||9,351||0.0207||PS|
|Subaru||XV Crosstrek Hybrid||7,926||0.0175||Other|
|Buick||Lacrosse Hybrid||7,353||0.0163||MH w IMA|
|Honda||Civic Hybrid||5,070||0.0112||MH w IMA|
|Honda||Insight||3,965||< 0.01||MH w IMA and CVT|
|Toyota||Highlander Hybrid||3,621||< 0.01||PS|
|Honda||CR-Z||3,562||< 0.01||MH w IMA and CVT or M6|
|Infiniti||Q50 Hybrid||3,456||< 0.01||PS|
|Nissan||Pathfinder Hybrid*||2,480||< 0.01||P2|
|Volkswagen||Jetta Hybrid||1,939||< 0.01||P2|
|Infiniti||QX 60 Hybrid*||1,678||< 0.01||P2 with CVT|
|Chevrolet||Malibu Hybrid||1,018||< 0.01||SS|
|Buick||Regal Hybrid||662||< 0.01||MH|
|Porsche||Cayenne Hybrid||650||< 0.01||P2|
|Chevrolet||Impala Hybrid||565||< 0.01||SS|
|Acura||ILX Hybrid||379||< 0.01||P2|
|Audi||Q5 Hybrid||283||< 0.01||P2|
|Lexus||GS 450h||183||< 0.01||PS|
|Infiniti||Q70 Hybrid||180||< 0.01|
|Acura||RLX Hybrid||133||< 0.01|
|BMW||Active (535ih)||112||< 0.01||Other|
|Chevrolet||Tahoe Hybrid||65||< 0.01||2-Mode|
|Lexus||LS 600h||65||< 0.01||PS|
|BMW||7-Series Hybrid||45||< 0.01||Other|
|Cadillac||Escalade Hybrid||41||< 0.01||2-Mode|
|GMC||Yukon Hybrid||31||< 0.01||2-Mode|
|Manufacturer||Model||Sales CY 2014||U.S. Hybrid Share 2014||Architecture|
|Volkswagen||Touareg Hybrid||30||< 0.01||P2|
|Chevrolet||Silverado Hybrid||24||< 0.01||2-Mode|
|Mercedes||S400HV Hybrid||10||< 0.01||P2|
|GMC||Sierra Hybrid||6||< 0.01||2-Mode|
|Overall Hybrid Share of LDV Market||2.75%|
*PHEV/BEV breakdown unknown.
|EVs and PHEVs|
|Manufacturer||Model||Sales CY 2014||U.S. PEV Share 2014||Architecture|
|Toyota||Prius Plug In||13,264||0.1118||P2|
|BMW||i3||6,092||0.0513||PHEV & BEV*|
|Porsche||Panamera S E-Hybrid||879||<0.01||PHEV|
|Honda||Accord Plug In||449||<0.01||PHEV|
|Porshe||Cayenne S E-Hybrid||112||<0.01||PHEV|
|Overall PEV Share of the LDV Market||0.72%|
SOURCE: Cobb (2015).
|2014 Models||Engine Size (L) and Type||Transmission||Certification City FE (mpg)||Certification Hwy FE (mpg)||Certification Combined FE (mpg)||Certification Combined FC (gal/100 mi)||Fuel Consumption Change (%)||Fuel Economy Improvement (% mpg)|
|Audi Q5 Regular||3.0L V6 T||S8||29.18||43.5||34.26||2.92||Baseline|
|Q5 Hybrid||2.0L I4 T||S8||30.40||39.9||34.05||2.94||0.6||-0.6|
|BMW 335i SS||3.0L V6 T||S8||27.45||45.8||33.5||2.99||Baseline|
|Active Hybrid 3||3.0L V6 T||S8||32.70||46.41||37.71||2.65||-11.2||12.6|
|BMW 750LI SS||3.0L V6 T||S8||24.07||40.14||29.36||3.41||Baseline|
|Active Hybrid 7L||3.0L V6 T||S8||28.21||43.13||33.41||2.99||-12.1||13.8|
|Infiniti Q60 FWD||3.5L V6||AV-S7||24.70||36.65||28.95||3.45||Baseline|
|QX60 Hybrid FWD||2.5L I4 SC||AV-S7||35.20||41.10||37.63||2.66||-23.1||30.0|
|Infiniti Q70 Regular||3.7L V6||A-S7||22.48||36.10||27.08||3.69||Baseline|
|Q70 Hybrid||3.5L V6||A-S7||38.25||47.90||42.06||2.38||-35.6||55.3|
|Nissan Pathfinder 2WD||3.5L V6||AV||24.64||36.98||28.99||3.45||Baseline|
|Pathfinder Hybrid 2WD||2.5L I4 SC||AV||34.87||40.82||37.31||2.68||-22.3||28.7|
|Porsche Cayenne Regular||4.8L V8||A8||19.50||31.60||23.56||4.24||Baseline|
|Cayenne Hybrid||3.0L V6 T SC||A8||25.10||33.10||28.16||3.55||-16.3||19.5|