Using electric vehicles for road transport

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Chapter 1

Using electric vehicles for road transport Malcolm D McCulloch and Justin D K Bishop and Reed T Doucette Abstract Road vehicles account for almost half of the energy used in all transport modes globally. Reducing energy use in vehicles is key to meeting the forecast increase in demand for transport, while improving energy security and mitigating climate change. Non-powertrain vehicle options may reduce fuel consumption by at least 15%. Electric motors are the significant powertrain option to reduce energy use in vehicles because they are more efficient than the internal combustion engine and can recover a portion of the vehicle kinetic energy during braking. Conventionally, batteries are used to meet both the power and energy demands of electric vehicles and their variants. However, batteries are well-suited to storing energy, while ultra-capacitors and high-speed flywheels are better placed to meet the bidirectional, high power requirements of real-world driving. Combining technologies with complementary strengths can yield a lower cost and more efficient energy storage system. Whereas pure and hybrid electric vehicles use less energy than internal combustion engine vehicles, their ability to mitigate climate change is a function of the emissions intensity of the processes used to generate their electricity. Malcolm D McCulloch Energy and Power Group, University of Oxford Department of Engineering Science, Parks Road, Oxford OX1 3PJ, United Kingdom e-mail: [email protected] Justin D K Bishop Institute for Carbon and Energy Reduction in Transport, Oxford Martin School c/o University of Oxford Department of Engineering Science, Parks Road, Oxford OX1 3PJ, United Kingdom e- mail: [email protected] Reed T Doucette Energy and Power Group, University of Oxford Department of Engineering Science, Parks Road, Oxford OX1 3PJ, United Kingdom e-mail: [email protected]

1.1 Introduction This chapter considers the design and impact of the advanced powertrains of electric vehicles (EV) and hybrid electric vehicles (HEV) along with their plug-in hybrid electric vehicle (PHEV) variants. All these powertrains have the potential to reduce transport's dependence on oil and the environmental impact. In 2008, fossil-fuels accounted for 97% of transport’s energy use [1]; lightduty passenger vehicles used 39 EJ [2], equivalent to 40% of the total energy consumption of transport [3]. Organization for Economic Co-operation and Development (OECD) countries have the highest per capita transport demand and are locked into using foreign fossil-fuel resources to supplement their domestic production [1]. Concerns arise, not only because of the security and stability of the energy supply, but also due to the increasing carbon dioxide-equivalent greenhouse gas (GHG) emissions from vehicle use. The emissions associated with the energy used by light-duty vehicles was 2.7 Gt GHG in 20081 [4]. The population of the developing world also has aspirations to use motorized transport. Consequently, the total energy used in road transport is forecast to increase to 73 EJ by mid century [2]. Associated GHG emissions are expected to nearly double to 5.1 Gt over the same period [4]. New vehicle technologies must be investigated, developed and commercialized to meet the growing demand for affordable transport within these resource and emissions constraints. This can be accomplished in three ways. Firstly, by reducing the energy used in vehicles by improving their efficiency. Secondly, by expanding the types of energy vectors so that the vehicles can make use of energy derived from an array of sources. Thirdly, by reducing both the number of vehicles on the roads and the distances that they travel. EVs, HEVs and PHEVs deliver on the first two ways through more efficient powertrains and by using electricity as an energy vector.

1.2 Energy and power Satisfying real-world driving demands requires periods of high power during acceleration and deceleration and high energy, when travelling at high velocity and over long distances. The New European Drive Cycle (NEDC), shown in Figure 1.1, is the standardized velocity-time profile used to determine vehicle fuel consumption and emissions in Europe. The combined NEDC consists of

1The 2008 value was determined by a linear interpolation on the 2000-2050 dataset of 2

2

energy and emissions, yielding correlation coefficients of R =0.997 and R =0.997, respectively.

four low speed, urban subcycles and a higher speed, exurban or motorway subcycle. The standard drive cycles used in the United States and Japan are the FTP and 10-15 cycles, respectively. Generally, such standard drive cycles underestimate energy use and associated emissions because they can fail to reflect factors which are important parts of realworld driving, including the presence of auxiliary loads (Figure 1.3) and changes in road gradient. For example, the NEDC can underestimate real-world cold and hot emissions by 30% and 50%, respectively [58,62].

Fig. 1.1 The combined urban and extra-urban New European Drive Cycle (NEDC) used to determine light duty passenger vehicle fuel use and CO2 emissions in Europe.

Figure 1.2 presents the power demands for a mid-size EV. It shows that such a mid-size car requires approximately 5 MJ to complete the 11 km combined NEDC, leading to an energy use of about 45 MJ/100 km. Accelerating to the NEDC’s top speed of 120 km/h from 100 km/h in 20 seconds requires a peak power of 40 kW. Therefore, such a medium-sized vehicle requires an electric motor of at least a 40 kW to complete the NEDC. Note that the NEDC is not a very aggressive drive cycle and that more realistic cycles would likely have higher power demands. To achieve a 300 km range, 135 MJ (38 kWh) of energy storage capacity is required. This energy is present in fewer than four litres of petrol or diesel when used in a conventional vehicle. The battery pack required to yield an all-electric range similar to that achievable with an equivalent conventional vehicle is both large and expensive (Table 1.2).

Fig. 1.2 Power required by a mid-size EV to complete the NEDC.

Over the entire cycle, the average (root mean squared) power is only 9 kW. Moreover, the power required of the internal combustion engine (ICE) only exceeds the average power for 18% of the time and for generally fewer than ten seconds in each instance. The ICE in a conventional vehicle spends much of its time in the NEDC operating under inefficient, part-load conditions. Moreover, the battery pack in a pure EV must be sized to supply the full energy and power demands, though it spends relatively little time supplying power over the average value. Hence, it is important to note that the nature of the energy and the power demands of a vehicle can lead to interesting technological solutions, such as hybridizing different types of energy storage and conversion technologies. In this way, components that are better suited to handling high power can be matched with those optimized to store energy.

1.3 Understanding energy losses To understand how the efficiency of vehicles can be improved, it is useful to describe the factors that affect vehicle losses. These factors can be disaggregated usefully into (1) the forces impeding motion and (2) the losses incurred by vehicles’ powertrain components. Losses associated with these mechanisms are illustrated using the best selling medium vehicle in the UK in 2010 [5]. However, the manner in which a vehicle is driven will directly affect most of the loss mechanisms. Driver behaviour can be represented by drive cycles which express the speed of the vehicle as a function of time. Many types of drive cycles exist. The losses which arise in a passenger vehicle following the United States Environmental Protection Agency (EPA) combined city/highway

drive cycle are illustrated in Figure 1.3. The remainder of this section will examine the potential for future vehicles to reduce the losses they experience in both of these areas.

1.4 Losses due to travelling When a vehicle is travelling on the road, it needs to overcome the force associated with displacing the air along its path - termed aerodynamic drag loss. As the tyres roll, they change shape - flatter when they are in contact with the road - which requires energy. This is termed the rolling resistance loss. When the vehicle is brought to a halt, the kinetic energy is lost as heat in the brakes. This is termed the kinetic energy loss.

1.4.1 Aerodynamic drag loss Aerodynamic drag comprises normal (pressure) and tangential (friction) forces acting on a vehicle shape as it moves through the air. The size of the drag force is dependent on the density of the air, the vehicle size, shape and square of the speed.

Fig. 1.3 Sankey diagram highlighting the magnitude of energy lost in the powertrain and non-powertrain components of a passenger vehicle on the US Environment Protection Agency (EPA) combined city/highway drive cycle. Adapted from [6].

Slower travelling vehicles experience smaller aerodynamic losses but, at least in the short term, it is unlikely that lower speed limits will be introduced. Typically at speeds above 55 km/h, aerodynamic drag becomes the dominant force opposing vehicle motion. Vehicle design can focus on reducing aerodynamic losses by reducing a vehicle's drag coefficient and frontal area which are primarily determined by a vehicle’s shape. For instance, the presence of a car boot leads to flow separation at the rear edge of the roof, spreading downwards. The horizontal distance from the rear edge of the roof to the rear edge of the boot, the height of the boot [7] and the resulting angle impacts the flow separation. Thus, fastbacks (coupés) generally outperform notch-backs (saloon cars), which in turn are better than square-backs (estate cars) [8–10]. Other sources of pressure drag arise due to wheel housings, external vehicle features and engine ventilation [11]. The extent to which vehicle aerodynamic drag may be reduced is constrained by functional, economic and aesthetic demands [8]. The desire for ve-

hicles in different size classes constrains their external dimensions and associated drag. Moreover, the aesthetics of low wind noise at high velocity due to the interaction of the airflow with external vehicle features, such as side mirrors and radio antennae, constrains the design of the vehicle shape. The effect of reducing the drag coefficient was simulated on the best-selling mediumsized vehicle in the UK using the Oxford Vehicle Model (OVEM). Reducing its drag coefficient 10% reduces its total energy use per kilometre by 4% on the NEDC and US EPA drive cycles. These findings are consistent with the estimates in the literature [12–15]. Aerodynamic drag may be reduced by: using vortex generators [9]; covering the vehicle underbody [14]; using body coatings to reduce friction drag; and reducing the frontal area.

1.4.2 Rolling resistance losses Rolling resistance acts in parallel to vehicle motion and along the road surface. It is dependent on the weight of the vehicle and the coefficient of rolling resistance of the tyre. At speeds below 55 km/h, rolling resistance is the dominant drag force and typically accounts for about a third of the energy at the wheels [16]. Reducing rolling resistance of the best-selling medium-sized vehicle in the UK reduces its energy use per kilometre by 4% on the NEDC and US EPA drive cycles. These findings are consistent with the estimates in the literature [12; 13; 16]. The tyre rolling resistance is a function of a number of factors, including: vehicle loading; inflation pressure; wheel diameter; speed rating and the road conditions for which the tyre is designed. The rolling resistance coefficients of modern tyres have decreased over the last 30 years by approximately 10% to a median value of 0.0099 [16]. There is certainly room for tyres to achieve lower rolling resistance coefficients in the future. However, that will be challenged by consumer desire for tyres which perform safely in all weather conditions and at high speeds.

1.4.3 Kinetic energy losses Every time a vehicle accelerates, kinetic energy is transferred to the vehicle. This energy is largely lost as heat in the brakes as the vehicle slows down. The kinetic energy is dependent on the mass of the vehicle and the square of the speed of the vehicle. Mass is the largest contributor to the intermittent power requirements of a vehicle and is the most important factor in determining the power rating of powertrain components and the amount of energy a vehicle must store on-board [17].

Lightweighting strategies often focus on material substitution and vehicle component re-design [18]. Every 10% of primary mass saved can reduce fuel consumption by up to 7% [19]. Since “mass begets mass,” saving primary mass often allows other components to be downsized to yield secondary mass savings [20], introducing a mass decompounding effect. For example, by reducing the mass of the vehicle body, the suspension, brakes and tyres may be downsized. Two thirds of the total vehicle curb mass is found to be gross vehicle mass-dependent, indicating the extent of the decompounding potential [18]. The vehicle subsystems which are affected by lightweighting are the suspension, engine, tyres and wheels, transmission, structure, steering and brakes, electrical and the exterior. Moreover, a lighter vehicle requires less power to achieve a given acceleration. Consequently, the vehicle can achieve similar performance with a smaller engine, which requires less massive structural supports. In general, mass decompounding yields 1.04 kg of secondary mass savings for each 1 kg of primary mass avoided [18], reducing the a vehicle's energy needs even further. Not all components of a vehicle are typically affected by such lightweighting strategies: the interior, information and controls, fuel, exhaust, closures and heating, ventilation and cooling are largely gross vehicle weight-independent [18].

1.5 Losses in Power Conversion Components Powertrain losses occur as a result of a powertrain’s components working to overcome the resistive forces impeding the motion of a vehicle. Powertrains consist of components that store energy and convert that energy from to kinetic energy. In the case of vehicular applications in EVs and HEVs, some of these components may be rather novel. These components can be arranged and the energy flows between them can be managed in ways that can produce significant improvements in vehicle efficiency. This section will present the currently available energy storage and conversion technologies that will likely shape future vehicle development.

1.5.1 Internal combustion engine The ICE is the conventional device used to convert chemical energy in transport fuels to shaft work. ICEs have good specific power (W/kg) characteristics and use energy dense fossil-fuels (MJ/l). Therefore, conventional vehicles have normally used ICEs as their only primary energy conversion device to meet their driving demands. The efficiency of the ICE is a function of its power output which is the product of its torque and speed. Losses arise due to thermodynamic limits, inefficient combustion of the fuel, friction, heat transfer to the cylinder walls during

the piston linear motion and in the exhaust gas. There is a combination of torque and engine speed, corresponding to the peak output power, which yields maximum efficiency, as shown in Figure 1.4. Under these full-load conditions, the peak efficiency is typically less than 45%. At normal driving speeds, whether in an urban setting or on the motorway, the ICE operates at part-load, where the losses are more significant [21]. A transmission is an essential auxiliary component to the ICE and is used to match the engine speed to that of the wheels. Typically, every gear of the transmission has a fixed ratio. For a given output power desired, the gear should be chosen to minimize energy losses in the engine. There are a number of measures available to improve the efficiency of ICEs under real-world driving conditions. Downsizing the ICE brings its peak power closer to that required when driving normally. Therefore, it can operate closer to its peak efficiency. In urban settings, an engine start/stop function avoids the inefficiency associated with idling. Increasing the number of gears in a transmission gearbox (ultimately to a continuous variable transmission) allows the engine to operate closer to its most efficient point, for a given power output. Turbocharging allows some of the work potential in the exhaust gas to be recovered and improves the performance of vehicles with downsized engines. In some HEVs, ICEs can either be completely decoupled from the road load, or only operated at certain points in their efficiency map. This allows them to produce power much more efficiently than in a conventional vehicle where they must respond to meet all the transient power demands of normal driving.

Fig. 1.4 Efficiency map of a diesel internal combustion engine, with efficiency shown as iso-contours. Adapted from [22].

1.5.2 Fuel Cells Fuel cells (FC) are another energy conversion technology with the potential to find an application in HEVs. Fuel cells work by converting energy stored in chemical bonds into electrical energy that can go onto be used by a vehicle’s energy storage media or electric motors. Many types of fuel cells exist, each with their own chemical processes and traits that come with distinct advantages and disadvantages. The fuel cell technology that has historically been used most frequently in light-duty fuel cell vehicles is the proton exchange membrane (PEM) fuel cell. PEM fuel cells use hydrogen to generate electricity and they are commonly used in passenger vehicles because of their relatively low operating temperatures, low mass and high durability in dynamic environments. The adoption of fuel cells has been hindered to a large degree

on account of unresolved issues related to their cost, resource constraints, refueling infrastructure and the storage of hydrogen. Please refer to chapter on batteries for more detail.

1.5.3 Electric motors Electric motors are a mature technology, with extensive use in industrial, domestic and power generation applications. However, they have not been widely deployed in mainstream, light-duty road passenger vehicles to provide tractive force. Electrification of a vehicle powertrain involves decoupling the shaft work output of the ICE from the rotational work required at the wheel. The three main reasons for considering the use of electric motors are that: they have a higher peak efficiency of converting potential energy to shaft work than ICEs when motoring; their torque profiles (100% torque from zero speed) are better suited to meet the demands of realistic driving without the need of a multi-speed transmission; and they can be used as generators to convert a portion of a vehicle’s kinetic energy to electrical current during braking [23–25]. In contrast with the ICE, the rotor is the only moving part in an electric motor. Losses are dominated by copper and iron losses. The copper losses occur because of the electrical resistance of the windings and is proportional to the square of the torque. Iron losses arise due to the interaction of the magnetic fields with the iron components, and is proportional to the speed. [26; 27]. An electric motor may not require a transmission to match its speed to that of the wheels. Moreover, the electric motor has a broad region of high conversion efficiency, when compared to an ICE of equal power rating (Figure 1.5). Finally, electric motors generally have a high power density (kW/l). This permits flexible packaging within the vehicle, such as in or near to the vehicle’s wheels.

Fig. 1.5 Efficiency map of a permanent magnet electric motor, with efficiency shown as iso contours [28].

The most common electric machines used for traction are induction machines (IM) and permanent magnet machines (PMM), see Table 1.1. IM are reliable and low cost, but have a limited constant-power range and suffer from high weights and low efficiency, particularly at high speeds [29]. PMM are more compact in size, lighter in weight and are efficient. However, the magnets are expensive, leading to a high cost per unit power [23; 30]. Despite these drawbacks, PMM have been identified as the most promising motor technology for modern EVs [30]. New topologies of PMM have been specifically developed to meet the needs of the EV market in a very high efficiency and an extremely lightweight solution [28]. Power electronic converters use high-power, efficient, reliable and fast-acting semiconductor switches to convert the output from the electrical system to a controlled input for use in the machine. A key advantage of the machine/converter sub-system is that bi-directional energy flow is possible. This implies that when the vehicle is slowed down, the kinetic energy of the vehicle can be recovered to an on board energy store, thus mitigating the kinetic energy loss. However a bi-directional energy store is needed. Table 1.1 Comparison of electric motor technologies for energy conversion in EVs [11; 28; 31– 34].

Electric motor technology

Specific power (W/kg)

Specific torque (Nm/kg)

Peak efficiency (%)

Permanent Magnet

660-1 120

22

92.5-97

Induction

563-2 128

1.43

90-90.5

1.6 Energy storage technologies Energy has traditionally been stored on board vehicles in the form of chemical energy of petrol. However the conversion process is not (readily) reversible. Therefore, an alternative reversible energy store needs to be on board the vehicle to exploit the recovery of the kinetic energy. This section will address the various technologies capable of storing energy on board a vehicle: batteries, ultra-capacitors and high-speed flywheels. The ideal characteristics of device energy storage in EVs and HEVs include high specific energy and power, long calendar life and cycle life and low cost. However, no single energy storage device currently satisfies all of these requirements to such an extent that efficient EVs and HEVs have been widely

adopted [23; 35]. Thus, energy storage remains the weak link in the electrified powertrain [35], and the cost and performance of energy storage devices seems likely to remain the key determinant in the future development and successful marketing of EVs and their variants [36].

1.6.1 Batteries Batteries convert chemical energy directly to electrical energy. The key technologies of interest here are those types that can do so reversibly. Not only can they then accept the recovered kinetic energy, but also provide the opportunity of using the energy from the electrical grid, with a route to a diversified energy mix. Battery storage has traditionally comprised rechargeable lead-acid (Pb-A), nickel- cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) cell technologies, with lithium-ion (Li-ion) and lithium-polymer now being the dominant technology. Their characteristics are found in Table 1.2. There is energy embodied in batteries which is present in their constituent materials and added during the manufacturing process [37]. This energy is small in comparison to the total energy used during the vehicle operation over its lifetime. Table 1.2 Comparison of battery technologies for energy storage in EVs [23; 35; 36; 38– 48].

Battery technology

Specific power (W/kg)

Specific energy (Wh/kg)

Roundtrip efficiency (%)

Lifetime (cycles)

Cost (€/kWh)

Lead-acid (flooded)

157-300

20-50

72-82.5

100-2 000

38-150

Nickelcadmium

150-300

20-80

72-78

5-1,500

110-600

Nickel metal hydride

154-1 500

46-120

70

300 – 3 000

766

Lithiumion

300 3000

35-190

90-95

300 – 3 000

422-1 000

The Pb-A battery is a mature technology with low costs that has been a commonly-used energy storage medium in previous EVs [23]. Despite their low specific energy and associated mass penalty, Pb-A batteries are appropriate for use in micro and mild hybrids [35] and may be paired with DC motors in

low power applications [49]. However, the low specific energy of the Pb-A renders them unsuitable as an energy storage technology for pure EVs and HEVs where the electric powertrain satisfies more of the driving load. Ni-MH and Ni-Cd batteries are usually thought of as occupying the next rung in the progression of batteries from Pb-A. The Ni-MH battery is both less expensive than the Ni-Cd technology and avoids the environmental issues associated with cadmium disposal [23; 35]. Moreover, Ni-MH batteries have high specific energy, are tolerant of abusive overcharge and overdischarge and display excellent thermal properties [35]. Lithium-based batteries are currently regarded as the state of the art in battery storage technology. There are many different forms of lithium-based batteries, each with a distinct chemical composition offering its own advantages and disadvantages. The high electrochemical reduction potential and low atomic mass of lithium allow batteries based on it to achieve a high specific energy. Li-on batteries comprise a number of cells. Cells are wired in series strings to satisfy the voltage requirements at the battery terminals. Parallel wiring of series strings allows the total battery energy capacity to be satisfied [23]. The higher the voltage of the battery pack, the lower the transmission current (and losses) along the powertrain to the motor. Operating voltage of existing EVs and HEVs are on the order 200 V [63,64]. This will increase to over 500 V, as with the new Toyota Hybrid Synergy Drive [65]. Li-ion batteries require a management system to ensure efficient pack operation and provide undervoltage, overvoltage, short-circuit and thermal protection [23,35]. Therefore, a li-ion battery is expensive on account of its management system and the elements used in its electrodes and electrolyte [23; 50]. These batteries also do not suffer the memory effects associated with Ni-MH chemistry [35]. The ability for Li-ion batteries to achieve lower costs in the future will be a function of the cost of the natural resources on which they are based, the potential to substitute one expensive natural resource for a less expensive one and the ability to achieve mass production. Supplying high power requires the battery to discharge at high current, which reduces the battery capacity faster than a slow current discharge. Moreover, the battery life is shortened with the number of fast discharges. Batteries are suited for storing energy but are an expensive, heavy and inefficient medium to satisfy the frequent, large bi-directional current flows associated with real-world driving. Most commercial EVs and their variants use batteries to satisfy both the energy and power demands. Consequently, the battery capacity is sized to meet both the maximum power demanded to satisfy real-world driving requirements and the maximum energy required to enable the vehicle to achieve a suitable range [36].

1.6.2 Ultra-capacitors Ultra-capacitors store charge on two plates separated by an insulating dielectric. See Table 1.3 for their key characteristics. The main benefit of using an ultra-capacitor is the large specific pulse power2, due to a short discharge time and low equivalent series resistance [47]. Ultra-capacitors have long calendar lives and up to 500 000 cycles [36]. Yet, ultra-capacitors have relatively low specific energies and are an expensive way to store large amounts of energy. Therefore, rather than a substitute or alternative, ultra-capacitors, as they currently exist, are commonly thought of as a natural partner to be hybridized with another form of energy storage or conversion. For instance, a battery and ultra-capacitor hybrid has the potential to yield an energy storage system which meets a vehicle’s energy and power needs at lower cost than batteries alone [45]. Some of the net cost savings from using ultra-capacitors may be reduced on account of the additional power electronics required to ensure correct operation of the more complex, hybrid energy storage system.

1.6.3 High-Speed Flywheels High-speed flywheels store energy mechanically. See Table 1.3 for there key characteristics. It is a technology that, through recent advances in highstrength, low density materials, has emerged as a low cost, robust and high specific power energy storage option [51]. In stationary applications, the energy capacity of a flywheel is typically increased by adding to its inertia. However, in mobile applications, where increased weight and inertia are detrimental, the energy capacity of flywheels can be increased by increasing their speed – hence, high-speed flywheels. High-speed flywheels can be integrated into EV and HEV powertrains in one of two ways: electrically or mechanically. In an electrical integration, the rotational potential energy is stored in the flywheel and converted to shaft work to drive an electrical generator. The electrical energy output is directed onto the vehicle’s electric bus where it can be used to power the motor or recharge another energy storage device, such as a battery. This type of flywheel integration is generally less efficient; however it allows for greater packaging flexibility. Alternatively, high-speed flywheels may be mechanically linked directly to the drivetrain, usually via a continuously variable transmission. In this configuration, flywheels avoid the losses associated with the additional, roundtrip

2 Not all ultra-capacitors outperform all batteries. Some high-power battery models have specific power performance that is comparable with ultra-capacitors [46].

conversion to and from electrical energy [52]. However, high-speed flywheels can have relatively high self-discharge characteristics and can be inefficient at storing charge for extended periods, unlike batteries3 [24] and ultra-capacitors. As with ultra-capacitors, high-speed flywheels are well-suited to handling high power loads, and they may be be hybridized with additional components which are better suited to meeting high energy demands. Table 1.3 Comparison of high specific power technologies to complement energy storage using secondary batteries in EVs [23; 34; 36; 45–47; 53–55]. Use £1.00 = 1.15e; £1.00 = US$1.50.

Technology

Ultra-capacitors Composite flywheels

Specific power (W/kg)

Specific energy (Wh/kg)

Roundtrip efficiency (%)

Cost (€/kWh)

700 – 5 000

2.4-5.6

95

713 – 4 900

2 000

200-350

98

253

1.7 Powertrain topologies The energy storage and conversion components that have been discussed can be arranged and managed in a number of ways once they are actually placed in a vehicle. Generally, these components are assembled in a manner based on one of the four powertrain topologies discussed below: pure EV, series HEV, parallel HEV and series-parallel HEV. Each of these topologies has its own strengths and weaknesses and, as the case studies in this chapter illustrate, there is usually no single answer as to which one is optimal.

1.7.1 Electric Vehicle Powertrain A pure EV uses an electric motor for traction and that motor is powered by an energy storage system (ESS), which is usually a battery. Figure 1.6 shows a schematic of an example of a common powertrain topology for an EV. In 2011, the energy storage system onboard EVs most commonly takes the form of a battery array – though the battery array has the potential to be hybridized with other technologies, such as ultra-capacitors or high-speed flywheels. EVs

3

Not all ultra-capacitors outperform all batteries. Some high-power battery models

have specific power performance that is comparable with ultra-capacitors [46].

have a tank-to-wheel efficiency that is typically higher than conventional vehicles, which rely solely on ICEs. Moreover, EVs produce no tailpipe emissions and use electrical energy which can be derived from any means - not just liquid fossil-fuels. One of the weaknesses of EVs is that in order to achieve a range comparable to a conventional vehicle, they need an energy storage system with a high energy storage capacity. Given the state of energy storage technology in 2011, such an energy storage system is typically heavy and expensive, especially if it is made large enough to achieve a range approaching that of a conventional vehicle. The Tesla Roadster from Tesla Motors is an example of a pure EV. It uses a Li-ion battery pack of mass 450 kg, which accounts for 36% of the curb mass of the vehicle, to achieve a range of 354 km on the US EPA combined city/highway drive cycle.

Fig. 1.6 Schematic of an example of an electric vehicle powertrain [56].

1.7.2 Series Hybrid Electric Vehicle Powertrain A series HEV is similar to an EV in that an electric motor provides all of its tractive force, and that motor is powered by an energy storage system. However, a series HEV also has an onboard power generator that can be used to recharge the energy storage system or supplement its power while the vehicle is driving. The onboard power generator, also known as an alternative power unit (APU), typically takes the form of an ICE connected to a generator or a fuel cell. Figure 1.7 shows a schematic of one example of a powertrain for a series HEV. The alternative power unit in a series HEV can often be downsized and operated at its most efficient point since it only has to recharge the energy storage system and it does not have to provide traction. The energy storage system can also be designed to meet the transient driving loads while the alternative power unit supplies the average power. This means the energy storage system can often be downsized since it shares the energy storage and power production load with the alternative power unit. The presence of the electric traction motor and energy storage system can improve the efficiency of

the vehicle by enabling regenerative braking. The energy storage system of a series HEV can also be charged from the electric grid. Such a vehicle is known as a plug-in hybrid electric vehicle (PHEV). The Vauxhall Ampera (or Chevrolet Volt, as sold in the United States) is an example of a series PHEV that uses an ICE as its alternative power unit. The Volt has a reported all-electric range of 64 km before it has to resort to using its alternative power unit, a four-cylinder ICE. According to EPA testing on the combined city/highway drive cycle, the Volt will achieve an equivalent of 2.4 l/100 km in electric-only mode and 6.4 l/100 km once its alternative power unit is engaged.

Fig. 1.7 Schematic of an example of a series hybrid electric vehicle powertrain [56].

1.7.3 Parallel Hybrid Electric Vehicle Powertrain In a parallel HEV, the tractive force is split between an ICE and an electric motor. Figure 1.8 shows a schematic of an example of a parallel HEV powertrain. This topology represents only one possible parallel configuration, but it illustrates the primary points of parallel HEV operation. In this configuration the electric motor typically propels the vehicle at low speeds since it has a higher efficiency than an ICE during the stop-and-go pattern characteristic of urban driving. The ICE is typically used to power the vehicle during high speed driving where there is less speed fluctuation and the engine has to deviate less frequently from operating at its most efficient point. Parallel hybrids, such as the Ford Escape Hybrid, are capable of an extended range by utilizing an ICE. Also, the ICE, motor and energy storage system can be downsized since neither one of them is solely responsible for the total power output of the vehicle. Parallel HEVs can also be PHEVs, but this is less common since their energy storage system generally have lower energy capacities. This is because since the energy storage system is only used to power low-speed driving, generally in start and stop conditions.

Fig. 1.8 Schematic of an example of a parallel hybrid electric vehicle powertrain [56].

1.7.4 Series-Parallel Hybrid Powertrain A series-parallel, also known as a dual or power split, HEV combines elements from both the series and parallel HEV configurations. Like a parallel HEV, the road load in a series-parallel HEV can be split between the motor and the ICE. Like a series HEV, the ICE can also function as an alternative power unit to generate additional power to recharge the energy storage system while the vehicle is driving. A series-parallel HEV can also be a PHEV. The Toyota Prius implements this powertrain topology. Figure 1.9 shows a schematic of a common layout for the powertrain of a series-parallel hybrid.

Fig. 1.9 Schematic of an example of a series-parallel hybrid electric vehicle powertrain [56].

1.8 Case Studies How much energy will vehicles require? How will they store and convert that energy? Where will that energy come from? The previous section provided a look at the technologies and topologies that will likely play a prominent role in answering these questions and shaping the future of automobile transport. This section looks at two studies that provide practical examples for how the

answers to these questions can intersect, sometimes to produce surprising results.

1.8.1 Case study 1: Comparison of batteries, ultra-capacitors, and high-speed flywheels functioning as the energy storage medium in a fuel cell-based series HEV Batteries are currently the most common form of energy storage onboard EVs and HEVs. However, their ubiquity belies the fact that they still have characteristics that make them less than ideal as a vehicle energy storage medium. For instance, their low power and energy density, low cycle life, and high cost could all be improved in order to increase the adoption of EVs and HEVs. This study compares high-speed flywheels and ultra-capacitors to the more conventional form of energy storage – batteries – on the bases of cost and fuel economy in a series HEV [57]. The results show that batteries may have some legitimate competitors, at least in certain applications. 1.8.1.1 Methods This study was conducted by using the OVEM computer simulation tool to model high-speed flywheels, ultra-capacitors, and Li-ion batteries each as the sole energy storage medium onboard a series HEV. The series HEV was simulated as using a fuel cell as its main power unit, so the fuel economy for the vehicle was expressed in km/kg H2. It should be noted that the fuel economy figures in this study would be qualitatively similar for an ICE. The batteries and ultra-capacitor arrays were integrated into the powertrain as shown in Figure 1.6 by connecting them to the electric bus. The high-speed flywheels were mechanically integrated into the powertrain via a continuously variable transmission, as seen in Figure 1.10. These three different technologies were simulated in a range of sizes in the standard series HEV powertrain. As the size of the energy storage media increased, so did their cost, mass, and energy storage capacity. The energy storage media were sized such that they would be able to meet the transient power demands of the drive cycle. The fuel cell was sized to meet the average power (load).

Fig. 1.10 Schematic of a mechanically integrated high-speed flywheel in a series HEV [56].

The simulation was run over two different drive cycles: the NEDC (Figure 1.1) and the Assessment and Reliability of Transport Emission Models and Inventory Systems (ARTEMIS) Combined Drive Cycle [58] (ACDC, Figure 1.11). The NEDC was chosen because it is used in Europe as the basis for the fuel economy ratings of commercially available vehicles. The ACDC was used because it is a more aggressive drive cycle better representing actual urban and highway driving. 1.8.1.2 Results The results from this study are presented in Figure 1.12 and Figure 1.13 below. Over both of the drive cycles, the batteries were shown to achieve the highest fuel economy. However, over both drive cycles, the ultra-capacitor and high-speed fly-wheel systems that achieved the highest fuel economy were within several percentage points of the best-performing battery array. Additionally, powertrains using the high-speed flywheel and ultra-capacitors have the potential to be cheaper than similarly fuel efficient powertrains using batteries, especially on the more realistic ACDC. Such a result may seem counterintuitive on account of the lower specific energy of both the high-speed flywheels and ultra-capacitors relative to the batteries, as one may envision that the alternative power unit would have to operate under a greater load to maintain the flywheel and ultra-capacitors at a useful state of charge. However, the results illustrate that the energy stored in both the flywheel and ultra-capacitor arrays is generally enough to power the vehicle through the transient spikes in road load enabling, the alternative power unit to operate at a high efficiency.

Fig. 1.11 The Artemis combined Driving Cycle (ACDC) [58].

The flywheels, ultra-capacitors and batteries used in this case study are commercially-available and the comparison represents the state of the technology at present. The flywheel system costs range from the current value to the future estimates by the manufacturer based on mainstream use. The cost range used for both ultra-capacitors and batteries are based on a survey of the literature and represent mainstream production for non-automotive applications primarily [57]. It is also interesting to note that there are regions of optimal sizing for the energy storage media. As the size of the energy storage increased there was a point where they reached their maximum fuel economy. As they continued to expand, their fuel economy decreased due to the effects of the increased mass of the vehicle. Therefore, the optimal size for each ESS will vary depending on the drive cycle, the ESS technology and the characteristics of the rest of the vehicle.

Fig. 1.12 Fuel economy as a function of ESS cost on the NEDC [57].

This study did not consider life cycle costs, but both high-speed flywheels and ultra-capacitors have long calendar lives that could result in significant cost savings over their operating lives. For these reasons, it appears that highspeed flywheels and ultra-capacitors may be more legitimate competitors to batteries than is widely perceived.

Fig. 1.13 Fuel economy as a function of ESS cost on the ACDC [57].

1.8.2 Case Study 2: Comparison of tank to wheel CO2 emissions from a conventional vehicle, an EV, and PHEVs

One of the key attributes of pure EVs is that they produce no emissions at the point of use. However, this does not mean that EVs cannot still be responsible for emissions. Complete well-to-tank accounting includes emissions resulting from the extraction, processing, and transportation of the raw materials and their conversion to the final fuel. However, in this case study, the scope of well-to-tank emissions is limited those emitted during the electric power generation process alone. If they obtain their electrical energy from a process that involves fossil-fuel combustion, then their emissions

are simply shifted from the vehicle to the power station. This fact is often overlooked, and it is also widely assumed that EVs produce significantly fewer tank-to-wheel CO2 emissions than any other vehicle powertrain topology – fewer than PHEVs, and certainly fewer than conventional vehicles. However, this notion may not always be true. Efficient ICEbased vehicles may actually have lower tank-to-wheel CO2 emissions than EVs if those EVs derive their electrical energy from CO2-intensive sources. This study examines the relationship between the CO2 emissions of ICE-based vehicles, PHEVs, and EVs given the varying CO2 intensity of different regions’ power generation mixes [59]. 1.8.2.1 Methods In order to make equitable comparisons across the three different types of vehicle topologies – conventional vehicle, EV, and PHEV – a single vehicle platform was needed. Since there is currently no single vehicle platform that uses all three of these topologies, this study compared the CO2 emissions from an actual ICE-based vehicle to those from similar EVs and PHEVs via the use of OVEM [60]. The first step was to select a widely used ICE-based vehicle to serve as the template for all three vehicle topologies. For this purpose, the Ford Focus was selected because it is a popular mid-size car in many markets around the world4. All of the relevant manufacturer supplied figures from the ICE-based Focus were recorded, chief among them, the vehicle’s tank-towheel energy use and CO2 emissions. Then the ICE-based version of the Focus was converted to a simulated EV and PHEV. This was done by replacing any components unique to the ICEbased vehicles with those components necessary to make the vehicle either an EV or PHEV. For instance, the ICE in each of the conventional vehicles was exchanged for an electric motor, and the multi-speed transmission was exchanged for a fixed speed gearbox. In the case of the EV, Li-ion batteries replaced the fuel tank. In the case of the series PHEV, a Li-ion battery pack was

4

In this study, the version of the Focus that was used was the 1.6 TDCi ECOnetic be-

cause it was the lowest CO2 emitting Focus at the time this study was conducted.

added along with a downsized ICE. The modelled EVs and PHEVs reflected the mass and efficiency characteristics of their new components, but retained all of the other relevant properties of the conventional vehicle upon which they were based. The EV’s battery pack was sized such that it would be able to achieve a range of 300 km over the NEDC. The battery packs of the series PHEVs came in four different sizes such that the PHEVs would be able to achieve an all-electric range of 20 km, 40 km, 60 km, and 80 km. The PHEVs operated by having their ICEs switch on to recharge their batteries when the state of charge of the batteries fell to 20% and the ICE switched off again once the state of charge reached 80%. It was assumed that the EVs and

PHEVs were able to utilize regenerative braking to recover energy from the wheels when decelerating. Such energy was still subjected to the powertrain loss mechanisms. For braking powers within the motor capacity, regenerative braking was used. Standard friction brakes were employed for braking powers in excess of the motor capacity. Table 1.4 shows the pertinent specifications for the ICE-based vehicles as well as the modelled EVs and PHEVs used in this study. The engine of the conventional vehicle operates the entire time the vehicle is driving, and the PHEV has two distinct modes: its electric-only mode where it is powered exclusively by its batteries and the other where its ICE is operating to power the car and recharge its batteries. It is interesting to note that the simulations showed all of the PHEVs use less energy than the EV in electric-only mode. This is primarily due to the fact that the PHEVs have lower mass, since the mass of their alternative power unit is offset by the fact that it reduces the vehicles’ need for a heavy battery pack. It is also worth noting that the CO2 emissions from the PHEVs during the phase when their ICEs are operating are all largely the same. This can be explained by the fact that the ICE in the PHEVs is decoupled from the road load, and hence is not as sensitive to changes in vehicle mass. The average CO2 intensity of a nation’s power generation mix is a measure of the amount of CO2 produced per unit of generated electricity, weighted by the amount of power obtained from each process – expressed in this study as g CO2/MJ. This study used the current average CO2 intensity of the national power generation mix for different key countries around the world – France, USA, and China – to see how the CO2 emissions from the EV and PHEVs varied depending on where they were recharging. France was selected because the CO2 intensity of its grid is low due to its widespread use of nuclear power. The USA was selected because the average CO2 intensity of its national grid is about in the mid-point of the range seen internationally and it is the second largest market for new vehicles in the world. China was selected because the CO2 intensity of its grid is high due to its heavy reliance on coal and it represents the world’s largest new vehicle market. It has even been projected that

by 2050 China alone will have nearly as many cars on its roads than the approximately 600 million that existed worldwide in 2005 [61]. Table 1.5 contains the data used for the CO2 intensity of these nations power generation mixes5. Table 1.4 Specifications for the actual ICE and simulated EV and PHEVs used in this study

Powertrain type

Battery pack mass (kg)

Conventional vehicle

APU mass (kg)

Curb mass (kg)

Energy Emisconsump- sions when tion in elec- ICE is opertric only ating (g mode CO2/km) (MJ/100 km)

-

-

1 340

-

11.4

EV

340

-

1 380

455

-

PHEV-

30

104

1 168

424

10.8

PHEV-

50

107

1 191

427

10.9

PHEV-

80

110

1 224

431

10.9

20 40 60 PHEV-

110

113

1 257

437

10.9

80

Table 1.5 CO2 intensity of the power generation mix of the countries examined in this study

Country

CO2 intensity of power generation mix (g CO2/MJ)

France

24

5

4 Data

from Carbon Monitoring for Action (CARMA), available online at

www.carma.org

USA

169

China

241

The tank-to-wheel CO2 emissions for the conventional vehicle, EV, and PHEVs were all derived in different ways. The operating emissions from conventional vehicles can be measured by what comes out of their tailpipe. In Europe, automobile manufacturers publish the grams of CO2 per kilometre (g CO2/km) emitted by conventional vehicles. Therefore in this study, that manufacturer reported value is used as the constant CO2 emissions from the conventional vehicle over its entire range. The emissions from EVs depend on their own energy use and on the CO2 intensity of the power generation mix from which the EVs energy was obtained. The CO2 intensity varies considerably depending on the composition of the power generation mix. By combining the energy use of the EV with the CO2 intensity of its power generation mix, its emissions in g CO2/km, EEV can be determined. The CO2 emissions from the PHEVs depend on the CO2 intensity of the power generation mix, the efficiency of the vehicle in its electric-only and its ICE-operating modes, and the distance travelled in both modes. Note that the energy consumption and hence CO2 emissions figures for the PHEVs were obtained by simulating the vehicle over multiple discharge-charge cycles as they were run over repeated iterations of the drive cycle. This enabled the study to obtain a reliable average value for their CO2 emissions that was not unfairly biased by a particular state of charge profile over a specific drive cycle (or segment thereof). 1.8.2.2 Results Figures 1.14 through 1.16 present the tank-to-wheel CO2 emissions from all three topologies as though they were operating in France, the USA, and China. Note that the conventional vehicle produces the same tank-to-wheel emissions regardless of where it is operating. The tank-to-wheel CO2 emissions from the EV depend directly on the CO2 intensity of the power generation mix from which it recharges. The CO2 emissions from the PHEVs in electriconly mode also depend directly on the CO2 intensity of the power generation mix. However, once the PHEVs enter their ICE-operating mode, their cumulative CO2 emissions averaged over their distance travelled pick up at the level they were at during electric-only mode before asymptotically approaching their ICE’s CO2 emissions value. Figure 1.14 shows how the average emissions profiles compare for the conventional vehicle, EV, and PHEVs when the EV and PHEVs charge in France. While the PHEVs were more efficient in their electric-only mode than the EVs,

which led to them having lower emissions than the EVs in their all-electric range, the difference was almost negligible. Figure 1.14 shows that given France’s low CO2 intensity, there was less than a 1 g CO2/km difference between the CO2 emissions from the most efficient PHEV and EV. What was only a small difference in emissions in electric-only mode becomes much larger once the PHEVs ICE turns on. As the average CO2 emissions from the PHEVs asymptotically approach their CO2 emissions from their ICEs, they move farther away from the low, constant CO2 emissions of the EV. Since the PHEVs still produce fewer g CO2/km than the conventional vehicle when their ICE is operating, and since in France their CO2 emissions in electric-only mode begin at a point below those from the conventional vehicle, their average emissions performance is lower at all points in their driving range than the conventional vehicle. From these results, the generalization can be made that if the CO2 intensity of the power generation mix is low, then EVs provide a more consistent option than PHEVs or conventional vehicles for lowering CO2 emissions in automobile transport.

Fig. 1.14 Tank to wheel CO2 emissions of various vehicle powertrain topologies in France.

Figure 1.15 shows the average emissions for the three topologies over the course of their driving range as if they were charging in the USA. Given the USA’s CO2 intensity, the PHEVs emit slightly fewer g CO2/km than the EV during their electric-only mode. This was expected because of the lower curb mass and higher efficiency of the PHEVs. The PHEVs even continue to emit less than the EV into their ICE-operating mode. For instance, the PHEV-80 continues to have lower emissions than the EV for several kilometres into its ICE-operating mode. There is clearly a range of distances over which the PHEVs emit less than the EV. Thus, in a country with a mid-range CO2 intensity, PHEVs offer the possibility of reducing CO2 emissions relative to EVs when the PHEV begins with a full charge and is not driven too extensively in its ICE-operating mode. Yet, once the PHEVs travel further in their ICE-operating

mode, they begin to emit significantly more g CO2/km than the EV. Both the EV and PHEVs emit less CO2 at all points in their range than the conventional vehicle. Thus, with power generation mixes of mid-range CO2 intensity, PHEVs may provide lower CO2 emissions if the driving distances between charges are short relative to the maximum range of the vehicle (as they typically are for many drivers). However, the emissions reductions from PHEVs in electric-only mode compared to those from EVs are not too substantial and should not be overstated. The EV produced low CO2 emissions over its entire range and it achieved over 90% lower CO2 emissions than the conventional vehicle. Ultimately, EVs appear to be a better option than PHEVs for lowering CO2 emissions in automotive transport given a power generation mix similar to the average one found in the USA.

Fig. 1.15 Tank to wheel CO2 emissions of various vehicle powertrain topologies in the USA.

When the CO2 intensity of the power generation mix is high, a different picture begins to emerge as shown in Figure 1.16. Given China’s power generation mix, the simulated PHEVs achieve lower tank-to-wheel CO2 emissions at all points over their range than the EV and conventional vehicle. This fact has been largely ignored in the literature and it demonstrates that PHEVs provide a stronger alternative than previously thought to lower the CO2 emissions from transport in countries with a highly CO2 intensive power generation mix. In these countries, PHEVs have the potential to be more than a mere stopgap in the transition from conventional vehicles to EVs – compensating for the current range deficiencies of EVs until battery technology improves. Depending on how the CO2 intensity of the power generation mix evolves, PHEVs may be able to play a more prominent role in a long-term solution to lower CO2 emissions in automobile transport. However, the reality of China’s power generation mix also shows that even EVs and PHEVs will not be able to significantly lower

the CO2 emissions from automobile transport in the short to medium term. The EV and PHEV-20 in electric-only mode achieved less than a 5% and 11% CO2 emissions reduction, respectively, compared to the conventional vehicle. This demonstrates that countries with highly CO2 intensive power generation mixes clearly have an extra incentive to reduce their CO2 intensity in order to maximize the potential of EVs and PHEVs to reduce CO2 emissions in automotive transport.

Fig. 1.16 Tank to wheel CO2 emissions of various vehicle powertrain topologies in China.

The specific results on display in these studies are a product of the assumptions and modelling techniques outlined earlier. As such, the above predictions should not necessarily be expected to hold true for all possible EVs and PHEVs. Nevertheless, these results illustrate the complex nature of the questions posited at the beginning of this section. Clearly, there is no simple answer as to what vehicle topologies and technologies are optimal at all times and for all places. A thorough understanding of the characteristics and limits of various automotive solutions will be crucial if we are to make informed decisions about the future of road transport.

1.9 Conclusions Road transport accounted for 40% of the total final consumption of energy across all transport modes globally in 2008. Energy use is expected to double by 2050 to satisfy growth in road transport demands. Much of the growth in demand for transport will be due to more extensive vehicle use in developed countries and the transition to motorized transport in developing countries. The consequence will be approximately 5 Gt GHG emitted in 2050 due to automobile transport alone. In order to ameliorate many of the environmental and eco-

nomic issues related to automobile transport, vehicles’ energy use must be reduced and the sources from which they can derive their energy should be diversified. The primary way that vehicle design can tackle these issues is through EV and HEV powertrains. EVs and HEVs are typically more efficient than conventional vehicles based on an ICE and they have the potential to make use of energy derived from an array of sources addressing the energy security issue. The sizing of vehicle powertrain components is strongly dependent on the vehicle mass. Reducing vehicle component primary mass allows additional, secondary mass savings. Consequently, a 10% reduction in primary vehicle mass can yield at least 7% reduction in energy use. Reducing other nonpowertrain resistive forces, such as aerodynamic drag and tyre rolling resistance, can lower energy use by up to 5% each. Most conventional vehicles use the mature and application-flexible ICE as the main energy conversion technology. However, many modern vehicles have ICEs with peak power outputs far in excess of what is required under normal driving conditions. The consequence is that the ICE generally operates under part-load which is when it is less efficient. This is compounded by many idling periods when driving in urban or congested environments. Most notably, all of the energy used to achieve a particular vehicle velocity is converted to heat by the brakes when the conventional vehicle eventually slows to a halt. Electric motors are also a mature technology, with widespread use in industrial, domestic and power generation applications. Motors have significantly higher peak efficiencies than ICEs and power profiles which favour part-load operation of the kind normally seen during realistic driving conditions. Moreover, an electric motor can be operated as a generator to recapture the vehicle’s kinetic energy during braking. Thus, electric motors via electrified powertrains are a more efficient alternative for delivering road transport. Many EVs, HEVs and their variants use batteries to meet the energy and power needs under normal driving conditions. In particular, batteries are used to satisfy the high power demands associated with sharp accelerations and the lower energy demanded when cruising. Batteries are better suited to storing energy than ultra-capacitors and high speed flywheels on account of their high specific energy. However, batteries are generally ill-suited to the high current bi-directional flows associated with large power demands. The result is lower roundtrip efficiency, reduced cycle capacity and shorter battery lifetime. Using high specific energy lithium-based batteries in this way incurs a large cost penalty. Ultra-capacitors and flywheels are alternative technologies that may be able to address some of the shortcomings of batteries. Both technologies are well-suited to frequent charge-discharge cycling, have high specific power, and can be used to provide a blended energy storage system which is more efficient and less expensive.

This chapter included two case studies that examined the complex nature of just some of the economic and environmental issues surrounding the future of automobile transport. The first study compared ultra-capacitors and highspeed flywheels to batteries as the energy storage medium in a series HEV. The results showed that though batteries allowed the vehicle to achieve the highest fuel economy, ultra-capacitors and high-speed flywheels have the potential to be nearly as efficient at less cost. The second study compared the tank-to-wheel CO2 emissions stemming from conventional vehicle, EV and HEV use. The results showed that PHEVs actually have the potential, in regions where electricity generation is CO2 intensive, to produce fewer tank-towheel CO2 emissions than similar conventional vehicles and EVs. Therefore, although EVs and their variants have more efficient powertrains than conventional vehicles, their tank-to-wheel emissions, and ultimate ability to mitigate climate change, is a function of the sources and processes from which they derive their electrical energy.

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