Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency Françoise Nemry, Guillaume Leduc, Almudena Muñoz
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JRC 54699 Technical Note
Luxembourg: Office for Official Publications of the European Communities © European Communities, 2009 Reproduction is authorised provided the source is acknowledged
Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency
F. Nemry, G. Leduc, A. Muñoz
JRC Technical Notes
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1 Table of contents 1 2 3
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Table of contents ............................................................................................................................ 2 Introduction .................................................................................................................................... 4 Technological components ............................................................................................................. 4 3.1 General definitions................................................................................................................. 4 3.2 Power train architecture ......................................................................................................... 5 3.3 Energy management............................................................................................................... 6 3.3.1 Advantages and disadvantages of PHEVs......................................................................... 8 3.4 Batteries ................................................................................................................................. 8 3.4.1 Key parameters .................................................................................................................. 8 3.4.2 State of the art and anticipated developments ................................................................. 10 3.4.3 Battery characteristics...................................................................................................... 13 Tank-to-wheel energy performance.............................................................................................. 15 4.1 Introduction.......................................................................................................................... 15 4.2 Literature review.................................................................................................................. 16 4.3 How to measure the final energy consumption of PHEVs?................................................. 19 4.4 Energy performance of EDVs: first estimations .................................................................. 22 4.4.1 Reference cars energy performance and cost .................................................................. 22 4.4.2 Battery specifications ...................................................................................................... 23 4.4.3 Energy efficiency............................................................................................................. 23 4.5 Need for further work .......................................................................................................... 25 Vehicle costs................................................................................................................................. 26 5.1 Literature review.................................................................................................................. 26 5.2 Vehicle cost comparison for the EU .................................................................................... 27 5.3 Need for further work .......................................................................................................... 28 Battery charging options and infrastructures ................................................................................ 29 6.1 Introduction.......................................................................................................................... 29 6.2 Battery charging options ...................................................................................................... 29 6.3 Expected recharging time and implied charging infrastructure ........................................... 31 6.4 Need for further work .......................................................................................................... 32 Impacts on, and role of electricity grid operators ......................................................................... 33 7.1 Introduction.......................................................................................................................... 33 7.2 From mono-directional to bi-directional power flow management ..................................... 34 7.3 Need for further work .......................................................................................................... 36 References .................................................................................................................................... 37
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Acknowledgement The authors of this paper would like to thank Christian Thiel (JRC/IE Institute), Adolfo Perujo and Biagio Ciuffo (JRC/IES Institute) for their suggestions on the draft version.
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2 Introduction Electric-drive vehicles (EDVs) have gained attention, especially in the context of growing concerns about global warming and energy security aspects associated with road transport. The main characteristic of EDVs is that the torque is supplied to the wheels by an electric motor that is powered either solely by a battery or in combination with an internal combustion engine. This covers hybrid electric vehicles (HEVs), battery electric vehicles (BEVs), and plug-in hybrid electric vehicles (PHEVs), but also photovoltaic electric vehicles (PVEVs) and fuel cell vehicles (FCVs). As part of its transport and energy modelling activity, IPTS initiated research work with a view to assess the economic and environmental impacts for the EU27 of a future market penetration of those car technologies, with a focus on BEVs and PHEVs. As a starting step, IPTS reviewed the literature and prepared this report which provides a summary description of the technology aspects, the current state of the research and development in the field. It also elaborates consistent sets of data about the vehicle technologies in view of the subsequent modelling work to undertake the assessment. The report also identifies a series of areas where more data and assessment are needed. This report also represents a first IPTS contribution to a JRC horizontal project involving IE and IES.
3 Technological components 3.1
General definitions
To be more precise, the following definitions are used this report: Battery Electric Vehicles refer to vehicles propelled solely by electric motors. The source of power stems from the chemical energy stored in battery packs which can be recharged on the electricity grid. The future of such vehicles strongly depends on the battery developments (performance and cost). Plug-in Hybrid Electric Vehicles refer to vehicles that can use, independently or not, fuel and electricity, both of them rechargeable from external sources. PHEVs can be seen as an intermediate technology between BEVs and HEVs. It can indeed be considered as either a BEV supplemented with an internal combustion engine (ICE) to increase the driving range, or as a conventional HEV where the all-electric range is extended as a result of larger battery packs that can be recharged from the grid. As an example, the IEEE1 (board of directors, 2007) defines a PHEV as ''any hybrid electric vehicle which contains at least: (1) a battery storage system of 4 kWh or more, used to power the motion of the vehicle; (2) a means of recharging that battery system from an external 1
http://www.ieeeusa.com/policy/POSITIONS/PHEV0607.pdf
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source of electricity; and (3) an ability to drive at least ten miles in all-electric mode, and consume no gasoline. These are distinguished from hybrid cars mass-marketed today, which do not use any electricity from the grid.'' A large range of options are currently developed that vary in terms of power train architecture, energy mode management, battery type, that can influence the energy performance and costs.
3.2
Power train architecture
A plug-in hybrid electric vehicle can be designed with the same types of technological architecture as current hybrid vehicles, namely series-hybrid, parallel-hybrid, or combined series-parallel hybrid (split): •
Series-Hybrid: this configuration is to be associated with electric cars since only the electric motor provides power to drive the wheels. Sources of electrical energy are either the battery pack (or ultra capacitors) or a generator powered by a thermal engine. An example of PHEV series is the famous Chevrolet Volt developed by General Motors2. Such vehicles are also called Extended-Range Electric Vehicles.
•
Parallel-Hybrid: in this case, both the electric motor and thermal engine can provide power in parallel to the same transmission.
•
Power split or series/parallel hybrid: this configuration combines the advantages of both parallel and series hybrid concepts. This is for instance the architecture implemented in the Toyota Prius model (Hybrid Synergy Drive). This relatively complex architecture allows running the vehicle in an optimal way by using the electric motors only, or both the ICE and the electric motors together, depending on the driving conditions.
Figure 1 provides an illustration of the PHEV configuration. PHEV HEV Regenerative braking
ELECTRICITY GRID
B A T T E R I E S
FUEL
Generator
ICE
ELECTRIC MOTOR
DRIVE TRAIN
Figure 1: Simplified representation of HEV/PHEV configuration (blue: series; red: parallel) 2
See also its EU version Opel Ampera.
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3.3 Energy management When driven, the BEV and PHEV State of Charge (SOC) – i.e. the fraction of total energy capacity remaining in the battery - varies within a certain window, given by the difference between maximum and minimum SOC. The technological advantage of PHEVs stems from its capability of driving on different energy modes, resulting in different SOC levels. Two basic modes are possible (see e.g. [Market and Simpson, 2006] and [EERE, 2007]): •
In the Charge Depleting operating mode (CD), the vehicle is powered only or almost only by the energy stored in the battery, and the battery's SOC gradually decreases up to a minimum level (which depends on the battery size). The vehicle thus mostly behaves as an electric car, which particularly suits to urban driving [Shiau et al., 2009]. This mode can actually operate in two ways: under the "CD blended mode", the ICE is turned on. Under the "CD all electric' mode", the ICE is turned off.
•
During the Charge sustaining mode (CS), the SOC over a driving profile may increase and decrease but will, on the average, remain at its initial level. The battery's SOC is maintained within an operating range and can be recharged through regenerative braking and from the ICE. In this case, PHEVs behaves as conventional HEVs.
Depending on the driving conditions, the two modes can be combined over the distance travelled in such a way as to reap the full advantage of the PHEV and extend the driving range. This is illustrated in Figure 2. The resulting discharge cycle will influence the total energy demand over the distance travelled and the environmental performance. In practice, the feasible combination will depend on how often and where the driver could charge his vehicle from the grid, on the driving cycles, etc. However, at some point the vehicle needs to rely on fuel to extend the driving range and then switches to the so-called 'Charge Sustaining' CS mode.
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Figure 2: Example of discharge cycles for BEVs, HEVs and PHEVs Source: [Anderson, 2008]
PHEVs are differentiated according to their All-electric range (AER) i.e. the distance driven electrically up to the point at which the ICE engine first turns on. It can be also defined as the distance travelled before the vehicle switches from charge-depleting to charge-sustaining operation [Gonder and Simpson, 2007]. This is measured for a reference driving cycle, usually on urban driving cycle. The notation "PHEVx" is commonly used to specify the PHEVs AER. For instance a PHEV30 corresponds to a PHEV with a 30 miles electric range. Typical PHEVs AER are in the range 20-60 miles. The PHEVx notation is more indicative for the case where, in practice, a PHEV would operate on all-electric CD mode over the first x kilometres, and after in CS mode3. But this definition is less appropriate if a PHEV operating in CD blended mode for which both electricity and gasoline are used to power the vehicle. In this case, it would be more convenient to define the suffix x as the equivalent distance of petroleum-based fuel displaced by electricity from the battery [Gonder and Simpson, 2007]. It has also to be noted that, in practice, the real world driving behaviour and energy management mode will influence the actual driven distance. Therefore, a given AER doesn't mean that the vehicle may or may not actually drive the corresponding distance electrically.
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This is the case for instance of the Chevrolet Volt (hybrid series) whose the ICE is used as backup source of energy to provide onboard electricity when the batteries reach their lower bound limit SOC.
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3.3.1 Advantages and disadvantages of PHEVs Figure 3 summarises the main advantages/disadvantages associated with PHEVs. These elements are analysed in the present report, with a special focus on fuel efficiency (section 4) and costs (section 5). PHEVs
!
+ Ability to run on Charge Depleting (as for BEVs) or Charge Sustaining mode (as for HEVs)
Fuel consumption/GHG emissions depends on many parameters (e.g. driving and charging patterns, AER, CD mode management, efficiency in the CS mode)
Grid connection capability Extended All-Electric Range (typically up to 60 km) can displace important quantities of fuel
Market penetration greatly depends on battery developments (performances and costs)
GHG emissions are generally reduced (depending on the electricity generation mix = f (space, time))
Additional weight (battery pack, etc.)
Driving range not limited as for BEVs Higher initial costs
Take advantages from HEV and BEV driving performances
Impacts on the electricity grid need to be carefully assessed
Can greatly improve local air quality V2G capability
Infrastructure costs (charging facilities, etc.)
Well suited to urban areas
Figure 3: Pros and Cons of PHEVs Note: some of the negative aspects of PHEVs vs. HEVs would be positive vs. BEVs (e.g. costs, battery weight etc.)
3.4
Batteries
Battery performance and cost are essential factors for the development of electric vehicles. The present chapter provides a brief description of current progress and potential evolution of the different aspects concerned.
3.4.1 Key parameters Batteries design variables include (see e.g. [Axsen, 2008; Anderson, 2008; Anderson, 2009]): Energy The energy storage capacity (kWh) is of high importance since it will directly determine the distance the vehicle can drive on the CD mode, as well as the mass of the battery pack. For PHEVs, the energy storage requirement considered in the literature typically varies from ~6 8
kWh to 30 kWh depending on the CD range (compared to 1-2 kWh for conventional hybrids and 30-50 kWh for BEVs). The energy storage capacity represents the 'available' or 'total' energy capacity depending on whether the SOC window is taken into account or not (e.g. a 10 kWh of total energy capacity operating with a 65% charge swing would have only 6.5 kWh of available energy). Generally, the battery usable energy increases linearly with the CD range [Rousseau et al., 2007]. Both high specific energy (Wh/kg) and energy density (Wh/l) – i.e. the ratio of the total energy (Wh) to the battery mass (kg)/volume (l) – is crucial to achieve high energy storage capacity without entailing significant additional mass/volume. Power The peak battery power (W) required primarily depends on the CD range, the CD energy management mode and on the total vehicle weight. For instance, a PHEVx operating in CD blended mode would require less power than the one operating in CD all-electric mode. The peak power is generally assumed to remain constant as the AER increases [Rousseau et al., 2007]. Note also that the P/E ratio decreases with increased AER (hyperbolic behavior). This is due to the linear relationship between the available battery energy and the AER, although the power remains nearly constant as AER increases (see e.g. [Rousseau et al., 2007]). Lifetime, safety, costs and others •
Calendar life: it is defined as the 'ability of the battery to withstand degradation over time' [Axsen et al., 2008]. 10-15 years is generally assumed to be a sufficient calendar life4.
•
Cycle life. Lifetime requirements depend on the energy management mode (i.e. CD and/or CS modes) and therefore the number of micro and full discharged cycles. As HEVs operate in CS mode, they require sufficient micro-cycles which is not the case for BEVs which need sufficient full (deep) cycle life (CD mode exclusively). PHEVs batteries must comply with both characteristics which is quite challenging. They should be able to undergo deep discharge cycles during the CD mode (probably 3-5 thousand deep discharge cycles are a reasonable target) and shallow cycles in the CS mode.
•
Safety and thermal management requirements (meet safety standards, crash worthiness, etc.).
•
Cost of the battery pack (in €/kWh and in €/kW). The battery cost increases with extending all-electric range. It remains the main barrier for the deployment of PHEVs.
•
Usable SOC window (%). The SOC window has slight impact on the fuel consumption but can significantly impact the costs. It has to be maximised while satisfying cycle and calendar life requirements [Markel and Simpson, 2006].
•
Recharge time (h). Fast recharge times are necessary for PHEVs, but this also affects the life of the battery.
4 As pointed out by [Pesaran, 2007] 'currently CARB requires 10 years warranty for AT PZEV batteries but most consumers expect the batteries to last the average life of vehicles, i.e. 15 years'.
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•
Life cycle issues (e.g. availability of lithium resource, recycling).
•
Influence of vehicle mass: the vehicle mass (through additional battery weight) slightly increases with the AER, but this effect is somewhat limited. Note that the electricity consumption (in Wh/km) increases linearly with the vehicle mass, around 6-7 Wh/km for every 100 kg in vehicle mass added [Rousseau et al., 2007].
3.4.2 State of the art and anticipated developments Battery performances Nickel Metal Hydride (NiMH) batteries are the current typical batteries used by car manufacturers in mass-produced HEVs (e.g. Toyota). However NiMH batteries are considered to have reached their maximum potential. For the future, experts do not expect significant new technical improvements and cost reductions (see e.g. [Anderman, 2008; Kalhammer et al., 2007; Kromer and Heywood, 2007]). Car makers are moving to lithium-ion5 batteries, especially because they offer energy density higher than what NiMH batteries do. They are also characterised by the absence of memory effects and low self-discharge rate. They are seen as the best option to meet the energy storage requirements not only for PHEVs, but also for BEVs and HEVs, at least in the short to medium term. Li-ion batteries offer a wide field of new developments and have not yet achieved the same maturity level as for NiMH batteries (see Figure 4). As underlined by the IEA [IEA, 2007] ''for PHEV, the key additional breakthrough appears to be lithium-ion battery technology, as the energy density has continued to improve in recent years. At the same time, the energy density of other battery technology has remained constant''. Even if R&D efforts are still needed to cope with longevity and safety problems (see e.g. [Kalhammer et al., 2007; Karden et al., 2007]) Li-ion batteries have been testing intensively worldwide and are already used on many PHEV prototypes.
Figure 4: Evolution of the energy density (Wh/litre) of Li-ion batteries, compared to NiMH and NiCD technologies Source: [IEA, 2007] (taken originally from Shinsuke Ito, EVS-22 Plug-in Hybrid Electric Vehicle Workshop) 5
Note that behind the term 'Li-ion batteries' there is a set of different technologies.
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Improvements in specific energy of Li-ion batteries are crucial for their deployment. Different goals regarding battery performances of PHEVs have been set by the U.S. Advanced Battery Consortium (USABC), the MIT and EPRI [Axsen et al., 2008]. The USABC considered two battery types: a high power/energy ratio battery providing 10 miles of AER (PHEV10) in a SUV vehicle of 1950 kg and a low-power energy ratio battery providing 40 miles of AER (PHEV40) in a midsize sedan of 1600 kg. The MIT analyzed the goals for a midsized sedan with a 30 miles CD range in blended mode. EPRI considered a PHEV20 and PHEV60 midsized sedan. Different driven cycles were considered: USABC used the Urban Dynamometer Driving Schedule (UDDS), MIT used UDDS as well as the Highway Fuel Economy Test (HWFET) and US06 schedules (part of the Supplemental Federal Test Procedure that is characterized by being more aggressive) and finally EPRI used a driven cycle that included the other ones (UDDS and HWFET). In all cases, the battery goals covered power, energy, life and cost goals. These goals are dependent on the assumptions made about PHEV design, drive cycle, vehicle and battery weight and recharge behaviour.
Table 1: PHEV assumptions and battery 'goals' (long-term development) Source: [Axsen et al., 2008]
Battery costs: current trend and expectations The cost of Li-ion batteries (i.e. at cell, module or pack level) includes material cost (e.g. anode/cathode materials), manufacturing cost and other costs (e.g. R&D, marketing, transportation). Material costs account for ~75% of the total battery pack cost while manufacturing and other costs represent around 5% and 20% respectively. For a more thorough analysis of battery-related costs, see e.g. [Anderson, 2009]. A list of current Li-ion battery costs is given in Table 2 [Petersen, 2009]. Current prices are in the range of 700-1000 $/kWh or even higher.
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Manufacturer
Chemistry
Current Price ($/kWh)
Target Price ($/kWh)
Enert1 (HEV)
Li-Polymer
660
N/A
Valence Technologies (VLNC)
Li-phosphate
1000
500
Altair Nanotechnologies (ALTI)
Li-titanate
1000
N/A
A123 Systems (power tool packs)
Li-phosphate
1228
N/A
2008 DOE SEGIS-ES Estimates (PV Solar battery packs)
Various
1333
780
2009 NEDO Survey Results (Average of Japanese Producers)
Various
2018
1000
Table 2: Current price of Li-ion batteries Source: [Petersen, 2009]
The high production volumes already achieved today suggest that Li-ion battery costs could significantly decline in the short term (see e.g. [Sanna, 2005]). It is for instance expected that Li-ion battery cost would fall as low as 395 $/kWh and 260 $/kWh for a PHEV10 and a PHEV40 respectively with 100000 units produced [Kalhammer et al., 2007]. The battery cost goal set by the USABC range from 300 $/kWh to $200/kWh for the PHEV10 and PHEV40 respectively. The MIT estimates that the commercialization of a PHEV30 requires a cost as low as 320 $/kWh. The U.S. Department of Energy' goal is 250 $/kWh by the year 2015. According to [Kammen et al., 2008], PHEVs would become cost efficient to consumers if battery prices would decrease from 1300 $/kWh to about 500 $/kWh (so that the battery may pay for itself). It is however not yet proven that costs will reduce in such a scale. Despite already important production volumes, costs remained constant over last 9 years [Petersen, 2009]. It is thus not guaranteed that the above-mentioned targets will be met. Li-ion battery costs are expected to remain lower than NiMH batteries but the range of 600700 $/kWh is seen more realistic in the short to medium term (see e.g. [Anderman, 2008]). Figure 5 shows a possible (rather optimistic) evolution of Li-ion battery pack cost for PHEV40 by 2010 and 2020.
Figure 5: Battery Pack Supply Chain Cost Breakdown Source: [Anderson, 2008]
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3.4.3 Battery characteristics The following (Table 3) proposes typical ranges for the key parameters for batteries in relation with different vehicle types to be considered later when considering their energy and cost performance (section 4.4 and 5.2 respectively). HEV
BEV
PHEV
0
150-200
10-60
NiMH
Li-ion
Li-ion
Total capacity (kWh)
1.3
30-60
4-30
Specific energy (Wh/kg)
46
110-160
110-160
200-350
400-600
400-600
AER (miles) Material (1)
Energy density (Wh/l) Peak power (kW)
27-35
Specific power (W/kg)
1300
1500
500-1500
Battery pack weight (kg)
29(2)
200-500
70-190
Calendar life (years)
10-15
10-15
10-15
Deep cycle life (number of cycles) Specific cost (€/kWh)
40-100
>2500 600
750-1500
Table 3: Typical battery performance for the near term (medium car) (1)
Obtained by multiplying the nominal capacity (in C or Ah) by the nominal voltage (V). For the Toyota Prius III, we find 6.5Ah*201.6V=1.31 kWh. (2) 28 modules weighting 1040 g each
Variation of parameters vs. AER All other things being equal, extending the AER of PHEV (e.g. from 20 to 60 miles) will modify some parameters as follows: • • • •
The peak power (W) is generally assumed to remain constant with the AER, but can vary depending whether we consider CD all-electric or blended mode. The energy capacity (kWh) varies linearly with the AER (see Figure 6), whose the slope is function of the vehicle mass (category). The battery pack weight increases with the AER, typically from 70 kg (PHEV20) to 190 kg (PHEV60). The electricity consumption (Wh/km) is assumed to increase linearly with the vehicle mass (around 6-7 Wh/km for every 100 kg added, see e.g. [Rousseau et al., 2007]) with a similar slope for each vehicle category.
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Figure 6: Usable energy battery requirements for different AER Source: [Rousseau et al., 2007]
Table 4 gives the typical capacity storage as required by different AER in the case of a midsize car. HEV
BEV
PHEV (all)
PHEV-20
PHEV-30
PHEV-40
PHEV-60
AER (miles)
0
150-200
10-60
20
30
40
60
Material
NiMH
Li-ion
Li-ion
Li-ion
Li-ion
Li-ion
Li-ion
Total capacity (kWh)
1.3
30-60
4-30
4-8
8-12
12-16
20-30
Table 4: Typical capacity storage as required by different AER (mid-size car case)
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4 Tank-to-wheel energy performance 4.1
Introduction
The comparison of the energy performance of EDVs with conventional ICE or with HEV technologies has to be made on a well-to-wheel (WTW) basis, considering the final energy required to drive the power-train (TTW), and the indirect energy consumption by the upstream energy transformation processes (WTT part). The WTT energy consumption from ICE cars accounts for all the processes from oil extraction to refinery processes. In the case of BEVs, the WTT energy use stems from the generation of the electricity used to drive the car. In the case of PHEVs, the WTT energy consumption from both fuel and electricity production has to be taken into consideration. The WTT energy use part - and resulting GHG emissions - attributed to electricity generation will depend on the power generation mix and on the time when the electricity is consumed to recharge the battery. These factors will be analysed in a subsequent assessment. Therefore, the following concentrates on the TTW aspects (or final energy consumption). As a result of its ability to drive a certain distance on CD mode, PHEVs have the potential to avoid fuel consumption. The "fuel displacement" is commonly referred to in the literature characterizing the energy performance of PHEVs6. This information is primarily relevant when energy security aspects are looked at. Fuel displacement - and, replacement with electricity consumption -, indeed means a more diversified energy supply mix. The sole indication about the fuel displacement may however be misleading as the fuel consumption avoided (compared to a reference car – be it a conventional ICE or a hybrid car) doesn't necessarily mean that the "fuel displaced" is substituted by a same amount of electricity. A full energy performance assessment of PHEV implies to quantify both the fuel and the electricity consumptions over the considered distance driven. Compared to BEVs (that exclusively use electricity) and to HEVs (for which fuel is the sole source of energy7), the final energy consumption equation for PHEV is more complex. As illustrated in Figure 7, it will depend on the distance travelled on all-electric range, which will be influenced by both the charging pattern and the driving behaviour.
f − f PHEV The fuel displacement rate is defined as: FD = REF where f REF and f PHEV denote the fuel consumption of the reference f REF vehicle and of the PHEV respectively. 7 For HEVs, an amount of fossil fuels is 'indirectly' displaced through on-board production of electricity (e.g. regenerative braking). 6
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HEV
PHEV
BEV
100% CS
xCD+(1-x)CS
100% CD
Charging pattern
Driving pattern
Annual distance performed on electricity = f (annual vehicle-km, AER capability etc.)
Driving cycles
l/km
Δl/km of fuel ‘converted’ in kWh/km
Driving cycles
kWh/km Tyres, body shape Weight Power train Electric motor Etc.
Tyres, body shape Weight Power train ICE efficiency Etc.
Figure 7: Level of complexity to consider when assessing the energy performance of PHEVs
Assessing the final energy consumption of PHEVs implies to take into account the following factors: • Driving cycle (urban, highways, combined) • The CD mode energy management (CD 'blended' or CD 'all-electric'). This is linked to the power train configuration (series, parallel, split). • The daily distance travelled on CD mode which depends on charging (location, time of the day, frequency) and driving patterns (e.g. average daily distance travelled). Also, the fuel displacement rate with PHEVs (i.e. based on the annual AER) will increase when the energy storage capacity is extended (PHEV60 offers more potential than PHEV20).
4.2
Literature review
Most of the research studies assessing the energy performance of PHEVs have been undertaken by American research bodies, notably by the Massachusetts Institute of Technology (MIT), the Electric Power Research Institute (EPRI), the National Renewable Energy Laboratory (NREL) of the U.S. DoE, the Argonne National Laboratory (ANL) but also by the International Energy Agency (IEA). Several studies (see Table 5) analysed the quantity of fuel displaced for different PHEV configurations, under different assumptions (e.g. charging time, driving models, market penetration rates, etc.). Different approaches are used, including tests and modelling (e.g. ADVISOR, PAMVEC models). All of them concluded that the use of PHEV would displace a large quantity of fuels.
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Reference [Parks et al., 2007] [Karner and Francfort, 2007] [Stephan and Sullivan, 2008] [Kliesch and Langer, 2006] [Samaras and Meisterling, 2008] [EPRI/NRDC, 2007]
[Shiau et al., 2009] [Bradley and Franck, 2009] [Silva et al., 2009]
[Simpson, 2006]
[MIT, 2008]
[Kammen et al., 2008]
[Elgomainy et al., 2009] [Dowds et al., 2009] [Clement-Nyns et al., 2007] [IEA, 2007] [Gonder and Simpson, 2007] [Gonder et al., 2007] [Kromer and Heywood, 2007]
Methodology / comments ADVISOR model Colorado case study Data from HEVs converted in PHEVs Based on Toyota RAV4 Based on converted Toyota Prius (CO2 and air emissions) PHEV10-60 fuel reduction under average driving conditions (US) No quantification of fuel displacement Life cycle Assessment of GHG emissions Potential long-term GHG emissions impacts of PHEVs in the U.S. (annual consumption, years 2010 and 2050). 9 scenarios (3 emissions intensity for the electric sector and 3 scenarios for PHEVs market penetration) Economic and environmental related impacts of battery weight and charging patterns of PHEVs Literature review on fuel consumption from PHEVs 2 PHEVs, 2 configurations (series and parallel), 4 driving cycles (CAFE, FTP75, NEDC, JC08). Alternative approach to the SAE J1711 recommended practice The ADVISOR software is used. Influence of driving and charging patterns on FC and air pollutants (U.S., EU and Japan) Fuel consumption, CO2 emissions, impact on electricity grid, costs. PAMVEC model Cost-benefit analysis FC (l/100km) and electricity consumption (Wh/km) for PHEV2,5,10,20,30,40,50,60 under two scenarios Deep analysis, US context UF is around 50% for a PHEV30 (for the US) Simulation results for time horizon 2035 WTW emissions and market penetration of PHEVs are analysed under different scenarios. Compared CV, HEV and 2 PHEVs (compact car and full-size SUV) GHG avoided estimated from the GREET model Cost-effectiveness analysis of PHEVs WTW energy used and GHG emissions from PHEVs GREET model and PSAT simulation Review on PHEVs fuel displaced, GHG emissions and their interaction with the infrastructure. Based on TREMOVE model Consumption of electrical energy (for Belgium). Deep analysis, using GREET model and PSAT simulations. Measurement of the fuel consumption of PHEVs. Discuss the SAE J1711 recommended practice. Influence of driving patterns on fuel displacement rate. Based on GPS real-world measurements. WTW energy use for PHEV30 Market penetration assumptions
Institute National Renewable Energy Laboratory (NREL) Idaho National Laboratory (US DOE) Ford Motor Company American Council for an Energy-Efficient Economy Carnegie Mellon University (PA) EPRI, NRDC
Carnegie Mellon University (PA) Georgia Institute of Technology, UC Davis IDMEC (Lisbon) University of Michigan (MI)
National Renewable Energy Laboratory (NREL) MIT
University Berkeley Argonne Laboratory IEEE
of
California,
National
Universite Catholique de Louvain IEA (Annex 7) Tesla Motors Inc., National Renewable Energy Laboratory (NREL) National Renewable Energy Laboratory (NREL) TIAX and MIT
Table 5: (Non-exhaustive) list of key references dealing with petroleum displacement of PHEVs
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The fuel displacement potentials reported in the literature were reviewed by [Dowds et al., 2009] and are shown in Figure 8. The fuel displacement rates are associated with PHEV10, PHEV20 and PHEV40 and were found to range from 42% to 78% relative to ICEs and from 12% to 66% relative to HEVs [Dowds et al., 2009]. Such ranges are also confirmed by other research works. It is worth mentioning that the different estimates obtained in a same study (see e.g. [EPRI/NRDC, 2007]) illustrate the large influence of the assumptions made.
Figure 8: Fuel displacement from PHEVs with varying all-electric ranges. Source: [Dowds et al., 2009] [7] = [EPRI/NRDC, 2007]; [11] = [Gonder et al., 2007]; [15] = [Kliesch and Langer, 2006]; [9] = [Letendre et al., 2008]; [12] = [Parks et al., 2007] [12](A) assumed that the PHEV charged once per day. [12] (B) assumed a PHEV charged whenever it was not in use. In scenario [12] (B), the PHEV is charged more frequently and a higher proportion of VMT is based on electricity, increasing the relative gasoline displacement.
Medium and long-term energy performances of PHEVs (2030-2050) have been analysed and compared with a reference vehicle. [MIT, 2008] estimated that in 2035, a PHEV50 would consume around 1.5 l/100km (TTW gasoline consumption only, electricity usage not included) which is roughly four times lower than a gasoline ICE at the same year and five times lower than a current gasoline ICE. The results of the EPRI study [EPRI/NRDC, 2007] are summarized in Table 6. Energy consumption 2010 Gasoline (l) Electricity (kWh) 2050 Gasoline (l) Electricity (kWh)
CV 1847
HEV 1198
1514
981
PHEV10 1049 467 859 382
PHEV20 609 1840 498 1504
PHEV40 406 2477 332 2024
Table 6: Energy consumption of different EDVs in 2010 and 2050 Source: adapted from [EPRI/NRDC, 2007] Annual mileage: 12000 miles (19312 km); converted into litres (1 gal = 3.78 l)
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4.3
How to measure the final energy consumption of PHEVs?
The ability of PHEVs to drive independently (or not) from electrical and/or chemical energy make their fuel efficiency assessment a complex task. As underlined by [Silva et al., 2009], there is no worldwide consensus on how to fairly calculate them. The SAEJ1711 recommended practice8 proposes a methodology for measuring the fuel consumption and standard emissions of PHEVs for different driving cycles. New improvements have been subsequently proposed by several research works [Silva et al., 2009; Gonder and Simpson, 2007]. In the following, the emphasis is put on key aspects for evaluating the energy efficiency of PHEVs. Influence of the CD mode energy strategy control •
CD all-electric
The CD all-electric mode means that no fuel is burnt over the first x kilometres driven on CD mode while it starts steadily increasing when the engine is turn on so as to achieve the energy performance of a HEV (CS mode). This results in an asymptotic behaviour, as shown in Figure 9. This is for instance the case of PHEV series (e.g. GM Chevrolet Volt).
Figure 9: Fuel consumption as a function of distance driven between two full charging events Source: [Shiau et al., 2009]
•
CD blended
The Argonne National Laboratory analysed the energy use of PHEVs [Elgowainy et al., 2009] based on different simulations9. They calculated the average fuel consumption of the engine in CD (blended) and CS modes based on weighting factors of 55% and 45% for the fuel consumption over an urban driving cycle (UDDS) and a highway driving cycle (HWFET) 8
SAE J1711-MAR1999., Recommended practice for measuring the exhaust emissions and fuel economy of hybrid electric vehicles, March 1999. 9 From the Powertrain System Analysis Toolkit (PSAT)
19
respectively. The results are presented in Table 7 and compared with the corresponding ICE and HEV as reference vehicles. AER 10 ICE refernce
7.26
HEV reference
AER 20
CD mode
CD mode
CS mode
AER 30 CD mode
CS mode
AER 40 CS mode
CD mode
CS mode
Motor
Engine
Engine
Motor
Engine
Engine
Motor
Engine
Engine
Motor
Engine
Engine
0.99
2.17
4.96
1.07
1.93
4.98
1.28
1.39
5.02
1.28
1.43
5.07
5.13
Table 7: Fuel consumption (l/km) of PHEVs over the CD (blended) and CS modes Source: [Elgowainy et al., 2009]
The average fuel consumption (in l/100km) can be derived for each PHEV configuration, from these figures. The resulting profiles are similar to those from Figure 9, at the exception that the CD mode is assumed here to operate in blended mode (i.e. not 100% electric, gasoline is also used to power the vehicle during the CD range). 8
7
Litres per 100 km
6
5
PHEV10
4
PHEV20 PHEV30
3
PHEV40 ICE
2
HEV
1 2
40
80
120
160
200
240
280
320
360
400
440
480
520
560
600
Distance (km)
Figure 10: Average fuel consumption (electricity excluded) as a function of distance (own calculations based on [Elgowainy et al., 2007])
Average daily distance travelled: Utility Factor The above estimations provide the fuel consumption as a function of the distance travelled. Realistic estimates of the average daily (or annual) fuel and electricity consumption by a PHEV will depend on the average daily distance travelled and on the likely distances driven in CD mode and CS mode respectively. For instance, [Silva et al., 2009] report the following average commuting distances: 30 km for the U.S., 5-30 km for Europe and 20 km for Japan. In their analysis, a 20 km average distance was assumed. Considering that around half the passenger cars in the U.S. drives less than 25 miles (40 km) a day and 80% less than 50 km (see e.g. [Sanna, 2005]), this distance could be easily covered by PHEVs running on the sole electric power (considering an AER of typically 30-100 km). They conclude that if the vehicle is daily recharged, large amount of fuel could be displaced annually. 20
Given a distribution of driven distances in a region considered, the share of kilometres travelled on all-electric over the year has to be determined to estimate the fuel displacement. The estimates made at national level in the literature introduce the Utility Factor (UF) which is defined as the ratio of the vehicle-km driven on all-electric over the total annual vehiclekm. UF =
VKTELEC VKTELEC + VKTFUEL
VKTELEC is the annual vehicle-km driven on electrical energy VKTFUEL is the annual vehicle-km driven on chemical energy The total fuel consumed (fuel equivalent) per year is then given by: FTOT = FCD * UF + FCS * (1 − UF )
where FCD and FCS are the fuel consumed during CD and CS operation modes respectively. Typically, the shortest the daily trips, the higher the utility factor will be since most of the distance will be driven on electric mode (depending of course on the AER capability of the PHEV). The typical utility factor in U.S. is estimated to be around 50% for a PHEV with a 50 km AER (see e.g. [MIT, 2008]). [EPRI/NRDC, 2007] assumes the utility factors to be 12%, 49% and 66% respectively for PHEV10, PHEV20 and PHEV40 (in miles). An example of UF curve is given in Figure 11 for the U.S. [Elgowainy et al., 2009]. These indicative UF could be used as a first estimate for Europe but EU specific estimations should be made later. A first indication to this end relates to the shares of the different road modes in the annual distance driven. The assumptions in TREMOVE for the year 2005 are as follows: 19%, 58% and 23% of vkm are driven on respectively urban motorways, non urban roads and urban roads. If urban and non urban roads are assumed to be linked to short distance trip, it can be concluded that ~80% of the total distance driven consist in short distance trips.
Figure 11: Typical Utility Factor curve used for the U.S. [Elgowainy et al., 2009]
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Influence of charging patterns As seen before, the charging pattern of PHEV users is of high importance. Questions are how often, where and at what time of the day would the battery be recharged? These factors introduce a non-negligible level of complexity for assessing the energy performance of PHEVs. Many studies assume that the vehicle will be recharged at home every day, mainly overnight. But such an assumption could significantly distort the results, and, possibly overestimate the benefits of PHEVs.
4.4
Energy performance of EDVs: first estimations
A simplified model has been elaborated in order to derive first estimates of the total energy consumption (electric and chemical energy) of different vehicles, and to provide a comparison with the current new conventional cars purchased in Europe. The following describes first, the reference cars considered, and then, the assumptions made to calculate the energy consumption of the corresponding PHEVs. Three AER values are considered, respectively 20, 40, and 60 miles. The estimates correspond to "type-approval values". In the following, the focus is on the medium-size car segment, which in terms of typical conventional car engine size is in the range 1400-2000 cc. However some background information about other vehicle size segments is also referred to (reference cars). This leaves the possibility to later extend the estimates for the other relevant segments. This should cover the upper segment (>2000 cc). On the other hand, one could wonder whether if PHEV will significantly penetrate the market in the lower segment (
