Tmnspn Res..A Vol 14A, pp. 415421 Pergamon Press Ltd.. 1980. Printed in Great Britain
ENERGY USE OF ELECTRIC VEHICLES? WILLIAM HAMILTON General Research Corporation, Santa Barbara, CA 93111,USA Abstract-Although electric vehicles are not more energy-efficient than conventional vehicles (of comparable performance), they do offer two substantial possibilities for conserving fossil fuels. First, much of the electricity used for recharging electric cars will be generated from non-petroleum sources. Second, if we consider the tradeoff between using coal to produce synfuels for conventional cars, or using the coal to produce electricity for electric cars, there is substantially greater transformation-efficiency in the production of electricity. Regarding the first point, projections of fuel use by U.S. electric utilities indicate that even for total electrification of light-duty vehicles, less than 25% of recharge power would be generated from oil in the 1990s.In many areas of the country, little or no petroleum would be used to generate recharge power. Thus the potential for petroleumconservationthrough vehicularelectrificationis immense.Regardingthe second point, cars powered by coal-produced electricity would require about 40%less coal than cars powered by coal-produced synfuels, because of the relatively low efficiency with which gasoline can be synthesized from coal. INTRODUCTION
Taken together, passenger cars and light trucks account for well over one third of the consumption of refined petroleum products in the U.S.-a little under 30% for cars, about 9% for trucks. Hence the overall benefit from reducing the petroleum consumption of these vehicles would be substantial and important. In this respect electric vehicles have great promise. They use no petroleum fuels directly, and the electricity to recharge them may be produced from coal, nuclear energy, geothermal energy, or any of the options available to electric utilities. On the other hand, electric utilities in the United States now rely on petroleum fuels to a substantial extent. Thus the petroleum savings which might result from electrification of cars and light trucks depends on the facilities available at electric utilities as well as on the fuel use of conventional vehicles, both of which are changing. This paper investigates the overall effects of vehicular electrification on energy use. First, it considers the simplified case in which all energy requirements are met from either oil or coal. This amounts to a comparison of the energy efficiencies of electric and conventional vehicles, including all inputs and losses between the primary resource and its eventual highway use. Then the paper estimates potential petroleum savings which might accrue in actual use, as conventional vehicles are replaced by electric vehicles recharged from the fuels and facilities which will be available at electric utilities. The paper is based on studies performed by General Research Corporation for the Office of Transportation Programs, U.S. Department of Energy, in 197679. The conservation of energy and petroleum by vehicular electrification is not a new subject. Especially in recent years, there has been great attention given to all aspects of electric vehicles. As one observer noted, there has been ‘I.. . a fantastic number of technical papers. .
In fact, it is almost impossible to say anything on the subject which has not already been printed” (Starkman, 1968). Previous investigations of energy and petroleum conservation, however, have not always been either complete or equitable. The object of the analysis discussed here has been to be thorough and comprehensive, assuming levels of performance and technology for electric and conventional vehicles which are as nearly equal as possible. Furthermore, published plans rather than hypothetical scenarios have been used in projecting the sources of the recharge energy which might be generated by U.S. electric utilities. OVERALLENERGYUSE
Coal and oil are the primary resources from which energy for motor vehicles will be derived in the remainder of this century. Gasoline has so far come almost entirely from petroleum, but production of synthetic gasoline from coal is being proposed for an important role in the future. Electricity to recharge electric vehicles can already be generated efficiently from either coal or oil. Table 1 shows overall energy requirements for electric and conventional vehicles in the simple case where all energy is derived from a single primary resource, either oil or coal. The energy required from the primary resource takes into account all energy inputs and losses in producing and transporting the fuels ultimately used by the vehicles. The fuel uses for the electric vehicles were estimated using a standard electric vehicle simulation, ELVEC, described below. The fuel uses for the conventional vehicles were estimated from formulas developed by the Interagency Task Force on Motor Vehicle Goals Beyond 1980 (U.S. Department of Transportation, 1976). The ELVEC simulation has been under continuous development since early 1976. It was originally constructed to estimate the range and energy use of future tThis research was supported under contract NO. DE-AC03- electric cars in a study of the impacts of future vehicular 76-CS51180from the U.S. Department of Energy. electrification (Hamilton, 1980). ELVEC calculates the 415
WILLIAM HAMILTON Table
from primary resources for urban operation of projected
Compact Pickup Truck Electric
Fuel Required Electricity, MJ/km
Energy Required from Primary source
Electric and ICE vehicles have equal payload volume, equal test payloads (136 kg). equal maximum payload capability (408 kg for cars, 10130kg for trucks), end equal acceleration with 136-kg test payload (O-64 km/s in 10 6).
Electric vehicles have representative future batteries and urban ranges of 160 km (on SAE 5227(a). Schedule D).
ICE vehicle fuel economies are estimated for the Federal urban Driving Cycle.
road load, drive train efficiency, and battery output
power required by a given car in a given driving cycle, integrating equations of motion for the vehicle to determine energy use. A battery discharge model is then employed to estimate driving range. The original version of ELVEC was subsequently improved and used to develop standards for electric and hybrid vehicles (Brennand, 1977).By the end of 1979it had been expanded to cover not only electric cars and light trucks, but various hybrid-electric configurations and conventional internal-combustion vehicles as well. It offers five alternative models of battery discharge and various levels of detail for each component of the propulsion system, depending on the level of information available. During the past three years, its development has been supported by the Jet Propulsion Laboratory, which makes ELVEC available nation-wide on a computer time-share system (Jordan, 1979). The vehicles implicit in Table I are intended to be as nearly comparable as possible. The electric vehicles differ only in their limited range from their internalcombustion-engine (ICE) counterparts. Both electric and conventional cars are subcompacts with weight-conscious design. They offer interior accommodations like those of the VW Rabbit and a relatively modest acceleration capability (O-64kmlhr or O-40 mph in IOset), the minimum considered acceptable for safe entry into freeways. The pickup trucks are compact in size, offer payload capability in the popular “one-ton” class, and again offer minimum acceleration capability. The ICE car achieves I4 km/l (33 mpg) in urban driving, about midway between the fuel economies already being delivered by the gasoline and diesel versions of the VW Rabbit. The ICE truck would achieve about I I.6 km/l (27.2 mpg) in urban driving. The overall energy requirements of the electric and ICE vehicles in Table I are about equal if provided from petroleum. If provided from coal, the overall energy requirements of the electric vehicles are much less, about 40% below those of the ICE vehicles. This is due
to the relative inefficiency of converting coal into gasoline. In Table 1, the following efficiencies are assumed for producing fuels from primary resources: Crude oil to gasoline Crude oil to electricity Coal to gasoline Coal to electricity
8% 28% 55% 30%.
These efficiencies are based on an analysis by SRI International (Hughes, 1976) which is summarized in Figs. I and 2. These figures show the efficiencies of the various steps in preparing petroleum and coal for automotive use, as well as additional energy required for transportation and processing (indicated in the triangles in the figures). The overall efficiency for producing synthetic gasoline in Fig. 2 is only an estimate. As improved processes are developed and proven by experience, higher efficiencies may result. On the other hand, substantial improvements in the efficiency of producing electricity from coal are also possible. Most of the energy in the coal resource is lost at the electric power plant, with typical present-day efficiency of 35%. With increasing fuel costs and the advent of improved technologies, electric utilities may build new facilities using combined-cycle or even magnetohydrodymanic generators with efficiencies of 40-50% or more. Construction of new electric plants is so difficult and expensive that they may not appear very rapidly, however. For the same reasons, the new mines and processing plants required for producing synthetic gasoline from coal may be equally slow in coming. The total energy required from primary sources in Table 1 also depends directly on the fuel requirements estimated for each vehicle. For the electric vehicles, the major determinants of fuel use are illustrated in Figs. 3-5. These figures are based on additional runs of the ELVEC simulator with inputs from a supporting model of vehicle weights, EVWAC (see Curtis, 1979).
r-, ELECTRICITY , AVAILABLE FOR , ELECTRIC CAR
TRANSMISSION LINE EFF = 90%
OIL-FIRED POWER PLANT EFF = 35%
TRANSPORT (PIPELINE) EFF = 100%
REFINERY EFF = 93%
Fig. 1. Energy flows in preparing petroleum for automotive
DISTRIBUTION/ STORAGE EFF = 99.5%
REFINERY EFF = 91%
TRANSPORT (PIPELINE) EFF = 100%
TRANSPORT (PIPELINE) EFF = 100%
1 1 ,
PETROLEUM REFINERY EFF = 91%
743 MJ TRANSPORT _ (PIPELINE) EFF = 100%
Fig. 2. Energy flows in preparing coal for automotive use
DISTRIBUTION AND STORAGE (PIPELINETTANK TRUCK/ SERVICE STATION) EFF = 99.5%
URBAN RANGE, km
Fig. 3. Effect
of design range on energy
Figure 3 shows the strong dependence of electric vehicle energy requirements on design range which in turn, depends basically on the fraction of vehicle weight devoted to battery. The points on the curves in Fig. 3 correspond to battery weights of 25, 30, 35 and 40% of
vehicle test weight (curb weight plus a nominal 136kg, about the weight of two passengers). The curves show energy versus range for two different projected propulsion batteries, both considerably improved over the best battery available today. Today’s best golf-car batteries provide about 0.1 MJ of output energy per kilogram of battery. The improved lead-acid battery in Fig. 3 produces about 0.17 MJ/kg, while the nickel-zinc battery produces 0.28 MJ/kg. Batteries with these levels of performance may result within a few years from development programs sponsored by the U.S. Department of Energy (see Hamilton, 1980). The recharge energy requirement in Table 1, 0.8MJ/km, is about halfway between the requirements in Fig. 3 for the lead-acid and nickel-zinc versions of the car with 160km range. Electric vehicle energy requirements also depend on the efficiencies of the electric propulsion system and the road load which must be overcome. These are illustrated in Fig. 4 for the lead-acid battery car with battery fraction of 30%. For simplicity, Fig. 4 illustrates the case in which regenerative braking is not used. An urban driving cycle with energy requirements like the Federal Urban Driving Cysle is assumed (Schedule D of SAE Recommended Procedure 227(a)). The efficiency and road load -1 I
ELECTROMECHANICAL LOSSES (0.15)
ROAD LOAD (0.36)
Fig. 4. Energy Rows and efficiencies in a representative
future electric car
Energy use of electric
Fig. 5. Weights of subcompact
estimates are relatively optimistic. The only substantial energy loss which can be avoided is the 17% of input energy dissipated in friction brakes during the frequent stops in the driving cycle. With regenerative braking, some of this energy could be returned via the drive train to the battery. This would reduce energy consumption about 15%, to the level shown in Table 1 and Fig. 3. The road load in Fig. 4 is principally due to the rolling resistance of the tires, since a low aerodynamic drag coefficient of 0.35 is assumed and the driving cycle involves a low maximum speed. Efficient radial tires are implicit in Fig. 4, with a rolling resistance which is 1.2% of vehicle weight. Because tire rolling resistance is proportional to weight, vehicle weight is the fundamental determinant of overall energy use. Weight breakdowns for the electric and conventional cars of Table 1 are shown in Fig. 5. Upper body weights of the two cars are the same because they offer equal accommodations for passengers. Nevertheless, the electric car is 60% heavier than the ICE car, primarily because of the weight of the battery, plus the additional structure and chassis weight required to support the battery. About a third of the total propulsion energy is required solely to transport the battery weight, which is about 500 kg, over 30% of vehicle curb weight. This battery weight suffices for a range between recharges of about 150km with improved lead-acid batteries, or 250 km with the advanced nickel-zinc batteries. Reducing the size of the battery would reduce energy use but is probably undesirable because it would further limit the utility of the electric vehicles. Long inter-city trips, which account for some lO-15% of U.S. auto travel, are beyond the reach of all foreseeable electric cars. Even in purely urban driving, without intercity travel, distances exceeding the ranges of the electric car in Fig. 5 are occasionally required. In Los Angeles, for example, the average driver travels over 150km within the metropolitan area on one day out of every 20, and daily travel distances in
many other areas appear to be equally large (Hamilton, 1980). The comparisons of Table 1 are limited because they assume energy for both conventional and electric vehicles comes from a single primary resource, coal or oil. They are directly useful, nonetheless, in important cases. They show, for example, that vehicle electrification would make far better use of coal resources than production of synthetic fuels for conventional vehicles. They also show that in areas relying solely on oil for electric power generation-such as Honolulu-vehicular electrification would not save petroleum. In general, electric utilities in the U.S. derive most electric power from resources other than petroleum, while most U.S. fuel for motor vehicles will be refined from natural petroleum for the remainder of this century. The effect of vehicle electrification on use of petroleum in this practical case is examined next. PETROLEUM CONSERVATION
At many utilities, where coal, nuclear and hydroelectric generating stations meet all needs, no petroleum would be used to generate recharge energy for electric vehicles. In areas served by these utilities, savings of petroleum through vehicular electrification could be complete. In some areas of the U.S., however, utilities derive at least a part of their electric power from petroleum. In these areas, savings of petroleum through electrification would be substantially less. The actual use of basic resources in the U.S. to produce electricity in 1976 was as follows (U.S. Bureau of the Census, 1977): -Nuclear 9.4%
-Coal - Oil - Gas -Hydro 46.5% 15.7% 14.4% 14.1%
Had electric vehicles been recharged in large numbers, however, petroleum use would have been considerably larger than in this year-long average. The reason for this
is that utilities ordinarily avoid burning petroleum because of its scarcity and expense. During the night, when demand for electricity may fall to half the lateafternoon level. oil-fired facilities are the first to be shut down. If electric cars were to increase late-night demand, of course, these idle oil-fired facilities would be restarted to generate the necessary electricity. This overall picture of the U.S. is, of course, complicated by great differences among regions. Many utilities burn virtually no petroleum, as in the Midwest or the Great Plains. Other utilities rely heavily on petroleum, as in New England, Southern California and Hawaii. To obtain a national average for petroleum use to recharge electric vehicles, each individual utility company must be considered since each has now, and will have in the future, a different mix of facilities and fuels available. Demand for electricity also varies from season to season, from utility to utility, and from day to day. This demand depends on climate, on industrial energy use, and on other factors which vary from time to time and place to place. For a given utility, summer peak demands may require generating electricity from oil in the late afternoon and early evening, while lesser spring peaks might not. Late at night, winter heating demand at some utilities may leave little non-petroleum capacity for re-charging electric vehicles, while in spring and fall much more non-petroleum capacity might be available. To deal with these complexities, a computer model RECAPS was developed (Hamilton, 1980)that treats 228 large utilities which collectively provide about 98% of all generating capacity in the contiguous United States. Inputs to the model include descriptions of each generating unit at each power station in each utility, at present and as planned for the next ten years, plus the regional projections of capacity by fuel type made by the National Electric Reliability Council for 10 to 20 years in the future. Input also includes recent demand, hour-byhour for an entire year, for about 50 separate small groups of utilities. The model extrapolates demand to future years, with and without electric car recharging each night, and then estimates the hourly use of fuel to meet that demand, assuming that petroleum facilities are operated as little as possible. An analysis and projection using the RECAPS model of electric vehicle recharging was completed in 1978, with projections for the future based on 1976data, trends and plans (Hamilton, 1980). The projection is now several years old and appears to call for more rapid growth in electricity production, and more rapid expansion of nuclear capacity, than presently seems likely. Accordingly, the picture it gives of the potential for electric vehicles is probably optimistic, but no more recent analysis is available. The projection may also be optimistic in its basic assumption: that electric vehicles will be recharged late at night to the maximum extent possible. At present, few U.S. utilities offer incentives to late-night recharging. Lacking such incentives, motorists are likely to begin recharging when they return home at the end of the day, before leaving their garages or carports. This would add to peak loads borne by utilities, and require maximum
use of petroleum for generating recharge power. Before electric cars are widely used, however, it appears likely that utilities will be able to offer peak and off-peak prices making late-night charging much more economical. In addition, there may be reduced rates for interruptable loads, such as battery chargers which the utility may shut off by remote control in case of excessive demand. Figure 6 shows the projected sources of recharge energy for electric car usage ranging from zero to 100% in the year 1990. The projection assumed that electric cars would be charged at the most favorable hours for minimizing petroleum consumption, i.e. overnight. It also assumed that utilities would use available coal and nuclear capacity wherever possible before turning to oil. The figure also includes a scale for electric light trucks. Electrification of all such trucks would require about as much recharge energy as electrifying 42% of passenger cars. Since fuel use for recharging trucks and cars would be interdependent, the curves in Fig. 6 apply to one or the other, not both at once. Given 1976 plans and trends, the 1990 generating capacity which would be available at off-peak hours is about sufficient for electrification of all passenger cars in the United States. With slower growth of electric utilities, fewer electric cars could be recharged; but in any case, the number which could be accommodated seems certain to be far greater than the number which could actually be built and put into operation. Even a very rapid conversion of most U.S. motor vehicles to electric propulsion would require several decades. At levels of electric vehicle use in Fig. 6 in the range 20-30%, the sources for recharge energy would be as follows: Nuclear --17%
Coal Oil Gas and Other 51% 26% 6%
This applies to the case where electric vehicles are
PERCENT CAR TRAVEL BY ELECTRIC CARS
50 75 100 PERCENT LIGHTTRUCK
TRAVEL BY ELECTRIC TRUCKS
Fig. 6. Sources of rechargeenergy vs use of electric vehicles, 1990.
Energy use of electric vehicles distributed uniformly in the contiguous United States. A
much greater reduction in petroleum consumption could be achieved by distributing electric vehicles first to the regions with the least reliance on petroleum-produced electric power. It appears that through this strategy, over 35% of U.S. light-duty vehicles could be electrified in the 1990s with no use of petroleum at all. In later years, when utilities expect to have added more coal and nuclear generating units, this percentage would be even higher. The overall conclusions are simple. Given recent trends and plans for electric utility demand and supply, future electric vehicles can reduce petroleum consumption 75% or more for each unit of conventional vehicular travel they displace. In many areas, savings could be nearly 100%. The potential of electric vehicles for conserving petroleum, then, is immense. If this potential is to be realized, large-scale sale and use of electric vehicles will be necessary. Even with improved technology, however, electric vehicles will continue to cost more than conventional vehicles, and they will continue to do less because of their limited range. Unless gasoline becomes much more expensive, or more difficult to obtain in relation to electricity, most drivers will probably continue to purchase conventional vehicles even after electric vehicles are available. The range limitation of electric vehicles is probably their most serious drawback since their substitution for conventional vehicles would substantially curtail travel and mobility. The hybrid-electric vehicle, however, is a promising development which may provide unlimited range without sacrificing the benefits of electrification. The hybrid-electric vehicle with greatest potential for petroleum conservation is basically an electric vehicle to which a small (roughly 25 horsepower) internal-combustion engine has been added. For driving within the electric range of the vehicle, the internal-combustion engine would not be operated at all. In most urban driving, then, this hybrid would operate exactly as would an all-electric vehicle, with the attendant benefits for petroleum conservation, air pollution, and so on. For long-distance highway travel, however, the internalcombustion engine would provide the same range capability and quick refueling offered by conventional vehicles. In this way, petroleum fuel would be required
only for travel exceeding the capability of the vehicle on batteries alone. Such hybrids offer a practical means for electrifying the preponderance of vehicular travel, which involves short-range trips, without any attendent sacrifice of longdistance travel or mobility. Furthermore, these hybrids may be no more expensive than comparable electric cars: because very long electric range is no longer required, the size and cost of the propulsion battery may be reduced sufficiently to offset the cost of the internalcombustion engine (Hamilton and Curtis, 1979). Due to their unlimited range, the hybrids will probably capture a much larger share of the U.S. auto market than electric cars (Morton, 1978). Thus the hybrids may be much more important than pure electric cars in conserving petroleum. RJXFERENCES
BrennandJ., CurtisR.,Fox H. and HamiltonW.(1977)Electric and hybrid vehicle performance and design goal determination study. SAN-1215-1, Energy Research and Development Administration, Washington, D.C. Curtis R. L. (1979) Electric vehicle weight and cost model (EVWAC). General Research Corporation IM-2202, Santa Barbara, California. Hamilton W. (1980) Electric Automobiles. p. 31. McGraw-Hill, New York. Hamilton W. and Curtis R. (1979) Projected Characteristics of Hybrid-Electric Cars. General Research Corporation IM-2200, Santa Barbara. Calfiornia. Hughes E. E., Walton B. L., Newgard P. L., Ryan I. W., Steele R. V., Halton P. M., Kohan S. M., Kouelman J. B. and Oliver E. D. (1976)Long term energy alternativds for automotive propulsion. Center for Resourceand EnvironmentalSystemsStudies,Rep. No. 5, Stanford Research Institute, Palo Alto, California. Jordan D. (1979) General Research Corporation Electric/Hybrid Vehicle Simulation Program (ELVECJ User’s Manual. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California. Morton A. S., Metcalf E. I., Strong S. T. and Venable A. (1978) Incentives and acceptance of electric, hybrid and other alternative vehicles. Arthur D. Little, Inc., Cambridge, Mass. Starkman E. S. (1%8) Prospects of electric power for vehicles. SAE Paper 680541, Society of Automotive Engineers, West Coast Meeting, San Francisco, California. U.S. Bureau of the Census (1977) Statistical Abstract of the United States: 1977, p. 599. U.S. Government Printing Office. U.S. Department of Transportation (1976)Report of a Panel of the Interagency Task Force on Motor Vehicle Goals Beyond 1980,Automotioe Design Analysis. Table B-I.