Thermodynamics and energy usage of electric vehicles

Thermodynamics and energy usage of electric vehicles

Energy Conversion and Management 203 (2020) 112246 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

454KB Sizes 0 Downloads 69 Views

Energy Conversion and Management 203 (2020) 112246

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Thermodynamics and energy usage of electric vehicles

T

Efstathios E. Michaelides Dept. of Engineering, TCU, Fort Worth, TX 76132, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Electric vehicles Batteries Exergy Transportation Carbon dioxide emissions

The global number of electric vehicles is exponentially rising, due to strong marketing efforts and governmental incentives that significantly lower the price of new vehicles. This shift of consumers from internal combustion engine vehicles to electric vehicles is actually a shift from petroleum to the primary sources that generate electricity. The motivation for this paper is the holistic analysis of electric vehicles and the determination of their benefits and detriments. Starting with an exergetic assessment for all road vehicles, this paper determines the electricity needed for the propulsion of electric vehicles. When the heating and cooling requirements of the vehicle’s cabin are included, the range of the vehicles decreases significantly. The electricity requirements of the vehicles are abridged to primary energy sources using the concept of well-to-wheels efficiency. Based on the regional mix of electricity generation, the effect of the shift to electric vehicles on greenhouse gas emissions is determined. Because the charging of the batteries of electric vehicles requires significant power, it was concluded that the simultaneously charging of a number of vehicles will strain the capacity of the electricity grid. The paper also examines the effects of electric vehicles on the further utilization of renewable energy sources.

1. Introduction The use of electricity for passenger transportation is not new. Its first known application was in 1839, when sir William Grove invented the first fuel cell and demonstrated its use in a small tractor. Centrally generated electricity has been successfully used for more than a century in rail transport – primarily in underground (subways) and aboveground railways (intercity trains) as well as in the trolleys and trams of several cities. In the 21st century environmental concerns with CO2 (carbon dioxide) emissions and other combustion pollutants, in combination with improved battery technology have ushered the new era of electric battery-powered vehicles. Governmental incentives in several countries (tax credits and rebates) have been adopted to promote the substitution of internal combustion (IC) engine passenger cars with electric vehicles (EVs). The incentives are most generous in countries with high renewable energy availability, such as Norway [1]. In parallel a network of publicly available EV charging stations is rapidly being constructed in most countries: while in 2008 there were only 47 charging stations, in 2018 this number mushroomed to 143,502 [1]. EVs are often portrayed as a panacea that has very high energy efficiency; will alleviate the global dependence on energy; will significantly reduce the emissions of the greenhouse gases (GHG) that contribute to the Global Climate Change (GCC); and would lead the way to sustainable development. While most of the information on EVs is derived from commercial and marketing publications, there are few

scientific and engineering studies that document the energy requirements of EVs, their environmental impact, and their effects on the regional electricity grids. Among the few scientific assessments of EVs, a recent study presented a methodology to estimate the energy use in EVs, based on data measured with an on-board portable laboratory [2]. The methodology, which is based on real-world data, has the potential to establish needed benchmarks for the performance of EVs that are free of untested commercial hyperbolae. The effects of EVs on electricity usage and the electric power grids were examined in simulations [3] that proposed a parallel optimization framework as a power-demand-unit-commitment problem. The study concluded that, if the charging of the EVs from fossil fuel sources is optimized, their proliferation will significantly benefit the efficiency of energy use (the capacity factor of the units) and will reduce the fossil fuel cost. A more recent study [4] looked at the impact of EVs on the energy mix of countries and the electricity grids and determined that the widespread adoption of EVs requires that strategic decisions be made on the development of sufficient generating capacity and the distribution networks. The pollution reduction benefits of EVs in urban settings and the replacement of urban pollution with (perhaps lesser) pollution in the vicinity of electricity generation units has been the subject of another study [5], which also examined policies to encourage the adoption of EVs as the primary means of urban transportation. The environmental effects of EVs in general and the increase of a country’s vehicle fleet fuel

E-mail address: [email protected] https://doi.org/10.1016/j.enconman.2019.112246 Received 21 August 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

early 20th Century. Based on the 2019 data and the uses of petroleum products, it was estimated that road vehicles emitted 31% of the global CO2 emissions, the main constituent of GHGs [14]. The total force, FT, which keeps a vehicle in motion on any type of terrain, is the sum of the friction forces (aerodynamic and road friction), the gravitational force (when the motion is on an inclined terrain) and the acceleration (when the vehicle velocity varies) [15]:

economy are recognized in [6]. This study also warns that encouraging consumers, who would otherwise purchase other types of fuel-efficient vehicle, to purchase EVs is not advisable as a national policy, because it does not lead to significant GHG emissions reduction. Another recent study examined the ecological effects of the batteries used in EVs and presented a more skeptical opinion based on the availability of rare earth metals and the heavy metals used in all the batteries [7]. A position paper summarized the costs and benefits of the high penetration of EVs in the national markets and outlined the effects this would have on electricity generation corporations and the needed changes in the infrastructure and the regulatory environment and public policy [8]. Several studies have also examined the energy usage of EVs. A rather general study [9] advocated the use of the exergy method (instead of an energy balance method) to improve the efficiency of several electric power technologies, including that of urban EVs. A subsequent study used the exergy method to analyze the thermal management system of EVs and hybrid cars (before the thermal engine engages). The exergy destruction and exergetic efficiency of several parts of EVs were to determine high irreversibility components that may be improved [10]. The thermal management system of EVs was also analyzed more recently using an exergy analysis [11] and recommended the replacement of resistance (called “positive temperature coefficient”) heaters heat pumps that have higher coefficient of performance. The thermal management of the cabin of EVs (for both heating and cooling) is the subject of another recent study [12] suggested the adoption of high efficiency Heating Ventilation and Air-Conditioning (HVAC) systems with a Tesla turbine1 replacing the expansion valve. This turbine, which will add to the complexity and cost of the system, recovers part of the exergy lost by the working fluid during the isenthalpic expansion process. While previous studies examined particular aspects of EVs (the HVAC system, the environmental effects, etc.) there has not been a study that examines in a holistic way the total energy usage of EVs, starting with the question: “Where is the energy (of fuels or batteries) used or dissipated?” This paper aims at a holistic analysis of the EVs from a thermodynamics point of view and starts with the friction/resistance forces that must be overcome by all road vehicles. The rate of exergy dissipation for road vehicles is first calculated and this yields the minimum amount of mechanical energy required for the operation of vehicles. Since EVs operate with electric energy, a tertiary form of energy, the well-to-wheels efficiency is introduced as the figure of merit of EVs that connects the exergy dissipation in all vehicles to the primary energy source consumption. Based on the mix of the primary energy sources used for the generation of electricity in several countries, the CO2 emission reductions are calculated when IC-powered vehicles are converted to EVs. Finally, the effects of charging a fleet of EVs on the electric power grid and the possible use of a large number of EV batteries for utility-level energy storage are examined.

FT = FF + mg sin(ϕ) + m

dV , dt

(1)

where FF is the friction/resistance force; m is the mass of the vehicle; g is the gravitational acceleration; ϕ is the angle of the road with the horizontal – the “grade” of the terrain; and V is the velocity of the vehicle. The last two terms correspond to the changes in the potential and kinetic energy of the vehicle and are conservative forces. The total power supplied by the engine of the vehicle is [16]:

dV Ẇ = FT V = ⎛FF + mg sin(ϕ) + m ⎞ V dt ⎠ ⎝

(2)

In principle, the reversible operation of vehicles would recover all their kinetic and potential energy when they complete a circuit and return to their original coordinates (a round trip). Modern vehicles equipped with regenerative brakes (all hybrid vehicles and most EVs have this system) recoup part of the potential and kinetic energy when they complete a circuit and come to a stop. For this reason, this study will concentrate on the non-conservative force FF, which is dissipative and is imposed on the movement of the vehicles by the terrestrial environment. All the power and energy associated with this force are dissipated in the environment. When vehicles are cruising at constant speed on level roads (ϕ = 0, dV/dt = 0), the mechanical power consumed is spent to overcome the rolling friction with the ground and the air drag of the vehicles. These two resistance forces are modeled as follows [15, [17]:

FF = FR + FD = CR mg +

1 CD ρAV 2, 2

(3)

where CR and CD are the rolling friction and the aerodynamic drag coefficients respectively; ρ is the air density; and A is the cross-sectional area of the vehicle. The rolling friction coefficient depends on the surface of the road and the tires of the vehicle [15]. The drag coefficient is a function of the Reynolds number of the moving vehicle [17]. Typical ranges of the two coefficients for cars are 1–2% for the rolling friction and 30–60% for the aerodynamic drag coefficient. Because the two forces are resistance forces, all the power consumed by the vehicles during round trips is dissipated as heat in the environment. The exergy dissipation (work lost, Ẇlost ) and the entropy production, Θ̇, during cruising are given by the expression [16]:

1 Ẇlost = T0 Θ̇ = V ⎛CR mg + CD ρAV 2⎞ 2 ⎠ ⎝

2. Power requirements for road transportation – exergy destruction

(4)

Eq. (4) shows that the power dissipation of vehicles is a monotonically increasing function of their speed. Fig. 1 depicts this relationship for a sedan-style road vehicle that weighs 1500 kg and has a frontal area 1.75 m2. For the calculations the rolling coefficient was constant at 0.018 and – because the flow becomes turbulent at this range of vehicle velocities [18] – the aerodynamic drag coefficient was also constant at 0.45. Vehicles powered by IC engines are subjected to Carnot limitations and, typically, have lower thermodynamic efficiencies – in the range 10–30% with the efficiency of some hybrid vehicles being as high as 45% [19,20]. The efficiency of the vehicle depends on the type of the vehicle (the so called “sports cars” have very low overall efficiencies). Fig. 2 depicts the effect of the cruising speed on the fuel consumption and the time of arrival for a road vehicle with the characteristics of that in Fig. 1. The overall thermal efficiency of the IC engine is taken to be constant at 21%, a typical value of mid-size sedan

The global transportation sector consumes approximately 36% of the Total Primary Energy Sources (TPESs) and is dominated by the road vehicles – primarily passenger cars and freight trucks. Road vehicles account for 89% of the global transportation energy use; air transport accounts for 7%; and water and rail transport together account for approximately 4% [13,14]. Liquid hydrocarbons (primarily gasoline and diesel) supply most of the energy used in the transportation sector. From the environmental point of view, the number and usage of passenger cars and freight trucks has been steadily increasing since the 1 A centripetal flow turbine without blades that was patented by Nikola Tesla in 1913. The patent has expired and the design of the turbine is now in the public domain.

2

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Fig. 1. Power requirements of a cruising vehicle as a function of the cruising velocity.

Fig. 2. Fuel consumption and travel time of a vehicle as a function of the cruising speed.

of the EV during a round trip is:

cars [19,20] that also includes the transmission losses. It is observed that the maximum fuel consumption drops from approximately 26 km/ L when it travels at 10 km/hr to 6.6 km/L when it cruises at 150 km/hr. It is also observed that the time to travel 1 km, which is related to the expected time of arrival at the destination, drops significantly. Despite the higher fuel consumption at lower speeds, the time to travel one km (close to 5 min at the optimum speed) is very long and inconvenient for most vehicle operators who will choose higher speeds for the convenience of earlier arrival.

̇ 1 ̇ + Q, E ̇ = V ⎛CR mg + CD ρAV 2⎞ + Weq 2 β ⎝ ⎠

(5)

̇ is the power needed for lights, instruments and auxiliary where Weq equipment; Q̇ is the absolute value of heat added to the interior of the vehicle by the heater (or removed by the air-conditioner in the summer); and β is the coefficient of performance of the heating/cooling ̇ is typically 50–70 W [21] when the vehicle equipment. The power Weq engine produces 8000–20,000 W while cruising at speeds 60–100 km/ ̇ is at least two orders of magnitude lower than the hr (Fig. 2). Since Weq other exergy requirements it may be neglected. When the ambient temperature is low, the rate of heat transfer and the additional power required for the heating of the vehicle are significant. The absolute value of the rate of heat that enters or leaves any segment of the vehicle may be calculated from the equation [22]:

3. Power requirements for electric vehicles Other expenditures of engine power are vehicle acceleration; changes in altitude; power for instruments, lights, and other equipment, such as air-circulation fans; and heating and air-conditioning. The first two are included in Eq. (2) and the last ones need to be modeled. In conventional IC vehicles heating is provided by the waste heat of the engine. This rate of heat is plentiful, always sufficient to keep the interior of the vehicle at a comfortable temperature, and does not burden the fuel requirements of the vehicle. The EVs lack a thermal engine that produces waste heat and all the heat for the cabin must be supplied by the stored energy in the battery. The most efficient method to supply the needed heat for the cabin (and for the battery itself if this is needed) is with the use of a heat pump, which doubles as air-conditioner in the summer. In this case the total rate of exergy, necessary for the operation

Qi̇ = UAi |Tin − T0|,

(6) th

where Ai is the area of the i segment; Tin is the desired cabin temperature; T0 is the outside (ambient) temperature; and the overall heat transfer coefficient, U, is given by the expression:

1 1 Δx 1 = + + , U hin k hout

(7)

where hin is the inside heat transfer coefficient, typically due to natural 3

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Fig. 3. Energy requirements for a cruising EV when ΔT = 25 K for different coefficients of performance of the heating system.

significant range improvement is achieved when a heat pump with β = 3 is used for the heating instead of resistance heating (β = 1).

convection; hout is the outside heat transfer coefficient, due to forced convection because the vehicle is moving in the ambient air, and is a strong function of the vehicle velocity; Δx is the thickness of the material (glass, metal, composite, etc.) that comprises the vehicle segment and k is the thermal conductivity of this material. The total rate of heat that enters or leaves the vehicle is obtained by summing over all the segments of the vehicle that are exposed to the ambient air. Calculations were performed for the energy requirements of a typical sedan-style EV with the characteristics of the vehicle in Figs. 1 and 2, when the difference between the interior cabin and ambient is 25 K. The results of the calculations are shown in Fig. 3, which depicts the power requirements when zero heat is supplied (Q = 0) and also when heat is supplied by resistance heating (β = 1) and by a heat pump (β = 2 and β = 3). It is observed in this figure that the addition of heat consumes a significant fraction of the stored electric energy, especially when resistance heating is used for the interior of the cabin. At the typical cruising speed of 100 km/hr, the additional heat is 37.5% of the propulsion energy when resistance heating is used (β = 1) and 12.5% when a heat pump with coefficient of performance β = 3 is used.2 For the convenience of travelers and the improvement of the vehicle range, it becomes apparent that the use of heat pumps is the most desirable method for EVs. When a relatively efficient heat pump with β = 3 is used, Fig. 4 depicts the absolute value of the additional power the battery must supply as a function of the cruising speed. The temperature difference between the interior cabin and the ambient is the parameter in this figure. It is observed that the heating and cooling of the EV imposes significant power requirements that must be met by the battery. This would decrease the distance travelled by the vehicle between two charging processes. The additional rate of exergy that must be supplied to the vehicle for heating and cooling has a significant effect on the range of the vehicle, the distance it may travel on one charge. All additional power expenditures reduce the range of the vehicle. Table 1 shows the results for the range reduction of a vehicle, in km, for four temperature differences between the interior and the exterior of the vehicle, ΔT, and the coefficient of performance of the heat pump, β, when the cruising speed is 100 km/hr and the vehicle is fitted with a, 80 kWh battery. It is apparent in this Table that the traveling range of the vehicle, given in km, is significantly reduced in cold weather trips, when a great deal of heat must be supplied to the cabin. It is also apparent that

4. Supply of exergy – the well-to-wheels efficiency IC-powered vehicles receive their energy from fossil fuels (gasoline, diesel, compressed natural gas, and synthetic gas), which is derived/ produced from a primary energy source, typically petroleum or natural gas. Relatively low amounts of energy and exergy are lost – typically on the order of 10% – during the transportation and production of the IC fuels in chemical refineries [23]. Electric vehicles run on electricity, which is a tertiary energy source and is produced from primary energy sources by a variety of processes [20]. Significant amounts of energy and exergy – from approximately 70% in nuclear power plants to 25% in hydroelectric power plants – are lost in the conversion of the primary energy sources to electricity [24], 20]. Additional losses occur in the electricity transmission process to the consumers as well as during the charging processes of the batteries [25]. Any comparison between ICpowered vehicles and EVs should adopt the primary energy sources as reference for energy. This implies that figures of merit must take into account the consumption of exergy during the entire chain of processes that powers the wheels of the vehicles starting with the primary energy sources. Such analyses are referred to as well-to-wheels (WTW) analyses and are frequently performed to assess the operation of vehicles [26], 20]. The chain of energy conversions that power the EVs and the range of efficiencies of these processes are: 1. Fuel transportation to the power plant and conversion to electricity, denoted by ηt. The efficiency of this process depends on the type of unit that produces the electric power. Typical values of ηt are [20, [27]: 18% for photovoltaics; 30% for nuclear and wind units; 35% for gas turbines; 40% for coal units; 50–64% for combined cycle gas turbines; and 75% for hydroelectric power plants. 2. The transmission of electric power from the power plant to the consumer. This includes the losses in the step-up and step-down transformers, the transmission lines, and the distribution network and is denoted by ηtr. Typical values of ηtr are in the range 90–95% in the OECD countries and slightly less (85%) in some developing countries [28,29]. 3. The charging of the battery, ηB, which also includes the losses in the a/c to d/c converter. Batteries have an internal resistance, which contributes to energy dissipation during charging. A recent study [30] compiled the testing results of 1,896 batteries used in commercially available EVs and examined four factors that affect the charging efficiency of batteries: a) the charging voltage (120 V and 240 V); b) the state of charge; c) the temperature; and d) the charging power. It was reported that the average charging efficiency at

2 Low temperatures also have a deleterious effect on the ability of the battery to provide power. This effect may be neutralized by supplying additional heat to warm the battery system.

4

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Fig. 4. Energy requirements for a cruising EV when β = 3 and for different ΔT.

main determinant of the ηww for EVs, especially when the electricity is produced from thermal power plants that are subjected to the Carnot limitations. When the electricity is generated from both conventional and renewable sources, the overall efficiency of EVs is comparable to those of conventional IC-powered vehicles. It must be noted that a few publications separate the well-to-wheels efficiency in two parts: the well-to-tank efficiency and the tank-towheels efficiency. The product of the two is equal to the ηww. Conventional and hybrid vehicles have high well-to-tank efficiency and low tank-to-wheels efficiency. On the contrary, EVs have low well-totank efficiency and high tank-to-wheels efficiency.

Table 1 Range reduction, in km, of an EV, fitted with an 80 kWh battery, at different temperature differences and coefficients of performance when V = 100 km/hr.

ΔT = 0 ΔT = 20 ΔT = 25 ΔT = 30

β=1

β=2

β=3

428 329 311 295

428 372 360 349

428 389 380 372

120 V is approximately 83.8%, while at 240 V the efficiency increases to 89.4%. The starting state-of-charge (SOC) of the battery plays an important role in the charging efficiency with batteries that started at 10% SOC having efficiencies close to 90% and several batteries that started at 85% SOC having efficiencies less than 10%. For the entire charging process typical values of ηB are range from 70% to 90%. It must be noted that the testing results in this compilation were at very low power (less than 0.2 kW). The fast charging processes – on the order of tens of kW to charge a 50–100 kWh battery at a reasonable time – entail higher thermodynamic irreversibilities and the charging efficiency may drop significantly. This is a consequence of the generalized force-generalized flux relationships of Non-Equilibrium Thermodynamics and has been observed in other independent studies on the charging-discharging efficiencies [31,32]. 4. The supply of power from the battery through the motor to the wheels, which includes the battery discharging losses and any friction in the motor and the gear box, ηM. EVs typically have one gear only and do not need a complete transmission to operate. Typical values of this combined efficiency are in the range 85–90%.

5. Effect on greenhouse gas emissions One of the principal attractions of the marketing of EVs is the public perception that they do not emit any pollutants. While this is correct for the actual vehicles that move in urban areas without emitting pollutants, the generation of electricity at the power plants that supply the motive power for these vehicles entails significant pollutant production. Fossil fuel combustion is still used in most of the countries for the production of a very high fraction of their electric energy demand: of the entire global electricity demand in 2016, 38.4% was generated by the combustion of coal, 3.7% by liquid hydrocarbons, and 23.2% by natural gas [13]. The combustion of these three fossil fuels emits carbon dioxide, which is the principal GHG, as well as other pollutants that must be accounted in any environmental assessment of EVs. It is apparent that, whether or not the substitution of IC-powered cars results in environmental benefits, depends to a large extent on the method of electricity production: if the electricity supplied to EVs is produced by non-carbon energy sources – nuclear, hydroelectric, solar, wind, biomass and waste products – there is significant CO2 avoidance by switching the transportation fleet to EVs. On the other hand, if a very high fraction of the electricity is supplied by coal and heavy liquid fuels, the substitution of IC-powered vehicles with EVs entails a net increase of CO2 emissions [20]. Because electric power production is a national affair and varies among countries and larger geographical regions, calculations were conducted for the CO2 emissions avoidance (or increase) for several countries that are global leaders in EV registrations. The calculations are based on the fact that, since the CO2 is emitted by the combustion process, one may calculate the heat supplied to the IC engines as well as the equivalent heat supplied to the fossil fuel power stations that generate the fraction of electricity supplied by the fossil fuels. The principal results of the calculations are shown in Table 2. The first column of the Table indicates the country and the next four columns show the fraction of electric energy generated by coal, natural gas, petroleum, and noncarbon emitting sources [33]. Biofuels were included in the non-carbon

When all these processes are taken into account, the well-to-wheels efficiency for EVs is:

ηww = ηt ηtr ηB ηM .

(8)

With the ranges of efficiency values in the energy supply chain given above, the WTW efficiency of EVs is in the range 9–58%; and 5.4% if a solar PV array is used for the generation of electric power.3 The lower part of this range is comparable to that of the powerful (and less efficient) sports cars and the upper part is slightly higher than that of hybrid vehicles. A glance at the values that determine the WTW efficiency proves that the efficiency of the electricity production is the 3 It can be argued that, since the solar insolation would have been dissipated in the environment if it were not used, the lower efficiency related to the PVs is not a disadvantage because it does not lead to primary energy consumption and dissipation.

5

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Table 2 Fraction of electric energy produced by fossil fuels and non-carbon sources and CO2 avoidance when gasoline-powered cars are substituted by EVs. Country

Coal

Natural Gas

Petroleum

Non-carbon

% CO2 avoidance

% CO2 avoidance 1.25 higher efficiency

% CO2 avoidance, 1.67 higher efficiency

Australia Belgium Brazil Canada Chile Estonia European Union France Germany India Japan Norway P.R. China Russia UK USA Global Average

0.636 0.031 0.045 0.093 0.381 0.838 0.226 0.019 0.422 0.748 0.337 0.001 0.682 0.157 0.093 0.314 0.384

0.196 0.262 0.098 0.093 0.149 0.006 0.188 0.063 0.127 0.048 0.392 0.017 0.027 0.478 0.422 0.329 0.232

0.022 0 0.026 0.012 0.037 0.021 0.018 0.005 0.009 0.016 0.082 0 0.034 0.01 0.005 0.008 0.037

0.146 0.707 0.831 0.802 0.433 0.135 0.568 0.913 0.442 0.188 0.189 0.982 0.257 0.355 0.48 0.349 0.347

−18.89 73.41 81.97 76.34 23.58 −34.89 46.85 91.24 21.97 −23.64 5.52 98.43 −13.44 34.57 49.84 22.37 16.24

4.89 78.73 85.58 81.07 38.87 −7.91 57.48 93.00 37.58 1.09 24.42 98.75 9.25 47.65 59.87 37.90 32.99

28.67 84.04 89.18 85.80 54.15 19.06 68.11 94.75 53.18 25.82 43.31 99.06 31.93 60.74 69.90 53.42 49.74

equivalent electricity prices, primarily because the liquid fuels are encumbered by higher local taxes. Table 3 shows the cost comparison of two similar vehicles when they travel on trips of 1000 km, the first with an unleaded gasoline engine that performs on the average 12.2 km/L (28 mpg) and the second with an electric engine that performs 19 km/ kWh. The annual savings of running the two vehicles, calculated for 15,000 km, is also shown in the Table, as well as the ten-year present value (PV) of the annual savings for two discount rates, r, 5% and 8%. The unleaded gasoline and electricity prices were obtained from [13] and pertain to the first quarter of 2018 (the price for electricity in Japan was obtained from the 2016 data of the same publication). It is observed in Table 3 that – at the current prices regime – the monetary savings from the substitution of gasoline-powered cars with comparable EVs results in significant monetary savings. When these savings are combined with the other monetary incentives – primarily tax credits and rebates offered by governments that range from $4000 in Sweden, to $8000 in the USA, and up to $20,000 in Norway [36] – that are available in most countries, the total monetary benefits offset the original price premiums of new EVs and make them competitive in the vehicle market-place. It must be noted, however, that the governmental incentives are not always guaranteed and may be faced out. For example, in the USA, the higher incentives will be faced out when 200,000 EVs are sold.

emitting primary energy sources because they absorb their carbon content from the atmosphere and are, therefore, carbon-neutral. For the calculations of the CO2 emissions avoidance the low-heating value (LHV) data was used for the gasoline and the average of the low- and high- heating values (HHV) was used for natural gas and petroleum [34]. This because the emissions of all IC-powered vehicles comprise water vapor (hence the LHV) while a fraction of the gas turbines that operate with natural gas and petroleum increasingly use combined cycles with preheaters that exhaust liquid water and not water vapor [27]. The first column on CO2 avoidance pertains to the substitution of IC-powered vehicles with EVs that have comparable WTW efficiencies. Negative values in this column indicate CO2 emissions increases. The numbers in this Table indicate that, if IC-powered vehicles in Norway are substituted with comparable EVs, there will be on the average 98.3% CO2 emissions avoidance, primarily due to the fact that a very high fraction of electricity in this country is generated from hydroelectric energy.4 On the contrary, in a country like Estonia, which generates approximately 84% of its electricity from carbon (tar sands) such a substitution would result in 34.89% increase of CO2 emissions. The values for the entire world were obtained from the averaged global data in [33]. The data indicate that, on the average, 16.24% CO2 emissions avoidance is achieved when IC-powered vehicles are substituted with EVs. The last two columns in Table 2 show the result of calculations when the WTW efficiencies of EVs are significantly higher than those of the IC-powered vehicles by factors 1.25 and 1.67%. Such efficiencies may be achieved in the future by a continuation of the following trends that have been observed in the last decade:

6. Effects on renewable energy utilization A salient conclusion from the data of Table 2 is that the environmental advantage of replacing a fleet of IC-powered vehicles with the equivalent fleet of EVs depends very much on the electric power mix of the country (or the region, if electricity grids are regional). The environmental benefit is much higher in countries with high fraction of nuclear and renewables – e.g. France, Belgium, Norway, and Canada. On the contrary, the benefit is diminished (and may, actually, become an environmental detriment) in countries that heavily rely on coal and other fossil fuels for their electric power generation. When a great deal of renewable energy is unused and available, it is beneficial to use part of this energy in the transportation sector. Norway, where the hydroelectric energy resources by far exceed the current power generation, is an example of a country that could readily substitute a high fraction of the IC-powered vehicles with EVs and achieve significant CO2 emissions avoidance in the process. Hydroelectric energy is readily available to be harnessed at any time there is demand. In addition, and because of the very high mass of water stored in the reservoir and the potential energy of water (mgΔz), hydroelectric power plants are capable of providing very high quantities of power, with the larger hydroelectric power plants having power

1. Manufacturing EVs with lesser weight. 2. Developing more efficient battery charging and discharging processes. 3. Improving the overall efficiency of the electric power production units by increasing the fraction of combined cycle units. 4. Improving the carbon footprint of the electric power industry by the substitution of coal power plants with renewables and combined cycle gas turbines [20,35]. The energy comparison of IC-powered and EVs notwithstanding, at present there are significant monetary savings in favor of purchasing EVs. The prices of liquid fuels (gasoline and diesel) are higher than the

4 Norway, which has a great deal of unused hydroelectric capacity, is also actively promoting the use of electric cars by offering significant monetary incentives to the owners.

6

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Table 3 Cost of running comparable gasoline and electric vehicles, annual savings and present value of savings over 10 years. r is the discount rate. Country

Australia France Germany Japan Korea Norway Spain UK USA

Cost of 1000 km, gasoline, $US

Cost of 1000 km, electricity, $US

Annual Savings, $US per 15,000 km

Present Value, 10 years, r = 5%, $US

Present Value 10 years, r = 8%, $US

95.66 146.97 135.02 108.76 142.06 127.41 124.63 137.32 61.78

45.03 35.72 65.17 48.07 20.71 21.47 55.67 38.38 24.51

759 1669 1048 910 1820 1589 1034 1484 559

5398 11,861 7448 6470 12,938 11,295 7353 10,548 3974

4744 10,425 6546 5686 11,371 9927 6462 9271 3493

Table 4 Power generated (in kW/m2) and PV area necessary to charge with 40 kWh the batteries of 1 EV and 500 EVs in four cities of the USA.

Dallas-Fort Worth, winter Dallas-Fort Worth, summer Los Angeles, winter Los Angeles, summer Miami, winter Miami, summer New York, winter New York, summer

Power Generated per m2, kW/m2

PV area for one EV, m2

PV area for 500 EVs, m2

0.934

48.7

24,333

1.172

38.8

19,392

0.992 1.626 1.170 1.205 0.648 1.004

45.8 28.0 38.9 37.7 70.1 45.3

22,911 13,977 19,425 18,861 35,073 22,637

of electric power is at a minimum [38]. This electric energy surplus may be consumed by EVs that would be charged during the nighttime. Pricing incentives – e.g. lower electricity prices during the nighttime hours, which are already in effect in several regions, – will assist with the nighttime charging of EVs by consumers and will facilitate the substitution of fossil-fuel powered vehicles with EVs at least for the shorter, commuting routes.

ratings in the thousands of MW [20]. This is not the case for solar and wind power, which are diffuse power sources; are not always available to produce power at demand, and require very large tracts of land for their operation. Solar energy is periodically variable and wind power is essentially intermittent. For this reason these two power sources may not be available when battery charging is desired by the consumers. It is often publicized that the EVs of the future will be charged during the workdays by PV panels in urban garages, while their owners are at work. However, this practice requires PV panel areas (and power ratings) that are exceedingly high and are not available in an urban setting: Typical multilevel urban garages have on the order of one thousand passenger vehicles parked. Table 4 shows the PV area that would supply with 40 kWh of electric energy the batteries of 500 of these vehicles if they were garaged between 8:00 am and 5:00 pm5 in four metropolitan areas of the USA: New York, Los Angeles, Dallas and Miami. The PV panel area is calculated from two values of the irradiance in the cities, one for the winter months and the second for the summer. The irradiance data for the four cities were obtained from the NREL’s National Solar Radiation Data Base [37]. The PV panels are stationary and facing south, at an angle equal to the latitude of the region, the optimum angle for maximum annual energy production [20]. The overall efficiency of the PV panels is 20% and the battery charging efficiency is 88%. In order to avoid weather-related fluctuations the averages of the same two weeks – first week of January and first week of July – were calculated over six years of the insolation data. The area required in other northern cities with lesser total (direct and diffuse) irradiance – e.g. Boston, Detroit, Chicago and Seattle – is significantly higher. With the roof area of the typical multilevel garage that houses vehicles in urban areas being on the order of 1000 m2, it is apparent from the calculations of Table 4 that the needed PV area for the charging of a large fraction of the garaged EVs is not available within the urban settings. The electric power for the charging of the EVs by solar power during the working day must be generated from solar farms outside the urban setting. The d/c-to a/c and back to d/c conversions and the transmission of electricity in the urban centers would entail additional and significant energy dissipation. The other widely available form of renewable energy, wind power, is intermittent. While wind power may supply a fraction of the energy to a grid, whenever it becomes available, it is not dispatchable and may not be relied to supply daily, the urban garages with electricity. An option that would supply non-carbon generated electric power to EVs is to increase the number of nuclear units that supply an electric grid. Currently, the addition of several nuclear power plants, which are base-load units, to an electric grid may create electricity generation surplus during the nighttime, when the residential and commercial use

7. Other effects on the electric grids With 36% of the global TPESs spent on transportation and 89% of this consumed by road vehicles, the latter currently consume 173 Quads of energy (174*1012 MJ), which is primarily supplied by liquid petroleum products. It becomes apparent that, even if a fraction of this energy consumption is substituted with electric energy, the global electricity supply – currently at approximately 23,000 TWh (82.8*1012 MJ) – must be significantly increased to satisfy the additional demand. With the current mix of global electric energy production, which includes thermal power plants, approximately 165*1012 MJ of primary energy sources should be diverted from the transportation sector to the electricity sector in order to accommodate the shift to EVs. While this is a very substantial increase in electric energy production, adequate planning and market forces can support the transformation. Several countries (the USA, Canada, Japan, and most European Union countries) already have the excess electric power capacity to supply the needed electricity during the nighttime, when the electricity demand is at the lowest. A study pertinent to the Japanese electric grid that used the MARKAL tool to project into the future, concluded that, while at present there is sufficient excess power capacity to charge the EVs during the nighttime, by 2050 there will be sufficient installed solargenerated electricity to facilitate the charging of EVs during four hours in the middle of the day [39]. Significant grid problems would arise at present if a large number of EVs were to be charged simultaneously and, particularly, at fast rates of charging. Several popular publications flaunt the idea that EVs, used in trips longer than their range, may be charged in stations along the way while the owners have a meal. This implies a charging time of approximately one hour and that the charging occurs within the habitual meal times – e.g. between noon and 2:00 pm for lunch. If it is assumed that there is a sufficient number of charging stations and that 40,000 EVs (a very conservative number in most metropolitan areas of the USA and several populous countries) with average battery capacity 65 kWh are charged in the area served by an electric grid during the one-hour lunch period. At charging efficiency 90% and transmission efficiency 90%, the grid must generate and transmit to the consumers an additional 3210 MW, during the hour of charging. This is a significant demand fluctuation that many regional electricity grids will not be able to meet. When the electricity demand is not met, brownouts and blackouts occur in the entire region served by the electric grid. In general, the widespread deployment of EVs will cause significant problems to the

5 This is the typical “working day” in the four cities that includes one hour for lunch.

7

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides

Declaration of Competing Interest

functioning and operation of current power grids, such as power quality issues, supply-demand mismatching, and overloading of distribution transformers, if proper corrective actions by the grid management and optimal charging of EVs by the drivers community are not implemented at a national/regional scale [40]. The high energy capacity of EV batteries and the fact that a large fraction of the EVs are idle (in urban garages) for several hours every day has fostered the idea to use EV batteries for large-scale (utilityscale) electric energy storage. This is a necessary condition for the avoidance of the U-shaped demand curve (also referred to as the duck curve) and the higher penetration of wind and solar energy in the electricity generation mix of a region [41,31,42]. The use of EV batteries for utility-level electric energy storage is, in general, accomplished with higher round-trip efficiencies than other large-scale energy storage methods – e.g. pumped hydroelectric systems (PHS) and advanced compressed-air systems (CAES) [20]. The process is often referred to as V2G (vehicles to grid) process, and the vehicles are referred to as Grid-Integrated Vehicles (GIVs). A recent experimental study on the electric energy losses in the GIVs determined that the round-trip efficiency of this method of energy storage is in the range between 53 and 62% [31]. While this range is slightly higher than the 50% typical values for PHS and CAES storage, the energy losses are still significant. A companion study [43] claims that the actual round-trip efficiencies of V2G systems are lower because of unfavorable temperatures that were not considered in the original testing [31]. The V2G is still an intriguing subject for energy storage that requires additional research work and optimization. It must be noted, however, that this is a short-term (diurnal) storage method and not a long-term, seasonal storage (e.g. spring, when wind power is at maximum, to summer) method that would facilitate the higher utilization of both solar and wind power.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was partly supported by the Tex Moncrief Chair of Engineering at TCU. References [1] Global EV. Outlook 2019 – Scaling up the transition to electric mobility, 2019. Paris: Int Energy Agency; 2019. [2] Alves J, Baptista PC, Gonçalves GA, Duarte GO. Indirect methodologies to estimate energy use in vehicles: application to battery electric vehicles. Energy Convers Manage 2016;124:116–29. [3] Wang Y, Yang Z, Mourshed M, Guo Y, Zhu X. Demand side management of plug-in electric vehicles and coordinated unit commitment: a novel parallel competitive swarm optimization method. Energy Convers Manage 2019;196:935–49. [4] Zhuk A, Buzoverov E. The impact of electric vehicles on the outlook of future energy system. IOP Conf Ser: Mater Sci Eng 2018;315:012032. [5] Van Wee B, Maat K, De Bont C. Improving sustainability in urban areas: discussing the potential for transforming conventional car-based travel into electric mobility. Eur Plann Stud 2012;20:95–110. [6] Xing J, Leard B, Li S. What does an electric vehicle replace? Working Paper 25771. Cambridge, Massachusetts: National Bureaus of Economic Research; 2019. [7] Racz AA, Muntean I, Stan SD. A look into electric/hybrid cars from an ecological perspective. Procedia Technol 2015;19:438–43. [8] H. Lee, A. Clark, 2018, Charging the Future: Challenges and Opportunities for Electric Vehicle Adoption, Harvard Kennedy School Report, August 2018. [9] Rosen MA, Bulucea CA. Using exergy to understand and improve the efficiency of electrical power technologies. Entropy 2009;11:820–35. [10] Hamut HS, Dincer I, Naterer GF. Exergy analysis of a TMS (thermal management system) for range-extended EVs (electric vehicles). Energy 2012;46:117–25. [11] Zhang K, Li M, Yang C, Shao Z, Wang L. Exergy analysis of electric vehicle heat pump air conditioning system with battery thermal management system. J Thermal Sci 2019;28:1–15. [12] P. Iora, A. Cassago, C. Invernizzi, A. Copeta, G. Di Marcoberardino, S. Uberti, D. Fiaschi, L. Talluri, L. Tribioli, 2019, Assessment of Energy Consumption and Range in Electric Vehicles with High Efficiency HVAC Systems Based on the Tesla Expander, SAE Technical Paper, 2019-24-0244. [13] International Energy Agency, 2018, Key World Statistics, IEA, Paris. [14] International Energy Agency, 2019, Key World Statistics, IEA, Paris. [15] Hibbeler RC. Engineering Mechanics – dynamics. ninth ed. New Jersey: Prentice Hall; 2001. [16] Kestin J. A Course in Thermodynamics vol. I. Washington DC: Hemisphere; 1977. [17] Munson DR, Young BF, Okiishi TR, Huebsch WH. Fundamentals of fluid mechanics. 6th ed. New York: Wiley; 2009. [18] Michaelides EE. Particles, bubbles and drops – their motion. New Jersey: Heat and Mass Transfer World Scientific; 2006. [19] Dunlap RA. A simple and objective carbon footprint analysis for alternative transportation technologies. Energy Environ Res 2013;3:33–9. [20] Michaelides EE. Energy, the environment, and sustainability. Boca Raton, FL: CRC Press; 2018. [21] Y. M. Tsui, 2005, The Physics Factbook – An Encyclopedia of Scientific Essays, in https://hypertextbook.com/facts/2005/YatManTsui.shtml, last visited October 10, 2019. [22] Incropera F, DeWitt DP. fundamentals of heat and mass transfer. 3rd ed. New York: Willey; 2002. [23] Wang M, Lee H, Molburg J. Allocation of energy use in petroleum refineries to petroleum products implications for life-cycle energy use and emission inventory of petroleum transportation fuels. Int J Life Cycle Assessment 2004;9:34–44. [24] El-Wakil MM. Power plant technology. New York: McGraw Hill; 1984. [25] Sullivan JL, Gaines L. A review of battery life cycle analysis. Report, Argonne, IL: Argonne National Lab; 2010. [26] N. Brinkman, M. Wang, T. Weber, T. Darlington, 2005, Well-to-Wheels Analysis of Advanced Fuel/Vehicle Systems — A North American Study of Energy Use, Greenhouse Gas Emissions, and Criteria Pollutant Emissions, ANL Report, May 2005. [27] Langston LS. Anticipated but unwelcome. Mech Eng 2018;140(6):36–41. [28] Electric power transmission and distribution losses (% of output), The World Bank, https://data.worldbank.org/indicator/EG.ELC.LOSS.ZS last visited October 10, 2019. [29] International Energy Agency, 2014, Electricity Transmission and Distribution, Energy Technology Analysis Programme, Technology Brief E12. [30] Kong N. Exploring electric vehicle battery charging efficiency. U-C Davis, California: The National Center for Sustainable Transportation; 2018. [31] Apostolaki-Iosifidou E, Codani P, Kempton W. Measurement of power loss during electric vehicle charging and discharging. Energy 2017;127:730–42. [32] Batteries, North America Clean Energy, 10, 3, 40–45.

8. Conclusions Cruising road vehicles require exergy expenditures to overcome the road friction and aerodynamic drag, two resistance forces that dissipate significant quantities of energy. EVs must store all this energy in their batteries. In addition, the EV batteries must supply any heating and cooling requirements of the vehicles and this significantly reduces the range of the EVs in adverse weather conditions. The results indicate that a significant part of the energy stored in the battery (37.5% at 100 km/hr) is spent on the heating of the vehicle with resistance heating. This is reduced proportionately when an HVAC system with higher coefficient of performance is used (12.5% with β = 3). For the correct assessment of the exergetic requirements of EVs the total consumption of primary energy sources is considered, a practice that introduces the well-to-wheels efficiency as the proper figure of merit for the performance of EVs. The study shows that the well-towheels efficiencies of EVs are comparable to those of vehicles with IC engines. When a significant fraction of the electricity is generated from noncarbon sources, the shift from IC-powered vehicles to EVs results in net CO2 emissions reductions. On the contrary, in countries where a high fraction of electricity is generated from coal combustion, a shift to EVs may actually increase the associated CO2 emissions. Under the current conditions, the average percentage CO2 avoidance for the entire world is 16.24%. Finally, the charging of EVs with solar energy requires high PV panel areas that are not available in larger urban centers, where land is at a premium. With the current trends of electric power demand, the charging of large numbers of EVs during the daytime will cause grid problems, including blackouts. However, several countries currently have sufficient unused electricity generating capacity during the nighttime for the charging of large number of EVs. The shift of high fractions of the current vehicle fleets to EVs would require additional regulations, incentives for charging during low demand hours, and optimization. 8

Energy Conversion and Management 203 (2020) 112246

E.E. Michaelides [33] International Energy Agency. Electricity Information – 2018. Paris: IEA; 2018. [34] Fuels - Higher and Lower Calorific Values, https://www.engineeringtoolbox.com/ fuels-higher-calorific-values-d_169.html, last visited August 2019. [35] Leonard MD, Michaelides EE, Michaelides DN. Substitution of coal power plants with renewable energy sources – shift of the power demand and energy storage. Energy Convers Manage 2018;164(2018):27–35. [36] International Energy Agency, 2016, Global EV Outlook 2016: Beyond one million electric cars, Paris. [37] Wilcox S. 2012. National Solar Radiation Database 1991–2010 Update: User’s Manual, Technical Report NREL/TP-5500-54824, August 2012. [38] Michaelides EE. Alternative energy sources. Berlin: Springer; 2012. [39] Masuta T, Murata A, Endo E. Electric vehicle charge patterns and the electricity generation mix and competitiveness of next generation vehicles. Energy Convers

Manage 2014;8:337–46. [40] Khan, AZ, Janjua, AK, Jan, ST, Ahmed, ZN. 2017. A comprehensive overview on the impact of widespread deployment of electric vehicles on power grid. In: Proceedings of 2017 IEEE International Conference on Smart Grid and Smart Cities, Singapore. [41] Castillo A, Gayme DF. Grid-scale energy storage applications in renewable energy integration: a survey. Energy Convers Manage 2014;87:885–94. [42] Freeman E, Occello D, Barnes F. Energy storage for electrical systems in the USA. AIMS Energy 2016;4:856–75. [43] Shirazi YA, Sachs DL. Comments on Measurement of power loss during electric vehicle charging and discharging – notable findings for V2G economics. Energy 2018;142:1139–41.

9