Evaluating the Potential of Platooning in Lowering the Required Performance Metrics of Li-Ion Batteries to Enable Practical Electric Semi-Trucks
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here is an explosion of interest in the potential of developing a long-haul electric semi-truck, and this interest has been amplified by Elon Musk outlining potential plans for long-haul electric semi-trucks.1 In an earlier Viewpoint, we delineated the performance requirements of the batteries to enable long-range electric semi-trucks.2 We identified that a fully electric semi-truck would require estimated battery pack sizes of roughly 1000, 2000, and 3100 kWh for driving ranges of 300, 600, and 900 miles, respectively, in a realistic driving scenario. Due to the specific energy limitations of current Li-ion batteries, an average of 240−280 Wh/kg at the cell level, the battery packs would weigh an enormous 8, 18, and 27 US tons for the three values of driving ranges considered. The large battery packs and the consequent pack weight restrict the payload capacity of electric semi-trucks due to the on-road gross vehicle weight (GVW) limit imposed on Class 8 vehicles.2,3 In terms of payload capacity, we found that only a short driving range of 300 miles would have a reasonable payload capacity of about 22 US tons, while 600 and 900 miles of range would require a battery pack that is comparable to or heavier than the total payload carried.2 In comparison, current diesel semi-trucks carry an average payload of 16 US tons across different applications while also being able to travel an average of 1000 miles before refueling.4 The large battery packs result in an electric semi-truck that is further bottlenecked by cost; a 300-mile-capable battery pack costs about $200 000, which is much higher than a diesel-powered semi-truck, which costs about $120 000, on average, for the entire vehicle.4 The results from our previous Viewpoint were widely covered5−7 and led to speculations regarding other systemlevel approaches to decrease the required energy,8,9 thereby lowering the battery weight and improving the ability to carry cargo. One of the avenues proposed to address the uphill challenge is to exploit the platooning of multiple trucks.8 In this work, we systematically evaluate the effect of platooning in altering the performance requirements of Li-ion batteries for semi-trucks, and a pictorial summary of our findings is shown in Figure 1. Platooning could be generally defined as two or more vehicles arranging themselves in a specific pattern to travel together. This leads to several benefits, including improved safety for the drivers as well as an improved overall efficiency of the vehicles involved.10 The increase in efficiency is largely due to a reduction in aerodynamic drag experienced by the participating vehicles (including the lead vehicle).11 An experiment performed under the PATH project studied the effect of vehicle spacing and the number of vehicles in a platoon © XXXX American Chemical Society
Figure 1. Comparison between the performance of a single truck and a platoon of trucks.
on the average coefficient of drag of the entire platoon.12 Using model vans, they determined the average drag coefficient in several configurations with different numbers of vehicles in the platoon and varying intervehicle spacing. The results for the drag coefficient reduction as a function of intervehicle spacing and number of trucks are shown in Figure 2, where vehicle spacing is the distance between the vehicles normalized by the length of a vehicle in the platoon. This model could be easily incorporated into a vehicle dynamics model where the potential of platooning in lowering the energy demands and, thereby, modifying the battery cost, weight, and associated cargo capacity can be evaluated. The parametric relationship for estimating the total energy required for a trip of distance D, at an average velocity of v, is obtained using the same approach described in our previous study2 and is given by ⎡⎛ ⎞ 1 1 E P = ⎢⎜ ρCdCd redAvrms 3 + CrrWTgv + tf WTgvZ ⎟ ⎝ ⎠η ⎢⎣ 2 bw +
⎛ 1 ⎞⎤⎛ D ⎞ 1 − ηbw ηbrk ⎟⎟⎥⎜ ⎟ WTva⎜⎜ 2 ⎝ ηbw ⎠⎥⎦⎝ v ⎠
(1)
The first term represents the aerodynamic drag, which also includes Cdred, the drag coefficient reduction factor obtained Received: October 18, 2017 Accepted: October 23, 2017
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DOI: 10.1021/acsenergylett.7b01022 ACS Energy Lett. 2017, 2, 2642−2646
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http://pubs.acs.org/journal/aelccp Cite This: ACS Energy Lett. 2017, 2, 2642-2646
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ACS Energy Letters Table 1. Bounds and Mean Values of Variables parameter
[bounds], mean
Cd Crr WV (1000 kg) v (m/s); (mph) vrms (m/s); (mph) SP f burden (Wh/kg) CostkWh ($/kWh)
[0.45,0.7], 0.63 [0.0045,0.01], 0.0063 [6,8], 7 [16, 21], 19; [36, 47], 43 [19, 24], 22; [43, 54], 49 [136, 200], 170 [150,300], 190
Table 2. Constant Parameters ρ (kg/m3) A (m2) WT (US tons) g (m/s2) Z ηbw ηbrk tf a (m/s2)
Figure 2. Average drag coefficient reduction per vehicle in the platoon as a function of the intervehicle spacing and the number of vehicles in the platoon. The drag coefficient reduction is expressed as the ratio of the average drag coefficient of the platoon to the drag coefficient of a single vehicle. The intervehicle spacing is determined by normalizing the distance between the vehicles by the average vehicle length. We can observe that while significant improvements are obtained by increasing the number of vehicles from a single vehicle to a few vehicles, the improvements saturate after a certain number of vehicles.
1.2 7.2 40 9.8 0.01 0.85 0.97 0.15 0.112
have an average drag coefficient reduction ratio of 0.81, while at a intervehicle spacing of 0.5 vehicle lengths, the average drag coefficient reduction ratio becomes 0.65. In addition, the average drag coefficient reduction ratio improves for any given intervehicle spacing as the number of vehicles in the platoon increases. As shown in Figure 2, the effect of adding additional vehicles to the platoon on the average drag coefficient reduction peaks at a intervehicle spacing of around 0.2 vehicle lengths. Thus, for this analysis, we assume that the platoon will maintain an intervehicle spacing of 0.2 vehicle lengths, which is 8 ft for a standard 40 ft semi-truck. The 8 ft distance is a relatively small following distance between trucks relative to what is considered safe by the second rule.17 However, truck drivers drive at this separation on the highway, and with intertruck communications, driving at this separation could become safer.18 In addition, we assume that the drive profile of the platoon is the same as that of a truck traveling by itself. This allows us to use the NREL DriveCAT velocity profile as the representative drive profile.19 We allow the number of trucks in the platoon to vary from one to seven. Because this analysis will be assuming an intervehicle spacing of 0.2, it can be seen in Figure 2 that the drag reduction saturates beyond seven trucks in a platoon; thus, having more than seven trucks in the platoon does not yield any additional benefit. The range was chosen to vary from 300 to 900 miles; 300 miles is the lower end of the range of miles traveled daily by long-haul semi-trucks, while 900 miles would bring an electric semi-truck nearly on par with the driving range of current diesel semi-trucks.4 The battery pack size, pack weight, pack cost, and maximum payload capacity as a function of the required range in miles and the number of trucks in the platoon can be seen in Figure 3. The contour shows that, in general, as the number of trucks in the platoon increases the required battery pack size for a given range decreases, causing the battery pack to decrease while also decreasing the battery pack weight and increasing the maximum payload. Also, a reduced pack size would have a lower pack cost. It can also be observed from Figure 3 that the benefits received in platooning level out as the number of trucks approaches seven. This is due to the saturation in drag coefficient reduction exhibited in Figure 2. Among the cases considered, the lowest battery pack requirement is 880 kWh for
from platooning, the second term represents rolling friction, the third term is associated with road gradient, and the last term represents the inertial component where we see that a large portion of the energy used to overcome the inertial forces could be recovered through regenerative braking.13 Equation 1 assumes a depth of discharge (DOD) of 100%. In practice, however, the DOD is generally much lower; thus, an oversizing fraction is added to account for the DOD as well as to accommodate for capacity degradation due to cycling.2 E The pack weight, WP, is given by WP = S f P , which is P burden
evaluated using a distribution of values for the specific energy SP at the cell level and a distribution of values for the packing burden factor.14 f burden is a mathematical representation of all noncell inactive materials including the weight of the thermal management systems, module hardware, and battery jackets.15 The maximum payload capacity of the vehicle, WL, is given by WL = WT − WP − WV (2) This is the available portion of the GVW after the weight of the pack, WP, and the empty vehicle weight, WV, have been included. The final equation determines the cost of the pack, E CostP, and is given by CostP = CostP , which is evaluated using kWh
the pack energy distribution obtained from eq 1 and the cost per kWh of the battery pack, CostkWh.16 Using the above equations, the battery pack size, battery pack weight, battery pack cost, and maximum payload can be calculated based on the range, number of vehicles in the platoon, and intervehicle spacing in addition to the other input parameters specified in Tables 1 and 2 using a standard Monte Carlo calculation. As mentioned earlier, Figure 2 shows how the drag coefficient is reduced as the ratio of the average drag coefficient of a truck in the platoon to the drag coefficient of a single truck as a function of the intervehicle spacing and the number of trucks in the platoon. It can be seen that as the vehicle spacing increases the effect of increasing the number of trucks to the platoon diminishes because the reduction in the drag coefficient decreases. For example, a platoon of five trucks traveling with an intervehicle spacing of 1.3 vehicle lengths will 2643
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ACS Energy Letters
which is about $120 000,4 given that the operational costs of electric trucks would be much lower than that of their diesel counterparts. From the perspective of commercial feasibility, this emphasizes the potential of platooning in tilting things in favor of electric trucks, while at the long range of 1000 miles, that diesel trucks would still remain a challenging target for electric semi-trucks. The enormous challenge in electrifying semi-trucks is restated in Figure 3 with estimates of the large battery packs required for achieving a range comparable to the 1000 mile driving range of diesel trucks. The large battery packs at this range would not only be a significant technical challenge but would also severely limit the payload of the truck. By specifying a limit of 16 US tons on the payload capacity, we observe that the maximum range achievable for electric semi-trucks with current Li-ion battery technology is 680 miles for a single truck, which can also be interpreted as the technological feasibility limit of electric semi-trucks. This range of 680 miles increases to 740, 760, 770, 780, 790, and 800 miles for platoons that have 2, 3, 4, 5, 6, and 7 vehicles in the platoon, respectively. The important point to note here is that each of these values for the driving range is achieved using the same battery pack size of about 2330 kWh, which implies that each truck can gain up to 60−120 miles with the same battery pack just by platooning. In addition to the effects discussed above, it is crucial to evaluate the consequent effect platooning has on the cycle life of the battery packs. Because platooning decreases the drag force on the vehicle, the power load on the battery pack is consequently reduced, leading to a reduced discharge current from the battery pack. In order to study the influence of the reduced discharge current on cycle life, we model the battery pack on the AutoLion-ST20 platform where the degradation submodel is implemented for each cell in the pack.21 Our approach to studying the effect of different drive cycles and their consequent power loads on the cycle life of battery packs is described in our previous work.22 The two baseline cases chosen are standalone semi-trucks with driving ranges of 300 and 600 miles with a GVW of 40 US tons. While the pack sizes are fixed at the single-truck scenario for each case, we need not make any assumptions of the specific energy of the battery packs because the specific energy of the cell or the pack does not directly influence the degradation model. The power load of the single-truck scenario is based on the NREL CARB HHDDT “Composite” drive cycle,19 and the power profile for each platooning scenario with an increasing number of trucks is obtained by incorporating the reduction in the drag coefficient based on the aforementioned results. For the sake of simulations, we model the cell with an NCA cathode coupled with a graphite anode within an 18−650 cylindrical cell geometry. The results of the cycling studies are shown in Figure 4, where we can observe two cases, one with the base case of a 300 mile range and the second with a 600 mile range. In Figure 4, we find that the effect of platooning with seven trucks increases the range per cycle by 52 miles for the 300 mile range case and by 106 miles for the 600 mile range case, while the battery pack size remains the same for each case, respectively, similar to the estimates obtained from the parametric energy equation. The end-of-life condition imposed on battery packs is degradation to a state-of-health (SOH) of 0.8 or 80% of the initial capacity.23 The degradation or reduction in SOH is studied as a function of the number of miles traveled by the vehicles. In the 300 mile range base case scenario, the 1000 kWh battery pack reaches its end-of-life at
Figure 3. Battery pack size (kWh), battery pack weight (US tons), maximum payload (US tons), and battery pack cost (thousand $) as a function of the range required (miles) and the number of vehicles in the platoon. As the number of vehicles in the platoon increases, the respective values for each of the evaluated parameters improves with respect to the single-vehicle case. The contour flattens out after the number of vehicles in the platoon reaches four, following the saturation that occurs in the drag coefficient reduction.
the case when the range is 300 miles (smallest range) in a seven-truck platoon. As stated previously, the battery pack required to power a single truck for 300 miles is about 1000 kWh. This means that by joining a platoon of six other vehicles each truck was able to reduce its battery pack requirement by about 15% or 120 kWh. Similarly, if a platoon of seven trucks were required to travel 900 miles, the required battery pack would drop from 3100 kWh to about 2600 kWh, a reduction of about 500 kWh (or about 15%) compared to the single-truck case. These values show that, while the absolute battery savings in the 900 mile case is larger, the percentage savings is the same when compared to the 300 mile case. This is due to the fact that the average drag coefficient of the platoon is the only parameter that is improved and the aerodynamic drag force does not depend on the vehicle weight, resulting in a constant fraction of energy savings. The effect of platooning on the payload capacity, as seen in Figure 3, shows an increase in the maximum payload capacity with the number of vehicles in the platoon. For the 300 mile case, we can see only a marginal increase in the payload, and it remains at approximately 25.5 US tons for seven vehicles as well. This is due to the fact that the reduction in pack size is minimal for the 300 mile case, which translates to a small reduction in pack weight. However, for the 900 mile case, because a reduction in pack size of about 500 kWh is more significant, we see an increase in payload capacity from 11.1 to about 14 US tons in a seven-truck platoon. Because the percentage savings in energy remain constant, there is a larger absolute reduction in battery pack weight for the longer range cases, and hence, we would observe the greatest effect upon increasing the maximum payload on the trucks that have to travel the furthest distances. To summarize a case for platooning in Figure 3, a 300-mile-capable seven-truck platoon would require an 880 kWh battery pack that weighs 5.5 US tons, costs about $158 000, and has a maximum payload of 25.5 US tons. The large payload capacity would put these vehicles well above the average diesel semi-truck payload of 16 US tons. In addition, the cost of the required battery packs would now be $158 000, which is closer to the cost of a diesel semi-truck, 2644
DOI: 10.1021/acsenergylett.7b01022 ACS Energy Lett. 2017, 2, 2642−2646
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ACS Energy Letters
with seven trucks in the platoon, thus effectively increasing the operating range of an electric semi-truck simply by having the trucks join a platoon.
Matthew Guttenberg Shashank Sripad Venkatasubramanian Viswanathan*
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Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b01022. A discussion of the configuration of the platoon, validation of the drag coefficient reduction data, fraction of total energy spent on overcoming drag forces, and the distribution of the required pack size for different number of vehicles in the platoon. It also includes a commentary on the potential for battery swapping and autonomous driving strategies. (PDF)
Figure 4. SOH studied as a function of miles traveled for the two base cases of 300 and 600 miles, respectively. Within each case, we compare the rate of degradation in SOH for a single truck (solid line) and that of a seven-truck platoon (dashed line). The 300-milecapable single truck can travel a longer range of 352 miles in a platoon of seven, and similarly, the 600-mile-capable single truck can travel 706 miles in a platoon of seven. For the 300 miles per cycle case, with a 1000 kWh battery pack, we observe an increase in miles to end-of-life of about 30 000 miles, and for the 600 miles per cycle case, we observe an increase in miles to end-of-life of about 60 000 miles.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Venkatasubramanian Viswanathan: 0000-0003-1060-5495
about 275 000, miles while a platoon of seven trucks with the same 1000 kWh battery packs would travel about 305 000 miles before reaching the end-of-life capacity. This effect is due to the increased efficiency and lower power requirements of the platooning scenario. The 600 miles per cycle base case, displayed in Figure 4, also shows a similar trend, where the single truck travels up to 550 000 miles before reaching the end-of-life capacity while the seven-truck platoon travels about 610 000 miles. It is worth highlighting that 120 000 miles is the average annual distance traveled by Class 8 trucks,4 suggesting that platooning could provide approximately an additional half year of battery life, which could have a significant effect on the operational and replacement costs of electric semi-truck battery packs. Given the cost bottleneck that exists for electric semitrucks, the lifetime of battery packs in operation is an extremely important aspect to consider, and as demonstrated above, platooning can have a significant impact on the cycle life. The analysis done in this paper suggests that platooning has the potential to alleviate some of the shortcomings of current Li-ion batteries in electrifying semi-trucks. Platooning has been shown to dramatically reduce the coefficient of drag of the trucks within the platoon and can do so without the need for an unrealistic amount of vehicles in the platoon. We could obtain up to a 15% reduction in battery pack size in a best-case scenario with seven trucks in the platoon. This means that an electric semi-truck that has a range of 300 miles can potentially be technologically and economically feasible. This case requires a battery pack of 880 kWh and can carry a payload up to 25.5 US tons, and the battery pack would cost about $158 000. In addition, if electric semi-trucks are required to have a 16 US ton payload, keeping them competitive with the payload of diesel semi-trucks, it is shown that the battery pack size and the cost will remain roughly the same at 2330 kWh and $420 000, respectively; however, the range increases from 680 miles with one truck, to 760 miles with four trucks, and up to 800 miles
Notes
Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.S. and V.V. gratefully acknowledge support from the Technologies for Safe and Efficient Transportation University Transportation Center. V.V. gratefully acknowledges support from the Pennsylvania Infrastructure Technology Alliance, a partnership of Carnegie Mellon, Lehigh University, and the Commonwealth of Pennsylvania’s Department of Community and Economic Development (DCED).
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ABBREVIATIONS PATH: Partners for Advanced Transit and Highway NREL: National Renewable Energy Laboratory DriveCAT: Drive Cycle Analysis Tool CARB HHDDT: California Air Resources Board - Heavy Heavy Duty Diesel Truck FHWA: Federal Highway Administration
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REFERENCES
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