Benefits of an Electric Road System for Battery Electric Vehicles
Round 1
Reviewer 1 Report
Dear authors,
Thanks for an important and insightful contribution. The presented paper addresses a difficult problem that may be key to accelerate the transition to an electric transport system, namely the relationship between battery capacity for passenger cars and the availability and type of charging infrastructure. In particular, the authors analyse how Electric Road Systems in conjunction with other forms of stationary charging can be used to reduce the battery capacity and thus the overall cost of such a system.
Addressing this problem requires modelling of an extensive system, which leads unavoidably to a number of simplifications that may affect the final results. Nonetheless, the authors have listed the most significant assumptions and simplifications made, and pointed out their potential impact on the results, which is appreciated.
Although it is hard to obtain accurate results when modelling the future, given the high level of uncertainty, the results confirm the trends that have been published in previous studies to a certain extent.
Please find below some specific comments to each section:
ABSTRACT:
The authors mention that ERS is an attractive option for electrifying long-haul trucking, which can also be of benefit for battery electric passenger vehicles with some modifications. While the first statement is true, the second one suggests that ERS are mostly beneficial for long-haul trucking and need to be adapted to be used with passenger cars, while your results (and some of the previous studies) indicate that making ERS available to passenger vehicles as well will bring along at least comparable savings. Moreover, there are several ERS technologies that have been developed to accommodate passenger cars without any additional modifications. Some of them have actually never been tested on trucks, although they will certainly work (e.g. Honda ERS, https://www.sae.org/publications/technical-papers/content/2018-01-1343/).
INTRODUCTION
Regarding the impact of home charging on the power grid, it should be mentioned that there are already several commercial chargers that have the ability to smartly adapt the charging hours according to the demand / price of electricity over the night, thus reducing the impact of this type of charging (as suggested in references 7 and 21).
https://myenergi.com/zappi-ev-charger/
https://zaptec.com/en/zaptec-go/
The use of real-world data is definitely interesting. However, there’s little real-world data from EVs yet, and the use of data from ICE-powered vehicles, although it can give a good insight of the possibilities, might lead to wrong conclusions. The ICE vehicles logged where used without any consideration to charging. Therefore, in this analysis, charging opportunities are determined based on the time slots in which the ICE vehicles are standing still. These may neglect further potential charging opportunities that the drivers might have utilised, should there have been a need for it.
In the literature review, page 2, line 70, it reads “c inspects the potential… “. I presume that a name / reference is missing here?
Finally, in the last paragraph of page 2, the authors state that none of the previous study consider the possible role of ERS as complementary to stationary charging. However, a study by Márquez-Fernández et. al. (https://ieeexplore.ieee.org/document/9374483) analyses the amount of static fast charging stations that would be necessary to complement a certain network of ERS in Sweden. In that study, a microscopic agent-based model of the Swedish transport system has been used to capture the behaviour of each vehicle rather than using general assumptions as in the previous studies.
METHODOLOGY
Electric Road System: when selecting which road segments to electrify the authors have consider those segments with higher (truck) traffic volume. However, are there any further constraints applied (e.g. minimum length of an electrified segment, presence of electric power grid in the area, etc)?
Car movement patterns: this is one of the key input data of this analysis. One of the most interesting results of the study is the cost savings resulting from the reduction on the battery capacity. However, the minimum “allowable” battery capacity is determined by the longest (worst-case) distance that the user would like to be able to cover between two consecutive charging events. The results then are valid as long as the dataset used (412 cars with between 30 – 80 logged days per car) also comprises these worst-case occasions. Can the authors elaborate a bit more about the nature of the trips in the input data?
Vehicle cost savings: The savings on the vehicle side originate mostly from the reduced battery size. However, in order to be able to utilise the ERS the vehicles must be provided with a current collector (pick-up) and, in most cases, a power converter interface adapting the voltage level supplied by the ERS to the voltage level in the vehicle traction system. Is the cost of that power interface included in the study? If so, is that included in the cost of the “pick-up system” of 1010 €? In reality the cost of such an interface will increase almost linearly with its power capability, thus the cost per vehicle in the 4e scenario will be 2 times the cost per vehicle in the 2e scenario. Given the high number of vehicles, the authors should clearly explain their assumption regarding this cost.
ERS costs: The upper cost figure for ERS seems quite high compared to previously published figures (see e.g. https://www.piarc.org/en/order-library/29690-en-Electric%20road%20systems:%20a%20solution%20for%20the%20future) The authors may highlight this in their discussion.
Treatment of fast charging: fast charging and ERS may be seen as competing solutions for charging when the cheaper home charging is not enough. Therefore, assuming that the existing fast charging infrastructure will be the same in all the scenarios analysed (with largely different ERS penetration) is not realistic. Moreover, the authors argue later in the paper that the cost of a comprehensive fast charging infrastructure is small compared to the cost of a comprehensive ERS network. This may be true to a certain extent if the power grid is already present at the charging locations and it is only the power transfer devices from the grid to the vehicles that are needed. However, a large part of the cost to deploy charging infrastructure lays on bringing the power grid to where it will be needed. This cost will be essentially the same for both ERS and fast charging. The authors also estimate the need for fast chargers according to ref. [55] in nine thousand charging points for 4.9 M vehicles. However, there are studies suggesting that a significantly larger number of chargers will be required if no ERS is present (1 fast charging station per 150 – 300 EVs, https://electrek.co/2022/02/02/ev-charging-ports-will-soon-outnumber-gas-stations-in-the-us-yet-it-wont-be-enough/). Moreover, later in the paper the authors argue that scalability of fast charging stations is easier and cheaper than scalability of ERS. This statement will also be challenged later.
Note: in several places in the paper the authors refer to Methodology with the wrong numbering (as if it was section 3). Please correct that.
RESULTS
The paper presents very insightful results for ERS placement and ERS driving share. In page 9, line 360, the authors state that charging at work or at relatively long stops (4 hours) does not make a significant difference on the required battery capacity. However, the authors do not reflect on why. It may be good to elaborate on the reasons for this, coupling it with the fact that the initial dataset was logged from ICE-vehicles, which are used with no regards to charging needs. The minimum battery capacity then is not decided based on the average “home-work-home” day, but on the occasional longer trip (e.g. on a weekend outing). As the authors mention in the discussion section, a dedicated commuting BEV will most likely benefit from the MixedSC scenario.
Nonetheless, the trends observed in the results for both the battery capacity needed and the cost savings are very interesting. I am curious as to how these results would look like with the added cost of the ERS – VE power converter interface.
Likewise, the results presented in figures 8 and 9 are both novel and insightful. Really good!
DISCUSSION
As mentioned earlier, it is appreciated that the authors state all the assumptions together with their potential effect on the results.
Regarding static fast charging infrastructure, I have already pointed out that their cost (especially if the power grid needs to be taken into account) is not negligible, especially in those scenarios with less ERS.
Moreover, in page 17, line 584, the authors state that, in comparison to an ERS, the fast charging system achieves “reasonable coverage at a relatively low cost. Further development is easily and cheaply scalable to charging needs with an expanding BEV fleet by adding more charging points.
These statements should be taken carefully for two reasons:
- the installation of fast chargers in areas where the power grid is weak or non-existent has a cost that is lower but comparable to ERS, and certainly not negligible
- while the scalability of the ERS is relatively easy (since roads have a certain traffic capacity, it’s a matter of designing the power supply system accordingly) the scalability of fast charging stations is not completely straight forward. The number of charging points needed will hardly fit in the existing designated charging places and will probably require SIGNIFICANT investments in terms of extra land space.
I really hope that my comments help you improve an already really good work. I’m looking forward to seeing the final version of this paper.
Best regards,
The Reviewer.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors,
I have now reviewed your paper with the title “Benefits of an electric road system for battery electric vehicles”. Overall, I think this is excellent research and a well-written paper. You address a precise and relevant research gap and selected suitable methods, which you applied in a rigorous way, resulting in valuable results and important conclusions. The paper has a clear structure, excellent figures, and you have done a good job in communicating in a transparent way, e.g. regarding assumptions, validity, and limitations. I have only minor comments, which I list below:
- Detail: Page 1 lines 33-35 can be misunderstood. “BEV-charging technologies and infrastructures, which include conventional stationary charging and solutions that allow for charging vehicles while driving—electric road systems (ERS), are considered central in this process [6].” The formulation leaves it unclear whether it is “BEV-charging technologies and infrastructures” or ERS that are “considered central”. It should be the first option. Please adjust the formulation to avoid misunderstanding.
- You write “Furthermore, drivers who use stationary charging prefer to mainly charge at home in the evening, when power demand is often already at its peak [7], [21].” This is interesting and probably true, but this might change in the future. Especially now that electricity prices in Europe are skyrocketing, people look for ways to save money, e.g. by choosing electricity contracts where you pay the specific price of electricity at the time of day that you use it. Home SC is also rapidly getting smarter. It is not unlikely that people will charge their cars during night time when the electricity price is the lowest. This would have beneficial effects for the grid and could be even better than ERS, which can be subject to peaks due to rush hour.
- Literature review: You cite many relevant references, but I am missing some literature, not least when it comes to studies which were carried out in a Swedish context. I want to recommend expanding your literature review and can specifically point to authors such as Stefan Tongur, Mats Engwall, Håkan Sundelin, and Jesko Schulte. Please do not feel forced to include any of their publications if you don’t find them adding value to your paper, but I recommend taking a look if you have not already done so.
- In the section “our contributions”, you very clearly describe the research gap you are addressing, but I think you can be more precise when it comes to your aim and contribution in relation to the gap. The second paragraph of the section is describing it, but not as crisp as I would wish. Also, when reading the first sentence of section 2.1 “This study investigates the optimal ERS placement in a large-scale implementation using road-traffic data […]” the reader can ask whether this is the contribution?
- You write “The extrapolated average annual travel distance for the cars is 22,155 km/year.”. This number appears very high to me, given that the average travel distance for passenger cars in Sweden on a national level is around 10.000 to 12.000 km/year. How come your number is so high? What does that say about the generalizability of your findings? Or do I misunderstand?
- You write “The study also assumes, as in Limb et al. [8], that annual operation and maintenance costs of about 0.01 M€/km [49] associated with road segments of ERS are equivalent to the operation and maintenance costs of conventional roadways; thus, no extra maintenance costs are included.” This sentence appears contradictory to me. You include annual operation and maintenance cost, but then you state that no extra maintenance costs are included? Surely there will be higher operation and maintenance costs for a road section with ERS than without ERS? Please clarify.
- You write “The long required range to specific drivers suggests that HomeSC only might not currently be a realistic charging scenario.” The tricky thing here is time. It might not be a realistic scenario for the present. However, a large-scale introduction of ERS is not realistic for the present either. This would take many years, maybe decades. By that time, a BEV range of 655km does not at all appear unrealistic, given the rapid developments in the battery and car industries. The question is what makes most sense from an economic and environmental perspective.
- Detail: Page 13 line 425. Please avoid casual language like “can’t”.
- The results and discussion sections are really good. Very interesting findings, good figures. Given all the data and analyses that you did, it was probably not easy to write it up in a short and concise way. You managed well. Still, I think it could improve even further. The sections are dense and it is not easy to comprehend all the findings, assumptions, scenarios and so on. At some point in the paper, I would have liked to see more synthesis to make it easier for the reader to grasp the main message(s). One way could be to more explicitly write the key factors or questions that this study identifies as uncertain but important for the analyses, e.g. the cost of ERS (uncertain, large interval, but important factor), the saving of battery capacity (very uncertain given market aspects, psychological aspects, etc. but also important for the calculations, and so on. Could your findings be synthesised and distilled even further?
- Potentially in conjunction with the previous point, I would like to see more about future research trajectories. This study poses so many interesting questions. In fact, I see one of the main contributions of this study as the fact that it reveals so many and important research questions to be investigated. E.g. how about the environmental impact of ERS vs. large batteries and fastSC, how about business models, how about threshold effects and lock-in effects, etc. etc.
I hope you find these comments helpful to further improve an already very well done paper.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
