Hydrogen vs. Battery-Based Propulsion Systems in Unipersonal Vehicles—Developing Solutions to Improve the Sustainability of Urban Mobility

Round 1

Reviewer 1 Report

The minor errors in spelling have been highlighted in the attached version of the paper. Please correct before final submission.

Brief summary:
The paper presents two implementations of electric vehicles, one battery powered, and a battery-hydrogen hybrid, with the corresponding tests for usage in public urban transport. The authors have demonstrated the advantages of the hybrid implementation (higher specific energy, more autonomy, and shorter recharge time), measuring parameters of both vehicles in two test scenarios. 

Broad comments:
- It is a bit unclear what the authors contribution is to the scientific world, through this paper: is it the testing platform for the two types of vehicles or the actual technologies (which is understood from the title)? Title and content seem to be a bit out of sync. 

Specific comments:
- The graphs that show the vehicle responses to the two tests have different time axes. Is there a justification for why the hydrogen-powered vehicle needs to run for 9000 seconds before taking the measurements? 

Comments for author File: Comments.pdf

Author Response

Reviewer(s)' Comments to Author:

Reviewer: 1

Comments R1.1:

• The minor errors in spelling have been highlighted in the attached version of the paper. Please correct before final submission.

Authors Reply: Authors thank the reviewer’s comment and the effort to correct all the mistakes one by one. All the suggested corrections have been resolved and included in the final manuscript. There are only a couple of suggestions that we wanted to detail:

• Line 92, word “necessary” is not correctly broken into syllables: In the Word version appears correctly written. Probably is a mistake during the conversion process. We hope this does not happen in the next conversion.
• Graph legend Figure 7, “and” can be removed: authors would like to maintain the figure caption, because they are two variables: motor voltage and battery voltage. Due to configuration, they achieve the same value, but they are two variables.
• Line 230, change "in" for "to": authors suggest delete both of them because it has been a reduction of 68 bar, in absolute terms (from initial pressure to final pressure).

Comments R1.2:

• Broad comments: It is a bit unclear what the author’s contribution is to the scientific world, through this paper: is it the testing platform for the two types of vehicles or the actual technologies (which is understood from the title)? Title and content seem to be a bit out of sync.

Authors Reply: Regarding the reviewer’s comments, in the paper titled "Hydrogen vs battery-based propulsion systems in unipersonal vehicles. Developing solutions to improve the sustainability of urban mobility", both technologies are compared with the aim of providing scalable sustainable solutions in unipersonal vehicles. For this purpose, an original test platform has been developed. The mobile platform developed provides a suitable environment for the comparison of propulsion systems in real tests in a controllable urban circuit.

Please, see text labeled as “Reviewer 1 Comment 2”; this sentence defines the paper’s contribution: As far as technological development is concerned and, in comparison with previous scientific works found in the literature, this paper presents important novelties such as the specific design of the test platform, the use of comparable propulsion systems and real tests in a controllable urban circuit.

Comments R1.3:

• Specific comments: The graphs that show the vehicle responses to the two tests have different time axes. Is there a justification for why the hydrogen-powered vehicle needs to run for 9000 seconds before taking the measurements?

Authors Reply: The reason why the experiments with batteries and with fuel cell have different time bases is because, in the tests with fuel cell, before starting the characterization measures, authors carried out a series of internal laboratory protocols aimed at testing the initial state of the fuel cell, reaching the minimum operating temperature and, above all, stabilizing the measurements of the system parameters. These measures are taken wirelessly, and in the on-road test, they tend to cause stabilization problems because two equipment must roll close to each other (the prototype and the one which takes the measurements). Experimental results presented in the papers reflect the system performance after the internal laboratory protocols have been successfully passed.

Reviewer 2 Report

I have read the article entitled “Hydrogen vs battery-based propulsion systems in unipersonal vehicles. Developing solutions to improve the sustainability of urban mobility” by J. M. Andujar et al submitted to Sustainability MDPI.

Since the invention of the automobile, its use and marketing are on a steady rise, which has created a high demand for fossil fuels. Climate change is one of the major problems that people face in this century, with fossil fuel combustion engines being huge contributors. Given this fact, new alternative energy, such as electric and hydrogen cars, present a solution that still has some efficiency and power problems unresolved. An important factor in the design of unipersonal vehicles is the aerodynamic design factor because it affects variables such as performance and stability. Currently, the battery-powered electric vehicle is considered the predecessor, while hydrogen vehicles only have an insignificant market share.

With this as the situation, authors in the submitted work evaluated the differences in hydrogen-based configuration versus the battery-based one. The submitted work is well presented (language can be improved at some places though including abstract; having long sentences) with supportive validations.  Nevertheless, moderate revision is required before rendering a final decision.

Following are my general and specific comments:

• In the notation and symbols; C-rates (charge/discharge rates) for battery storage also to be included.
• While calculating the available energy, I hope authors have considered the weight, tire, and air friction that can slow the vehicle down. The available energy depends on the vehicle specifications as well as the breaking speeds and deceleration
• Lines 50 -52 (introduction section); For efficient energy generation (or) energy efficiency and to use energy at minimal emission level; storing the energy efficiently and use is therefore becoming increasingly important since they play a large role in both energy and industrial applications. In this aspect, new energy storage sustainable materials recently developed should be mentioned to reflect a recent research direction emphasizing the reduction of CO2 emission (ChemistrySelect 5 (2020) 1597; RSC Adv 9 (2019) 26981).
• Line 60 -71 (introduction section); the function structure of the hydrogen system, as well as the electric (battery) one, needs to be provided in detail. Like, for instance, hydrogen is produced through water splitting (electrolyzer and its efficiency) and stored in batteries/supercapacitors (including the material and efficiency). This would provide a better picture to the reader. This also can be included in Table 1.
• In figure 1 and 5, Hydrogen tank, battery/fuel cell that has the same components as an electrolyzer in the power range of a few kW as used in current hydrogen vehicles takes up all the space and become heavy. Is the reviewer correct? Please justify.
• What would be the energy storage (otherwise capacity) fade after many cycles of use (in the cycle)?
• From the sustainability point of view, how hydrogen has been produced (completely green?)
• In table 5, cost and CO2 emissions can be compared, if possible.
• In Fig. 7a the y-axis unit; voltage should be expressed in Volts rather than Amps.
• Section 3.3 The discharge rate (heat transfer and the influence of temperature) for a battery cell has also been available in the literature (Energies 13 (2020) 1477) by designing a module with an indirect cooling system, which is a first-order demonstration. Please compare this prototype model and discuss.

Author Response

Reviewer: 2

Comments R2.1:

• In the notation and symbols; C-rates (charge/discharge rates) for battery storage also to be included.

Authors Reply: Authors thank the reviewer’s comment. It has been included in the new version.

Comments R2.2:

• While calculating the available energy, I hope authors have considered the weight, tire, and air friction that can slow the vehicle down. The available energy depends on the vehicle specifications as well as the breaking speeds and deceleration.

Authors Reply: Of course, as reviewer suggests the mentioned parameters have been considered. The comparison between both technologies has been carried out with the same test platform, so the results obtained can be compared.

Comments R2.3:

• Lines 50 -52 (introduction section); For efficient energy generation (or) energy efficiency and to use energy at minimal emission level; storing the energy efficiently and use is therefore becoming increasingly important since they play a large role in both energy and industrial applications. In this aspect, new energy storage sustainable materials recently developed should be mentioned to reflect a recent research direction emphasizing the reduction of CO2 emission (ChemistrySelect 5 (2020) 1597; RSC Adv 9 (2019) 26981).

Authors Reply: Reviewer’s suggestion has been included and commented as new references.

Comments R2.4:

• Line 60 -71 (introduction section); the function structure of the hydrogen system, as well as the electric (battery) one, needs to be provided in detail. Like, for instance, hydrogen is produced through water splitting (electrolyzer and its efficiency) and stored in batteries/supercapacitors (including the material and efficiency). This would provide a better picture to the reader. This also can be included in Table 1.

Authors Reply: Authors thank reviewer’s suggestions. The new version of the manuscript has been updated taking into account your comments. Anyway, it is not the scope of the article to go into details of production and storage of H₂. Details about this subject have been already developed in other works:

(https://0-doi-org.brum.beds.ac.uk/10.3390/electronics9071074; https://www.osti.gov/etdeweb/biblio/20599461; ...)

Comments R2.5:

• In figure 1 and 5, Hydrogen tank, battery/fuel cell that has the same components as an electrolyzer in the power range of a few kW as used in current hydrogen vehicles takes up all the space and become heavy. Is the reviewer correct? Please justify.

Authors Reply: Regarding reviewer’s comment, questions regarding weights and comparative of both versions are detailed in Table 5, as well as the specific energy ratio (Wh/kg) of both propulsion systems. The powertrains developed in each case (battery and hydrogen-based) weight 3,6 kg and 4,7 kg, respectively. It is not completely correct to assert that hydrogen tank, battery/fuel cell has the same components as an electrolyser. An electrolyser needs a balance of plant different to a fuel cell. Even in case the weights were similar, mobile applications do not require to integrate the hydrogen production plant. Usually, the hydrogen production plant is placed on the recharging point, not in the mobile platform.

Comments R2.6:

• What would be the energy storage (otherwise capacity) fade after many cycles of use (in the cycle)?

Authors Reply: According to the technical data sheet of the used battery, the maximum charge and discharge cycle is 300.

On the other hand, the fuel cell has not this limitation. It would only be affected in case the inlet hydrogen flow incorporates impurities.

Comments R2.7:

• From the sustainability point of view, how hydrogen has been produced (completely green?).

Authors Reply: Thanks for this interesting question. INTA has been working in hydrogen generation from renewable sources for the last 30 years. Yes, all the hydrogen consumed in our facilities is green hydrogen. It has been specified in the text and new references have been included.

Comments R2.8:

• In table 5, cost and CO2 emissions can be compared, if possible.

Authors Reply: The aim of the paper is to show the technological advantages of the performance of certain vehicles. The current cost of hydrogen-based technologies will always be higher until their use is more widespread. This is reflected in line 64, “But also disadvantages as current cost of hydrogen technology and availability of recharging points”. CO₂ emissions have not been considered since they do not occur during the use of the vehicle. The study of emissions in the manufacture and disposal of materials is outside the developed context.

Comments R2.9:

• In Fig. 7a the y-axis unit; voltage should be expressed in Volts rather than Amps.

Authors Reply: Authors apologize this mistake. It has already corrected in the new version of the manuscript.

Comments R2.10:

• Section 3.3 the discharge rate (heat transfer and the influence of temperature) for a battery cell has also been available in the literature (Energies 13 (2020) 1477) by designing a module with an indirect cooling system, which is a first-order demonstration. Please compare this prototype model and discuss.

Authors Reply: Following with reviewer’s recommendation, authors have incorporated it to the new version. 

Reviewer 3 Report

• The paper should be shown detail the speed of vehicle when the dynamic road test was conducted.
• The paper should be mentioned the initial experimental conditions in detail.
• The paper has a number of errors in format, so the paper should be corrected for formatting errors.
• The paper has some grammatical errors, so the paper should be improved in English.
• In “Introduction” includes a very poor literature review as well as very limited references.

In general, there are still editorial and technical errors in the manuscript including formatting.

Author Response

Reviewer: 3

Comments R3.1:

• The paper should be shown detail the speed of vehicle when the dynamic road test was conducted.

Authors Reply: Authors thank the reviewer’s comment. In section 3.4 the speed at which the tests have been developed is detailed: “The on-road autonomy test has covered a total of 48 km at an average speed of 12 km/h”.

The dynamic laboratory test consists in applying acceleration over the motor and next, provoking friction or manual braking. For dynamic road test, a real driving test is applied over the unipersonal electric vehicle in movement at an average speed of 12 km/h.

Comments R3.2:

• The paper should be mentioned the initial experimental conditions in detail.

Authors Reply: Regarding Reviewer’s suggestion, the initial experimental conditions are detailed in the initial state of charge (SOC) and in the initial pressure of the hydrogen tank. These are shown in following graphs:

• Figure 7: initial SOC = 85%
• Figure 8: initial SOC = 80%
• Figure 9: initial SOC = 60%; initial pressure = 154,6 bar
• Figure 10: initial SOC = 93%; initial pressure = 149,3 bar

Comments R3.3:

• The paper has a number of errors in format, so the paper should be corrected for formatting errors.

Authors Reply: Authors apologise the spelling error. We have revised the whole manuscript, with the aim to correct them.

Comments R3.4:

• The paper has some grammatical errors, so the paper should be improved in English.

Authors Reply: Authors thank the reviewer’s comment. All the found grammatical errors have also been corrected and included in the new version.

Comments R3.5:

• In “Introduction” includes a very poor literature review as well as very limited references.

Authors Reply: A new literature review has been done and new references have been included in the Introduction Section.

Comments R3.6:

• In general, there are still editorial and technical errors in the manuscript including formatting.

Authors Reply: Authors apologise the editorial and technical errors. We have revised the manuscript, and we hope new version is free of them.