Experimental Validation of Hydrogen Fuel-Cell and Battery-Based Hybrid Drive without DC-DC for Light Scooter under Two Typical Driving Cycles

Round 1

Reviewer 1 Report

The manuscript describes the design and experimental evaluation of a passive electrical system for managing the energy on board a fuel cell/battery hybrid electric scooter without the use of a DC/DC converter. Both the title and abstract are accurate indications of the contents and the work appears to be original, relevant to the journal and of interest to academics and engineers working in the area.

The concept of the work is very interesting and demonstrates the practical performance of a passively controlled fuel cell hybrid system – particularly relevant to low-cost markets. However, the methodology lacks rigour and comparison to active systems is very limited (“similar to an active fuel cell/battery hybrid power system” and a single reference from 6 years ago). The analysis of the system performance is limited to positive aspects and fact such as the SOC of the battery falling during the Chinese Standard driving cycle is not acknowledged. Also, although the energy management strategy is described briefly, the results only show a single operating state of the Finite State Machine – that of the fuel cell active – which it appears to be the case for both cycles tested. This suggests that the fuel cell is likely undersized for the application. Again, this fact is not mentioned by the authors.

The authors have included some mention of transient operation in their results, but this should also be included as a potential downside of passive systems in the introduction and literature review at the beginning of the manuscript.

In summary, the system shows some promise, but it also has a number of potential disadvantages. The authors do not discuss the issues with such a system, and therefore do not present an unbiased evaluation of the proposed design.

Author Response

Q1：The manuscript describes the design and experimental evaluation of a passive electrical system for managing the energy on board a fuel cell/battery hybrid electric scooter without the use of a DC/DC converter. Both the title and abstract are accurate indications of the contents and the work appears to be original, relevant to the journal and of interest to academics and engineers working in the area.

R1：Thank you for your kind review.

Q2：The concept of the work is very interesting and demonstrates the practical performance of a passively controlled fuel cell hybrid system – particularly relevant to low-cost markets. However, the methodology lacks rigour and comparison to active systems is very limited (“similar to an active fuel cell/battery hybrid power system” and a single reference from 6 years ago).

R2：Thank you for your advice. Our works are financed by the Bosch China (RBCN) and the Natural Science Foundation of Shanghai. For the reasons of intellectual property, some key algorithms are not published in this paper. Some new reference (38,39) has been added in the line 278.

Q3：The analysis of the system performance is limited to positive aspects and fact such as the SOC of the battery falling during the Chinese Standard driving cycle is not acknowledged.

R3：Thank you for this good advice. The fact of the battery SOC falling during the Chinese Standard driving cycle is added and emphasized in the paper from line 288 to 290 shown as “However the high output power demand of the motor results in the falling of the battery’s SOC, and the output current of the fuel cell will increase and result in more energy loss on the power resistor”.

Q4：Also, although the energy management strategy is described briefly, the results only show a single operating state of the Finite State Machine – that of the fuel cell active – which it appears to be the case for both cycles tested. This suggests that the fuel cell is likely undersized for the application. Again, this fact is not mentioned by the authors.

R4：Thank you for this good advice. The fuel cell is likely undersized for the application has been added in the line 303.

Q5：The authors have included some mention of transient operation in their results, but this should also be included as a potential downside of passive systems in the introduction and literature review at the beginning of the manuscript.

R5：Some literature(new reference 21 and 22) review of transient operation for the downside of passive systems has been added at the beginning of the manuscript from the line 40 to 43.

Q6：In summary, the system shows some promise, but it also has a number of potential disadvantages. The authors do not discuss the issues with such a system, and therefore do not present an unbiased evaluation of the proposed design.

R6：Some potential disadvantages has been discuss in the end as “However, it is highlighted that there are some disadvantage of this system such as the low velocity, thermal balance problem, etc.” from the line 330 to 322.

Author Response File: Author Response.docx

Reviewer 2 Report

As far as I am concerned, the work presented by the authors is publishable in the following form, due to its content.
Just a note on the quality of the figures: figure 2 should be enlarged to make readable the text inside it; in figure 4(a) you can see a black border that is not there in figure 4(b);
in figure 5 the two graphs should be better aligned and the legends moved so as not to be superimposed on the data;
Figure 7(b) should show the same parts as figure 7(a);

Equation 1 is not centred with respect to the centre; in fact after = it goes to the next line.

The conclusions should contain some considerations for the future development of this activity.

Hoping that my comments will help to improve the presentation of the results, I wish you good luck with your research!

Author Response

Q1:As far as I am concerned, the work presented by the authors is publishable in the following form, due to its content.

R1：Thank you for your kind review.
Q2: Just a note on the quality of the figures: figure 2 should be enlarged to make readable the text inside it;

R2：The figure 2 has been enlarged.

Q3: in figure 4(a) you can see a black border that is not there in figure 4(b);

R3：The figure 4 has been modified.
Q4: in figure 5 the two graphs should be better aligned and the legends moved so as not to be superimposed on the data;

R4：The figure 5a and 5b have been modified.
Q5: Figure 7(b) should show the same parts as figure 7(a);

R5：The figure 7b has been modified as the same part of the figure 7(a).

Q6: Equation 1 is not centred with respect to the centre; in fact after = it goes to the next line.

R6：The eq.1 has been modified.

Q7:The conclusions should contain some considerations for the future development of this activity.

R7: some considerations have been added as “The efficiency of the passive hybrid system is closely related to the fuel cell operating point that could be carefully controlled by means of advanced control system. Therefore, the energy management of the scooter will be furtherly optimized at different operating parameters and in different power demands by the scooter depending on the type of the driving cycles.”

Q8: Hoping that my comments will help to improve the presentation of the results, I wish you good luck with your research!

R8: Thank you for your kindness. 

Author Response File: Author Response.docx

Reviewer 3 Report

• serious flaws regarding methodology and Research design, novelty not laid out properly
• results aren't put into any relation towards the state of the art - no Evaluation possible
• extensive language editing required

Author Response

• serious flaws regarding methodology and Research design, novelty not laid out properly

R1: The manuscript describes the design and experimental evaluation of a passive electrical system for managing the energy on board a fuel cell/battery hybrid electric scooter without the use of a DC/DC converter. The title and abstract are accurate indications of the contents and the work is original, relevant to the journal and of interest to academics and engineers working in the area.

Moreover, a novel configuration of the passive fuel cell/battery hybrid power system for an e-scooter for Asia market with a power resistor and a diode is proposed for the low cost, and in order to match the power output feature of fuel cell stack and battery for the same voltage. Simultaneously the test bench is self-developed and set up to experimentally evaluate the power demand and efficiency under two driving cycles with a realistic and complicated traffic condition.

Our works are financed by the Bosch China (RBCN) and the Natural Science Foundation of Shanghai. For the reasons of intellectual property, some key algorithms are not published in this paper.

• results aren't put into any relation towards the state of the art - no Evaluation possible

R2: The experimental results showed that the output power of fuel cell stack is stable, which contribute good durability of the light scooters on road, and it can also well hold a good consistency of the stable output power of fuel cells under two typical driving cycles of this passive hybrid power system for the electric scooter.

Moreover, the effective efficiency analysis of fuel cell stacks in the passive hybrid power system is evaluated, the average efficiency of the passive hybrid power system is relatively high (88% in the Shanghai urban driving cycle and 89.5% in the Chinese standard driving cycle). Based on this evaluation, the passive fuel cell/battery hybrid system without DC-DC converters is practical for the commercial scooters.

• extensive language editing required

R3: the main problems of language have been modified

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Dear authors, thanks for improving the contribution. Especially adding reference in line 294 to assess your results was helpful.

Before Publishing I suggest once again to check spelling, wording and grammar. P.e. , in lines 28, 33 and 391 "Energy conversation" is used (even referring to a non-existing journal). Such, and further, obvious mistakes really should be fixed.

Author Response

Thank you for your kind review.

1. In line 28, “energy conversation resource” has been modified as “alternate energy resource”.
2. In line 33, “energy conversation resource alone” has been modified as “energy resource alone”.
3. In line 41, “caused by” is added.
4. In line 49, “in energy conversion system design.” has been modified as “in powertrain system design”.
5. In line 58, “is” has been modified as “was ”,and “can” was replaced by “could”.
6. In line 67, “is” has been modified as “was ”
7. In line 103, “for scooters” has been deleted.
8. In line 130, “Figure 2. Architecture of the passive fuel cell/ battery hybrid power system for electric scooter”has been modified as “Figure 2. Architecture of the passive fuel cell/ battery hybrid powertrain system for electric scooter”
9. In line 131, to avoid the repetition, “the hydrogen-flow, air-flow, main electric circuit, sensor and control signals are individually presented. C” has been deleted.
10. In line 138, add´s after scooter
11. In line 165, “(frequent acceleration in cities)”has been deleted.
12. In line 173, “would ”has been replaced by “will”.
13. In line 223, “energy conversion system” has been modified as “performance”
14. In line 257, “power” is added.
15. In line 329, “combined with the driving range” has been deleted.
16. In line 391, “Energy Conversation and Management” has been modified as Energy Conversion and Management

Author Response File: Author Response.docx