Identification of the Optimal Passenger Car Vehicle Fleet Transition for Mitigating the Cumulative Life-Cycle Greenhouse Gas Emissions until 2050

Round 1

Reviewer 1 Report

Authors have provided an interesting study on the optimization of fleet turnovers for the German case study to minimize GHG emissions. The findings from study can help decision makers over the 30 years planning horizon to achieve the IPCC emission reduction goals.

Although the manuscript is eloquent and easy to follow, it is verbose. There are too many figures and tables which are suitable for a technical report but not a journal article. It is good to explain every detail, but this can backfire and dissuades the readers to follow the paper to its conclusion. please try be more concise. 

Summarize the introduction in a more scientific literature review manner. I expect to see more scientific literature review such as this paper in Transportation Research Part A:

Stasko, Timon H., and H. Oliver Gao. "Developing green fleet management strategies: Repair/retrofit/replacement decisions under environmental regulation." Transportation Research Part A: Policy and Practice 46.8 (2012): 1216-1226.

The steady share of solar and hydropower in total capacity of German energy sector over 30 years depicted in figure 5 does not seem reasonable. Can you please double check on that?

Regarding the load factor, figure 6, one can assume that technological advancement, in particular for solar power, will affect the load factors in future? Is this correct? And if yes, how assuming constant load factors over year can be justified? Maybe revise how you define the load factor.

it is hard to follow the logic behind the energy sector scenarios. Does figure 5 depicts the neutral scenario? Figure 9 a and b parts show a different installed capacity for the 2050? It is reasonable to assume different share for different sources under different scenarios, but the total supply should match the demand in the future regardless s of the composition.

The assumption of VKT reduction over 30 years planning horizon does not seem correct. With the existing trend in vehicle usage and population increasing over years, we can assume that VKT will increase over years. Can you please explain your assumption on the optimistic scenario in more details? It is very important since the sensitivity analysis show how result are sensitive to transportation activity.

Can you add more details on tailpipe emission on figure 15 for ICEVs? with 12.5 years traveling and 173,906 Km of driving and assuming 40 km/hr and the ball park estimate of 400 gr/km (US EPA) for ICEV this translates to 69,200 kg GHG emissions over the lifetime and when divide by 12.5 for 1 year, it would be interesting to compare with PHEVS and BEVs.

The paper heavily relied on the result section and less on the conclusion and policy implications. I expect to see more elaborate conclusion and discussion section. As author stated the BEV has lowest life cycle GHG emission so their early introduction is an obvious strategy. But what are the hinders and how the results from this study help to overcome them?

Minor comments:

Line 69, why LCA is a good approach, either explain or cite to another study.

The emission during EoL phase need more explanation from line 393.

The optimization framework should be included in the text.

The manuscript will be benefit from a editing service. For example, line 499 shouldn’t start with “So far”.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper aims to find the fleet transition in Germany while considering optimal ways to reduce GHG emissions. Topic is worth pursuing. However, I have several comments below about how the analysis is framed and presented. 

Below are a few comments that may be useful to expand on the authors' work.

The introduction could be streamlined to include more relevant studies. It is better that the authors include more studies in the introduction and make the case for why this paper is different and how it contributes to the existing research. The statements like “A series of studies have performed…” should be referenced properly.      The authors discussed about the meta-analysis of the life-cycle GHG emissions of batteries. However, more detail is needed. For example, it is not clear how included studies in the meta-analysis are selected (selection criteria, inclusion and exclusion criteria), how and when the search is done, etc.  My major concern is related to the proposed scenarios. For battery production scenarios, how do the authors define three different scenarios and how they are different from each other? They mentioned that the pessimistic or optimistic scenario are defined so that their emission reduction rates to be slower or faster. In what extent?  Again, for Energy Sector Scenarios, how the pessimistic or optimistic scenarios are different than the neutral or business as usual? It is not clear how the authors came up with three scenarios for hydrogen production path. Please remember this is a scientific paper and there should be a scientific reason for any assumption. I am not convinced with the way that authors detailed the scenarios.  It is totally unclear how the sharing mobility which is increasing rapidly, is reflected in the mobility scenarios. Why the authors assumed that VKT will be decreased over time and I am not sure about it. I recommend authors to do research about the travel demand modeling results for Germany for more accurate estimation of VKT.  For optimization modeling, the theoretical framework is missing. The authors need to build a theoretical framework to lay foundations for modelling. The authors may provide some policy implications in the end. There are 21 figures in the paper, and it is hard to follow all of them.  Further, the readability of this paper is affected by several typos, grammatical errors, and punctuation issues.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

This is a really well-written paper, and the authors have done a meticulous job in designing the sensitivity analysis and presenting the results. A couple of minor comments:

The entire problem statement is expressed in text form in several places. An equation or a set of equations showing the objective function/constraints would be helpful for the readers to better understand the entirety of the problem in consideration, as it involves several parameters (and it's difficult to keep track of them). A better pedagogy on the assumptions on sensitivity analysis might add more value to the paper. For eg. under Mobility trend scenarios, starting from line 310, the authors mention that they generate the three scenarios for passenger-distance demand. It's understandable that these are pessimistic, neutral and optimistic scenarios--but how were these numbers assumed? It is based on any literature or an educated guess? The same applies to several scenario assumptions in previous sections. Any discussion on the cost of carbon for these scenarios or a commentary on what that would look like? I feel the 'Conclusion' section could be better written/rearranged. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I believe the revised manuscript is in a good shape and is ready for publication. However, I think it might be valuable to add some explanation on how your findings for Germany can be used for other areas as well.

Reviewer 2 Report

Please proofread the text.