The Carbon Footprint of Hydrogen Produced with State-of-the-Art Photovoltaic Electricity Using Life-Cycle Assessment Methodology
Round 1
Reviewer 1 Report
Dear Authors,
the Review is quite interesting and relevant, but the the number of articles observed is not enough and should be expanded to one hundred. Also more papers of 2021-2023 should be observed.
Good luck!
Dear Authors,
please carefully check the spelling.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This paper evaluated the carbon footprint of PV-based hydrogen production. A very exhaustive literature review is first presented with tables and charts based on 22 publications. The analysis identified the background and foreground data used in the studies, as well as the module types and efficiencies assumed. A statistical analysis of global warming potential values was conducted, comparing a baseline scenario with two updated scenarios. Updating the life cycle inventory and using real data for electricity production led to significant reductions in global warming potential values, with a decrease of 59% and 52% for single-crystalline silicon and multi-crystalline silicon photovoltaic systems, respectively. The analysis highlighted the importance of photovoltaic system-related factors in the carbon footprint of hydrogen production. The study emphasized the need for state-of-the-art photovoltaic modules and highlighted the advantages of installing systems in regions with higher irradiance.
Overall, this paper is well written with a good literature review, detailed analysis, and clear illustration. The English language is also good and easy to understand. I recommend it to be published in the journal of energies after the authors fix the following small issues:
1. In the abstract and conclusion, the authors claimed that "any advancement in the PV industry and any saving in the electrolyzer’s electrical demand can further decrease the carbon footprint of PV-based H2", which, in my opinion, is too definite to be credible. Please try to avoid using "any" in a scientific paper.
2. Page 2 Line 66, PEM is not explained where it first appeared.
3. The pie chart in Figure 2 is not easy to read. Some shares are too small. I suggest the authors add numbers behind each item in the legend.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
In this paper, LCA of the carbon footprint on PV hydrogen production was investigated in detail and the GWP values of H2 production process were updated as well as the PV panels and PV module supply chain . In addition, Life Cycle Assessment for Italian case study was presented for all the evaluation.The findings also prove that any further advancement in the PV industry can further decrease the carbon footprint.
This paper is well organized and written. Abundant data and methods were shown, which gives a guideline to further reduce the carbon emission of PV based H2 production.
Minor editing of the English language required
Author Response
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Author Response File: Author Response.docx
Reviewer 4 Report
Review for the article: The carbon footprint of H2 produced with SoA PV electricity using LCA methodology.
Dear authors, editor.
Please find below my review for the article. I would like to commend the authors with a the production of a work that is reflective of a substantial time spent and has educational merit. However, I regret to recommend against publication in Energies as a scientific article in its current shape.
My comments center around two main critiques as regards to the scope and methods employed in the article. The article purports to be a review of carbon footprint estimates for hydrogen production based on electrolysis and powered by PV generated electricity. Simultaneously, the authors present original LCA work, with results based on an updated Life Cycle Inventory and inclduing scenarios and sensitivity parameters. Unfortunately, this mix of «two studies in one» means neither part reaches the expectation threshold for scientific publication.
As a community LCA practitioners, modellers and scientists, we have a responsibility to ensure that the results and claims we make can be independently validated and that results can be reused in further research and innovation activities. This implies that methods can be reliably replicated and will generate the same or similar conclusions, and that data, models, and results can be found, accessed, and reused in an interoperable manner.
In relation to the review part of the article, the authors identify 22 publications in Table 4. To me, it is unclear how this sample was constructed. The review protocol is missing, there is no search query on search engines such as Web of Science (WoS), lens.org, or Google scholar. Neither is it explained how the many results one gets when querying WoS for relevant studies were reduced to the 22 indicated studies. While I believe there is value in presenting an illustrative example set of relevant studies (which, in lieu of the above review protocal documenting reproducibility, this is at the moment) due to e.g. time constraints, the current review cannot claim give a comprehensive overview. (N.B. For a basic introduction to literature review in environmental sciences, I recommend the work of Haddaway at the Stockholm Environment Institute. Many of the approaches can be generalised for more «technology oriented» reviews as well.)
In relation to the LCA part of the article, the authors do provide key system characteristics, such as PV system lifetime assumptions and annual specific yields. However, this is an insufficient overview of the inventory, which is not shared (as Supplementary Information, or downloadable from a public repository such as Zenodo) with the article. Likewise, the (raw) LCIA results are missing, including results for other impact categories than Climate Change, though I respect the authors decision to limit their impact category scope to a single indicator. The authors do not justify the reason for withholding their inventory model or LCIA results.
At minimum, the article would greatly benefit from a system diagram documenting key foreground processes, main emissions and resource use, and their link to specific Ecoinvent background database processes. Secondly, to improve the understanding of the environmental hotspots in the value chain process-based contributions, advanced foreground contributions, and perhaps even emission based contributions can be expected in an LCA study. (Please note the latter is particularly relevant when splitting the article in a review article and an LCA article).
Finally, conceptually, the authors seem to mix «generically» applicable LCI inventories, with location, or regional-specific LCIs, so to me it comes at no surprise that carbon footprint results differ based on location, region, or parameters such as PV efficiency. An open LCA model reflecting these updated parameters in a high-quality inventory warrants publication in itself, but the current article lacks these elements.
While I recognise that there are ample journal articles published in the last 2 decades that do not meet the standard I hold the authors to, including many of the articles reviewed for this study. However, I sincerely hope the authors will agree with me future studies should be published in line with Open Science and FAIR principles, as is also reflected in the dissemination and exploitation requirements of e.g. European Union funded projects
I would like to wish the authors success in the deciding how to take their work further and hope I will be able to see an updated version of their study in the literature in the future.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Good luck!
Reviewer 4 Report
Dear authors, and editors.
Thank you for the major revision. While I still believe that the literature review protocol could be worked out in more detail, you have made a great step in allowing your work to be taken up by other researchers.
I therefore recommend publication.
