Towards a Safe Hydrogen Economy: An Absolute Climate Sustainability Assessment of Hydrogen Production

Round 1

Reviewer 1 Report

It is an interesting work with useful results that will be clearly of interest to the journal readership.

In general, I agree (or I don’t disagree) with most of the conclusions of the study and the reasoning behind it as presented by the authors in the manuscript. It is a useful attempt to assess and quantify (based on current estimations of several parameters used as input to the study) the prospects of hydrogen economy to operate in a safe space (in relation to achieving net zero GHG emissions) by 2050. In my opinion, the clearest and best presented section of the paper are the Conclusions. There, the limitations of the work are recognized, the main results are summarized and the authors’ general stand relative to hydrogen is clarified.

However, in the sections before the Conclusions, the presentation does not look to me as clear in several points. To start with, the question posed by the authors right from the beginning (introduction) is how to assess the role of hydrogen in keeping humanity in a net zero, safe operating mode by 2050. The adopted methodology to deal with this question is provided along with certain purposes that the study serves (e.g. avoid unwanted lock-ins and/or misallocation of priorities away from energy efficiency (EE) and direct electrification (DE) based on RES).

I think that this is not the only question to ask presently. The story should begin by asking what are the existing alternatives (besides hydrogen) particularly for the “hard-to-abate” sectors. Everyone agrees about the importance of EE and DE but these are simply not enough or not possible (technically speaking) especially for these sectors. What would be the results in terms of consumption of remaining carbon budget, “overshoot” (whatever that means) etc, if such alternatives are to be followed?  And make no mistake, any alternative has to be first developed for large scale and after that it will take at least a decade for a wide implementation in the market. Do we have as humanity this time? Clearly no, 2050 approaches fast. Hydrogen technologies are not unproven. On the contrary, they have been developed and tested at large scale and are (technically) ready for wide employment if the economic conditions will allow for the development of the required infrastructure etc. Only after considering the possible alternatives (if any), we can start making critical statements and I think that this aspect has to be added to the paper. We should worry about avoiding lock-ins but we are as humanity very close to the point-of-no-return and the fear of lock-ins makes sense mainly if viable alternatives exist.

The authors also (correctly in my opinion) point out the scale of electricity that will be needed to power the hydrogen economy configurations (and this should come from a big increase in RES use). This is a well known issue but equally well known is the answer: What is the alternative presently? And, I repeat, there is practically no time to develop new alternatives. We all accept that our best shot today is to enhance dramatically RES penetration globally (how exactly this might happen is a technical aspect that should be resolved, ideas exist).

Subsection 3.5 shows useful results (including figure 3). It should be stressed in the paper (even before 3.5) that the present EU-taxonomy and CertifHy values are not meant to remain constant in the future. They represent current estimates that should decrease significantly with time (even by one order of magnitude, e.g. in 2040-2050). Otherwise, it is evident that hydrogen will improve things but will not bring the targeted benefits. The means to achieve this dramatic decrease of emissions per unit H2 are presently thought to be both technical and economic. This aspect should be clearly included in early parts of the paper.

The arguments about the role of possible de-growth and demand side alterations are very useful and in the correct direction, I think. However, it is obvious that while helpful, they cannot solve the problem. It is clear that they should be part of the solution though. An interesting point here would be to estimate what levels of de-growth / reductions in demand would be required by 2050 in order to appreciably affect the overall balance.

Blue hydrogen is accounted for often in the paper. It is clear now that most experts and organizations (like the EC) do not consider blue H2 (not even CCS) as a viable option in the long run. Low carbon H2 is brought into play only with regard to the creation of a H2 market in the coming years. So, I would suggest to reduce significantly the role attributed in the study to blue or low carbon H2.

The major uncertainties associated with the particular LCA studies (input – output) should have been discussed in the manuscript in a more pronounced way.

It is clear that public awareness and acceptance are key aspects for the wide adoption of hydrogen. Social studies of the kind proposed in the conclusions section are important and very useful. They should be carried out in parallel with the technical developments for hydrogen infrastructure because we have NO time available as humanity. Generally, this aspect of how critical the situation is timewise is not adequately addressed in the paper. It is important as it may affect many decisions and modes of moving forward.

Other points / comments that may improve the manuscript:

·     -  The term “climate overshoot” that is used in several places (e.g. 500-800% by 2050) should be better defined.

·      -  The present target of 1.5 C is thought to be by many “history”, i.e. there exist no credible chance to achieve this anymore (despite the fact that for “political” reasons it is still officially used). A target of 2 C seems more likely now and this may affect the remaining carbon budget.

·  - It is evident that many of the parameters involved in the study are characterized by considerable uncertainties. They should be better pointed out and I think that it would be interesting to propose (or even conduct if possible) sensitivity studies for some of these parameters to assess the potential impact of the uncertainties.

·        -  English is generally good. Only few typos exist and should be corrected by the authors.

 

Author Response

Please see the attachment where your comments are labelled as Reviewer 1

Author Response File: Author Response.docx

Reviewer 2 Report

As the authors correctly state in the conclusions, the study is quite simplistic, but it does show the challenges of adopting hydrogen as an energy vector in the just transition. The limitations should be made clear upfront - in the title and the abstract. It is not an absolute environmental sustainability assessment. Although land use is highlighted, it is largely focussed on carbon emissions. It is also not an assessment of the hydrogen economy. It is focussed on hydrogen production only. How hydrogen will be used in the economy will have significant implications, which the authors do mention in the paper. Indeed, if entire value chains are considered a 'safe' hydrogen economy may not be achievable.

Other issues:

The authors should not refer to hydrogen as a technology. As highlighted from line 67, it is an energy carrier, or vector. Perhaps rather use the term commodity, as introduced in line 70.

The grammar needs attention. For example, in line 145 it should read: "are described".

Footnote 3 - in Table 2 - is not shown.

Lines 568 to 584 should be removed. Although true, the content of the paper does not support these viewpoints.

Author Response

Please see the attachment where your comments are labelled as Reviewer 2

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

My comments have been satisfactorily addressed by the authors and I wish to thank them for that.

Author Response

We thank you very much for your thoughtful and constructive comments.

Reviewer 2 Report

The revisions to the paper are noted, but the following should still be addressed:

The title should probably read: Towards a 'safe' hydrogen economy: An absolute climate sustainability assessment of hydrogen production

Line 15 in the abstract: climate overshoot of at least 5.4-8.1x - since the focus of the paper is on hydrogen production only and not the rest of the value chains.

The other aspects in the content of the paper have sufficiently been addressed.

Author Response

Please see that in the manuscript the described changes have been made.

The title was changed in a different version and was accidentally not included in the revision.

We thank the reviewer for their time and thoughtful comments.