Standard, Point of Use, and Extended Energy Return on Energy Invested (EROI) from Comprehensive Material Requirements of Present Global Wind, Solar, and Hydro Power Technologies
Round 1
Reviewer 1 Report
The authors present a very interesting study on the energy return on energy invested for different renewable technologies for electricity generation. The paper is very well-written and of high interest, since it deals with a very important aspect. The huge work performed by the authors is evident, but I think it could be better exploited by improving the quality of the paper.
The paper seems a little longer than usual, the authors may consider the possibility of moving some parts to the appendices to make the reading more fluent (especially on the methodology and hypotheses).
The authors provide single results for the EROI of each technology, which I think is a little limited, given the multiple assumptions that they present in their work. I think it should be important to try to assess the level of accuracy of their results, through a sensitivity analysis on the main hypotheses or at least incorporating an assessment of the potential uncertainty of their evaluations.
Some other recommendations are the following:
The literature review on EROI methods (in particular in lines 71-81) is an important aspect, and additional information could be given on the main discrepancies and aspects that lead to the non-comparability of different studies. This is important to support the innovative contribution of this work, which is well described at the end of the Section 1.
Figure 4 is not very clear, as I guess a significant part of the “energy used” (represented in green) is actually electricity (which is represented in blue). Please clarify the figure.
Line 349, footnote. I agree with the authors on the importance of accounting for the actual diesel upstream energy consumption. It seems that the authors have calculated this “g” factor, but they haven’t applied it in their analysis. Why?
Lines 360-364: it is not clear why the ratio of final energy consumption vs primary energy consumption should tend to 1 with increasing electrification. Power plant losses and grid losses will always exist. Moreover, all these calculations are made under the hypothesis that for RES the primary energy consumption is the same as the electricity output (as done by IEA, Eurostat, etc.). I am not sure this is a good way to consider potential improvements of efficiency, since for instance both PV and wind have an actual efficiency in converting available primary energy (i.e. radiation and wind energy). What do you think about this aspect?
Why the CF of PV and CSP is different? I understand that the storage may allow for using energy during more hours, but the capacity factor should be defined as the energy produced over the maximum theoretical energy generation (i.e. nominal power times hours per year). Thus, I expect that these values should be similar for these two technologies, since they rely on the same energy input (i.e. solar radiation). In the same way, if you add a battery to a PV system you may increase the flexibility of the plant, but the CF should not change (and it should even decrease due to the battery losses). Moreover, current CSP are installed in locations with average higher radiation than PV (which are more diffused), but this aspect is not strictly related to the technology itself. Please elaborate on these issues.
Section 3.4. Since this aspect seems to be important, the authors should justify their choice of 10,000 km by ship and 500 km by truck. For some technologies, the majority of installations may occur in the country/region of manufacturing (e.g. PV in China, hydropower using cement produced locally, off-shore wind in Europe relying on European manufacturers, etc.). In particular, these numbers may vary significantly in time, depending on the location of new power plants and the evolution of the supply chains in different countries. While I understand the complexity of obtaining world average numbers, those approximations should at least be mentioned.
Figure 5: The lines should be removed from the chart, since they have no significance, given the fact that the points represent different countries and conditions.
There are minor English errors in the manuscript, please double-check it before the final submission.
Some minor corrections.
Iine 266: “1MW = 106 J/s.” the “^” symbol is missing. (but I would avoid the definition of “MW”…)
Line 306: “wheel-to-wheels” should be “well-to-wheels”? How have 1.09 and 1.19 obtained?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper presented on the Energy Return On Energy Invested (EROI) for the five different renewable energy systems. Overall the paper is presented well, however, the following points are needed to consider:
- Validation of the proposed model with some published work, if possible, maybe done for better presentation.
- The conclusion needs to be more concise and a little more quantitative.
- Well-written recommendations for possible improvement of the existing RE system would be helpful to attract more readers.
- What are the additional findings that this work considers as that of the earlier work "Capellán-Pérez, I.; de Castro, C.; Miguel González, L.J. Dynamic Energy Return on 1131 Energy Investment (EROI) and material requirements in scenarios of global transition to renewable energies. Energy Strategy Reviews 2019, 26, 100399, doi:10.1016/j.esr.2019.100399".
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
In the paper presented for evaluation, authors consider the current global EROI for the five RES energy technologies regarded to have the highest theoretical potential for electricity generation (solar PV and CSP, wind onshore, wind offshore and large hydroelectricity). EROI is determined at standard or primary and point-of-use or final. This approach allows to identify those steps where more energy investments are required, as well as a consistent inter-comparison between the levels of different technologies.
Paper contributes to the debate by estimating the current global-average EROIst (standard farm-gate), EROIfinal (consumer point-of-use) and EROIext (extended, including indirect investments). The same methodology, based on an extensive and comprehensive literature review, is applied in order to collate data about the material requirements for the power plants’ construction and operation of each technology, which allows for an internally consistent inter-comparison between the EROI levels of different technologies.
The paper covers the topic of EROI very extensively. It forms a kind of a manual for persons interested in splitting the different costs into the fundamental ones. The paper is well structures and very comprehensive. Although it is a very long paper it is not "boring" to read. I recommend that paper for publishing in Energies in the submitted form.
Author Response
Thank you
Round 2
Reviewer 1 Report
The authors have properly addressed all my comments. I recommend the publication of the manuscript in its present form.
