An unjustified exclusion
In 2022 was published the paper:
[1] Murphy, David J., Marco Raugei, Michael Carbajales-Dale, y Brenda Rubio Estrada. «Energy Return on Investment of Major Energy Carriers: Review and Harmonization». Sustainability 14, n.o 12 (enero de 2022): 7098. https://doi.org/10.3390/su14127098.
which claimed to have achieved “a much-needed update and harmonization of the EROI literature, and it advances the conversation surrounding the viability of renewable resources in the energy transition process.” It seems the paper has gained considerable popularity, and we have frequently received inquiries about it in relation to our own EROI work.
The authors of [1] set up a systematic literature review (see details in section 2.5 ‘Literature review’ of the paper) selecting databases, time period and key words. Our 2020 paper ‘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’ [2] was excluded from that work despite it fulfilled all the conditions to be included, as it:
- was published between 2017-2020
- computes original estimates of standard EROI
- includes in its title the word “EROI”
- is published in an indexed journal
Given the lack of transparency in the paper about the identification and screening processes (37 papers were finally analysed from an initial list of 113), we reached out to the authors directly to request an explanation.
We had some interesting correspondence with the main author and one of the co-authors between summer 2022 and summer 2023, during which they were unable to explain why our study was excluded from the analysis. The conversation progressed and their conclusion was that adding one study to the sample would anyway not change the “general trends observed”.
In the email exchange, we received some general critiques to our work in [2] which may have prevented to be included in the review & harmonization study [1] which however do not hold, notably, to our EROIfinal definition. This critique do not hold because in our 2020 paper we also computed the EROI at standard (conventional) point of view (in fact it is a previous step to computing the EROIfinal). We very clearly differentiated the boundaries both in data (supplementary material) and results. So, there is no scientific nor practical reason to have excluded our paper from the review.
Our conclusions are the following:
- The validity of the review [1], and hence the extracted conclusions, is not assured. At least one paper matching the keywords and requirements is not there without an explanation, while for example a paper from authors’ colleagues was manually added (“one exception to this search was made, and that is the inclusion of Leccisi and Fthenakis 2021”). Paradoxically one of the said criteria for exclusion was “lack of transparency”… If one study could be an “outlier” as mentioned by the authors, a proper harmonization study should analyse this work with particular detail to confirm (or not) and explain that outlier hypothesis. Overall, the performed review does not follow basic proper scientific method.
- 3/4 authors of the review have in the past already misrepresented (and hence, cited) our work (cf. Reply to Fthenakis et al., (2022) (refused to be published by Energies, MDPI) [3])1. So our work is not unknown to the authors (which is not surprising given that our work on EROI [2,4,5] accumulates 180 JCR citations in the last 5 years). So, the feeling is one of disregard for our work.
- The low quality of MDPI publications is again confirmed. The article is even tagged as “Editor’s choice”, ie., articles “based on recommendations by the scientific editors of the MDPI journals from around the world”. We deeply regret having published in MDPI Energies our work on EROI.
(In)consistent comparisons
The topic of the comparison between the EROIfinal of electric renewables and that of liquid and gas fossil fuel products was also raised. The critique in the words of one of the authors is that “for the comparison to be even meaningful, at the very minimum the same type of energy carrier would have to be considered. And if “electricity at point of consumption” were consistently selected as the supply chain stage of choice for the purposes of this comparison, then not only would oil incur in the investments for refining and transport […], but it would also incur in the additional energy investments for converting the heavy fuel oil to electricity, plus, finally, ALL the exact same energy investments associated to the O&M of the grid as previously calculated for renewable technologies.” And they added later: “direct electrification (coupled with low-C electricity generation) IS a key strategy to speed up the decarbonization of our societies in general, and many/most key high-energy-using sectors CAN BE and ARE BEING converted to electric (among which, passenger vehicles, domestic heating/cooling/cooking, etc.). E-fuels should be considered either as a “temporary fix” to tide us over, or as a “last resort” for ONLY those sectors for which direct electrification isn’t feasible (like long-haul aviation and oceanic maritime transportation)”.
To which we responded:
“About your critique to the “comparison”, which is not related to our query, but I can answer anyway with a counterexample: let’s think that we bring the comparison to common reference “liquid fuel” (since I hope we can agree that not all machines in the world -notable those requiring very high power- will be able to work with electricity). Then, if we start the chain from the electricity from renewables, this would mean: electricity -> H2 -> synthetic fuel, whose EROI should be much lower than that of the initial electricity. So we can arrive to very different conclusions comparing EROIs by choosing a different reference, which is then not robust comparison.” And later we added: “Storage systems and e-fuels can drain enormous amounts of net energy from the system even if a majority of the energy mix is electric.”
These so-called ‘hard-to-abate’ sectors (synonym in fact for ‘unfeasible-to-large-scale-electrify’) are generally a blind spot in EROI studies comparing RES and fossil fuels, as for example in the recent ‘Estimation of useful-stage energy returns on investment for fossil fuels and implications for renewable energy systems’ [6]. However, despite [6] took [1] as reference for the EROI of solar PV and wind in their main paper, our work was at least considered in their Supplementary Material.2
As we argued in section “5.1. On the Role of Future Technological Change” in de Castro & Capellán-Pérez (2020) [2], in the future there will be some very relevant factors which will tend to offset potential technical improvements as the renewables progressively scale-up at large levels and gain a substantial share in the energy mix.
Of those and particularly related with the aforementioned email exchange, we are in fact currently working on the implications for the EROI of the full system to introduce hydrogen and hydrogen-fuels in those “minority” ‘hard-to-abate’ sectors.3 Preliminar results are already confirming that the combination of the fact that these “minority” ‘hard-to-abate’ sectors consume in reality significant amounts of energy, the low efficiency of the hydrogen supply chains, the non-linear nature of EROI and dynamic factors all together tend to alarmingly worsen the system’s net energy returns. As we have argued since 2019 [5], the relevant EROI is the dynamic, system-level one, rather than the conventional static, technology-specific metric. We hope to publish some work on these topics in the near future. Stay tuned if you’re interested!
Overall, we hope that these notes help shed some light on the ongoing dicussions around EROI.
Iñigo Capellán-Pérez & Carlos de Castro
1. This approach of criticizing our work without substantive arguments and demonstrating a lack of basic understanding is, unfortunately, far from uncommon, see eg. [7]: “Ref. [174, Ref [2] in this post] estimated an EROI of 8.7 for the standard offshore wind technologies. This low number could be the result of pessimistic assumptions that need to be tested, including the assumption that “indirect investments of Renewable Energy Systems constitute at least 100% of the total direct energy investments estimated” by typical LCA analyses.”
The authors vaguely mention “pessimistic assumptions”, and the only specific critique which is addressed to the indirect investment’s estimation does not in fact affect the standard EROI computation (but the EROI extended one). Note that this also highlights a failure in the review process (once again, with MDPI involved).
2. In the future we may comment on [6], which in our opinion represents a very interesting method, although it also includes several important limitations.
3. This effort is partially being devoted within the HYDRA EU Horizon project in which we participate.
References
- [1] Murphy DJ, Raugei M, Carbajales-Dale M, Rubio Estrada B. Energy Return on Investment of Major Energy Carriers: Review and Harmonization. Sustainability 2022;14:7098. https://doi.org/10.3390/su14127098.
- [2] de Castro C, Capellán-Pérez I. 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. Energies 2020;13:3036. https://doi.org/10.3390/en13123036.
- [3] Fthenakis V, Raugei M, Breyer C, Bhattacharya S, Carbajales-Dale M, Ginsberg M, et al. Comment on Seibert, M.K.; Rees, W.E. Through the Eye of a Needle: An Eco-Heterodox Perspective on the Renewable Energy Transition. Energies 2021, 14, 4508. Energies 2022;15:971. https://doi.org/10.3390/en15030971.
- [4] Pulido-Sánchez D, Capellán-Pérez I, Castro C de, Frechoso F. Material and energy requirements of transport electrification. Energy Environ Sci 2022;15:4872–910. https://doi.org/10.1039/D2EE00802E.
- [5] Capellán-Pérez I, de Castro C, Miguel González LJ. Dynamic Energy Return on Energy Investment (EROI) and material requirements in scenarios of global transition to renewable energies. Energy Strategy Rev 2019;26:100399. https://doi.org/10.1016/j.esr.2019.100399.
- [6] Aramendia E, Brockway PE, Taylor PG, Norman JB, Heun MK, Marshall Z. Estimation of useful-stage energy returns on investment for fossil fuels and implications for renewable energy systems. Nat Energy 2024:1–14. https://doi.org/10.1038/s41560-024-01518-6.
- [7] Samsó R, Crespin J, García-Olivares A, Solé J. Examining the Potential of Marine Renewable Energy: A Net Energy Perspective. Sustainability 2023;15:8050. https://doi.org/10.3390/su15108050.