A Review of Energy Consumption in the Acquisition of Bio-Feedstock for Microalgae Biofuel Production

Round 1

Reviewer 1 Report

This manuscript reviewed the energy consumption of the processes leading to microalgal biomass acquisition for biofuel production. Overall, the paper presents fairly meaningful information on the energy consumption of cultivation processes but lacks novelty in the way forward. The manuscript requires further improvement on the recent technologies to enhance the energy efficiency of the operations. The paper should be revised before it can be considered for publication. Some specific comments are listed below:

1. Abstract: Include the results and conclusions of your findings.
2. Figures should be inserted after the text containing the figure citation and not the other way around.
3. In Fig. 3, change ‘sollar array’ to ‘solar array’, ‘Medium’ to ‘medium’ and ‘Water’ to ‘water’. How does the culture return to the degasser if the line designated as ‘harvest’ is used for harvesting.
4. Figure 3 is not cited in the text.
5. In Figure 4: Correct words as indicated for Figure 3. Indicate which Figure is ‘a’ and ‘b’ in the figure itself. Figure caption should be titled appropriately as this is under flat-panel pbr.
6. It is also necessary to mention the types of cooling systems that can be applied to pbrs and how this choice can affect the system's energy consumption.
7. Figure 5 should be cited in the text. The schematics should be distinguished with the letters a, b, c, and d, and defined in the figure legend.
8. Table 1: Change ‘PH’ to ‘pH’. What about O2 build-up in the tubes?
9. Table 2 is missing in the text.
10. In Figure 6, the quality of x-axis labels is poor and should be improved.
11. In table 3 title, change ‘radio’ to ‘ratio’
12. In Table 3, rows 3&4 after the heading row, change ‘Open’ to ‘open’ and it is usually called open raceway pond and not raceway open pond.
13. In Table 3, names of algae species should be written in italic and empty cells in the table should be indicated with a symbol and defined in footnotes.
14. In Figures 8&9, include the sample size, n, in the figure legends and these figures should be cited in the text.
15. In Table 4, names of microalgae should be in italic.
16. The reference format in Figures and Tables is different from the in-text citation style. Please, uniform reference style should be maintained.
17. Photovoltaic coupled-photobioreactors are currently emerging as energy-efficient systems. Please, include information on this innovation, their energy consumption, and compare with conventional systems. See Morales M, Hélias A, Bernard O. Optimal integration of microalgae production with photovoltaic panels: environmental impacts and energy balance. Biotechnology for biofuels. 2019 Dec;12(1):1-7., Nwoba EG, Parlevliet DA, Laird DW, Alameh K, Louveau J, Pruvost J, Moheimani NR. Energy efficiency analysis of outdoor standalone photovoltaic-powered photobioreactors coproducing lipid-rich algal biomass and electricity. Applied Energy. 2020 Oct 1;275:115403., Tredici MR, Bassi N, Prussi M, Biondi N, Rodolfi L, Zittelli GC, Sampietro G. Energy balance of algal biomass production in a 1-ha “Green Wall Panel” plant: How to produce algal biomass in a closed reactor achieving a high Net Energy Ratio. Applied Energy. 2015 Sep 15;154:1103-11, etc.
18. Authors should proofread the manuscript carefully for English.

Lines 17-18: It is unclear what authors meant by the “concentration of culture medium.” culture medium is a multi-component mixture. How does it affect the culture methods and, subsequently energy consumption? Did you mean microalgal concentration in the culture medium?

Line 20: Define abbreviations at first mention and use consistently, e.g., PBR.

Lines 28-29: Please include in parenthesis after the word “ago” the actual value of the world energy consumption and add a reference at the end of the sentence.

Lines 29-30: It makes sense to provide either the current value of the rate of burning fossil fuels or compensation provided by nature in parenthesis and add references at the end of the sentence.

Line 44: microalgae biofuels cannot replace traditional fossil fuels. In reality, they can complement and not replace this fuel source.

Lines 48-50: Reference is required at the end of the sentence.

Line 80: Change the comma after ‘[11]’ to a full stop.

Line 70: Please, explain in the text why biogas production in Fig. 1 is a reversible process.

Lines 119-120: It is unclear why this example for a PBR is in the section for the open ponds system. Please, explain.

Lines 121-122: It is not clear how the use of freshwater or seawater affects energy consumption?

Line 183: Change ‘System’ to ‘system’

Line 193: use a different word instead of ‘adsorb’.

Line 202: Write ‘P. tricornutum’ in full. Change ‘The’ to ‘the’.

Line 206: Use sentence case.

Line 209: ‘…has been widely concerned’ makes no sense. Revise.

Line 246: Poor punctuation. Use full stop instead of a semi-colon.

Line 425: Using ‘results’ here makes one believe that this is a research review, in which you will be required to incorporate a methodology section. To avoid this requirement, it makes sense you replace ‘Results’ with ‘The energy consumption ratio of cultivation’.

Line 430: Fig. 1 shows the production process for microalgal fuels and not the energy consumption ratio. Double-check and correct appropriately. Further, be consistent in the use of the word ‘figure’.

Lines 430-431: Please, justify in the text the choice of the energy consumption ratio (ECR) metric (and not other metrics) as a basis of comparison. What does the low or high value of this parameter mean? In addition, explain how you got the energy consumptions of the systems. Make it clear if the authors calculated ECR values or extracted them from the literature. What is the relationship between ECR and NER?

Line 441: As stated in Line 430, Fig 2 is incorrectly cited.

Line 461: Double-check if the word profitably is correctly used.

Line 494: This sentence does not make sense. See comment for Line 430 and previous.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript Number: sustainability-1295924

The manuscript is very relevant to ongoing research on Microalgae Biofuel Production. The manuscript is relevant to the scope of the Journal. However, several issues need to be addressed adequately before accepting this manuscript for the Journal. Therefore, I strongly recommend the manuscript to publish after a major revision.

Detailed comments:

• The authors should explicitly specify the novelty of their work. For example, what progress against the most recent state-of-the-art similar studies was made in this study? Mention this in the revised manuscript sections, including abstract, introduction, and conclusions.
• Since many similar studies have been conducted on the same topic, how is this study different from what has been published or studied earlier?. Authors must clearly state this in the introduction outlining what has been done before and what has been left that authors are considering to examine in this study. The present form does not have sufficient arguments to justify the novelty of the paper.
• It is strongly recommended to add a new subsection under results and discussion, 'Practical implications of this study,' outlining the challenges in the current research, future work, and recommendations.
• It is recommended to discuss and explain the appropriate policies based on the findings of this study. In addition, the results should be further elaborated to show how they could be used for real applications.
• Currently, the conclusion section contains both the concluding marks and recommendations. The future work-related points can be grouped under the newly added subsection, as mentioned above.
• Please eliminate those multiple references. After that, please check the manuscript thoroughly and eliminate all the lumps in the manuscript. Also, check with the Journal for reference formatting/style.
• It is recommended to extend the comparison of the study findings with other similar published work under the results and discussion section.
• It is much recommended to provide a comparative studies table (literature review table) with similar factors/parameters used in this study.
• The last paragraph or closing lines of the introduction section always highlight the novelty aspects of the study with the clear aim of the study and the importance/significance of the study findings. The manuscript is missing such key/vital information in its introduction.
• Overall, the manuscript is too short and contains very little information in the Introduction and Results and Discussion sections, which need to be extended by following the examples of good related articles published in similar and other high-impact journals.
• The quality/visibility of the figures should be improved by giving them more attractive styles/formatting.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments of Reviewer.

In the article presented the interesting and quite new review, study and analysis results of energy consumption in the acquisition of bio-feedstock for microalgae biofuel production. The authors of this article collected and analyzed important research data of many scientiscs, that helped to obtain these results. This article has been prepared in sufficient quality, it is prepared and structured according to the requirements for authors, and the quality of figures and tables is sufficient high. But some minor inaccuracies were observed in this article.

Comments and Remarks of Reviewer:

1) In the Abstract is mentioned the „four types of closed PBR systems“. But it is not clear, what the abbreviation PBR means?

2) In the article, at the end of “Introduction” chapter the aim and novelty of presented work should be highlighted more clearly.

3) At the end of chapter 2 (2. Microalgae Fuels Production Process), at least a brief description or summary of this chaptere should be provided.

4) Figures 6 and 7 are of insufficient quality, the inscriptions in them are too small.

5) Tables 1 and 2 provide a lot of information and important data, but there are not indicated, from which sources of information they are taken?

6) This paper analyzes only energy consumption and energy consumption ratio, but it would be interesting to make at least a rough and economic assessment? But such work is complex, may be it could be presented in another article?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The content of the manuscript titled “A Review of Energy Consumption in the Acquisition of Bio-feedstock for Microalgae Biofuel Production’ (Manuscript ID: sustainability-1295924)” has been significantly improved by the authors and should be accepted after minor revisions.

See specific comments:

1. Point 6 in the response to reviewer 1 comments is not properly addressed. (i.e, Point 6: It is also necessary to mention the types of cooling systems that can be applied to pbrs and how this choice can affect the system's energy consumption. Response 6: "As far as we know, cooling is generally realized through the circulation of culture medium, and no additional cooling system is required, so it is not particularly emphasized in this study").

Cooling options for pbrs are (a) heat exchange system involving culture circulation, (b) placement in a greenhouse, (c) immersion in a thermoregulated pool, (d) freshwater passive evaporative cooling involving freshwater spray on pbr surface, (e) spectral filtration involving the removal of heat-inducing infrared radiation from sunlight. Energy consumptions of these cooling strategies are different and should affect the ECR of operations.

2. Authors should carefully proofread the manuscript for spelling errors e.g., Line 543 change “quantified” to “quantify”, Line 458 change “ration” to “ratio”, Line 503 change “serval” to “several”, Line 543 change “wether” to “whether”, etc.
3. Lines 450&454, Change “chapter” to “Section”.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have verified the usefulness of the results and data and significantly improved the manuscript after revision. The manuscript is within the scope of the Journal. The manuscript is important to publish because of the topic/contents importance, precise methodology, clarity, and novelty. Based on my comments, I recommend the manuscript to publish as it is.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf