Simulation Data of Regional Economic Analysis of OTEC for Applicable Area

Round 1

Reviewer 1 Report

I appreciate contribution of authors to the journal and the research on OTEC systems. This paper describes about the finance of the ocean thermal energy conversion to produce the electricity and the desalinated water on the worldwide locations in tropical areas. The authors investigate the tariff of power and water in each region and conduct the economic analysis using a benefits/cost analysis, a net present value and an internal rate of return of OTEC. Authors also compare the OTEC system among a simple open cycle, double-stage open cycle, and closed cycle in the economic analysis. And the finally mentioned the feasibility of OTEC in some regions.

Considering the OTEC characteristics, the calculation of the products has large effect on this finance report, then, in the calculation of the products, Aspenetch Hysis are used for heat and mass balance. However, the balance does not optimize due to inadequate assumptions. In addition, the sensitivity analysis will be required if you assume the cost of initial expenditure, and the operation and maintenance because as authors claim that there are no actual OTEC project in MW capacity. 

Therefore, the followings are the recommendations for the authors:

 [Major points]

1. In the introduction, there are few OTEC technology development, although there are many papers on OTEC for even a decade ago. Except reference 5, 6, 7, and 10, self citation papers are described on OTEC. So, please add the current research papers on OTEC and/or desalination and discuss the comprehensive background of the research.
2. In the introduction, there are no explanation and reference in figure 1. Please add.
3. In the introduction, the authors should clarify the purpose of this research. Why are the economic comparisons in many regions required ?
4. In section 2, Eqs.(1)–(3) and Eqs.(9),(10) show the Helmholtz free energy equation. Please add the explanation why authors describe these equations as a basic equations, and add the appropriate equations for the calculation of the heat and mass balance, not only Eqs.(5)–(8), (11) and (12). In addition, there are no efficiency of the equipment in Eqs. (5)–(8), (11) and (12), although the assumptions in table 3 have efficiencies of equipment. Please add in the equations.
5. In open cycles, the brine discharge pump are required to transfer the brine to ocean or any other place because of the vacuum condition in the evaporation chamber. The brine discharge pump may consume the highest internal power in the system.
6. In the open cycle, please consider the boiling temperature change and non-equilibrium temperature change, which are not negligible due to small available temperature difference in OTEC.
7. In line 154, the authors claims that the closed cycle OTEC has lower power efficiency. Please add the reason or reference because they are ideally same in terms of thermal efficiency.
8. In the calculation of the heat and mass balance, the authors assume a constant temperature change (5^oC) of seawaters in the evaporator and the condensers. However, this assumptions effect the power output and desalination product flow rate. The optimized heat and mass balance may differ from your results. Because the authors uses commercial software, Aspentech HYSIS, the optimization in each region seawater temperature would possible. Please optimize if possible and/or show the sensitivity analysis to let readers know the uncertainty of the results.
9. In OTEC system, the on-shore and the off-shore types are totally different cost break. Please add the explanation of the assumption of type and capacity dependency to calculate the cost of equipment.
10. Even closed cycle, the combination of OTEC and desalination by distillation is possible, which is a kind of hybrid type. The hybrid type would be interested for the comparison between OC-OTEC and CC-OTEC.
11. The cost estimations are valuable. However, there are many assumptions based on few cost estimation reference. However, the authors should add the sensitivity analysis in the assumed costs shown in table 2 because the results are one of the case.
12. In table 2: Is the surface temperature show annual mean temperature? And the price of electricity and water tariff may change individual use or industrial use. Please add the condition.
13. Please consider the number of significant digits in table 4–6.

[Minor points]

14. Line 35, in Abbreviations. Please check the spelling of Benefic.
15. In Abbreviations, please add phi(s).
16. Line 71. "3%" is a freshwater in the earth, and most of them are the ice in the poles. If you count the resource, "0.01%" (river and lake) is used in general.
17. Line 86-88, Please check the sentence. It may miss the verb.
18. Line 86. Reference [5] may not meet to the description. Please check the reference number.
19. Line 89. 80MW would be 50MW.
20. Spell out the abbreviation in the first time such as OTEC in line 84 and VMD-OC in line 96.
21. Line 113. Authors introduce the direct condenser results in Ref.[10], however, it can not produce desalinated water. The appropriate explanation is required.
22. Line 115-117. Please add the reference.
23. Line 120. Open cycle is not a Rankine cycle. Please correct.
24. In figure 2 and figure 3, blue lines may show seawater and water, red lines may show electricity. Please add symbols.
25. Line 200. The assumption 3 means is not clear. Are there no benefit in first 20 years?
26. Line 205. "project" may be "installation" or "commissioning". Please check.
27. Line 206. Please add reasonable reason or reference for the assumption of 5% of initial cost as an operating cost.
28. Line 218, 219. A mixed-case sentences and heading of a figure. Please correct.

Thank you for your considerations.

Author Response

Thank you for pointing out the paper and reviewing it.

We will reply to the point you indicated with correction as attached.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present a study about open cycle OTEC plant for electricty and fresh water production.

The models are showns in this paper are simple with some hypothesis on water properties or on the efficiency of pumps for example. But it permit to take into account that is required for the study. They also take into account economic parameters. The economics model are well present and permit to understand the results. The conclusion shows where it better to implant these kind of technology. 

But the authors don't speak about the required ground. An 50 MW open cycle plant will take some place (based on Vega work's in the 1990's) and it's can be a problem for many island countries where it's will be put in place. Can the authors estimate the size of the different power plant present in this paper?

Some corrections need to be done :

Line 312 : the sentence need to be above the figure 7

Author Response

Thank you for pointing out the paper and reviewing it.

We will reply to the point you indicated with correction as attached.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

We appreciate kind comment reply from the authors. I checked the response from authors, and some minor recommendations are remaining. Please check the following items.

Poin-1: Noted.

Piont-2: Noted, but please move the sentence "Figure1 shows...." (line 112–115) to the beginning (for instance, insert to line 93). It's quite strange to explain the figure in the last of the introduction.

Point-3: Noted.

Point-4:

1) I understand the author try to explain the Helmholtz free energy equations. However, the listed equations are not enough to describe all the properties for the refrigerant. So, please cite appropriate reference for the equations and delete Eqs.(1)–(3), (12), and (13).

2) In addition, again, please add the efficiency of the equipment shown in Eqs.(4)–(8) because in line 282–283 discuss about the efficiency without the definition.

3) Net efficiency should be "Net thermal efficiency" in Eqs.(11) and (16). Please revise.

4) This paper only discusses the steady state condition to calculate the power and water production. Eq.(11) is not appropriate equation. Please revise.

Point-5: Ref.[9] mainly focused on the performance of the system. But this paper discusses about the economy. Therefore, the required power for the management of the brine is should be considered. If you assume to neglect it, please add the assumption.

Point-6: There are no standardized experimental values. If you applied the same value of a reference, please cite the reference.

Point-7: What is the power efficiency? And if the open cycle turbine is not suitable for constructing large-scale thermal power plant, why do the authors discussing about the economy of 10–50MW OTEC using open cycle? I'm wondering if readers will confuse in this comment that the open cycle OTEC is not suitable for constructing large-scale thermal power generation because this study consider up to large capacity of 50MW OTEC. And the reference should be different from [15].

Point-8: Noted. Please add the mentioned common condition such as water depth, pipe length, and water temperature.

Point-9–28: Noted.

Thank you for your considerations.

Author Response

Dear reviewer.

First of all, thank you for leaving interest and review until the thesis is completed. The contents you pointed out are summarized in the attached document. We ask for a review again.

Thank you.

Author Response File: Author Response.docx