Heat-Integration of Solar-Heated Membrane Distillation and Fuel Cell for Desalination System Based on the Dynamic Optimization Approach
Round 1
Reviewer 1 Report
Manuscript presents the results of optimization of a system consisting of a Direct Contact Membrane Distillation (DCMD) unit integrated with a Solar Collector and heat storage tank. The Authors analyzed two system configurations: with an integrated Proton Exchange Membrane Fuel Cell (PEMFC) and without PEMFC. The optimized variable was the Unit Production Cost. Before considering publication of the presented paper, the Authors should address several issues listed below.
0) There are some discrepancies in the page and line numbering in the pdf file I downloaded from SUSY. In my file, the fourth consecutive page is labeled as page 2 of 28. In my review, I will provide consecutive page numbers rather than page numbers from headings. In the comments, I will also try to indicate the specific portions of the text to which a particular comment applies.
1) page 1, lines 39-40, "There are the potentials for MD systems to be combined with renewable energy such as solar energy or industrial waste heat [4-10]" - too many references are merged together. In my opinion, Authors should briefly summarize each of the papers.
2) page 2, lines 17-20, "Lai et al. [9] proposed a hybrid PEMFC and Direct Contact Membrane Distillation (DCMD) module to simultaneously use the waste heat generated by PEMFC for desalination. The operating parameters of the hybrid system were analyzed and optimized with a genetic algorithm." - What is the difference between this work and the mentioned work by Lai et al.? This should be clarified.
3) page 3, lines 1-9 - These two paragraphs are virtually identical. In my opinion, there is no need to explain in the introduction what is contained in each section of the manuscript. I suggest removing this entire section.
4) page 3, section 2.1 - Why did the authors not consider other system configurations? Did the Authors consider placing the PEMFC upstream of the hot water storage tank inlet or parallel to the Solar Collector? The use of water with a lower initial temperature to collect the excess heat from the PEMFC could lead to recovery of more excess heat energy. In the second case, the excess heat would have been delivered directly to the hot water storage tank. Both cases could potentially allow more heat to be recovered.
5) page 5, line 85, Table 1. - The specific heat of pure water has the unit J/kg*degC, but the specific heat of a PEMFC has the unit J/kg*K. The authors should be consistent in the units used. Also, with the notation used, the entire denominator of the units used should be in parentheses if there is more than one symbol in the denominator, e.g., J/(kg*K), or using the power notation: J*kg^-1*K^-1.
6) page 6, lines 98-99, "The preliminary simulation results are shown in Fig. 3(a), that T_fo can reach the highest temperature of 93 °C" - in the Figure 3a the T_fo temperature reaches almost 110 degC.
7) page 9, line 152, "and the details are shown in Fig. 7" - I think it should be Fig. 5.
8) page 10 - the numbering of the equations in the text starting from equation (21) is not consistent with the numbering shown with the equations presented.
9) pages 24-27, "Nomenclature" - In my opinion there are too many symbols listed in the nomenclature. Perhaps some of them could be broken down into a symbol and subscript (e.g. specific heat, molar flow rate at anode/cathode, pressure)? In addition, I suggest that each symbol should be described in the text at the place where it first appears. At this point, the symbols A_sc and U_sc in equation (1) are not being explained in text.
Author Response
The authors would like to thank the reviewer for his/her thorough comments to improve the quality of the manuscript. The following are the authors' itemized replies to the reviewer's comments.
Manuscript presents the results of optimization of a system consisting of a Direct Contact Membrane Distillation (DCMD) unit integrated with a Solar Collector and heat storage tank. The Authors analyzed two system configurations: with an integrated Proton Exchange Membrane Fuel Cell (PEMFC) and without PEMFC. The optimized variable was the Unit Production Cost. Before considering publication of the presented paper, the Authors should address several issues listed below.
0) There are some discrepancies in the page and line numbering in the pdf file I downloaded from SUSY. In my file, the fourth consecutive page is labeled as page 2 of 28. In my review, I will provide consecutive page numbers rather than page numbers from headings. In the comments, I will also try to indicate the specific portions of the text to which a particular comment applies.
The authors would like to thank you for the reviewer's thoughtful way to address the page number issues.
1) page 1, lines 39-40, "There are the potentials for MD systems to be combined with renewable energy such as solar energy or industrial waste heat [4-10]" - too many references are merged together. In my opinion, Authors should briefly summarize each of the papers.
The authors have separated the references so that they are not bulked together and avoid confusion. The changes are marked in red on Page 1 of the second paragraph.
2) page 2, lines 17-20, "Lai et al. [9] proposed a hybrid PEMFC and Direct Contact Membrane Distillation (DCMD) module to simultaneously use the waste heat generated by PEMFC for desalination. The operating parameters of the hybrid system were analyzed and optimized with a genetic algorithm." - What is the difference between this work and the mentioned work by Lai et al.? This should be clarified.
The work by Lai et al. [9] addressed the operating parameter optimization of the given hybrid system. Unfortunately, the economic aspects were not addressed and there were no constraints implemented on maintaining the daily production continuity of the hybrid system. In our work, we optimize the overall PEMFC-DCMD system with economic objectives which obviously include simultaneous equipment sizing. Therefore, rigorous dynamic optimization and daily production continuity constraints are proposed to ensure the operability of the optimal design.
3) page 3, lines 1-9 - These two paragraphs are virtually identical. In my opinion, there is no need to explain in the introduction what is contained in each section of the manuscript. I suggest removing this entire section.
The authors would like to thank the reviewer for the sharp review of the paragraphs. The authors have removed the corresponding paragraph in concern.
4) page 3, section 2.1 - Why did the authors not consider other system configurations? Did the Authors consider placing the PEMFC upstream of the hot water storage tank inlet or parallel to the Solar Collector? The use of water with a lower initial temperature to collect the excess heat from the PEMFC could lead to recovery of more excess heat energy. In the second case, the excess heat would have been delivered directly to the hot water storage tank. Both cases could potentially allow more heat to be recovered.
The authors would like to point out that the proposed configuration is chosen as shown in Figure 1(b) to ensure that the DCMD module feed water temperature can be maintained and supported directly by the PEMFC waste heat. If the PEMFC is installed in the solar collector closed-loop (series or parallel), the dynamic delay in the hot storage tank may slow the action of the heat transfer and temperature change on the DCMD module feed water temperature. The heat recovered would arguably be similar albeit more heat transfer area is required due to lower temperature difference for the PEMFC module downstream of the hot storage tank. The larger heat transfer area of the PEMFC module will also be arguably compensated with a smaller hot storage tank and quicker control action.
5) page 5, line 85, Table 1. - The specific heat of pure water has the unit J/kg*degC, but the specific heat of a PEMFC has the unit J/kg*K. The authors should be consistent in the units used. Also, with the notation used, the entire denominator of the units used should be in parentheses if there is more than one symbol in the denominator, e.g., J/(kg*K), or using the power notation: J*kg^-1*K^-1.
The authors would like to thank the reviewer for the sharp review of the units. The authors have made the necessary modification as suggested.
6) page 6, lines 98-99, "The preliminary simulation results are shown in Fig. 3(a), that T_fo can reach the highest temperature of 93 °C" - in the Figure 3a the T_fo temperature reaches almost 110 degC.
The authors would like to thank the reviewer for the sharp review of Figure 3. The authors put the wrong figure in the previous version and have made the necessary modification as suggested.
7) page 9, line 152, "and the details are shown in Fig. 7" - I think it should be Fig. 5.
The authors would like to thank the reviewer for the sharp review of the corresponding typo. The authors have made the necessary change.
8) page 10 - the numbering of the equations in the text starting from equation (21) is not consistent with the numbering shown with the equations presented.
The authors would like to thank the reviewer for the sharp review of the equation numbering. The authors have made the necessary changes.
9) pages 24-27, "Nomenclature" - In my opinion there are too many symbols listed in the nomenclature. Perhaps some of them could be broken down into a symbol and subscript (e.g. specific heat, molar flow rate at anode/cathode, pressure)? In addition, I suggest that each symbol should be described in the text at the place where it first appears. At this point, the symbols A_sc and U_sc in equation (1) are not being explained in text.
The authors would like to thank the reviewer for the suggestion. The authors would like to opt for retaining the nomenclature list for the sake of brevity. The authors use a subscripts table to explain the symbols of Asc to be the unit area of solar collector in Table 2 and the nomenclature list, whereas Usc parameter description can be found in Table 1 and also in the nomenclature list as collector overall heat loss coefficient. The authors have tried their best to explain the symbols both in the Tables and in the nomenclature list.
Reviewer 2 Report
The article is well written with a good flow of information. The results are well stated with sound knowledge on the outcomes with evidence. I didn't find anything added or deleted to make it more profound. So I recommend the article acceptance in the current form.
Author Response
The authors would like to thank you for the reviewer's positive feedback.
Round 2
Reviewer 1 Report
I accept all revisions made and all responses to issues presented in my review.
