A Comparative Economic Feasibility Study of Photovoltaic Heat Pump Systems for Industrial Space Heating and Cooling
Round 1
Reviewer 1 Report
The authors present a comparative study of various system configurations involving PV-driven heat pumps. Although the article is indeed of significant scientific and practical relevance, the authors must improve the quality of the manuscript before it can be recommended for publication. The authors are requested to address each of the following points before the manuscript can be reviewed in further detail. Note that because of the errors with the cross references, the reviewer has only been able to read a very small part of the entire manuscript.
- The English should be improved. Among other errors, the authors use an excessive (grammatically incorrect) amount of commas, which interrupts the text flow and difficults reading the manuscript and following the argumentation. The manuscript should be reviewed in detail by an English native speaker before resubmitting.
- The authors should reduce the use of acronyms. In the current version, acronyms are used excessively, which makes it very hard to follow the text as one is forced to remember the numerous acronyms used in the text.
- The methodology applied to dimension the thermal energy storage should be explained in further detail.
- The conclusions section should be improved. It should include a brief description of the technical solution proposed by the authors such that the conclusions can be understood without having to read the entire manuscript.
Author Response
We thank the reviewer for his/her comments and corrections, that have contributed to improve the quality of the paper. We have addressed each of those comments as follows (we have marked in red text the changes in the paper and the answers here):
- The English should be improved. Among other errors, the authors use an excessive (grammatically incorrect) amount of commas, which interrupts the text flow and difficults reading the manuscript and following the argumentation. The manuscript should be reviewed in detail by an English native speaker before resubmitting.
Answer: Following the suggestion of the reviewer, the text has been revised by an English native speaker.
- The authors should reduce the use of acronyms. In the current version, acronyms are used excessively, which makes it very hard to follow the text as one is forced to remember the numerous acronyms used in the text.
Answer: The use of the acronyms RA (Rabbit), PO (Poultry), St (Static) and Tr (Tracking) and Only Grid-Powered (OGP) has been avoided during the discussion of the results and the sensitivity analysis. We believe that those were the acronyms that made it harder to follow the text as they are not very common in literature, but we are open to any further suggestion. Also, we have avoided the use of acronyms for variables that were mentioned at a big distance from where they were defined (for example, Initial Investment Cost, Power Cost, Energy Cost…). All these changes are indicated in the text in red.
- The methodology applied to dimension the thermal energy storage should be explained in further detail.
Answer: An explanatory text has been added in lines 111-118 for trying to explain the dimensioning of the thermal energy storage with more detail. However, if there is any particular aspect that the reviewer wants to clarify, we will be glad to extend our explanations.
- The conclusions section should be improved. It should include a brief description of the technical solution proposed by the authors such that the conclusions can be understood without having to read the entire manuscript.
Answer: We have expanded the initial paragraph of the conclusions section, briefly explaining the technical solution proposed by the authors (see lines 448-455). Additionally, we have added a sentence in the introduction (see lines 51-53).
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper deals with an interesting subject however as presented it has some flaws:
-Could authors add some figures showing the system?
-What are the details of the farms? surfaces, heating, cooling needs?
-Technical informations are missing like tank volume, PV surface for each solution; The work is too focused on economic analysis and it misses technical information about system operation and sizing;
Author Response
We thank the reviewer for his/her comments and corrections, that have contributed to improve the quality of the paper. We have addressed each of those comments as follows (we have marked in red text the changes in the paper and the answers here):
-Could authors add some figures showing the system?
Answer: For confidentiality reasons, we cannot show any image of the farms. However, we have included a figure showing the PV-HP system components for the two studied configurations (see lines 107-108 and 121-122)
-What are the details of the farms? surfaces, heating, cooling needs?
Answer: We have included the surface of the two farms and the indoor temperature ranges (see lines 91-94).
-Technical informations are missing like tank volume, PV surface for each solution; The work is too focused on economic analysis and it misses technical information about system operation and sizing;
Answer: we had not included information about the tank volume or the PV surface because, as the length of the paper is limited, we would need to remove part of the economic analysis to include more technical information, which is not the focus of this work. In any case, in order to follow the suggestion of the reviewer, we have included the surface occupied by the PV generator with both structures, static and north-south horizontal tracker as well as the thermal tank volume. These data have been included in Table 3. For any additional technical information, we have included the reference to the paper containing the technical details of the Autonomous solution without batteries, developed by the same authors, is referenced: C. Lorenzo, L. Narvarte, R. H. Almeida, and A. B. Cristóbal, "Technical evaluation of a stand-alone photovoltaic heat pump system without batteries for cooling applications," Solar Energy, vol. 206, pp. 92-105, 2020, doi: 10.1016/j.solener.2020.05.097.
Author Response File: Author Response.docx
Reviewer 3 Report
Dear authors please find below my comments:
-
- Line 14 lack of batteries is not a clear statement may be you mean "avoiding battery system" !.
- In page 2 and 3 the table of nomenclature needs to be moved out of the introduction under separate title.
- In equation 1 why only E_night is used and not the day time energy as well ?
- Line 119 remove the error sentence.
- Table 1 and 2 are not was not described or mentioned in the text
- In Figure 1 shows energy for cooling, What about the energy consumption for heating in winter is not part of the simulation ?
- In line 195 remove the error statement.
- In 217 remove error statement.
- No comment was found in the text about table 3.
- No comment was found in the text about table 4.
- No comment was found in the text about Figures 2-4
- In line 286 remove error statement.
Author Response
We thank the reviewer for his/her comments and corrections, that have contributed to improve the quality of the paper. We have addressed each of those comments as follows (we have marked in red text the changes in the paper and the answers here):
- Line 14 lack of batteries is not a clear statement may be you mean "avoiding battery system" !.
Answer: we have modified sentence form line 14 according to the reviewer’s suggestion.
- In page 2 and 3 the table of nomenclature needs to be moved out of the introduction under separate title.
Answer: we have created a new section with the nomenclature table. See lines 86-89.
- In equation 1 why only E_night is used and not the day time energy as well ?
Answer: The thermal storage system is only needed at night, because the PV generator is dimensioned to provide the mean daily consumption. Hence, the thermal energy that needs to be stored is calculated from the nightly mean electric consumption.
- Line 119 remove the error sentence.
Answer: We apologize for this mistake. However, it was caused by a problem during the editing process of the journal and the authors were not involved.
- Table 1 and 2 are not was not described or mentioned in the text
Answer: We apologize for this mistake. However, it was caused by a problem during the editing process of the journal and the authors were not involved. Tables 1 and 2 are described in lines 133-135.
- In Figure 1 shows energy for cooling, What about the energy consumption for heating in winter is not part of the simulation ?
Answer: The simulation also includes the heating consumption in winter. However, we only included summer consumption in Figure 1 because it is the relevant one for the sizing of the PV generator. In this work, we design the system so that it can be stand-alone in summer; in winter, the grid or a diesel generator are needed.
- In line 195 remove the error statement.
- In 217 remove error statement.
- No comment was found in the text about table 3.
- No comment was found in the text about table 4.
- No comment was found in the text about Figures 2-4
- In line 286 remove error statement.
Answer: We apologize for these mistakes. However, they were caused by a problem during the editing process of the journal and the authors were not involved. Table 3 is mentioned in lines 208-210. Table 4 is mentioned in lines 250-251. Figures 2-4 (now 3-5) are mentioned in lines 261-262.
Author Response File: Author Response.docx
Reviewer 4 Report
Review of: Comparative economic feasibility study of photovoltaic heat pump systems for industrial space heating and cooling
Overall comments:
A well-written and relevant paper, but two major corrections needed.
- I think you need to include a sensitivity analysis on the lifespan of PV cells, as 20 years is often assumed. You could look at 15-30 years. (I assume the relevant graph can be easily generated from your equations.) One of the many papers that discuss the complex question of PV cell array durability is given below. The problem is not simple: performance degradation means that the equivalent life span is reduced.
Dirk C. Jordan, Timothy J. Silverman, Bill Sekulic and Sarah R. Kurtz. PV degradation curves: non-linearities and failure modes. Prog. Photovolt: Res. Appl. 2017; 25:583–591.
- The paper heavily emphasises monetary costs. But for energy these are not very reliable, given the subsidies—especially the unpaid costs for CO2 emissions from fossil fuels. See the reference below for global subsidies for fossil fuels, but even renewable energy is subsidised.
Coady, D.; Parry, I.; Sears, S.; Shang, B. How large are global energy subsidies? World Dev. 2017,91, 11–27.
The justification for the research is to reduce the ‘environmental impact’ of energy systems. Thus, the paper needs to look at CO2 emissions from the various options as well. Section 3.1 does discuss energy yield for the various options, which can be converted into CO2 emissions.
Some minor corrections/typos:
Line 189...with the diesel...
Line 257. drop the 'a'
Line 314. Assuming it is not an error, remove the bold type. Perhaps write: 'From Table 3,...'
Line 342. ..guaranteeing...
Author Response
We thank the reviewer for his/her comments and corrections, that have contributed to improve the quality of the paper. We have addressed each of those comments as follows (we have marked in red text the changes in the paper and the answers here):
- I think you need to include a sensitivity analysis on the lifespan of PV cells, as 20 years is often assumed. You could look at 15-30 years. (I assume the relevant graph can be easily generated from your equations.) One of the many papers that discuss the complex question of PV cell array durability is given below. The problem is not simple: performance degradation means that the equivalent life span is reduced.
Dirk C. Jordan, Timothy J. Silverman, Bill Sekulic and Sarah R. Kurtz. PV degradation curves: non-linearities and failure modes. Prog. Photovolt: Res. Appl. 2017; 25:583–591.
Answer: following the suggestion of the reviewer, we have included the results of the sensitivity analysis on the lifespan of the system, from 15 to 30 years. We have included both the relevant graphs and a short discussion (see lines 432-446 and 490-492).
- The paper heavily emphasises monetary costs. But for energy these are not very reliable, given the subsidies—especially the unpaid costs for CO2 emissions from fossil fuels. See the reference below for global subsidies for fossil fuels, but even renewable energy is subsidised.
Coady, D.; Parry, I.; Sears, S.; Shang, B. How large are global energy subsidies? World Dev. 2017,91, 11–27.
The justification for the research is to reduce the ‘environmental impact’ of energy systems. Thus, the paper needs to look at CO2 emissions from the various options as well. Section 3.1 does discuss energy yield for the various options, which can be converted into CO2 emissions.
Answer: as suggested, we have included a new column in Table 3 with the expected savings in terms of CO2 emissions if installing a PV-HP system. We have also included some explanatory sentences in the Materials and Methods section (lines 108-109) and in the Results section (lines 228-232)
Some minor corrections/typos:
- On lines 119, 195, 217, 286 and 308, the appropriate reference needs to be added.
Answer: We apologize for these mistakes. However, they were caused by a problem during the editing process of the journal and the authors were not involved.
- Line 189...with the diesel...
Answer: The mistake has been corrected (see line 201)
- Line 257. drop the 'a'
Answer: ‘a’ has been removed in line 275)
- Line 314. Assuming it is not an error, remove the bold type. Perhaps write: 'From Table 3,...'
Answer: We are sorry, but we could not find what the reviewer refers to in line 314 of the previous version of the document.
- Line 342. ..guaranteeing...
Answer: We have corrected the mistake (see line 335).
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Authors replied to reviewer comments
Reviewer 3 Report
No comments, my previous questions were addressed.
