A Hybrid Solar–Thermoelectric System Incorporating Molten Salt for Sustainable Energy Storage Solutions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is interesting and the design of the solar system with molten salt is original. Using the latent heat of melting to energy storage increases the efficiency of the system. The usage of Peltier cells to electricity generation is standard. I can recommend the article for publication, but I have some comments, see below. I think, the article need major revision and the english need correction.

Comments:

- The quality of English should be improved.

- I think, there would be better to write „Solar radiation“ than „Sun light“ (see for example line 248, 255, 263, 345).

- Line 324 – The output current is probably 2,7 A.

- Line 326 – There should be 56x56 mm².

- Line 345 - I think, the system with parabolic mirrors need tracking system with the higher tracking accuracy. For example, the article doi: 10.1016/j.solener.2015.12.054 describes such solar tracker. I think, it could be inspiration for authors and it could be mentioned in the references.

- Line 354 - Why is 310 days/year included in the calculation? There is usually 310 sunny days per year in UAE?

- Line 355, 356, 365 - The efficiency of 75% seems excessive to me. How was such a high efficiency determined?

- Table 1 and the next text - The symbols of physical quantities should be written in italica font, the symbols of physical units should be written in standard font. It is mostly correct but not strictly, see for Table 1 and lines 396-423.

- The calculations in section 4 are very simplified and are only indicative. I have nothing against it, but it should be written that this is only a rough estimate. Really the solar radiation is not constant during 5.5 hours per day.

- Most of references are not mentioned according the standard. There is written the publisher and not the journal name. Only at the end of the list are a few references mentioned correctly (authors, title, source-journal, year, volume, pages). Although articles can be searched by doi number, but the citations should be according to the standard.

Comments on the Quality of English Language

Some formulations are not clear. Some physical terms are not accurate.

Author Response

RESPONSES TO EDITOR AND REVIEWER COMMENTS

 

Manuscript ID: technologies-3449845

 

Title: Local Storage of Solar Energy Using Molten Salts

New title: "A Hybrid Solar-Thermoelectric System Incorporating Molten Salt for Sustainable Energy Storage Solutions"

 

Thank you so much for your thoughtful suggestions and insights, which have been extremely helpful towards enriching the manuscript and producing a better and more balanced research account. I hope the revisions will be acceptable to you and the reviewers. Based on the instructions provided in your letter, I have uploaded the revised manuscript file. Revisions in the text are highlighted in red colour. Responses to reviewers’ comments are provided below.

 

Reviewer: 1

 

The article is interesting and the design of the solar system with molten salt is original. Using the latent heat of melting to energy storage increases the efficiency of the system. The usage of Peltier cells to electricity generation is standard. I can recommend the article for publication, but I have some comments, see below. I think, the article needs major revision and the English need correction.

Response: thank you

Comments:

• The quality of English should be improved.

Response: Thank you for your comment. The manuscript has been thoroughly reviewed and edited for language clarity and accuracy. We have made significant improvements to the English language throughout the document to ensure it meets the journal's standards.

• I think, there would be better to write „Solar radiation“ than „Sun light“ (see for example line 248, 255, 263, 345).

Response: Thank you for this suggestion. We have replaced "Sunlight" with "Solar radiation" in the relevant sections.

• Line 324 – The output current is probably 2,7 A.

Response: Thank you for pointing this out. We have corrected the output current to 2.7 A.

• Line 326 – There should be 56x56 mm².

Response: Following your comments, we have corrected the dimensions to 56 mm x56 mm.

• Line 345 - I think, the system with parabolic mirrors need tracking system with the higher tracking accuracy. For example, the article doi: 10.1016/j.solener.2015.12.054 describes such solar tracker. I think, it could be inspiration for authors and it could be mentioned in the references.

Response: We appreciate your suggestion. We have incorporated the reference in the revised manuscript. We have also included a more detailed explanation of the tracking system and how this system can align with our study in Design methodology section (line 388-396).

• Line 354 - Why is 310 days/year included in the calculation? There is usually 310 sunny days per year in UAE?

Response: Thank you for this observation. The 310 is the minimum number of sunny days in the UAE after excluding the rainy, cloudy, and dusty days. But we have revised the calculation to reflect a more accurate number of sunny days per year in the UAE. The value has been updated to 365 days/year, since we used an average value of 5.84 peak hours/day that takes into account the variations in solar radiation throughout the year, including sunny, cloudy, and even dusty days and the corresponding calculations have been adjusted accordingly in the Data Analysis and Results section, line 400 (lines: 399-605, red color).

• Line 355, 356, 365 - The efficiency of 75% seems excessive to me. How was such a high efficiency determined?

Response: Thank you for your comment regarding the efficiency value. The efficiency of 75% is based on the performance ratio (PR) measured for different solar systems in the UAE, as discussed in the literature (Emziane & Al Ali, 2015). This value was used to account for real-world losses and inefficiencies, such as temperature, shading, panel age, and dust. However, we agree that this value may appear optimistic. In the revised manuscript, we have clarified this in the Data Analysis and Results section (starting from line 401), where we explain that the efficiency assumptions are based on average performance ratios observed in the UAE. We have also revised the efficiency of the proposed trough system to a more conservative value better reflect the thermal, optical and TEG efficiencies, and the corresponding calculations have been updated accordingly. This adjustment ensures a more realistic representation of the system's performance. This change is reflected in Section 4 in red color.

• Table 1 and the next text - The symbols of physical quantities should be written in italica font, the symbols of physical units should be written in standard font. It is mostly correct but not strictly, see for Table 1 and lines 396-423.

Response: Thank you for pointing this out. We have ensured that all symbols of physical quantities are in italic font, and units are in standard font throughout the manuscript, including Table 1 and Table 2.

• The calculations in section 4 are very simplified and are only indicative. I have nothing against it, but it should be written that this is only a rough estimate. Really the solar radiation is not constant during 5.5 hours per day.

Response: We appreciate your comment. We have clarified that it is an average value and use different approach considering all factors that may affect this value. The updated calculations are reflected in section 4, first and second paragraphs (red color).

• Most of references are not mentioned according the standard. There is written the publisher and not the journal name. Only at the end of the list are a few references mentioned correctly (authors, title, source-journal, year, volume, pages). Although articles can be searched by doi number, but the citations should be according to the standard.

Response: Thank you for this comment. The used style was the 7^th edition of the APA style  (https://apastyle.apa.org/style-grammar-guidelines/references/examples/journal-article-references#1). We have revised the reference list to ensure that all references follow the journal's standard format, including the journal name, volume, and page numbers where applicable.

• Comments on the Quality of English Language: Some formulations are not clear. Some physical terms are not accurate.

Response: thank you for the insight comment. We have carefully reviewed the English Language all over the paper as per your valuable comment and if still further revision is needed, we will send it for MDPI Author Services - language editing for the necessary proofreading.

 

Once again, we greatly appreciate the time and effort they have dedicated to reviewing our work and providing detailed comments. We have carefully revised the manuscript to address all the concerns raised, and we hope the changes meet your expectations. We are grateful for the opportunity to resubmit the revised manuscript and look forward to hearing from you. Thank you once again for your time and consideration.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Local Storage of Solar Energy Using Molten Salts

Below are my comments regarding the manuscript:

1. The abstract should include some numerical values of the key results to provide a clearer summary of the study's findings.
2. The novelty section requires improvement, as it currently lacks sufficient emphasis on the unique contributions of this work.
3. The authors should incorporate more technical results and a physics-based discussion to strengthen the manuscript before a final review decision can be made.

Comments on the Quality of English Language

ok

Author Response

RESPONSES TO EDITOR AND REVIEWER COMMENTS

 

Manuscript ID: technologies-3449845

 

Title: Local Storage of Solar Energy Using Molten Salts

New title: "A Hybrid Solar-Thermoelectric System Incorporating Molten Salt for Sustainable Energy Storage Solutions"

 

Thank you so much for your thoughtful suggestions and insights, which have been extremely helpful towards enriching the manuscript and producing a better and more balanced research account. I hope the revisions will be acceptable to you and the reviewers. Based on the instructions provided in your letter, I have uploaded the revised manuscript file. Revisions in the text are highlighted in red colour. Responses to reviewers’ comments are provided below.

 

Reviewer: 2

Local Storage of Solar Energy Using Molten Salts

Below are my comments regarding the manuscript:

1. The abstract should include some numerical values of the key results to provide a clearer summary of the study's findings.

 

Response: Thank you for your valuable suggestion. We have updated the abstract to include key numerical results, such as the target electrical energy output (2.067 kWh/day), thermal efficiency (90%), optical efficiency (90.2%), and TEG efficiency (8%).

 

1. The novelty section requires improvement, as it currently lacks sufficient emphasis on the unique contributions of this work.

 

Response: Following your comment, we have revised the introduction and added a more detailed discussion of the novelty of our work, particularly the integration of parabolic trough mirrors, molten salt storage, and thermoelectric generators (TEGs) for off-grid applications in the UAE. The novelty of the work also presented in the last paragraph of the literature review.

1. The authors should incorporate more technical results and a physics-based discussion to strengthen the manuscript before a final review decision can be made.

 

Response: Thank you for your valuable suggestion. We have expanded the technical discussion in Section 4, providing more detailed calculations and a physics-based explanation of the system's operation. Specifically, we have added a subsection dedicated to the thermoelectric generator's (TEG) performance and efficiency, the parameters used in these calculations, such as thermal efficiency, optical efficiency, and TEG efficiency, and output energy are summarized in Table 2 for clarity. These revisions aim to provide a more robust and comprehensive analysis of the system's performance. We hope these additions address your concerns and strengthen the manuscript. Thank you again for your insightful feedback.

Comments on the Quality of English Language: ok

 

Once again, we greatly appreciate the time and effort they have dedicated to reviewing our work and providing detailed comments. We have carefully revised the manuscript to address all the concerns raised, and we hope the changes meet your expectations. We are grateful for the opportunity to resubmit the revised manuscript and look forward to hearing from you. Thank you once again for your time and consideration.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

From reading the abstract, it appears that the paper focuses on proposing a new hybrid system for electricity storage and generation in the United Arab Emirates (UAE), using solar energy as the primary source. In the ABS it is stated that “..the system integrates Solar Photovoltaics with Molten Salt Energy Storage “.

However, after reading the article, it becomes evident that the central element is the proposal of a new type of high-temperature collector with integrated thermal storage and its enhancement through thermoelectric cells based on the Seebeck effect.

Therefore, frankly, the article is misleading both in its title and abstract. The title, "Local Storage of Solar Energy Using Molten Salts", suggests local storage similar to that used in power plants, whereas it actually refers to storage integrated within the collector component. In fact, the article does not mention any characteristics of storage external to the component. The abstract states that "the system integrates Solar Photovoltaics with Molten Salt Energy Storage," which is likely a typographical error, as thermoelectric cells are involved instead, given that photovoltaics do not appear in the proposed component. Therefore, it is suggested that the title and abstract must be  revised.

In general terms, it should be noted that the proposed component, given the still limited adoption of thermoelectric systems compared to traditional photovoltaic/battery systems, represents a niche application with a low Technology Readiness Level (TRL). Specifically, since it is designed to utilize solar energy, which has significant intensity and temporal variability, it absolutely requires experimental testing on prototype systems to verify its performance and reliability. Conducting a feasibility and comparative analysis with photovoltaic systems at this stage seems premature, with the risk of emphasizing applications and benefits that remain unproven.

It is well known that concentrating solar power systems use only direct radiation and are highly sensitive to thermal shocks, effects that are more manageable in large-scale plants. Not surprisingly, these systems have typically been applied in large-scale power plants with high thermal inertia. For example, in the 1980s, LUZ built parabolic trough power plants in the USA using thermal oil, and more recently, several large molten salt storage parabolic trough plants have been built in Spain. Therefore, developing domestic-scale components for electricity production presents a complex challenge. Additionally, it should be noted that solar thermal systems for electricity generation have seen a decline in interest in recent years due to strong competition from photovoltaic systems, which have significantly reduced component costs and offer greater simplicity.

The article’s introduction lacks a general framework on the state of technology, its applications, and its diffusion. To gain a broader understanding of the dissemination of high-temperature solar technologies, in addition to scientific articles, it is recommended to consult strategic platforms such as the International Energy Agency (IEA), particularly the Energy Storage Technology Collaboration Programme. 

Here are some relevant links:

https://iea-es.org/publications/

https://www.solarpaces.org/

https://www.solarpaces.org/research-reports/publications-and-reports/

Regardless of the above, the article presents several critical issues, some of which are summarized below:

• The title and abstract need to be revised to better align with the content, including references to the thermoelectric application.

• The introduction needs to be revised as it is redundant in several places and contains repeated sentences that should be removed.

• The Literature Review section needs to be revised and expanded. It does not adequately reference the state of the art in solar thermal technology and thermoelectric systems, including different system types and efficiencies.

Regarding section 3, “Design Methodology,” while the collector design is very interesting, there are doubts about whether it can realistically achieve such high temperatures. This is due to both the low concentration ratio of the system and the low rigidity, which could lead to imperfect focusing. Additionally, the high precision required for the optical system could make it particularly expensive, especially for small systems. These aspects should be verified through careful experimental activities; otherwise, the design remains purely theoretical. This is particularly important given the system’s intended application. The article states that “..the design aims to develop an efficient and effective solar energy system for electricity generation and storage, also economically viable and scalable in power, for application in ordinary homes, commercial establishments, and businesses, even in remote and off-grid areas.”

Analysing the schematic in Figure 2, the following aspects need clarification:

• The absorber is said to be vacuum-sealed within a glass tube, but the vacuum level, sealing mechanism, and containment of thermal deformations are not specified.

• The absorber is described as a copper tube without further details. The characteristics, particularly whether a selective surface is present, need to be specified, along with its optical properties at different temperatures. It is well known that copper's absorption properties depend significantly on surface type and operating temperature, with a substantial decrease in absorptance at high temperatures. These details should be integrated.

• The thermal storage system is not described in detail, for example: the required volumes per unit of irradiated surface, the management of thermal transients in the absence of sunlight, the stored energy, and the temperature variations allowed between sunny and non-sunny hours, etc.

• Another missing aspect is the system’s management, particularly during startup and extended periods without sunlight. For example, if there are 2-3 days of rain and no thermal energy is available, how is the solidification of the salt and system restoration managed, especially for off-grid systems?

Regarding the thermoelectric generator:

• The characteristics of a commercial system are mentioned, assuming a hot-side temperature of 300°C and a cold-side temperature of 30°C. However, it is not specified how the module is applied to the storage unit, how heat is transferred on the hot side, or how heat is dissipated on the cold side.

• The generator’s electrical characteristics are provided, but the inherently low electrical efficiency of thermoelectric technology is not emphasized. The proposed cell, with an input thermal flow of approximately 325 W, generates about 13 W of electrical power, corresponding to an efficiency of approximately 4%, which is extremely low. This suggests that such applications are more suitable when waste heat is available for free, such as on high-temperature pipes in industrial plants that need insulation or cooling. Designing a dedicated solar collector to generate high-temperature heat to power a thermoelectric generator initially appears to make little sense. If this assessment is incorrect, a detailed counterargument should be provided.

• Tracking systems are described superficially. It is unnecessary to reiterate that such systems, particularly if they require dual-axis tracking for daily and seasonal adjustments, are complex elements that should not be overlooked, as they can significantly impact costs, especially for small-scale systems.

• The evaluations carried out in the "Data Analysis and Results" section appear to be highly simplified and approximate. If correctly interpreted, the aim is to hypothesize the pre-feasibility of a system based on thermodynamic solar technology capable of producing the same energy as a 400 Wp photovoltaic panel, assuming it generates approximately 512 kWh of electricity annually. The conclusion is that it could be replaced by the proposed collector, which has a capture surface of approximately 0.53 m² (Table 1), along with the use of 24 commercial thermoelectric modules from Solid State Power Generators, specifically the TEG1-12611-8.0 model, with dimensions of 56 mm x 56mm.

Honestly, these results raise some doubts. First of all, it seems very difficult to install such a large number of TEG modules on the storage unit of a collector that is only 1 meter long. In fact, the application of 24 TEGs would result in a theoretical linear development of approximately 1.4 meters and a width of 5.6 cm.

Additionally, there are concerns regarding the collector’s dimensions based on the following simple considerations. According to the Global Solar Atlas, the Direct Normal Irradiance (DNI) for the EUA  is around 2000 kWh/m² per year.

https://globalsolaratlas.info/map?c=21.698265,49.57493,5&s=23.80545,54.290306&m=site

It is true that one should consider the Global Irradiation at the optimal angle, which is approximately 2400 kWh/m² per year. However, for an approximate evaluation, assuming that the total annual direct energy (2000 kWh/m²) is fully absorbed by the collector, and considering the absorber surface area (0.53 m²), this corresponds to an available thermal annual energy of approximately 1060 kWh per year.

Therefore, taking into account the efficiency of the selected thermoelectric cells, which is around 4%, the annual producible energy would be approximately 42 kWh. This value is significantly lower than the expected value of 511.5 kWh. In summary, it seems that the collector's surface area is at least one order of magnitude smaller than what is required. If this assessment is incorrect, a detailed counterargument should be provided.

In summary, the article needs to be thoroughly revised, including a review of its calculations.

Apologies if this evaluation seems overly critical, but the goal is to highlight the necessary in-depth analyses to avoid unrealistic expectations and premature proposals in an energy-important  country like the UAE, which is heavily involved in the energy transition. While the collector prototype concept is very interesting, at present, it remains an approximate theoretical scheme that can only be evaluated after the development of an experimental prototype and a rigorous component qualification process.

Author Response

RESPONSES TO EDITOR AND REVIEWER COMMENTS

 

Manuscript ID: technologies-3449845

 

Title: Local Storage of Solar Energy Using Molten Salts

New title: "A Hybrid Solar-Thermoelectric System Incorporating Molten Salt for Sustainable Energy Storage Solutions"

 

Thank you so much for your thoughtful suggestions and insights, which have been extremely helpful towards enriching the manuscript and producing a better and more balanced research account. I hope the revisions will be acceptable to you and the reviewers. Based on the instructions provided in your letter, I have uploaded the revised manuscript file. Revisions in the text are highlighted in red colour. Responses to reviewers’ comments are provided below.

 

 

Reviewer: 3

 

From reading the abstract, it appears that the paper focuses on proposing a new hybrid system for electricity storage and generation in the United Arab Emirates (UAE), using solar energy as the primary source. In the ABS it is stated that “..the system integrates Solar Photovoltaics with Molten Salt Energy Storage “.

However, after reading the article, it becomes evident that the central element is the proposal of a new type of high-temperature collector with integrated thermal storage and its enhancement through thermoelectric cells based on the Seebeck effect.

Therefore, frankly, the article is misleading both in its title and abstract. The title, "Local Storage of Solar Energy Using Molten Salts", suggests local storage similar to that used in power plants, whereas it actually refers to storage integrated within the collector component. In fact, the article does not mention any characteristics of storage external to the component. The abstract states that "the system integrates Solar Photovoltaics with Molten Salt Energy Storage," which is likely a typographical error, as thermoelectric cells are involved instead, given that photovoltaics do not appear in the proposed component. Therefore, it is suggested that the title and abstract must be revised.

In general terms, it should be noted that the proposed component, given the still limited adoption of thermoelectric systems compared to traditional photovoltaic/battery systems, represents a niche application with a low Technology Readiness Level (TRL). Specifically, since it is designed to utilize solar energy, which has significant intensity and temporal variability, it absolutely requires experimental testing on prototype systems to verify its performance and reliability. Conducting a feasibility and comparative analysis with photovoltaic systems at this stage seems premature, with the risk of emphasizing applications and benefits that remain unproven.

It is well known that concentrating solar power systems use only direct radiation and are highly sensitive to thermal shocks, effects that are more manageable in large-scale plants. Not surprisingly, these systems have typically been applied in large-scale power plants with high thermal inertia. For example, in the 1980s, LUZ built parabolic trough power plants in the USA using thermal oil, and more recently, several large molten salt storage parabolic trough plants have been built in Spain. Therefore, developing domestic-scale components for electricity production presents a complex challenge. Additionally, it should be noted that solar thermal systems for electricity generation have seen a decline in interest in recent years due to strong competition from photovoltaic systems, which have significantly reduced component costs and offer greater simplicity.

The article’s introduction lacks a general framework on the state of technology, its applications, and its diffusion. To gain a broader understanding of the dissemination of high-temperature solar technologies, in addition to scientific articles, it is recommended to consult strategic platforms such as the International Energy Agency (IEA), particularly the Energy Storage Technology Collaboration Programme. 

Here are some relevant links:

https://iea-es.org/publications/

https://www.solarpaces.org/

https://www.solarpaces.org/research-reports/publications-and-reports/

Regardless of the above, the article presents several critical issues, some of which are summarized below:

• The title and abstract need to be revised to better align with the content, including references to the thermoelectric application.

Response: Thank you for your valuable feedback. We have revised the title to "A Hybrid Solar-Thermoelectric System Incorporating Molten Salt for Sustainable Energy Storage Solutions" to better reflect the content of the manuscript. Additionally, the abstract has been updated to emphasize the thermoelectric application and its role in the system. Furthermore additional references were included as per your valuable comment. We hope these changes provide a clearer alignment between the title, abstract, and the manuscript's content.

 

• The introduction needs to be revised as it is redundant in several places and contains repeated sentences that should be removed.

Response: Thank you for pointing this out. We have carefully revised the introduction to eliminate redundancy and improve clarity. The introduction now provides a more concise and focused overview of the research problem, objectives, and contributions. We appreciate your suggestion, which has helped us streamline this section.

 

• The Literature Review section needs to be revised and expanded. It does not adequately reference the state of the art in solar thermal technology and thermoelectric systems, including different system types and efficiencies.

Response: Thank you for this insightful comment. We have expanded the literature review to include a more comprehensive discussion of the state of the art in solar thermal technology and thermoelectric systems. We have also added references to recent advancements in these fields, including different system types and their efficiencies. These revisions aim to provide a stronger foundation for research and better contextualize our work within the existing body of knowledge. The updates are presented in the literature section (red color).

 

Regarding section 3, “Design Methodology,” while the collector design is very interesting, there are doubts about whether it can realistically achieve such high temperatures. This is due to both the low concentration ratio of the system and the low rigidity, which could lead to imperfect focusing. Additionally, the high precision required for the optical system could make it particularly expensive, especially for small systems. These aspects should be verified through careful experimental activities; otherwise, the design remains purely theoretical. This is particularly important given the system’s intended application. The article states that “..the design aims to develop an efficient and effective solar energy system for electricity generation and storage, also economically viable and scalable in power, for application in ordinary homes, commercial establishments, and businesses, even in remote and off-grid areas.”

Response: Thank you for raising this important concern. We have addressed this issue by adding a more detailed discussion of the system's design in Section 3, including the measures taken to ensure high-temperature operation. Specifically, we have highlighted the use of high-reflectivity mirrors, a vacuum-sealed copper tank, and the role of the tracking system in maintaining optimal focusing. We have also acknowledged the challenges and limitations in achieving high temperatures, providing a more balanced perspective on the system's capabilities.

 

Analysing the schematic in Figure 2, the following aspects need clarification:

• The absorber is said to be vacuum-sealed within a glass tube, but the vacuum level, sealing mechanism, and containment of thermal deformations are not specified.
• The absorber is described as a copper tube without further details. The characteristics, particularly whether a selective surface is present, need to be specified, along with its optical properties at different temperatures. It is well known that copper's absorption properties depend significantly on surface type and operating temperature, with a substantial decrease in absorptance at high temperatures. These details should be integrated.
• The thermal storage system is not described in detail, for example: the required volumes per unit of irradiated surface, the management of thermal transients in the absence of sunlight, the stored energy, and the temperature variations allowed between sunny and non-sunny hours, etc.
• Another missing aspect is the system’s management, particularly during startup and extended periods without sunlight. For example, if there are 2-3 days of rain and no thermal energy is available, how is the solidification of the salt and system restoration managed, especially for off-grid systems?

Response: Thank you very much for your thorough and insightful review. We greatly appreciate the time and effort you have taken to provide detailed feedback on our manuscript. Below, we address your specific concerns regarding the above design aspects:

1. The vacuum-sealed absorber is a critical component of the system, and its design parameters were carefully considered when estimating the overall efficiency. The vacuum level, sealing mechanism, and thermal deformation management are implicitly accounted for in the thermal efficiency (η_thermal = 90%) value used in the calculations.
2. The copper tank is designed with absorptance value of (α ≈ 0.95) and thermal emittance (ε ≈ 0.05). These properties are reflected in the optical efficiency (η_optical = 90.25%) value used in the calculations.
3. The tank size that is mounted in the hollowed portion between the mirrors has w length similar to the mirror length and has w semicylindrical shape that is appropriate for the molten salt volume.
4. The techniques of overcoming these issues and maximizing these efficiencies are discussed in the subsection of the overall system efficiency.
5. The system is designed to operate efficiently even during periods of limited sunlight. In the UAE, the minimum number of sunny hours per day during winter is 6 hours, which is sufficient to generate the required thermal energy. The system uses insulated thermal storage to retain heat for more than one day, ensuring continuous operation during short periods of reduced sunlight.

Regarding the thermoelectric generator:

• The characteristics of a commercial system are mentioned, assuming a hot-side temperature of 300°C and a cold-side temperature of 30°C. However, it is not specified how the module is applied to the storage unit, how heat is transferred on the hot side, or how heat is dissipated on the cold side.
• The evaluations carried out in the "Data Analysis and Results" section appear to be highly simplified and approximate. If correctly interpreted, the aim is to hypothesize the pre-feasibility of a system based on thermodynamic solar technology capable of producing the same energy as a 400 Wp photovoltaic panel, assuming it generates approximately 512 kWh of electricity annually. The conclusion is that it could be replaced by the proposed collector, which has a capture surface of approximately 0.53 m² (Table 1), along with the use of 24 commercial thermoelectric modules from Solid State Power Generators, specifically the TEG1-12611-8.0 model, with dimensions of 56 mm x 56mm.

Honestly, these results raise some doubts. First of all, it seems very difficult to install such a large number of TEG modules on the storage unit of a collector that is only 1 meter long. In fact, the application of 24 TEGs would result in a theoretical linear development of approximately 1.4 meters and a width of 5.6 cm.

Response: Thank you for this feedback. We revised the 4rth section to encounter all these issues.

1. The TEG modules are mounted directly onto the outer surface of the copper tank using a thermal interface material to ensure efficient heat transfer. The hot side of the TEG is in direct contact with the tank, while the cold side is connected to a heat sink with an integrated fan to dissipate heat effectively. As mentioned in the second paragraph in the 3^rd section; When the tank is heated, it transfers this heat to the molten salt inside, then the TEGs start generating electrical energy by converting the thermal energy generated by the molten salt using the phenomenon of Seeback. TEGs have two sides; hot and cold sides, the hot side is connected with the heat source and the cold side with the heat sinks. When the heat passes through the TEG from hot side to cold side, the TEG material exposed to temperature difference which allows the electrons to move from the hot side to cold side, this movement generates an electric current. As the temperature difference increases, the current generation increases which means more electrical energy generation. In this study, the hot side of TEGs in connected with the molten salt, and the cold side is connected with a heat sink and fan to ensure the required temperature difference.
2. The difference of 220℃ across the molten salt is selected to ensure the stability of molten salt as mentioned in the study of (Chen & Zhao, 2017), also to consider the maximizing the efficiency of the TEG, and to ensure the high performance of the heatsink and fan that affected by the ambient temperature of UAE (up to avg. max. temperature of 34.4°C for the years 2013-2023 according to (Statista, n.d.).
3. The proper electrical connection of TEGS
4. A modified method of enhancing the efficiency of TEGs to generate energy efficiently.
5. The collector’s surface area (5.13 m²) is sufficient to achieve the target energy output of 2.067 kWh/day. We have included a more detailed explanation of these calculations in Section 4 to address your concerns. (Red Color)

 

• Tracking systems are described superficially. It is unnecessary to reiterate that such systems, particularly if they require dual-axis tracking for daily and seasonal adjustments, are complex elements that should not be overlooked, as they can significantly impact costs, especially for small-scale systems.

Response: Thank you for this observation. The tracking system in our design is a single-axis tracker with a ±0.5° accuracy, which balances cost and performance for small-scale applications. While dual-axis tracking could improve efficiency, it was deemed unnecessary for this system due to the UAE’s consistent solar path and the system’s compact design. We have added a cost-benefit analysis of the tracking system in Section 3 to provide a clearer justification for this choice.

• Additionally, there are concerns regarding the collector’s dimensions based on the following simple considerations. According to the Global Solar Atlas, the Direct Normal Irradiance (DNI) for the EUA is around 2000 kWh/m² per year.

https://globalsolaratlas.info/map?c=21.698265,49.57493,5&s=23.80545,54.290306&m=site

Response: Thank you for referencing the Global Solar Atlas. We agree that the DNI for the UAE is approximately 2000 kWh/m² per year. However, our calculations are based on the daily DNI of 6.2 kWh/m²/day, which aligns with the average solar radiation conditions in the UAE. This value is derived from local meteorological data and is consistent with the assumptions used in similar studies (Soomro et al., 2019). We have clarified this in Section 4 and included a more detailed discussion of the solar radiation assumptions.

 

• It is true that one should consider the Global Irradiation at the optimal angle, which is approximately 2400 kWh/m² per year. However, for an approximate evaluation, assuming that the total annual direct energy (2000 kWh/m²) is fully absorbed by the collector, and considering the absorber surface area (0.53 m²), this corresponds to an available thermal annual energy of approximately 1060 kWh per year. Therefore, taking into account the efficiency of the selected thermoelectric cells, which is around 4%, the annual producible energy would be approximately 42 kWh. This value is significantly lower than the expected value of 511.5 kWh. In summary, it seems that the collector's surface area is at least one order of magnitude smaller than what is required. If this assessment is incorrect, a detailed counterargument should be provided.

Response: Thank you for this detailed analysis. We appreciate your concerns regarding the collector’s surface area and energy output. However, there seems to be a confusion in the assumptions used for the calculation. The system’s design is based on daily energy generation rather than annual energy absorption. The target daily energy output is 2.067 kWh/day, which corresponds to an annual output of approximately 754 kWh/year (assuming 365 days of operation). The detailed calculations presented demonstrate that the system is capable of meeting the target energy output with the proposed collector area. The discrepancy in your assessment arises from the assumption of a 4% TEG efficiency, whereas our design uses 8% efficiency, as specified by the manufacturer. Additionally, the system’s overall efficiency (6.5%) accounts for optical, thermal, and TEG efficiencies, ensure a realistic energy output.

We have revised Section 4 to include a more detailed explanation of these calculations and assumptions, ensuring clarity and transparency. We hope this addresses your concerns and provides a clearer understanding of the system’s performance.

In summary, the article needs to be thoroughly revised, including a review of its calculations. Apologies if this evaluation seems overly critical, but the goal is to highlight the necessary in-depth analyses to avoid unrealistic expectations and premature proposals in an energy-important country like the UAE, which is heavily involved in the energy transition. While the collector prototype concept is very interesting, at present, it remains an approximate theoretical scheme that can only be evaluated after the development of an experimental prototype and a rigorous component qualification process.

 

Response: Thank you for your comment. We have revised the "Data Analysis and Results" section to provide a more detailed and accurate analysis of the system's performance. The calculations now include a physics-based explanation of the system's operation, and we have clarified that the results are indicative and based on average values. We have also acknowledged the limitations of the simplified approach, ensuring that the pre-feasibility analysis is presented with appropriate context and transparency.

 

Once again, we greatly appreciate the time and effort they have dedicated to reviewing our work and providing detailed comments. We have carefully revised the manuscript to address all the concerns raised, and we hope the changes meet your expectations. We are grateful for the opportunity to resubmit the revised manuscript and look forward to hearing from you. Thank you once again for your time and consideration.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I think, my comments were accepted and the article was improved. I can recommend the article for publication. During the final checking, there could be corrected some formal graphical accuracy. In the table 2 and few times in the text the physical units are written in italica font. The units should be written in standard font.

Author Response

I think, my comments were accepted and the article was improved. I can recommend the article for publication. During the final checking, there could be corrected some formal graphical accuracy. In the table 2 and few times in the text the physical units are written in italica font. The units should be written in standard font.

Response: Thank you for your positive assessment and recommendation for publication. We appreciate the feedback on the graphical accuracy. In the revised manuscript, we have corrected the formatting of physical units in Table 2 and throughout the text, ensuring they are now presented in standard (non-italic) as per conventional scientific standards.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

I appreciate the clarifications provided for the numerous questions raised; however, some important doubts still remain, namely:
•    First of all, upon reviewing the new version, it is noticeable that the new collector opening area has become 5.13 m² compared to the numerical value of 0.533 m² indicated in the previous version. This is approximately ten times larger than the dimensions stated in the previous version, which naturally raises doubts about all the calculation done and the quality of the paper; 
•    In some parts of the article, there seems to be an ambiguity suggesting that an experimental prototype is already available. This is a fundamental point; it should be explicitly clarified both in the introduction and, most importantly, in the conclusions that this is a pre-feasibility study and that the results require validation through the construction of a prototype, followed by its characterization and experimental qualification.
•    Regarding the electrical efficiency of the thermoelectric modules: According to the manufacturer's specifications for the EG1-12611-8.0 module, the declared electrical power output is 13 W, while the thermal power irradiated is 325 W, which corresponds exactly to 4%. Why do you state that the manufacturer declares 8%? Have you used a different module? If so, you need to correct the module description in the previous sect
•    To provide a clearer perspective, the study should be supplemented with an economic assessment of the required investments, ultimately defining the cost per kWe produced with the solution proposed. In addition, if possible, need a comparative analysis with the KWe produced by stand-alone photovoltaic systems. 

Author Response

I appreciate the clarifications provided for the numerous questions raised; however, some important doubts still remain, namely:

• First of all, upon reviewing the new version, it is noticeable that the new collector opening area has become 5.13 m² compared to the numerical value of 0.533 m² indicated in the previous version. This is approximately ten times larger than the dimensions stated in the previous version, which naturally raises doubts about all the calculation done and the quality of the paper;
• In some parts of the article, there seems to be an ambiguity suggesting that an experimental prototype is already available. This is a fundamental point; it should be explicitly clarified both in the introduction and, most importantly, in the conclusions that this is a pre-feasibility study and that the results require validation through the construction of a prototype, followed by its characterization and experimental qualification.
• Regarding the electrical efficiency of the thermoelectric modules: According to the manufacturer's specifications for the EG1-12611-8.0 module, the declared electrical power output is 13 W, while the thermal power irradiated is 325 W, which corresponds exactly to 4%. Why do you state that the manufacturer declares 8%? Have you used a different module? If so, you need to correct the module description in the previous sect.

To provide a clearer perspective, the study should be supplemented with an economic assessment of the required investments, ultimately defining the cost per kWe produced with the solution proposed. In addition, if possible, need a comparative analysis with the KWe produced by stand-alone photovoltaic systems.

Response:

We appreciate your keen observations and feedback and are pleased to address the remaining concerns in the revised manuscript:

• Pre-Feasibility Clarification is now stated to eliminate any ambiguity exists. This is a pre-feasibility study, now explicitly stated in Section 1: "This pre-feasibility study proposes a hybrid system, requiring prototype validation," and Section 6: "This pre-feasibility design awaits experimental qualification via prototype construction," clarifying the theoretical nature of our work.

 

• Regarding TEG efficiency. The manufacturer’s specification for the TEG1-12611-8.0 module indeed indicates 4% efficiency (13 W ÷ 325 W), and our initial claim of 8% was not intended as a manufacturer-stated value but as a design target. No different module was used. We have revised Sections 4 (red color) to reflect the 4% baseline efficiency, with 8% (26 W) adopted as a theoretical target supported by advanced BiTe research (e.g., ηmax∼8% at ΔT=220∘C, Faizan et al., 2020; Faizan et al., 2023) and enhancements and porous media (Farhat et al., 2022; Mansour et al., 2023). These changes ensure accuracy while maintaining our design assumptions, with validation pending experimental confirmation.
• Regarding the economic assessment and comparison: Section 5 now includes an economic assessment (LCOE $0.117–$0.166/kWh for the hybrid system) and comparisons with a PV-battery system (LCOE $0.075–$0.130/kWh, 2.2 m², 400 W) and standard trough system (LCOE $0.168–$0.232/kWh).

Author Response File: Author Response.docx

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for addressing my comments as well as those from the other reviewer and for the revisions you have made. The modifications and additions have significantly improved the manuscript. I believe that in its current form, the article has been enhanced and can now be accepted for publication