Nanoparticle-Enhanced Phase Change Materials (NPCMs) in Solar Thermal Energy Systems: A Review on Synthesis, Performance, and Future Prospects

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper systematically reviews the applications of nanoparticle-enhanced phase change materials (NPCMs) in solar thermal energy systems. The manuscript is relatively comprehensive with substantial content and extensive references, but there is room for improvement in formatting standardization, content depth, and logical coherence. It is recommended that the authors revise the paper addressing the following issues, with particular emphasis on strengthening mechanistic analysis and improving the completeness of techno-economic data to enhance the academic value and reference significance of this review.

 

1. Abstract Section
   The statement "NPCMs integrate nanoparticles into traditional phase change materials" is overly general. It is suggested to specify representative nanomaterials (e.g., Cu, Al₂O₃, graphene) and their core functions (e.g., thermal conductivity enhancement, photothermal conversion promotion) to highlight the uniqueness of NPCMs.
2. Tables and Figures

 

• In Table 1, the first row uses full names for parameters, but "TC" (thermal conductivity) appears as an abbreviation without explanation. Consistency should be maintained by using full names or adding abbreviations in parentheses (e.g., "Thermal Conductivity (TC)"). Other tables should be checked for similar issues.
• The color labeling of ions in Figure 3 is overly complex, reducing readability. It is recommended to simplify the color scheme (e.g., using grayscale gradients or 2-3 contrasting colors) while ensuring clear differentiation of key components (e.g., CuO NPs, water molecules).

 

3. Reference Formatting
   Inconsistencies exist in author presentation (e.g., some entries list full author names, others use "et al."). All references should be standardized:

 

• For works with ≤3 authors, list all names; for ≥4 authors, use the first author + "et al."
• Ensure completeness of journal names, volume numbers, issue numbers, page ranges, and DOI links. Missing information (e.g., in Refs. [80]-[87]) must be supplemented.

 

4. Content Logic and Depth

 

• Introduction: The current version is overly lengthy with redundant discussions on general renewable energy challenges. It should be streamlined to focus on: (1) limitations of traditional PCMs in solar thermal systems; (2) specific advantages of NPCMs (e.g., thermal conductivity, photothermal efficiency); (3) clear research gaps addressed by this review.
• Classification of NPCMs (Section 2): The five categories (Thermophysical NPCM, Photonic NPCM, etc.) lack clear defining criteria. For each category, it is recommended to add:
  • A concise definition (e.g., "Thermophysical NPCM: Nanoparticles (e.g., Cu, Al₂O₃) dispersed in PCMs to enhance thermal transport via phonon conduction").
  • Typical application scenarios (e.g., "Photonic NPCM is primarily used in photovoltaic-thermal (PVT) systems to improve light absorption").
• Mechanistic Analysis (Section 3):
  • In the "Molecular Dynamics Simulation" subsection, the rationale for selecting Cu NPs (over Ag or Al) is missing. Supplement with comparative analysis (e.g., "Cu NPs were prioritized due to their balanced cost, thermal conductivity, and chemical stability in paraffin-based PCMs compared to Ag (high cost) and Al (susceptibility to oxidation)").
  • The "Density Functional Theory" subsection (Refs. [80]-[87]) includes cases weakly related to NPCMs (e.g., lignin-fatty acid capsules). It should focus on DFT applications in NPCM design, such as "DFT calculations revealing interface charge transfer between graphene and paraffin, which enhances thermal stability".
• Redundancy and Relevance: Some content (e.g., excessive details on magnetic field coupling models in Section 3.2.4) is tangential to solar thermal systems. It is recommended to streamline descriptions and emphasize connections to core themes (e.g., "Magnetic field regulation of Fe₃O₄-NPCMs in solar collectors to optimize melting dynamics").

 

5. Techno-Economic Analysis (Section 5)
   The current cost-benefit analysis lacks quantitative comparisons (e.g., NPCM vs. traditional PCM lifecycle costs). It is suggested to add a summary table comparing key indicators (e.g., unit production cost, payback period, CO₂ reduction) across different applications (solar stills, PVT systems) to enhance practical reference value.

 

By addressing these issues, the manuscript will achieve greater clarity, depth, and academic rigor, better serving as a critical review of NPCMs in solar thermal applications.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper covers important topics and has a detailed account of nanoparticles enhanced PCMs in solar thermal energy systems. The authors have done a great job by covering a lot of topics like synthesis methods, simulation, characterization and economic analysis. At the same time, there are many references which help the reader understand in depth of what result outcome came from which paper. It’s easy to follow the paper. It can be overwhelming but very useful.

• However, I would suggest incorporating the Temperature History (T-history) method, which is a widely recognized technique to evaluate supercooling behavior in PCMs, particularly in the presence of various nano-additives. Several relevant studies use T-history and could be cited here for completeness.

Review of the T-history method to determine thermophysical properties of phase change materials (PCM) - ScienceDirect

• Additionally, it would be beneficial to mention dynamic testing using heat flow meters on tiles or container-integrated PCMs. This approach simulates real-life application scenarios and allows for larger sample analysis, providing complementary insights to DSC-based measurements.

C1784 Standard Test Method for Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products

• There are many experimental works explained with their benefits however if it comes with trade-offs, or whether long-term stability is preserved is not mentioned. Maybe doing that will kind of give an honest review of your cited works.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The review looks to be quite comprehensive and would be ustful as an introduction into the subject. As the main drawback one sould note that the authors just list the works performed earlier but almost do not give any analysis and physical explanation of their results. I would recommend to add some analysis of the works mentioned in the manuscript. One should also note rather low attention of the authors to the usage of carbon nanoparticles (carbon nanotubes, graphene etc.) as a dopand to PCMs for enhancement of the thermal conductivity. This direction seems to be very perspective and attracts many research groups. The article can be published after corrresponding corrections.   

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

1. Core Value of the Manuscript： V2 has significantly enhanced the scientific rigor, focus, and practicality of the review by correcting data errors (e.g., PCM composition), eliminating logical redundancies (e.g., irrelevant energy source comparisons), and supplementing technical details (e.g., T-history characterization methods). It has established a comprehensive analytical framework covering *“NPCM materials-technology-industry in solar thermal management”*, which offers substantial reference value for researchers in the field.
2. Potential for Improvement & Recommendations： If the authors further supplement **classical models of nanoparticle thermal transport mechanisms** (e.g., correlation analysis between the Maxwell model and experimental data) and **specific case studies of algorithm optimization** (e.g., comparison of prediction accuracy of machine learning models for NPCM thermal properties), the manuscript could evolve from a *“high-quality review”* to a *“landmark contribution”* in the field. It is recommended to invite the authors for a **Major Revision** to deepen these two dimensions.
3. In summary, the manuscript demonstrates outstanding potential after revision. It is suggested to refine the details through a major revision, and ultimately, it can be recommended for acceptance.

Comments on the Quality of English Language

The English in Version 2 demonstrates notable improvements in clarity and consistency: • Strengths: Unified terminology (e.g., consistent use of nAl₂O₃), simplified sentence structures (removal of redundant comparisons), and more precise academic phrasing.

• Areas for refinement:

1. Complex sentence construction: Long, multi-clause sentences (e.g., “…greenhouse gas emissions that drive global warming, ecosystem disruptions [1], and extreme weather events [2]”) should be split to enhance logical flow.
2.  Technical specificity: Vague descriptions (e.g., “enhance thermophysical properties”) could be strengthened by adding quantitative context (e.g., “improved thermal conductivity by 30% via graphene dispersion”) or referencing empirical data.
3.  Field-specific vocabulary: Incorporate terms like “thermal hysteresis”, “phase change kinetics”, or “effective medium theory” to replace general expressions, boosting technical precision.

Author Response

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Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors After two rounds of revisions, the manuscript has made noticeable progress in structure and coherence. However, to truly reflect the academic value and practical reference significance of this review, some details still need polishing.

1. Abstract and Introduction: Need for Greater Precision and Focus The abstract still fails to fully distill the unique contributions of the research. Although the authors have updated the description of NPCMs to "incorporate high-performance nanoparticles," they have not yet clarified which specific nanomaterials are involved and what their core functionalities are. The introduction already includes examples such as Cu, Al₂O₃, and graphite, but the abstract does not summarize the key advantages of NPCMs (e.g., how they enhance thermal conductivity or improve photothermal conversion efficiency). This vagueness makes it difficult for readers to distinguish between NPCMs and traditional PCMs.

Suggestion: Within the word limit of the abstract, add 1–2 specific examples of nanomaterials (e.g., "graphene enhances thermal conductivity, and copper-based materials optimize photothermal conversion"). Highlighting these "nano-enhancement" mechanisms will make the academic distinctiveness of the research more prominent.

2. Classification and Applications of NPCMs: Closer Alignment with Solar Thermal Systems The authors have supplemented application directions for each category of NPCMs, but some descriptions remain overly general and lack close connection to the theme of "solar thermal systems." For instance:

• Section 2.2 (Photonic NPCMs): It only states that photonic NPCMs "enhance the performance of photovoltaic-thermal (PVT) systems" but does not specify their actual roles in solar thermal systems—such as acting as absorber coatings to improve light-harvesting efficiency, or serving as thermal storage media to cool PV panels;
• Section 2.5 (Phase-engineered nanomaterials): Examples like Pd-Se and CuₓS-Ag₂S are not linked to the practical needs of solar thermal systems (e.g., high-temperature heat storage, photothermal conversion characteristics), which weakens the focus on the theme.

Suggestion: For each category of NPCMs, add at least one specific application scenario in solar thermal systems (e.g., "photonic NPCMs used as absorber coatings in concentrated solar power systems" or "phase-engineered nanomaterials for regulating the crystal-phase stability of molten salt storage"). Incorporating real engineering cases will help the review stay more closely anchored to the core of "solar thermal systems."

3. Techno-Economic Analysis: Supplement Quantitative Data and Practical Cases Section 5.1 lists data on performance improvements (e.g., "233% increase in productivity" and "574.1% improvement in energy efficiency"), but the economic analysis remains insufficient—we still do not know: What is the cost of NPCMs per kilogram? How do their costs compare to traditional PCMs? What is the payback period? Additionally, there is no cross-comparison of key indicators across different application scenarios (e.g., solar stills vs. PVT systems), making the techno-economic analysis incomplete.

Suggestions:

• Create a comparative table (e.g., "Table 15: Techno-Economic Comparison Between NPCMs and Traditional PCMs") covering metrics such as unit production cost (\(/kg), lifecycle cost (\)/system), payback period (years), and annual CO₂ reduction (kgCO₂/year), categorized by application scenarios (e.g., solar stills, PVT systems);
• Supplement 1–2 real engineering cases (e.g., "Cost-benefit comparison between NPCMs and paraffin in a 10 kW solar thermal system") and cite specific data from literature (e.g., "NPCMs have a 30% higher initial cost but reduce lifecycle costs by 25% due to efficiency improvements") to make the analysis more practical and referenceable.