Advances in Solid Particle Thermal Energy Storage: A Comprehensive Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The present paper is a review of solid particle based thermal storage systems. After showing the technology, the authors first resume the heat transfer correlations, then the practical solutions, say for instance shell-and-tube based heat exchangers, are included, discussed, and compared.

The reviewer thinks that this is a really interesting topic to be covered with a review. It is then suggested to consider the paper for publication if the attached points are addressed by the authors

• In Fig. 2, the heat storage capacity is shown. Is this far from very novel systems like thermochemical storage?
• What is surprising here is that the authors describe carefully packed bed, showing also heat transfer correlations in Table 1. However, they must report a figure that shows how the moving bed is done, how it stores/release heat and so on
• Generally speaking, the present paper lacks in mathematical modeling, say there are no equations that for instance show how to model packed bed storage systems
• Innovative configurations are resumed in Fig. 4. When changing the shape, which is the constraint to compare the solutions? Are the outcomes from the reviewed system better than the ones coming from [doi.org/10.1016/j.tsep.2024.102538], where the system is based on latent storage?
• Also in Fig. 6, the authors should report the meaning of the heat transfer coefficient. Is this referred to the interfacial solid/fluid heat transfer coefficient, or to an overall heat transfer coefficient that resumes the heat transfer within the moving bed?
• To remark the fact that the reviewed solutions is valuable compared to similar ones, the authors should spend some lines about the difference between the reviewed system and other ones like latent storage systems to similar problems that could be comparable in terms of geometries employed, say tube-in-tube storage system [doi.org/10.1016/j.heliyon.2024.e36105]
• What the authors mean with the average scores in Table 2? Some mathematical expression should be reported so far
• In a review, people would like to see more outcomes and comparison among solutions. In such storage system, which is the maximum energy that could be stored in practical applications? Which are the pro/cons with similar storage solutions?
• A brief paragraph that shows what could be done for this technology in the future must be introduced. The authors just focus on frontiers for numerical modeling, but actually the potential improvement for the technology in general must be included too

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study conducts thorough analysis of three representative reactor configurations: packed beds, moving beds, and fluidized beds, revealing the governing role of particle thermophysical properties in regulating heat transfer processes and storage performance. There are some comments:

1. In part “2. Advances in solid particle characteristics in different bed reactors”, the maximization of heat storage capacity per cost-volume ratio for TES solid particles in different industries is shown, but the thermal conductivity is lack. In fact, thermal conductivity plays a decisive role in particle selection applied in thermal energy storage.
2. The correlation of heat transfer in moving bed is listed in Table 1,what about correlation of heat transfer in packed bed and fluidized bed?
3. There are 82 references, to strengthen this manuscript, the authors should add more references.
4. The clarity of Figure 6 is too poor and the reading friendliness is low

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Review of the Manuscript: "Advances in Solid Particle Thermal Energy Storage: A Comprehensive Review"

The manuscript "Advances in Solid Particle Thermal Energy Storage: A Comprehensive Review" by Guang Zeng et al. presents a timely and technically detailed examination of solid‐particle thermal energy storage (TES) technologies. Focusing on three principal reactor configurations—packed beds, moving beds, and fluidized beds—the authors analyze how particle thermo‐physical properties, reactor design, and heat transfer mechanisms affect TES performance. Given the growing importance of TES for renewable energy integration and decarbonization, this synthesis is both relevant and necessary. The manuscript’s strengths lie in its thorough reactor‐specific analyses and clear treatment of heat transfer dynamics. However, several critical issues limit its overall completeness and practical utility.

1. Superficial Treatment of Storage Materials
   A truly comprehensive review of solid‐particle TES must include an in‐depth discussion of the storage materials themselves. Although the manuscript notes general thermo‐physical properties (e.g., specific heat, thermal conductivity) and references a material database [32], it stops short of detailing which materials are most relevant or promising. In particular:

   • Sensible Heat Storage Materials: Solids such as alumina, silica, and concrete form the backbone of many existing TES systems. Their heat capacity, thermal stability, and cost profiles warrant explicit comparison to illustrate trade‐offs between energy density and economic feasibility.

   • Phase Change Materials (PCMs): Latent‐heat storage options—such as encapsulated salts or waxes—can significantly increase overall storage density. A comprehensive overview should at least acknowledge PCM integration strategies, typical operating temperature windows, and cycling performance in particle‐based systems.

   • Thermochemical Materials: Metal oxides (e.g., CaO, Co₃O₄) can deliver very high energy densities via reversible redox cycles, but they demand specific reactor conditions (temperature, atmosphere) and considerations around reaction kinetics. The current omission of thermochemical options leaves readers without a clear sense of the upper limits of TES capacity.

2. Lack of Comparative Illustrations for Material Properties
   Without tabular or graphical comparisons, it is difficult to evaluate which materials best balance performance, durability, and cost. For example:

   • A table mapping key metrics—specific heat, thermal conductivity, typical operating temperature range, and approximate cost—for a representative set of materials (e.g., alumina, silica, simple concrete formulations, selected PCMs, and at least one or two metal oxides) would immediately clarify relative advantages and drawbacks.

   • A simple chart (e.g., energy density versus cost, or temperature stability versus mechanical robustness) would help readers visualize trade‐offs and inform practical decision‐making.
     The single figure on cost‐volume ratios [32] does not suffice to capture the multidimensional nature of material selection.

3. Neglected Material–Reactor Interactions
   The manuscript addresses particle size and shape, but it does not connect material composition to reactor‐specific performance and longevity. For instance:

   • Attrition in Fluidized Beds: Brittle ceramics may fragment under continuous collisions, generating fines that alter fluidization behavior and heat transfer rates. An assessment of attrition resistance across common materials would identify which solids perform reliably under fluidized conditions.

   • Thermal Expansion in Moving Beds: Materials like concrete or certain metal oxides can undergo non‐uniform thermal expansion, potentially leading to channeling, bridging, or uneven flow unless reactor geometry and operating profiles are carefully matched to material behavior.

   • Reactivity and Stability in Packed Beds: Thermochemical materials may undergo volumetric or morphological changes during repeated redox cycles, which can result in channeling or sintering if the packed bed is not designed to accommodate such changes.
     A thorough review should explicitly link material characteristics—hardness, brittleness, expansivity, chemical reactivity—to each reactor configuration (packed, moving, or fluidized), demonstrating how these factors influence system performance and lifecycle costs.

4. Disorganized Presentation of Particle Characteristics
   As currently written, Section 2 moves abruptly between particle size, shape, distribution, and thermo‐physical properties without a clear thematic structure. Consequently:

   • The transition from typical packed-bed particle diameters (1–100 mm) to fluidized-bed particle size limits (≤ 0.8 mm) appears sudden, with no narrative explaining why these distinct size ranges are critical for each configuration.

   • Discussions of shape, surface area, and porosity are interwoven with thermo‐physical metrics (specific heat, thermal conductivity), making it difficult to discern which particle attribute drives which aspect of reactor performance.
     To guide the reader logically, particle attributes should be grouped into discrete categories—(a) size and its effects on pressure drop and heat transfer, (b) shape and surface characteristics influencing packing and fluidization behavior, and (c) intrinsic thermo‐physical properties (heat capacity, conductivity, density). Within each category, the manuscript should note reactor‐specific implications (e.g., how size influences pressure drop in packed beds versus minimum fluidization velocity in fluidized beds).

The manuscript "Advances in Solid Particle Thermal Energy Storage: A Comprehensive Review" delivers a strong, timely analysis of reactor configurations and heat transfer phenomena in solid‐particle TES systems. Its focus on packed beds, moving beds, and fluidized beds provides valuable insights for researchers and engineers alike. However, the review falls short of being truly comprehensive due to its limited treatment of storage materials, lack of comparative visual aids, omission of material–reactor interactions, and disorganized presentation of particle properties. A more detailed examination of sensible‐heat, latent‐heat, and thermochemical materials—accompanied by side‐by‐side comparisons and explicit linkage of material traits to reactor performance—would fill critical gaps. Similarly, restructuring Section 2 to follow a clear hierarchy of particle attributes would strengthen the manuscript’s readability and coherence. Addressing these issues will transform the work into a more complete, authoritative reference on solid‐particle TES.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper can be accepted as it is in the revised version

Reviewer 3 Report

Comments and Suggestions for Authors

The careful revision made the manuscript considerably better, thus it could be accepted in the present form.