Thermal Performance Analysis of Porous Foam-Assisted Flat-Plate Solar Collectors with Nanofluids

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study deals with the numerical analysis of a flow that passes through a channel with porous inserts including a nanofluid too. In particular, the authors are considering particular boundary conditions since they refer to a flat-plate solar collector application. Different porous shapes, as well as different nanoparticles, have been considered in this study. The predictive model is built up with the local thermal equilibrium assumption and validated with literature data. A large bunch of results in terms of Nusselt number, friction factor, thermal efficiency, and performance evaluation criteria, is shown. The authors conclude by remarking which is the best solution to maximize heat transfer for this application, that is generally referred to the one with the lowest Darcy number and the highest nanoparticles percentage.

The topic treated is really recent, since improving the heat transfer characteristics of engineering devices by using extended surfaces like porous media or nanoparticles is something really actual and useful to optimize thermal performances. The paper is well presented and rigorous in terms of numerical modeling. Therefore, it is suggested to consider this for publication after the authors answer to the following points

-        Together with Fig. 1, a figure that resumes the investigated configurations (triangular, trapezoidal, rectangular) is suggested

-        When writing governing equations, the authors assume local thermal equilibrium between the two phases, with no justification to be sure that a local thermal non equilibrium model is here necessary. Is this a reliable assumption for this study?

-        Permeability expression is shown in Fig. 7. Isn't the hydraulic diameter squared? If not, Darcy number would not be dimensionless

-        The authors are here studying a flat plate solar collector which is - per definition - a strongly time-dependent device because of the variable incoming solar radiation. Therefore, is the steady state assumption here done reliable?

-        It is suggested to provide a better description of the boundary conditions employed. First of all, how did the authors treat the porous/free flow interface? Secondly, if they assume that there is some emitted solar radiation, why didn't they consider this in Eq. 12 via Stefan-Boltzmann law?

-        The expression for thermal conductivity shown in Eq. 13 seems to be pertinent to porous foams, so the authors implicitly assume that the porous media is a foam. If so, why didn't they perform the same assumption when describing the pressure drop within the porous media via Carman-Kozeny equations?

-        In Eq. 30 the authors define the Nusselt number. Since conduction is involved within the equivalent medium (Fig. 1), why didn't they consider the Biot number - or a scaled transmittance - instead of the Nusselt number?

-        In Fig. 4a, the light blue and green curves seem to overlap at some point. Since this is the only case if one considers both Figs. 4a and 4b, more comments are suggested.

-        To which shape is Fig. 6 making references? Besides, why at higher Darcy - so higher permeability - the Nusselt number trend changes? Is this because at very high permeability conduction starts having a role?

-        In this contribution, the main novelty consists in combining nanoparticles and porous materials for solar applications, making heat transfer better because of higher heat transfer area, effective thermal conductivity, and flow mixing. The authors forgot to mention that combining solutions is something that has been already proposed in literature, in particular in similar studies on thermal storage systems based on embedded tubes (doi.org/10.1016/j.enconman.2023.117268) or coaxial heat exchangers (doi.org/10.1016/j.applthermaleng.2023.121139). In order to remark novelty from the present paper compared to other similar studies from the literature, the authors should mention all this within the state of the art review.

-        In Fig. 7, it is suggested to plot the pressure drop as a scaled form like pressure drop over kinetic energy in order to have a fully-scaled figure

-        In Fig. 9, why the friction factor becomes higher for the rectangular shape? One would expect more impact from a triangle, unless the authors are considering different volumes for triangles and rectangles. This would make the cross-section area for flow different

-        It is not really clear how did the authors compute the 25-75% area in Fig. 11. Could they report more information about this point? Why there is a need to run a statistical analysis? Besides, please use the scientific notation for the x-axis values

-        Generally speaking some more comments about the physics observed are suggested. Why the efficiency (Fig. 14) is higher for lower Darcy numbers and higher mixing volume fractions? Is this because the porous media is more permeable to the flow - making heat transfer better - and the higher percentage of nanoparticles make everything more conductive? Please report more comments like this

-        At the end of the results section, the authors should also report some more details about what happens when the present solution is integrated within an energy system. Which are the limitations due to nanoparticles deposition, operation costs, limited energy storage, weather conditions, and so on? All this discussion should be included in order to show how the present study could be useful for whom studies solar collectors integrated in energy systems

-        It is generally suggested to reduce the conclusions length. In particular, the authors could just focus about in which conditions one achieves the maximum thermal efficiency, the maximum PEC (that is, in turn, a percentage), and so on. In general please just report the most relevant outcomes from the paper

Author Response

Please refer to the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Please find attached comments and suggestions for authors.

Comments for author File: Comments.pdf

Author Response

Please refer to the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper by Lin et al. presents a comprehensive investigation into the thermal performance of flat-plate solar collectors (FPSC) assisted by porous foam and nanofluids. The study explores various parameters affecting thermal efficiency, including porous block design, permeability, Reynolds number, nanoparticle type, volume fraction, and mixing ratio. Through numerical analysis, the authors determine the impact of these factors on heat transfer and pressure drop within the FPSC channel, concluding with a Performance Evaluation Criteria (PEC) to identify optimal FPSC configurations for enhanced thermal efficiency. The paper's key findings underscore the significance of flow state, porous block characteristics, nanoparticle addition, and their combined influence on FPSC performance.

paper presents a meticulous investigation into FPSC thermal performance, examining various parameters' effects on heat transfer and pressure drop. The findings underscore the delicate balance between improved heat transfer and energy consumption, emphasizing the importance of porous block characteristics, nanoparticle types, and their combinations. The paper's detailed insights and comprehensive analysis provide a valuable resource for future advancements in FPSC design and optimization, serving as a solid foundation for commercial applications in sustainable solar energy technologies.

 The paper exhibits significant merit in elucidating the complex interplay of parameters influencing FPSC thermal performance and deserves strong consideration for publication in the journal Sustainability.

Comments on the Quality of English Language

Minor editing, and rephrasing may improve quality.

Author Response

Please refer to the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewer thinks that the paper can be now accepted for publication as it is

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have successfully addressed the comments and suggestions. The paper is now ready for publication. Congrats authors; well done! Merry Xmas and happy new year!