An Evaluation of a New Building Energy Simulation Tool to Assess the Impact of Water Flow Glazing Facades on Maintaining Comfortable Temperatures and Generating Renewable Energy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents and validates a novel building energy simulation tool designed to predict the thermal behavior of Water Flow Glazing (WFG) in building façades. The study has a clear application background and practical significance, particularly in the context of promoting nearly zero-energy buildings and sustainable design. However, the article has the following issues:

1. The current validation is limited to “software-to-software” comparison (the new tool vs. IDA-ICE), without any experimental data serving as ground truth. If both models deviate in the same direction, software agreement does not necessarily indicate proximity to the real physical process. It is strongly recommended to supplement at least one set of experimental data for validation—for example, monitoring data from a WFG prototype panel or from a system installed in a laboratory/real building (at minimum including inlet and outlet water temperature, indoor air temperature, solar irradiance, outdoor weather conditions, instantaneous flow rate, and surface temperature). If experimental data are not available, publicly accessible monitoring datasets should be selected and properly referenced (including source and time period), or at the very least, comparable cases validated by observations in the literature should be cited.

2. The use of ASHRAE Guideline 14 thresholds such as NRMSE ≤ 30% to judge model accuracy is problematic, since this standard is primarily intended for model calibration against measured data rather than inter-software comparison. The paper should explicitly clarify the scope of applicability of this standard and avoid treating it as an absolute acceptance criterion. Additional error metrics should be reported (e.g., MAE, bias, R², residual distribution), and the situation in Case 3 where NRMSE exceeded the threshold (32.69%) should be discussed, identifying possible numerical or physical reasons.

3. Several critical assumptions in model development have not been sufficiently justified, such as neglecting thermal resistance between glass and water layers, simplifying diffuse radiation, and assigning fixed values for convective heat transfer coefficients. It is recommended to list these assumptions explicitly in the methodology section and to quantitatively evaluate their influence through sensitivity analysis. Parameter scanning or uncertainty analysis should be conducted for hi/he, flow rate, inlet water temperature, radiation components, and spectral absorptivity. Sensitivity rankings with respect to indoor temperature and WFG outputs should be provided, supported by figures and tables.

4. Numerical implementation details are insufficiently described. Important information such as time step size, spatial discretization, solver type, convergence criteria, and computational cost are not provided. These details are crucial for reproducibility and practical application. The methodology or appendix should include a comprehensive description of the numerical implementation, and numerical sensitivity tests (e.g., effect of different time steps on results) should be performed. Computational efficiency should also be reported, for example, the runtime required for a full-year simulation on a specific computer, to substantiate the claim of suitability for early design phases.

5. Reproducibility is inadequate. The manuscript mentions an open interface, but the Data Availability section states “Not applicable,” and essential model files or examples are missing. It is recommended to provide at least a minimal working example or key files, including code/scripts, sample input files (.idm), spectral files (.plt), the source of the weather file, and the CSV datasets used to generate figures. If releasing the full code is not feasible, a reduced set of example and key files should be shared, with clear instructions in the Data Availability statement (e.g., via a GitHub repository or Zenodo DOI), to allow independent replication.

6. Section 2.1 refers to the “Simplified model” and the “Complete model” but does not sufficiently explain their mathematical basis or cite relevant references. In particular, the solution method for the partial differential equations in the “Complete model” requires further clarification or literature support.

7. The conclusions are expressed in overly generalized terms. For example, the abstract states that “a 4:10 WFG area-to-floor ratio ensures comfort throughout the year,” but this claim lacks broad data support. Conclusions should be restricted to the studied case (Madrid climate, south-facing orientation, 7 × 7 × 3 m room, specific inlet water temperature and flow rate), and universal statements should be avoided. If generalization is intended, additional parametric studies across climates, orientations, and room sizes should be conducted to test applicability.

8. The scope of case studies is too narrow, as only Madrid with a single orientation and room size was analyzed. Cross-climate and cross-building-type validations are lacking. It is recommended to extend the study by including simulations for cold and hot climates or other representative cities, and to consider different orientations and room geometries. If expansion is not feasible, the discussion should clearly acknowledge the limitations of the study.

9. Figures 7, 8, and 9 have unclear labels and legends. Coordinate axis units, legend explanations, and explicit references in the main text should be added. All figures (Fig. 1–Fig. 9) should be checked to ensure variables and units are complete, captions can be understood independently, and table footnotes specify data sources and calculation methods. Error bands or confidence intervals could be added to key figures to facilitate intuitive assessment of reliability.

10. Symbols and units are inconsistently used. For example, mass flow rate is expressed as “2 L/(min·m²),” whereas it should be standardized in SI units (e.g., kg·s⁻¹·m⁻² or kg/(s·m²)), with density or conversion factors noted in table footnotes. Tables 2, 4, 5, 6, and 7 should include explicit units in headers, and symbols should be explained upon first occurrence in the text.

11. The output curves of the new tool and IDA-ICE differ in slope, but the manuscript does not explore the underlying reasons. A differential attribution analysis is recommended, whereby input conditions (irradiance model, convective coefficients, spectral parameters, time step, etc.) are progressively harmonized across the two tools, and the resulting changes are compared to identify the source of discrepancies. A dedicated “Difference Analysis” section could be added, supported by tables or figures.

12. The validation framework lacks internationally recognized benchmark tests. It is recommended to supplement the study with BESTEST or similar standardized test cases to provide additional validation. Even if full execution is not possible, reference to benchmarked results in the literature should be included to contextualize model performance.

13. The manuscript header/footer shows inconsistent years (e.g., “Sustainability 2022” in the header vs. “2025” in the citation line). References should also be carefully checked to ensure consistency in year, DOI, and formatting. Such metadata inconsistencies undermine the professionalism of the manuscript.

14. Language and style require improvement. Several statements are overly absolute, and inconsistencies in English spelling and grammar are evident (e.g., British vs. American spelling). Professional English editing is recommended, with particular focus on enhancing the logic and academic tone of the abstract, conclusions, and discussion sections, while avoiding absolute or colloquial expressions.

15. The derivation of model equations and boundary conditions is incomplete. The appendix should include the full mathematical derivations, boundary and initial conditions, and provide pseudocode or a flowchart to enhance transparency and reproducibility.

16. The conclusion refers to Case 3 as the “best” choice for summer, but the definition of “best” is unclear (e.g., lowest energy consumption, best thermal comfort, or highest system efficiency). A more comprehensive multi-objective discussion is recommended, combining WHG and indoor heat gains (qi).

17. The indoor convective heat transfer coefficient (hi) and outdoor coefficient (he) are modeled as constants (8 and 23 W/m²K). The basis for selecting these specific values should be explained (e.g., whether derived from standards or empirical sources), and the implications of this simplification, especially in transient simulations, should be discussed.

18. The description of indoor air temperature calculation in the room simulation is overly simplistic (e.g., “the tool must provide a method to calculate the indoor temperature”). A brief explanation of the indoor heat balance equation should be included, covering how longwave radiative exchange between internal surfaces and non-WFG heat gains are considered. This is critical for understanding the overall thermal performance of the room.

19. Since the paper claims that the model is suitable for early design stages, a “quick-start guide” and a set of recommended default parameters should be provided. A step-by-step minimal example (e.g., a five-step workflow to run a basic simulation) can be included in the appendix or supplementary materials, along with default parameter values and their recommended ranges of applicability.

Author Response

Thank you for your valuable comment. You can find our responses in the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

I wanted to reach out and express my sincerest appreciation for your efforts in writing this paper.

The present MS is very interesting as it presents simulated tests of a Water Flow Glazing (WFG) panel that could potentially replace full glazing or air glazing panels. From an AEC stakeholder perspective, this could be groundbreaking if the experimental outcomes are outstanding and demonstrate novelty in the glazing sector. Unfortunately, this is only a simulation, although it is quite encouraging for a real-world pilot test to validate these outcomes, despite the authors' efforts to validate it against the outcomes of similar software (IDA-ICE). The developed tool for WFGs seems interesting and well-conceived, and I must congratulate the authors for it. I am not certain whether Sustainability has the best scope for this article, as it focuses more on software and tool development rather than the WFG screens performance and impact on thermal and energy performance of the whole building itself, but I will leave that decision to the editors (nonetheless I am sharing this with you).

The paper is well-structured and follows a classical approach. However, the literature review needs better framing and is outdated as far as I can see it. The cited references are mostly older than five years (24 out of 44), which suggests that the authors may be working with outdated data. This scientific area is fertile with new research, with novel studies emerging almost daily. I wonder what the authors' rationale is for relying on such outdated studies. 

The Methodology section requires work, as it is the most important part of the article. Some specific comments:

• Line 133-135: Did the software already include a WFG composition in its catalog, or did you have to create one?
• Line 142-143: Which EPW file was used? Where did you get it? Please provide the location and file name. Only at line 405 did I realize you were working in Madrid.
• Can you provide readers with an image of the simulated building and its baseline conditions? It is poorly described. These baseline conditions will definitely affect the WFG panel performance. Or if you have arguments of the opposite please state them in the paper.
• Line 245: When you refer to "the climate must be placed," do you mean the EPW file? Please make terminology uniform and coherent throughout the paper.
• The Methodology needs to be rewritten for greater uniformity and to avoid repeating the same information. Some elements are missing, such as initial boundary and baseline conditions. However, there is a significant effort by the authors to describe in detail the step-by-step creation of the WFG panel, that shows commitment and the seriousness of the authors work.

All figures are not displayed properly: they appear to be cut off on the right side of the page, which hinders their interpretation. Although, the figures appear to be properly referenced in the text.

Finally, it represents interesting experimental development work and is definitely publishable once you address the Methodology section issues.

Keep up the good work and all the best!

Author Response

Reviewer 2:

I wanted to reach out and express my sincerest appreciation for your efforts in writing this paper.

The present MS is very interesting as it presents simulated tests of a Water Flow Glazing (WFG) panel that could potentially replace full glazing or air glazing panels. From an AEC stakeholder perspective, this could be groundbreaking if the experimental outcomes are outstanding and demonstrate novelty in the glazing sector. Unfortunately, this is only a simulation, although it is quite encouraging for a real-world pilot test to validate these outcomes, despite the authors' efforts to validate it against the outcomes of similar software (IDA-ICE). The developed tool for WFGs seems interesting and well-conceived, and I must congratulate the authors for it. I am not certain whether Sustainability has the best scope for this article, as it focuses more on software and tool development rather than the WFG screens performance and impact on thermal and energy performance of the whole building itself, but I will leave that decision to the editors (nonetheless I am sharing this with you).

 

The paper is well-structured and follows a classical approach. However, the literature review needs better framing and is outdated as far as I can see it. The cited references are mostly older than five years (24 out of 44), which suggests that the authors may be working with outdated data. This scientific area is fertile with new research, with novel studies emerging almost daily. I wonder what the authors' rationale is for relying on such outdated studies.

 

RESPONSE:

Thank you for this comment. Some references are international standards such as ISO and ASHRAE. Literature on Water Flow Glazing is scarce. There are some research teams working on this topic, especially in Hong Kong, but most of their articles are more that 10 year old. The team of authors responsible for this article has published several scientific papers on Water Flow Glazing over the past four years. However, we attempt to avoid self-citation due to editor’s advice.

We have included more updated references on innovative glazing systems.

Sorooshnia, E., Rashidi, M., Rahnamayiezekavat, P., Mahmoudkelayeh, S., Pourvaziri, M., Kamranfar, S., Gheibi, M., Samali, B., Moezzi, R. A novel approach for optimized design of low-E windows and visual comfort for residential spaces, Energy and Built Environment, 2025, 6(1), 27-42, https://doi.org/10.1016/j.enbenv.2023.08.002.

1. Nur-E-Alam, M. Vasiliev, B.K. Yap, M.A. Islam, Y. Fouad, T.S. Kiong, Design, fabrication, and physical properties analysis of 567 laminated Low-E coated glass for retrofit window solutions, Energy and Buildings 318 (2024).
2. Jia, C. Xiang, Smart solar windows for an adaptive future: A comprehensive review of performance, methods and applications, 569 Energy and Buildings 346 (2025).

M.N. Mustafa, M.A.A. Mohd Abdah, A. Numan, A. Moreno-Rangel, A. Radwan, M. Khalid, Smart window technology and its 571 potential for net-zero buildings: A review, Renewable and Sustainable Energy Reviews 181 (2023).

M.N. Mustafa, M.A.A. Mohd Abdah, A. Numan, A. Moreno-Rangel, A. Radwan, M. Khalid, Smart window technology and its 571 potential for net-zero buildings: A review, Renewable and Sustainable Energy Reviews 181 (2023).

Rashevski, M., Slavtchev, S., Stoyanova, M. Natural and mixed convection in a vertical water-flow chamber in the presence of solar radiation, Engineering Science and Technology, an International Journal, 2022, 33, 101073, https://doi.org/10.1016/j.jestch.2021.10.005.

 

 

The Methodology section requires work, as it is the most important part of the article. Some specific comments:

• Line 133-135: Did the software already include a WFG composition in its catalog, or did you have to create one?

RESPONSE:

As per your suggestion, we have included the following clarification:

“The tool features four different WFG configurations. Nevertheless, the user can calculate the thermal performance for any configuration of water flow glazing, with the restriction of only one water chamber.”

 

• Line 142-143: Which EPW file was used? Where did you get it? Please provide the location and file name. Only at line 405 did I realize you were working in Madrid.

 

 

RESPONSE:

Thank you for pointing out this inconsistency. We downloaded the EPW file from Madrid. We have changed the text and included a reference to the EnergyPlus website.

“Figure 1 shows the parametric curves of potential solar energy (kWh) absorbed and the impinging solar power (kW) on different surfaces (facades and roofs) in Madrid, Spain on July 5th, from the EnergyPlus Weather (EPW) file for Madrid [30].”

“The case studies described in this research are placed in Madrid, Spain. The EPW file was downloaded from the EnergyPlus website [30]”

[30] https://energyplus.net/weather-location/europe_wmo_region_6/ESP/ESP_Madrid.082210_IWEC

 

 

• Can you provide readers with an image of the simulated building and its baseline conditions? It is poorly described. These baseline conditions will definitely affect the WFG panel performance. Or if you have arguments of the opposite please state them in the paper.

 

 

RESPONSE:

Thank you for this suggestion. All reviewers considered necessary the experimental validation of the software tools. The research team bult a small prototype in Madrid to test WFG facades. The results from this prototype will be a topic of further research work and publication. However, two new sections have been included in the article showing preliminary results from the prototype.

• 4. Description of the experimental prototype.
• 3. Experimental validation in a prototype with a Water Flow Glazing facade. Transient-State Case 3.

 

• Line 245: When you refer to "the climate must be placed," do you mean the EPW file? Please make terminology uniform and coherent throughout the paper.

 

RESPONSE:

We totally agree that the sentence is confusing. The final sentence is “the EnergyPlus Weather (EPW) file must be placed in the installation IDA-ICE folder”

 

 

• The Methodology needs to be rewritten for greater uniformity and to avoid repeating the same information. Some elements are missing, such as initial boundary and baseline conditions. However, there is a significant effort by the authors to describe in detail the step-by-step creation of the WFG panel, that shows commitment and the seriousness of the authors work.

RESPONSE:

Thank you for this suggestion. We have revised exhaustively the Methodology Section.

 

 

All figures are not displayed properly: they appear to be cut off on the right side of the page, which hinders their interpretation. Although, the figures appear to be properly referenced in the text.

 

RESPONSE:

Thank you for pointing out this issue. We have changed the format of Figures to fit the images on the page.

 

Finally, it represents interesting experimental development work and is definitely publishable once you address the Methodology section issues.

Keep up the good work and all the best!

 

RESPONSE:

Thank you for these encouraging words. Your comments helped improve the quality of the article. We will continue our research and publish in open-access journals to learn from our peers' feedback.

Reviewer 3 Report

Comments and Suggestions for Authors

The article presents the development and validation of a new building energy simulation (BES) tool designed specifically to analyse the thermal behaviour of water flow glazing (WFG) systems integrated into building façades. The tool addresses the inaccessibility of existing BES software, which is often too complex and expensive for architects and engineers to use in the early stages of design.

The new tool provides two modes — steady-state and transient simulations — to evaluate the performance of WFG systems. It incorporates spectral and thermal models and features graphical interfaces for intuitive use. It also allows users to test different glazing compositions under real climatic conditions.

The topic of this paper is interesting and aligned with the scope of the journal.

Nevertheless, I recommend that the authors conduct a substantial revision, taking into account the following observations.

Firstly, I recommend improving the literature review. The papers presented in the introduction are not very recent, and the literature analysis lacks discussion of interesting research concerning innovative glazing systems for building insulation, such as glazing systems with aerogel, glazing systems with photovoltaics, vacuum façades and solar films, or other important applications.

The interesting aspect relates to the analysis of water flow glazings, an innovative technology with dual roles (thermal regulation and renewable energy potential), which makes the study highly relevant to sustainable building design. The study provides both steady-state and transient simulations, offering flexibility in terms of complexity and computational effort. While validation against a recognised commercial tool (IDA-ICE) strengthens the credibility of the results, the validation was limited to a few case studies (three glazing configurations, one room type and a single climate file). Therefore, the results may not be applicable to complex buildings, diverse climates or real-world monitored data. Furthermore, deviations above the ASHRAE threshold (particularly in water heat gain for case 3) highlight potential limitations in the model. Could you try to address these issues by improving the simulation analysis?

Certain boundary conditions (e.g. constant convective coefficients and neglecting diffuse irradiance in some cases) may oversimplify real-world behaviour. Have you case studies to be analysed and compare to the results?

Validation was mainly carried out through simulation-to-simulation comparison (with IDA-ICE), rather than against experimental or field data, which limits practical verification.

Although the tool provides both simplified and complete thermal models, the differences between the approaches were said to be negligible without substantial quantitative evidence. This may result in performance gaps in more complex scenarios being underestimated.

The study only considered WFG as a standalone façade system, and the integration of full HVAC systems and energy management strategies remains unexplored. Can you add this aspect to your simulation analysis?

In general, further research with experimental validation and broader case studies would strengthen its practical adoption. Can you improve the conclusions by discussing also these aspects?

The paper is accepted after a MAJOR REVISION.

Comments on the Quality of English Language

English form could be improved in some parts.

Author Response

Reviewer 3:

REVIEWER’S COMMENT

The article presents the development and validation of a new building energy simulation (BES) tool designed specifically to analyse the thermal behaviour of water flow glazing (WFG) systems integrated into building facades. The tool addresses the inaccessibility of existing BES software, which is often too complex and expensive for architects and engineers to use in the early stages of design.

The new tool provides two modes — steady-state and transient simulations — to evaluate the performance of WFG systems. It incorporates spectral and thermal models and features graphical interfaces for intuitive use. It also allows users to test different glazing compositions under real climatic conditions.

The topic of this paper is interesting and aligned with the scope of the journal.

RESPONSE:

Thank you for these encouraging words. Your comments helped improve the quality of the article. We will continue our research and publish in open-access journals to learn from our peers' feedback.

 

 

REVIEWER’S COMMENT

Nevertheless, I recommend that the authors conduct a substantial revision, taking into account the following observations.

Firstly, I recommend improving the literature review. The papers presented in the introduction are not very recent, and the literature analysis lacks discussion of interesting research concerning innovative glazing systems for building insulation, such as glazing systems with aerogel, glazing systems with photovoltaics, vacuum façades and solar films, or other important applications.

RESPONSE:

Thank you for this comment. Some references are international standards such as ISO and ASHRAE. Literature on Water Flow Glazing is scarce. There are some research teams working on this topic, especially in Hong Kong, but most of their articles are more that 10 year old. The team of authors responsible for this article has published several scientific papers on Water Flow Glazing over the past four years. However, we attempt to avoid self-citation due to editor’s advice.

We have included more updated references on innovative glazing systems.

“In the context of innovative glazing solutions to enhance the performance of building envelopes, several emerging technologies include aerogel-infused materials, glazing systems integrating photovoltaic cells, and electrochromic facades.”

• Sorooshnia, E., Rashidi, M., Rahnamayiezekavat, P., Mahmoudkelayeh, S., Pourvaziri, M., Kamranfar, S., Gheibi, M., Samali, B., Moezzi, R. A novel approach for optimized design of low-E windows and visual comfort for residential spaces, Energy and Built Environment, 2025, 6(1), 27-42, https://doi.org/10.1016/j.enbenv.2023.08.002.
• Nur-E-Alam, M. Vasiliev, B.K. Yap, M.A. Islam, Y. Fouad, T.S. Kiong, Design, fabrication, and physical properties analysis of 567 laminated Low-E coated glass for retrofit window solutions, Energy and Buildings 318 (2024).
• Zhuoying Jia, Changying Xiang, Smart solar windows for an adaptive future: A comprehensive review of performance, methods and applications, Energy and Buildings, 2025, 346, 116227, https://doi.org/10.1016/j.enbuild.2025.116227.
• Wu, S., Sun, H., Duan, M., Mao, H., Wu, Y., Zhao, H., Lin, B. Applications of thermochromic and electrochromic smart windows: 574 Materials to buildings, Cell Reports Physical Science, 2023, 4(5), 101370, DOI: 1016/j.xcrp.2023.101370
• Zhang, S., Liu, J., He, E., Li, D., Si, W., Wei, B., Wang, G. Photothermal performance investigation of a reversible window combining paraffin and silica aerogel, Energy, 2025, 329, 136646, https://doi.org/10.1016/j.energy.2025.136646.

• Rashevski, M., Slavtchev, S., Stoyanova, M. Natural and mixed convection in a vertical water-flow chamber in the presence of solar radiation, Engineering Science and Technology, an International Journal, 2022, 33, 101073, https://doi.org/10.1016/j.jestch.2021.10.005.

 

 

REVIEWER’S COMMENT

The interesting aspect relates to the analysis of water flow glazing, an innovative technology with dual roles (thermal regulation and renewable energy potential), which makes the study highly relevant to sustainable building design. The study provides both steady-state and transient simulations, offering flexibility in terms of complexity and computational effort. While validation against a recognised commercial tool (IDA-ICE) strengthens the credibility of the results, the validation was limited to a few case studies (three glazing configurations, one room type and a single climate file). Therefore, the results may not be applicable to complex buildings, diverse climates or real-world monitored data. Furthermore, deviations above the ASHRAE threshold (particularly in water heat gain for case 3) highlight potential limitations in the model. Could you try to address these issues by improving the simulation analysis?

RESPONSE:

We have rewritten the Discussion section to include an analysis of the simulated data with measured results from a prototype. The following section explains the steps to compare measured and simulated results:

4.3. Experimental validation in a prototype with a Water Flow Glazing facade. Transient-State Case 3.

We have included the following text to clarify the differences between simulation tools and the experimental results in the new section and the conclusions:

“The main difference between recorded and simulation data lies in the peak indoor temperature. This discrepancy may be attributed to several factors, such as the prototype size, the interior thermal mass assigned in the simulation, and the limitations of the Peltier device in maintaining a steady inlet temperature. The prototype was constructed using the same materials specified in the simulation, with an opaque enclosure made of aluminum panels with thermal insulation. The size of the simulated case study was 7 m by 7 m by 3m, whereas the actual prototype's volume was less than 1 m³. This size difference resulted in a lower thermal mass and reduced capacity to absorb heat. On the other hand, the inlet temperature set in both simulation tools was 17 °C. The Peltier device used in the experimental prototype began operating at noon, maintaining an inlet temperature between 22 °C and 18 °C.”

“Another goal of this article was to validate the results from the newly developed tool, which was explicitly designed to assess the thermal behavior of Water Flow Glazing, against the modeling approaches of the commercial dynamic simulation tool, IDA-ICE. The analysis of indoor temperature results revealed that both simulation tools produced identical maximum and minimum values. However, a notable divergence in the slope of the resulting graphs was observed. This variation suggests that the two tools may employ differing methods for considering interior thermal mass.”

 

 

 

 

REVIEWER’S COMMENT

Validation was mainly carried out through simulation-to-simulation comparison (with IDA-ICE), rather than against experimental or field data, which limits practical verification.

RESPONSE:

Thank you for this suggestion. All reviewers considered necessary the experimental validation of the software tools. The research team bult a small prototype in Madrid to test WFG facades. The results from this prototype will be a topic of further research work and publication. However, a new section has been included in the article showing preliminary results from the prototype.

This section describes the experimental setup:

2.4. Description of the experimental prototype.

The new section that presents the experimental results is:

4.3. Experimental validation in a prototype with a Water Flow Glazing facade. Transient-State Case 3.

 

REVIEWER’S COMMENT

Certain boundary conditions (e.g. constant convective coefficients and neglecting diffuse irradiance in some cases) may oversimplify real-world behaviour. Have you case studies to be analysed and compare to the results?

 

RESPONSE:

The following text has been included in section 3.1.2. Thermal results:

Exterior boundary conditions (Outdoor temperature and solar irradiance) are determined by the EPW file described in the Methodology section. The opaque enclosure and the transparent WFG facade define indoor boundary conditions. The opaque envelope features include orientation of the simulated prototype, indoor mass and thermal inertia, the thermal transmittance, the interior convective coefficient, and the absorption coefficients (αNIR and αFIR) of the indoor surfaces. The boundary conditions in the transparent WFG panel include interior and exterior convective coefficients, as well as the transmittance and absorptance of each WFG component (glass, water, and gas chamber).

 

REVIEWER’S COMMENT

Although the tool provides both simplified and complete thermal models, the differences between the approaches were said to be negligible without substantial quantitative evidence. This may result in performance gaps in more complex scenarios being underestimated.

RESPONSE:

We totally agree that comparing the simplified model with experimental results is key to understanding the feasibility of the proposed tool. We have included the following paragraph in the Methodology section:

“This work focuses on formulating an algebraic model to obtain the outlet temperature of the water chamber, the temperature profile, and the heat flux in each layer. By doing so, the tool calculates the energy absorbed by the water chamber, enabling the analysis of its energy performance over the course of a year. The simplified model enables a shorter simulation time with a suitable level of precision. An experimental setup has also been constructed to generate measured data as a benchmark for comparing and validating the results derived from the proposed simplified model.”

We acknowledge the limitations of this procedure and state that in conclusions.

 

 

REVIEWER’S COMMENT

The study only considered WFG as a standalone façade system, and the integration of full HVAC systems and energy management strategies remains unexplored. Can you add this aspect to your simulation analysis?

RESPONSE:

The authors of this research have been testing Water Flow Glazing devices in various locations. The most fruitful outcome of this research was participating in the INDEWAG project, where multiple prototypes were tested in Madrid, Spain, and Sofia, Bulgaria. The energy performance of Water Flow Glazing has been studied in several open-access articles across various locations, spanning a range of mild to extreme cold weather conditions. The researchers developed the studied tool, in part, thanks to the INDEWAG research project. The novelty of this article lies in the comparison of the tool with commercial software such as IDA ICE. However, due to reviewers' suggestions, we have included a new section to describe the results from prototypes, comparing the outcomes with simulation results from both the studied tool and IDA ICE.

A scope of future research will be to compare simulation results from the proposed tool and IDA ICE with measured outputs from the prototypes. The editors advised us to reduce self-citations. Nevertheless, you can find information about previous research in these references:

• del Ama Gonzalo, F.; Moreno Santamaría, B.; Hernández Ramos, J.A. Assessment of Water Flow Glazing as Building-Integrated Solar Thermal Collector. Sustainability 2023, 15, 644. https://doi.org/10.3390/su15010644
• Del Ama Gonzalo, F., Hernandez, J.A., Moreno, B., 2017. Thermal Simulation of a Zero Energy Glazed Pavilion in Sofia, Bulgaria. New Strategies for Energy Management by Means of Water Flow Glazing. IOP Conf. Series: Materials Science and Engineering 245 (2017) 042011 doi:10.1088/1757-899X/245/4/042011.

 

 

REVIEWER’S COMMENT

In general, further research with experimental validation and broader case studies would strengthen its practical adoption. Can you improve the conclusions by discussing also these aspects?

RESPONSE

Thank you for this suggestion. All reviewers considered necessary the experimental validation of the software tools. The research team bult a small prototype in Madrid to test WFG facades. The results from this prototype will be a topic of further research work and publication. However, a new section has been included in the article showing preliminary results from the prototype.

2.4. Description of the experimental prototype.

Focused on evaluating the thermal performance of water flow glazing in comparison with software tools, a scaled prototype was built and tested in Madrid, Spain. The prototype’s dimensions are 100 cm in height, 100 cm in width, and 75 cm in depth. A Peltier unit connected to a buffer tank regulates the temperature of the water inlet. The prototype consists of a steel structure with roofs, floors, and opaque walls. A white aluminum sandwich panel incorporating 100mm of extruded polystyrene was placed as the opaque envelope. For the transparent south facade, a Water Flow Glazing panel, as defined in Case 3, was selected. Figure 6 illustrates a schematic view of the prototype and its main components.

4.3. Experimental validation in a prototype with a Water Flow Glazing facade. Transient-State Case 3.

The sensors deployed in the prototype measured the water heat gain of the WFG module. Inlet and outlet temperature probes were located to determine the flow rate and the total thermal power of the module. Then, the data monitored by the prototype was compared with the simulation results. Figure 11 illustrates a three-day analysis, where the outdoor temperature matched the temperature of the file used for simulation. The inlet and outlet temperatures of the water were determined by the operating time of the Peltier device. Between 12:30 and 8:00 pm, the outlet temperature exceeded the inlet temperature. The outdoor temperature ranged from 38°C to 15°C. The Peltier device was unable to maintain a fixed inlet temperature throughout its operating time. The inlet temperature ranged from 22°C at the start of operation to 18°C at the end.

A comparison between Figures 9 and 11 showed similarities in the experimental and simulated outputs. The Water Flow Glazing facade maintained a comfortable indoor temperature during the hottest hours of the day. The water heat gain (WHG) is calculated based on the mass flow rate of 0.028 kg/(s m2), the fluid's specific heat, and the difference between the inlet and outlet water temperatures, as described in Equation (5). The WHG in kWh is 2.95 kWh over three days, which aligns well with the simulation results.

However, notable differences between the measured and simulated results were observed. The measured outdoor temperature on the first day of data collection was warmer than that used in the simulation file. The simulated temperature ranged between 19°C and 36°C, whereas the measured data showed peaks of 38°C. When the outdoor temperature reached those peaks, the indoor temperature was close to 38°C. When the outdoor temperature remained below 35°C, the recorded peak indoor temperature was 24°C, slightly above the maximum indoor temperature in the simulation, which was 22°C. The variation can be attributed to the working period of the Peltier device, which starts at noon. If the Peltier device had started its operation earlier, the indoor temperature peak would have been reduced. Another difference noted in the results was that the simulated water inlet temperature was set at 17°C. In comparison, the observed results ranged between 18°C and 22°C due to the constraints of the Peltier device in achieving a steady water temperature.

The conclusions have been rewritten to include all these aspects.

Reviewer 4 Report

Comments and Suggestions for Authors

This article  develops  a new tool that allows the performance of buildings to be evaluated at an early stage. This tool includes water-flow glazing (WFG) as a construction element that forms part of both the facade and the building's heating and cooling system. The tool was validated by comparing the results of the new tool with those of the commercial tool BES Indoor Climate and Energy IDA-ICE.I find the work very interesting and worthy of publication.
The work is well written, the methodology is sufficiently described, and the conclusions are well supported by the research results.
I have no comments to make and believe that the work can be published in its current form.
Just one small observation for the authors to consider: line 154 and Fig. 1 present results for July 5. I believe that the authors are referring to this day in the epw file for Madrid, but I think that the use of this file should be expressed more clearly here.

 

Author Response

Reviewer 4

This article  develops  a new tool that allows the performance of buildings to be evaluated at an early stage. This tool includes water-flow glazing (WFG) as a construction element that forms part of both the facade and the building's heating and cooling system. The tool was validated by comparing the results of the new tool with those of the commercial tool BES Indoor Climate and Energy IDA-ICE.I find the work very interesting and worthy of publication.
The work is well written, the methodology is sufficiently described, and the conclusions are well supported by the research results.

Thank you for these encouraging words. Your comments helped improve the quality of the article. We will continue our research and publish in open-access journals to learn from our peers' feedback.

I have no comments to make and believe that the work can be published in its current form.
Just one small observation for the authors to consider: line 154 and Fig. 1 present results for July 5. I believe that the authors are referring to this day in the epw file for Madrid, but I think that the use of this file should be expressed more clearly here.

Thank you for this input. We have changed the sentence describing Figure 1 to clarify that the data comes from the EPW file. The corrected sentence is:

“Figure 1 shows the parametric curves of potential solar energy (kWh) absorbed and the impinging solar power (kW) on different surfaces (facades and roofs) in Madrid, Spain on July 5th, from the EnergyPlus Weather (EPW) file for Madrid.”

Reviewer 5 Report

Comments and Suggestions for Authors

The article, titled "Assessment of a new Building Energy Simulation tool to predict the thermal behavior of water flow glazing in facades," aligns with the journal Sustainability. The purpose and scope of the work are clearly defined. The overall structure and layout of the article meet the standards of scientific papers and the journal's requirements. After clarifying certain ambiguities and making corrections, the article can be considered for publication. Comments are provided below.
1. A weakness of the article, in terms of methodology, is the lack of validation with actual experimental data. How was the sensitivity and correct calibration of the model assessed?
2. What innovation does the discussed modeling method introduce compared to other tools commonly used in this industry?
3. What parameters characterized the adopted boundary conditions and how were they determined?
4. Was validation attempted for other weather conditions?
5. How was diffuse radiation taken into account?
6. Please clearly indicate the limitations of the tool used. 7. The results are discussed primarily in the context of Madrid's climate and a simple room model – there is no discussion of whether and how they can be extrapolated to other climates or more complex buildings.
8. Taking into account the above comments, the conclusions require further verification.

Editorial Notes:
1. Figures 5 and 6 were incorrectly inserted, and therefore do not fit entirely on the sheet.

Author Response

Reviewer 5:

The article titled "Assessment of a new Building Energy Simulation tool to predict the thermal behavior of water flow glazing in facades," aligns with the journal Sustainability. The purpose and scope of the work are clearly defined. The overall structure and layout of the article meet the standards of scientific papers and the journal's requirements. After clarifying certain ambiguities and making corrections, the article can be considered for publication. Comments are provided below.

Thank you for these encouraging words. Your comments helped improve the quality of the article. We will continue our research and publish in open-access journals to learn from our peers' feedback.

1. A weakness of the article, in terms of methodology, is the lack of validation with actual experimental data. How were the sensitivity and correct calibration of the model assessed?

Thank you for this suggestion. All reviewers considered necessary the experimental validation of the software tools. The research team bult a small prototype in Madrid to test WFG facades. The results from this prototype will be a topic of further research work and publication. However, two sections were included in the article describing the prototype and results from measuring devices.

2.4. Description of the experimental prototype.

Focused on evaluating the thermal performance of water flow glazing in comparison with software tools, a scaled prototype was built and tested in Madrid, Spain. The prototype’s dimensions are 100 cm in height, 100 cm in width, and 75 cm in depth. A Peltier unit connected to a buffer tank regulates the temperature of the water inlet. The prototype consists of a steel structure with roofs, floors, and opaque walls. A white aluminum sandwich panel incorporating 100mm of extruded polystyrene was placed as the opaque envelope. For the transparent south facade, a Water Flow Glazing panel, as defined in Case 3, was selected. Figure 6 illustrates a schematic view of the prototype and its main components.

4.3. Experimental validation in a prototype with a Water Flow Glazing facade. Transient-State Case 3.

2. What innovation does the discussed modeling method introduce compared to other tools commonly used in this industry?

This is a relevant question, and it should have been explained in the original manuscript. We have included the following text at the end of section 2.1. Description of the proposed simulation tool.

The discussed modeling method introduces significant innovations compared to other Building Energy Simulation tools. A simple interface with limited options simplifies the user experience. It requires less time and expertise than sophisticated tools available in the market. Selecting the right component of the HVAC system at the early stages does not impact the final design of a building. This approach prevents the issues that may arise when the design is complete, yet the mechanical systems have not been considered. This innovation enables faster decision-making processes. However, one limitation is the lack of connections with other building systems, especially the ventilation strategies.

3. What parameters characterized the adopted boundary conditions and how were they determined?

The following text has been included in section 3.1.2. Thermal results:

Exterior boundary conditions (Outdoor temperature and solar irradiance) are determined by the EPW file described in the Methodology section. The opaque enclosure and the transparent WFG facade define indoor boundary conditions. The opaque envelope features include orientation of the simulated prototype, indoor mass and thermal inertia, the thermal transmittance, the interior convective coefficient, and the absorption coefficients (αNIR and αFIR) of the indoor surfaces. The boundary conditions in the transparent WFG panel include interior and exterior convective coefficients, as well as the transmittance and absorptance of each WFG component (glass, water, and gas chamber).

4. Was validation attempted for other weather conditions?

The authors of this research have been testing Water Flow Glazing devices in various locations. The most fruitful outcome of this research was participating in the INDEWAG project, where multiple prototypes were tested in Madrid, Spain, and Sofia, Bulgaria. The energy performance of Water Flow Glazing has been studied in several open-access articles across various locations, spanning a range of mild to extreme cold weather conditions. The researchers developed the studied tool, in part, thanks to the INDEWAG research project. The novelty of this article lies in the comparison of the tool with commercial software such as IDA ICE. However, due to reviewers' suggestions, we have included a new section to describe the results from prototypes, comparing the outcomes with simulation results from both the studied tool and IDA ICE.

5. How was diffuse radiation taken into account?

Excellent question. The mathematical model of the tool has been the scope of previous research. A detailed model of direct and diffuse solutions along with the problem of multiple reflections in the room have been stated in the following reference. We have included the following text in section 3.1.2. Thermal results:

“The authors of this article have conducted extensive research on long-wave solutions, short-wave solutions, solar energy distribution, the transmitted diffuse radiation by the glazing, and the reflection of solar beam radiation entering through the glazing. In addition, multiple reflections can be grouped into two sets: direct reflections between the glazing and parallel surfaces, and indirect reflections between the glazing, parallel surfaces, and perpendicular surfaces. These simulation models are taken from previous research [28].”

[28] Moreno, B., Hernández, J.A. Analytical solutions to evaluate solar radiation overheating in simplified glazed rooms. Build. Environ. 2018, 140, 162–172.

 

6. Please clearly indicate the limitations of the tool used.

We have included the following paragraph about the limitations of the tool in the Conclusion section:

“The simulation software tool is currently in the experimental phase, so further development is necessary, especially regarding the user interface. The incorporation of more Water Flow Glazing options and case studies requires proficiency in programming languages such as FORTRAN and C++, so lay users cannot create their own WFG panel. Therefore, the users of this tool can only work with limited WFG options. A closer relationship with industry stakeholders and glass manufacturers is essential for selecting valid WFG compositions suitable for each climate and energy strategy.”

7. The results are discussed primarily in the context of Madrid's climate and a simple room model – there is no discussion of whether and how they can be extrapolated to other climates or more complex buildings.

The response to this comment is similar to the one provided for point 4.

The authors of this research have been testing Water Flow Glazing devices in various locations. The most fruitful outcome of this research was participating in the INDEWAG project, where multiple prototypes were tested in Madrid, Spain, and Sofia, Bulgaria. The energy performance of Water Flow Glazing has been studied in several open-access articles across various locations, spanning a range of mild to extreme cold weather conditions. The researchers developed the studied tool, in part, thanks to the INDEWAG research project. The novelty of this article lies in the comparison of the tool with commercial software such as IDA ICE. However, due to reviewers' suggestions, we have included a new section to describe the results from prototypes, comparing the outcomes with simulation results from both the studied tool and IDA ICE.

A scope of future research will be to compare simulation results from the proposed tool and IDA ICE with measured outputs from the prototypes. The editors advised us to reduce self-citations. Nevertheless, you can find information about previous research in these references:

• del Ama Gonzalo, F.; Moreno Santamaría, B.; Hernández Ramos, J.A. Assessment of Water Flow Glazing as Building-Integrated Solar Thermal Collector. Sustainability 2023, 15, 644. https://doi.org/10.3390/su15010644
• Del Ama Gonzalo, F., Hernandez, J.A., Moreno, B., 2017. Thermal Simulation of a Zero Energy Glazed Pavilion in Sofia, Bulgaria. New Strategies for Energy Management by Means of Water Flow Glazing. IOP Conf. Series: Materials Science and Engineering 245 (2017) 042011 doi:10.1088/1757-899X/245/4/042011.

 

8. Taking into account the above comments, the conclusions require further verification.

We have rewritten the conclusion section. It draws conclusions from experimental data and states the limitations of the proposed methodology.

Conclusions:

The article described a graphical tool developed by the authors that includes a Fortran library to simulate Water-Flow Glazing facades, enabling future developers to integrate Water-Flow Glazing envelopes into existing energy simulation tools. The feedback provided by the tool enables users to iterate with design decisions, such as orientation, dimensions, and glazing properties.

The steady-state analysis provided a preliminary assessment of the efficiency of each Water Flow Glazing (WFG) panel. Three case studies were described and analyzed in this article. Cases 2 and 3 were selected for analysis under transient conditions in both winter and summer, serving as a facade of a room. Finally, an experimental setup was built in Madrid, Spain, to compare the simulated results of Case 3 with recorded data from the prototype.

Case 2 achieved a tenfold increase in water heat gain compared to the other two options for winter conditions. On the other hand, Case 3 was the optimal choice for summer conditions, as it shielded and reflected solar energy. The performance of Case 3 was similar to that of Case 2 in terms of solar energy transmission. However, it is essential to note that Case 2 required over twice the energy of Case 3 to achieve a similar cooling capacity.

Another goal of this article was to validate the results from the newly developed tool, which was explicitly designed to assess the thermal behavior of Water Flow Glazing, against the modeling approaches of the commercial dynamic simulation tool, IDA-ICE. The analysis of indoor temperature results revealed that both simulation tools produced identical maximum and minimum values. However, a notable divergence in the slope of the resulting graphs was observed. This variation suggests that the two tools may employ differing methods for considering interior thermal mass.

A quantitative validation method suggested by ASHRAE Guideline 14–2014 for hourly values was used to assess the results from the newly developed tool. The value from IDA-ICE was set as the reference value, and the Normalized Root Mean Square Error (NRMSE) was used to understand the degree of agreement between the tools. The results from Cases 2 and 3 under transient conditions revealed acceptable NRMSE values, which were below the ASHRAE maximum allowable error of 30%, in the assessment of the room's indoor temperature. The values of the Water Heat Gain were slightly above the limit stated by ASHRAE due to the different implementation of the interior thermal mass in both tools.

Comparing the simulation tools with an experimental setup yielded similar results in terms of Water Heat Gain over three days in summer, when the EPW weather file used for simulation replicated the measured outdoor conditions. Significant discrepancies were found between the measured and simulated indoor temperatures, with an initial coefficient of determination (R²) of 0.41. Two iterations improved the simulation; adjusting the room geometry resulted in an R² of 0.81, and integrating measured inlet temperature data increased it to 0.97.

The simulation software tool is currently in the experimental phase, so further development is necessary, especially regarding the user interface. The first limitation is that incorporating more Water Flow Glazing options and case studies requires proficiency in programming software such as FORTRAN and C++, so lay users cannot create their own WFG panels. Therefore, the users of this tool can only work with limited WFG options. A closer relationship with industry stakeholders and glass manufacturers is essential for selecting valid WFG compositions suitable for each climate and energy strategy. Another limitation of the proposed tool is the use of a simplified model that disregards the thermal mass of the various WFG layers and resolves a system of algebraic equations. A complete model that acknowledges the thermal mass and considers each layer as a partial differential equation requires greater computational effort. The tool must be tested against more empirical data from prototypes in various locations to validate the accuracy of the simplified model.

Overall, the findings of this report contribute to a deeper understanding of the diverse capabilities of different building energy simulation (BES) tools. Further studies must continue to carry out these comparisons to establish a set of parameters that can help building designers increase their confidence and understanding of using Building Energy Simulation software. Additionally, the proposed methodology must be tested in buildings with more complex heating, ventilation, and air conditioning systems and compared with recorded values from monitoring systems.

 

Editorial Notes:
1. Figures 5 and 6 were incorrectly inserted, and therefore do not fit entirely on the sheet.

Thank you for pointing out this issue. We have changed the format of Figures 5 and 6 to fit the image on the page.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

This article has been revised in detail, and the author has also completed the experiment. It is a very meaningful research. I hope to continue to see your research in the future.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

I wanted to reach out and express my sincerest appreciation for your efforts in reviewing the MS. I can confirm that you:

• addressed the literature review insufficiencies by adding new references that strengthens the existing body of knowledge, particularly from the last five years.
• clarified whether the WFG panel solution was native to the software and provided appropriate disclosure.
• addressed concerns regarding the EPW file, specifically its location and data source.
• The inclusion of "Section 4: Description of the Experimental Prototype" and "Section 3: Experimental Validation in a Prototype with a Water Flow Glazing Facade—Transient-State Case 3" substantially improves the overall quality by enabling direct comparisons. Figure 12 is particularly enlightening. While the prototype differs from the computational model in volume at this stage, it would be valuable to achieve identical conditions in future research.
• addressed my previous comments regarding the figures cuted/hidden part. The figures now display properly, and the methodology has undergone extensive revision, appearing more robust. The addition of the “physical prototype” with photos and results significantly strengthened this aspect.

Thank you and all the best in your future research!

Reviewer 5 Report

Comments and Suggestions for Authors

The authors provided comprehensive responses to the comments in the review. The article has been significantly improved. I recommend its publication in its current form.