A Comprehensive Case Study of a Full-Size BIPV Facade

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a very meaningful and interesting job, integrating photovoltaic buildings to make them more beautiful and efficient. 

1. The investment cost of this plan also needs to be considered

2. For high-rise buildings, different heights and building clusters, this solution also has a significant impact on the integration of photovoltaic buildings

Author Response

Comment 1: The investment cost of this plan also needs to be considered

Response 1: We thank Reviewer 1 for his contribution to this review and the comments made. The building under investigation is a highly technical laboratory building with multiple clean rooms. The overall building costs do not stand in comparison to standard residential or office buildings. Thus, they are not mentioned. The differences in investment costs of a standard facade without PV and a PV- facade have been investigated in Table 2. These costs are comparable to other buildings of the same size and shape.

Comment 2: For high-rise buildings, different heights and building clusters, this solution also has a significant impact on the integration of photovoltaic buildings

Response 2: Thank you for pointing this out. We added an explanatory line and quote in line 57-60:

“The variety of options in design and construction process led to many unique and often hardly comparable systems. Only few attempts were made to develop a universally applicable evaluation methodology [7].”

Reviewer 2 Report

Comments and Suggestions for Authors

This paper discusses comprehensive factors for BIPV technology, including module positioning, ventilation, and shading and so on. The paper’s structure is clear. However, there are also some issues in the paper that require further improvement.

1. The problems and main contributions addressed in this paper need to be further summarized.
2. How to analyze the impact of temperature and shadingon photovoltaic power generation, and what are the specific methods used?
3. The technical and economic feasibility of installing photovoltaics in the north facadeis poor. What are the conclusive suggestions in the paper?
4. What is the load curve of the laboratory building?What is the proportion of photovoltaics? Do you need to configure energy storage?
5. What is the overall PV investment payback period?

Author Response

Comment 1: The problems and main contributions addressed in this paper need to be further summarized.

Response 1: We thank Reviewer 2 for his time and comments. Even though no chapter was specifically named “summary”, those points have been written into chapters 6 and 7. Also, additions were made in introduction and conclusions to further clarify the aims and contributions of this paper (see line 72-74 and line 541-543):

„There is still a lack of experience and understanding of real-world projects which prevents a widespread application of BIPV. “

„Now that the feasibility and practicability of BIPV systems has been demonstrated, this information must be communicated primarily to architects and builders to ensure widespread use. “

Comment 2: How to analyze the impact of temperature and shading on photovoltaic power generation, and what are the specific methods used?

Response 2: Natural shading by surrounding objects occurs only rarely in winter on this specific facade. Shading by clouds could easily be spotted by looking at the generation profile. An in-depth shading analysis with artificial shading will be subject of future investigations, and we mention this in the outlook. The influence of the module temperature on the module performance is quantified in the temperature coefficient (0,4 %/K). The module temperature was investigated based on real measurements in Figures 10 and 11 in chapter 4.3.

Comment 3: The technical and economic feasibility of installing photovoltaics in the north facade is poor. What are the conclusive suggestions in the paper?

Response 3: Thank you for pointing this out. This topic will now be explained in both introduction and outlook in line 96-101 and 554-560.:

„The majority of the modules (248) is installed on the south facade, while the west facade only contains 56 modules. The north facade is equipped with the same number of modules as the west facade, even though only small yields are expected. At this part of the building’s façade, diffuse irradiation as well as reflection and albedo will in future be extensively investigated.”

“The closely monitored system also enables detailed studies on the influence of shading on module degradation as well as the influence of module soiling on the yield. Diffuse irradiation and albedo measurements will be closely monitored at the north facade, which will be subject of further investigations. Installations on a North-directed facade still may pay off, assuming long lifetimes of cheap modules and generation times might make them interesting under current circumstances. “

Comment 4: What is the load curve of the laboratory building? What is the proportion of photovoltaics? Do you need to configure energy storage?

Response 4: As stated in line 486-488, the laboratory building is part of a big campus grid that includes an energy-intense electron storage ring. Therefore, all the generated PV power (~30 MWh/a) is fed into this non-public grid, which supplies the necessary energy for multiple buildings in the near vicinity (30 GWh/a). All the energy provided by this PV system is used in the campus grid. Therefore, no energy storage is needed.

Comment 5: What is the overall PV investment payback period?

Response 5: Any building with a ventilated curtain-wall facade needs a cladding material. This can either be PV modules or a common cladding material like aluminum. Thus, we have chosen to provide a calculation based on the difference between this non-PV option and the PV installation. Facade-integrated PV systems should not be economically evaluated like add-ons, like a common rooftop PV-system but rather as alternative building material. The payback periods for the extra costs are listed in Table 4.

Reviewer 3 Report

Comments and Suggestions for Authors

The research is significant because it provides real-world data from a large-scale installation, which is relatively rare in the BIPV literature, as most studies focus on smaller test installations or theoretical analyses. However, there are some suggestions:

1. The paper lacks a clear comparison of the BIPV system's performance metrics with other similar BIPV facade installations. The authors should include quantitative comparisons with other case studies to better contextualize their results, particularly for:

• Energy yield per m² for different facade orientations
• Module temperature ranges

2. Section 4.1 (Simulation) shows discrepancies between measured and simulated data that are not adequately explained. The authors should:

 

• Quantify the deviations between simulated and measured values
• Discuss potential sources of these differences

     3. Technical specifications of measurement equipment need more detail: Data acquisition system specifications.

     4.  Section 3.3: The air flow measurement setup doesn't account for potential turbulence effects. Authors should: Justify sensor placement positions

     5. The paper's discussion of facade integration methods would benefit from citing work on prefabricated BIPV wall systems, particularly studies on BIPV walls for multi-story buildings. These references would provide valuable context for their own facade implementation and highlight different technical approaches to facade integration. I recommend including citations to the following key papers:

1. "Reimagining Building Facades: The Prefabricated Unitized BIPV Walls (PUBW) for High-Rises"

 

 

Author Response

Comment 1: The paper lacks a clear comparison of the BIPV system's performance metrics with other similar BIPV facade installations. The authors should include quantitative comparisons with other case studies to better contextualize their results, particularly for:

• Energy yield per m² for different facade orientations
• Module temperature ranges

Response 1: In order to be able to provide a comparison, we installed the same modules and black reference modules in a separate module test-field. There, we investigate the influence of the installation angle on module temperature and energy yield, as well as the difference between facade-integrated and free-standing installations. We added this information in line 391-397:

“In a separate module test field at HZB, the difference of a free-standing installation in comparison to the facade-integrated installation was thoroughly investigated [21]. By using the same CIGS module in the free-standing installation, only external factors resulting from the installation influenced the module temperature. It was shown that integrated modules operate at similar temperatures in winter, while being 10 K warmer in May. The integration therefore only led to a relatively small power loss, when taking the temperature coefficient of the module of 0,39 %/°C into account.”

The specific PV module installed has huge influence on energy yield. Only a few buildings are comparable to our living lab (in terms of the installed module) and none of those were as closely monitored. Usually, BIPV systems outside the research environment do not publish specific yield data for different facade orientations. When using an evaluation method [7] to put our overall energy yield into perspective, our building generates not as much energy as other BIPV systems.

Comment 2: Section 4.1 (Simulation) shows discrepancies between measured and simulated data that are not adequately explained. The authors should: 

• Quantify the deviations between simulated and measured values
• Discuss potential sources of these differences

Response 2: Thank you for showing us that our writing was misleading. We do mention but not clearly enough in chapter 4.2, that a TMY was used for the simulation. We added explanatory information in lines 310-314:

“This standard weather data set is commonly used in the building sector in planning and preplanning processes. A TMY does not show the actual weather data from a specific year, but rather a constructed data set to minimize the influence of yearly weather fluctuations. Therefore, deviations occur to the measured results when looking at a specific year. “

Comment 3: Technical specifications of measurement equipment need more detail: Data acquisition system specifications.

Response 3: The string data is recorded via a rs485 interface, while the meteorological data is captured using a data logger (CR1000). The airflow measurements are recorded in a database with an Industrial Edge Computing Gateway (Servicerouter V3). The AC values are exported from the inverters. The installed sensors are named and described in chapter 3.

Comment 4: Section 3.3: The air flow measurement setup doesn't account for potential turbulence effects. Authors should: Justify sensor placement positions

Response 4: Our air flow sensors are specifically designed to handle ventilation measurements from every angle to accurately measure even turbulent air flows.

Comment 5: The paper's discussion of facade integration methods would benefit from citing work on prefabricated BIPV wall systems, particularly studies on BIPV walls for multi-story buildings. These references would provide valuable context for their own facade implementation and highlight different technical approaches to facade integration. I recommend including citations to the following key papers:

"Reimagining Building Facades: The Prefabricated Unitized BIPV Walls (PUBW) for High-Rises"

Response 5: Thank you for that suggestion. We added respective information in line 59-60:

“In the last years, an increasing number of test-fields started operation, investigating functionality and efficiency of BIPV systems [8] [9]. Some of those aim to simplify and standardize the construction process of BIPV systems [10]. “

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors After carefully reading the revised paper, I currently have no other comments.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript makes a valuable contribution to the field by providing detailed performance data from a real building installation.

• The paper presents comprehensive data and analysis from a full-scale BIPV facade installation, addressing an important knowledge gap in real-world performance data
• The methodology is sound, with extensive monitoring of multiple parameters including temperature, ventilation, and power output
• The results provide valuable insights about BIPV facade performance across different orientations and conditions
• The economic analysis considering amortization periods adds practical value
• The paper is well-structured and clearly written overall