Application of the Guidelines for the Integration of Photovoltaics in Historic Buildings and Landscapes to Evaluate the Best Practices of the Historic Building Energy Retrofit Atlas

Abstract

The challenge of transforming historic buildings and city centers into energy-self-sufficient environments requires innovative solutions. The research project “BiPV meets History” addressed this challenge by providing comprehensive guidelines for assessing the integration of photovoltaic (PV) systems in protected historic architectural contexts. To validate these guidelines, this study conducts a thorough examination of best practices through the mentioned guidelines, developing an application tool. Recognizing the power of well-communicated best practices in overcoming obstacles to integrated photovoltaic adoption, this tool is used to assess PV integration quality with respect to the best practice contained in the HiBERatlas database. The analysis of 17 successful refurbishment cases highlighted the robustness and reliability of the proposed methodology, considering aesthetic, technical, and energy aspects. This study emphasizes the potential of the guidelines for achieving a harmonious integration of renewable energy solutions with historic architectural heritage and landscape and improving usability through the developed tool.

1. Introduction

Bringing a sustainable future to our historic buildings and city centers will be a major challenge in future decades [1]. This means not only reducing their carbon emissions with improved energy performance of the envelope and renewable energy sources but also increasing occupants’ thermal comfort and reducing the risk of energy poverty. Climate protection and heritage conservation are not antonyms, and the best way to ensure that historic buildings have a continued use over time is by providing occupants with levels of comfort adapted to current standards [2]. However, the building stock is renovated very slowly at a rate of below 1% per year [3], and the pace urgently has to be increased if we want to keep climate change in bearable limits. Excluding historic buildings from renovations is not an option since they make up for a consistent share of the total stock—with 14% dating from before 1919 and 12% from between 1919 and 1945 [4]. At the same time, especially if it comes to historic buildings, private owners do often not know “what to do, where to start, and which measures to implement in which order” ([5], p. 4). Informational barriers do limit implementation with as with any actual lack of technical solutions and guides [6]. Another barrier is the concern about the availability and expertise of professionals [7] as well as the compatibility of proposed solutions with the historic values of peoples’ homes [8]. One approach to help the construction sector develop toward more sustainable practices is to share examples of best practices [9]. “Good examples” are a powerful tool that has been successfully used to demonstrate what can be achieved and consequently to influence building design practices. Another approach is the formulation of concerted design criteria and guidelines—as has been shown with the integration of active solar technologies by Lucchi et al. [10]. The possibility of reaching positive-energy building (PEB) standards [11] depends on a difficult balance between building design and energy performance [12]. Moreover, the landscape is also considered as a potential space for renewable energy sources in several applications [13].

With the aim of supporting widespread and high-quality integration of photovoltaic systems in historic buildings and landscapes, this study brings together the best practice documentation achieved in the HiBERatlas (Historic Buildings Energy Retrofit atlas) [14] with the Guidelines for the integration of photovoltaic systems in historic buildings and landscapes [15] developed within the Interreg IT-CH project BiPV meets History [16]. These two powerful tools will be deepened afterward in Section 1.1 and Section 2.2, respectively.

1.1. Best Practice Database: A Tool to Promote and Foster PV Integration in Built Heritage

The success of best practice and demonstration projects is generally dependent on the nature of the information provided. The assessment of energy interventions in historic buildings requires a level of detail and a targeted query strategy [17].

Although none of them are specific to BIPV practices, several online databases exist today, which contribute to disseminating building refurbishment best practices. The most widespread are as follows.

• Sustainable Traditional Buildings Alliance (STBA) [18]—noted for its exclusive focus on energy-efficient renovation of historic buildings, though limited to only two project examples.
• dena [19]—specific to Germany, mentioned for incorporating buildings of cultural value, either generally under renovations or as a special category, with associated filter functions.
• Construction21 database [20]—highlighted as one of the most comprehensive surveys of energy-efficient renovations but lacking sufficient consideration of specific requirements of historic buildings.
• IEA SHC Task 37 examples collection [21]—noted for its comprehensive surveys of energy-efficient renovations but lacking in detailed consideration of the unique features of historic architecture.
• EffiBUILDING database [22]—referenced as an example of a dedicated product database for standard construction solutions but deemed inadequate for illustrating the connection between the particularities of a building and the detailed solutions employed.

On the basis of this experience, a more modern database was elaborated by the interdisciplinary panel of the research projects IEA SHC Task 59 “Renovating Historic Buildings Towards Zero Energy” [23] and Interreg Alpine Space ATLAS [24]. In this database, HiBERatlas [10], the use of visual information (photographs, icons, charts, and construction drawings) and short texts makes the experience accessible to unexperienced users (e.g., building owners), while detailed information is available in a second instance for more specialized users (e.g., architects and engineers). The information displayed in the HiBERatlas database is structured in four categories. Examples of best practices should inspire users, and therefore, the first section starts with images of the whole building and some key figures summarizing some generic information about the intervention. In the next section, the information is focused on the renovation process with the overall aim of describing the context and the rationale behind all the solutions adopted, thinking especially of those who want to learn more about the entire decision-making process. The different retrofit solutions implemented are presented in the third section, ranging from wall, window, and roof improvement over airtightness and ventilation to the building services and integration of renewable energy systems. Lastly, the result of the intervention is evaluated in terms of energy efficiency, internal climate control, financial assessment, and environmental impact.

1.2. Existing Guidelines for the Integration of PV in the Protected Environment

Several attempts were made to establish agreed criteria for the integration of the PV into the protected environment in research projects and in the legislative framework. The integration of renewable energy systems (RESs) in the landscape is generally encouraged [25] when minimizing any negative aesthetic impact and preserving the integrity of heritage buildings and natural sites. Many countries within the European Union have established national guidelines that outline the installation of RES in buildings and landscapes based on a common strategy outlined in [26]. Thus, obtaining proper authorization from heritage authorities is mandatory when installing RESs in cultural heritage sites, particularly in the case of historical and rural buildings, historical towns and settlements, and areas designated for landscape protection. The final approval from the local authority for cultural heritage is required in these instances. The legislative framework pertaining to RES integration can vary across different countries, with some adopting more conservative approaches than others. However, there is a noticeable shift among authorities and legal entities toward a more open-minded approach, whereby basic criteria and guidelines are established to ensure compliance. This trend promotes greater permissibility and empowers municipalities to search for appropriate solutions that align with the landscape and construction characteristics of urban areas and to conduct thorough evaluations of specific and unique cases when necessary. While the guidelines provide several examples of best practices, specific aesthetic or technical criteria for assessment are only outlined in a limited number of cases.

Among the other tentative attempts made to define those technical criteria, the guidelines for the Integration of Photovoltaics in Historic and Landscape Contexts [12], developed through a collaboration between the Lombardy region in Italy and Eurac research within the BIPV Interreg research project [13], provide a comprehensive perspective on the key issues associated with this topic. The compatibility criteria for the integration of the PV in the protected architecture or landscape can be subdivided into three categories: (i) aesthetic, (ii) technological, and (iii) energy-related criteria [10,27].

1.3. Aim of the Study

The review performed outlines existing guidelines for integrating photovoltaic systems in historic buildings and in protected environments, outlining evaluation criteria related to aesthetic, technological, and energy aspects. Moreover, the review presents several databases for best practice dissemination, emphasizing the HiBERatlas for its potential to present visual information and detailed texts in four categories to make the experience accessible to various kinds of users. The present study aims to explore the potential of the presented guidelines in guiding a harmonious integration of renewable energy solutions with historic architectural heritage, paving the way for a sustainable and aesthetically pleasing future. To validate this approach, the study proposes a thorough examination of 17 successful refurbishment cases within the HiBERatlas through the principles of the described guidelines to check their robustness and reliability. This is carried out through the development of an application tool that implements the described methodology, which is addressed to designers, researchers, and public administrators who need to evaluate the quality of the integration of a BIPV intervention.

2. Materials and Methods

As mentioned above, a reliable and complete formulation of guidelines for the integration of PV systems was delineated in the document titled “Integration of Photovoltaics in Historic and Landscape Contexts”. These guidelines emerged from a collaborative effort between the Lombardy region in Italy and Eurac research as part of the Interreg research project “BIPV meets history”. The proposed criteria for evaluating the compatibility of PV integration within protected architectural and landscape contexts can be broadly classified into three distinct categories, which are extensively described in the following subsections: aesthetic category, (Section 2.1), technological category (Section 2.2), and energy-related category (Section 2.3).

Table 1. Categories of Integration (Aesthetic, Technological, Energy) with respective evaluation criteria according to the Guidelines [12].

Aesthetic Integration	Technological Integration	Energy Integration
Color Color ranges of panels and frames must be compatible with the colors of traditional materials and the original building being worked on and with the wider urban or natural environment.	Weather protection Integrated PV systems must provide protection from the weather, preventing damage to the building envelope and avoiding negative effects on indoor comfort.	Exposure The side of the building most exposed to solar radiation throughout the day must be identified, giving preference to the installation of panels on the parts of the building that are less visible from the main viewing points.
Reflection PV systems characterized by low reflection must be preferred.	Noise protection The integration of photovoltaic modules must be designed with sound-insulating materials in order to create an acoustic barrier, helping to mitigate or deflect unwanted background noise.	Shadows Panels’ positions must be as free as possible from obstacles; a shadow calculation must be carried out during the design phase. Moreover, a constant cleaning of the modules is crucial.
Texture Textures can help to make the PV panel look as similar and consistent as possible with the tactile and visual qualities that characterize the surrounding building materials.	Thermal insulation PV systems can be designed in combination with insulating materials or including the back ventilation of panels. The use of special PV modules, equipped with heat-recovery systems on the back side, can be considered.	Materials Depending on the type of material and chemical structure used, one can choose among three different systems of PV panels: first, second, and third generation.
Pattern Some patterns can make the PV cell less visible; it is possible to obtain both monochromatic and homogeneous surfaces and surfaces characterized by different designs created on request.	Light and Shadow Visual contact with the outdoors, make sure to control glare, meet light contrast requirements and adhere to certain luminance and illuminance levels to ensure a comfortable and functional environment.	Ventilation Integration between the ventilated roof and PV panels must be provided, avoiding high temperature loss in performance.
Transparency Different levels of transparency can be reached depending on the location of the photovoltaic elements in relation to the historic building and depending on the requirements to be met.	Resistance Roof-integrated photovoltaic systems must ensure resistance to snow and wind loads and to loads for maintenance, and they must be able to meet specific requirements for walkable glazing and laminated glass properties. Those applied vertically in the façade must provide increased bending stiffness and resistance to high wind loads.
Planarity The inclination of the architectural element must be matched, avoiding an arrangement of the panels that alters the general lines.	Hygiene and health The use of toxic materials or significant quantities of rare materials should be limited to encourage recycling.
Geometry PV panels must be grouped with the aim of achieving a uniform geometry and avoiding visual fragmentation, respecting the main compositional patterns of the architecture and also paying attention to the junction points.	Safety Fire safety must be considered, and the regular maintenance of the system must be carried out by verifying the correct functionality and integrity of each of its components.
	Installation Installation must be carried out by qualified operators with the technical and professional requirements.
	Durability A regular maintenance of the system must be ensured. Also, the accessibility of the PV system for activities cleaning, maintenance, and replacement of individual modules or panels must be warranted.
	Quality The presence of the certifications and guarantee on the products must be checked.

presents a detailed summary of the criteria evaluated for each of the three categories mentioned.

2.1. Aesthetic Criteria for the Integration of the PV

When considering the integration of photovoltaic systems into architectural structures, aesthetic considerations play a crucial role. In order to ensure a harmonious blending of the modules with the existing building and the surrounding environment the several aesthetic criteria must be taken into consideration, involving chromatic, reflection, texture, pattern, transparency, planarity and geometrical issues.

First, chromatic integration can be achieved by choosing colors for panels and frames that match the existing materials of the building or urban or natural surrounding environment. As a general concept, frameless panels have to be preferred for a seamless and integrated look. Moreover, reflections have to be reduced to a minimum: several modules exist that have low reflection properties; also, using anti-reflection coatings is possible to avoid glare and disruptive reflections. PV modules with a coarse grain or texture are available on the market for a matte appearance and reduced light reflection. To visualize building morphology and PV visibility during the design phase, 3D simulation tools are available for architects; during this assessment, it is possible select texture of the existing surrounding building materials, especially the external finishing, and print a specific pattern on the modules to match the surrounding environment. This technology allows for creative solutions in urban areas and modern structures, upgrading outdated buildings while maintaining visual integrity. It is also possible to create transparent or semi-transparent surfaces in architectural elements like greenhouses, skylights, and canopies. This approach applies to various structures, including energy infrastructure, noise barriers, parking canopies, bus stops, railings, fences, dividers, lighting elements, and advertising structures. If the designer is considering photovoltaic glass, the purpose must be clarified at the beginning: shading, visual screening, or illumination. The transparency level, in fact, can be planned based on the photovoltaic elements, prioritizing greater transparency when it is needed and lightness at points of contact with the existing architecture. Transparent modules are in fact ideal for skylights, porches, greenhouses, and winter gardens. Semi-transparent modules work well for solar shading, arbors, parking canopies, fences, and dividers. In these cases, planar integration is crucial for seamlessly incorporating photovoltaic panels into historic and landscape contexts: the mentioned guidelines suggest avoiding panel positioning that disrupts the building’s aesthetic lines, maintaining the inclination of technical elements and installing panels in less visible areas. Thinner panels and frameless designs are usually preferred. Finally, to achieve a geometrical integration, panels should be grouped in a compact and regular design, respecting the building’s compositional patterns and paying attention to junction points.

2.2. Technological Criteria for the Integration of the PV

Technological criteria are essential in ensuring the successful integration of photovoltaic systems into historic buildings and landscape contexts, encompassing systems’ performance issues, durability, safety, and environmental impact issues.

The module needs to be waterproof, airtight, and wind- and snow-resistant; when useful, additional moisture protection systems help to prevent condensation and related issues. Noise generated by the photovoltaic system under windy conditions should be minimized. Proper integration and design, coupled with sound-insulating materials, can act as an acoustic barrier, reducing unwanted background noise and enhancing acoustic comfort. Another interesting integration strategy on roofs and façades includes the coupling of photovoltaic systems with insulation materials to improve the thermal performance of the envelope. Moreover, designing modules’ back ventilation can help to control system and environmental overheating. Consideration can also be given to specific photovoltaic modules equipped with heat-recovery systems on the rear side.

Balancing natural light penetration while managing glare and maintaining visual comfort are crucial issues when designing integrated photovoltaic systems. As mentioned, for roof-mounted photovoltaic systems, ensuring mechanical resistance to snow, wind, and maintenance loads is important; walkable glass elements also exist, which can be applied to specific contexts. Similarly, vertically applied photovoltaic systems on facades should exhibit enhanced bending stiffness, resistance to higher wind loads at elevated heights, and improved resistance to point loads from shading devices. The product should guarantee environmental sustainability: it is important to prioritize the presence of non-toxic materials, minimizing the use of rare materials, and it is important to design products that are easily separated for recycling purposes at the end of their life. Proper adherence to fire safety guidelines, together with regular periodic maintenance, is also important to reduce fire risks. Importance is also given to qualified and trained operators during installation and maintenance operations and to the presence of warranties on performance, typically for 25–30 years. Analyzing the technical datasheets provided by manufacturers aids in comparing and selecting the most suitable photovoltaic panels based on characteristics and reliability.

2.3. Energy-Related Criteria for the Integration of the PV

The most important skill of a renewable energy production system remains, obviously, to produce a satisfactory amount of clean electric energy; various factors related to exposure, shade, materials, and ventilation have a determining role in the performance and efficiency of photovoltaic systems. Identifying the building’s surface with the highest solar irradiation is the first step. Ideally, PV modules should face south for optimal productivity. However, for perfect integration, installing panels on areas less visible areas from major observation points is preferable. Thus, when south-facing installation is not feasible, an orientation between southeast and southwest, deviating no more than 45° from south, can be considered. Based on this orientation, the solar potential productivity of the specific surface can be assessed to calculate the decrease in panel efficiency relative to optimal orientation conditions. Shading considerations must be given particular attention; the utilization of simulation software that includes shading calculations is important. If shading is a problem, bypass diodes, optimizers, or microinverters can enhance module performance even under shading conditions. The regular cleaning of modules to remove dirt, dust, and biological material helps maintain optimal energy supply. Choosing the appropriate photovoltaic module is essential, considering the distinct characteristics of each technology and cell type. Common options include monocrystalline silicon, polycrystalline silicon, thin-film, amorphous silicon, CIS, and CIGS. These materials are classified into three generations of photovoltaics: the first generation refers to traditional silicon-based panels, the second generation encompasses cost-effective thin-film modules with slightly lower efficiency, and the third generation [28] represents innovative technologies still in development, such as multi-junction cells and organic photovoltaic devices. The possibility of integrating a ventilated roof with photovoltaic modules has to be taken into consideration: by treating the panels as roofing tiles, they become an integral part of the ventilated roof structure. Finally, the temperature coefficient has to be taken into consideration: it indicates the efficiency loss of a module per degree of temperature increase; this coefficient varies depending on module quality: lower-quality modules typically have a coefficient of around 0.5% per degree Celsius, while higher-quality panels can achieve improvements of approximately 0.25% per degree Celsius.

2.4. The HiBERatlas Database

HiBERatlas, an atlas of energy retrofitting for historic buildings, stands out as a key tool in this context. It presents concrete examples of how historically significant buildings can be renovated to achieve high levels of energy efficiency while simultaneously preserving their historical significance. These “best examples” have proven powerful in showcasing what can be achieved, positively influencing design practices in the construction industry. Sharing these examples is crucial to inspire and guide the industry toward adopting more sustainable approaches. These practical examples demonstrate the possibility of designing buildings with reduced environmental impact, increased internal comfort, and superior energy efficiency. Furthermore, they highlight how innovation, the adoption of advanced technologies, and the consideration of environmental and social aspects can be successfully integrated into construction projects. Within the HiBERatlas database, buildings of historical significance are considered, irrespective of the level of protection, ranging from medieval structures to those from the 1920s and post-World War II architecture. Basic requirements for best practice examples include project completion, the comprehensive renovation of the entire building, a significant reduction in energy consumption, an assessment of the compatibility with heritage solutions, and the availability of documentation on technical solutions.

HiBERatlas organizes information into four distinct categories to provide a comprehensive view of best practice examples in building renovation:

• Images and General Information: offers a broad overview of the intervention through images of the entire building and summaries of key information, inspiring users and providing an overall view.
• Context and Motivations: explores the decision-making process, describing the context and motivations behind choices made during the project for those seeking to understand the “why” behind the decisions.
• Retrofit Solutions: presents in detail the various retrofit solutions implemented in the building, such as improvements to walls, windows, roofs, airtightness, ventilation, and the integration of renewable energy systems, providing a clear view of measures adopted.
• Results Evaluation: analyzes the results obtained post-intervention, assessing parameters like energy efficiency, internal climate control, financial analysis, and environmental impact, offering a comprehensive view of the interventions’ effectiveness and the benefits achieved. Since its release in September 2019, the tool described has been primarily available to designers and private clients, who can explore design solutions among the open-source best practices. It was honored with the European Solar Prize 2021 in the “Education and Vocational Training” category.

The database is also correlated with the HiBERtool [29], developed within the same framework to provide solutions for the energy-efficient retrofit of historical buildings. It is a comprehensive resource that documents a wide range of solutions for the refurbishment of different architectural element of a building, including windows, walls, ventilation, heating, and solar energy systems. It includes a decision tree feature that filters all documented solutions to find the most relevant ones for a particular project. Users can then download detailed documentation of these solutions in PDF format.

2.5. Creation of an Automated Tool

As mentioned above, in this study, the mentioned guidelines [12] were experimentally tested through a critical review of the best practices cataloged in the HiBERatlas database [10]. Within the mentioned database, all the case studies that include BIPV interventions were selected. These case studies underwent a meticulous evaluation based on the criteria outlined in the guidelines, which concern the main Categories of Intervention—aesthetic, technological, and energy—which are collected in Table 1. In order to conduct this assessment, a specific tool was developed, enabling the comprehensive evaluation of photovoltaic system integration. Within this tool, each building was uniquely identified using an ID, accompanied by the name, the designer, and a link to the database. Subsequently, the criteria described for assessing various types of interventions were analyzed. Each criterion was assigned a numerical score ranging from 0 to 2, where 0 indicates inadequate or poor integration, 1 represents an average or moderate evaluation, and 2 indicates good or excellent integration. Each category of integration (aesthetic, technological, or energy) was weighted equally, contributing equally one-third to the final evaluation. Finally, a Key Performance Indicator, representing a general indicator for BIPV Integration quality, was obtained in base 10. At the end of the performed assessment, it was possible to identify a classification of the best interventions within the tool. This tool can be useful to all the potential users of the mentioned guidelines, e.g., researchers, public authorities, professionals who want to evaluate the quality of PV integration.

3. Results

Next, the work carried out with the tool described in Section 2.5 is reported. The buildings identified as following best practices within the HiBERatlas database have been cataloged, identifying the project by its ID, name, and the designer of the retrofit intervention. Subsequently, each criterion listed in Table 1 is evaluated, assigning a score; then, the general score of its category of integration (aesthetic, technological, and energy integrations) is calculated through the methodology declared. Finally, a general Key Performance Indicator (KPI) for BIPV Integration quality is obtained out of 10.

Table 2. The outcomes from the application of the methodology are presented. The data entered into the tool are reported, progressing from left to right, starting with the project identification through an ID, name, and designer. Subsequently, scores for aesthetic, technological, and energy-integration categories are provided. These values are derived from an average of the evaluations of the respective integration criteria (aesthetic, technological, and energy). Lastly, the rightmost column illustrates the total value of photovoltaic integration quality. This is derived from the summation of the three integrations.

ID	Name	Designer	Aesthetic Integration	Technological Integration	Energy Integration	KPI
1	Villa Castelli	Valentina Carì	1.4	1.5	2.1	5.0
2	Castello di Doragno	DeltaZERO SA—De Angelis—Mazza Architects	2.9	3.0	3.3	9.2
3	Solar silo in Gundeldinger Feld	Baubüro	2.6	3.0	3.3	8.9
4	Residential and commercial building Feldbergstrasse	Viridén + Partner AG	1.4	2.8	2.9	7.1
5	House Breuer	Bernhard Breuer	1.9	2.7	3.3	7.9
6	Platzbon	Benno Graus	0.7	2.4	3.3	6.4
7	Ansitz Mairhof	Manuel Benedikter	1.4	2.5	3.3	7.3
8	St. Franziskus Church	Daniel Studer-Studer Architekten	1.2	3.0	3.3	7.5
9	Musikschule Velden	Arch + more ZT GmbH	0.2	2.0	3.3	5.6
10	Ryesgade 30 A-C	Krydsrum Architects and Rönby.dk (Leif Rönby)na	1.9	2.0	2.1	6.0
11	Kindergarten and apartments	Pfleger + Stöckli Architektur GmbH	1.2	3.0	2.5	6.7
12	Single-family house	Beat Wermuth und Partner Architekten GmbH	2.1	2.7	3.3	8.1
13	Single-family house	Gehret Design GmbH	1.9	3.0	3.3	8.2
14	Glaserhaus	Anliker Christian, Innenarchitekt SWB	1.9	3.0	3.3	8.2
15	PalaCinema Locarno	AZPML + DFN (architects consortium)	0.5	2.0	3.3	5.8
16	Ritterhof	Michael Felkner	0.5	2.0	2.9	5.4
17	Farm house Huber	Lorenz Pobitzer	0.5	2.2	3.3	6.0

below collects the outcomes from the application of the methodology presented.

Table 3. The table presents the ID for project identification within the HiBERatlas database and the commentary developed following the analysis of individual projects.

ID	Comments on the Total Score
1	The replacement of the traditional tile roof with metal sheeting was carried out simultaneously with the integration of a photovoltaic system. The integration of the photovoltaic system preserved the form, original dimensions, and uniformity of the appearance of the new metal roof. However, some critical aspects in the roof’s design emerged, including material loss and alterations in appearance, encompassing color, texture, and pattern. It is noteworthy that the intervention is irreversible, and perceptually, it does not seamlessly integrate with the surrounding context, thereby impacting the local identity. Despite the transformative impact on the building envelope, the photovoltaic integration adheres to most recommendations provided by the guidelines. The aesthetic integration can be deemed satisfactory. However, a critical analysis of the color indicates a negative impact, as despite the module appearing camouflaged in the roofing, a standard coloring was utilized. This suggests an adaptation of the roof to accommodate the panel, rather than a genuine aesthetic integration of the module. A notable weakness pertains to technological integration, as the photovoltaic modules, applied to the roofing, do not fulfill the functions of weather protection, noise reduction, and thermal insulation. The case study analysis underscores the necessity of a comprehensive assessment of the entire retrofit project.
2	This is an exemplary instance of architectural design and energy retrofitting. The project demonstrates that the past and innovation can coexist harmoniously. A well-thought-out and meticulous design leads to a successful photovoltaic integration that blends positively with the context, morphologically integrating and fully respecting the historical value. Alternatively, the incorporation of photovoltaics within the expansive glass windows could have been considered. This approach exemplifies the synergy between modern technology and historical context.
3	This is an exemplary instance of integration and innovation. The photovoltaic panels integrated into the roof and facade fully adhere to established guidelines and meet the criteria for energy, technological, and aesthetic integration. Given that the facades did not possess a specific aesthetic value to preserve, innovative colored frameless photovoltaic modules were employed through meticulous facade design. However, the chosen colors do not particularly resonate with the surrounding context or the identity of the location. An alternative option could have been the integration of photovoltaics within the expansive glass windows.
4	The revision of the unconstrained facade covering was carried out without specific attention to the visual continuity of color, heights, textures, and patterns that the previous roof covering exhibited. The proposal of the two larger dormers in the protected facade is noteworthy, as photovoltaic modules have been discreetly incorporated within them.
5	The integration of the photovoltaic system has preserved the form, original size, and uniformity of the appearance. Weaknesses in the roof project concern the loss of material and alterations in appearance, including changes in color, texture, and pattern. From a perceptual standpoint, it does not seamlessly integrate with the surrounding context, altering the local identity. Nevertheless, it can be regarded as a noteworthy example of a Building-Integrated Photovoltaics (BIPV) application.
6	The adopted solution involves the installation of a decentralized system apart from the building. This represents a conservative approach to the structure; however, it lacks distinctive visual or innovative features. The utilized modules are standard and equipped with frames, and they exhibit a basic coloration. There is no notable design effort for aesthetic or technological integration. Nevertheless, it constitutes a valid additional approach.
7	The intervention involves the decentralization of the system. It satisfactorily meets the requirements for aesthetic, technological, and energy integration. The presence of photovoltaic modules is not visible from the street and seamlessly integrates into the hosting structure. It serves as an excellent example of this non-invasive approach.
8	The integration of the photovoltaic system has preserved the form, original size, and uniformity of the appearance. Weaknesses in the roof project include the loss of material and alterations in appearance, involving the color, texture, and pattern. Despite significantly changing the perceptual aspect of the building, the photovoltaic integration adheres to most of the recommendations provided by the guidelines.
9	The approach used for the energy retrofit does not affect the building, placing the system in the non-conservation subject extension. The flat roof has made possible an intervention that is not perceptible from the surrounding context. However, the poor aesthetic quality of the extension negatively affects the judgment. It is perceptively unfavorable because it does not integrate with the surrounding context, altering the local identity. Despite the unacceptable impact of the extension, the integration of photovoltaics complies with most of the recommendations provided by the guidelines. The integration can thus be considered moderately satisfactory.
10	The building’s configuration has made it possible to implement the system on the roof without compromising the aesthetics of the building or the surrounding context; the modules’ application is not visible from the street. The assessment of technological integration is low because the modules are not technologically integrated, meaning they do not serve as a building component.
11	Applying the guidelines objectively reveals that the aesthetic, material, and chromatic compatibility with the existing roof is not optimal. However, it can be asserted that in the presence of an overall valid project like this, it is demonstrable that, at times, through innovation, a solution can be found to enhance the pre-existing conditions, improving their perception and quality. From an energetic and technological standpoint, it is well integrated.
12	This approach, which is respectful of the identity and historical value of the building, is an integrated intervention in the roof, but it is applied as the ventilated system does not serve as cladding. Through meticulous design, the geometry and morphology of the roof have been respected.
13	The building is located in an isolated context of great naturalistic value. The intervention respects the roof’s geometry, planarity, and the dark color seems to evoke the surrounding peaks. It can be assessed as aesthetically, technologically, and energetically integrated. It may appear visually impactful, but the compromise between historical identity and technological innovation is acceptable.
14	The building is situated in an isolated context of significant natural value. The intervention respects the roof’s geometry and planarity, and the dark color seamlessly integrates without imposing a substantial visual impact from above. A moderately innovative module was employed, yet it integrates well aesthetically (being perfectly embedded in the roof, respecting the morphology of the existing architectural typology), technologically, and energetically. It can be considered an acceptable compromise between historical identity and innovation.
15	The solution has been integrated into an expansion and restoration project that has altered the facade of the building. It involves the application of a standard system with black/blue monocrystalline cells on a flat roof. The approach is valid as the integration does not aesthetically compromise the perception of the building; however, it does not effectively integrate either aesthetically or technologically.
16	The approach used in this case is conventional, involving the application of modules directly onto the roof. There is no distinct emphasis on aesthetic and technological integration; the choice is evidently geared toward maximizing energy efficiency.
17	The adopted approach involves decentralized energy production from the building, with photovoltaic technology integrated into a nearby structure. This approach maintains the appearance of the historic building. However, the modules used are traditional and do not fully meet the criteria for aesthetic and technological integration. It is a reversible intervention, representing a more conservative approach to the historical identity of the building and its surroundings.

reports an extensive evaluation for each considered best practice, which led to the assignment of the ratings proposed in the table above.

4. Discussion

This comprehensive analysis of best practices has led to the identification of various approaches to the integration of photovoltaic systems. This reflection has enabled further categorization, primarily identifying three approaches, which are listed and described below.

• Approach 1: In this approach, the integration of Building-Integrated Photovoltaics (BIPV) addresses all three aspects of integration. It emphasizes seamless integration with the surrounding landscape, establishing a direct or indirect dialogue with the environment. This approach primarily involves the incorporation or direct integration of photovoltaic modules onto historic buildings. It is particularly applicable to structures in states of disrepair, damage, reconstruction, or replacement. A notable example is the renovation of Doragno Castle in Rovio, Switzerland, and its photovoltaic system integration (an example is shown on the left of Figure 1). The matte black rooftop harmoniously melds with the dark forest vegetation, respecting the historical values of the original building, conveying collective memory, and generating green energy through the complete integration of photovoltaic and solar systems into the roof’s slopes. For historic buildings, rooftop integration is the most prevalent. In the case of industrial structures, photovoltaic modules are integrated as facade-mounted systems, emphasizing the notions of transformation and innovation.
• Approach 2: These integrations exhibit limited consideration for context; they do not seek a dialogue between the photovoltaic installations and the historic building or landscape. The identified approach entails situating the photovoltaic system on a nearby or connected building, thereby enabling decentralized energy production in the immediate vicinity. This approach preserves the historical value of the primary building (an example is shown in the middle of Figure 1).
• Approach 3: These integration attempts deviate from a holistic approach that addresses aesthetics, technology, and energy integration simultaneously. Instead, the focus is directed toward specific aspects, neglecting the comprehensive consideration of all three elements. This indicates a more specialized or targeted emphasis on specific aspects rather than a unified and synergistic integration strategy (an example is shown on the right of Figure 1).

Among these various approaches, Approach 1, which encompasses interventions with the highest evaluations, stands out. In general, these evaluations made through guideline applications underscore the individuality of each building: each structure possesses a unique historical narrative and distinctive character that necessitates different approaches to intervention. Existing context, materials, construction techniques, and local building should influence the proposed interventions. In fact, in some instances, these interventions may represent appropriate and acceptable innovations, while in others, the same approach and type of intervention may appear unacceptable. For this reason, a robust preliminary phase is also essential, enabling a clear understanding of the building’s historical evolution, its surrounding context, and the changes it has undergone. This knowledge forms the basis for making an appropriate and well-motivated design choice, fostering the ideal balance between historical preservation and innovation.

During this analysis, possible weaknesses in the methodology are also found, making room for possible improvements. First, aesthetic integration was shown to be the fundamental category to be fulfilled from the discussion. In this respect, this study showed that the three categories for integration may need different weights. It also reflects the different number of criteria within each category (aesthetic 7, technological 10, and energy 4). Assigning the same quantitative weight (a solution adopted following the definition of the methodology) can lead to unbalanced ratings.

It was found to be more difficult to evaluate some criteria: hygiene and health, Safety, installation, durability, and quality. Although it is important and necessary that these criteria are followed and adhered to by the designer at the design stage, there is a general difficulty in finding information.

5. Conclusions

In this study, 17 best practices of building integration of PV systems, selected within the HiBERatlas, are evaluated through the application of the guidelines developed within the context of the research project “BIPV meets history”. To do so, a tool has been developed for the assessment of the PV integration quality through the mentioned guidelines.

The study has shown how, through the application of the described guidelines and through the developed tool, it is possible to reliably assess the quality of the PV integration. Moreover, for the analyzed case studies, three different design approaches have been identified, recognizing the superior validity of one of them. Further analysis revealed strengths and weaknesses of the methodology proposed by the developed guidelines: it is crucial to have a robust preliminary phase that can provide a clear understanding of the evolutionary history of the context in which it is situated to drive well-motivated design choice. On the other hand, some criteria were difficult to evaluate, and the aesthetic category of integration seemed to deserve more influence on the final KPIs.

Future outlooks may involve improvements to the guidelines by considering different types of buildings in order to deliver a more tailored analysis and some balancing of the adopted criteria.