Renovation of Typological Clusters with Building-Integrated Photovoltaic Systems ^†

Abstract

The current climate emergency makes it imperative to take action to halt the irreversible destruction of the planet, with the renovation of existing buildings playing a crucial role. In Europe, particularly in Spain, energy efficiency improvements in existing buildings are undertaken in only a small fraction of cases. This gap presents a valuable opportunity to implement measures that encourage such interventions. To enhance energy production and tackle this issue from a distributed energy perspective, building-integrated photovoltaic (BIPV) systems emerge as a key solution. In this context, the primary objective of this research is to enhance the visibility and promote the adoption of BIPV systems in building energy retrofitting through the development of a standardised action framework for their installation across distinct typological clusters. To achieve this objective, a comprehensive and systematic analysis was undertaken to construct a classification that most accurately and exhaustively represents the Spanish building stock. The analysis resulted in the identification of 15 typological clusters, which, based on shared formal attributes, were consolidated into 3 principal clusters. For each of these three primary groups, a tailored action guide for BIPV system implementation was developed, addressing their specific characteristics and highlighting the critical factors to be considered in each case. To illustrate the practical application of the proposed framework, a representative case study was selected and subjected to an in-depth analysis, resulting in a detailed proposal for BIPV system installations on both the façade and the roof. In this regard, this research develops an initial procedural framework that comprehensively represents diverse building typologies, providing a structured protocol for the integration of BIPV systems within the context of energy retrofit interventions.

1. Introduction

Climate change and environmental degradation represent the most pressing global threats of the contemporary era. Addressing these challenges demands a comprehensive and concerted international commitment to mitigate the progression of environmental deterioration, as society has reached a critical point from which recovery may no longer be feasible. In response to this urgent issue, the European Union (EU) has introduced the European Green Deal, an ambitious initiative aimed at making Europe the first climate-neutral continent by 2050 [1]. To achieve this long-term objective, the EU has set an ambitious mid-term target as follows: a 55% reduction in greenhouse gas emissions by 2030. In parallel, the ‘Fit for 55’ package of measures has been developed to support this objective [2,3]. This study focuses on two key measures of the Fit for 55 framework, which play a pivotal role in the energy transition. The first measure sets a target of increasing energy consumption from Renewable Energy Sources (RESs) by 42.5%, ensuring that at least 49% of the energy used in buildings originates from renewable sources by 2030 [4,5,6]. The second measure stipulates that all buildings must achieve zero-emission status by 2050, as buildings are responsible for 36% of global greenhouse gas emissions and account for 40% of total energy consumption [7]. Official statistics indicate that approximately 75% of the European building stock is energy inefficient, while less than 1% of buildings undergo annual energy performance renovations [8]. In the Spain context, a similar trend is observed, with fewer than 1% of target buildings identified in 2014 having undergone renovation [9]. This highlights the urgent need for large-scale building renovation, both as an opportunity and a necessity.

To accelerate the rate of energy renovations, the EU introduced the Renovation Wave initiative, aiming to renovate 35 million buildings by 2030, thereby improving energy efficiency and enhancing the quality of life [10]. The policy measures within the Fit for 55 framework present a crucial opportunity to drive the renovation of existing buildings through the integration of renewable energy technologies. Solar energy, in particular, has emerged as one of the most widely used and well-established RES [11]. Building-integrated photovoltaics (BIPV) systems offer significant potential, as they can be seamlessly incorporated into building structures, serving as an integral component of the building envelope while simultaneously generating energy.

Examples of BIPV solutions for building renovation have shown the versatility of BIPV to fit the energy, construction, and aesthetic needs of buildings in different climates and urban contexts [12]. The energy and economic profitability of renovating buildings with BIPV have been analysed in a variety of contexts and building types [13,14,15,16]. Although recent decreases in PV costs and increases in PV efficiency make BIPV solutions even more economically attractive, there is still a path to go. Advancing methods that facilitate the decision-making process for each specific case is another important step [17].

1.1. Photovoltaic Systems in Buildings

In the context of solar energy systems for buildings, it is crucial to examine the two primary systems currently in operation.

• Building-Attached Photovoltaic (BAPV) systems: These consist of conventional photovoltaic modules designed exclusively for the conversion of solar radiation into electricity. Typically mounted on roofs or façades, BAPV installation utilises supporting structures—commonly made of metal or wood—that are affixed to the building’s structural framework. In this configuration, the photovoltaic system remains an additional element rather than an integral part of the architectural design or building envelope. Consequently, BAPV systems do not contribute to the building’s structural, protective, or aesthetic functions, instead operating as independent technological elements, similar to ventilation, lighting, and heating systems [18]. While BAPV has a longer market history and greater recognition compared to BIPV systems, its adoption is increasingly constrained by its lack of integration with the built environment. This limitation often results in its rejection in architectural and urban planning contexts.
• Building-Integrated Photovoltaic (BIPV) systems: Unlike BAPV configurations, BIPV systems are designed to function both as energy-generating systems and as integral components of the building envelope. In contrast to conventional photovoltaic setups, BIPV components replace traditional building materials, thereby contributing not only to energy production but also to the structural, functional, and aesthetic characteristics of the building. Integrated into the building’s outer layer, BIPV elements fulfil multiple essential roles, including protection against environmental factors, fire resistance, noise reduction, shading to mitigate solar heat gain, enhancement of thermal insulation, and facilitation of natural lighting. Due to their multifunctional nature, BIPV systems transcend the conventional role of photovoltaics, emerging as key architectural elements that harmonise energy efficiency with sustainable design [18,19,20].

In these systems, which constitute integral components of the building envelope, any necessary replacement must be carried out in a manner that ensures the installation of an alternative construction element capable of fulfilling the same function while maintaining habitability standards. Consequently, BIPV modules can be regarded as both structural and photovoltaic elements within a building [21]. A notable example of this dual functionality is observed in the case of solar roofs, which constitute a BIPV system. In this scenario, the solar tile, functioning as a BIPV component, necessitates substitution upon removal in order to preserve the building’s structural and functional integrity. BIPV components are affixed to the supporting structure, that is, the building itself, through a mechanical anchoring system employing longitudinal and transversal guides composed of metal or wood. This configuration ensures structural stability while allocating minimal space that fulfils a dual purpose as follows: providing a ventilation chamber to enhance module efficiency and concealing all electrical components for aesthetic and safety considerations. A significant advantage of BIPV technology is its adaptability, allowing for seamless integration across a wide range of architectural applications, including façades, roofs, skylights, balustrades, parapets, windows, and urban infrastructure. In certain architectural elements, such as canopies, skylights, and pergolas, there is potential for incorporating bifacial modules in BIPV installations. These photovoltaic modules can capture solar energy on both sides of the photovoltaic cell surface, thereby significantly increasing energy generation [22,23,24]. The proper characterisation of bifacial canopies and the variability of the rear irradiance are singular aspects to be studied in rooftop canopies or carports [25]. This highlights the adaptability and versatility of the modules employed in BIPV systems, further enhanced by the aesthetic customisation options available, as these modules can be produced in various colours [26,27].

A comparative analysis of these two photovoltaic systems reveals certain similarities, with both technologies utilising solar energy to generate electricity for on-site consumption, thereby reducing dependency on grid-supplied power. Nevertheless, the disparities between these systems are considerably more pronounced than their shared qualities. BAPV systems prioritise energy efficiency and performance over aesthetics, resulting in a lack of integration with the built environment. In contrast, a fundamental characteristic of BIPV systems is their customisability, encompassing modifications in colour, texture, dimensions, and shapes [28]. This level of design flexibility is achieved by concealing photovoltaic (PV) cells, a strategy that inevitably incurs a trade-off in the form of reduced energy efficiency and increased production costs. Nevertheless, BIPV systems are widely recognised as one of the most promising solutions in sustainable architecture, due to their dual capacity to seamlessly integrate into building structures while simultaneously generating energy. The increasing adoption of BIPV in recent years further underscores its potential, with numerous implementations evident in both new construction projects and building renovations [29].

In analysing the technical aspects of the modules used in BIPV systems, it is essential to establish as a basic premise that the energy output of a module is influenced by a myriad of factors. In terms of the expected energy output, this value is subject to variation due to both the intrinsic characteristics of the module itself and the quality and quantity of solar irradiance falling on it. Consequently, electricity production will also depend on the geographical location of the installation, with higher output in regions characterised by higher levels of solar flux and direct sunlight, as opposed to those located in areas with lower levels of solar exposure. Factors such as latitude, climate, topography, terrain, and air pollution will inevitably affect energy production. As these systems are integrated into the built environment to ensure seamless alignment with the surrounding context, BIPV systems face an increased number of constraints. Phenomena such as reflections, dirt accumulation, shading effects, and irregular irradiation patterns induced by urban environmental elements have a direct impact on the operational efficiency of BIPV installations. Taking this into account, the efficiency of a monocrystalline silicon module—one of the most common in this type of installation—should fall within a range of 19% to 22%, as defined under Standard Test Conditions (STCs) [12]. A notable advantage of the modules used in BIPV systems is that they can be manufactured in a range of colours and textures. Unfortunately, this comes with the inherent risk of a significant reduction in efficiency, which in extreme cases can be as high as 45% [28]. The treatments and colours in BIPV may have a significant impact on the effective irradiance used in the PV conversion [30].

The durability and overall life cycle of both the modules and the overall system depend on several internal factors, such as the manufacturing processes of the individual components, as well as external influences, including extreme climatic conditions and fire. In terms of those factors controlled by the manufacturing process and materials, these will determine the robustness and durability of the system in terms of thermal–mechanical and electrical performance. In this context, environmental factors such as temperature, solar radiation, wind, snow, humidity, and atmospheric pollutants play a significant role in the degradation of the modules, resulting in a reduction in their electrical efficiency. In general, integrated photovoltaic systems require regular maintenance to ensure the continued quality and optimal performance of the installation. In terms of life cycle and taking into account the composition of the system’s various components, a BIPV installation has a life cycle of up to 100 years for the supporting structure, between 25 and 30 years for the modules [31], and around 15 years for the electrical components, such as inverters [12].

1.2. BIPV Market

The current state of the BIPV market is promising, particularly considering the challenges encountered during its initial development phase. Significant advancements have been achieved in both BIPV module efficiency and installation methodologies. These innovations, combined with a progressive reduction in costs, have contributed to increasing BIPV systems within the construction sector. Indeed, the initial high costs associated with these installations, which constituted a formidable barrier to market entry, have experienced substantial fluctuations over time. To further encourage the integration of BIPV, various policy frameworks have been introduced across different countries, primarily aimed at reducing carbon emissions and enhancing energy efficiency.

Despite this progress, BIPV systems remain in the early stages of development, as reflected in current market dynamics, where significant advancements are still required in terms of large-scale implementation and diffusion. Several barriers continue to hinder the extensive deployment of these systems. Within the construction sector, there is a pronounced deficiency in professional training, affecting both the design phase and the availability of specialised labour for installation. This knowledge gap extends to the general population, where widespread misconceptions and a lack of awareness regarding BIPV persist. A particularly notable issue is the prevailing aversion to the use of photovoltaic systems in renovation projects, especially BIPV solutions. This reluctance largely stems from the association of BIPV with outdated visual perceptions reminiscent of conventional BAPV modules, leading to the frequent dismissal of its implementation [32]. Consequently, BIPV remains in an early developmental stage, and a radical transformation in market perception is improbable in the near future, primarily due to the competition from BAPV and a prevailing preference for traditional construction materials. Nonetheless, a gradual increase in BIPV utilisation is anticipated over time, due to its distinctive characteristics and advantages. The adaptability of BIPV to the technical requirements of individual buildings is a crucial factor in its long-term viability. This inherent flexibility is likely to be a decisive consideration in the development of positive energy buildings, where the integration of BAPV solutions is not feasible.

The analysis of the BIPV market presents a multifaceted challenge due to the interplay of numerous factors, including the presence of economic incentives and subsidies, as well as its applicability across diverse infrastructures and sectors. This versatility is evidenced by the market’s extensive range of offerings, which include modules with varying degrees of transparency, spanning from opaque modules designed for ventilated façades to semi-transparent variants intended for use as skylights. In this manner, the integration of BIPV solutions is feasible across a wide spectrum of building typologies. Given the variability of market values according to the application, the European BIPV market ranged between 300 MW and 500 MW annually in 2022, with a global estimation of up to 2 GW [26]. It is important to acknowledge that these values remain estimations, as a significant proportion of BIPV systems are customised, resulting in substantial variations in market data when compared to glass-to-glass BIPV solutions. While customised BIPV systems may appear to be more successful in the market, the most widely deployed solutions remain those with standardised dimensions. Nevertheless, a significant advantage of BIPV systems lies in the extensive array of manufacturing solutions, enabling their adaptation to diverse architectural and functional requirements. The diversity of colours, sizes, and shapes is a crucial factor driving the commercialisation and increasing market penetration of BIPV. This trend is reflected in the increasing market share observed in recent years, spurred by the entry of China and various globally recognised corporations, underscoring both the current and projected growth of the BIPV sector [26,33].

The objective of this research is to enhance the visibility and promote the utilisation of BIPV systems in building renovation. To achieve this, a comprehensive analysis was undertaken to develop an intervention framework using BIPV systems, adapting these actions to specific typological clusters. The research is based on the classification of Spanish building typologies established by the Ministry of Development of the Spanish Government [34]. This classification is derived from an extensive examination of the residential building stock in Spain, aimed at identifying recurring architectural and structural characteristics to facilitate a representative and equitable categorisation of the national building inventory. In this context, the typological clusters are systematically evaluated for their suitability for BIPV integration, leading to the formulation of targeted renovation strategies. Each proposed strategy is meticulously adapted to the specific morphological attributes of the respective clusters, thereby ensuring a tailored and appropriate implementation of BIPV systems in the built environment.

2. Methodology

The investigation initiated with an extensive review of the diverse typologies of clusters present in Spain. Subsequently, an exhaustive analysis of these clusters was conducted, aiming at identifying their main characteristics and proposing an innovative renovation strategy incorporating photovoltaic systems. This approach seeks to adapt to their morphological characteristics while at the same time enhancing them. Through the use of comprehensive datasets and case studies on the implementation of BIPV, standardised BIPV interventions were formulated and designed for each typological cluster. Beyond the main objective of this research, this study aims to develop standardised action frameworks that serve as reference guidelines for users, companies, and professionals [35].

The methodology utilised in this research consists of four principal phases, which are delineated as follows:

• Phase 1: Establishment of criteria for categorising the entire building stock in the most representative manner.

To establish a classification that accurately and equitably represents the entire building stock in Spain, this research adopts the two key parameters employed in the reference study as a basis for grouping the clusters [34]:

• The number of floors above ground level: The application of this criteria has led to the categorisation of buildings into three primary groups, namely, single-family dwellings with one or two stories, multi-family buildings with up to three floors, and multi-family buildings with four or more stories.
• The year of construction: The temporal ranges selected for construction periods ensure comprehensive coverage of the entire building stock in the most methodologically appropriate manner, namely, before 1940, 1941–1960, 1961–1980, and 1981–2007 and between 2008 and 2011.

The resulting classifications consist of buildings that share analogous structural, constructional, and aesthetic characteristics and similar levels of energy efficiency. This categorisation facilitates the assessment of the necessity for renovation and enables the identification of the most urgent interventions required to address their current demands.

• Phase 2: Systematic classification of the building stock based on the defined criteria.

The application of these criteria has resulted in the identification of 15 distinct typological clusters, which have subsequently been categorised into three overarching groups based on the number of floors above ground level. To enhance the comprehension of the classification, each group has been assigned a corresponding letter as follows:

• “U” for single-family dwellings. Typically comprising one or two stories, either attached or detached on individual plots, with two or four available façades. Pitched roofs predominate, featuring slopes ranging between 20° and 40°, commonly configured as gable or hipped roofs.
• “C” for multi-family buildings with up to three stories. Ranging in height from 6 to 8 m, these structures are attached to adjacent buildings on both sides. The rear façade generally opens onto a small courtyard designated exclusively for ventilation and lighting, making the main façade the primary element of interest for a solar intervention. Pitched roofs are predominant, with slopes varying between 40° and 10°.
• “D” for building blocks with four or more stories. Characterised by significant heights and located in areas of urban expansion, typically either detached or semi-detached. Various typologies can be distinguished within this group as follows: linear blocks, which lack interior courtyards and feature two external façades; H-shaped buildings, incorporating small internal courtyards; and modern urban blocks, occupying entire city blocks and integrating large interior courtyards. Flat roofs are the predominant roofing typology in this cluster, although sloped roofs with inclinations of 20° and 10° are also present.

The classification is graphically represented in

Table 1. Classification of the different typology clusters.

Clusters	<1940	1941–1960	1961–1980	1981–2007	2008–2011
U
C
D

, which illustrates the external appearance associated with each of the 15 typological clusters identified in Spain.

• Phase 3: Implementation of various BIPV interventions tailored to each typological cluster.

The classification of typological clusters facilitates the formulation of renovation strategies incorporating BIPV technologies for each grouping. A comprehensive analysis has been conducted for each typological category, with particular emphasis on their formal characteristics. Based on this analysis, an action framework has been developed for clusters of types U, C, and D, specifying the optimal BIPV systems installations and the corresponding implementation methodology while accounting for the distinct attributes of each cluster. The rationale for proposing energy renovation strategies using BIPV for these three major clusters lies in the high degree of similarity among the building typologies within each group. However, certain cases may arise in which individual buildings are not fully represented within the cluster classification, necessitating a tailored and specific BIPV installation. Despite these variations, the research provides a methodological approach to identifying the typological cluster of a given residential building and offers a reference framework for the application of BIPV systems.

• Phase 4: Selection of a representative case study for in-depth analysis.

A real case study, representative of one of the previously identified clusters, is selected to illustrate the practical application of the performance framework, incorporating BIPV systems in the building renovation process. This selection ensures that all relevant specifications and contextual factors are duly considered.

Consequently, when analysing and designing renovation interventions incorporating BIPV, the specific characteristics of each cluster—both in terms of construction and architectural form—are meticulously considered, along with the deficiencies identified in the buildings. In most cases, the factors contributing to a building’s energy inefficiency are related to its period of construction. For instance, a predominant issue in older structures is inadequate thermal insulation, which is often either obsolete or absent due to historical construction practices that did not incorporate such measures [36]. This study, therefore, advocates for renovation strategies that integrate BIPV systems within the building envelope, aiming to reduce interior energy demand through a dual approach as follows: capturing solar radiation for electricity generation and utilising materials that enhance energy efficiency while maintaining indoor thermal comfort. To determine the optimal locations for integrating photovoltaic technology, a comprehensive analysis of the building’s morphology and the shadows cast—both by the structure itself and by surrounding buildings—is required. In general terms, the most effective surfaces for BIPV systems are roofs and façades, as they constitute the primary areas suitable for photovoltaic installation. Pitched roofs offer the highest potential for solar energy capture [37], whereas flat roofs present a viable alternative when the optimal inclination is achieved through the implementation of an auxiliary structural system.

Each typological cluster represented in the classification table possesses distinct characteristics while also sharing common features with other clusters within its broader category. This enables the classification of these clusters into overarching typological groups. By categorising the building stock into three principal groups, characterised by common attributes and similar morphology traits, it becomes possible to establish standardised renovation strategies for each group. These strategies are sufficiently adaptable to accommodate the specific requirements of individual cases. The renovation solutions proposed in this study are designed to maximise the use of each building typology’s intrinsic characteristics while avoiding actions that could compromise the efficiency of the photovoltaic systems. In this regard, employing BIPV-based renovation strategies must be tailored to the specific characteristics of each cluster, requiring detailed analysis to ensure that their full potential is effectively exploited.

3. Results

The consolidation of building typologies into three primary clusters facilitates the grouping of similar typologies and the detailed consideration of their specific attributes, thereby enabling the development of renovation strategies incorporating BIPV. These interventions serve as a model for all typological clusters within each group. Nevertheless, it is essential to acknowledge the heterogeneity within each category. In this regard, the present study conducts a comprehensive analysis of the three possible intervention approaches. Before examining the specifics of each, the following fundamental premises must be considered:

• The visibility of the BIPV system can be a challenge in certain cases, particularly in protected areas. In the case of high-rise residential buildings, such as those classified within clusters D, where roofs are not visible from the street, BAPV systems may present a more economically viable and cost-effective alternative.
• As previously noted, a comprehensive analysis of both the building and its urban context is imperative. This assessment determines whether the structure is subject to any form of heritage protection, which, in turn, delineates the allowed interventions and regulatory approvals required. In instances where the building under renovation is legally protected, existing studies have demonstrated the feasibility of integrating BIPV systems within the renovation process of historic buildings, ensuring the preservation of the building’s characteristics and intrinsic heritage values [38,39,40];
• In renovation strategies that incorporate BIPV on façades, it is essential to acknowledge their continuous visibility. In cases where façades are located along narrow streets, certain areas of the building may remain in shadow due to the proximity of adjacent buildings or urban elements. Consequently, where adequate solar resources are available, BIPV represents the most effective solution. However, if an assessment indicates that BIPV integration is insufficient, a passive renovation approach should be pursued. This ensures the preservation of the building’s architectural integrity while concurrently enhancing its energy efficiency, in alignment with the principles of sustainable development.
• Two specific interventions are applicable to all typological clusters: terrace parapets and BIPV skylights. In buildings with spaces conducive to such applications, these interventions merit serious consideration. The photovoltaic modules employed in these systems utilise semi-transparent glass, facilitating the penetration of natural light while concurrently capturing solar radiation for energy generation.

Prior to the analysis of each cluster, it is essential to reaffirm that the overarching objective of this research is to promote and prioritise renovation strategies employing BIPV, with the dual aims of enhancing energy generation and optimising thermal performance.

3.1. Clusters U

For the typological clusters within group U, classified as typologies U < 1940, U 1941–1960, U 1961–1980, U 1981–2007, and U 2008–2011, the characteristics of roofs and façades play a pivotal role in determining the feasibility of BIPV interventions. To initiate the analysis, it is essential to first establish the specific nature of the building and its characteristics. The predominant morphological configuration within this cluster is that of the traditional single-family dwelling, typically comprising one to two stories, with a median height of approximately six metres. This typology of building is generally semi-detached or, less frequently, fully detached. In cases where isolated dwellings are present, an in-depth assessment of solar exposure and shadow projects is required to ascertain the viability of façade interventions. In select circumstances, all façades may be considered for intervention. In general, the façades orientated toward the south, east, and west are deemed optimal for photovoltaic integration, while the potential applicability of north façades must be rigorously evaluated based on prospective energy yield.

In contrast, the most common scenario within this cluster is constituted by semi-detached single-family dwellings, where the scope for intervention is inherently constrained by the availability of only two façades. In such circumstances, the primary façade is generally the most suitable site for BIPV installation. The rear façade, which frequently delineates the inner courtyard area, may also be utilised, contingent upon a meticulous assessment of shading conditions to ensure optimal module performance. In both scenarios, intervention should be carried out across the entire available height of the façade, provided that shading effects from adjacent buildings and surrounding obstructions do not significantly impair energy generation. The BIPV installation in these areas will follow the same construction process as a ventilated façade, employing a structural system composed of metal guide rails arranged both longitudinally and transversally across the surface. These guide rails will be anchored to the supporting structure, serving as the infrastructure for the photovoltaic modules, which will constitute the final protective and functional layer of the building envelope.

In relation to roofing systems, the most common types include gable roofs, observed in instances of semi-detached housing; hip roofs, seen in detached residences; and flat roofs, typically seen in more contemporary constructions. Consequently, the potential applications of BIPV can be categorised into the following two distinct approaches:

• For pitched roofs, it is required to conduct a detailed assessment of the roof’s orientation, as slopes facing south, southwest, and southeast provide the most favourable conditions for solar energy capture, while north-facing slopes should be excluded unless performance calculations substantiate their viability. The installation of BIPV in these cases will be implemented as a ventilated roofing system, whereby photovoltaic modules, functioning as solar tiles, are directly incorporated into the uppermost layer of the building envelope, ensuring both structural integrity and energy efficiency.
• For flat roofs, the installation of a photovoltaic system necessitates the incorporation of an auxiliary structural framework designed to optimise the orientation and tilt angle of the modules, thereby maximising solar radiation absorption. In such instances, it is crucial to prioritise the integration of the photovoltaic system into the existing built environment, thereby moving away from conventional BAPV systems towards fully integrated solutions.

3.2. Clusters C

The five typological clusters identified within group C—categorised as C < 1940, C 1941–1960, C 1961–1980, C 1981–2007, and C 2008–2011—present similar characteristics, with the primary distinctions being the construction period and the number of exposed façades available for intervention. The C cluster typology pertains to buildings predominantly located within central urban areas, typically comprising three stories, with an average height ranging from approximately 9 to 10 m, and accommodating one or two residential units per floor. Typically situated within urban centres, these areas frequently occupy spaces within the historic districts of cities. The urban planning of these areas is characterised by narrow streets, which, when combined with the height of the buildings, results in the exclusion of direct natural light from penetrating the interior of these streets, resulting in constant shadow. Morphologically, these buildings are distinguished by their predominant use of pitched gable roofs. Their configuration is typically characterised by a single external façade, as they are adjoined by neighbouring structures on both sides. In instances where an interior-facing façade is present, it is generally the result of small internal courtyards.

The analysis underscores that the most advantageous placement for BIPV systems is on building rooftops, leveraging the inherent inclination of these surfaces to optimise solar energy capture. This configuration also ensures minimal exposure to shading, a critical factor that can adversely affect system performance. However, in densely built urban environments, the presence of neighbouring structures with varying heights and architectural features, such as dormer windows on mansard roofs, can introduce shading effects that may impair the efficiency of BIPV systems on adjacent buildings. The integration of BIPV systems into building façades presents considerable challenges, particularly in the context of renovation projects, thereby substantially limiting the scope of potential interventions. Notably, the installation of BIPV on interior-facing façades, commonly known as “light wells” or courtyard façades, is immediately precluded due to the spatial constraints inherent to these areas. Originally designed to fulfil essential residential functions—primarily ventilation and natural illumination—these confined spaces are perpetually shaded, rendering them unsuitable for solar energy capture. Nevertheless, given that the objective of this research is to enhance the habitability of the buildings, it is possible to renovate these façades using an ETICS (External Thermal Insulation Composite Systems) solution [41]. Consequently, the principal façade emerges as the only viable surface for BIPV integration within the scope of the renovation project. Despite this, the integration of photovoltaic systems on primary façades is fraught with significant challenges, particularly within historic urban centres. The spatial constraints imposed by narrow streets—typically ranging between 7 and 10 m in width—combined with the variable heights of surrounding buildings frequently result in persistent shadows, which predominantly affect the upper sections of building façades. Therefore, whenever feasible and following a comprehensive analysis to underpin this determination, it is recommended to utilise the BIPV system on the entire façade. In instances where the analysis indicates insufficient performance of BIPV systems in specific areas, a viable solution may involve intervention on the surface of the upper floor. This is due to the fact that this area will experience increased solar incidence, thereby reducing the risk of shading.

In instances where one portion of the façade is equipped with a BIPV installation while another remains free from photovoltaic intervention, it is essential to meticulously define the scope of the BIPV retrofit to ensure seamless integration with both the existing façade and the overall structure. Although this type of intervention presents a challenge, it is achievable due to the extensive variety of BIPV components available in the market, offering diverse sizes, shapes, colours, and textures. The involvement of professionals with expertise in architecture, photovoltaics, and energy ensures the development of an optimal design tailored to each specific case.

As with clusters U, photovoltaic ventilated roofs and façades are the design solution for BIPV installations.

3.3. Clusters D

The clusters comprising Group D exhibit a broader diversity in external appearance while retaining fundamental shared characteristics. Variations in their exterior design are predominantly influenced by their construction period, enabling the classification of distinct typological patterns. Specifically, the linear block typology is emblematic of the 1941–1960 period, the H-block typology characterises the 1961–1980 period, and the modern block typology is typical of the 1981–2007 and 2008–2011 periods. Among these clusters, only those constructed prior to 1940 are commonly located within city interiors and historic urban centres. In contrast, the remaining building typologies are situated in areas undergoing urban expansion, characterised by wider roadways that facilitate increased penetration of natural light into the interior spaces of the dwellings. Despite their varied external and formal attributes, these structures typically comprise six floors, with an approximate height ranging from 18 to 19 m, and accommodate between two and four residential units per floor. The predominant roofing style among these typological groups is flat; however, pitched roofs are also present, albeit to a lesser extent, typically exhibiting slopes of less than 10°. Regarding the façades available for potential modification, these buildings generally feature two external façades, while the remaining sides are buildings. An exception arises within the D 1981–2007 and D 2008–2011 clusters—classified as modern blocks—which are distinguished by their four accessible façades. This includes interior façades that open onto expansive inner courtyards, which, in certain instances, may be repurposed for functional interventions.

In the case of buildings belonging to this group, it is possible to incorporate integrated photovoltaic systems into both the roof and the façade, although the latter option is subject to more limitations. In the case of BIPV installations on flat roofs, auxiliary structures composed of self-supporting concrete blocks, with the requisite inclination, are employed. These structures serve to anchor the PV modules, thereby functioning as a secondary roof. In the case of flat, non-trafficable roofs, it is recommended to prioritise the performance of the installation to produce as much energy as possible and include suitable building materials to reduce the internal demand. A viable solution to this problem is the integration of external thermal insulation into the concrete blocks. In the context of walkable roofs, it is imperative to incorporate the principles of sustainable design into the design of the installation. This involves the implementation of measures that not only reduce energy consumption within the building but also ensure that the residents are able to enjoy and utilise the available space. In such cases, priority must be given to solutions that generate energy and enable the optimised utilisation of the roof, such as photovoltaic pergolas. In instances where a pitched roof with a slope conducive to the installation of PV panels is present, the existing roof is to be removed, and the BIPV installation is to be executed through the implementation of a photovoltaic ventilated roof, which is to be integrated with the building.

A comparative analysis of the possible façade interventions in these typologies and the multi-family housing complexes previously analysed reveals a greater possibility of including BIPV systems in the renovation of these spaces, given their two façades with views to the exterior. To determine the viability and performance of a proposed design, a comprehensive analysis of the urban structure and the potential shadow patterns is imperative. In such cases, it is essential to consider not only the height of the adjacent buildings and the separation of the streets but also the potential impact of urban elements, such as trees, which may pose a problem. Following a comprehensive data analysis, if the findings indicate the feasibility of intervening on the entire free height of the façade, this action will be given the highest priority. In the context of modern blocks, the number of available façades increases to four.

In such instances, it is imperative to employ a methodologically rigorous approach, encompassing a comprehensive analysis of shadow patterns generated by adjacent buildings and urban elements. This analysis must be accompanied by a critical evaluation of the feasibility of interventions on the northern façade, which is anticipated to pose the greatest challenges due to its suboptimal solar orientation. Furthermore, particular attention should be directed towards the façades facing the expansive interior courtyards characteristic of this typology, as these areas represent highly favourable opportunities for BIPV systems integration. A thorough shadow analysis is equally indispensable in this context to determine which façades are most suitable for the installation of photovoltaic modules, as the potential for shading may present more significant impediments compared to external façades. Following the strategy outlined for Clusters C, in the event of unfavourable shading analysis outcomes, the installation of BIPV systems should be prioritised on the uppermost sections of the façades, where solar exposure is typically optimised.

4. Case of Study

The selected case study is a real building represented by one of the selected typologies in order to analyse its characteristics and demonstrate the application of the methodology established in this study. The case study is situated in the district of Villaverde, Madrid, Spain. The building, constructed in 1970, is a residential structure with a linear block typology and five floors, thus measuring between 16 and 18 m in height and comprising two dwellings of approximately 74 m² each. This information indicates that the building is represented by cluster D 1961–1980 (Figure 1). In terms of external characteristics, the roof is pitched, with a north-south orientation and an inclination of 20°. This falls within the range of inclination between 20 and 40° [42], a classification that will facilitate and streamline the comprehensive study of the urban fabric in which it is situated for future analyses. The building is distinguished by its two exterior façades, orientated towards the south and north, which present a potential for the implementation of interventions employing BIPV, given the proximity of the east and west sides to residential buildings. The combination of these factors presents a significant opportunity to propose a renovation incorporating integrated photovoltaic systems.

The annual irradiation data on the roof and façade surfaces, with a north-south orientation, have been calculated with the PVGIS programme [44], within which shading and irradiation analyses have heretofore been conducted to ascertain the extent of the available solar resource. The results obtained were as follows:

• North roof slope with 20°: 1380 kWh/m².
• South roof slope with 20°: 2021 kWh/m².
• North façade with 90°: 320 kWh/m².
• South façade with 90°: 1395 kWh/m².

As expected, data gathered from the southern sides exhibit higher values compared to those from the northern, as anticipated, thereby designating the former as the primary areas for renovation utilising BIPV systems, though not the exclusive ones. A detailed analysis of the irradiation levels on the northern façade reveals a marked disparity when compared to the southern façade, leading to the conclusion that the northern façade should be excluded as a viable site for the installation of a photovoltaic system. Nevertheless, although the northern slope of the roof demonstrates lower values than its southern counterpart, these values are still deemed sufficiently favourable to justify the consideration of BIPV installation in this area. Furthermore, the lower the roof pitch, the smaller the difference in solar resources between the two slopes.

As has been announced throughout the research, when addressing the proposed interventions for each cluster, the BIPV solutions proposed for the areas with the best irradiation values are the BIPV ventilated façade and the BIPV ventilated roof (Figure 2). The procedure for both solutions is analogous as follows: a layer of thermal insulation is applied to the existing structure, followed by the installation of a system of primary metal rails fixed to this support structure. In turn, this system of rails serves as the foundation for a secondary structure of metal guides, arranged both longitudinally and transversely across the full height of the façade, which provides the necessary support for and to which the photovoltaic modules are affixed. The dimensions of the materials vary by manufacturer, while the material and thickness of the thermal insulation are regulated in Spain by the Código Técnico de la Edificación, in the Documento Básico “Ahorro de Energía” (CTE DB-HE), which specifies maximum values for buildings located in Madrid, specifically within the D zone [45]. In order to meet the thermal transmittance requirements of less than 0.35 Wm⁻²K⁻¹ on the roof and 0.41 Wm⁻²K⁻¹ on the façade, it is necessary to ensure that the minimum thickness is approximately 10 cm, with a thermal conductivity of 0.04 Wm⁻²K⁻¹ used. The integration of a ventilation chamber, facilitated by the supporting structure of primary and secondary metal guides, helps reduce the rear-face heating of the PV modules, thereby enhancing their performance. This space also conceals the electrical system and allows for easy maintenance, as the panels can be removed in case of failure. Furthermore, the modules used in BIPV systems can be aesthetically customised to blend with the urban environment, with hidden cells and the required colours and textures. This feature is a significant advantage of BIPV systems, applicable in both rooftop and façade installations.

5. Discussion

BIPV systems are characterised by their remarkable flexibility and adaptability to a diverse range of specifications, as demonstrated by the proposed solutions for each typological cluster. The construction approaches recommended for each scenario share a common framework, integrating ventilated façades and roofing systems. This design strategy seeks to facilitate the incorporation of BIPV systems with minimal material inputs while maintaining a strong resemblance to traditional building systems, thus promoting their widespread adoption. These interventions are standardised and serve as the fundamental basis for the design and implementation of the installation.

As evidenced by extensive research and numerous studies, the highest energy yields from BIPV systems are generally achieved when installed on rooftops, due to the higher levels of solar irradiance in these areas [46,47,48,49]. As demonstrated through the various intervention proposals presented in this study, rooftops are the least susceptible to risks associated with shading effects from surrounding structures, which can otherwise detract from the efficiency of photovoltaic systems. Furthermore, due to their relatively low visibility, rooftop renovations, including the incorporation of RES, are typically less complex, even in buildings subject to heritage protection. In contrast, façades present greater challenges in integrating photovoltaic technology, primarily due to shading issues and the need for careful integration with the surrounding built environment. The construction method proposed for the installations remains consistent across all three typological clusters, intending to promote clarity and, given that all the buildings in question are residential, providing a cohesive solution that incorporates ventilated roofs and façades. This mounting system is versatile, adaptable to various morphological characteristics of the buildings, and characterised by ease of assembly and design. Moreover, the performance of the BIPV system is enhanced by the creation of a ventilation chamber between the supporting structure and the rear face of the modules, thus facilitating the ventilation of the rear surface of the modules [50]. Nevertheless, it is important to emphasise that rooftops are not the sole viable locations for BIPV installations—other potential areas include skylights, parapets, and sunshades, all of which present considerable opportunities for the integration of photovoltaic systems.

The successful implementation of BIPV systems necessitates the comprehensive involvement of a multidisciplinary team throughout the entire renovation process. The design and execution of photovoltaic installations of this magnitude require the seamless coordination of all project components, ensuring stringent adherence to pertinent legislation and standards at every phase. While this research establishes a foundational framework and serves as a key reference point for addressing BIPV systems, it is imperative to supplement it with the expertise of professionals from a diverse range of fields. Such interdisciplinary collaboration is critical to achieving an optimal implementation that effectively addresses the technical, urban, and policy dimensions inherent in the installation process [51].

In the context of implementing an energy renovation, a range of interventions are deemed appropriate for reducing electricity consumption within buildings, which must be executed concurrently with the integration of any RES during the renovation process. The most suitable construction solutions for reducing energy demand encompass the following: the enhancement of thermal insulation, both in floors and façades—interior and exterior—and on the interior surface of roofs; the replacement of original windows with those that ensure airtightness and proper insulation; and the modernisation of outdated heating, hot water, and ventilation systems, where applicable [36].

This study has focused primarily on photovoltaic systems in the context of RES employed in energy renovation, though other technologies such as solar thermal—one of the most widely used—geothermal, biomass, and wind energy are also prevalent. The deployment of any RES is advantageous both environmentally and in terms of reducing energy consumption within buildings. A comparison of these renewable energy solutions with BIPV systems demonstrates considerable advantages for the latter in consideration of the needs of contemporary society. BIPV is distinguished by its capacity for integration with the built environment due to its inherent flexibility, and it is the only technology that enables professionals to transform buildings into energy generators while preserving the formal characteristics of the urban fabric. Furthermore, it is among the few renovation interventions that are currently mature enough to generate on-site energy, along with BAPV and micro-wind energy, though the latter remains significantly less developed. Despite this, BIPV systems remain in the developmental phase, with a market that is less advanced, resulting in high costs, limited incentives to promote their adoption, and a lack of professionals with expertise in such installations.

From a scientific perspective, it is essential to continue advancing research and promoting the integration of BIPV installations to facilitate their widespread adoption. Currently, substantial resistance to these systems persists, both within the construction industry and among the general public. This resistance is predominantly attributed to a lack of sufficient knowledge and comprehension regarding the functionality and benefits of BIPV technology. To overcome this barrier, it is imperative to implement comprehensive educational initiatives aimed at fostering greater awareness of renewable energy sources and the far-reaching consequences of their inadequate implementation. Achieving this objective requires the incorporation of renewable energy education into foundational curricula, the provision of specialised courses tailored to the broader population, and the development of targeted training programmes at the tertiary level.

In the interest of transparency, it is imperative to address the limitations encountered during the research process to ensure the rigour of the analytical approach. The selection of parameters was conducted with the objective of attaining the most comprehensive representation of the residential building stock in Spain. While the classification system has been found to encompass most constructions, it is acknowledged that there are numerous specific cases whose architectural characteristics do not align with the defined classification criteria. The applied methodology, therefore, presents the inherent limitation of being unable to encompass the entirety of building typologies. Rather, it focuses on generating typological clusters that effectively illustrate prevailing patterns across the building stock. As a result, a quantitative analysis of all potential typological clusters has not been feasible. Similarly, certain cases may arise in which the proposed guidelines for BIPV system implementation cannot serve as a standardised approach, thereby necessitating bespoke solutions. Accordingly, a pertinent avenue for future research would be to refine and extend the classification framework to encompass a broader spectrum of building typologies, thereby facilitating the development of BIPV system proposals for newly identified clusters.

To illustrate this, the case study presented was selected to exemplify the application of the proposed action guidelines, starting with an initial analysis of its formal characteristics, followed by a proposal for BIPV system installations on both the façade and the rooftop. The formulation of these BIPV proposals was primarily guided by annual irradiance values, focusing the analysis on environmental conditions rather than delving into the technical specifications of the BIPV systems themselves. As no specific module model was defined for the proposed installation, it was not possible to conduct a detailed analysis from either an energy performance or economic perspective. The principal aim of this research is to enhance the visibility and promote the implementation of BIPV systems; therefore, in-depth technical and economic analyses were intentionally excluded from the scope of this study. This limitation highlights an opportunity for future investigations, where the proposed classification framework could be applied to facilitate comprehensive analyses of BIPV system performance and the technological components constituting such systems.

6. Conclusions

As the climate crisis intensifies, implementing effective strategies to mitigate greenhouse gas emissions has become a global imperative. The European Union has established ambitious targets for achieving carbon neutrality, with renovating existing buildings playing a key role in this transition. Upgrading the current building stock to meet modern environmental standards is essential for enhancing energy efficiency and reducing emissions.

Beyond conventional renovation strategies aimed at reducing energy consumption and improving indoor comfort, promoting the use of BIPV systems within these projects presents a significant opportunity. Grid-connected BIPV systems enable buildings to generate low-emission energy on-site, reducing reliance on grid electricity and thus contributing to a lower carbon footprint.

This research highlights the practical potential of BIPV systems. It provides a structured classification of residential building stock into 15 typologies, further consolidated into three overarching categories based on shared formal characteristics. This framework facilitates the development of adaptable BIPV solutions that can be customised to various building types, enhancing their feasibility for large-scale implementation.

The versatility of BIPV systems, as demonstrated in this study, highlights their adaptability to various architectural typologies while maintaining compatibility with conventional building methods. The proposed construction strategies, which incorporate ventilated façades and roofing systems, facilitate efficient BIPV integration with minimal material use and structural modifications. Notably, rooftop installations remain the most effective placement for photovoltaic elements due to their higher solar irradiation and reduced shading risks. However, other interesting integration opportunities, such as façades, parapets, skylights, and sunshades, should also be explored to maximise solar energy generation potential across different building configurations.

The findings of this study underscore the urgent need to accelerate energy renovations, not only to reduce energy demand but also to establish decentralised renewable energy generation at the local level. Integrating BIPV systems into renovation projects enhances energy security, supports the decarbonisation of the energy mix, and fosters a more decentralised and equitable energy landscape. Renovation initiatives that do not integrate local renewable electricity generation risk missing a crucial opportunity to accelerate progress toward zero-emission buildings, districts, and cities, promoting sustainable urban development.

From a policy perspective, this study highlights the need to integrate BIPV solutions into energy renovation regulations and incentive schemes. Promoting the widespread adoption of BIPV technology requires targeted financial support, streamlined permitting processes, and greater awareness among key stakeholders, including architects, policymakers, and building owners. In fact, ensuring the successful deployment of BIPV systems requires a multidisciplinary approach, encompassing technical, regulatory, and economic considerations. Additionally, fostering interdisciplinary collaboration among researchers, industry professionals, and policymakers is essential to addressing challenges such as high installation costs, a shortage of skilled professionals, and limited market maturity.

Future research should aim to expand the classification framework to include a broader range of building typologies, facilitating a more comprehensive approach to BIPV integration. Additionally, further studies on the technical performance, economic viability, and long-term benefits of BIPV systems will contribute to refining best practices and reinforcing the case for their large-scale adoption. Addressing these challenges, along with leveraging policy-driven incentives, will enable BIPV technology to play a transformative role in developing a resilient, energy-efficient, and carbon-neutral built environment.