**Flavio Rosa**

CITERA, Interdepartmental Centre for Territory, Building, Conservation and Environment, Sapienza University of Rome, 00137 Rome, Italy; flavio.rosa@uniroma1.it

Received: 13 June 2020; Accepted: 4 July 2020; Published: 14 July 2020

**Abstract:** In this work, we investigate the potential of using last generation photovoltaic systems in traditional building components of historical buildings. The multifunctional photovoltaic components also open new application and implementation horizons in the field of energy retrofitting in historical buildings. Some of the Building-Integrated Photovoltaics (BIPV) solutions lend themselves optimally to solving the problems of energy efficiency in historical buildings. For the next few years, Italian legislation foresees increasing percentages of energy production from renewable sources, including historical buildings. The opportunities and constraints analysed are presented through a specific approach, typical of building processes for innovative technological BIPV solutions on historical buildings.

**Keywords:** building-integrated photovoltaics—BIPV; building heritage; energy efficiency; traditional materials

#### **1. Introduction**

The heritage building stock of which Italy is particularly rich must be studied with an innovative approach for acceptable energy retrofitting solutions. In historical buildings, energy efficiency requirements, in synergy with conservation and protection, show methodological and operational limits that cannot be generalized.

Decarbonization policies framed by the European Union (EU) in the emissions reduction roadmap involve all the economic sectors, such as the civil and construction sector that includes heritage building stock [1]. The need to reduce the incidence of emissions in this sector has stimulated European legislators and international research centres to reduce energy requirements using nearly Zero Energy Building (nZEB) solutions [2–7].

The International Energy Agency (IEA) attributes over 40% savings in expected heating and cooling energy demands under a low-carbon scenario directly to improvements in the building envelope [8].

Driven by policies towards Zero-Energy Buildings and subsequently Plus Energy Buildings (PEB), design and innovation with new Building-Integrated Photovoltaic (BIPV) materials, concepts and combinations of energy-efficient building materials, with BIPV have become essential parts of the development strategies of both, the photovoltaic (PV) sector and the building sector [9–11].

The Strategic Energy Technology Plan (SET Plan) defines renewable technologies as being at the heart of the new energy system, with photovoltaic solar energy as the main pillar [12].

The heritage building stock can provide a contribution to decarbonization policies using Renewable Energy solutions (RES) installed on the building envelope with BIPV appropriate solutions. Most of the Italian heritage building stock does not present stringent protection restrictions as required by current regulations [13]. They can be classified as Traditional Historical Buildings (THB) that are not directly included in the Maintenance and Restoration category of the Code of Cultural and Landscape

Heritage [14]. The historical importance of the building is herein regarded in the function of the specific Italian post-unification period (1871–1942). Urban design, building types, construction techniques and the technology of the material used are not included in any category of particular interest or value to guarantee its safeguarding as foreseen by Cultural Heritage codes. In Rome, over 50% of the existing building stock falls under THB. In Europe, architectural and planning regulations for protected historical buildings lead to major technical constraints in integrating renewable energy, such as photovoltaics. These problems call for innovative and creative solutions for BIPV that must apply both aesthetic and photovoltaic technology to historical buildings that represent the artistic and cultural heritage of a city.

The Energy Performance of Buildings Directive (EPBD) prescribes the Member States to increase the number of buildings that not only fulfil current minimum energy performance requirements, but are also more energy-efficient, thereby, reducing both energy consumption and carbon dioxide emissions [15,16].

In Italy, Ministerial Decree 26 June 2015 completes the transposition of the European EPBD 2002/91/EC. This legislative measure comprises: (a) the application of calculation methodologies for energy performance and the definition of the minimum requirements for buildings; (b) requirements of nearly zero energy buildings; (c) sets the new minimum Energy Performance EP; (d) defines single component requirements to enter into force starting October 2015 [17].

Historical buildings represent a good reference paradigm by which we can investigate energy retrofit approaches and methodologies using BIPV solutions [18].

It is more difficult to apply an adequate retrofitting scheme on historic and/or listed public buildings, particularly in Italy, because there are strict regulations on the changes of this type of building. In the case of Rome, solar energy harvesting in Italian urban scenarios were analysed, by considering the geographical and morphological constraints with respect to the Sustainable Energy Action Plans (SEAP) [19].

The production of energy with solar systems as well as depending on local radiation is highly conditioned by the solar conversion technology adopted and the orientation of the modules. These factors are even more stringent on THB, as not all optimal exposures can be used for mounting modules due to the magnitude of impact. The THBs considered in this work can be assimilated to the provisions of the Annex 3A-B for the Built Heritage or Historic Urban Landscape of the International Council on Monuments and Sites ICOMOS Guidelines, where a grading scale for assessing the value and magnitude of the impact of Heritage assets is proposed. ICOMOS Guidance on Heritage Impact Assessments for Cultural World Heritage Properties [20,21].

In relation to the public heritage building stocks, it is possible to identify thresholds beyond which nZEB targeted non-reachability allows for a more defined vision of the issues, and to implement specific retrofitting strategies [22].

The requirements in the improvement of energy efficiency in cultural heritage buildings using active solar systems in historical constructions establishes the use of the general principles of restoration, including the reversibility and non-invasiveness of interventions on historical structures [23]. This approach can also be reworked for interventions on THBs, as illustrated below, starting from a holistic point of view and by examining the relationship between energy efficiency and preservation of the THB. This approach can also be reworked for interventions on THBs as illustrated below. PV technology in architecture has two types of solutions: Building Attached Photovoltaics (BAPV), in which the element is mounted on the casing using various techniques; and BIPV modules that form a building component and provide a function as described in EU regulation 305/2011, that defines the seven basic requirements for construction that have to be fulfilled, in addition to the electrical requirements [24]. PV Active Solar components are also defined as multifunctional as they must simultaneously perform functions so that the active architectural element of the building system produces and distributes energy [25–27]. In addition to being a source of electricity, several other purposes can be achieved, such as weather protection, thermal insulation, noise reduction and daylight modulation.

In recent years, BIPV applications in Europe have been applied to one-third of all renovation projects and two-thirds of new building construction [28]. The photovoltaic productivity on roofs and façades in Europe was estimated at around 1 TWp by 2030 [29]. For integrated solutions like BIPV, it has been calculated that Italy can reach 40% of the national electricity needs [30–32]. BIPV is a solution that can be adapted to the historical building stock to satisfy solutions calibrated on these particular buildings. When changes and adaptations to sustainability standards are proposed, even the slightest alterations, particularly external, can be damaging [33]. BIPV Solutions should be assessed as opportunities in the energy efficiency processes of historical buildings. At the same time, constraints must be assessed to mitigate the risks of impact on architectural and landscape heritage of merit and value.

The major problem of BIPV on THB solutions is the visual impact determined by the color of the photovoltaic cells that affect the level of the overall insertion on valuable casings and traditional materials. The color of a solar module is determined by the color of the cells in the module. [34]. Layering techniques to camouflage the PV element and solar damage to traditional materials, such as brick, tile and plaster developed in nanoscale, are in an advanced phase of the study and solutions are already on the market [35].

BIPV applications with high integration and almost no visual impact are already on the market [36]. Solutions for the treatment of glazed surfaces, Window-Integrated Photovoltaics (WIPV), with nanotechnologies are also very interesting [37]. The colored glass is selective, designed to reflect a narrow spectral band of visible light to provide color. The rest of the solar spectrum is transmitted to the solar device and converted into energy.

This duality in a historical building clearly influences the evaluation of materials and technologies, restricting solutions first and foremost to the aesthetic factor. The visual impact component conditions the materiality of the element that is firmly correlated to the materials with which they are made.

In this work, we want to investigate the opportunities and constraints of BIPV solutions in historical buildings, evaluate the potential applications in the regeneration of historical and/or traditional building heritage, without detriment to conservation and protection. Three macro-factors, technologies, market, and innovative solar-integrated solutions that describe state-of-the-art BIPV are analyzed beforehand. A final evaluation scheme of the opportunities and constraints of BIPV solutions on historical building enclosures is then proposed.

Then, a synthesis approach is defined to meet the needs of the energy retrofit, the protection and preservation of the historical building stock and the BIPV solutions, named Heritage Building Energy Solar Solution Technologies (hBESST).

The typical structuring modalities of the BIM-based building design process [38–40] and the correlations with BIPV solutions on historical buildings will be briefly analyzed.

#### **2. Materials and Methods**

#### *2.1. Historical Buildings and Traditional Heritage Buildings (THB) Retrofitting*

The relation between sustainability and heritage cannot be reduced to the mere energy efficiency of the buildings, simplifying a complex problem into an exclusive element of energy savings [41].

In Italy, over 7 million buildings are over 50 years old, which is equal to 61% of the building stock. From these, over 2 million, or 18%, are in a state of conservation ranging from mediocre to bad. The planned objectives of greenhouse gas (GHG) reduction in the construction sector, introduced by international and national regulations, must be compared to find economically sustainable solutions for each type of user.

The guidelines for improving energy efficiency in cultural heritage indicate that no solution can be considered in itself decisive and that the perspective, in which to move, can only be that which pragmatically proceeds on a case-by-case basis. The current approach is to reduce visual impact, using shields which obstruct the view of the modules to obtain a low magnitude of impact [21,42]. However, the approach cannot be simply one of concealment. Layering technologies allow overlapping layers, of which the last one is visible, made with finishes similar to historical and traditional building materials that hide the underlying photovoltaic element [43].

In principle, multi-layering techniques modify the appearance of a solar cell through the variation of the Anti-Reflection Coatings ARC [44]. The placement of stacks with large numbers of layers is a challenge, especially in a high-throughput, low-cost production environment [34]. *Energies* **2020**, *13*, x FOR PEER REVIEW 4 of 29

The processes used in manufacturing the new transparent PVs are environmentally friendly and not energy-intensive. The coatings are placed at nearly room temperature so the transparent PV can be laid on essentially any type of surface [45]. There is no need to use glass, which is costly in the manufacturing of conventional systems [46]. In principle, multi-layering techniques modify the appearance of a solar cell through the variation of the Anti-Reflection Coatings ARC [44]. The placement of stacks with large numbers of layers is a challenge, especially in a high-throughput, low-cost production environment [34]. The processes used in manufacturing the new transparent PVs are environmentally friendly and not energy-intensive. The coatings are placed at nearly room temperature so the transparent PV can

All Italian historical building stock, bound and not, will be increasingly subject to interventions of energy retrofitting in the coming years. In Italy, Legislative amendment 28 of 2011 in the implementation of Directive 20-20-20 (2009/28/EC) introduced the requirements for renewables in buildings (RES) in the event of major renovations. Regarding buildings that fall into the category of historical centres, as defined by the legislation [13], the percentage of RES coverage is expected to be 25% starting on 1 January 2018. It can also be obtained through technological combinations. be laid on essentially any type of surface [45]. There is no need to use glass, which is costly in the manufacturing of conventional systems [46]. All Italian historical building stock, bound and not, will be increasingly subject to interventions of energy retrofitting in the coming years. In Italy, Legislative amendment 28 of 2011 in the implementation of Directive 20-20-20 (2009/28/EC) introduced the requirements for renewables in buildings (RES) in the event of major renovations. Regarding buildings that fall into the category of

In the building sector, photovoltaic technology has taken over as a low-level integration technology element, with installations, usually adhering to the enclosure, known as Building Attached Photovoltaic (BAPV) [47,48]. historical centres, as defined by the legislation [13], the percentage of RES coverage is expected to be 25% starting on 1 January 2018. It can also be obtained through technological combinations. In the building sector, photovoltaic technology has taken over as a low-level integration technology element, with installations, usually adhering to the enclosure, known as Building

The problems of technology and protection integration are addressed as follows: "doing something about historical buildings with energy efficiency measures (obviously compatible with the cultural characteristics of the artefacts) is the first significant step for the real conservation of said heritage, so widespread, so fragile, so difficult and expensive to preserve [49]." In historical buildings, listed and not, the installation of photovoltaic systems and components has always had to deal with the visual impact on traditional materials and urban and landscape contexts. Attached Photovoltaic (BAPV) [47,48]. The problems of technology and protection integration are addressed as follows: "doing something about historical buildings with energy efficiency measures (obviously compatible with the cultural characteristics of the artefacts) is the first significant step for the real conservation of said heritage, so widespread, so fragile, so difficult and expensive to preserve [49]." In historical buildings, listed and not, the installation of photovoltaic systems and components has always had to deal with

The production of electricity from photovoltaic sources is one of the technological solutions that can be integrated into the building with plug and play solutions. In geographical areas, such as Southern Europe, the annual productivity in the face of well-exposed available surfaces allows for the coverage of high levels of electricity requirements: see Figure 1. the visual impact on traditional materials and urban and landscape contexts. The production of electricity from photovoltaic sources is one of the technological solutions that can be integrated into the building with plug and play solutions. In geographical areas, such as Southern Europe, the annual productivity in the face of well-exposed available surfaces allows for the coverage of high levels of electricity requirements: see Figure 1.

**Figure 1.** Theoretical PV production by nation (Source: 2018 snapshot of Global Markets—IEA PVPS)**. Figure 1.** Theoretical PV production by nation (Source: 2018 snapshot of Global Markets—IEA PVPS).

architectural property [50–53].

Energy THB retrofitting with BIPV solutions has been an important subject in an R and D

Energy THB retrofitting with BIPV solutions has been an important subject in an R and D laboratory of ideas, which are developed to create solutions for the support and guardianship of the architectural property [50–53]. *Energies* **2020**, *13*, x FOR PEER REVIEW 5 of 29

The increase in the production of electricity from solar sources is constantly on the rise thanks to progressively improved high-performance materials and technologies [54,55]. The active surfaces on the building envelopes, using efficient materials and technologies, will allow for an increase in the density of electricity from solar sources to single buildings, and in the single exposed surface of the casing. The energy density factor in BIPV is decisive in the choice of photovoltaic technologies to optimize the available surfaces concerning the production of electricity, as shown in Figure 2 [56–58]. The increase in the production of electricity from solar sources is constantly on the rise thanks to progressively improved high-performance materials and technologies [54,55]. The active surfaces on the building envelopes, using efficient materials and technologies, will allow for an increase in the density of electricity from solar sources to single buildings, and in the single exposed surface of the casing. The energy density factor in BIPV is decisive in the choice of photovoltaic technologies to optimize the available surfaces concerning the production of electricity, as shown in Figure 2 [56–58].

**Figure 2.** Power density (W/m2) BIPV modules by the installation**. Figure 2.** Power density (W/m<sup>2</sup> ) BIPV modules by the installation.

The use of integrated low visual impact solutions and the increased availability of electricity from RES will permit the use of the primary energy from the sun for the production of electricity with H2G (Hydrogen to Gas) hybrid solutions [59,60] and the possibility of achieving the objectives set for The use of integrated low visual impact solutions and the increased availability of electricity from RES will permit the use of the primary energy from the sun for the production of electricity with H2G (Hydrogen to Gas) hybrid solutions [59,60] and the possibility of achieving the objectives set for nZEBs.

nZEBs. The energy response (using a network of active and passive systems and technologies) from a historical building (or a "cultural landscape", using due caution in terminology), can be improved The energy response (using a network of active and passive systems and technologies) from a historical building (or a "cultural landscape", using due caution in terminology), can be improved through appropriate and well-balanced solutions [61].

through appropriate and well-balanced solutions [61]. Plant components have become an increasingly invasive part of the building-plant system since the Second World War [62,63]. Moreover, historical buildings have undergone the highest number of (sometimes damaging) alterations due to the inclusion of new plant solutions over the years as per Figure 3. Plant components have become an increasingly invasive part of the building-plant system since the Second World War [62,63]. Moreover, historical buildings have undergone the highest number of (sometimes damaging) alterations due to the inclusion of new plant solutions over the years as per Figure 3.

Technological infrastructures require technical spaces such as cavedium, traces, and niches that are often obtained without preliminary studies on stone and/or masonry bearing structures. Historical heritage in real estate, as buildings, must always be able to testify to their historical importance through all the material components that constitute it. Conservation can be implemented to all the activities and work carried out to control the conditions of the cultural property and maintain the integrity, functional efficiency and identity of the property and its parts [13] through appropriate maintenance methods.

With a view to an integrated process between HB retrofitting and solar energy solutions like BIPV, Heritage Building Information Modeling (HBIM) can offer solutions that allow framing information flows and critical factors. The application of BIM design process to existing buildings it will become mandatory in the coming years for all types of design especially in regards to maintenance and large refurbishments [40,64,65], but these applications do not contemplate the historical and cultural legacy of the buildings and sites [66].

The BIM approach is a 'workflow', in which all of the building objects that combine to make up the building design coexist in a single database. This concept is important because it allows us to

0.5%

23.0%

27.6% 31.0% 31.9%

75.9%

understand the entire building lifecycle (encompassing design, build and operation) from a single, central data storage unit. Therefore, in theory, a BIM implementation should facilitate a single, logical, consistent source of information associated with the building. from RES will permit the use of the primary energy from the sun for the production of electricity with H2G (Hydrogen to Gas) hybrid solutions [59,60] and the possibility of achieving the objectives set for nZEBs.

The use of integrated low visual impact solutions and the increased availability of electricity

6.0% 3.4% 3.4%

13.8%

4.6% 6.9%

**Figure 2.** Power density (W/m2) BIPV modules by the installation**.** 

0-50 W/m2 50-100 W/m2 100-150 W/m2 > 150 W/m2 Unvailable

71.9%

*Energies* **2020**, *13*, x FOR PEER REVIEW 5 of 29

BIPV power density W/m2 between installed power and occupied area

BIPV FACADE BIPV ROOF BIPV Module

The increase in the production of electricity from solar sources is constantly on the rise thanks to progressively improved high-performance materials and technologies [54,55]. The active surfaces on the building envelopes, using efficient materials and technologies, will allow for an increase in the density of electricity from solar sources to single buildings, and in the single exposed surface of the casing. The energy density factor in BIPV is decisive in the choice of photovoltaic technologies to optimize the available surfaces concerning the production of electricity, as shown in Figure 2 [56–58].

The definition of Level of Definition (LOD) within the workflow aimed at the HBIM, and specifically the PV components, is the first methodological step to be implemented. One of the first objectives in developing a BIM model of infrastructure is to define its LOD. Design phase outputs require a very detailed definition of project components to enable the user to extract automatic configurations of element quantities and related costs, and also to generate two-dimensional (2D) technical drawings [67]. The energy response (using a network of active and passive systems and technologies) from a historical building (or a "cultural landscape", using due caution in terminology), can be improved through appropriate and well-balanced solutions [61]. Plant components have become an increasingly invasive part of the building-plant system since the Second World War [62,63]. Moreover, historical buildings have undergone the highest number of (sometimes damaging) alterations due to the inclusion of new plant solutions over the years as per Figure 3.

**Figure 3.** Implementation of somewhat controversial photovoltaic systems on heritage buildings**. Figure 3.** Implementation of somewhat controversial photovoltaic systems on heritage buildings.

Technological infrastructures require technical spaces such as cavedium, traces, and niches that are often obtained without preliminary studies on stone and/or masonry bearing structures. Historical heritage in real estate, as buildings, must always be able to testify to their historical importance through all the material components that constitute it. Conservation can be implemented There are still gaps in the know-how that forestall insights into the future development of methods and tools of Historical Building Information Modelling for refurbishment projects. Therefore, it will prevent a complete automated diagnosis of the residual performances and designs of energy retrofitting using BIPV solutions.

to all the activities and work carried out to control the conditions of the cultural property and maintain the integrity, functional efficiency and identity of the property and its parts [13] through appropriate maintenance methods. With a view to an integrated process between HB retrofitting and solar energy solutions like BIPV, Heritage Building Information Modeling (HBIM) can offer solutions that allow framing information flows and critical factors. The application of BIM design process to existing buildings it In this work, we present a series of fundamental elements for energy retrofitting interventions on THBs that are strongly integrated with each other. The process presented requires active involvement of the stakeholders. The innovations in the design process of the BIM-HBIM is transformation in all this phases throughout the entire process: Design, management, maintenance, decommissioning. The multi- or trans-disciplinarily that governs energy retrofitting is even wider in the case of Built heritage or Historic Urban Landscape.

will become mandatory in the coming years for all types of design especially in regards to maintenance and large refurbishments [40,64,65], but these applications do not contemplate the historical and cultural legacy of the buildings and sites [66]. For the above mentioned, and, this paper proposes a holistic approach to the entire design process starting from the assessment of the conservative impact in relation to BIPV energy efficiency solutions on Built Heritage. This approach is presented with a preliminary integrated assessment of

central data storage unit. Therefore, in theory, a BIM implementation should facilitate a single,

logical, consistent source of information associated with the building.

technical drawings [67].

of energy retrofitting using BIPV solutions.

the case of Built heritage or Historic Urban Landscape.

The BIM approach is a 'workflow', in which all of the building objects that combine to make up

The definition of Level of Definition (LOD) within the workflow aimed at the HBIM, and specifically the PV components, is the first methodological step to be implemented. One of the first objectives in developing a BIM model of infrastructure is to define its LOD. Design phase outputs require a very detailed definition of project components to enable the user to extract automatic configurations of element quantities and related costs, and also to generate two-dimensional (2D)

There are still gaps in the know-how that forestall insights into the future development of methods and tools of Historical Building Information Modelling for refurbishment projects. Therefore, it will prevent a complete automated diagnosis of the residual performances and designs

In this work, we present a series of fundamental elements for energy retrofitting interventions on THBs that are strongly integrated with each other. The process presented requires active involvement of the stakeholders. The innovations in the design process of the BIM-HBIM is transformation in all this phases throughout the entire process: Design, management, maintenance, decommissioning. The multi- or trans-disciplinarily that governs energy retrofitting is even wider in

For the above mentioned, and, this paper proposes a holistic approach to the entire design process starting from the assessment of the conservative impact in relation to BIPV energy efficiency solutions on Built Heritage. This approach is presented with a preliminary integrated assessment of

the benefits for energy efficiency, heritage impact and intervention costs of, highlighting opportunities and constraints.

#### *2.2. BIPV Standardization*

The concept of a multi-functionality for a photovoltaic module poses a problem shared definition between technological or construction component.

It is important to remember that any use of PV modules must adhere to the specific standards in force in the country of use. The European standard EN 50583 for BIPV that applies to photovoltaic modules used as construction products, was published in 2016. This new standard consists mainly of the compilation and modification of existing standards related to BIPV. To be suitable for building integration, PV products must fulfil both the standards of the PV sector and the construction sector as presented in Table 1 [11,68].


**Table 1.** Electrical and building reference standards for PV modules [69].

The scope of EN 50583 is to implement technical requirements for photovoltaic modules used as building components that are subjected to both electrical standards; the Low Voltage Directives 2006/95/EC and IEC/CENELEC, and the European Construction Product Regulation 305/2011 [70].

The standard is divided into two parts: (1) Photovoltaics in buildings: Panels, two-part systems, based on three levels of differentiation as reported in Table 2:


**Table 2.** Levels of differentiations EN 50583.

Until the norm EN 50583-1/2 "Photovoltaics in Buildings" was published, BIPV modules and systems were not regulated through any unified European standard. PV panels mounted on the building envelope in either, one of the five mounting categories, stated in the EN 50583 standard, were thus treated both as an electrical component and a building product. However, the EN 50583 standard does not focus largely on BIPV modules. They are mentioned briefly in part 1 with regards to categorization dependent on the material (glass/polymer/metal sheet/other), and in part 2, with regards to categorization dependent on the installation method.

To be suitable for building integration, PV products have to fulfil both the PV sector and construction sector standards. Most of the time, the requirements for these standards are related to a country or a region [68].

In Italy, a definition of BIPV is given by the GSE Gestore Servizi Energetici (Energy Service Management). The photovoltaic model that is possible and effective for applications of an architectural type alone, is the building element itself [71]. This document extends particular relevance to the concept of integration for which the photovoltaic surface, together with the assembly system (in the case of a special component), replaces traditional building elements, and in addition to the production of electricity, guarantees the following functions typical of a building envelope: Water tightness, mechanical seal comparable with that of the replaced building element, and thermal resistance that does not compromise the performance of the building casing.

Building-Integrated Photovoltaics is classified as a new building system technology drive as established in the UNI 8290 standard [25]. The problem of the classification of photovoltaic models to be supplemented in the covering of a building has also been described from a fire protection perspective. This has highlighted a normative gap in all PV systems not recognized as construction products. This was dealt with in EN 15583.

A shared definition of the solutions for the integration of photovoltaic technologies on the building envelope are the following:


The action of harmonizing CENELEC and ISO regulations is the subject of a study conducted by the IEA Pvps Task 15 to subtask C to achieve:


The work on BIPV standardization has gained new impetus due to the decision by IEC/TC 82 to create a new Project Team (PT 63092) to prepare an international BIPV standard [72].

Building-Integrated Photovoltaics on historical buildings is an approach that must take into account the peculiarities of traditional materials and visual impact in a logic of high integration and reversibility.
