*2.3. Three Macro Factors for Building-Integrated Photovoltaics*

The reduction of consumption and the consequent lowering of the energy requirement is chosen based on high-efficiency plant elements.

The adoption of multifunctional BIPV innovative solutions and colored active PV components with high energy efficiency will allow remedying, or at least mitigate, the historical conflict between technological systems and the safeguarding and protection of the Architectural historical building.

Three factors determine favorable conditions for the development of BIPV as illustrated in Table 3: Technology, market and innovative solar-integrated technologies. The first two can be measured and evaluated directly. The third factor must be assessed by less deterministic formalities through a Building Management Approach for Heritage (BMAH). The first two are exogenous factors independent of the historical importance or otherwise the building and materials with which it was built. The third one must be analyzed with all its components. This analysis can be conducted through a case-by-case approach, but with a holistic methodology that respects the valorization and historical building energy retrofit.


**Table 3.** Macro factors of BIPV on historical buildings.

In constrained or THB buildings, BIPV is the bottom-up approach. The building process is based on inductive reasoning: from specific to universal, through analyses and solutions that must be adopted using an innovative methodology.

The three factors listed above are analyzed individually, highlighting their significance in the case of BIPV on heritage buildings.

## 2.3.1. Photovoltaic Technologies

Research in the PV field still focuses greatly on silicon-based materials. Since no single technology, either established or in development, offers benefits on all fronts, researchers recommend scaling up current silicon-based systems quickly. While, continuing to work on other technologies to increase efficiency, decrease materials used, and reduce the manufacturing complexity and cost [45]. PV classification is arranged by generations on the grounds of the evolution of the technologies [73]:


Emerging technologies encompass advanced thin films and organic solar cells (OSC). The latter is about to enter the market through niche applications.


Table 4 shows the targets foreseen by the Technology Roadmap for photovoltaics up to 2050.


**Table 4.** General technology target (Source: Technology Roadmap, solar photovoltaic energy, IEA 2010).

For the next few years, Crystalline Technologies will still represent a reference point, given their significant presence of maturity and technology on the market.

The main objectives of research on photovoltaic technologies are:


Many studies are addressing the need to synthesize BIPV solutions with nZEB references, highlighting that each new intervention, including energy efficiency, has to maintain the values mentioned, and further achieve suitable landscape integration within the urban context [7,78–81].

The availability of primary energy from PV provides a fundamental contribution to the net-zero energy balance that can be determined, either from the balance between delivered and exported energy or between load and generation. Import and export balance and the latter load/generation balance [82,83].

National strategies towards climate-neutral buildings have to reflect the (future) climate, the building standard and energy system as well as the associated (future) energy grid infrastructure. Individual strategies differ depending on the climate, the resources for renewable power in the grid and the heating and cooling grid infrastructures but they must keep in mind development towards a more networked European energy grid in the future (smart European grid) [4].

The visual impact of BIPV is a problem that encounters more difficulties in the solution due to the synthesis of issues to be addressed within the photovoltaic architecture of valuable envelopes. In the present work we provide a multidisciplinary approach that is considered unavoidable as a result of the convergence of issues regarding antithetical appearance.

#### 2.3.2. The Market

The cost of photovoltaic technologies is constantly changing. By 2025, the global weighted average levelized cost of electricity (LCOE) of solar photovoltaics (PV) could fall as much as 59% [55]. Moreover, together with the achievement of Grid Parity in Italy [78], they are a valid driver for the energy efficiency of the historical building stock [78,79,84,85].

Building-Integrated Photovoltaics is seen as one of the five major tracks for large market penetration of PV, in addition to price decrease, efficiency improvement, lifespan, and electricity storage [86].

The increase is justified through the rapidly plunging installation cost per watt; enhanced aesthetics of BIPV; improving the efficiency of c-Si modules as well as flexible thin-film panels, and the unabated desire among residential and commercial owners to "go green" and to reach the national energy efficiency target [87].

Building-Integrated Photovoltaics is still too costly, especially when compared to its rival technology, building-added PV (BAPV) since its added value as a multifunctional building element is only now beginning to be recognized [88].

It is expected that the BIPV/BAPV prize will induce the integration of new photovoltaic systems in protected historical urban districts by accelerating innovations in photovoltaic technologies and improving the architectural enhancement needed for sustainable use in European protected historical urban districts [89].

The growth opportunities for the BIPV market analyzed by the NREL [90–92] for the residential sector, are among the factors to be enhanced in the future through government support in maintaining historical/cultural building designs and through incentives.

The levelized cost of energy (LCOE) from PV systems is already below retail electricity prices (per-kWh charge) in several countries [93]. Innovations in the solar technologies field, with more and more politically efficient and appearance incentives, have introduced the Grid Parity including an LCOE between 200-300 USD/MWh in Italy.

LCOE of PV and BIPV are different. From 2010 to 2018 the BAPV LCOE in Italy suffered an 80% drop [94].

At the basis of this reduction is obviously 26% to 32% the drop-in prices of crystalline silicon modules. BIPV lowering of the LCOE is related to the reduction of costs in project management, due to the strong integration of BIPV in the building project, design and construction and performance cost reductions of technological components. For example, some savings derive from the elimination of the cost of BIPV mounting hardware.

The Energy Pay Back Time (EPBT) is the time it takes for the PV system to generate as much energy as has been used to produce it. High energy return on energy investment (EROI) corresponds to a short EPBT as reported in Table 5.


**Table 5.** Mean harmonized EPBT and EROI for multiple insulations in the Italian area.

An EPBT of one year and a life expectancy of 30 years corresponds to EROI of 30:1 [8]. EROI is simply a measure of the marginal amount of additional energy that certain technologies can provide in society for a given energy investment [95,96]. In the following table, the results of research, conducted in the systematic review, are re-established and meta-analysis of embedded energy, energy payback time and energy return on energy invested for the crystalline silicon and thin-film photovoltaic systems [97]. The results highlight, depends primarily on their embedded energy and not their efficiency.

The following table, elaborated by the author on data from Defne et al. [97,98], shows the relative results given the mean harmonized EPBT values for the following insolation (kWh/m<sup>2</sup> /yr): 1700 (Italian average), 1436 (Central Italy), and 2032 (Sicily in Southern Italy) for roof mounted PV.

A PV system with a multicrystalline module in Sicily has an EPBT of around one year. Considering a lifespan of 20 years this system can produce twenty times the energy needed to produce it [99,100].

#### 2.3.3. Innovative Solar-Integrated Technologies

Low impact integration on traditional elements of historical buildings, following the innovations on the single photovoltaic cells, opens new perspectives with important repercussions for the valorization and protection of the historical building heritage.

produce it [99,100].

2.3.3. Innovative Solar-Integrated Technologies

valorization and protection of the historical building heritage.

The innovative BIPV solutions rely on the multifunctional qualities that determine the level of integration on building envelopes, be they modern or historical [86,91,92,99,100]. The two factors define a specific "Photovoltaic architecture" approach as proposed by Antec Solar in Figure 4. innovative design and management solution for the entire workflow [101–106]. This analysis is not detailed in this paper and will be addressed in the second phase of research. The main components that determine the innovations and integration of BIPV solutions on historical buildings are shown below.

solutions. In this case, the BIM/BIPV approach on historical buildings and/or THB represents an

Building envelope characteristics; (2) function and performance; (3) product customization.

*Energies* **2020**, *13*, x FOR PEER REVIEW 12 of 29

A PV system with a multicrystalline module in Sicily has an EPBT of around one year. Considering a lifespan of 20 years this system can produce twenty times the energy needed to

Low impact integration on traditional elements of historical buildings, following the innovations on the single photovoltaic cells, opens new perspectives with important repercussions for the

The innovative BIPV solutions rely on the multifunctional qualities that determine the level of integration on building envelopes, be they modern or historical [86,91,92,99,100]. The two factors define a specific "Photovoltaic architecture" approach as proposed by Antec Solar in Figure 4.

Photovoltaic architecture is a discipline called to find innovative, energy sustainable solutions with a low magnitude impact on sensitive construction like THB. They demand more and more about the BIPV components to enable designers to develop appropriate design and effective integration [8]. The multi-functionality of the design must provide BIPV and THB solutions considering (1) Heritage

The various retrofitting levels on buildings testify to the cultural, historical and aesthetic values

Photovoltaic architecture is a discipline called to find innovative, energy sustainable solutions with a low magnitude impact on sensitive construction like THB. They demand more and more about the BIPV components to enable designers to develop appropriate design and effective integration [8]. The multi-functionality of the design must provide BIPV and THB solutions considering (1) Heritage Building envelope characteristics; (2) function and performance; (3) product customization. 2.3.4. Multifunctional PV Solutions If BIPV solutions are designed and installed to provide multifunctional solutions like solar components with low impact on traditional materials on THB envelopes. They must also guarantee

The various retrofitting levels on buildings testify to the cultural, historical and aesthetic values that need to be implemented through an appropriate workflow of knowledge, information and solutions. In this case, the BIM/BIPV approach on historical buildings and/or THB represents an innovative design and management solution for the entire workflow [101–106]. This analysis is not detailed in this paper and will be addressed in the second phase of research. The main components that determine the innovations and integration of BIPV solutions on historical buildings are shown below. results and performances in the area of valorization and conservation of cultural architectural heritage. The steep increase in the levels of production of innovative materials opens new avenues to materials, technologies and solutions in the Construction Management Process (CMP) even on historical buildings. The field of nanotechnologies seems to be the most promising, since it is expected that active photovoltaic supports will reach the thickness of paint [45,107–109].

**Figure 4.** Multi-color solar wall Antec Solar headquarters in Arnstad Germany (source: [37]). **Figure 4.** Multi-color solar wall Antec Solar headquarters in Arnstad Germany (source: [37]).

#### 2.3.4. Multifunctional PV Solutions

If BIPV solutions are designed and installed to provide multifunctional solutions like solar components with low impact on traditional materials on THB envelopes. They must also guarantee results and performances in the area of valorization and conservation of cultural architectural heritage.

The steep increase in the levels of production of innovative materials opens new avenues to materials, technologies and solutions in the Construction Management Process (CMP) even on historical buildings. The field of nanotechnologies seems to be the most promising, since it is expected that active photovoltaic supports will reach the thickness of paint [45,107–109].

However, the BIPV concept raises the likelihood that a thin layer of PV-active material, possibly lain as a paint, could become a standard feature of building elements, such as roofing tiles, façade materials, glass, and windows, just as double-glazed windows have become standard in most countries [8].

There are photovoltaic films already on the market, which allow the glazed components of the enclosures to be active solar surfaces [110]. Power generation, through window coatings, is a relatively new idea, and is based on the use of semi-transparent solar cells as windows [77]. Transparent PV additive technology has a limited mechanical flexibility is a high cost of the modules [111]. Flexible or light-weight PV modules are also part of R and D efforts. BIPV applications are expected to become a major market for flexible PV modules [55]. In the area of walkable materials, solutions are available on the market, which includes the solarization of roads, floors and terraces [112]. Some solutions, like

buildings.

countries [8].

countries [8].

those by Onyx Solar in the Figure 5, still represent a solution for THB because the invisibility of the intervention also allows chromatic solutions not typical of historical materials. Solvable problems with external finishes and colors more similar to those of historical buildings. solutions are available on the market, which includes the solarization of roads, floors and terraces [112]. Some solutions, like those by Onyx Solar in the Figure 5, still represent a solution for THB because the invisibility of the intervention also allows chromatic solutions not typical of historical materials. Solvable problems with external finishes and colors more similar to those of historical [112]. Some solutions, like those by Onyx Solar in the Figure 5, still represent a solution for THB because the invisibility of the intervention also allows chromatic solutions not typical of historical materials. Solvable problems with external finishes and colors more similar to those of historical

expected to become a major market for flexible PV modules [55]. In the area of walkable materials,

solutions are available on the market, which includes the solarization of roads, floors and terraces

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

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

However, the BIPV concept raises the likelihood that a thin layer of PV-active material, possibly lain as a paint, could become a standard feature of building elements, such as roofing tiles, façade materials, glass, and windows, just as double-glazed windows have become standard in most

However, the BIPV concept raises the likelihood that a thin layer of PV-active material, possibly lain as a paint, could become a standard feature of building elements, such as roofing tiles, façade materials, glass, and windows, just as double-glazed windows have become standard in most

There are photovoltaic films already on the market, which allow the glazed components of the enclosures to be active solar surfaces [110]. Power generation, through window coatings, is a relatively new idea, and is based on the use of semi-transparent solar cells as windows [77]. Transparent PV additive technology has a limited mechanical flexibility is a high cost of the modules

There are photovoltaic films already on the market, which allow the glazed components of the enclosures to be active solar surfaces [110]. Power generation, through window coatings, is a relatively new idea, and is based on the use of semi-transparent solar cells as windows [77]. Transparent PV additive technology has a limited mechanical flexibility is a high cost of the modules

**Figure 5.** Walkable photovoltaics solution for the flat roof (source: Onyx Solar). **Figure 5.** Walkable photovoltaics solution for the flat roof (source: Onyx Solar). **Figure 5.** Walkable photovoltaics solution for the flat roof (source: Onyx Solar).

Interestingly, the PVT prototype developed by Greppi et al. [113] for a new hybrid solar panel, as per Figure 6, can be used as a tile to pave driveways, areas, and terraces and to cover roofs due to its particular robustness and compactness. The main feature, that characterizes the new hybrid tile, is its walkability and the simple laying procedure. Interestingly, the PVT prototype developed by Greppi et al. [113] for a new hybrid solar panel, as per Figure 6, can be used as a tile to pave driveways, areas, and terraces and to cover roofs due to its particular robustness and compactness. The main feature, that characterizes the new hybrid tile, is its walkability and the simple laying procedure. Interestingly, the PVT prototype developed by Greppi et al. [113] for a new hybrid solar panel, as per Figure 6, can be used as a tile to pave driveways, areas, and terraces and to cover roofs due to its particular robustness and compactness. The main feature, that characterizes the new hybrid tile, is its walkability and the simple laying procedure.

The problem of laying in historical buildings subject to difficult retrofitting must be fully assessed also with regards to the additional loads that are applied to the underlying structures and the technical bulky structures. The problem of laying in historical buildings subject to difficult retrofitting must be fully assessed also with regards to the additional loads that are applied to the underlying structures and the technical bulky structures. The problem of laying in historical buildings subject to difficult retrofitting must be fully assessed also with regards to the additional loads that are applied to the underlying structures and the technical bulky structures.

**Figure 6.** Photovoltaic cells, heat sink enclosed in transparent and opaque resins with on left hydraulic and electrical connectors [113]. **Figure 6.** Photovoltaic cells, heat sink enclosed in transparent and opaque resins with on left hydraulic and electrical connectors [113]. **Figure 6.** Photovoltaic cells, heat sink enclosed in transparent and opaque resins with on left hydraulic and electrical connectors [113].

Organic solar technologies seem to be a very promising cell-based solution. The 6% efficiency is, of course, a big minus, however, through the continued development in technology and research, its efficiency is gradually increasing. One of the advantages of organic cells is that, when compared to CIGS cells (copper indium gallium selenide solar cell), organic cells don't require rare elements such as Indium.

The lifespan of these solar modules is approximately 15 years. This technology is expected to be a bit less durable than glass modules, but a 15-year lifespan is not bad at all [114].

#### *2.4. Energy E*ffi*ciency*

The main objective of the research in the photovoltaic sector is to achieve a high solar energy conversion efficiencies using widely available raw materials, and sustainability both, from an economic and environmental point of view.

As reported in Table 6, the growth and development forecasts for the next few years of photovoltaic technologies are still very much linked to Silicon-based components.

As already mentioned above, the main limitation of BIPV on the listed and THB is the visual impact; mainly the PV cell.

**Table 6.** Technology goals and key R and D issues for crystalline silicon technologies (Source: Technology Roadmap, solar photovoltaic energy, IEA 2010).


#### *2.5. Colored Cells and Glazing Modules*

One of the main problems in BIPV solutions on traditional materials are factors, related to module/cell color, pattern, texture, and visible materials [115].

The concept of a low impact solution for photovoltaic components on historical materials is based on preserving the history and value of the product, and the final result obtained. Reversibility to the original state is fundamental if the BIPV solutions are replaced or removed.

The first and most important element pointed out by market surveys was the need to change the color of the PV module from the blue-purple tones of standard technologies to a more terra-cotta like color matching traditional roofing materials [116].

The need to make photovoltaic components "invisible" on building envelopes is a very advanced research objective. The color of the external finish is one of the main obstacles of BIPV solutions.

Reflection losses limit all types of photovoltaic devices. The first reflection loss occurs at the glass-air interface of the photovoltaic module. If no light trapping mechanism is used, about 4% of the solar energy is lost on this surface [117]. The most commonly used techniques in reducing reflection, include texturing of the glass surface and the application of an anti-reflective coating (ARC). The simplest ARC consists of a single layer of refractive index matching material [118]. High quality anti-reflection (AR) coatings have become a vital feature of high-efficiency silicon solar cells. Solar cell efficiency can be improved by antireflection gratings. The silicon material has a high refractive index. Understanding how to develop broadband and omnidirectional antireflection is a key technology for increasing solar energy efficiency [119].

Most of these ARCs are manufactured by film deposition techniques such as chemical vapour deposition (CVD), sputtering, or evaporation, as well as possible lithography steps on perovskite solar cells [120].

Therefore, RCs are of great importance in improving the efficiency of the solar cell by reducing loss due to reflection. ARCs containing a single layer can be non-reflective only at a single wavelength, generally up to the middle of the visible spectrum. Whereas, ARCs containing double layers are effective over the entire visible spectrum [121].

The predominantly dark color of the photovoltaic cells is designed to reflect as little light as possible. This way the solar cell will produce the maximum power output. The color of the solar cells can be changed by varying the thickness of the anti-reflection coating. By reducing the thickness of the anti-reflection layer, the overall reflection will increase and the efficiency decrease by 15–30%, depending on the color.

Using antireflection coatings (ARCs) processes suitable for large-scale manufacturing, many authors have explored new materials and process modifications to systematically reduce conversion losses in devices manufactured on low-cost glass substrates [122].

More precisely, colored modules with high saturation, angle independent color appearance, and a minimized color, induced targeted efficiency loss. Homogeneous colored layers have a high transmittance in wavelength range with non-negligible spectral responsivity of the PV-cells [123].

BIPV latest generation solutions allow access to solutions with high integration and low visual impact even on historical buildings enclosures.

The technological solutions in the field of colored and transparent solar technologies represent one of the most promising for application on historical buildings. The colored solutions, including different crystalline Si cells and Thin-film technologies have been available on the market for several years.

The coloring of the active PV component and a specific design process offer opportunities and innovative BIPV solutions that can also be implemented on valuable enclosures.

There are two very promising and innovative solutions for heritage building envelopes: transparent solar technologies and selective materials, with low molecular density solar radiation that reproduce the components and colors of traditional material.

Further advances have been made in applied research on the color of materials used in the production of photovoltaic cells. Market projections predict that in 2020 the price of these modules will decrease by 20% concerning 2013.

The adoption of innovative technologies for colored photovoltaic cells with a high energy yield will make it possible to correct, or at least mitigate, the historical conflict of technological systems versus safeguarding and preserving the historical and architectural heritage.

#### *2.6. Glazing*

The glazing application on a THB envelope represents one of the more interesting opportunities in energy retrofitting solutions. The minimum glazing available surface on a building is approximately 30%. The potential of these solutions on historical buildings from an nZEB perspective offers a vast scope of very interesting opportunities in future years when the current more promising solutions will have acceptable EPBT. Window-integrated Photovoltaic (WIPVs) is a system that generates electrical energy from glass and windows [124].

#### *2.7. Light Selective Compound for Traditional Materials (multilayering)*

Any alternative energy technology that is supposed to address the problem of energy sufficiency and security and climate change must be: Simple, easily scalable and inexpensive.

This results from the controlled and coherent integration of the solar collectors simultaneously from all functional, constructive, and formal (aesthetic) points of view [115].

These so-called "invisible" solutions are available on the market [23]. The silicon cells are completely embedded in the body of the same as these are made of polymeric compounds loaded with natural powders. The result is that they are similar in appearance to traditional materials, and at the same time, allow sunlight to filter through the outer surface (as if it were transparent) and reach the photovoltaic cells.

The principle is stratification (layering) achieved by superimposing low molecular density polymeric materials over the PV cell [35]. In this case, the layering technique does not concern the cell but the external finish, with particular optical properties that allow camouflaging the tile on the components of the building envelope.

The single roof coppo-tile, as per Figure 7, has a power of 4.5 Wp. To obtain a power of 1 kWp, 223 roof tiles are necessary for a surface of 15 m<sup>2</sup> . Technological innovations prove that multifunction optics open up very interesting solutions. It is possible to propose low impact BIPV solutions, not

only on roofs, but also on architectural technological units and historical, as well as traditional building materials. optics open up very interesting solutions. It is possible to propose low impact BIPV solutions, not only on roofs, but also on architectural technological units and historical, as well as traditional building materials.

223 roof tiles are necessary for a surface of 15 m2. Technological innovations prove that multifunction

*Energies* **2020**, *13*, x FOR PEER REVIEW 16 of 29

The principle is stratification (layering) achieved by superimposing low molecular density polymeric materials over the PV cell [35]. In this case, the layering technique does not concern the cell but the external finish, with particular optical properties that allow camouflaging the tile on the

**Figure 7.** PV coppo with a polymeric compound developed to encourage the photon absorption (source [35]). **Figure 7.** PV coppo with a polymeric compound developed to encourage the photon absorption (source [35]).

A reference model of BIPV solutions is proposed as functional for technologies, visual impact, A reference model of BIPV solutions is proposed as functional for technologies, visual impact, the level of integration per architectural unit and potential integration on traditional materials.

the level of integration per architectural unit and potential integration on traditional materials. Solutions that can have win-win a result on the extended historical buildings stock not subject to restrictions and in non-prestigious urban areas. Such an extension of active solar surfaces contributes to an increase in the production of electricity from a primary source for each building beyond certain protection constraints. Some architectural elements of historical buildings, such as cornices, terraces, vertical walls and traditional roofs in Roman tile, are presented in this paper for their photovoltaic solarization. Solutions that can have win-win a result on the extended historical buildings stock not subject to restrictions and in non-prestigious urban areas. Such an extension of active solar surfaces contributes to an increase in the production of electricity from a primary source for each building beyond certain protection constraints. Some architectural elements of historical buildings, such as cornices, terraces, vertical walls and traditional roofs in Roman tile, are presented in this paper for their photovoltaic solarization.

#### **3. Results 3. Results**

The availability of BIPV multifunctional components opens up the possibility of low-visual impact energy retrofit interventions on the heritage buildings stock. The availability of BIPV multifunctional components opens up the possibility of low-visual impact energy retrofit interventions on the heritage buildings stock.

The limits of integration are not of a technological or aesthetic nature to respect the historical value of the enclosures. Given the current levels of integration achieved, in the present work, an analysis was carried out comparing BAPV and BIPV approaches for heritage buildings to highlight opportunities and constraints. This analysis provides an effective means of 'mapping' the current situation and identifying chances for future developments. In the present work, practical involvement is explored not only from a holistic point of view but The limits of integration are not of a technological or aesthetic nature to respect the historical value of the enclosures. Given the current levels of integration achieved, in the present work, an analysis was carried out comparing BAPV and BIPV approaches for heritage buildings to highlight opportunities and constraints. This analysis provides an effective means of 'mapping' the current situation and identifying chances for future developments.

also to examine the relationship between supportability and conservation in the field of the energy efficiency applied to the historical building property as per Figure 8. In the present work, practical involvement is explored not only from a holistic point of view but also to examine the relationship between supportability and conservation in the field of the energy efficiency applied to the historical building property as per Figure 8.

The solutions supplied by BAPV to BIPV on historical buildings need an innovative methodological approach that involves guardianship organs, seekers and planners for a reverse photovoltaic design. In this work, we propose the Heritage Building Energy Solar Solution Technologies (hBESST) as a synthesis approach to meet the needs of the energy retrofit, the protection and preservation of the historical building stock and the BIPV solutions.

A solution to the integration of integrated solar technologies on a historical building is proposed in Table 7 through various levels; from a high-impact BAPV to a low-impact BHESST concerning the opportunities and constraints exposed above.

The holistic approach frames define and quantify the opportunities and constraints that emerged using a simple 3-value scale of Low (\*), medium (\*\*) and High (\*\*\*). Subsequently, to provide typical indications of the building process, through BIPV design solutions and practical implementation, it was possible to define an initial example on how to intervene on typical architectural elements of THBs already available on the market.

*Energies* **2020**, *13*, x FOR PEER REVIEW 17 of 29

**Figure 8.** (Source: www.lofsolar.com) and Perovskite transparent PV cell. **Figure 8.** (Source: www.lofsolar.com) and Perovskite transparent PV cell.

The solutions supplied by BAPV to BIPV on historical buildings need an innovative methodological approach that involves guardianship organs, seekers and planners for a reverse **Table 7.** Approaches and criteria within the Heritage Building Management Process for different BIPV solutions.


**Table 7.** Approaches and criteria within the Heritage Building Management Process for different 3-value scale: Low (\*), Medium (\*\*), High (\*\*\*).

BIPV solutions. **Criteria Approach BIPV-THB Building Management Process Project Management BIPV Design Construction Visual Impact Codes and Regulation**  The methodology of the building process for heritage buildings in the field of energy retrofit shows clear limits of replicability given the specificity of each case on which to intervene. However, this does not mean that we can generalize the process with the detriment of the principles of protection and safeguarding, by identifying three objectives: BIPV, Heritage Buildings (Hbuil) and nZEB.

**BAPV**  Within the hBESST process, three BIPV, nZEB and hBuil targets were defined and four interactions were identified, as shown in Figure 9.

**Building Attached (Applied/Added) PV**  \* \* \* \*\*\* \*\*\* **BIPV**  Point 4 represents the maximum number of integration conditions required to establish a structured process that goes from the needs of energy sustainability up to the development phase on the scale of the single architectural element on historical buildings, including THB.

**Building Integrated PV**  \*\* \*\* \*\*\* \*\* \*\*\* **BIPPV**  For each interaction, through a holistic approach, the hBESST has made it possible to highlight 23 factors that can be assessed according to opportunities and constraints as reported in Table 8.

**Building Integrated Project**  \*\*\* \*\*\* \* \* \*\* The following table provides comparative solutions for laying surfaces by evaluating BIPV and hBESST solutions.

**PV hBESST**  The technologies are presented based on PV technology, the efficiency of electricity production and annual producibility per 1 m2 installed.

**Building Heritage Energy Solar Solutions Technology**  \*\*\* \*\*\* \* \* ? 3-value scale: Low (\*), Medium (\*\*), High (\*\*\*) Table 9 shows how some hBESST solutions on building envelope components on historical buildings can be developed with technologies already available on the market. For the glazing and WIPV solutions, market solutions with acceptable EROI and EPBT are not yet available. In 2030, the solarization of glazed surfaces is expected to reach levels of economic acceptability.




3-value scale: Low (\*), Medium (\*\*), High (\*\*\*).



3-value scale: Low (\*), Medium (\*\*).

**Table 9.** Comparison between traditional BIPV and BHESST solutions on historical building envelopes and technological and visual impact parameters.

interactions were identified, as shown in Figure 9.

The methodology of the building process for heritage buildings in the field of energy retrofit shows clear limits of replicability given the specificity of each case on which to intervene. However, this does not mean that we can generalize the process with the detriment of the principles of protection and safeguarding, by identifying three objectives: BIPV, Heritage Buildings (Hbuil) and

**Figure 9.** Four interactions between three objectives to obtain the heritage building management **Figure 9.** Four interactions between three objectives to obtain the heritage building management system hBESST.

system hBESST. Point 4 represents the maximum number of integration conditions required to establish a structured process that goes from the needs of energy sustainability up to the development phase on A bottom-up approach was adopted which, starting from the level of integration on the historical building component, has allowed formulating interactions and factors related to energy retrofit on the enclosures of historical buildings [86].

the scale of the single architectural element on historical buildings, including THB. For each interaction, through a holistic approach, the hBESST has made it possible to highlight 23 factors that can be assessed according to opportunities and constraints as reported in Table 8. Figure 10 shows the bottom-up approach concerning the hBESST and hBMP objectives. *Energies* **2020**, *13*, x FOR PEER REVIEW 21 of 29

**Figure 10.** Bottom-up approach schematization of the hBESST procedure.

**Figure 10.** Bottom-up approach schematization of the hBESST procedure. Related adoptions have been assessed and incorporated in this work.

photovoltaics

• Fitted solutions

• High RES production

• High respect for traditional materials

• Designing the PV as a building material with high architectural integration that meets technical, functional and aesthetic

• Enabling mass realization of nZEB by BIPV • Develop BIPV module and system design concepts that enable fast and highly

between the photovoltaic industry and the

• HBIM process in an innovative workflow

requirements at acceptable costs

• Innovation and integrated solutions

automated installation,

building industry.

process

**hBESST** 

Related adoptions have been assessed and incorporated in this work. Table 10 shows the main opportunities and constraints of hBESST regarding know-how, training and economic barriers, and to a lesser extent, technological barriers. The need to carry out case studies with innovative and low impact solutions starting from the unconstrained historical building stock Table 10 shows the main opportunities and constraints of hBESST regarding know-how, training and economic barriers, and to a lesser extent, technological barriers. The need to carry out case studies with innovative and low impact solutions starting from the unconstrained historical building stock can also accelerate overcoming barriers linked to codes and conservation rules.

can also accelerate overcoming barriers linked to codes and conservation rules. **Table 10.** Compare BIPV and hBESST and related opportunities and constraints. The process outlined in this paper can be implemented on that part of a historical building stock, such as THB in urban contexts, without constraints and architectural and landscape protection. An hBESST approach allows you to tackle solutions with a low visual impact on traditional materials and surfaces.

 **Opportunities Constraints BIPV**  • Larger collection surface • High integration • Smart grids will increase the use of • Additional complexity in building design • More complex system design • Visual impact • Lack of cooperation in the design workflow Heritage BIM solutions and applications for energy retrofitting, and in particular for BIPV solutions, represent a huge potential for numerous researches. HBIM applied to the specificity of THB defines a new relationship between BIPV, traditional materials and optimization of a workflow, where BIPV, nZEB and the Heritage/Historical buildings represent an opportunity and not a constraint.

• Higher costs compared to market alternatives

• Crossing barrier on regulations, technologies

• Limited availability on the market of highly integrated BIPV solutions on traditional

• Strengthen research, development and demonstration (RD&D) efforts for BIPV to

• Transform the niche market of BIPV into a

• LOD define for specific Heritage on PV materials in BIM/HBIM design process

and technological solutions

• Define a specific business model

historical building materials • Few realizations of BIPV on historical

further reduce costs.

buildings

mass market

The BIM informative process is classified in seven levels of which the first three are linked to the geometry of the objects while 4D is applied to scheduling, 5D is applied to estimation, 6D is applied to sustainability and 7D is applied to facility management applications. The energy component turns out to be transversal for all seven levels but determining for levels from 4D to 7D.

The BIM and HBIM solutions in a multilevel and interoperable digitization process represent obligatory paths as they allow for the analysis and evaluation of hBESST choices, thereby facilitating the choice at all levels of design for BIPV solutions.

The constraints concern the LOD definition, the parametric digitization of all the BIPV components within the typical workflow of the BIM processes.

This first phase of the research will be integrated later in the second phase with an evaluation procedure on the components of BIPV on historical buildings through votes and opinions formulated by experts in the field.


**Table 10.** Compare BIPV and hBESST and related opportunities and constraints.

#### **4. Discussion**

The process outlined in this paper can be implemented on that part of a historical building stock, such as THB in urban contexts, without constraints, architectural and landscape protection. An hBESST approach allows you to tackle solutions with a low visual impact on traditional materials and surfaces.

Heritage BIM solutions and applications for energy retrofitting, and in particular for BIPV solutions, represent a huge potential for numerous researches. HBIM, applied to the specificity of THB, defines a new relationship between BIPV, traditional materials and optimization of a workflow where BIPV, nZEB and the Heritage/Historical buildings represent an opportunity and not a constraint.

The BIM informative process is classified in seven levels of which the first three are linked to the geometry of the objects while 4D is applied to scheduling, 5D is applied to estimation, 6D is applied to sustainability and 7D is applied to facility management applications. The energy component turns out to be transversal for all seven levels but determining for levels from 4D to 7D.

The BIM and HBIM solutions in a multilevel and interoperable digitization process represent obligatory paths as they allow for the analysis and evaluation of hBESST choices, thereby facilitating the choice at all levels of design for BIPV solutions.

The constraints concern the LOD definition, the parametric digitization of all the BIPV components within the typical workflow of the BIM processes.

This first phase of the research will be integrated later in the second phase with an evaluation procedure on the components of BIPV on historical buildings through votes and opinions formulated by experts in the field.

## **5. Conclusions**

The present work aims to investigate the opportunities and to outline the limits and the opportunities that BIPV design approaches offer during architectural and energetic upgrading interventions on historical buildings. The interactions of the three main objectives, BIPV, Hbuil and nZEB, have been analyzed using traditional Heritage Building Solar Solutions Technologies (hBESST). Subsequently, through a bottom-up process for individual materials and PV technologies on enclosures of historical buildings, we defined a Heritage Building Energy Solar Solution Technologies (hBESST) as the first approach of synthesis to the needs of the energy retrofit, BIPV solutions, and protection and preservation of the historical building stock.

The challenge in hBESST design is to find the best combination of design strategies that will face the energy performance problems of a THB and future listed buildings.

The PV's maximization producibility on a historical building property cannot be the only methodological reference. The technical solutions are shown in the present paper. An hBESST approach can be defined if, for example, cornice solarization and roof floors are opportunely inserted into project integration management. Integration interventions on other traditional materials can be designed to keep the natural balance and protect the landscape, the historical patrimony and the energetic sustainability. The latest generation PV components, colored and highly invisible, open very interesting design perspectives.

The results highlighted have been submitted to give indications and cues in evaluating the limits that RES solutions, and in this case BIPV, can present on the historical building stock.

**Funding:** This research received no external funding

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
