4 = Corresponding author: Elena Lucchi, Eurac Research, Institute for Renewable Energy, Via A.Volta 13/A,
39100 Bolzano, IT, firstname.lastname@example.org
A conceptual framework on the integration of solar energy
systems in heritage sites and buildings
E Lucchi1,4; C S Polo Lopez2; G Franco3
1 Eurac Research, Institute for Renewable Energy, Via A.Volta 13/A, 39100 Bolzano, IT
2 University of applied sciences and arts of southern Switzerland, Department for environment
construction and design Institute for applied sustainability to the built environment, Canobbio, CH
3 University of Genoa, Architecture and Design Dep., Stradone S. Agostino 37, 16123 Genova, IT
Abstract. The integration between solar energy systems and building components is highly
critical in sensitive heritage contexts. On the one hand there is the need for finding a balance
between the preservation of the aesthetic appearance and the historical values, but on the other
hand, finding the space where to effectively integrate the systems might be quite challenging.
The solar systems can be divided in photovoltaic (PV) and solar thermal (ST) systems. Building
Integrated Photovoltaics (BIPV) and Building Integrated Solar Thermal (BIST) are PV or ST
panels integrated into the building envelope, combining the energy generation with other
functions, such as noise, weather protection, thermal insulation, sun shadow, and other aspects.
Nowadays, the dynamism of the market allows to design highly compatible products which look
like traditional architecture materials. This situation fosters the integration of these products in
the BIPV and BIST systems within the heritage sites, especially thanks to the use of advanced
customisation processes, special and low-reflecting glasses, and innovative cost-competitive
coatings. There is a limited number of studies on the application of these technologies in heritage
contexts, due to the presence of architectonic, conservative, and cultural barriers. This paper aims
to conduct a comprehensive review of the available literature on the integration of renewable
energy sources (RES) in heritage sites and buildings, which would foster the preservation of their
cultural and natural values as well as reducing primary energy consumption, increasing comfort
levels, minimizing environmental impacts, and improving technical quality and economical
outlays. A common framework will thus defined to support restorers, historic conservators, and
energy experts and to facilitate the diffusion and application of RES in heritage contexts. This
conceptual framework will provide industries and academics with operative strategies and will
encourage their diffusion and application in sensible contexts.
Historical buildings constitute a considerable part of the European stock. In numerical terms, more than
40% of the residential houses have been constructed before the 1960s  and more than 50% before the
1970s . The constructions built before 1945 and worthy of being preserved represent 30-40% of the
whole EU stock . The EU households contribute to 68% of the total final energy use in houses, which
is mainly related to heating, cooling, hot water, cooking, and appliances . Existing buildings are the
main cause of 30% of solid waste production, of about 35% of the total emission of pollutant emission
in atmosphere, and 39% of the global energy consumption . Most of these buildings were built in the
past when proper technologies were not yet available. Some of them fell into disuse or have been
abandoned. Energy-retrofit interventions aimed at the reuse of such spaces can provide comfort to their
occupants . On 30 May 2018, the European Parliament issued the Directive (EU) 2018/844 on the
energy performance of buildings, introducing a specific legislative framework to cut carbon dioxide
emissions (hereinafter, CO2 emissions) by 2020, to increase the share of renewable energy sources
(hereinafter, RES), and to enhance the energy performances of existing buildings . These exalted
objectives seem difficult to achieve when renovating heritage buildings, where the energy saving
measures are hindered by the restrictions on preservation [5; 6]. Although it is not always possible to
comply with the current energy standards, trying to improve as much as possible the energy efficiency
of heritage buildings is considered compulsory and essential . RES can play a fundamental role in
achieving this goal, allowing a rational use of energy (hereinafter, RUE). In case of major renovations,
the EU legislation  requires RES to cover 50% of the energy produced for domestic hot water, heating
and cooling. This measure is mandatory for existing buildings, contrary to listed buildings in the case
that an aesthetic impact or a a damage is generated . This situation is very critical in heritage contexts,
especially for the protection of their aesthetic appearance and historic values. In general, the installation
of traditional photovoltaic (hereinafter, PV) and solar thermal (hereinafter, ST) systems is not
recommendable for historic and vernacular buildings, in order to preserve their valuable fronts and roofs
. The use of integrated RES could be allowed using highly compatibility products designed to appear
similar to traditional architectonic materials thanks to the advanced customisation and to the presence
of several colours, patterns, special and low-reflecting glasses, and innovative cost-competitive coating
[9; 10]. Building Integrated Photovoltaics (hereinafter, BIPV) and Building Integrated Solar Thermal
(hereinafter, BIST) solutions are respectively PV and ST panels integrated into the building envelope,
combining electricity generation with weather and noise protection, thermal insulation, and sun shadow
. The “integration” implies the substitution of the constructive element: BIPV or BIST elements
have to be incorporated into the building as functional or constructive components, replacing the
traditional building element (e.g. window, cladding or architectural element). Therefore, the new BIPV
or BIST element becomes a functional and constructive element of the building envelope, similarly to a
conventional material. The balance of technical and aesthetic aspects becomes a priority in terms of
architectural functionalities and construction requirements (e.g. dimensional flexibility, easy mounting,
safety and reliability, thermal stability and comfort, fire security, climate protection, maintenance and
durability over time, etc.) . These solutions appear appropriate also for heritage contexts due to the
technological advances, ensuring low-rate reflection, compact shapes, and mimetic appearance. Despite
this, authorization requirement remains valid for historical centres, areas of landscape protection,
vernacular and cultural heritage buildings [5; 6]. Their construction involve a high level of commitment
of the experts appointed to execute the work, under the careful direction of the institutions responsible
for protecting the buildings [5; 6; 12; 13; 14; 15; 16; 17]. Uncontrolled, although minor, alterations to
the envelope, windows, facings, or installations may undermine the drive for conservation [12; 13; 15;
16]. However, a careful assessment of the benefits and feasibility of each case, together with the
exploitation of the technological innovations currently available on the market, may lead to a number of
potential solutions for RES integration in historical contexts [12; 13; 16; 17].
Aims and methodology
This paper discusses the integration of RES in heritage sites and buildings, aiming at the establishment
of a conceptual framework on the application of RES systems for the retrofit of heritage buildings and
sites, preserving their historic, aesthetic and natural values as well as lowering energy bills, increasing
comfort, architectural and technical quality, economic and environmental sustainability. The conceptual
background has been developed analysing international research projects, academic studies (i.e.
scientific papers, conference proceedings, and books), and “grey literature” (i.e. guidelines, technical
reports, awards, and internet sites from government, Heritage Authorities and PV and ST companies)
focused on the application of solar systems in heritage contexts in Europe and the United States of
America. This allows to consider both practical and scientific approaches. The legislative and normative
framework has not been considered because, apart from the EU framework, it is strongly connected to
local laws, regulations, and policies. Thus, it requires a deep analysis focuse only on the comparison
among different legislative approaches. The selection of the studies is based on technical keywords
(“RES”, “Solar Energy”, “Solar Architecture”, “Building Integrated Photovoltaic”, “BIPV”, “Building
Integrated Solar Thermal”, “BIST”) combined with heritage keywords (“Historical building”, “Historic
Building”, “Heritage”, “Protected building”, “Landscape”, “Heritage site”, “Sensible context”, and
“Energy retrofit”). Common searching engineering (i.e. google) have been used to identify research
projects, awards, guidelines, and “grey literature”. Then, academic studies have been selected in
academic databases (i.e. Scopus, Sciencedirect, google scholar) and conference proceedings. Finally,
the references of the scientific literature have been considered.
Framework on RES application in heritage contexts
The study presents the most important research projects, awards and prizes which reward historical
buildings RES. Furthermore, the analysis gives a brief outlook of the main guidelines which advise on
heritage renovation, preservation and sustainability. The main contribution constitutes in a conceptual
framework which offers a comprehensive view on the development of the integration of the solar energy
systems in historic buildings and sites. The selection of these three tools (research projects; awards and
prizes; guidelines and policies) is based on the most prominent aspect related to RES integration
emerged from the literature review.
1.1. Research projects
RES integration in heritage contexts is deeply examined within international, EU and local research
projects (Table 1). The most popular strategy in cultural heritage is the integration of BIPV systems into
building components, despite the above-mentioned architectural barriers [14; 17]. Initially, researches
focused on the acceptability of PV and BIPV technologies in heritage context . Few years later, a
solid knowledge base was developed on BIPV and BIST solutions in building renovation, especially to
demonstrate their advantages, e.g. technical competitiveness, economic savings, and building protection
[14; 15; 18], or to achieve high-quality standards of solar architecture [20; 21; 24]. In several cases, an
ad hoc design of BIPV and BIST systems was encouraged to preserve original shapes, features, and
values [9; 13; 16; 18; 29; 21; 27]. The introduction of active collaboration with local and heritage
administrations was a fundamental step for researchers to find unexplored solutions based on local
legislations and policies (i.e. localisation on alternative structures close to heritage sites) [9; 12; 22; 27;
30]. Geographical Information Systems (hereinafter, GIS) has been used to calculate the solar potential,
to interpret spatial data patterns, and to combine several spatial datasets [13; 30]. Several studies focus
only on the RES integration in historic buildings [21; 22; 27; 29], towns [26; 29; 30], and landscapes
[29; 30; 31]. Lately, the projects pay attention to the reduction of their impact on the electricity grid [28;
30]. Finally, outdoor test-beds and living labs have been created to investigate the application of
innovative products in buildings and landscapes[28; 31].
Table 1. Research projects on RES integration in historical buildings and sites (non-exhaustive list).
To develop marketable modules
with an innovative design for
historical buildings and sites; To
study the PV acceptability
To develop a solid knowledge
base on the advanced housing
renovation with RES, comfort
To promote the integration of
RES & RUE measures in
To demonstrate the advantages
of technical competitiveness,
economic savings and
protection through the
application of integrated RES
and energy distribution systems
To achieve high quality
architecture for RES integration
To install less visually intrusive
commercial products, with the
commitment of heritage
To promote energy retrofitting
of historic buildings with RES
To develop technical and
architectural guidelines for RES
To apply ST in major
renovations and protected urban
To develop a solid knowledge
base on renovation of non-
residential buildings towards the
To assess the solar potential in
historic towns, preserving their
To support urban planners,
authorities and architects to
achieve architectural integration
of RES in urban areas
To promote solar technologies at
urban level in a pilot project for
municipalities to produce RES
and preserve heritage sites
To enable a framework for the
acceleration of BIPV
To improve the application of
RES in historical buildings
To adopt RES in historical and
public buildings, with a focus on
low-income families; To
develop living labs; To reduce
RES impact on the electricity
To identify and to assess several
RES compatible with historic
To create a value chain for BIPV
in historical buildings and
landscapes; To develop a
platform with building
examples, products, guidelines
To enhance skills related to RES
spatial planning, analysis and
decision-making methods in
landscapes; To understand the
legal frameworks and practice in
the implementation of RES in
To set up an outdoor test-beds to
verify the application of BIPV in
1.2. Awards and prizes
Several prizes on RES integration in heritage contexts have been established in the last decade. They
reward historical buildings or settlements to effectively meet the challenge for innovation using solar
systems, considering architectural and planning rules. Prizes include innovative and creative solutions
for BIPV and BIST that must combine aesthetic aspects and energy performances (Table 2).
Table 2. Awards and prizes on RES integration in historical buildings and sites.
1.3. Guidelines and policies
Several countries define national guidelines that include RES installation in sensitive buildings and
landscapes (Table 3). The American NREL gathers best practices of solar guidelines and policies of
different US municipalities, giving some clear and practical guidelines to ensure RES successful
installation, balancing energy efficiency and preservation . Scottish guidelines propose a
comprehensive approach on micro-RES, considering PV, ST, wind, biomass, etc. [32; 34], while taking
into account conservation, energy, costs, and maintenance aspects. The Italian guideline on energy
efficiency in historic buildings  provides operative indications for evaluation and improvement of
the energy performance in listed buildings, with reference to the national regulations. It mainly provides
designers with a tool to assess the energy performance of historic buildings on the basis of the national
regulations, and personnel working at the Ministry with a useful tool to communicate with technicians.
This document also focuses on the need of common and agreed criteria to evaluate and authorize the
energy improvement of historic buildings. Some general principles are currently commonly shared, such
as the maximum conservative attitude and the integration of RES where historic matter and materials
are irremediably damaged or lost. On the other side, difficult to assess is the architectural quality of an
intervention that depends also on non-objective criteria, based on personal sensibility or the level of
product innovation, as some projects shown several years ago . Quality evaluation, at least in Italy,
remains an open question. In Switzerland non-compulsory but indicative reference documents are active
at national and federal scale  or local level (cantonal/regional) . These documents are directed
to specialists in the field of design, architecture, physics of construction and energy consulting as well
as to the services of Public Authorities involved in the preservation of monuments, energy issues and
release of building permits. The handbook of SolarKultur  illustrates how municipalities can
Solar architecture and
s für Energie
BIPV in EU protected
historic urban districts
reconcile the use of RES with heritage culture, highlighting the importance of planning larger territorial
units to achieve good solutions. The document is based on the global solar planning of the city of
Carouge , a Swiss ISOS site of national heritage significance in the Canton of Geneva, which aims
at reconciling the protection of the building stock with the RES installation (mainly PV and ST). In
addition to that, several Swiss Cantons published guidelines to evaluate the RES installation (i.e. in
Ticino RES installation on roof is possible when are coplanar, protrude < 0.2m, compact and rectangular,
have appropriate colour and no visible connections or pipes) . Overall, these guidelines suggest to
insert the solar panels on: sites of a historic resource, new constructions, non-historic buildings,
additions, areas that minimise their visibility from a public thoroughfare or natural site [32; 33; 34; 35;
37]. The solar panels installation must not create permanent loss of heritage features, obstruct views of
significant architectural or decorative characters, permanently transform the historic fabric, or create
disjointed and multi-roof solutions [33; 34; 35; 36; 37; 39; 40; 41]. Furthermore, to reduce their
appearance is necessary: (i) to have solutions and low aesthetical profiles [33; 34; 37; 40; 41]; (ii) to
preserve the distinctive materials, features, finishes, construction techniques, or examples of original
craftsmanship [33; 34; 37; 39; 40; 41]; (iii) to ensure the integrity of the historic property and its
environment [33; 34; 37; 40]; and (iv) to guarantee the chemical or the physical compatibility with old
fabrics . Finally, historic features with severe deterioration can be replaced with new features that
match the old in design, colour, texture, and materials while missing features can be substantiated by
documentary and physical evidence [33; 37; 39; 40; 41].
Table 3. Guidelines on RES integration in historical buildings and sites.
Guide to facilitate the reduction of fossil
fuel in historical/historic homes with RES
Criteria that balance historic preservation
and energy production
Definition of a scale of compatibility of
different RES technologies
Discussion on the use of micro-RES, with
examples and considerations
Production of handbooks with technical
solutions to reduce energy consumption,
even in historic buildings
Definition of best practices on PV
integration in historic buildings
RES installation should be clarify with
Definition of specific rules for the
aesthetical and technical integration of
Illustrations for reconciling PV and
quality of constructions
Discussion and conclusions
This study shows that solar technologies have high barriers from the conservative point of view, but
there are significant market innovations and long-term benefits. The BIPV products are very mature to
allow a good integration with heritage buildings and landscapes. As previously shown in Table 1, several
research projects have been approaching this topic since a long time. At national, regional and local
level, authorities and legal framework already contemplates the delicate issue related to the integration
of RES systems in heritage sites and buildings. Nonetheless, a small number of examples are well-
known and only few of them are shown as exemplary case studies, recognised mainly at international
level (IEA tasks). Solar prizes, instead, intend to reward historical buildings or towns using RES
technologies and considering architectural, planning, and technical constrains. These prizes aim to
recognise private and public investment and to encourage them to replicate similar solutions. The EU
Horizon prize dedicated only to the integration of new PV systems in protected historic urban districts,
also generating and supplying electricity for the district consumption is of a particular interest. It aspires
also to foster the development of the best suitable architectural and aesthetic design in combination with
optimal technical solutions, low visible impact, and minimal intrusion into the buildings structure.
Unfortunately, only five applications has been submitted but, nevertheless, none was found to be eligible
according to the “rules of contest”. However, national and local guidelines define clear criteria and rules
for RES integration, trying to balance preservation and energy efficiency requirements. These guidelines
suggest three different criteria : (i) “localising criteria”, related to project siting and location; (ii)
“qualitative criteria”, mainly referring to the minimal intervention on materials, features, spaces, and
spatial relationships and to the mitigation of the visual impact, with the correct selection of colours,
texture, anchoring, arrangement and alignment; and (iii) “quantitative criteria”, driven by system
performance (dependent on wheather, type of technology, site characteristics, impact of shade, and
orientation, tilt angle, surface extension) and economics (related to initial, operating and maintenance
costs, availability of incentives, discount and fuel escalation rate). These criteria show different
approaches to maintain the balance of preservation, usage of energy sources, and economic and energy
cost in different levels. Although national and local guidelines clearly define criteria and rules for RES
integration, it looks like that there are not enough examples of perfect BIPV systems in historic urban
To conclude, energy transition needs clear policies and systems regulations, updated guidelines, simple
rules, professional trainings and widespread dissemination. Dissemination of pilot projects and
interventions, in which the concept of integration is not a simple juxtaposition of solar panels on an
existing surface, is fundamental. On the building level, it is necessary to work on projects where the
word ‘integration’ involves a more deeply relationship between old and new solutions at the level of
detail, recalling the Brandi’s idea on pictorial restoration and integration of the so called ‘lacunae’ (the
lacking part of frescos decoration on walls or vaults: how do we “restore” missing parts? Should we
paint what we think the original drawing or could we interpret in a more contemporary way?). By
demonstrating that a technological element can become an integral part of a conservative project (unless
it is the cause of an unnecessary demolition), it will be possible to eliminate some taboos or, at least, to
open some paths in that direction. On the territorial level, several EU countries (e.g. Switzerland,
Sweden, Germany, Austria, Denmark) developed a new approach to solar planning which show that it
is possible to achieve optimal use of solar energy, preserving the heritage and architectural quality of a
site. Conversely, in Italy, a lively debate took place after that some medium-sized installations in
prestigious territories were implemented, favoured by the State economic incentives. This led to
consider this technology as one of the least appropriate for architectural and landscape protection. The
ability to insert new technological devices in such a way that respect both the aesthetical appearance and
the physical matter of the historic building, will contribute to opening up the world of architectural
restoration to research and innovation. In the near future, synthetic organic photovoltaic films on large
glazed surfaces as well as graphene sheets and materials of new generation will be able to create smart
cities, capable also of preserving the inheritance from past generations. Clearly, this topic is still under
discussion, but the first steps in the right direction have been taken and hopefully this challenge will be
met in next years, thanks to innovative, excellent and high-quality products under development in the
solar industry market. This study provides an inspiring view on the need for environmental, economic
and cultural sustainability in the field of heritage renovation and preservation, and addresses the
importance of being aware of eco-friendly techniques and policies in heritage preservation.
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Operation co-financed by the European Union, European Regional Development Fund, the Italian
Government, the Swiss Confederation and Cantons, as part of the Interreg V-A Italy-Switzerland
Cooperation Program for the Project “BIPV meets history. Value-chain creation for the building
integrated photovoltaics in the energy retrofit of transnational historic buildings” (ID n. 603882). The
authors express their gratitude to the IEA SHC and EBC Executive Committees for supporting the