Renewable Energy Sources are becoming increasingly important in the power markets; it is calculated that almost 70 % of the global capacity additions to 2040 will be attributed to renewables. Among these, solar energy is considered to be the most promising because it is clean, abundant, inexhaustible, accessible and does not produce pollutants during its use. Nonetheless, in densely built urban environments, the supply of onsite solar energy generation with traditional ground-mounted PV installations is challenging. In this context, Building integrated photovoltaics (BIPV) technologies can support the transition towards a low-carbon energy system, unlocking the solar potential of a large set of vertical and horizontal envelope surfaces (both roof and façade) currently not exploited, promoting onsite energy production and enhancing self-consumption, i.e. the share of the onsite produced electrical energy directly consumed by the building.

Building Integrated Photovoltaics, a dual purpose : serving as a building envelope and as power generation system

Building Integrated Photovoltaic (BIPV) systems represent a technical solution to integrate Renewable Energy Systems (RES) in buildings. Building integrated photovoltaics technologies can have a dual purpose: serving as a building envelope component, providing a function related to the building construction (e.g., structural integrity, thermal insulation, solar shading, daylighting control, safety, and security), and as power generation system at the same time, harvesting solar energy for onsite energy production.  Despite the high potential for BIPV applications, there is the need to overcome technical, social, and economic barriers to reach a wider uptake of BIPV applications, improving their economic profitability.

Cost and aesthetic acceptance have been the main limitations to the spread of BIPV. In the case of cost, there has been a consistent decrease recently. However, aesthetic acceptance is still considered a barrier to BIPV uptake. The high tech appearance of PV used in buildings does not represent a problem per se since it could very well suit modern architectural styles. Nevertheless, when it comes to PV integration in existing buildings, especially in architecturally and naturally sensitive areas (e.g., historical centres, heritage sites, archaeological areas or heritage landscapes), special care must be given to the aesthetical integration and the balance of technical and aesthetic aspects is indeed a priority for BIPV system in terms of architectural functionalities and construction requirements.

Towards higher aesthetical integration to promote social acceptance of BIPV technologies

Although multiple studies are focusing on the aesthetic acceptance of BIPV / PV technologies, the integration of BIPV / PV in the existing building stock and in heritage sites is still debated. Several countries published national guidelines defining the architectural criteria for RES installation according to national legislation, local authorisation processes and specific heritage features. Nowadays, these criteria are always devoted to the protection of historic and distinctive materials, features, spaces, finishes, construction techniques, and it is crucial that BIPV / PV panels must not create permanent losses or transformations in the historic fabric. According to those guidelines, both PV and BIPV technologies can be inserted, even in historic buildings, while matching the original designs, materials, finishing colours, and texture.

Hence, in the case of renovation of historic buildings, there are two main integration parameters to be considered from the aesthetic point of view:

• Geometrical uniformity, meaning the visual impact of PV panels should be as uniform as possible, and thus 100% coverage of the entire area is preferable.
• Colours of the modules, that should foster the chromatic integration with traditional materials, e.g. using terra-cotta cells for clay roof tiles, anthracite or green-grey cells for slate or stone, white cells for plaster or high-resolution images as marble or wood.

A possible turning point in the aesthetic acceptance of BIPV applications has been the development of modules that can hide the PV cells behind coloured patterns, which block the sight on the original material of the cells, making the modules appear as standard construction components. Different customization techniques to obtain coloured or textured BIPV modules are currently used for modules available on the market, including:

1. Solar cells with anti-reflection coating,
2. Semi-transparent and/or coloured PV-active layers,
3. Layers or interlayers containing solar filters, coloured or patterned coatings,
4. Coloured polymeric encapsulant films, and
5. Printed, coated or alternatively finished front glass. These kinds of coloured BIPV modules have recently shown market growth, and their application is increasing considerably.

In this respect, the customisation of coloured BIPV modules and their expansion on the market would contribute to the social acceptance of PV where their aesthetics are important to integrate them in buildings. The high level of customisation constitutes the strength and the uniqueness of BIPV products but is also a limiting factor, since it obstructs the series production of the modules and, thus, provides less room for cost reduction.

Credits: arch.Valentina Carì/Studio Progetto Serr@. Source: https://www.bipvmeetshistory.eu/caso-studio-villa-castelli/

Projects

In an international context, several projects focused on this topic, leading to important results. At the beginning of the past decade, many research projects focused on technology development such as new PV modules, new integration substructures, new components, and new systems. Over those years, several projects addressed this challenge, as for example the FP7 project named Solar Design that developed new technologies (in particular flexible PV modules) to address architectural requirements.

Another project was the International Energy Agency task 41 on solar energy and architecture, providing good examples on how PV can be integrated with high architectural value in the built environment. It highlighted that architects are very important stakeholders of the BIPV value chain who must be involved in order to succeed in boosting the penetration of PV in buildings.

Currently, the project BIPV meets History is addressing the problem of integration of PV in historical contexts; the challenge is to meet the requirements of local, national and European policies while respecting the heritage and landscape values of the Italian-Switzerland territory. The approach is based on the creation of synergies between the expertise of Public Authorities, research and industrial companies of construction and PV sectors, to open new markets and to offer interesting financial and productive profits to all agents in the value chain.

The project is gathering exemplary case studies from the industry and implementation point of view to define criteria and guidance tools to promote BIPV diffusion. This repository of case studies is the basis for the development of a digital platform and new business models and funding mechanisms for the stakeholders involved. In addition, several dissemination activities (focus groups with stakeholders, events with industries, etc.) are improving the technological and cultural knowledge transfer providing a replicable case for different transnational regions.

In the projects mentioned above, the focus was very much on the architectural integration of BIPVs in buildings. Recently, another aspect of integration that becomes important is the energy integration, meaning the ability of PV to be integrated not only into the envelope of the building but also to be efficiently integrated in the overall energy system of the building.

On this topic, the Institute for Renewable Energy of Eurac Research is currently coordinating a research project named EnergyMatching that is addressing energy integration and, in particular, the matching between renewable energy onsite production and buildings/districts consumption. The project aims to maximise renewable energy (RES) harvesting from BIPV in the built environment by developing and demonstrating cost-effective active building skin solutions as part of an optimised building energy system, connected to the local energy grid and managed by a district energy hub implementing optimised control strategies. A comprehensive economic rationale balances the objectives and performance targets of both private and public stakeholders.  

Interesting outcomes of the EnergyMatching project are the Solar Window Block and the EnergyMatching platform. The Solar Window Block is an example of a prefabricated insulated system that can integrate photovoltaic modules, dynamic and automated shading systems and decentralised ventilation units with the aim of maximising indoor comfort without adding electrical loads to the building. The EnergyMatching Platform, is an online platform that supports various stakeholders (e.g. architects, designers, engineers, ESCOs, government housing bodies, sustainable technology providers, real estate, building owners, research centres etc.) in maximising the RES harvesting in their built environment.

The platform offers users the chance to run an optimisation tool that suggests optimal configurations of BIPV systems and provides inspiring examples of active building skin solutions developed within the project (e.g. BIPV click&GO solution, Solar Window Block, building and district EnergyHub solutions).

BIPVBOOST is a project that focuses on bringing down the cost of multifunctional BIPV systems, limiting the over-cost with respect to traditional, non-PV construction solutions and non-integrated PV modules. The project aims to achieve this through the effective implementation of short and medium-term cost reduction roadmaps addressing the whole BIPV value chain and demonstrating the contribution of the technology towards mass realisation of nearly Zero Energy Buildings. With huge involvement from industrial partners in the consortium, BIPVBOOST looks at pursuing a 50% reduction of the additional costs of BIPV modules in 2020, and 75% reduction in 2030, and thus result in a substantial increase of market deployment of BIPV technology.

Conclusion

BIPV technology development is an interdisciplinary activity that calls for heterogeneous expertise, as the BIPV modules multifunctional features relate to the aesthetic and technological integration in the building envelope, as well as to energy integration both in the building systems and in the electric grid. All these aspects are important to succeed in BIPV design, and a joint effort between different stakeholders belonging to PV, building, and energy sectors is needed to ensure successful PV integration in buildings and districts, also considering EU policy towards energy sharing and energy communities. Coloured BIPV modules could enhance the market growth of BIPV technologies, by increasing the aesthetic integration in the built environment and the social acceptance of PV in buildings.

Customization is undoubtedly the main strength of coloured BIPV, which can support onsite energy production and enhance self-consumption for renovated buildings, even in heritage sites. The cost of such technologies is decreasing and the potential to balance the difference in cost with standard PV modules lies in the exploitation of larger building surfaces, including roof and façades, previously not considered for PV installation.