ARCHITECTURAL INTEGRATION OF PHOTOVOLTAIC CELLS OVERVIEW OF MATERIALS AND PRODUCTS FROM AN ARCHITECTURAL POINT OF VIEW

The building skin is often called the third skin of a human being after the body skin and clothes since it provides protection from the elements, creates privacy and provides contact between the indoor and outdoor space. Moreover, it is the most important element regarding the total energy balance of buildings. Solar energy systems, and in particular solar cells, have an important role to play in reducing energy needs of buildings. Several building surfaces are ideally suited for the use of solar panels, but high costs as well as technical and aesthetical considerations have long kept building owners and architects from using even a small part of this potential. The paper is the result of a study that investigates the architectural potential of existing photovoltaic materials and product development trends. The main goal is to give an overview of current possibilities and discuss their relevance for future development regarding architectural integration. 1. The Building Skin with Integrated Photovoltaics In most European countries, buildings account for approximately 40% of the total energy use. The situation is not improving, despite the fact that developed technologies and stricter building codes are making the buildings more energy efficient. This is due to higher comfort requirements and to the fact that the use of electrical equipment is increasing [Hestnes 2000]. Subsequently, architects have a great responsibility in reducing the energy use in renovation of existing and in design of new buildings. The conscious design of the building skin is one of the main tools to achieve a lower-energy building. 1.1 Building Envelope The principal task of the building envelope is to create a comfortable shelter that provides protection from the elements. Therefore it is often called the third skin of a human being after the body skin and clothes. The building envelope also creates privacy and provides contact between the indoor and outdoor space through different layers with specific functions (e.g. wind-, water protection). Throughout history people decorated the building skin as lovingly as if it was their own clothing. It created identity and the feeling of belonging to a certain tribe, community. At the same time, ornamentation gave the possibility for people to express their own creativity. The European Antiquity transformed the façade into a unique showcase with which public buildings presented themselves in the urban space. In the Renaissance, especially, façades began to appear separated from the building; that is, they were placed in front of an old church or palace as a “new” cloak [Schittich 2001]. They fulfill a primarily aesthetic purpose: attractive packaging with a strong status and symbolic meaning. Most recently the surface – and hence its materials – has become the central focus of investigation. Some architects turn back to the use of traditional natural materials such as timber, massive stone or brick masonry, demonstrating the solid physical presence of a building in an increasingly virtual world. Others 1 Department of Architectural Design, History and Technology, Norwegian University of Science and Technology, Trondheim, Norway, klaudia.farkas@ntnu.no 2 SINTEF Building and Infrastructure, Trondheim, Norway, Inger.Andersen@sintef.no 3 Department of Architectural Design, History and Technology, Norwegian University of Science and Technology, Trondheim, Norway, annegrete.hestnes@ntnu.no reflect the consumer society’s information age by focusing on intensive perception through colorful, flickering media facades and illuminated screens [Schittich 2001]. Among these extremes lies an equally contemporary and innovative possibility that deserves increasing attention: the building skin as a responsive component of a low energy concept [Schittich 2001]. Several building surfaces are ideally suited for solar systems, especially for solar panels, that can play an important role in reducing the energy demand of buildings. In these cases a balance has to be found between functional, technological and structural features based on energy-conscious aspects as well as on social, psychological, ecological and aesthetical aspects. They are all interdependent and bear a direct influence on each other. 1.2 Photovoltaics and Building Integration Aspects The solar cell or photovoltaic (PV) cell is a device that converts light energy into electrical energy. A PV panel is a collection of photovoltaic modules (a number of cells electrically connected and packaged) that are mechanically fastened together, wired, and designed to be a field-installable unit. The term Building Integrated Photovoltaics (BIPV) refers to products with photovoltaic cells that are integrated parts of the building envelope. Building integration refers to several issues [Hagemann 2002]. The energy concept and the financial budget are two basic determining aspects. The first generation products of solar cells used ON buildings were just elements added to the building skin. During the planning and the installation of these components electric and economical issues were prioritized. In that case we cannot really talk about building integration. As a second step functional, constructional and structural aspects came into focus. The integration of the solar cells INTO building elements rather than adding them on later as separate systems became also a key factor for greater market penetration. Solar cells that replace other building elements and that thereby serve dual functions are also more cost effective. Recently building integration of PV covers mainly these issues. Today’s market already offers a broad range of photovoltaic modules. These are suitable for integration in different parts of the building skin (roof, facade, glazing, shading.etc.). A basic problem is still that aesthetical, cultural and social issues are neglected in the development of PV products. This is setting back the spread of this technology and the acceptance among architects and clients. It is important to highlight that all these aspects, like energetic, economical, functional, constructional, structural, aesthetical, cultural and social are interrelated. In every project there is a need for prioritizing and balancing between these issues to achieve a successful building integration. 2. PV Materials and Technologies One of the major issues regarding the building integration of photovoltaic elements is the attitude how we approach these components. In most cases solar panels are still considered to be electric devices representing a high-technology-based added element on the building skin. However, products that integrate photovoltaic cells should be considered like other building components. They have general physical, chemical and visual characteristics. In addition, they possess some specific characteristics due to the used materials, technology and function (in this case for instance that they produce electricity). The language how these components are presented verbally and virtually is one of the main keys to the acceptance of these elements among architects and lay people. The following paper will discuss photovoltaic cells and products from an architectural language point of view starting from the scale of materials to architectural products. 2.1 Overview of Materials and Technologies Used in Current BIPV Products Certain characteristics of the material determine the further possibilities of building components using this material. In the case of photovoltaics the technology used to produce the physical form is a crucial point that needs to be understood. It is not a new phenomenon: look at for instance how many types of bricks we can get from lime and sand with different processes. The basic raw material that still rules the PV market is silicon. It is the second most common chemical element on Earth after oxygen. It is a very hard, grey material that glitters like metal. In nature it can be found in oxidized form for instance in gravel, sand, clay or quartz. The use of silicon came into focus due to its semiconductor properties, in other words its ability to conduct electricity. Currently there are two main technological processes used to create solar cells from silicon. These two technologies provide significantly different characteristics to the cells, hence it is important to distinguish between them. Wafer-based technologies, often called first generation photovoltaics, cut crystalline silicon ingots to get solar cells. This results in cells with specific thicknesses, sizes and shapes. The second generation, thin film technology uses amorphous silicon to create a homogeneous thin layer. Consequently there is no limitation on the size of the cells. One of the greatest advantages is that this process uses much smaller amounts of silicon. Other materials are used as well, like CIS, CIGS, CdTe but still silicon is dominating the market. The third generation photovoltaics are a range of novel alternatives to the previous ones mentioned above. The third generation cells that are used currently in building integration are dye-sensitized solar cells using biomimics through artificial photosynthesis. In this case the semiconductor material is TiO2. It is a very promising technology that significantly reduces the initial costs of solar cells. Table 1 Solar Cells Used in BIPV Monocrystalline Multicrystalline Thin film opaque Thin film translucent Dye Solar Cell TYPE OF CELL