Photovoltaics (PV) are solid-state, semiconductor type devices that produce electricity when exposed to light. Electrons in the photovoltaic material are knocked free by light to flow out of the device as an electric current. The more intense the sunlight, the stronger the electric current.
This phenomenon was discovered in the mid-1800s, yet important applications followed much later—for satellites and applications where extending the electric grid is cost prohibitive. Architectural applications have only recently become prominent. With Building Integrated Photovoltaics (BIPV), for instance, photovoltaics are integral to the building skin: the walls, roof, and vision glass. The envelope produces electricity, which flows through power conversion equipment and into the building's electrical system.
Photovoltaic vision glass integrates a thin-film, semitransparent photovoltaic panel with an exterior glass panel in an otherwise traditional double-pane window or skylight. All the PV types can be integrated and/or laminated in glass, but only thin-film photovoltaics will be translucent. Electric wires extend from the sides of each glass unit and are connected to wires from other windows, linking up the entire system. If the PV cells are part of the vision glass, various degrees of transparency are possible—as in frit glass—since the PV cells offer shade and produce electricity. In some cases, the PV panels are placed in spandrel panels, rather than the vision glass. Smaller PV systems can be used to power facade equipment directly instead of being connected to the electrical grid in the building. Vertically oriented PV panels are optimally not positioned toward the sun. One approach to position the PV panels more perpendicular to the sun is to place them into fixed shading devices on the facade or on movable shading panels, using the generated power to track the shades to the optimal solar angle.
Current PV production technologies, pricing structure, and energy rates limit BIPV use to prominent, prestige buildings (although PV-integrated cladding costs are comparable to marble). Of course, as these factors are in constant flux, BIPVs are receiving increased attention that is justified by the promise of a building envelope that can generate energy in addition to providing shelter.