The use of photovoltaics (PV) is not only limited to generating electricity but can, in fact, also play a major part in energy-saving strategies. With respect to daylight control, in particular, semitransparent photovoltaics provide a wide range of building façade applications. The transparency of crystalline-based solar modules is determined by the distance between each cell: the closer the cells to each other, the higher the total photovoltaic output of the panels.
In the course of a research project carried out at the Karlsruhe Institute of Technology (KIT, Germany), scientists have investigated and compared the energy efficiency of three buildings with façade-integrated PV, located in Fortaleza (North Brazil), Florianopolis (South Brazil), and Frankfurt a. Main (Germany). All façades of the three buildings contained windows with integrated semi-transparent crystalline solar cells. Since the transparency of all PV windows was only 30 percent, significant energy savings were achieved during the summer months due to a reduced need for cooling. At the same time, more energy had to be spent on artificial lighting inside the building. “However, we were able to counter this side effect by installing a lighting control system,” explains Evelise Didoné, one of the researchers at the KIT.
An entirely different way to control the transparency of photovoltaic building façades involves the use of dye-sensitized solar cells (DSCs). Unlike common silicon solar panels, the photoactive layers of DSCs use photoactive dyes to convert light into electricity. The fact that the organic photoactive layers used in DSCs are extremely thin enables the manufacturing of solar panels with different transparency levels determinable by selecting different colors and thickness of materials.
Rossella Corrao of the Department of Architecture at the University of Palermo, Italy, explains: “Although DSC modules have only eight percent efficiency compared to twenty percent of silicon panels, they can actually generate up to 15 percent more energy annually, compared with crystalline panels of identical KW. The reason for this is that DSC modules are more efficient under low-light conditions. Unlike silicon-based systems, the efficiency of DSCs does not depend on the light’s angle of incidence. Therefore, DSC modules don’t necessarily have to be pointed towards the sun; they can also be mounted in vertical or horizontal orientation without losing out on efficiency.”
Moreover, using DSCs in conjunction with glass façades “enables a two-way application of the façade-integrated PV elements by utilizing sunlight sources outside a building as well as any artificial light sources inside the building, thereby significantly extending the daily operating period of the system,” Corrao adds.
Computer simulations predicting the direct impact of color and transparency on the photovoltaic efficiency of DSCs are an important tool when it comes to the design of zero-energy buildings. Janne Halme, professor at the Aalto University in Espoo, Finland, investigates the optimization of DSCs in terms of usability and power generation. In this context, Halme focuses on new ways of integrating DSCs in building designs rather than trying to optimize the cell efficiency. Transparency of the PV windows and the role of the esthetic appeal of the façade prevail over efficiency.
All three scientists will present their results in detail at the 7th Energy Forum on Solar Building Skins, to be held in Bressanone, Italy, Dec. 6-7, 2012. The conference also features presentations by Dieter Moor (Arconsol, Linz, Austria) on strategies for optimizing semi-transparent PV windows under performance and cost considerations, and by Livio Nichilio (University of Toronto, Canada) on strategies to increase the energy efficiency of buildings by using façades with integrating PV and heat-mirror glass. A detailed program of the conference is available online