Development of bi-functional photovoltaic modules for building integration
In the BIMODE-Project, designers, architects and scientists were working closely together for the first time to enhance the aesthetic appearance of PV-modules. A range of designs was proposed using solar cells in different colours and shapes on mono- and multi-crystalline wafers. Six prototypes were produced: A triangular module containing multi-crystalline triangular laser grooved buried grid (LGBG) cells of dark blue, steel blue, gold and magenta colour. For these cells a novel gridline pattern and interconnection method were developed to make cell interconnection without the use of busbars. A rectangular module consisting of square and rectangular multi-crystalline LGBG cells in the colours magenta and pale gold. The rectangular cells enhance the striped appearance of the module. Three rhombic modules with mono-crystalline hexagonal LGBG cells of dark blue, steel blue and pale gold colour. The three modules give a strong three dimensional impression of a cube when placed together. A hexagonal module with mono-crystalline hexagonal cells of green colour and a sinusoidal bus bar pattern. The green cells were produced using screen-printing technology. The prototypes have module efficiencies between 6.3% and 12.1%. The BIMODE project was very successful. For the first time artists and architects had a major input into the design of PV modules for building integration. All the designs and not only the prototypes have a very high level of visual appeal and can be regarded as works of art. At the same time the prototypes are photovoltaically functional. Therefore the prototypes are the realisation of the philosophy which was especially developed within the project and was behind all the designs: "form follows energy". Architects prefer the appearance of large grain multi-crystalline Silicon wafers for building integration. This is a clear outcome of the project. Bayer delivered such wafers and did a great deal of research into tailoring their production process towards larger grain sizes. Simulations showed that this can be achieved maintaining the production throughput and the wafer quality and therefore maintaining low wafer costs. A lot of research had to be done on the cell level in order to accommodate the design wishes of the group at KHM. Novel grid structures had to be developed and tested for both screen printing and laser grooved buried grid technologies. A completely new interconnection design was successfully developed and incorporated into the triangular module. With this new interconnection bus bars could be removed completely from the cell surface. This was one major demand of the architectural community. On the screen printing technology a completely different approach was used. Here the bus bars were designed in a way to show a special pattern when cells are assembled into modules. This new technology was successfully incorporated into the hexagonal prototype module. On the idea of directly connecting to the grain boundaries on multi-crystalline wafers significant progress was made. However a lot more work needs to be done to develop this technology further if it is to be applied in volume production.