Any architect who has tried to integrate photovoltaic modules into a curved building envelope will know it is not easy. Most PV modules available on the market are bulky, rigid panels based on glass or metal sheets. Even when flexible modules can be found, their standardised sizes inevitably become a big constraint for the architect’s design.
An innovative company based in Selkirk, Scotland, has been developing photovoltaic textiles with thin film modules that are directly deposited onto the woven polyester fabric at the manufacturing stage, rather than being merely stitched on afterwards. The company is Power Textiles Limited (PTL) and it is directed by a multidisciplinary team with composite expertise ranging from Mechanical Engineering to Chemistry and Physics. Among the directors there are Dr Robert Mather, expert in technical textiles, and Professor John Wilson from Heriot-Watt University, expert in the physics of photovoltaics.
The PV fabric is composed of layers that are produced with coating technologies based on plasma enhanced chemical vapour deposition (PECVD), thanks to which the size limitations of conventional PV modules can be overcome. As the silicon is formed from a gaseous chemical source, the process uses considerably less energy and wastes less material than producing crystalline silicon does, although the performance of thin film amorphous silicon cells is lower than that of crystalline silicon cells. In fact, forming thin film amorphous silicon requires temperatures of 200-250°C, while forming crystalline silicon requires temperatures over 1400°C. Polyester fabric is particularly suitable as a substrate not only for being a widespread material, but also because it is stable to the deposition temperature of amorphous silicon.
Like traditional PV modules, the thin film cells of the PV fabric are multi-layer devices, but unlike crystalline solar cells they maintain their conductivity when flexed thanks to a conductor composed of a deposited conductive polymer and a thin a thin evaporated layer of aluminium.
Besides the lightness and flexibility, PV fabrics present other advantages. The most interesting from the designer’s perspective is probably the possibility to alter the shape of the cells to compose the desired patterns, keeping in mind that the output current, depending on the area of the cells, must be the same for all the cells connected in series, while parallel connections can mitigate the differences due to the different degrees of exposure.
At the moment a relevant drawback of PV fabrics is the low efficiency of the solar cells, which can be simply overcome with a larger area of amorphous silicon. Some of the potential architectural applications of PV textiles could be tensile membranes, agricultural constructions, disaster relief shelters, scaffold cladding, shade awnings, perhaps even window blinds and advertising banners. Given its relatively low output, the PV fabric could be integrated in designs which include low-energy devices such as LEDs, sensors and microcontrollers.
Despite its potential versatility, the PV fabric hasn’t been applied in any completed designs yet, “but some are in-hand with PV/textile researchers”, says prof. John Wilson. Some of the reasons that might be delaying a real-scale application of the PV fabric in architecture could be its durability and its disposal at end-of-life, which haven’t been fully assessed yet, although some mechanical stretch tests have been made and none of the materials involved seem to have any environmental issues. But the main drawback is probably the lack of funding: “We need investment to be able to make samples on a larger scale than the lab-based equipment can at present achieve”, says prof. John Wilson.
In the light of this information, we can hope that the versatile technology developed by Power Textiles Ltd. will be soon sponsored and made available for architectural applications, reducing the environmental impact in the construction industry.