You are in:
Rotary screen spins solar cells into view
Specialising in solar cells for renewable energy applications, Switzerland-based VHF Technologies is proving the importance of rotary screen printing for industrial applications.
The company is using a 20in wide Stork Rotary Screen Integration (RSI) printing module during the production of new thin, lightweight and durable solar panel.
With environmental concerns high on the political agenda, there is growing demand for greener energy sources. Solar power is one such resource that could be exploited on a greater scale. However, in many situations, there has been no practical solution for harnessing this free resource: conventional panels can be too heavy, fragile and bulky.
The VHF Technologies team found a solution when they developed a method of applying very thin layers of amorphous silicon to plastic substrates, using very-high-frequency plasma technology.
The result is a photovoltaic (PV) cell, called Flexcell, comprising a 50µm thick, flexible polyester film plasma-coated with thin silicon layers. Because the cells are printed on flexible film, they offer advantages over conventional solid crystalline, heavy glass types. Crucially, the film can be laminated. A Teflon coat offers resilience against wear, tear and shock. Furthermore, it provides resistance to extreme weather and corrosion. Applications vary from compact portable battery chargers to large membranes that can cover entire rooftops.
Precision is required at every stage of this complex plating, printing and laminating. The Stork unit is used to print a continuous grid of conductive silver paste, that interconnect seamlessly.
Seamless quality
VHF Technologies’ CTO, Diego Fischer, said: “The rotary screen process is an important step. The grid of silver paste is needed to enhance the conductivity of the panel, which is able to deliver about 7W/0.2m2 in normal light conditions. We chose Stork’s module because it could clearly deliver the necessary seamless quality and it had the potential for high productivity. We were also impressed with the resilience and cost-saving attributes of the nickel screens, which can be reused when necessary.”
Rotary screen printing was the clear choice thanks to its consistent quality of reproduction. The Swiss team faced many challenges. The silver lines had to be 50µm wide by 20µm thick. Stork Print’s technical team, led by Ben Robesin, worked closely with the company. It advised on the design and installation of drying and curing equipment and worked jointly on finding ways to achieve optimum productivity without compromising the performance of the materials and substrate. The challenge was to achieve conductivity of 25ohm/square/mil, without reaching the substrate’s melting point and thus damaging the substrate.
The filmic panels can be walked on and rolled up when not required, making them ideal for energy saving situations wherever space is a premium, such as rollable battery chargers for camping, caravanning and yachting.
The power generated by the panels is roughly half that of conventional types, yet the PET panels are said to achieve energy payback within a year: about four times as fast.
With their ultra light weight and weatherability, Flexcell cells suited one application area where heavy panels could not go: structures. A PET membrane can cover complete building tops (especially flat industrial roofing) without overexposing the structure to pressure. Such a feature offers building owners the opportunity to reduce energy costs or even ‘harvest’ energy for forward selling to the electricity grid or end-users. With environmental concerns growing, especially in Europe, legislators are more likely to favour those who reduce reliance on the dirty fuels responsible for most of our energy needs. Tax reductions for green energy sources are one possibility. In Germany, it is possible to sell solar-derived energy at up to e0.50 per kWh.
VHF Techniologies' pioneering work has been recognised by the Swiss government. They received the Eco-Trophy from the city of Lausanne in 2004 and the CTI-Start-up Award from the Swiss Federal Department of Economy in 2000 as acknowledgement of its leading position in the innovation and industrialisation of renewable energies.
Ramp up production
Now, having successfully made its case to its investors, the financial resources required to ramp up production and market Flexcell effectively are finally there. Distribution will take place through B2B partnerships with building materials manufacturers, who will integrate Flexcell foil into their existing building material products. These PV activated products are designed to deliver high added-value by combining roofing and energy production.
The company is currently producing about 2,000m2 of PET panels per year but believes it can take this up to over 50,000m2 in 2008 and up to 500,000m2 in 2009. The beauty of rotary screen printing is its potential to achieve fast speeds and maintain high quality. Of course, in this particular production configuration, the overall speed is determined by a host of other processes. However, the Stork team are actively supporting its Swiss customer in finding ways to keep boosting productivity.
Diego concluded: “We believe there is enormous potential for this application. And with the productivity of the Stork Prints system, we are well placed to make this energy-saving system accessible to the market.”
