By Kay Rehberg
The ‘washing cycle’ to clean the silicon has, up until now, interrupted the production flow and, hence, prevented an efficient throughput. DECKER Anlagenbau GmbH’s development team saw it as a challenge to integrate this process step smoothly into the procedure of efficient recycling production plants which are used for solar silicon. As soon as the team stopped to optimize the washing barrel solution, the eureka moment came: The silicon separation moves swiftly from the band instead of endlessly in the barrel in this newly developed silicon granulate etching system. The German company recently received an award for this system at the Intersolar Europe in Munich in the PV Production Technology category. The new system is already successfully implemented for practical application and is distributed. One of the largest technology suppliers has been an early adopter of the DECKER invention and uses the technology to recycle the valuable silicon waste which occurs during the production of the solar cells. Modules which get broken or offcuts which already occur during the wafer cut can be recycled and reused in the production. This becomes increasingly important and more cost-efficient due to growing production numbers at the solar module manufacturers. A further market segment is also the recycling of old solar plants which promises to be an area with rapid growth for the future. It doesn’t matter whether the recycled silicon can compete with the spot market prices for silicon but the fact that the manufacturer is responsible for the recycling of the electronic waste. Cost-efficient recycling becomes significant in future.
Module Recycling Is a Business Worth Billions
A small group already thinks ahead to the future while the solar sector busies themselves to cover the world with PV plants--driven by climate protection goals and government subsidies. In order to illustrate the scope; for the first time, a large number of older PV plants will be replaced by newer and more efficient models in the coming years. The waste quantities are still manageable--about 800 tons per year in Europe. However, this is going to change in the near future. In 2010 alone, solar modules with a total weight of 500,000 tons were installed in Germany and it is estimated that those 1st generation modules will have to be replaced in two decades. In the same year, solar plants with a performance of 22 GW were installed which corresponds to about 1.5 million tons of material.
About 220,000 tons of module waste will accrue yearly until 2010 in the European Union alone according to a WEEE-study (Waste Electrical and Electronic Equipment). By 2050, this will be about 10 million tons. Recycling this waste could cost the solar industry billions. Since photovoltaic modules are electronic waste, they will most likely be included into the WEEE-directive, as this has already been recommended by a study commissioned by the European Union, or the manufacturers pledge it themselves. Both means, that the manufacturers of solar modules are responsible for the recycling of their product and have to pay for the recycling in the same way as mobile phone and computer manufacturers. The sector currently benefits from an exemption rule but this will not last forever.
The recycling concept gains in importance when viewed in the conext of environmental protection. And it also important for the green image of solar companies. Recycling is the first choice both from an ecological as well as economical point of view since waste materials must not have an impact on the environment and have to be disposed of properly. Recycling management has to be established. It is the high time to launch innovative recycling methods.
About 90% of the silicon waste occurring during module recycling consists today of crystalline silicon. Despite the increasing market share of thin-film cell which have a lower share of silicon, experts still expect that the effects of the waste quantity will only be reflected in the second or third generation of solar modules. The actual recycling process of a module consist of several steps: First the plastic, glass and frame parts are removed, then the wafer is shredded in order to reclaim solar silicon from the silicon breakage or granulate. Before the broken silicon can be reprocessed into blocks for wafer manufacturing, the granulate has to be cleaned in an etching bath. Up until now, the granulate was filled into a barrel and cleaned like in a washing machine. This interrupted the production flow. Hence, the search was on to find an etching system which could be integrated into the production flow.
High Throughput at Low Costs
Cleaning of small parts or granulate is a technical challenge due to the large surface area and the required degree of cleanliness. Plant manufacturers have to provide solutions that meet the solar sectors requirement in highly purified silicon and at the same time provides a cost-efficient process.
Moving the cleaning process to a band in the way DECKER’s developers have done is ground-breaking on several levels. The belt filter option also has a positive effect on process parameter: The developers are convinced that up to 50% of cost and time can be saved. Energy and chemical savings also have a positive effect on the environment. The most important progress, however, is that the technology now allows the use on a large scale. Cleaning of silicon production waste is only possible when using this technology. Standard cleaning methods are reaching their limits in this setting since they drastically reduce the production throughput. In comparison, the typical scale for the throughput of the new belt filter is about 1,000 tons per year. Therefore, the belt filter is the ideal alternative to existing cleaning methods, which use standard product carriers such as barrels or baskets. Quality problems, in addition to time and cost factors, are inherent to the system since the etching processes take longer inside the baskets than they do on the surface. Costs for DECKER-belt filter depend on different parameters; return on investment within one year can be realised under certain constraints.
Fast Cleaning Down to the PPBW Zone
Cleaning silicon surfaces is a complex task. There a many different possible contaminations, which range from finger prints to metallic residues which arise during milling and transport processes. Cleaning the relatively large surface becomes a challenge for grain sizes ranging from 100 micrometre and four millimetre. Belt filters remove all contamination completely to the required level which is within the PPBW zone (PPBW = Parts Per Billion by Weight). The cleaned silicon has a purity degree of 99.9999%. At the moment, the required average surface quality for high quality polysilicon granulate is at a value of less than 50 PPBW for metals such as iron. The purity of silicon is a decisive factor when it comes to the efficiency of solar cells. Metal contamination shorten also their lifespan. Recombination capable contamination such as iron, copper and titanium reduce the efficiency of solar cells.
Contamination causes on the surface as well as directly underneath are:
-The fast diffusion of metal atoms such as nickel or copper into the silicon at room temperature
-Silicon tears and breaks as well as smallest silicon particles
-Contamination due to transport, crushing, storage and packaging
The market requires more and more a silicon granulate with a high degree of surface cleanness in order to manufacture modules with better efficiency. The surface quality of silicon granulate has to keep pace with increasing demands into the quality of solar modules. Valuable solar silicon is accrued as waste or scrap in different forms (powder, granulate, lumps, wafer) during the production of solar wafers in almost every single step during the production process. This can be reused in the production of silicon ingots which in turn reduces the manufacturing costs. Almost all types of silicon waste can be cleaned in an entirely closed system with integrated exhaust gas and particle filter with hardly any loss of valuable silicon.
The optimization of plants to increase the production whilst at the same time reducing costs is of utmost importance for the future of solar companies. Silicon granulate etching system with band filter enables manufacturers to recycle solar silicon quickly and cost-efficiently.
Kay Rehberg is Managing Director at DECKER Anlagenbau GmbH (www.decker-anlagenbau.de) in Berching, Bavaria, Germany. He optimizes and develops wet-chemical processes for large solar technology suppliers as customer-specific turnkey solutions.
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