MANUFACTURINGMATERIALS
edge on the extremely competitive thin-film market.
The key to more efficient manufacturing is a so- called fluorine-on-site-generator made by the Linde company and connected to the supply lines of the plant. Malibu’s modules are made by vapour- depositing silicon onto glass in vacuum chambers. Since plenty of material ends up on the chamber walls in this process, these need to be purified after each coating cycle. The generator introduces the fluorine which then reacts with the silicon to form gaseous silicon tetrafluoride, which is pumped off, captured and reacted off.
The new method reduces climate risks and is fast: usually cleaning accounts for more than 10 % of the total process time in a vacuum chamber but fluorine reduces this time by half thanks to its high avidity, explains Linde-Manager Andreas Weisheit. This improves the line throughput and cuts costs.
Plenty of Chemistry and Energy This makes Malibu one of the pioneers in the industry seeking not only rapid but also clean growth. Triple Green ideally means: factories built using organic materials and supplied with energy from regenerative sources producing silicon, wafers, cells and modules while emitting scarcely any carbon dioxide or waste materials anymore and consuming minimum energy, gas, acid and water.
The ambitious green motivation of the solar industry goes back to one reason in particular: its clean image is at stake because its rapid growth goes hand in hand with increased resource consumption and emissions. Over the past two years global module output has doubled to eight gigawatt (GW). And this growth is predicted to continue at the same rate according to forecasts. As Eric Maiser of VDMA, the German Engineering Federation, explains silicon generation requires high levels of chemical and energy resources: producers melt down sand to metallurgical silicon and then use hydrochloric acid to reduce it to liquid trichlorosilane. This then is thermally decomposed and in the process grows into solar- grade silicon in the form of ingots. The processes run at high temperatures and are “power guzzlers”. In most cases this power comes from nuclear and fossil-fuel fired power plants because conventional power is simply cheaper for producers and more easily available than that from regenerative sources.
The following wafer and cell manufacturing processes are just as resource consuming: when cutting the wafers out of the ingots nearly half the silicon is lost and must be melted back down for it to be re-used. For cleaning wafers producers often use hydrochloric acid and for etching their surfaces poisonous fluoric and nitric acid as well as potash lye are deployed. The busbars applied as a rule consist of silver and aluminium. To electrically orient the crystals, companies use phosphoric acid. At the end of the day all of these chemicals end up in the waste water of the solar factories. Although this is treated, pollutants such as heavy metals and nitrates find their way into the sewage system.
Thin-film production also requires high amounts of energy and chemicals. CIS or CdTe modules are made at high temperatures and in long processes from copper, indium, toxic selenium, cadmium and sulphuric acid or cadmium telluride. For thin-film silicon manufacturers cleaned their chambers with NF3 until now. And however carefully they do this – they cannot prevent 100% of the greenhouse gas emissions. “17 % are released into the atmosphere during the product life cycle,” says Linde Manager Weisheit who goes on to say that this is also the reason for the rising demand for Linde’s fluorine generators in the photovoltaics industry.
Suppliers with Clean Solutions However, solar system producers can do a lot more than just change their gas suppliers. Upstream suppliers offer them a number of possibilities to make their production greener. State-of-the-art production equipment increases
Ökowerk: The power required for module production is obtained by the Freiburg-based company Solar-Fabrik from cells on the façade and roof of its own factory. (Photo: Solar-Fabrik)
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Energy-intensive: For electrical wafer orientation producers introduce phosphorus into the silicon at high temperatures in diffusion furnaces. (Photo: Q-Cells)
www.solar-pv-management.com Issue VI 2010
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