The thin-film silicon manufacturer Malibu, based in the German town of Osterweddingen, is one example of a company trying to make its solar manufacturing processes greener by cleaning its process chambers with fluorine rather than the hazardous greenhouse gas nitrogen trifluoride (NF3).
Although this does not sound too spectacular it does have major ecological benefits. If inadvertently released into the atmosphere NF3 is 17,200 times more of a global warming threat than carbon dioxide. Fluorine, by comparison, has no greenhouse gas potential. “It allows us to avoid any emission risks,” says Malibu Production Manager Antje Bönisch.
This turn towards ecological processes became attractive to the company because it significantly lowered operating costs. “We are saving a six-digit sum every year,” says Bönisch, adding that it has given Malibu a competitive edge on the extremely competitive thin-film market.
The key to more efficient manufacturing is known as a ‘fluorine-on-site-generator’, which is made by the company Linde 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. This is then is pumped off, captured and reacted off.
The new method reduces climate risks and is fast; cleaning usually accounts for more than 10% of the total process time in a vacuum chamber but fluorine reduces this by half thanks to its high avidity, explains Andreas Weisheit, Manager at Linde. This improves the line throughput and cuts costs.
The ‘Triple Green’ motto ideally encompasses several factors: 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, and consuming the minimum amounts of energy, gas, acid and water.
The solar industry's green ambitions are important; its clean image is at stake because rapid growth has gone hand-in-hand with increased resource consumption and emissions. Over the past two years, global module output has doubled to 8 GW and this is predicted to continue at the same rate according to forecasts.
Eric Maiser of the German Engineering Federation (VDMA) explains that silicon generation requires high levels of chemical and energy resources. Producers melt sand to metallurgical silicon and then use hydrochloric acid to reduce it to liquid trichlorosilane. This is then thermally decomposed and in the process grows into solar-grade silicon in the form of ingots. The processes run at high temperatures and use a large amount of power. In most cases this power comes from nuclear and fossil-fuel fired power plants because conventional power is cheaper and more easily available than that from renewable sources.
The 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. Producers often use hydrochloric acid for cleaning wafers, and for etching their surfaces they use poisonous fluoric and nitric acid as well as potash lye. 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 process 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 still find their way into the sewage system.
Thin-film production also requires a large amount 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. Until recently, thin-film silicon manufacturers cleaned their chambers with NF3, and however carefully they do this – they cannot guarantee 0% greenhouse gas emissions. “17% are released into the atmosphere during the product life cycle,” says Weishei.
Suppliers with clean solutions
Solar system producers can do a lot more than just change their gas suppliers, however. Upstream suppliers offer them a number of possibilities to make their production greener. New production equipment increases the yield while reducing consumption levels. Suppliers of thin-film production equipment such as Applied Materials or Von Ardenne offer machines that apply the absorber material faster to larger surfaces.
Machinery manufacturers specialising in crystalline systems such as Schmid supply plants that can process thinner silicon wafers and there are also more and more recycling specialists offering their services to the solar PV industry. For instance, the Italian company Saita has recently begun offering cell producers a system that recycles 96% of the process water for reuse in a closed loop system. This reduces the fresh water requirements for cell production by 75%, explains Marketing Manager Carlo Enrico Martin. He adds that thanks to this recycling no waste water ends up in the sewage system.
The factory model
Berlin factory planner Ib Vogt goes one step further; it has developed a so-called ‘Greenfab’ that is built and operated ecologically, and can produce up to 1 GW of solar energy. The energy required, explains Project Manager Lino Garcia, is generated by solar and geothermal systems on site. Waste heat is used for heating and cooling and less polluted water ends up in the sewage system as a large proportion is recovered. Integrated logistics and transport concepts shorten distances and also boost energy efficiency.
However, as big as the benefits of green fabrication are, the eco-breakthrough is a gradual rather than a sudden one. Although it was developed three years ago, Ib Vogt has not yet sold a single complete Greenfab, only single manufacturing components such as concepts for waste-water recycling.
“In contrast to the chip industry, PV has not embarked upon the green manufacturing voyage yet,” says Carlos Lee of SEMI, the global semiconductor association.
Green in small doses
So what is inhibiting the breakthrough of Triple Green in the solar sector? A decisive aspect is the high investment required for sustainable production. A Greenfab with 1 GW capacity is “definitely somewhat more expensive” than a regular 1 GW factory, explains Garcia.
This investment pays off through energy and raw material savings, he says, but the exact amortisation period differs from case to case. What is key though is that those investing millions of euros must know the timeframe for return on their investment. A period of 10 years would probably be too long as factory design and fittings would be outdated considering the high speed of innovations in solar PV.
Moreover, the recession has put a brake on investment and many producers have faced declining sales and profits. “At a time like this major spending is taboo,” says Kevin Reddig of the Fraunhofer Institute for Manufacturing Engineering and Automation in Stuttgart, Germany.
The introduction of green manufacturing technologies and processes currently suffers the same fate as the implementation of novel cell concepts; the commercialisation of complex solutions associated with high financial spending such as back contact cells are being postponed. Instead, manufacturers are focusing on the less expensive optimisation of standard cells and contenting themselves with moderate efficiency gains.
Since going ‘green’ is expensive, its profitability is vague and the sector is forced to reduce costs, we will see a rather gentle transition to the Triple Green mantra. In the words of Maiser from VDMA, “Green will come carefully dosed.”
For example, it took STMicroelectronics, one of Europe's largest semi-conductor producers, 15 years to reduce its CO2 emissions by 65%, energy consumption by 54%, water consumption by 70% and waste by 71%.
Today, the solar sector is in the same position that the chip industry was 15 years ago. Solarworld does not use a green production system yet, but has started improving the transparency of its sustainability report, which contains all relevant environmental data and that of its upstream suppliers, thereby paving the way for green investment.
Module maker Solon pursues a similar line; it has spent €200,000 on a new environmental management system to gain an overview of where sustainable solutions make business sense.
It appears the industry is at least making a start.