Industry report 2011 - Suppliers for Photovoltaics: Ecology

Ecology

Effective environmental management during the production process enables companies to achieve sustainable reductions in expenditure and cut costs. Recycling also mitigates the impact of increases in the prices of raw materials, which are expected to become considerably more expensive. A green product, sustainable manufacturing and systematic recycling of raw materials are the three pillars of the “Triple Green” concept.

Less materials, less energy

Leading manufacturers have opted to install large solar farms on the roofs of their factories in order to keep their energy cycle green. Using waste heat is increasingly becoming standard practice, even in the manufacture of glass. In new solar glass factories, waste heat is used to generate electricity using turbines and to produce heat.

Water usage is another important issue: A modern glass factory uses around 1,000 cubic meters of cooling water every hour. Water is kept in a closed circuit in order to compensate for losses when the glass is quenched. Solar cell manufacturers are also bearing down on water costs. Acids, for example, are needed in order to etch the silicon wafers used in crystalline solar cells. These acids are then mixed with chemicals to neutralize and purify the waste water. At an early stage during the design of a factory, processes are optimized so that 70 percent of the water used in the etching process can be re-circulated.

Using materials and energy efficiently is also the key to cutting manufacturing costs. While solar cells are becoming thinner and thinner, manufacturing them requires a lot of energy. Semiconductors are produced by fusing quartz sand at extremely high temperatures; this metallurgical process consumes huge amounts of energy. Various acids and gases are also needed in order to manufacture semiconductors. If machines slice the pure silicon into wafers, roughly 40 percent of the silicon is wasted in the form of off-cuts. This method has been the pre-eminent process for manufacturing crystalline solar cells worldwide until now.

Innovative fluidized-bed reactors are the latest way of manufacturing solar-grade silicon. They save up to 80 percent energy by continuously passing silane process gas through a funnel with raw silicon. This process does save energy but involves material losses that are similar to those of the process described above. In contrast, the string-ribbon process generates less waste. In this process, wafers are directly formed by pulling two wires through the molten silicon.

When producing thin-film silicon modules, process gases alone, such as silane, nitrogen trifluoride and ammonia, account for around a fifth of total manufacturing costs. Manufacturers of modules containing extremely thin layers of amorphous silicon have been able to reduce their consumption of silane, the process gas, by three quarters within a year by making more effective use of the gas during vapor deposition.

Particular issue: process materials

: Thin-film production waste for reprocessing

: Recyclable PV glass

Silicon-based thin-layer modules require special gases in order to clean unwanted deposits from the process chambers used to deposit extremely thin silicon layers. Nitrogen trifluoride or sulfur hexafluoride are used as sources of activated fluorine. Releasing one kilogram of nitrogen trifluoride into the atmosphere is equivalent to emitting 17.5 tonnes of carbon dioxide, while one kilogram of sulfur hexafluoride has the same environmental impact as 23 tonnes of carbon dioxide. The standard procedure is to discharge these gases from the process chambers through an airtight pipe system after the cleaning process and chemically neutralize them. A new technique that uses hydrofluoric acid renders hazardous cleaning gases superfluous. This does, however, demand additional investment. Because fluorine is classified as extremely volatile and corrosive, it requires double-walled pipes for security reasons.

Comprehensive recycling

: Employee at the filtration plant

The solar sector has developed closed-loop systems for its products to a greater extent than almost any other industry. Members of the PV CYCLE Recycling Initiative account for around 75 percent of the European market for solar cells and modules, and also include companies based in Asia. PV manufacturers have made a commitment to the European Commission in Brussels to devise their own recycling solutions. By 2015, there will be a Europe-wide network for recycling solar modules.

: Removing the metal layers

Since late 2009, a nationwide scheme for the return and recycling of old modules has been running in Germany. A pilot recycling line for crystalline solar modules was operated in Freiberg (Saxony) until the end of 2010. The modules were removed from their frames and the connection boxes were separated. The glass-glass laminate was then heated for several hours in order to burn off composite plastics. Glass, string connectors and silicon could then be separated and recycled. Etching was used to remove the metal coatings of the cells and doped layers among other components. The silicon was chemically purified. Breaking up the wafers produced high-purity silicon which could be remelted into ingots.

PV CYCLE is currently discussing the possibility of scaling this technology up to an industrial size. This will also open up the market for new technologies and disposal contractors. Recycling is an attractive line of business: The annual quantity of end-of-life modules in Europe is estimated at around 5,000 tonnes, and this figure is expected to rise sharply from 2020 onwards.

Early days for CIS and CIGS

So far, there has been hardly any recycling of solar modules made of compound semiconductors containing copper and indium (CIS or CIGS) because mass manufacture of these panels is still in its infancy. The crucial aspect of recycling is indium; the solar industry may possibly need around 580 tonnes of indium by 2050. Indium has previously been difficult to recover. Increasing demand would therefore have to be met from new deposits. Much the same is true for gallium, the material needed for CIGS solar modules. By 2030, gallium consumption will be equivalent to six times current production volumes. It is only a question of time until new recycling technology suppliers enter the market in order to fill this gap. As the quantity of discarded modules increases, recycling of solar modules will develop into an independent industrial sector that will also offer opportunities to suppliers of plant equipment.

Heavy metals in components

Amendments to the EU “Restriction of the use of certain hazardous substances” (RoHS) environmental protection Directive 2002/95/EU were debated in Brussels in the fall of 2010. It prohibits heavy metals such as cadmium or lead in portable electronic devices. Solar modules will continue to be exempted until 2014 because power plant engineering is generally not covered by RoHS. Nevertheless, the debate made one thing clear: The solar industry will increasingly be expected to reconcile economics with ecology. This is why many module manufacturers are opting to replace leaded solder pastes for stringers with lead-free technologies. In the case of inverters, which contain a lot of electronics, new devices from some suppliers already use lead-free solder throughout. Cadmium is currently used as a buffer layer in CIS and CIGS modules and as a semiconductor in modules containing cadmium telluride.

: Plastic scrap from laminated

: Glass pellets from laminated modules

: A detached and dried PV thin-film

: Recycled indium

Cadmium telluride

There has been a proprietary take-back scheme for modules containing cadmium telluride for several years now. Decommissioned modules are first shredded into centimeter-sized pieces in hammer mills. This breaks up the lamination. Dilute sulfuric acid is then used to dissolve and wash out the semiconductor layers. The solid materials are then separated from the process fluid. The metal-charged fluid is filtered and concentrated in several stages. The resulting filter cake provides cadmium telluride for new modules. As much as 95 percent of the cadmium telluride can be recycled.

The further development of solar modules containing cadmium telluride is expected to boost demand by up to 700 tonnes by 2050. Demand for tellurium is set to grow to around 300 tonnes by 2030. The limited supply of tellurium – 132 tonnes in 2006 – highlights the economic necessity of recycling.

Don’t underestimate the toxicity of the materials

: Establishing the concentration of metals released in the recycling process

Because cadmium, whether in the form of cadmium telluride (cell material) or as cadmium sulfide (buffer layer and contact layers), is a heavy metal that is toxic to humans and harmful to the environment, companies have to handle it with special caution. Care is required above all during manufacture and when handling waste water that contains cadmium.

The manufacture of solar equipment also requires various ancillary chemicals whose use is governed by the EU “REACH” Regulation (Registration, Evaluation, Authorization and Restriction of Chemicals) Directive. Photovoltaics is hardly affected by this as yet, but more stringent measures are only a question of time. Discussions are currently underway in Brussels on merging RoHS and REACH into a single regulation in the medium term.

In any event, the production of solar cells, solar modules, and components produces toxic waste. Disposal of this waste costs a lot of money and makes manufacturing more expensive. For instance, the potassium hydroxide solution used during wet etching of solar cells is only partly spent (two percent) when it is discarded as process waste. For the fabrication of solar modules made of thin silicon layers, for instance, explosive or toxic gases that contain silane or phosphane are used. Here too, the challenge is to use chemicals more efficiently and manage them in a closed circuit.

Environmental management is a must

: Commissioning of an automated line

Introducing an environmental management system is an integrated approach to identifying specific manufacturing costs and systematically looking for potential improvements. Most manufacturers apply international DIN EN ISO 14001 or EMAS standards. DIN stands for “Deutsche Industrienorm” (German Institute for Standardization), EN stands for “European Standard”, ISO stands for “International Organization for Standardization”, and EMAS stands for “Eco Management and Audit Scheme”.

DIN EN ISO 14001 lays down the requirements placed on an environmental management system. The main emphasis is on the continuous improvement of environmental targets, specified by the company itself. The DIN EN ISO 14001:2005-06 standard applies in Germany.

EMAS is also referred to as “eco-audit”. The scheme was developed by the European Union and is used for eco-auditing and environmental management. Independent experts scrutinize the environmental statements made by participating firms at regular intervals. EMAS II incorporates additional specifications from ISO 14001 and enables manufacturers who have various plants to be validated in a single procedure. EMAS III came into force in early 2010 and essentially concerns concessions for small and medium-sized enterprises.

Social standards

Renewable energy currently enjoys a strong position of trust. This is why the public is particularly concerned with the situation of factory workers. The solar industry would therefore be well advised to insist on high levels of social responsibility from producers and their suppliers – all over the world. In Germany, the industry’s task is to achieve levels of pay and worker participation that are at least equivalent to those in the automotive industry. Many companies are moving in this direction, setting up works councils and negotiating collective wage agreements. Only then will the solar industry be able to exploit its full potential and push ahead with the social and ecological transformation of the industrial society in exemplary fashion.

Market Segment Recycling

The market for module recycling is slowly becoming established with an increasing number of independent technology providers entering the scene, all searching for economic module recycling solutions. At present, research focuses on a special material that allows EVA films and other reusable materials to be removed and recycled without the need for heat or manual work. Another method uses acids to dismantle old modules. At the Brandenburg University of Technology (BTU) in Cottbus/Germany, one area of research focuses on the disposal of hazardous waste in environmental technology. The German Federation of Industrial Research Associations (AiF) recently sponsored a project at the BTU where a test facility was built that enables semiconductors and contact materials to be fully recovered from thin-film modules. This method uses vacuum ejector technology. In addition, a new type of high-performance blasting lance has been developed that allows the recyclable materials to be removed layer by layer. At the German Federal Institute for Materials Research and Testing (BAM), systematic tests on the concentration of recyclable materials were performed using a physico-chemical flotation technique. This technique can now also be applied in practice.