The Value and Feasibility of Proactive Recycling
V. M. Fthenakis and P.D. Moskowitz
Environmental and Waste Technology Center
Brookhaven National Laboratory, Upton, NY 11973
Photovoltaics (PV) technology has definite environmental advantages over competing electricity generation technologies, and so far these advantages have driven market penetration. The PV industry follows a pro-active approach to preserve its safe and environmentally friendly nature. Industrial ecology considerations raise the issue of what to do with the PV modules at the end of their useful life. One option is recycling. This paper discusses the value of proactive recycling and compares several alternatives.
It is known that large-scale market penetration by photovoltaic systems will bring great environmental benefits because their operation does not generate noise, toxic-gas emissions, or greenhouse gases. To safeguard the environmental friendliness of photovoltaics, the industry follows a pro-active, long-term environmental strategy involving a life-of-cycle approach to prevent environmental damage by its processes and products from "cradle to grave". Recycling manufacturing waste and spent PV modules is part of this strategy. The industrialized world is moving towards recycling in all areas. Although the PV industry will not face this problem in a large scale before the year 2020, today’s material selection and module design may very well set a precedent for the future.
2. The Value of Proactive Recycling
The value of recycling lies in environmental benefits, market acceptability and support, environmental regulations, and resource availability.
The environmental benefits of recycling products are, in general, related to savings in landfill space, energy, emissions and raw materials. Savings in energy use are translated to savings of CO2 emissions and give recycling an "added value"; perhaps recycling will qualify in the future for support from funds earmarked for reducing global warming. Public support of PV energy is driven by the environmental benefits derived by substituting fossil fuel plants with PV systems. Recycling PV systems at the end of their useful life adds to the environmental benefits and can further enhance market support. Also, recycling answers public concerns about some materials in PV modules (e.g., CdTe), which can create barriers to market penetration.
Photovoltaic modules may contain small amounts of regulated materials, which vary from one technology to another (e.g., Cd, Pb, Se, Cu, Ni and Ag). Environmental regulations can determine the cost and complexity of dealing with end-of-life PV modules. If they were classified as "hazardous" according to Federal or State criteria, then special requirements for material handling, disposal, record-keeping and reporting would escalate the cost of module decommissioning. Therefore, there is an economic incentive to design modules that will not be hazardous, or to design them in such a way that can be recycled at a reasonable cost.
Lastly, some PV materials (e.g., In, Te) may reach availability constraints in multi-GW production levels.
3. Existing Infrastracture
In this section, we briefly describe the experience from recycling similar products in other industries and then formulate a feasible recycling plan for solar cells. More details on the existing recycling infrastructure can be found elsewhere (2, 3).
The electronics and telecommunications industries recycle a wide range of used and unused products through a plethora of collection and processing channels. Large companies (e.g., AT&T) combine in-house collection with collection by "reverse logistics" companies, who collect, consolidate, pre-process and ship the products to a service center. During this stage, the items are regarded as used products, not wastes, and waste-handling or processing permits are not required. The service center either refurbishes the used equipment for resale; or disassembles the unit for spare parts; or dismantles the unit to reclaim the materials. The economics of recycling electronics and telecommunications is driven by the value of the usable components salvaged from recycled units and by their precious metals content.
The NiCd battery industry collectively moved towards recycling because of regulatory requirements and market acceptance concerns. A consortium of NiCd battery manufacturers funds and oversees a non-profit take-back program administered by the Rechargeable Battery Recycling Corporation (RBRC), using dedicated collection and recycling facilities. Participating commercial and institutional generators agree to return spent NiCd batteries to designated consolidation facilities. Participating retailers receive recycling kits; they set up collection boxes, and send full boxes to the International Metals Reclamation Company, Inc. (INMETCO). City and county municipal collection centers also gather batteries and send them to consolidation centers which, in turn, send them to INMETCO. INMETCO recover nickel and iron from NiCd batteries and use them in Fe-Ni-Cr alloy which they sell to the stainless steel industry, and also recover high purity cadmium which is returned to the NiCd industry. The experience of RBRC’s operation here and in Canada showed that recycling programs might encounter barriers related to the difficulty of allocating cost among participants, non-uniformity of state laws, restrictions from antitrust laws, national laws and international agreements affecting transportation, and additional overhead caused by tax laws. The program to be effective was expanded to involve distributors, retailers, end-users, and the government, in addition to manufacturers (4).
3.1 Possibilities for Collecting & Recycling Solar Cells
Recycling solar panels is more complicated than that of the above products, because of the decades-long interval between installing and discarding modules, their low content of valuable materials, and their geographical dispersion. The collection of spent modules is a challenge. In a previous study(3,5) we outlined three generic paradigms for an institutional collection infrastructure: the utilities, electronics, and battery paradigms.
1) In the utility paradigm, large end-users (e.g. electric utilities) would be the primary owners and servicers of large PV systems, and, therefore, the entities primarily responsible for getting the end-of-life modules to the recyclers. PV-module recycling would be integrated with other utility programs and recycling charges would be imbedded in the rates charged by the utility. 2) In the battery paradigm, manufacturers would be collectively responsible for collecting and transporting modules to recyclers through the incorporation of a collectively supported PV-module recycling entity. Reverse retail channels and consolidation entities would be responsible for collecting, consolidating, and transporting the modules which would be recycled by dedicated dismantlers, and materials recyclers. Goods collected through reverse-retail channels could be sent directly to smelters, or other recycling facilities, under pre-paid shipping arrangements. Consolidation entities could collect goods from municipal recycling centers and large commercial and institutional generators. Recycling services might be paid for by industry dues to the collective recycling entity. 3) In the electronics model, recycling of solar panels would mimic that of electronics and telecommunications products. Manufacturers would be individually responsible for collecting, consolidating, and transporting obsolete modules to the recyclers; this would likely involve reverse-logistics companies, and recycling would be carried out by integrated dismantlers (i.e. not exclusive to solar panels) and materials recyclers. Recycling services might be paid for by the generator, the manufacturer, or an escrow fund set aside when the PV systems were originally purchased.
These examples are useful in guiding collection programs that are likely to work for solar panels: 1) Paralleling the utility paradigm, on-site collection of decommissioned solar panels is practical and economically feasible for large, centralized installations. 2) Paralleling the battery paradigm, reverse-retail channels and periodic pick-up by consolidation (reverse logistics) companies may be the best strategy for collecting dispersed modules in small, remote installations and from consumers (3).
4. Feasiblity of Recycling Solar Cells
A typical photovoltaic module manufacturing facility generates a significant amount of scrap at the start of its operation but, within a few months to a year, reaches a steady-state level of production generating relatively little waste. In discussing PV recycling, we should distinguish between near-term and future needs and capabilities because of the long lapse between the start of manufacturing and decommissioning, and the corresponding differences in scale and technology. Near-term needs can be met by either centralized or de-centralized approaches, whereas future, large-scale needs would be more economically served by centralized strategies.
4.1 Centralized Strategies
Large smelters (e.g., Noranda, ASARCO), routinely recycle circuit boards, computer monitors, consumer electronics and telecommunication equipment to recover metals. Such facilities might incorporate the recycling of spent PV modules; today they recycle the relatively small amounts of scrap produced by PV manufacturers. The low concentration of metals in photovoltaic modules and scrap does not give them any significant recycling value, but their glass content has a certain value to smelter operators who buy silica for their fluxing operations. Glass credit, therefore, reduces the costs of smelting down to $200 per ton (~2˘/W for thin-film PV modules) for large deliveries of shred material (e.g., 20 ton containers). CdTe and CIS panels can be treated in copper smelters. The glass content of the shred is used up in the fluxing operation of the smelter, the ethylene vinyl acetate (EVA) and plastic backsheet decompose into vapors (that need to be treated), and the furnace anodes collect molten copper and the metals dissolved in it. These anodes are processed at the copper refinery, where metallurgical grade of tellurium and (if the supply is sufficient) of selenium are recovered electrolytically. Contact metals (e.g., Al) accumulate in the solution and are removed in the purification and acid-recovery phases. Cadmium does not dissolve in molten copper but remains in the waste stream of the copper smelter.
The average cost of transporting large deliveries of non-hazardous waste to smelters from US locations is estimated as $220 per ton ($30 to $400 per ton depending on location and quantity); this is equivalent to an average of 2.3 ˘/W for thin film modules. The projected cost of collecting and transporting thin-film solar panels from dispersed installations is about 8 to 10 ˘/W (5). Therefore, the total cost of recycling in smelters thin-film PV modules or scrap from large installations is 4 to 5 ˘/W; from dispersed installations it is estimated to be 10 to 12 ˘/W. These figures do not include credit for avoiding potential future environmental liability for disposal, and are likely to go down as separations technology and collection infrastructure mature. For comparison, the current total cost of disposal of large deliveries of trash in a local non-regulated landfill is about $100/ton (equivalent to 1 ˘/W). The current cost of disposing of hazardous waste is about $0.50/lb for 55-gal drums; this is equivalent to 35 ˘/W for thin-film modules (6). Although the concentration of metals like cadmium and lead in a solar panel is minuscule, it may cause a hazardous waste classification, with all the associated cost implications. It would make sense to separate these metals from the glass (which is the bulk of a solar panel), if it can be done economically. The de-centralized recycling strategies discussed below follow this approach.
4.2 De-centralized Recycling Strategies
4.2.1 CdTe & CIS Modules
Solar Cells Inc. (SSI) developed an operation for recycling CdTe modules, which starts with disassembly of a module and recovery of lead wires. Then, the module is crushed in a hammer mill. The module’s parts (e.g., mounts, coated glass and most of the EVA) are separated during different times of the milling process. The crushed glass is stripped of metals in successive steps of chemical dissolution, mechanical separation, and precipitation or electrodeposition. At the end, the mounts, glass, and EVA are completely recovered. The recovery of tellurium is 80% or better, and it can be sold as commercial grade (99.7% Te). The remaining metals (e.g., Cd, Te, Sn, Ni, Al, Cu) are contained in a Cd-rich sludge which is currently sent to INMETCO where Cd is recovered and eventually used as feedstock for NiCd batteries. The estimated total cost for this operation is approximately 4-5 ˘/W, excluding transportation (6).
Drinkard Metalox Inc. (DMI) developed operations for recycling CdTe and CIS-modules. Their operations include chemical stripping of the metals and EVA, skimming off the EVA from solution, and successive steps of electrodeposition, precipitation, and evaporation to separate and recover the metals. DMI reports recovery of 95% or better of Te and 96% or better of Pb from CdTe modules. Chemical stripping leaves the SnO2-conducting layer intact on the glass substrate, potentially allowing the re-use of the substrates for PV deposition. They project a processing cost of 9 ˘/W (7) or less.
Both of these operations are based on hydro-metallurgical principles, and are effective in recovering Pb and Te from CdTe modules in re-usable purities; the current recovery from SSI cells is lower than that from DMI. The following are the major differences (relative advantages/disadvantages) in these operations: The SSI operation uses sulfuric acid, whereas the DMI uses nitric acid for stripping; HNO3 creates acid fumes (which can be controlled by scrubbing), whereas H2SO4 does not. On the other hand, HNO3 is the stronger of the two solvents, and its use allows the stripping of full intact modules, if necessary. SSI’s crushing/milling process recovers polyurethane mounts and potting and most of the EVA in dry form, whereas DMI’s wet process recovers these materials as sludge.
4.2.2 Crystalline-Si Modules
Recently, SSI, Pilkington Solar International (PSI) and DMI reported methods to delaminate x-Si PV modules and recover crystalline Si wafers or functioning Si solar cells (6,8). SSI worked with PV coupons (not complete modules). They recovered most of the Tedlar backsheet and functioning x-Si cells. Their method starts by gently heating and manually peeling off the backsheet. Then inert atmosphere pyrolysis at about 500 oC vaporizes the EVA lamination layer. SSI recovered functioning cells with slightly lower electrical efficiency than the original ones. Efficiency dropped from 12.8% to 10.73%, but SSI thinks that this can be improved. Si-cell recovery was estimated to cost about 13 ˘/W, for an operational scale of 150,000 x-Si cells per year. For comparison, a new x-Si cell costs at least $1.50/W to produce today (6).
PSI used pyrolysis to delaminate x-Si modules (8). They worked with 706 full-size modules and they reported recovery of 60% of the wafers being processed. These wafers were reprocessed into cells, which had slightly better efficiency than the original ones. PSI did not attempt to recover functioning Si cells, and did not estimate costs for their process. The PSI pyrolysis takes place in air instead of an inert (nitrogen) atmosphere; air likely causes oxidation of the cell’s silver grid and makes cell recovery problematic. Also it is possible that air creates surface carbonization and places the requirement for longer pyrolysis times to clean-up residuals. SSI’s delamination of 10 cm by 10 cm PV coupons is reported to be completed within 90 minutes, whereas PSI’s delamination of PV modules takes 4 hours. Both processes generate EVA decomposition products that can be controlled with combustion devices.
DMI’s process is based on hydrometalurgical processes using stripping with a HNO3 solution, similarly to their processes for treating CdTe and CIS.
The photovoltaic industry and the DOE follow a pro-active long-term environmental strategy to preserve the environmental friendliness of solar cells. Accordingly, options are being investigated to recycle used solar cells and manufacturing waste. The current paper showed that such recycling is technologically and economically feasible. A recycling program was outlined, based on current collection and recycling infrastructure and on emerging recycling technologies. Metals from used solar panels in large centralized applications can be reclaimed in metal-smelting or refining facilities which use the glass as a fluxing agent and recover most of the metals by incorporating them into their product streams. In dispersed operations, small quantities and high transportation costs make this option relatively expensive. Separating the PV materials from the glass reduces the amount of waste generated by three orders of magnitude. Effective and economical methods of such separation have been developed that can be used in both small-scale (in-house) and large-scale recycling.
Currently, economic incentives may be inadequate to move the PV industry into voluntary recycling. However, this may change in future, as more economic incentives may be given to developing clean technologies, preventing pollution and reducing CO2 emissions.
This work was supported by the Photovoltaic Energy Technology Division, Conservation and Renewable Energy, under Contract DE-AC02-76CH00016 with the US Department of Energy. The authors thank J.Bohland, Solar Cells; C. Eberspacher, UNISUN, and R. Goozner, Drinkard Metalox for their review of and contributions to this paper.
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Last Modified: June 18, 2008
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