Towards a circular supply chain for PV modules: Review of today's challenges in PV recycling, refurbishment and re‐certification

Photovoltaic (PV) waste, associated to the exponentially growing PV installations on global scale, presents today an emerging environmental challenge but also brings unprecedented and multifold value creation opportunities. In this context, significant PV business and research and development (R&D) efforts shift towards establishing a more sustainable, environmentally friendly and economically viable end‐of‐life (EoL) management for PV modules: including recycling, recovery of raw materials, repair/refurbishment and even re‐use of decommissioned or failed PV modules. In the CIRCUSOL project, PV partners aspire to formalize the repair/refurbish and re‐use value chains in the PV industry and propose a circular business model, based on a product‐service system (PSS). Towards these goals, this review study introduces the relevant research groundwork, a status overview and today's R&D and business challenges in PV recycling, repair/refurbishment and re‐certification aspects for second‐life PV modules. The topics and the relevant reported literature are examined from both circular economy and technology perspective. The review indicates a considerable technological and operational know‐how in PV EoL management that already exists and continuously evolves in mature PV markets. On the other hand, R&D in repair/refurbishment of decommissioned and/or failed PV modules remains scarce, and best practices and commercial services for reliability testing/re‐certification and trading of second‐life PV modules are neither standardized nor consolidated into any PSS or business model.

this massive growth of PV installations is translated into an estimated global PV waste of 1.7-8 Mtons by the end of 2030 and up to 60-78 Mtons cumulative by 2050. 2 Further, these projections consider neither PV waste at production level nor waste from decommissioned PV for economic reasons, such as insurance claims and repowering.
Thus, in reality, PV "waste" volume can even be much higher. recovery/recycling ratio of waste PV modules by mass to be recycled from 2019 onwards. In addition to such a regulatory scheme, it is obvious that EoL and recycling technologies must evolve and become available to meet the increasing requirements of WEEE for the case of PV waste. Yet limited research has been done on optimal EoL management considering recycling and/or re-use due to the long operational lifetime of PV modules (>20 years) and inverters (10 years or more) on one hand, and the limited predictability of defects or failures in fielded PV components on the other hand. For instance, it might be incorrect to extrapolate the current recycling data or systemize repair and re-use practices as it can be argued that they are rather minor issues today 3 ; besides, relevant R&D efforts are still in progress, whereas PV technologies are rapidly evolving.
In this context, the exponentially growing PV waste particularly presents an emerging technical and environmental challenge; though, it also brings unprecedented, multifold value creation opportunities.
One can envisage e.g. new financing mechanisms and multiple revenue streams across the whole PV value chain. PV recycling, recovery of raw materials, repair or refurbishment of decommissioned, failed or degraded PV modules and their recommissioning (second-life PV modules) are indispensable for a more sustainable, environmentally friendly and economically viable solar PV energy-based future. To unlock the benefits and maximize the impact of these opportunities, thorough knowledge and research foundation, along with real-field experience feedback, must be laid down at first. Likewise, streamlined decision-making and business models based on circular economy should be adopted.
Recently, Wade et al. 4 presented a comprehensive decision tree ( Figure 1) depicting the different options for EoL management of decommissioned PV systems. A significant number of scientific studies 5-10 and public reports, 3,11,12 as well as research projects 13,14 and collaborative platforms 15 have attempted over the last years to shed light on these different EoL paths and processes, with a major focus on PV recycling technologies, high-value material recovery, downstream EoL management models (collection-transportationrecycling) and life cycle analyses. So far, insights from the reported literature have been rather fragmented and somewhat one-sided, largely focusing on PV recycling processes and relevant innovation efforts. As such, the potential environmental and economic value of PV re-use remains relatively unexplored; also, knowledge or best practices in repair/refurbishment, reliability and certification/qualification of second-life PV modules have been scarce. Besides, even the most recent EoL management approaches have been mostly examined from the perspective of conventional product-based single-path business models; thus, missing out to address additional value creation opportunities stemming from PV re-use or recycling within circular business models.

| THE CIRCUSOL PROJECT: RATIONALE AND METHODOLOGY/VISION
Today, by default, once PV modules are decommissioned, they enter the waste stream and are either disposed or-in the best case-recycled.
However, it is estimated that up to 80% of the PV waste stream can consist of product defects upon production, transportation or infant failures over the first 4 operational years, 12 instead of products that actually reach the end of their designed technical life. CIRCUSOL 16 partners and experts 15 estimate that about 45%-65% of these PV modules can be repaired or refurbished. Therefore, up to nearly 50% of the PV waste can be diverted from the recycling path. In practice, such ratio is likely to be even higher because decommissioned, though functional, PV modules currently also enter the "waste" stream.
On the other hand, re-use, repair and/or refurbishment remain rather informal and certainly neither systemized nor standardized in the PV industry and overall PV value chain today. In fact, these CIRCUSOL aspires to formalize the recycling, repair/refurbish and re-use segment in the PV value chains and to propose adapted technical standards/regularity framework for these emerging EoL business pathways for PV. This will be the foundation to develop and validate a product-service system (PSS) in the PV sector (next to the batteries sector, which is also addressed in CIRCUSOL), to enable the implementation of circular business models. The proposed PSSbased circular business model and its underlying value-creation goals throughout PV EoL are depicted in Figure 2. As such, PV modules can be designed for both recyclability and circularity towards second-life paths, i.e. for re-use, re-manufacturing and/or refurbishment. Moreover, in this way, decision making for the optimal life path for each PV module can be consolidated and carried out by product service providers, who are also responsible for co-creating value propositions to the PV end-users.

| PV recycling in the c-Si PV segment
In the c-Si PV segment, which represents by far the largest share of PV installations, PV CYCLE comprises a Joint Producer Responsibility system and a recycling pioneer, which has contracted a number of PV recycling companies and technology developers, over the last years.
As such, PV CYCLE was the first to establish a downstream PV module recycling and waste logistics process throughout the EU. In 2016, their contractors' recycling processes achieved a record recycling ratio of 96% for c-Si PV modules (fraction of solid recycled), 18 meaning that not only glass and aluminium is recycled but also silicon. In particular, after the removal of the cables, junction box and frame from the PV modules, PV CYCLE's process includes shredding, sorting and separation of the module's materials, allowing the latter to be sent to specific recycling processes associated with each material. is directed towards aluminium -based refinery; polymeric materials are recycled to fuel used in cement industry; silicon cells to precious metal sectors; cables and connectors are crushed and sold in the form of copper shots; whereas glass, recovered as clean cullet, is used in the glass manufacturing industry. As a side note on the latter, the glass recovered from such PV recycling process gets eventually contaminated with iron (Fe)-due to the involved shredding process -and therefore cannot be considered as a low-Fe (i.e. high value) glass product. Veolia aims to build more similar installations, as PV waste ramps up in the coming years. 19 SolarWorld is another pioneer and key actor in PV recycling that has a well-established c-Si recycling programme and process, based on a thermal processing method, with which EVA is eliminated through burning, followed by manual separation of metals, silicon and glass. Then, Si cells are re-etched, and at the end of such process clean wafers can be re-used. SolarWorld's recovery ratios typically exceeded 84% of the module weight, namely, 90% of the glass and 95% of the semiconductor materials. 8 Yet it should be clarified that SolarWorld and its recycling subsidiary are no longer in operation.
On the other hand, Loser Chemie's recycling process for c-Si PV starts with crushing and mechanical separation. 20 Afterwards, chemical treatment is performed, followed by solid-liquid separation. As a last step of the recycling process, glass is chemically treated, whereas aluminium -based metallization is recovered.
At early R&D/pilot stage, a novel PV recycling process line has been investigated by Sasil, in Italy, in the framework of EU LIFE's project FRELP. 14 Experimental research outcomes, 6 supported by experts' insights, resulted in a recycling process-prototype, where c-Si PV modules are mechanically disassembled, followed by multiple processing steps, i.e. glass separation, cutting, sieving, acid leaching, filtration, electrolysis, neutralization and filter press. After the latter step, any remaining waste is in liquid and sludge form, containing unrecovered metals and residual calcium hydroxide (hazardous). Yet, in 2016, Sasil announced that due to the low amount of PV waste that time and economic limitations, a PV recycling plant based on the aforementioned process will not be built.
In Japan, NPC Group has developed a recycling process for c-Si PV modules which can recover glass without any destructive process. 21 Recycling starts with automatic separation of aluminium frames after processing PV modules by means of a hot-knife/cutter blades. The glass, the solar cells matrix and the EVA sheets can be directed for further recycling. Key (relative) advantages of such process are (i) its fully In China, on the other hand, ordinary (pilot) PV recycling plants are recycling c-Si PV waste employing abrasive machining under cryogenic conditions and electrostatic separation. 22 The frame and junction box of PV modules are separated mechanically; then, the rest of the module is smashed and grated in cryogenic temperatures, and the mixture gets electrostatically separated into EVA particles, a powder mixture of Si, Ag, Al and Tedlar particles. It is however reckoned that, due to its consequent low purity, the Si recovered through such pilot plants and recycling process is not suited for reprocessing new Si wafers.
Details on the actual capacity (rate) and the intrinsic cost of the described PV recycling processes are rather limited. The claimed recycling capacity for Sasil's prototype accounts for 8000 t/a (i.e. tons per year), whereas Veolia and NPC are capable to recycle PV modules at a rate of 4000 t/a and 2400 t/a, respectively. 23 Moreover, NPC's reported cost of PV recycling is determined to be 780 €/t.

| PV Recycling in the thin films (compound) PV segment
In the thin film PV segment, First Solar is at the recycling technology forefront, running today a downstream scheme of collection, transportation and recycling for its CdTe modules. 24 In overall, First Solar's commercial recycling process recovers 90% of the glass from CdTe modules, for re-use in new glass products, and 95% of the semiconductor materials (i.e. Te and Cd, after the metal composites being processed and purified by a third party) for use in First Solar's new thin film PV modules. 24 An overview of the thin film PV recycling process steps and workflow, adopted by First Solar, is depicted in Figure 4. The   furnace-base pyrolysis of polymer sheets. 8,25 In addition, similarly to First Solar's recycling technology, ANTEC Solar designed a pilot plant for recycling of CdTe/CdS thin film PV modules. 5,26 As shown in Figure 4, ANTEC Solar's recycling process starts with the physical disintegration of the module into fragments, which are then processed through oxygen-containing atmosphere, at a temperature of at least 300°C, resulting in pyrolysis of EVA. In the dry etching process step that follows, the fragments are exposed to a chlorine-containing gas atmosphere, at a temperature of more than 400°C. During this process, CdCl 2 and TeCl 4 are generated which are then condensed and precipitated during cooling.

| PV REPAIR/REFURBISHMENT AND RE-CERTIFICATION: CURRENT STATUS AND CHALLENGES
In the course of PV modules' operational lifetime, it is not uncommon  In this context, details regarding the reliability/qualification testing of second-life PV modules, that are adopted and applied by the aforementioned actors, are not publicly disclosed. As a result, claimed duration of warranty periods for refurbished PV modules may be judgement-based, somewhat subjective and often misleading or misinterpreted. Most importantly, re-certification and quality standardization for such modules practically neither exist nor are under any development, as TÜV Rheinland and IEC experts reckon. 35,36 This, in turn, highlights the importance, ambition and novelty of the relevant research activity planned in the framework of CIRCUSOL Project, on setting up the technical groundwork for the development of (optimal) reliability testing and "re-certification" protocols, for second-life PV modules traders and/or refurbishment service-providers. However, with a significantly higher number of PV installations and modules expected to reach end-of-life, further R&D challenges will emerge towards the need for

| Technical innovation and opportunities in PV recycling
• even higher recovery/recycling ratios; • cost-efficient and environmentally friendly processes; and • recovery of higher grade, quality/value materials and/or materials for PV re-manufacturing or re-use (second-life PV).  mass-treatment, on-site processing);

Exemplary innovations and material
• streamlining collection-transportation networks and global-scale reverse logistics; • ensuring operational viability (need for sufficient PV waste, i.e. bankability); and • implementation of sustainable and circular business models, namely towards re-use or second-life PV. Next to above, one should note that there is a considerable volume of fielded PV modules that-although being non-failed ("healthy")-are still decommissioned in view of economic and/or technical reasons, e.g. insurance claims, repowering or lack of spares.

| R&D gaps towards second-life PV business
In principle, such modules (especially the "younger" ones) are considered as very promising candidates towards PV re-use (second-life) business. In this direction, systemizing appropriate labelling as well as time-and cost-efficient characterization and reliability/qualification testing comprise the central R&D gaps to be addressed.
• Product efficiency and reliability towards market confidence. In practice, the (remaining) efficiency of repaired/refurbished PV modules will depend on the years of their field exposure (thus power degradation rate), at the moment of the repair. In other words, efficiency-wise, repairing relatively "young" PV modules, i.e. with infant failures, has higher added-value potential. Besides, because PV modules in failed state degrade much faster, 45 timely and efficient detection of failed (yet repairable) modules in a PV system is another critical aspect. Next to product efficiency, another major challenge towards the bankability of second-life PV business is the lack of market confidence or "trust" in the reliability (and safety) of refurbished PV modules. Evidently, the latter stems from the lack of relevant regulatory framework and standardized reliability testing, as it has been also discussed in Section 4. In fact, considering that a PV module's warranty is intrinsically lost once a refurbishment/repair action is conducted, there is a need to somehow "certify" that the repaired, second-life module is safe and can re-gain the trust of the end-user.

| CONCLUSION-SUMMARY
In this review study, we presented a status snapshot on PV recycling, refurbishment and recertification as key aspects and second-life paths throughout a PSS-based circular economy model for PV. Publicly available literature, best practices, commercialized solutions and R&D gaps or emerging challenges, associated to each of these topics, were examined from both a circular economy and technology point of view.
It is well understood that the growing PV waste represents a new environmental challenge, which, however, also comes with new opportunities to create new services and pursue new economic avenues. As we presented in Section 3, although the clear lack of high-value PV recycling, there is substantial technological and Through this review, we also identified and listed, in Section 5, certain knowledge gaps, potential innovations and opportunities in both PV recycling and second-life PV business sectors, focusing on a circular business model perspective. On the basis of this groundwork, CIRCUSOL aims to formalize the recycling and repair/re-use segments in the PV value chain, through the following main R&D pathways: • assessment and validation of PV design-for-recyclability and design-for-reliability concepts; • development of tailored, cost-efficient reliability testing and characterization protocols for both failed/repaired and "healthy"/decommissioned, second-life PV modules; and • cost-profit and life cycle analysis for the PV re-use (i.e. second-life) business case.
Ultimate goal of PV partners in CIRCUSOL is to develop and validate market mechanisms, such as service-based business models, to enhance circularity in the solar PV power sector.