Circular economy value chains for decommisioned wind turbine blades

Wind turbine blades are large, complex products made from a mix of materials but consist mainly of glass fiber composite materials that are difficult to reuse or recycle in a circular economy. For these reasons, the blades reaching their end of life have been landfilled, which is unsustainable and causes a loss of resources. Thus, new systems and value chains must be designed and implemented for end-of-life wind turbine blades in accordance with circular economy principles. Yet, the operationalization of circular economy is sparingly described in academic literature.

The aim of this PhD dissertation is therefore to answer the following main research question: How can value chains for end-of-life wind turbine blades be designed, operated, and industrialized in accordance with a circular economy? This is supported by several objectives and four research sub-questions. The research sub-questions answered are: 1) Why do circular end-of-life value chains for wind turbine blades not exist today? 2) Which end-of-life value chain routes are potential end-to-end solutions for wind turbine blades and what technologies and processes are included in the design of these solutions? 3) How can fully functioning value chains for end-of-life wind turbine blades be operated at an industrial scale to support a circular economy? 4) How can it be evaluated which circular value chains for end-of-life wind turbine blades should be industrialized and what variables influence this decision? To provide a comprehensive answer to the posed research questions, this PhD dissertation combines seven individual research papers and is based on a mixed-methods approach.

The first sub-research question is addressed in Chapter 5 by exploring and elaborating on barriers for establishing circular value chains through a combination of the results presented in Paper I combined with additional literature review and analysis. Paper I applies a mathematical modeling using a Weibull distribution to the master data register for wind turbines in Denmark to identify the future waste masses from wind turbine blades. Its findings show how the average time for decommissioning of wind turbine generators in Denmark is 29 years, which is 9–11 years longer that previous studies have predicted. In addition, the results of Chapter 5 include the identification of eight barriers for implementing circular value chains for end-of-life wind turbine blades that must be solved or mitigated for circular solutions for waste management of wind turbine blades to become a reality.

The second sub-research question is answered based on Paper II and a separate study of future research agendas and emerging technologies in Chapter 6. In Paper II, a systematic literature review methodology is combines with a meta-analysis of literature resulting in the development of a roadmap for sustainable value chains for wind turbine blades. The findings of Paper II were combined with a separate research study of 17 ongoing collaborative research projects on end-of-life blades to map expected technological developments. The results of the two studies were consolidated in a modified roadmap for sustainable value chains for endof-life wind turbine blades that was discussed and validated by a group of academic and industrial experts. The roadmap includes eight interlinked process steps for the design of end-to-end value chains of six sustainable end-of-life routes for wind turbine blades. Five of the routes pertain to circular principles of repurposing and recycling and include cement-co-processing, mechanical recycling, solvolysis, pyrolysis, and structural repurposing. The sixth route of incineration with energy recovery is excluded, since it does not provide a sufficient level of material circularity. 

The third sub-research question is answered based on Paper III and Paper IV in Chapter 7, in which empirical findings are presented and consolidated from four industrial case studies of wind turbine blades at their end of life that have been recycled though circular value chains. The case study research methodology was applied and based on a comprehensive research protocol including research design, preparation, data collection, data analysis, and data sharing. Data was obtained from key actors through semi-structured interviews, site-visits, photos, product information data sheets, and other relevant data, and followed by a comprehensive analysis within each case and across cases. The findings include the mapping of four end-of-life recycling value chains, all of which include more than ten metric tons of blade material being recycled through cement co-processing, pyrolysis, and mechanical recycling. Across the cases, it is found that end-of-life value chains must be designed as a complete system that include up to eight different value chain processes and multiple transportations. The findings are found to validate the roadmap developed in Chapter 6 and demonstrate that functioning recycling value chains for wind turbine blades are both technically and operationally feasible at an industrial scale. Additional, findings from Paper III are consolidated and presented in a framework for decommissioning of large complex products.

The fourth sub-research question is answered based on findings from Paper V and Paper VI in Chapter 8. In Paper V, a structured literature review approach was applied and based on the findings of a three-step framework for sustainable decision-making. The framework was developed and adopts a multi-criteria decision-making approach. The framework was successfully validated through an application and test with a Danish waste management organization to identify the preferred technology for wind turbine blade sectioning on-site. Paper IV adopted a structured literature approach followed by scenario development and a framework for future value chain assessment using life-cycle-assessment. The findings from both Papers I and VI include the development of two frameworks for sustainable assessment of processes and technologies (Paper V) and of full future value chains (Paper VI). The findings also include the identification and presentation of a complex set of variables that should be included in these assessments.

The collective results of this dissertation are collected in a cohesive model for circular value chains for end-of-life wind turbine blades which summaries all of the identified value chain processes and technologies while considering the technological readiness level and circularity level. The results do not point out one preferred value chain route but illustrate that five end-of-life routes are feasible and present appropriate solutions for wind turbine blades at end of life. Nevertheless, most of the identified processes, including (1) on-site demolition, (2) on-site operations for sectioning, (3) first pre-processing, (4) landfilling (of non-recyclable parts), and (5) second pre-processing, are almost identical across the four assessed recycling routes. Thus, it is beneficial to optimize and standardize these processes across value chains.
The findings from this dissertation lead to the conclusion that circular value chains for end-of-life wind turbine blades can be successfully achieved by (1) applying a value chain approach to system development; (2) applying the model of circular value chains for end-of-life wind turbine blades to design and implement industrial facilities; (3) assessing value chain routes using life-cycle-assessment and multi-criteria decisionmaking methods based on specific case variables; (4) working on standardization, optimization, and automatization of common value chain processes between end-of-life routes to reduce complexity and cost; (5) investing in research and development to improve technological readiness levels of pyrolysis and solvolysis; and (6) ensuring collaboration between value chain actors, including sharing of knowledge and material data.

The novel contributions of this dissertation include several frameworks for design, assessment, operationalization, and industrialization of end-of-life value chains for wind turbine blades and for other large, complex products. In addition, this dissertation provides a cohesive model for circular value chains for end-of-life wind turbine blades that set a new state-of-the-art for the research topic of circular economy implementation the in composite and wind energy sectors. The applied value chain perspective and use of empirical data from industrial cases provides a novel contribution to both academia and practice. The results also contribute to the literature on circular economy and its implementation through the design and development of successful value chain solutions based on collaboration between the involved stakeholders.