Incorporating critical material cycles into metal-energy nexus of China’s 2050 renewable transition

Applied Energy 253 (2019) 113612

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Incorporating critical material cycles into metal-energy nexus of China’s 2050 renewable transition Peng Wanga,e,g, Li-Yang Chenb, Jian-Ping Gec, Wenjia Caid, Wei-Qiang Chena,f,g,

T

⁎

a

Key Lab of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, PR China Guanghua School of Management, Peking University, Beijing 100871, PR China c School of Economics and Management, China University of Geosciences, Beijing 100083, PR China d Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University, Beijing 100084, PR China e Sustainability in Manufacturing and Life Cycle Engineering Research Group, School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney 2036, Australia f University of Chinese Academy of Sciences, Beijing 100049, PR China g Xiamen Key Lab of Urban Metabolism, Xiamen, Fujian, 361021, China b

H I GH L IG H T S

minerals constraints on China’s renewable energy transition were evaluated. • Critical cycles can provide more accurate and holistic view on metal-energy nexus. • Material power in China may be limited by critical minerals availability. • Solar power in China may be limited by rare earth production scalability. • Wind • China should adjust its renewable pathways according to its critical mineral endowment.

A R T I C LE I N FO

A B S T R A C T

Keywords: Critical minerals M2E nexus Material flow analysis Industrial ecology Renewable

Renewables rely heavily on critical materials. Such material (metal)-energy nexus thinking is critical to guarantee global renewable transition. As the largest energy consumer, China aims to promote the unprecedented installation of renewables to significantly decarbonize energy system till 2050. However, the material constraints to those renewable targets have been widely neglected by current stakeholders in China. In this paper, a quantitative framework is proposed to identify and quantify the corresponding material constraints on energy transition from a material cycle perspective. Accordingly, the required critical material demand for China’s 2050 renewable transition and its flow, loss, and stock along the life cycle are quantified. It is found that the critical materials (i.e. Cadmium, Tellurium, Indium, Gallium, Selenium, and Germanium) required by solar power in China are all under high shortage and supply risk. Their cumulative demand from 2015 to 2050 will exceeded the present national reserve by 1.4–123-fold. Approximately 804–1056 thousand tons (kt) of Neodymium and 66–85 kt of Dysprosium are required to support the growth of wind power, which account for around 10% with the current reserve in China. Nevertheless, the limited scalability of rare earth production in China may still constrain wind power development. Hence, China should adjust its renewable pathways (e.g. more wind, less solar) based on the critical mineral endowment. Furthermore, recycling is preferred but has limited impact on material criticality mitigation before 2030, and it is then suggested more actions should be made on the international trade and material efficiency improvement along the life cycle to support future renewable needs.

1. Introduction Renewables are the key to a climate-safe world [1]. As the world’s largest energy consumer and carbon emitter, China has made the

⁎

development of renewables one of its green economic policy priorities to achieve its carbon mitigation goals [2]. With decades of continuous effort, China is galloping ahead in the global race for renewable investment, manufacturing, employment, and generation, according to

Corresponding author at: Key Lab of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, PR China. E-mail address: [email protected] (W.-Q. Chen).

https://doi.org/10.1016/j.apenergy.2019.113612 Received 13 January 2019; Received in revised form 29 May 2019; Accepted 22 July 2019 0306-2619/ © 2019 Published by Elsevier Ltd.

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makes it difficult to develop mitigation strategies [29]. (2) Energy-metals nexus and critical metal requirement. The nexus approach has been applied widely to offer a holistic view of the interdependence of various systems, such as water-energy nexus [30], food-energy-water nexus [31]. Recently, the attention on metal(minerals)-energy nexus is growing. Most of the metal-energy nexus studies focus on the energy requirement on the production of various minerals [32] and its impact on social-economic systems [33]. Meanwhile, given the renewables are more metal intensive than fossil fuels, various studies have explored the metal requirements on national and global renewable transition under various scenarios [34,35]. In those studies, the metal requirement can be quantified through two approaches: a material-based approach and a stock-based approach. In a material-based approach, the material demand is directly quantified by integrating material intensity (i.e., material use per service unit) with required future applications. For example, Viebahn P et al. [36] estimated the total demand for Lithium in meeting future energy needs in Germany as 6240 tons (t) by Lithium-ion battery through multiplying the material intensity of Lithium (i.e. 0.12 kg per kWh) with the total future electricity demand (i.e. 52 GWh). Such method has been widely used in various studies for solar photovoltaic (PV) (including [37,38,39,40]), wind energy([41,36]) and other renewable options. Nevertheless, it has been criticized by several authors [42] from an industrial ecology perspective due to the hierarchical relationship between the material and electricity demand. Consequently, a stock-based approach is highly recommended as it can provide more accurate material demand estimation and generate more insights to better manage the infrastructure and material. Nevertheless, the focus of these two methods is primarily on the in-use stage with a limited view of the full life cycle. (3) Critical materials along its anthropogenic cycle. Dynamic material flow analysis (MFA) has been widely adopted to trace the material stocks and flows along its anthropogenic cycle from mining, production, in-use, to recycling [43]. With the help of material flow analysis, the anthropogenic cycle of several critical materials has been quantified, including indium [44], gallium [45], tellurium [46], selenium [47], rare earth minerals [48], and others [49]. This anthropogenic cycle quantification can not only provide thorough information on the stock and flow in each life cycle stage. More importantly, a more accurate demand can be obtained by taking the various dissipative resource losses into account, which is absent in most previous studies. These dissipative resource losses have substantial impacts as resource loss for critical materials is usually very large, and the amount of reserve depletion can be 10–100 times more than the direct demand in the in-use stage for some cases as revealed in reference [49]. (4) Critical materials shortage assessment. One of the most common assessments on whether there is a shortage of critical materials in meeting future demand is conducted by comparing the amount of future demand with the national or global reserve. Herein, reserve represents the part of the resource that can be economically extracted or produced at the time of determination. However, as previously mentioned, there may be some degree of underestimation on the material shortage, since resource losses are excluded when applying the demand in the in-use stage with resource extraction. Meanwhile, the secondary resource from material recycling and corresponding production from the end-of-life stage could become an alternative future resource. Hence, a comprehensive assessment from the life cycle perspective would be necessary for this comparison, which is unavailable within the current literature. Furthermore, there is another consideration (i.e. in the mining stage) in the critical material shortage assessment that needs to be taken into account. As most critical material is produced with other host materials as by-/co-products and requires a long lead period (i.e. 5–10 years to start a new mine) [50], recent studies

REN21 [3] and IRENA [4], and China remains the global leader in the renewable installations with approximately 545 GW (GW) capacity in 2016, and most of the increase appeared in solar and wind power. Nevertheless, its share of renewables in its national energy mix (i.e. 3%) is still below the world average (i.e. 4%) in 2018, and the associated pollution from coal-based power in China still remain high and severe as revealed in a recent study [5]. Hence, many more renewables are inevitably required for China to secure an ever-increasing energy needs, mitigate associated environmental impacts, and sustain economic growth [6]. Given this context, a series of ambitious global targets have been proposed by the Chinese government. After the implementation of Renewable Energy Law passed in 2005, a clear goal was set in 2007 to have renewables accounting for 15% of the energy mix by 2020 [2]. This target was renewed in the newly released 13th Five-Year Plan, which expects 27% of total electricity to be sourced from renewables by 2020 [7]. The National Development and Reform Commission (NDRC) recently published the “China 2050 High Renewable Energy Penetration Scenario and Roadmap Study” [8] and “China Renewable Energy Outlook 2017” [9] to provide feasible pathways towards a high renewable (mainly the wind and solar power) penetration up to 2050. Meanwhile, some institutions, such as CNREC [6], IRENA [10], WWF [11], and other institutions [12] have also had similar optimistic projections on the rapid growth of renewables in China. The development of renewables to such high levels requires a substantial amount of diverse materials [13], most of which are not only indispensable but also extremely scarce and under high risk in their supply chains (commonly known as critical material [14]). These issues raise global concerns about whether or not the shortage of material supply constraints the development of renewables [15]. However, the primary focus in China remains on assisting this energy system transition regarding issues such as cost, environment [16], policy mechanism [2], etc. For instance, Zou P. et al. [7] explored the market competitiveness of various power technologies in achieving China’s 2050 renewable targets, and Dai H. et al. [17] quantified the economic benefits from such rapid renewable transition in China. However, information on critical material related to emerging renewable technologies is rare in China’s case. By contrast, the security of critical material supply has been highly valued and relatively well examined in developed countries. For instance, the U.S. National Research Council (NRC) identified the supply risk of various critical minerals [18]. Furthermore, its Department of Energy (DOE) also issued two Critical Materials Strategy reports in 2010 [19] and 2011 [20], respectively. Meanwhile, the European Commission also published three reports investigating critical materials of great importance to the EU in 2010 [21], 2014 [22], and 2017 [23], respectively, with most of those materials relating to renewable technologies. Other nations, such as Australia [24] and Japan [25], have also expressed strong concerns regarding the supply risk of critical materials. Correspondingly, there is an increasing body of academic literature in assessing material criticality and exploring material availability issues related to the rapid deployment of these emerging technologies. This study sorts them generally into the following four major topics: (1) Critical materials screening. Most studies have attempted to screen the critical elements from the periodic table through a criticality matrix or supply risk indices, which can be found in the Yale Criticality Project [14] and the studies from NRC [18] and EU Critical Raw Materials [26]. These index-based studies can help to effectively identify criticality risk based on many metrics including supply risk, economic importance, and environmental impacts [14]. Various studies have also applied such index-based approach to screen critical materials in the energy applications (e.g. renewables [27], green transportation [28]). However, the lack of in-depth analysis of particular material among some particular applications 2

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based approach is applied in this framework to capture dynamics among different layers in the in-use stage, the latter three layers of which is based on previous study [42]. The service layer is introduced and linked with other layers for two purposes:

([51–53]) suggest that there may be a high supply risk to expand the mining capacity following the rapid deployment of technologies, such as solar power [51] and wind power [52]. Hence, there is a two-fold aim to be served in this study. For one thing, there is an imperative need to fill the information gap on exploring metal-energy nexus through a comprehensive assessment on how materials are extracted, produced, used, and recycled along with energy system transition, which can (a) inform stakeholders in each stage to improve the global use of critical materials, and (d) form mitigation strategies to lower critical material risk in global low carbon transition. For another, there is an urgency for China to assess its critical material demand and formulate corresponding strategies, and such assessment is critical to the international critical material community as China is the global leading producer and owner of most critical materials [14] and has great influence on global supply chains [15]. In this context, this study aims to fulfill such needs in two stages: firstly, this study will introduce a framework by integrating (a) the anthropogenic cycle quantification on the critical materials, and (b) the hierarchical model in connecting service with material among in-use stage. Secondly, this framework will be applied to provide the first-ever investigation on critical metal demand in achieving China’s 2050 green electricity targets (i.e. the development of wind and solar power among two creditable scenarios). This framework will be fully explained in Section 2, and six types of wind and solar PV technologies with eight types of corresponding critical materials (i.e., Nd, Dy, Cd, Te, In, Ga, Se, and Ge) will be selected for our analysis. Section 3 will provide our results regarding the annual change at the infrastructure level and the stocks and flows of the related materials from 2000 to 2050. Section 4 will assess whether there will be a shortage of material in achieving those targets. In addition, some crucial issues and international implications from China’s renewable transition will be discussed in this section. Finally, the conclusions of the study are presented in Section 5.

(a) In most cases, the scenario settings only provide the service demand (e.g., 85 kWh electricity demand per year) rather than infrastructure stocks. Hence, the service layer is necessary to fit the service demand directly from scenario settings. (b) Service can be provided with less material if the efficiency of the intermediate layers (i.e., infrastructure layer and component layer) is improved. To improve this understanding, the dynamics of technical improvement and management among those layers should be captured for high attention for stakeholders. This is particularly important for emerging services such as solar and wind power with rapid improvements in their research, development, and management. Hence, this type of efficiency is introduced as “service efficiency (SE)” to capture such improvements, which can be defined as the amount of service delivery per in-use infrastructure capacity. 2.1.2. Anthropogenic Material Cycle Quantification This study extends the critical material investigation from in-use stage to its entire life cycle, namely its anthropogenic material cycle. This full material cycle quantification can (a) provide a comprehensive view of how the required material is produced, manufactured, used and recycled throughout its full life cycle, (b) give an accurate assessment on material supply by taking the resource loss in each stage and end-oflife resource into account, and (c) form more strategies from a life cycle perspective in managing critical materials rather than the in-use based ones. The anthropogenic material cycle quantification can be conducted by a dynamic material flow analysis [55,56], which is a state-of-art method to trace the material flows and stocks within a system or throughout a process. This study quantifies the material flows stage by stage initially from the inflows to the outflows of the in-use stage described as follows:

2. Method and materials This section starts with the description of our life-cycle framework, which links service demand with critical materials from an anthropogenic cycle perspective. This framework can be applied to any emerging service needs, such as low carbon transportation and energy. This study also further outlines the application of this method to renewable technologies. Furthermore, the energy scenario in China (from 1990 to 2050) and corresponding data sources for material use are also explained.

(a) The outflow of the stock is determined by the historical inflow to the in-use stage and its lifetimeτ :

Outflow (t ) =

∫t

t

Inflow (k , t ) × f (τ , t , w ) dk

0

(1)

where f (τ , t , w ) is the probability densities of the lifetime distribution function (assumed to follow the Weibull distribution [55]) with the current time t and lifetime distribution parameter w .The initial stock at time t0 is assumed to be uniformly distributed across its lifetime.

2.1. Life cycle framework for metal-energy nexus investigation

(b) The inflow to stock at the current time is based on the mass balance of outflow and stock change during in-use stage:

This study introduces a life-cycle framework (Fig. 1) to estimate the material stocks and flow among its anthropogenic cycle driven by future critical service demand. Herein, the life cycle framework aims to assist the life cycle thinking in the management on critical infrastructure and materials by quantifying material stocks and flows among four major life cycle stages: material production, manufacturing, in-use stage, and end-of-life. This framework also includes three important modules: the hierarchical model for in-use stage, anthropogenic cycle quantification for other life cycle stages (Fig. 1A), and critical materials selection (Fig. 1B).

Inflow (t ) = Stock (t ) − Stock (t − 1) + outflow (t )

(2)

where those stock levels are determined by the proposed hierarchical model described in the framework (A). (c) The inflow to other life cycle stages are based on the mass balance of outflow to the connected downstream stage and its resource loss:

Inflow (t ) = Outflow (t ) + Loss (t ) = Outflow (t )/ RE (t )

(3)

where resource efficiency (RE) is determined by the mass ratio of outflow to inflow, which is usually termed as metal recovery rate in the metal production stage, or recycling rate in the end-of-life stage. This study will trace the resource efficiency in those stages from their corresponding reports or studies.

2.1.1. Hierarchical model for the in-use stage This life cycle framework is triggered by the in-use stage, which links the service demand hierarchically with material in-use stocks. Herein, the in-use stock is a key concept used in industrial ecology to represent the amount of the product or material in active use [54]. As shown in Fig. 1A, a hierarchical model is introduced in the in-use stage with four distinct layers: service, infrastructure, technology (represented by critical technical components), and material. The stock-

2.1.3. Critical Materials Selection Critical materials specifically refer to the materials with the high 3

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Fig. 1. Life cycle framework to quantify anthropogenic cycle of critical materials related to critical service demand (Note: A. Life cycle framework to link critical service demand and anthropogenic material cycle. B. Critical material selection for various types of renewable technology).

scenario selected for China’s case will be further described in Section 2.3. This study only captures the service demand from solar PV and wind power in China as (a) they will make up a substantial share of China’s future energy supply, and (b) they are based on several most critical material intensive technologies. (2) Linking Service to Infrastructure Stocks. The second step is to obtain the infrastructure stock or the capacity of renewable infrastructure (in the unit of gigawatt, GW). The concept of service efficiency is introduced to transfer the service to infrastructure stock. This is similar to the parameter - “capacity factor”, which reflects the share of the nameplate installed capacity (GW) of infrastructures to its actual annual electricity outputs (TWh). The improvement of SE for each technology is captured by various related parameters such as capacity factor and PV cell efficiency. For both wind and solar power, many technologies can be applied to their critical components. This study focuses on those technologies with significant consideration to the shortage of critical materials. For solar power, as the crystalline silicon has limited demand on critical materials in the solar cells [38], three other technologies in thin film are studied: amorphous silicon germanium solar cells (a-SiGe), copper-indium-gallium-selenium (CIGS), and cadmium-tellurium (CdTe) [38]. As for wind power, three major types of technologies are included: permanent magnet direct drive (PMDD), permanent magnet gear middle speed (PMMS), and permanent magnet gear high speed (PMHS) [36]. (3) Linking Infrastructures to Critical Material. The types of critical materials and their corresponding infrastructures are carefully selected. As shown in Fig. 1B, the solar cell is composed of various

importance of use in critical applications but under high supply risks [14]. It should be acknowledged that the energy generation infrastructure requires various materials. Some of those materials such as steel, cement, and aluminum are out of this study’s scope since they have neither high impact on the core purpose of infrastructures nor receive high supply disruptions [49]. Hence, this study selects the critical materials through a two-stage approach: the first stage is to identify the critical technical components that deliver the designed service, and the second stage is to identify critical materials among those technical components based on the existing criticality studies. Consequently, as shown in Fig. 1b, around eight types of critical materials (i.e., Nd, Dy, Cd, Te, In, Ga, Se, and Ge) associated with various solar and wind technologies have been selected, which will be further given in Section 2.2. 2.2. Renewable energy case This section provides information regarding the application of our framework to the renewable energy case. In general, four steps are involved to obtain critical material flows and stocks related to future renewable energy services: (1) Obtaining Service Demand from Valid Scenarios. Many scenarios regarding future energy supply have been made in various national and international reports, which can be applied to the starting point of our study to obtain the service demand. Service provided by the energy infrastructure can be described as the electricity demand (normally in the unit of tera-watt hour, TWh). The details of the 4

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Table 1 The material composition, intensity, lifetime, and production parameters for studied wind and solar technologies. Service

Technical options

Critical component

Lifetime (yr)

Metals (t/GW)

Primary production RE (%)

Secondary production RE (%)

Wind power

PMDD

Turbine

20 ± 5

Dy: 21% [64]; Nd: 21% Same with Dy as they are co-products Fabrication RE of Turbine: 90% [65]

EoL-RR: 92%; SP-RE: 75%[66]

PMMS

Turbine

20 ± 5

PMHS

Turbine

20 ± 5

CdTe

Panel

28 ± 5 [63]

CIGS

Panel

28 ± 5 [63]

Cd: 46% [68] Te: 4.5% [46] Ga: 2.2% [45] Se: 4.5% [47] In: 43.3% [44] Ge: 1.7% [44] Solar cell fabrication RE: 99%[69]

EoL-RR for Cd and Te: 75% [70] In, Ga, Ge, and Se: 90% [71] SP-RE for Cd and Te: 90% [70] In, Ga, Ge, and Se: 75% [71]

a-Si

Panel

28 ± 5 [63]

Nd: 165 (130–200); Dy: 13.4 (11.8–15) [36,63] Nd: 40.8 (32–49.6); Dy: 3.3 (2.9–3.7) [36,63] Nd: 20.4 (16–24.8); Dy: 1.6 (1.4–1.8); [36,63] Cd: 84 (17–166); Te: 132 (19–182); [51,60,67] In: 13 (7–28); Ga: 10 (2.3–19.7); Se: 67 (17–161) [51,60,67] Ge: 73 [38]

Solar power

The results of the future electricity demand of wind and solar power under two scenarios are shown in Fig. S1 and Table S1 from supporting information. A rapid increase in wind and solar power demand in China is found in all scenarios. The solar PV power will be increased by approximately 52–79 times to 2010–3057 TWh/year in 2050 with 569–1282 TWh/year in 2030 depending on the scenario settings, and the wind power will be approximately 2344–3650 TWh/year in 2030 and 5358–6963 TWh/year in 2050, which is around 29–37 times of the level in 2016. Given the large benefits of those studied technologies in generating electricity [59], relatively high penetration of those critical material intensive technologies is applied to China’s case. More specifically, for wind power, the division of an onshore and offshore application for wind power is based in CREO2017 [9]. The technology mix of PMDD, PMMS, and PMHS for onshore and offshore application is adapted from [36]. As for solar power, the detailed division, regarding CSP, a-Si, CIGS, and CdTe, is adapted from [60] and [61] for the years leading up to 2050. The starting point of service efficiency (TWh to GW) in the studied wind and solar technologies is based in CREO2017 [9]. The future improvement and the change of related parameters are shown in Table S1. Different data sources are applied herein: the annual improvement of wind power are based on reference [62], and the parameters in power PV technologies in terms of cell efficiency, layer thickness, etc. are based on the roadmap studies from IEA [59]. In total, eight commonly-assessed critical materials (i.e., Dy, Nd, Cd, Te, In, Ga, Se, and Ge) are included for each technology in this study. The material intensity and their ranges in each technology are collected based on various publications (Table 1). A large variance is found for metal intensity, so the medium amount is applied in this study. The parameters for each metal in conjunction with their respective references for their anthropogenic material cycle quantification are also listed in Table 1. For the material production and manufacturing stage, the production technologies for each studied metal and the corresponding metal recovery rate are studied by collecting the data from creditable anthropogenic metal cycle studies (as listed in Table 1). The resource efficiency (RE) of primary production for each metal is based on global material flow analysis studies as listed in Table 1. As for the end-of-life stage, information regarding the existing recycling activities of those critical materials in studied applications is unavailable in China. Nevertheless, as the interests and benefits of recycling increase, this study assumes a rapid growth in the recycling of those materials to their practical potentials in China (Table 1).

critical materials in different technologies. For instance, the a-SiGe only requires Ge, while the other two technologies require In, Ga, Se, Cd, and Te. As for wind turbines, all studied technologies required Nd and Dy with varying material intensities [36]. Furthermore, the material intensity (in unit tons per giga-watt, t/GW) and its range for each critical component along with their technical parameters (e.g. lifetime, capacity factor), is obtained based on credible publications such as government reports, journal papers, and LCA (Life Cycle Assessment) case studies. (4) Critical Material and its Life Cycle. As introduced in the anthropogenic material cycle quantification found in Eqs. (1)–(3), the resource efficiency (RE) of each stage is vital to tracing the material flow from the in-use stage to other stages. Based on this anthropogenic material cycle quantification, this study first obtains the material flows and stocks of the studied critical materials and then maps the results onto the Sankey diagram. As shown in Fig. 1A, this diagram provides a visual presentation of the linkage of each flow as well as a quick comparison of the scale of flow in each stage as presented in [57,58]. 2.3. China’s 2050 renewable energy scenarios and corresponding critical materials Amid various roadmaps regarding the feasible development of China’s renewables, the China Renewable Energy Outlook 2017 (CREO2017) [9] is selected to create two scenarios for future estimation due to the following three reasons: (a) this is recently performed by Energy Reform Institute (ERI) in NDRC, which is one of the most important energy policymakers in China; (b) the results were obtained based on a comprehensive model, which captured the drivers from social-economic development, and economic, environmental, and carbon emission constraints according to China’s unique features; and (c) it provided feasible energy scenarios and a detailed roadmap up to 2050, which could strongly guide China’s future energy planning and policy development toward such routes. Two scenarios (i.e. Stated Policies Scenario, SPS; and Below 2 °C Scenario, B2S) are chosen to provide the low variant and high variant estimation of future critical material consumption, respectively. More specifically, (a) the Stated Policies Scenario (SPS) presents the future, which extends the current government policies and targets to 2050 and can also have 78% of renewables in future electricity by 2050; (b) the Below 2 °C Scenario (B2S) follows the pathway of China’s commitment to the Paris Agreement in reducing the greenhouse gas emissions to a below 2 °C global temperature increase with the share of renewables in total electricity in 2050 as 85% with 37% of solar and wind energy in total renewables.

3. Results Estimates of the annual change in each layer related to studied metals and infrastructures from 2000 to 2050 are presented in this 5

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B1. Infrastructure Stocks – Photovoltaic technologies

2500 2000

Infrastructure in-use stocks (Unit: GW)

Infrastructure in-use stocks (Unit: GW)

A1. Infrastructure Stocks – Wind technologies

365-474 fold

1500

B2S

1000

SPS

500

0 2010

2020

2030

2040

Newly-added Infrastructure (Unit: GW)

Newly-added Infrastructure (Unit: GW)

21-31 fold 100

B2S SPS

2010

2020

2030

2040

2050

SPS

200

0

2010

2020

2030

2040

2050

40 721-936 fold 30

20

B2S SPS

10

0

2020

2030

2040

2050

B3. Infrastructure Outflow – Photovoltaic technologies

150

40 End-of-service Infrastructure (Unit: GW)

End-of-service Infrastructure (Unit: GW)

B2S

400

2010

A3. Infrastructure Outflow – Wind technologies

68-92 GW

100

B2S 50

0

5583-8491 fold

B2. Infrastructure Inflows – Photovoltaic technologies

150

0

600

2050

A2. Infrastructure Inflow – Wind technologies

50

800

SPS

2010

2020

PMDD Frist Colum: SPS

2030 PMMS

2040

30

20 6-13 GW 10

SPS

B2S

0

2050

2010

PMHS

2020

CIGS

Frist Colum: SPS

Second Colum: B2S

2030 CdTe

2040

2050 a-SiGe

Second Colum: B2S

Fig. 2. Infrastructure change of stock capacity (A1-2), newly-added capacity (B1-2), and end-of-service capacity (C1-2) related to future wind and solar power scenarios in China.

section. We firstly present future changes at the infrastructure level as it is the main driver for material demand. Next, the demand for metals required for infrastructure installation is presented. Finally, the anthropogenic cycle of those metals is mapped in the type of Sankey diagram to clarify the flows and stocks in each stage.

based quantification, the magnitude of installed capacity, newly-added capacity, and end-of-service capacity for studied technologies (i.e., PMDD, PMMS, and PMHS for wind power, and CdTe, CIGS, and a-SiGe for solar power) under two designed scenarios can be obtained in Fig. 2. Several important results are highlighted as follows:

3.1. Infrastructure change

(1) The technical improvement is critical in determining the installation of infrastructure. The future infrastructure stock (or installed capacity) of all studied wind power ranges from 1669 GW to 2166

The infrastructure is the proxy of material requirement. Using stock6

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corresponding materials has increased to the level shown in Table 2. It is found that the wind-associated materials were 843 t for Nd and 67 t for Dy. The discrepancy between Nd and Dy is mainly due to their respective material intensities in studied technologies as shown in Table 1. Meanwhile, the cumulative demand of Cd, Te, Se, and Ge in solar power ranges from 170 to 280 t, and the cumulative demand of In and Ga is less than 40 t because of their low material intensity. A large amount of critical materials is required to support China’s future renewable goals. Table 2 gives the cumulative demand in the period from 2016 to 2050, which is demonstrated for each studied material in two scenarios. If the 2-degree climate commitment is implemented, (a) around 259 kt of Nd and 21 kt of Dy will be required cumulatively for scaling wind power to 2050; (b) Cd and Te will be consumed by around 13 kt and 21 kt in total during the studied period, respectively, (c) the growth of CIGS in solar power will increase the demand for Se, In and Ga to 6.3 kt, 1.2 kt, and 938 t, respectively; and (d) Ge will be applied with around 24 kt for a-SiGe in the future. Although parameters differ in each scenario, the cumulative material demand shows a similar magnitude up to 2050 with a variance of around 25% for wind power. However, the SPS scenario will require nearly half of that amount of metals in B2S. The amount of material contained in the installed capacity at the end of the studied period is shown in Table 2, which is obtained by stock dynamics in Eq. (2). It is found that the in-use stock of materials occupies the accumulative demand by around 70% for wind power and 90% for solar power. The remaining amount of accumulative demand is attributed to end-of-life material as shown in Table 2. Hence, this study provides strong evidence that a material-based approach would underestimate the accumulative material demand. This is because the materials are not only required to construct those installed capacity but also needed to replace the end-of-life (EoL) materials in End-of-Service capacities.

GW, including 645–839 GW of PMDD, 328–412 GW of PMMS, and 696–915 GW of PMHS. Meanwhile, that amount of all studied solar power in 2050 is 670 GW in B2S and 440 GW in SPS, including 88–133 GW of CIGS, 469–206 GW of CdTe, and 217–330 GW of aSiGe. Compared to the stock levels in 2016, the total capacity dramatically increases by 35–47 orders of magnitude in 2050. This study reveals the importance of technical development in the required future infrastructure stock. The magnitudes of installed capacity, which are based on the same service level as ours, are also available in CREO2017 [9]. The magnitude of installed capacity in wind power is similar (i.e., less than ± 1.5% in variation) with ours because of the slow technical development of PMDD, PMMS, and PMHS (Table S1). However, given our radical, the technical development of solar PV panels settings, a much lower increase (i.e., around 64% less) is found in the magnitude of installed capacity in this study compared with [9]. This highlights the great importance and urgency to prompt technical improvement in wind and solar technologies to reduce the need for infrastructure to provide the same energy needs. (2) Higher newly-added capacity will be required with a stock dynamics approach. The trend of newly-added capacity from 2000 to 2050 is shown in Fig. 2. The cumulative newly-added capacity for all studied solar technologies from 2000 to 2050 is 498 GW in SPS and 777 GW in B2S with 105–234 GW accumulated before 2030. As for the wind technologies, the accumulative amount is 3404 GW in B2S and 2498 GW in SPS with 70% of installation occurred after 2030. Compared with previous studies [9,22,72], which obtained newly-added capacity solely based on the change of installed capacity, a major difference is found in this stock dynamics approach: the magnitude of newly added capacity is around 1.2 to 1.6 times higher than the magnitude of installed capacity. This is because the demand of newly-added capacity is driven by both changes in infrastructure stocks and the replacement of end-of-service capacity, and the later requirement has been widely neglected in those previous studies [9]. As a result, there has been an underestimation of material demand in the present literature. (3) China will have a substantial increase in end-of-service infrastructure from 2030. After a period of use, many infrastructures cannot maintain their functionality and will enter the end-of-service phase. Furthermore, the embodied materials and components would also enter the end-of-life stage for further treatment. As shown in Fig. 2C, the end-of-service capacity continues to increase in all scenarios for all studied technologies. It is found that the magnitude of end-of-service capacity stays at a low level (i.e., around 1 GW for solar power, and 55 GW for wind power in B2S) in the early period from 2010 to 2030, but the magnitude would rise to a significate level from 2030 to 2050 (i.e., 57–106 GW of solar power, and 786–1182 GW of wind power). This trend can be found in China as well as renewable cases in US [52], Germany [73], and the world [74,75]because (a) those technologies are emerging technologies with a short period of application and a minimal amount of use, and (b) it would take a long period for them to enter the end-of-service phase as their lifetimes are normally longer than 20 years.

3.3. Mapping anthropogenic metal cycle related to China’s development of renewables Anthropogenic metal cycle quantification is a common tool to trace the material flow along its life cycle (i.e., production, fabrication, inuse, and end-of-life), which can help to reveal production technologies, material loss, and efficiency of each life stage [49]. The anthropogenic cycles of studied metals have been mapped in this study with four particular treatments: (a) the results are presented in the type of Sankey diagram in which the relative scale of the cumulative flows in the period from 2016 to 2050 is mapped; (b) the metal recovery rate in the production stage is kept constant in the future; (c) a high recycling rate is assumed to be applied in China for those metals in their applications; and (d) the flows in entire life cycle are quantified to solely meet the demand from studied applications, and the demand for other end-users is neglected in this study. The results are shown in Figs. 3–4. Fig. 3 maps the anthropogenic cycle of Nd and Dy related to the wind power deployment in China. Approximately 804–1056 kt of Nd and 66–85 kt of Dy from mineral resources are required to support the future growth of wind power, which is estimated to be about 5 times higher than the cumulative metal demand in the fabrication stage despite taking the recycled amount into account. This is because of the large material loss during the production stage. These Sankey diagrams also reveal the significant Nd and Dy flows in the end-of-life stage. With a high recycling rate, the number of recycled flows can substitute approximately one-third of raw resources during the fabrication stage. The other six metals (i.e., Cd, Te, Ga, Se, In, and Ge) related to the solar cell are mapped in the Sankey diagram in Fig. 4. To meet the future targets of solar power, a large amount of material is required from mineral extraction, including 16–28 kt of Cd, 264–425 kt of Te, 22.7–38.2 kt of Ga, 74–125 kt of Se, 1.5–2.5 kt of In, and 2246–3477 kt of Ge. The model predicts an optimistic estimation on the future recycling of solar cells, with approximately 75% of Cd and Te in CdTe

3.2. Material demand The demand of studied materials has been obtained annually from 1990 to 2050 with their ranges in two scenarios, which is calculated based on the stock dynamic approach in our framework. The selected main numbers are listed in Table 2. The material intensity of material in technology is of great importance in determining the demand for associated materials. In the past 5 years from 2011 to 2015, China has expanded its renewable infrastructures at an unprecedented rate, and both wind and solar power have increased significantly with an annual rate of 33% and 177%, respectively [76]. Consequently, the demand for 7

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Table 2 Cumulative demand of studied materials under B2S (Below 2 °C Scenario) and SPS (Stated Policies Scenario). Material

Scenarios

Nd

Dy

Cd

Te

In

Ga

Se

Ge

Previous total demand in 2011–2015 (unit: tons)

B2S SPS B2S SPS B2S SPS B2S SPS B2S SPS

843 843 258,986 193,867 67,242 44,203 173,905 133,994 86,053 60,845

67 67 20,935 15,676 5422 3565 14,066 10,838 6947 4915

173 173 13,751 8403 5592 2455 11,399 7212 2528 1367

272 272 21,609 13,205 8788 3858 17,912 11,333 3972 2148

37 37 1219 720 610 263 1053 642 203 115

28 28 938 554 469 203 810 494 157 89

190 190 6284 3713 3146 1360 5426 3310 1049 593

178 178 24,366 15,538 5915 2546 22,097 14,438 2443 1274

Total future demand (2016–2050, unit: tons) Total future demand (2016–2030, unit: tons) Material in-use stock in 2050 (unit: tons) Total future EoL material in 2016–2050 (unit: tons)

mineral shortage by the primary demand-based approach compared to the direct demand-based approach conducted in most of the previous studies [75,78]. If the production loss is not considered, there is a shortage of reserve existing only in Te and Ge in scenario B2S, but this demand can be fully met by the global reserve. However, if the production loss is considered, the reserve of Te, Ga, Se, In, and Ge in China is not enough for China to meet its future wind and solar power target. Moreover, the fraction of primary demand in China’s reserve for Nd, Dy, and Cd exceeds their shares among all the end-users. Hence, this study gives a “low” shortage ranking in the assessment. As for the metals like Ga and In, their primary demand can be met by the global reserve, which is marked as a “medium” shortage. For those metals, China should export minerals from other producers. The metals of Ga and Ge are marked with high and very high shortages, for their future demands in B2S would exceed the existing global reserve by approximately 1.3 and 38 times, respectively. Hence, China should adjust the deployment targets in their corresponding technologies of CIGS and aSiGe. Another recommended approach is to reduce the potential production loss of Ga and Ge primarily in coal fly ash production [44]. With resource efficiency of primary production increased to their technical potentials (i.e., 60% for Ga and 4545% for Ge), the total primary demand of Ga and Ge in B2S is 1.4 kt and 27 kt, respectively, which can be fully met by the global reserve.

cells and 90% of Ga, In, and Se in other solar cells being recycled. These recycled metals help to reduce EoL waste and the demand for the raw resource. For instance, approximately 20% of demand can be met by supply from secondary sources from end-of-life infrastructure in the cases of Ga and Se. Nevertheless, the recycling plays quite a limited role in substituting primary resource for the other three metals (i.e., Cd, Te, and In). As shown in Fig. 4, there is an enormous production loss in the processing of those metals, especially for Te, Ga, Se, and Ge. For every unit product from the production stage, approximately 21 units of Te, 44 units of Ga, 21 units of Se, and 58 units of Ge would be lost due to the low metal recovery rate in production.

4. Discussion 4.1. Is there a shortage of mineral in meeting China’s renewable goals? For consistency with a previous assessment, this study first compares the cumulative direct demand of each metal related to studied renewables with the available geological resource in China. This approach is called the direct demand-based approach. Herein, one of the most common estimations on the available geological resource can be obtained from the reserve from the USGS Minerals Yearbook. Furthermore, as indicated in Figs. 3–4, the production loss is tremendous. Hence, based on the life cycle investigation, the demand for reserve (primary demand) is compared directly with the reserve supply, which is known as the primary demand-based approach. The results are shown in Table 3. Herein, the information of reserve and annual production in China and across the globe are collected. Most of this data is obtained from the USGS Minerals Yearbook, and data from other sources are cited in Table 3. It is noted that the Nd and Dy are mixed in the rare earth, and the division in China’s case is based on [77]. This study reveals the significant differences in assessing the

a.Nd

Primary Production [mining, Beneficiation, REO processing, Magnet Production ,etc.]

B2S:1056 kt SPS:804 kt

Fabrication

In-use

B2S:200 kt SPS:169 kt

B2S:173 kt SPS:134 kt

Byproduct mining refers to the supply of metal that is recovered predominantly as minor by-/co-products throughout the mining and refining operations of host metal [29], which is the case of all studied metals as marked in Table 3. Given there is high risk for the capacity expansion in the primary production of those metals [50], in this study,

b.Dy

EoL

[turbine installment etc.] B2S:222 kt SPS:169 kt

4.2. Required scalability of byproduct mining supply in meeting future demand

B2S:6.8 kt SPS:4.9 kt

[landfill etc.]

Primary Production [mining, Beneficiation, REO processing, Magnet Production ,etc.]

B2S:85 kt SPS:65 kt

Fabrication

B2S:18 kt SPS:14 kt

B2S:16 kt SPS:12 kt

Recycling:

B2S:8.3 kt SPS:7 kt

B2S:0.56 kt SPS:0.39 kt

[landfill etc.]

Recycling:

B2S:79 kt SPS:56 kt

Reserve

EoL

In-use

[turbine installment etc.]

B2S:6.4 kt SPS:4.5 kt

Reserve Production loss B2S:68 kt SPS:52 kt

Production loss B2S:837 kt SPS:638 kt

Ratio of flow:

Ratio of flow: 40 kt

Co-production with Fe, and other rare earths

Co-production with Fe, and other rare earths

4 kt

Fig. 3. Anthropogenic cycle of (a) Neodymium and (b) Dysprosium in achieving targets of wind power in China (Note: the flow is given as cumulative amount during the period from 2016 to 2050). 8

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a. Cd

Fabrication [solar cell fabrication, etc.]

B2S:11 kt SPS:7 kt

B2S:12 kt SPS:7.5 kt

Primary Production [mining, Cu refining, Te refining+etc.]

b.Te

[landfill etc.]

B2S:0.6 kt SPS:0.3 kt

Fabrication [solar cell fabrication, etc.]

B2S:19 kt SPS:12 kt

B2S:18 kt SPS:12 kt

B2S:425 kt SPS:264 kt Recycling: B2S:1.9 kt SPS:1.0 kt

By-production with Zn (80%) and Cu (20%)

EoL

In-use

B2S:18 kt SPS:11 kt

[landfill etc.]

B2S:0.9 kt SPS:0.5 kt

Recycling: B2S:3.0 kt SPS:1.6 kt

Reserve

Reserve

B2S:12 kt SPS:8 kt

B2S:26 kt SPS:16 kt

EoL

In-use

Production loss

Production loss

Ratio of flow:

B2S:14 kt SPS:9 kt

Ratio of flow:

B2S:406 kt SPS:252 kt

20 kt

3 kt By-production with Cu (90%) and others(10%)

c.Ga

Primary Production [mining, Alumina Bayer refining, Ga refining,etc.]

B2S:38.2 kt SPS:22.7 kt

Fabrication [solar cell fabrication, etc.]

B2S:841 t SPS:499 t

B2S:832 t SPS:494 t

EoL

In-use B2S:810 t SPS:494 t

Primary Production [mining, Electrolytic Cu refining,Se refining, etc.]

d.Se

[landfill etc.]

B2S:16 t SPS:9 t

B2S:125 kt SPS:74 kt

B2S:5.6 kt SPS:3.3 kt

B2S:5.6 kt SPS:3.3 kt

Reserve

Reserve

Recycling: B2S:147 t SPS:119 t

Fabrication In-use [solar cell fabrication, etc.]

Production loss B2S:37 kt SPS:22 kt

Production loss B2S:120 kt SPS:71 kt

2 kt

Reserve

B2S:2.5 kt SPS:1.5 kt

Fabrication [solar cell fabrication, etc.] B2S:1.1 t SPS:0.6 t

Ratio of flow: 8 kt

By-production with Cu (~90%) and others(~10%)

By-production with Al (~99%) and Zn(~1%)

Primary Production [mining, Zn refining, In recovery, etc.]

[landfill etc.]

B2S:105 t SPS:59 t

B2S:5.4 kt SPS:3.2 kt

Recycling: B2S:944 t SPS:534 t

Ratio of flow:

e.In

EoL

B2S:1.1 kt SPS:0.6 kt

B2S:1.1 kt SPS:0.6 kt

f.Ge

EoL

In-use

B2S:20 t SPS:12 t

0.2 kt

In-use B2S:22 kt SPS:14 kt

EoL B2S:244 t SPS:127 t

[landfill etc.]

Recycling: B2S:2.2 kt SPS:1.1 kt

Production loss By-production with Zn (~70%) and flying ash(~30%)

Ratio of flow:

B2S:23 kt SPS:15 kt

B2S:1350 kt SPS:872 kt

Reserve

By-production with Zn

B2S:1.3 kt SPS:0.8 kt

Fabrication [solar cell fabrication, etc.]

B2S:23 kt SPS:15 kt

[landfill etc.]

Recycling: B2S:183 t SPS:104 t

Production loss

Primary Production [mining, Hydrometallurgy, Se refining, etc.]

Ratio of flow:

B2S:1326 kt SPS:857 kt

100 kt

Fig. 4. Anthropogenic cycle of (a) Cadmium, (b) Tellurium, (c) Gallium, (d) Selenium, and (e) Indium in achieving solar power targets in China (Note: the flow is given as cumulative amount during the period from 2016 to 2050).

China has enough reserve in Nd and Dy, China would still face severe supply challenges, for the production capacity of Nd and Dy must be expanded 7–58 times higher in the coming 5 years to meet the growth path in the designed scenarios. However, according to the latest estimate by [82], the future rare earth production capacity will remain still in the next few decades. It is worth noting that our projections have not considered the growth of other end-users such as electric vehicles. For this reason, the supply of Nd and Dy should be expanded earlier than our projected time.

the required production expansion rate under B2S (Below 2 °C Scenario) and SPS (Stated Policies Scenario) relative to the existing capacity in 2011–2015 is explored in Fig. 5. Notably, it usually takes around 5–10 years to start a new mine [50]. For a conservative estimation on the supply risk caused by the scalability constraint, this study selects 5year as a period for our analysis. It is quite clear that the metal demand will rise significantly in the future, and the scale of this expansion ranges from 1.8 to 58. The highest growth is attributed to Nd and Dy in B2S and SPS. Although

Table 3 Mineral shortage assessment of six critical metals based on demand, production and reserve estimations. Material

Nd

Dy

Cd

Te

Ga

Se

In

Ge

Reserve in China (unit: kt) Global reserve [36] (unit: kt) Annual Production in China-2015 (unit: tons) Global annual Production-2015 (unit: tons) Share of renewables in all end-users Fraction of direct demand in China’s reserve

6784 12,800 15,215 17,937[81] 1% [77] 3% 1% 16% 12% Fe, other rare earth Low

792 1100 1142 1328[81] 1% [77] 2% 1% 11% 8%

92 500 8090 24,200 6% [45] 13% 2% 28% 17% Zn, Cu Low

6.6 31 280 410 40%[52] 2.8 39% 64 40 Cu High

16.3 [79] 560 [79] 550 730 8% [79] 5% 0.1% 2.3 1.4 Al, Zn Medium

26 98 920 3270 10% [52] 22% 3% 4.8 2.9 Cu High

1.3 12 350 759 8% [44] 85% 5% 1.9 1.2 Zn Medium

10.9 [80] 36 [80] 115 160 15% [52] 2.1 42% 123 80 Zn, coal fly ash Very High

Fraction of primary demand in China’s reserve Co-/by-production host metals Shortage

B2S SPS B2S SPS

Low

9

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Expansion rate of Future mining capacity related to that of 2010-15: 57

B2S

27

35 26

17

14

29

26

SPS

13

10

10

10

9

8 5

5 14

13

7

10

9

7

B2S

6

10 5

4

2

2

2

2

2

2

2

2

2

2

2

2

8

3

3

3

3

3

3

3

3

3

3

SPS

6

4

3

3

3

3

SPS

7

6

4

4

3

3

B2S 13

8.Ge

6

SPS 4

4

6

4

8

6

4

B2S

8

6

SPS

10

5

B2S

3

29

27

21

21

19

3

SPS

7

5 2 2015

28

25

43 31

6

7.Se

9 10 20 30 40 50

6

B2S

6.Ga

8

26

17

6 SPS

6

5.In

7

42

25

B2S

4.Te

6

47

36

3.Cd

5

34

58

15

4

31

26

B2S

2.Dy

3

SPS

25

15

2

46

36

1.Nd

1

25

22

13

11

2 2020

2025

2030

2035

2040

2045

2050

2015

2020

2025

2030

2035

2040

2045

2050

Fig. 5. Required production expansion rate under B2S (Below 2 °C Scenario) and SPS (Stated Policies Scenario) relative to the existing capacity in 2011–2015.

the primary resource. As a result, with less raw primary resource use, recycling can (a) alleviate the shortages of critical material supply [77], (b) reduce the environmental impacts associated with mining and processing of ores [50]; and (c) improve the resource efficiency of metal use by reducing the metal loss in primary production as well as its entire life cycle [49]. Indeed, most of the previous studies also highlighted the urgent needs (e.g. the EU report [83] and others [84]), technical feasibility [85] and benefits [70,77] to have more recycling of critical metals in studied applications. However, based on our results, it is found that benefits of recycling are quite limited in mitigating supply risk of studied metals, especially in the next 1–3 decades. The options in the production and in-use stages (i.e., technical development and better management in applications, resource efficiency improvement in production) should be given greater attention due to the following reasons:

Although the required expand scale (i.e,. ranging from 1.8 to 25) of Cd, Te, In, Ga, Se, and Ge in solar power is lower compared to that of Nd and Dy, the situation of those metals is still severe. This is because those metals are not found in high enough concentrations to generate sufficient revenue to cover their full mining costs. The scalability of the supply capacity relies highly on the mining of hosts such as Fe, Zn, Cu, Al, and coal. A recent paper has predicted the future growth of Zn, Cu, and Al in four potential scenarios, which predicts the highest expansion rate of those metals as approximately 3.5 for Al, 5 for Cu, and 4.5 for Zn. Consequently, there would be an issue for CdTe deployment under the B2S scenario as the required scale of Cd and Te is higher than that of Zn and Cu. Similarly, the deployment of a-SiGe would be limited to the scalability of Zn. However, the growth of host metals (i.e., Al, Zn, and Cu) can nearly support the targeted deployment of CIGS under two scenarios and CdTe under scenario SPS.

(a) The available recycling resource is quite limited in the short term. As shown in Eq. (1), the available recycling resource, or EoL flow, is determined by the historical inflows and the lifetime of these resources. The lifetime of renewable infrastructure is always longer than 20 years, and the installment of infrastructure is relatively

4.3. Ineffectiveness of recycling in critical materials management for China renewable cases Recycling has been always considered as a promising alternative for 10

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contrary, they are highly concentrated in limited nations or producers [23] (e.g. rare earth 90% in China,), which could pose a threat to the global renewable transition. Consequently, such nexus thinking can help to identify the potential supply risks and enhance international energy cooperation policymaking. Thirdly, apart from renewables, the critical minerals are required by various industries such as advanced manufacturing, green transportation, military applications. The development of renewables can increase the mineral price, which may further the pressure on other industries as well as the entire social-economic systems. Meanwhile, the high mineral demand from other industries could constrain the availability of minerals to renewables. To avoid burden shifting from energy and other industries to metals as well as unexpected impact from critical metals on the promotion and competitiveness of various technologies, it is highly recommended to have trace the metal supply chain from the production, applications, to end-of-life for a holistic view on metal usages and its impacts on metal-energy nexus. Finally, the global renewable transition will boost the unprecedented mineral demand, which makes international trade of critical minerals increasing important to support such transition. In this study, large amounts of different materials will be required to support China’s rapid renewable growth, and the demand for most minerals (i.e. Cd, Te, In, Ga, Se, and Ge) will exceed its domestic reserve. Thus, international trade of those minerals becomes imperative. On the global level, those critical minerals are not equally distributed on national borders but highly concentrated in limited nations. Thus, the exchange of renewable technologies and those critical minerals across national boundaries will outstand as a key element in climate change mitigation, which deserves more attention and cooperation.

limited in the early years. Hence, as shown in Figs. 3–4, the recycling resource is quite limited from 2016 to 2030, remains at a low level from 2030 to 2040, and increases to a relatively higher level in the next decade. Nevertheless, the total amount of EoL flow is much less compared to the demand and stock level. Therefore, the availability of resources for recycling is quite limited in China. (b) In the studied period from 2015 to 2050, China will experience a rapidly growing demand for renewables and corresponding large demands for critical materials. Even though an optimistic recycling situation is given, the share of the recycled resource is still less than 20% on average due to the limited EoL flow and a large amount of material demand. Hence, the resource from primary ores would still dominate the future demand in China, and the security of the primary resource supply should be secured. Furthermore, better management in the mining and processing of the primary ores can help to reduce resource loss and improve the overall resource availability. (c) Recycling is not always a better option in critical material cases. The critical materials are always used with a small amount but coupled with other materials in a complex way in those components [86]. Thus, the amount of effort spent in collecting, separating, and purifying critical material from other compositions could be larger than extracting new ones in some cases [87]. Hence, recycling is not always technically, environmentally, or economically desirable and/or feasible. Meanwhile, much more attention should be put on the following options: (a) Improving the management of infrastructures can help to deliver more renewables services and prolong its lifetime and efficiency during the in-use stage. Consequently, less new infrastructures would be required, and critical material can be saved; (b) More developed technologies can help to reduce the intensity of critical materials. Hence, from a material perspective, more R&D should be put on improving technical development on those technologies; and (c) although the end-of-service infrastructures are limited, options such as reusing and remanufacturing critical components should be prioritized over material recycling. It should be noted that the critical material in the end-of-life stage is expected to be abundant from 2040 on. Hence, associated with the above options, this study urges that efforts should still be allocated to recycling to make this option more desirable and feasible.

4.5. Uncertainty and limitations Similar to other studies [89], our estimation on the critical material requirement in the deployment of emerging technologies is under a certain degree of uncertainty from parameters settings in future energy demand, energy and technology mix, material intensity, technology improvement, and efficiency in the material cycle, etc. Nevertheless, this study should not be viewed as estimation on actual future material demand but as a what-if analysis on future potential trends. Notably, this study still gives a conservative estimation of the supply risks of critical materials associated with future requirements in China’s solar and wind power because of the following assumptions: (a) at the infrastructure level, in our scenario analysis, a high technical efficiency is applied to studied technologies, especially for solar power as explained in Section 3.1 and Table S1; (b) In this study, a relatively lower material intensity is applied to calculate the material demand, and a high efficiency in production and recycling is also applied to quantify the material cycle; (c) the international trade of material and infrastructure is not considered in this study, which can increase the total demand given that China is the global leader in wind and solar manufacturing and may manufacture corresponding facilities for other nations [90]; and (d) the growth of other end-users for those critical materials is not considered. Still, there may be a relatively lower demand for material if the market share of critical material-intensive technologies is also lower. Although the actual number of material flows has considerable uncertainties from the above-mentioned factors, the expansion scales in critical materials’ demand and supply, together with the corresponding supply risks, are still validated and could be larger given the total renewables demand will be increased by an order of magnitude of approximately 50–150 from 2015 to 2050.

4.4. Metal-energy nexus and international implications Global climate mitigation requires unprecedented scale-up of renewable infrastructure all over the world. Given the renewables are more (critical) metal-intensive, the global energy system transition will shift quickly from fossil-based to metal-based. Such enhanced metalenergy nexus will bring various key implications on both energy and mineral systems: Firstly, the pressing mineral constraint will have a huge impact on the global energy transition. China is the global largest producers of various minerals such as rare earth elements, gallium, indium, germanium, etc. the mineral constraints on China renewable transition is still severe, not to mention other nations (e.g. United States, EU) without such critical mineral endowment [23]. Hence, such mineral constraints should be highly alerted by global and national policymakers in energy system planning and its feasibility assessment, otherwise, the climate targets may be stalled with the limited available mineral resource. Secondly, the cooperation between nations with an endowment in renewables and minerals is the key to the global energy transition. The present emphasis of global energy governance is mainly about the supply risks in oil, coal and other fossils producers, while renewable technologies were widely considered (e.g. in IEA report [88]) to enhance global energy security. As introduced in this study, critical minerals are not equally distributed across national borders. On the

5. Conclusion The rapid deployment of emerging renewable technologies in China requires a thorough investigation of corresponding critical materials. Herein, this study provides the first-ever investigation on 8 types of 11

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Appendix A. Supplementary material

critical materials (i.e., Neodymium, Dysprosium, Cadmium, Tellurium, Indium, Gallium, Selenium, and Germanium) in the deployment of six types of wind and solar PV technologies. The future scenarios related to China’s 2050 renewable targets were explicitly chosen according to the latest “China Renewable Energy Outlook 2017” [9]. Such a thorough investigation was provided by a novel methodological framework. This life cycle-based framework combines two well-established modules: (a) firstly, it explores the in-use stage through a hierarchical model in connecting required service with material; and (b) secondly, the flows and stocks of these critical materials along the anthropogenic cycle are quantified by dynamic material flow analysis and mapped in a Sankey diagram. By doing so, the hierarchical nature of the in-use stage can be further explored, and many important insights in the in-use stage and other life cycle stages can also be revealed. Various results and insightful implications were obtained in this study. Firstly, the annual change of newly-added, stock, and end-ofservice infrastructure related to future scenarios is estimated from 2000 to 2050. Secondly, the total demand for metals required for infrastructure installation is obtained. More specifically, our results suggest that the future demand would increase by significant orders of magnitude relative to historical consumption, which is 230 to 312 times more for wind-associated materials and 20 to 137 times more for solarassociated materials. In addition, compared to other nations (German [36], US [52]), China requires a much greater amount of critical materials to support future renewable goals. Lastly, the anthropogenic cycle of these materials was mapped in the type of Sankey diagram with a stage by stage quantification. These results reveal many insights that have not been explicitly considered by existing studies. The most significant implications are that the material required by solar power in China is under high shortage and supply risk, and while the material required by wind power is sufficient for the rapid deployment, but their supply may be limited by rare earth production scalability. Hence, from the critical mineral perspective, China should adjust its present renewable roadmap (i.e. more wind power and less solar power) to achieve the designed energy and climate targets. Furthermore, the life cycle quantification and mapping revealed a significant resource loss in the material cycle of required materials. Many strategies are provided for China to manage its supply of critical materials based on this life cycle investigation. It is found that while recycling has poor short-term impacts, it has many apparent benefits and is widely believed as an important strategy to mitigate the supply risks of critical materials [91]. This is because the availability of recyclable resources as a secondary resource is quite limited, and there are many limitations associated with the recycling process. However, we found huge potential in other life cycle stages, especially in the production and in-use stages. Consequently, the priority of supply risk mitigation should be given to improving the material efficiency by intensive material research, better design, and better infrastructure management. Likewise, mineral production also has a great deal of potential in increasing resource efficiency through the improvement of material recovery from its production losses. Finally, since these raw materials still dominate future supply and, consequently, a much higher extraction rate would be required for those materials, China should pay more attention to monitoring and managing these resources and their host minerals such as copper and zinc.

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Acknowledgements This study is supported by the general program of NSFC (41671523), NSFC (71774149), Leading Project of Fujian Science and Technology Department (2017Y0080), and the CAS Pioneer HundredTalent Program. We thank Ashley Thornton and Shiva Abdoli from the University of New South Wales for their efforts to proofread this manuscript. 12

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