A framework for sustainable evaluation of thermal energy storage in circular economy

A framework for sustainable evaluation of thermal energy storage in circular economy Mohamed Hany Abokersh a , Masoud Norouzi b , Dieter Boer a , Luisa F. Cabeza c , Gemma Casa c , Cristina Prieto d , e , Laureano Jim enez b , Manel Vall es a , * a Departament d’Enginyeria Mec anica, Universitat Rovira i Virgili, Av. Països Catalans 26, 43007, Tarragona, Spain b Departament d’Enginyeria Química, Universitat Rovira i Virgili, Av. Països Catalans 26, 43007, Tarragona, Spain c GREiA Research Group, Universitat de Lleida, Pere de Cabrera s/n, 25001, Lleida, Spain d Abengoa, Energía Solar 1, 41014, Sevilla, Spain e Department of Energy Engineering, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain article info Article history: Received 15 March 2021 Received in revised form 20 April 2021 Accepted 24 April 2021 Available online 8 May 2021 Keywords: Life cycle assessment (LCA) Circular economy Material circularity index Thermal energy storage (TES) Concentrated solar power Sustainability abstract The circular economy can be promoted as a solution to support the sustainability market position of renewable energy systems. To design a circular and sustainable system, a structured approach is needed. The present study develops a methodology framework for sustainable circular system design (SCSD), aiming to assess thermal energy storage (TES) technologies from a sustainable perspective. To this end, a composite indicator, namely, environmental sustainability and circularity indicator (ESC) is provided. This indicator combines the environmental impacts of the TES system via the conduction of a life cycle assessment and its circulatory performance using the product-level material circularity indicator (MCI). The developed methodology is applied to a case study of high-temperature TES using molten salts as a part of a concentrated solar power plant. The SCSD embraces the analysis for the most relevant processes through proposing different ecological scenarios including, increasing the recycling rates (Modest Scenario), increasing the reuse rates (Medium Scenario), and a combination of both (Optimistic scenario). The circularity analysis showed that for the Modest, Medium and optimistic scenarios, the MCI moves from 20.6% for the current situation to 30.3%, 38.6%, and 46.4%, respectively. Accordingly, the optimistic scenario showed the most environmentally sustainable and circular scenario with ESC of 7.89%, whereas the Modest and Medium scenarios exhibited ESCs of 1.20% and 2.16%, respectively. A major obstacle for substantial improvement of the circulatory and ESC is the high share of unrecyclable molten salts in the system and therefore, any effort to improve the circulatory and the environmental benefits of this system can be reached by using more environmentally friendly alternative materials. The study concludes that the integration of reusing and recycling at the initial design should be sought in order to achieve a more environmentally sustainable and circular outcome. ©2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction A new series of social hurdles, owing to the massive stress on organizational resources and the massive expansion of the worldwide population. The consumption of oil, food, water, components and other resources has risen, and suppliers are coping with this development [1]. The exponential progress of the world and the rapid growth of developing markets indicate that energy demand is growing, and costs have typically risen after the new century [2]. The European Union approved a new clean energy framework for a sustainable transition from fossil fuel usage to renewable energy following up the EU 2030 greenhouse gases reduction targets [3]. An important step to spread on the clean energy transition in the European Union and its Member States is renewable energy usage. Out of all renewable energy-based systems, solar thermal is the technology that can be expanded from residential applications to urban applications [4]. Concentrating solar power (CSP) plants are becoming the best *Corresponding author. E-mail addresses: [email protected] (M.H. Abokersh), masoud. [email protected] (M. Norouzi), [email protected] (D. Boer), [email protected] (L.F. Cabeza), [email protected] (G. Casa), [email protected] (C. Prieto), [email protected] (L. Jim enez), [email protected] (M. Vall es). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2021.04.136 0960-1481/©2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Renewable Energy 175 (2021) 686e701 option to produce clean thermal energy [5]. However, they present a challenge as temporal fluctuations are experienced based on seasonality and daily patterns. However, this challenge can be overcome by integrating energy storage, in this case, thermal energy storage (TES) [6]. The efficiency, as well as the flexibility of thermal solar applications, can be greatly increased with the help of TES systems [7], where the excess energy produced by the system is stored and then used later when the thermal energy is needed [8]. Flexibility between supply and demand can be managed using large scale TES in CSP plants [9]. TES are classified into the following technologies [10]: (1) use of chemical reactions or physical sorption, which is known as thermochemical storage; (2) the use of phase change materials (PCM), which is known as latent heat storage; and (3) the use of the heat stored in liquid media or solid media, which is known as sensible heat storage. Due to favorable properties such as higher specific heat, lower cost, mechanical properties, and easy processing, concrete is the material often chosen for sensible heat storage at high temperatures [5]. Whereas for the liquid media, materials such as mineral oils, molten salts, and synthetic oils can be used [11]. Due to the density difference between the hot and cold fluids, the material maintains natural thermal stratification. Due to the melting enthalpy, heat can be stored nearly isothermally in some materials using latent heat [12]. According to the temperature range and the application, the correct PCM should be chosen [13]. The correct PCM in each system has to be chosen depending on the application and its working temperature range [14]. According to a recent study by Palacios et al. [10], publications regarding TES have increased exponentially recently. However, most of those publications have neglected the sustainability aspect and have only addressed the technical aspects such as new control, new applications, new enhancement in technologies, and new materials [15]. Following the IEA energy storage roadmap [16], the importance of TES was also highlighted, with a potential for CO 2 emissions reduction estimated to be 2.6 Gt. Furthermore, a recent study shows that TES have a significant environmental impact [17]. One reason for the high impact is the absence of the closing of material loops in addition to the lack of sustainable redesigning concepts for thermal energy storage. Thus, a comprehensive environmental assessment such as a life cycle assessment (LCA) [18] can be a key aspect to fulfil the European legislation [19] regarding the necessary decision support to minimize the environmental burdens associated with the utilization of the TES. Furthermore, technologies in integrated waste management systems (WMS) such as waste treatment technologies and a combination of recycling and reused concept can also be useful in analyzing TES [20]. In this context, Guarino et al. [21] and Abokersh et al. [22,23] examined seasonal TES and its environmental impact in a complete district heating infrastructure. Pelay et al. [24] presented a LCA of a hypothetical tower CSP plant with a Rankine power cycle with thermochemical energy storage (TCES) with calcium hydroxide. The result of the comparison of the LCA with the reference plant without storage concluded that the additional environmental impact due to the TCES system was relatively small. In addition, Gasa et al. [25] proposed a detailed LCA of a CSP tower plant with molten salts storage in a baseload configuration is carried out and compared with a reference CSP plant without storage. While, during the manufacturing and operation phase of the three different TES types for solar power plants, their impact on the environment was compared by Or o et al. [26]. Furthermore, the LCA of two TES Nomenclature C R Fraction of mass of a product being collected to go to a recycling process (%) C U Fraction of mass of a product going to component reuse (%) DAM d Indicator result for damage category d E c Efficiency of the recycling process used for the portion of product collected for recycling () E f Efficiency of the recycling process used to produce recycled feedstock for a product (%) F R Fraction of mass of a product's feedstock from recycled resources (%) F U Fraction of mass of a product's feedstock from reused resources (%) FðXÞUtility factor built as a function of the utility factor X () LActual average lifetime of a product (years) L av Actual average lifetime of an industry average product of the same type (years) LCA T Normalized and Weighted Results of the LCA () LFI Linear Flow Index () L p Lifespan of a product (years) L sys lifespan of an industrial product (years) MCI Material Circularity Indicator (%) MCI p Product Level Material Circularity Index () MCI * p Material Circularity Indicator for a product () MðxÞTotal Mass of the product x(kg) PCI Product Circularity Indicator () RCP ReCiPe 2016 aggregated impact factor (Pt/MWh) W c Waste generated to produce recycled content (kg) WWaste generated during the recycling process (kg) W F Waste generated during the recycling process (kg) W o Unrecoverable waster amount (kg) VOverall virgin component for a product (kg) VðxÞVirgin feedstock for the product x(kg) X i Product Utility for product i() Greek symbols d d Normalization factor for damage category d ε d Weighting factor for damage category d q ei Damage component with respect to the elementary flow iimpact factor e Abbreviations CE Circular Economy CSP Concentrating solar power ESC i Environmental Sustainability and Circularity Indicator LCA Life Cycle Assessment LCIA Life Cycle Impact Assessment MFA Material Flow Analysis PCM Phase Change Materials SCSD Sustainable Circular System Design TES Thermal Energy Storage WMS Waste Management System Indices d damage category e impact factor i elementary factor M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 687 designs, namely indirect thermocline and indirect molten salt are compared by Heath et al. [27]. Most of the life cycle assessments of TES systems have focused on the production, operation, as well as the end life as a single value [28]. Thus, all these studies did not focus on environmental analysis due to the potential of recycling and reuse of materials. Regarding this limitation and to revise the current linear economy with a 'take disposition' model, circular economy (CE) is one of the techniques endorsed by the United Nations Environmental Program (UNEP) and the Ellen MacArthur Foundation for decoupling [29,30]. CE can be defined in large part as an industrial system that is intentionally and in terms of design restorative or regenerative. The Ellen MacArthur Foundation replaces the idea of end-oflife with reorganization and shifts to the use of sustainable resources and to eliminate utilization by a more advanced level of substance, processes, and business designs of chemical pollutants that impact reusability and waste reversion [31]. This substitutes the notion of a deficit with the regenerative framework to differentiate financial growth from the use of new environmental assets. There are numerous definitions of circular economy in the literature. For example, CE can be explained with the help of 10 circular strategies (R0: refuse, R1: rethink, R2: reduce, R3: reuse, R4: repair, R5: refurbish, R6: remanufacture, R7: repurpose, R8: recycle, R9: recover), based on Potting et al. [32]. CE has many links to sustainable development for economic prosperity, social equity, and environmental quality [33]. Globally, circularity can be achieved by implementing a clean cycle strategy which may start by improving material circularity through improving its environmental life cycle [34]. To assess CE, different methodologies have been proposed and applied, most prominently material flow analysis (MFA) and LCA [35,36]. MFA is a tool that helps to understand the flow of a waste management system, as well as it is a starting point of the environmental assessment. Furthermore, MFA reveals opportunities for improving and monitoring the recycling targets [37]. On the other hand, LCA can be used to assess the system environmental performance [26]. However, a complete LCA is laborious and complex as well [38]. Another approach is to use a shortened LCA form, also known as streamlined LCA [39]. Moreover, the project PRO SUITE [40] has presented another approach that takes into account all activities and their effects on the economy, society, and environment. The assessment proposed by them is based on the five pillars, which are (1) impact on exhaustible resources, (2) social wellbeing, (3) prosperity, (4) natural environment, and (5) human health. However, the sustainability of TES has not been assessed using these methods. Following Cobo et al. [41], other assessment methodologies were proposed to assess CE, including the life cycle costing [42], economic and environmental optimization [43], and the combination of MFA and LCA in multi-objective optimization [44]. Blum et al. [45] suggested an appraisal for all CE activities based on the light of economic, environmental, and social sustainability. Furthermore, Tomi c and Schneider [46] tracked each energy vector and calculated coverage of energy needs inside the analyzed systems. This method secures an analysis for the energy recovery of waste to evaluate its circularity. Even though the previously mentioned strategies can be utilized for the appraisal of CE parts of TES, an extensive and organized strategy for the frameworks that grasp CE features while decreasing ecological effects for the TES is lacking. This study's main novelty is to develop a method for sustainable circular system design (SCSD) that ecologically assesses the sustainability of TES technologies. This approach can be promoted as a solution to support the sustainability market position of renewable energy systems. Furthermore, the developed method can be utilized to increase circularity and the sustainability of those derived measures. TES is included in this new CE approach of considering our actions, as energy is part of the concept. Going ahead, when developing TES and materials, CE principles should be considered. These principles are territorial ecology, "functionality" economy, reuse, second use, recycle, reparation, industrial, eco-design, and valorization. The technique draws from the pool of conceivable outcomes CE offers, minimal ecological damage, material quality aspects, product design considerations, circularity strategies within the production chain (R0-R9), and waste hierarchy. The developed SCSD methodology will be tested on hightemperature TES technology using a liquid media system with molten salt as a part of a concentrated solar power plant. Furthermore, SCSD embraces the analysis for the most intensive processes within TES technology through proposing the different ecological scenario, including (i) increasing recycling rates, (ii) increasing the reuse rates, and (iii) a combination of both. The paper structure is as follows: A general outline of the SCSD methodology aspects is proposed in section 2; section 3describes the application of the methodology in a high-temperature TES application; section 4 presents the paper results and their relevant findings; and finally, the results summary and work conclusion are shown in section 5. 2. The sustainable circular system design (SCSD) framework The proposed methodology framework for evaluating the circularity of TES is illustrated in Fig. 1. The framework is structured in a 3-phases approach for sustainable evaluating of TES. The first phase (A) is mapping the overall waste movements and management procedures through introducing the material flow analysis (MFA) for the TES bill of materials. Besides, it shows the residuals and contaminations need to be considered. Once this evaluation is implemented, the life cycle assessment (LCA) presents the environmental evaluation of the TES materials. In the second phase (B), the circularity metric is proposed to estimate product preservation through recycling and reuse techniques. The second step in-phase (B) is to evaluate the overall sustainability of TES through combining the LCA indicator with the CE index. Finally, the third phase (C) offers a complete analysis for the effect of various scenarios on the TES circularity. 2.1. MFA/LCA assessment This phase entails the environmental impact of the product (TES), where the proposed MFA/LCA module has been implemented following Haupt et al. [18] guidelines. 2.1.1. Material flow analysis (MFA) The MFA offers the transfer factors for all residues processes in multiple entities that can be used for LCA calculations. To fabricate an input dependent model of waste management using LCAs, MFA can be used as the base. Knowledge of the transfer coefficients for all treatments (recycling, incineration, and landfilling) permits designing process inventories for arranging and reusing according to the MFA and ensures the preservation of the mass balance in the framework. MFA includes the comprehensive measurement of the content input and output flows into space at a time specified framework. The system in which input flows are equivalent to the output flows in addition to the accretion of substance in the system [47], as illustrated in Fig. 2, is handled as the black box. MFA starts by establishing the boundaries of time and space. This strategy includes the examined process as an operation in the anthroposphere. The input flows involve all raw resources mined from nature and the domestic extraction (DE). Hidden flows are products not evident from economic records but needed to produce the resource used at the end, for example, earth-removed products M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 688 and overburdened mining materials or wood harvest reductions in forestry. Hidden flows, called environmental backpack or “Rucksack”, are measured by means of the Material Intensity per Unit of Service (MIPS) database, given by the Wuppertal Institute [49]. MIPS is a measuring unit designed by the Wuppertal Institute that monitors the material intensity of different products and services with respect to a particular product unit. MIPS measures how products are different from their normal extractions, as Ritthof et al. [50]. All content used is measured in relation to natural resource usages during the production, usage and recovery or disposal. The MIPS definition is based on the perception that the capacity for the product environmental impact can be measured by the Material Input (MI) across its lifespan. Lesser raw materials utilized; lesser environmental impacts faced. As per Ritthof et al. [50], such items have their ecological impact as an intangible "ecological rucksack," i.e. as per the MIPS definition. The eco-backpack can be measured by extracting the product's total weight from the MI (Ecological Backpack ¼MI-net weight). 2.1.2. Life cycle assessment (LCA) LCA is a method for the environmental evaluation connected with material, products or operation under which resources, energy and atmosphere (from the cradle to the grave) are defined and quantified [51]. LCA contributes, including the production, utilization and landfill process, for all resource and energy inputs and outputs of a product over its lifetime. The environmental impact measured by conducting an LCA practice must be transformed into a singular dimensionless value. The proposed procedure includes two sub-processes: standardization and weighting, two additional components of the LCA method of lifecycle impact assessment, as defined in the ISO 14044 International Standard [52]. Standardization is the magnitude measurement for the reference information of the section indicating findings. This will also help convey details on the relative value of the indicator effects [53]. Weighting is a procedure using numerical variables to transform the findings of the impact category metrics, enabling the converted metrics to be aggregated further [54]. LCA intends to examine the unique influence of a product or service on the environmental burdens in its various life cycle phases. The ISO 14040:2006 and ISO 14044:2006 [52,55,56] sets out four interrelated measures: (1) goal and scope definition; (2) inventory analysis; (3) impact assessment; and (4) interpretation which identifies relevant issues and formulates recommendations. These phases are explained in detail in the next subsections. a. Goal and scope definition Three key regions, system borders, and impact groups are included in this process. The whole product development (cradle to grave idea) must be evaluated on the system border. The environmental effects of the impact groups are measured using the ReCiPe 2016 [57] impact value. Eighteen metrics of the middle point effect are chosen and clustered in three groups of endpoint damage. They Fig. 1. The SCSD outlines. Fig. 2. Schematic representation for MFA [48]. M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 689 are classified into three categories: (i) ecosystem quality, (ii) Human health, and (iii) natural resources. These results are given in classifications of impact and by specific measures determine the environmental burden. The ReCiPe is a measure that is broadly utilized for comprehending damage. The decision-maker considers this kind of measure as more comprehensible than the so-called mid-point concept [4]. b. Inventory analysis It takes into account input, performance, and energy use in connection with product in the second phase of the LCA sequence. In the impact evaluation step, they are further interpreted into waste and pollution. The material distribution and delivery to the factory are regarded during the entire project time. Besides the product effects and the energy usage (natural gas and electricity) in the framework, the product-related reserves have been taken from the Ecoinvent 3.7 directory during its lifespan [58]. c. Impact assessment The inventory records are transcribed into sustainability reports during this step. As already stated, three separate areas of harm are covered: the environment, health care, and risk to infrastructure centered on the 2016 ReCiPe framework [57]. The assisted system classification may be carried out for the midpoints depending on the endpoints scale. The damage can be represented as follows numerically for each impact section. IMPe¼X i q ei:LCIToT ice(1) where LCI ToT i is the life cycle inventory related to production, resource logistics and plants execution, correlated with the primary flow i. q ei showed the damage material with respect to the elementary flow iimpact factor e. This aspect encourages a connection between the inventory and the classifications of damage. DAMd¼X e2ID d IMPecd(2) RCPd¼X d d dεdDAMdcd(3) where ID d describes a classification of endpoint effects in damage classification d. RCP is the standardized ReCiPe 2016 measure and d d , and ε d are the specific weight and normalization elements. The normalization element is calculated based on the EU damage computations for land use and emissions calculations [59]. The weighting variables are calculated based on suggested values as described in the ReCiPe 2016 [60]. d. Interpretation In addition to several suggestions that help improve system efficiency, this step offers an evaluation of the findings. In this sense, policymakers can recognize the weak points conveniently along the way, where additional effort must be made to decrease the environmental impact. Even so, there are no specific boundaries on how the decrease can be achieved. Besides, because of the range of possibilities accessible and differing targets (i.e. impact classifications) in many situations, assessment is highly complicated. The LCA findings can be implemented as variables in the statistical model in order to address these constraints. 2.1.3. MFA/LCA combination Both MFA and LCA include production, use, and end-of-life of the products within the system, to allow for deriving measures at every step of the life cycle. The functional unit for both MFA and LCA should consider the service the investigated system provides over a determined time span [37]. The direct link between the MFA and the LCA is established through the product-process-matrix proposed by Haupt et al. [18], where each process entails a certain environmental impact. Combining the MFA and LCA is used to facilitate waste management and provide environmental impact data on multiple mass flow situations. The integration of regional MFA and LCA makes environmental assessment evaluations, thus maintaining continuity between factors such as the transition parameters of MFA and LCA process models considering the capacity constraints on waste management systems and the accessibility of waste resources. Besides, the input reliance on process output and the related environmental impacts may be observed, both for specific waste management processes like urban waste combustion and for alternate materials used as feedstocks. 2.2. Circular economy (CE) assessment The CE aims to represent the circularity of product consisting of product level Material Circularity Indicator (MCI). These structures then comprise a collection of products and materials, including functionality and interlinked behaviour. The 'ideals' of the CE should be correlated to all such metrics; (1) planning of the waste (2) product resilience by variety, (3) dependency on electricity from sustainable resources, (4) thought of "systems," and (5) wastes as fuel, are some of the recommendations that can be followed from comprehensive work carried out by the Ellen MacArthur Foundation [61]. This part concludes the advancement of a circularity evaluation method and the design composition. Part of this assessment is based on the methodological work undertaken on the 'circularity metrics ea guide to the calculation of circularity' by the Ellen MacArthur Foundation and Granta [30]. They established metrics that could be applied to designers as help for implementing choices, along with being utilized to various other ends, including acquisition choices and company assessment rates. The metrics rely mainly on non-renewable sources of technological cycles and resources. Since the research topic does not respond to Ellen MacArthur Foundation &Granta methods, further growth of a product evaluation technique is made. 2.2.1. Assessment methodology design The MCI expresses the amount to which virgin feedstock is minimized, and, as contrasted to a similar industrially average product [62]. The MCI consists fundamentally of three main features: (1) mass of virgin raw materials used in the manufacturing phase, (2) mass of unrecoverable waste assigned to this product, and (3) utility factor, which represents the duration and severity of MCI usage of this product. Fig. 2 describes the numerous MCI parameters. MCI p is the fundamental step towards the product level material. A product MCI can be classified by taking into consideration the product linear flow index (LFI) and the factor FðXÞ, which is constructed as a function F of the utility X, which decides the impact of the product utility on its MCI. The formula of Ellen MacArthur and Granta [48] used for measuring the MCI for a material is: MCI* P¼1LFI$FðXÞ(4) M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 690 MCIp¼max0 MCI* P(5) 2.2.2. Formulating product circulatory metric The Ellen MacArthur Foundation &Granta [48] evaluation process is the guiding principle. The CE tests the degree to which linear flows have been reduced, and restoration flows amplified and how frequently and rigorously it is utilized contrasted with a comparable system-average product. The concept behind the CE is to analyze the application, features and outcomes. The materials and products and the linkages, and the assembly of the structure should be taken into account in the implementation of the CE independently. Products and materials are recovered in the market in technical intervals to the best appropriate standard and by replacements, maintenance, reuse, restoration, analysis, and recycle for as long as feasible. However, non-toxic materials in a wide range of applications are preserved. The development of CE is confined to the material's technology process. The organic process is not considered because the evaluation of this period is entirely different. In developing the evaluation model, the first step is to develop a product level material circularity indicator (MCI) by evaluating the product's input, output, and effectiveness. The "theoretical" circularity attribute for a system without configurations and only the product itself may also be represented. The MCI will finally produce an MCI for a product for each material assessment. The input and output of materials include the content of virgin or non-virgin materials and then reusable or non-reusable materials. 2.2.3. Material circularity indicator (MCI) The MCI for a product depends on the assumption that construction is an ensemble of materials attached in a particular manner where every product has its circularity and characteristics. The basic principle for circularity is 100% non-virgin content at endof-life and 100% recycled. The MCI for a product measures the degree the linear flow has been mitigated, the restorative flow of basic elements amplified, and how much longer (often an estimate) the item is used, particularly in comparison to the systemic layer valuation the construct [62]. The MCI is mainly focused on the following features (see also Fig. 3): Mass of virgin feedstock content in development. Mass of waste that is recognized as unrecoverable after the operation. The utility factor Xwith the item's lifespan/functional value. These features are mainly utilized to calculate the Linear Flow Index and the Material Circularity Indicator (MCI p ). For the determination of MCI, a differentiation between a fully linear product and a complete 100% circular product should be considered. From a single product perspective, it is considered a linear product when 100% of the virgin feedstock goes to the landfill. Furthermore, it is considered as a circular product when 100% non-virgin feedstock is utilized. All cumulative effects may be derived from this differentiation as an inference, which are the two extremes of MCI within a spectrum [0, 1], from 0% (linear) and 100% (circular). In order to evaluate MCI p as part of the final product, various materials are required to be evaluated using comprehensive information of parts and specifications in a product. Thus, it is critical that the bill of material is a complete and fully accurate material breakup. The MCI p is established first by analyzing the input and output of the material and then by analyzing the utility factor of the product. The amounts 'linear flow index' and 'material circularity measure' are successively explored in evaluating the products input, material performance, and utility factor. a. Determination of the material input As previously stated, there was a difference between virgin (raw material) or non-virgin (reused, revamped, repaired or rehabilitated) material production. All materials that are a part of a product may be applied to a production process, by Virgin Feedstock, NonVirgin Feedstock, Fraction Recycled materials, and Fraction Remanufactured materials. A product is then produced with a variety of parts: subassemblies, sections and/or materials. A Bill of material can define all materials centered on the scale of specifics. The Material Circularity Indicator can summarize all materials input predicated on all subassemblies, elements, and/or materials ðxÞ. The virgin materials feedstock is presented for every assembly, portion and/or material (x): VðxÞ¼MðxÞð1NVRCðxÞÞ(6) where VðxÞis a portion of virgin feedstock for each manufacturing process, MðxÞis the whole assembly mass, NV RCðxÞ is the feedstock component for each assembly from the non-virgin material. The quantity of all distinct subsystems, materials or raw materials are the overall virgin component for a product (V): V¼X x VðxÞ(7) If the Virgin Feedstock is equivalent to zero, the entire intake comes from a material that has a full circular input. b. material performance evaluation The material output is the target after the lifetime of the product. Again, the products reusable portion is the maximum amount of resources found for a second, a third or at least a next life without distinguishing between the reused, renovated, remanufactured and recycled item. The other alternative is to use it to produce energy or to detect when it is unable to consider taking the next lifespan. A differentiation between the reusable percentage and the waste should be taken in this situation. The recovery of resources and/or deposition is then known as waste, and the other proportion is seen to be reusable. The waste (W) in an incinerator (energy recovery) or goes to the landfill can be expressed as: W¼WOþWCþWF 2(8) where W O is the unrecoverable waster amount, W C waste generated to produce recycled content, and W F is the waste generated during the recycling process. When the waste (W) is equivalent to 0, then it offers all materials a subsequent existence (second or subsequent existence), implying a circular output. c. The material utility factor The utility of a product is related to the materials and the product lifetime. The period of the product use phase (L P ) is the lifespan of a product. The period aspect reflects any decline (or increase) in waste streams for products that have a prolonged (or shorter) life cycle than certain products from various manufacturers over a given number of years. If a product lifespan is doubled, M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 691 waste is generated, and the virgin materials used every year are cut in half. The secondary element is the (L sys ) which is linked to the industrial lifespan of an industrial product. The determination of the Xutility can be done as: X¼LP Lsys (9) This means that if a product lifetime (L P ) is extended, a beneficial effect can be reflected in the utility. It is necessary to bring the product into perspective and to be part of a system that is either intensively or insufficiently utilized in the event of request adjustments (end of the deal or 'only' shifting requirements), and the ties to other products. A substance produced for excessive lives (utility Xis significant), which ends up as energy rehabilitation or landfill, is not a circular but instead a gradual linear operation. There seems to be a debate between environmentally-efficiency and eco-effectiveness. Consequently, eco-efficiency, which reduces the need for resources, has a constructive significance for this highlighted scenario compared to eco-efficiency, which has a negative connotation (to do good instead of less). The aim of ecoeffectiveness is not to reduce the flow of material from cradle to grave, but to create cradle-to-cradle metabolisms that allow materials to sustain their role as a resource. The connection between economy and ecology is, therefore, strengthened in a positive way. This tool could include either eco-efficient or eco-effective collectively with productivity, based on the prerequisites. The presumption of the highest longevity in good materials is the best option. d. The linear flow index The Linear Flow Index (LFI) stands for the portion of materials that have a linear flow where 100% of the virgin stock feed goes to the landfill or energy recovery units at the end of the material lifetime, and it can be expressed as follows: LFI ¼ðVþWÞ 2MðxÞþW F W C 2 (10) If all the products are reused, remanufactured and/or recycled, the LFI is in the reverse direction, and the entire path is circular. e. Determination of the material circularity indicator By evaluating the input, value and performance, the material circularity measure of material can now be calculated. The MCI is established for a product: MCI* P¼1LFI$FðXÞ(11) where, LFI P is the Linear Flow Index (from the Virgin Feedstock and Waste), FðXÞis the function of the utility factor X. FðXÞ¼a X(12) where:FðXÞ¼ a X with ais a constant Ellen MacArthur Foundation [48] established a ¼0.9. Accordingly, the usefulness of a product (e.g. through longer use) affects the MCI as it does reuse of materials which, in an amount of time (eco-efficiency), result to an equal amount of reduced virgin material use and waste not retrieved. 2.2.4. Development of the environmental sustainability and circularity assessment indicator The final phase is the interpretation of sustainability through combining their overall circularity and environmental impact. The Fig. 3. The material circularity indicator (MCI) Workflow [63]. M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 692 circularity of the product is not taken into account in the LCA structure, and the MCI quantification structure does not take into account the ecological consequences of the respective manufacturing process. As already stated, efforts to increase the circularity of product may ultimately have negative environmental impacts; and efforts to mitigate the ecological consequences of the developed product may, theoretically, contribute to a decline in circularity. Therefore, an index indicator is normalized and aggregated effects of the LCA as its base value (LCA T ) at the power of the value (1-MCI p ). Below you can find the equation defining the index [63]: ESCi¼1 LCATð1MCI p Þ100 (13) The environmental effect of the analyzed product and its material index must be measured in order to measure the Environmental Sustainability and Circularity Indicator as defined over the last sections. Consequently, in terms of sustainable development and circularity, this technique can be used to grade the alternative options of a certain product. ESC is various from 0% to 100% where the higher ESC value, the better it is contrasted with the lowerranking indicators. An important factor of ESC is that it follows the same system borders for either type of evaluation, environmental sustainability and circularity. It is an essential factor that requires all tests to be combined into a single metric. 2.3. Development of scenarios Following the SCDS approach for assessing the product technologies circularity based on using the MFA/LCA in the first phase and the CE in the second phase, three future scenarios were proposed and compared to the current product circularity situation (baseline scenario) to complete the SCDS approach. These scenarios can highlight the possible improvement potential in the WMS, including increasing the recycling and reuse rates in order to create new treatment pathways. It is important to keep in mind that the developed scenarios are ordered based on the level of required changes where the third scenario would introduce radical changes in the WMS, whereas other scenarios can present changes which can be attained in the near future. 2.3.1. Increase recycle rates scenario (Modest Scenario) Due to the current low circularity for the TES materials, an option to increase recycling is proposed as an initiation to support the sustainable deployment for TES technologies in the near future. In the scenario, all materials entered the closed-loop recycling pathways where the recycling rates of the most relative environmental impact materials in TES technologies is increased by 70%, following the EU 2030 target for waste management [64]. 2.3.2. Increase low impact material use (Medium Scenario) In the reuse increment rates scenario, we would propose the EU 2030 targets of increasing the reuse rates of relative materials by 30%. This scenario can contribute to the reduction in TES raw materials extraction and subsequently increase its circularity. The selection of the reuse construction materials and their relative reuse efficiency is shown in the case study section. 2.3.3. Optimistic scenario In the third scenario, we propose a combination of the two previous scenarios where the recycling rates are increased by 70% in parallel with the reuse concept to contribute toward the EU 2030 targets. This scenario preserves an ambitious scenario where radical changes in climate change and human toxicity impacts can be reached by improving the current WMS situation. 3. SCSD application dTES cases studies This paper demonstrates the developed approach by an application to high-temperature TES technology as a part of concentrated solar power plant. This TES have been employed from the literature; this case has been built in a pilot plant scale at Andasol, Spain [65] , as shown in Fig. 4. The functional unit for both the LCA and circularity assessment will be carried out per kWh of storage material. The proposed TES technologies used NaNO 3 and KNO 3 based molten salts to store sensible heat in the system [12]. 3.1. Liquid media system description This article is based on the thermal storage system in the research performed by Gabbrielli et al. [66]. This project is proposed as an upgrade in their conventional parabolic trough commercial plants. The proposed Solar TES consists of two molten salt storage tanks. The first storage (Hot Tank) holds the salt at a high temperature up to 388  C, whereas the other storage (Cold Tank) holds the salt at a low temperature of 288  C. Both tanks are identical in dimension and shape identical to avoid complexity. These tanks have a cylindrical shape with a diameter of 22.4 m and a height of 11 m. The volume of each tank is around 4335 m 3 as it needed to store 5500 tons of molten salt to store 600 MWh th . As shown in Fig. 5, the molten salt storage tanks are constructed into different layers. For the lateral walls (inside out): A stainless steel flexible protective liner is used, next is insulating firebricks’ layer, a carbon steel made tank shell, an insulating layer of ceramic fiber, exterior insulation of ceramic fiber, and an aluminum sheet. The tank bottom also has different layers, a stainless steel flexible protective liner, a layer of insulating firebricks, carbon steel tank shell, fine sand, insulating firebricks, foamglas®, reinforced concrete with a water-based cooling system, poor concrete, and foundation piles. The tank roof is also made up of different materials: a stainless steel flexible protective liner, ceramic fiber insulation, ellipsoid-shaped carbon steel sheet, and ceramic insulating material. The main characteristics of these storage tanks are shown in Table 1. Several equipment are required in the TES, where each molten salt tank is connected to an electric pump in addition to water cooling and heating systems. The temperature of the concrete base is maintained below 100  C through the water-cooling system. While the salt solidification is prevented through the heating system. Furthermore, molten salts pumps are installed to transfer the salts between the hot and cold tanks. 3.2. LCA and CE input data The proposed methodology application is illustrated through the molten salt TES where its material inventory is shown in Table 2. Based on the proposed approach, the LCA includes the production, utilization, recycling, and landfill process, for all resource and energy inputs and outputs of a TES over its lifetime. The LCA data were retrieved from the Ecoinvent 3.7 database [68]. This database comprises the production, recycling and disposal stages of the proposed TES based on the ReCiPe 2016 methodology. A summary for the total normalized damage categories of the TES comprising three main stages: (i) material market, (ii) recycled waste market, and (iii) disposal waste market is shown in Table 3 where no data is found regarding the recycling of molten salt as well as firebricks, ceramic fibre, sand and foamglas. Moreover, the CE of molten salt storage is estimated based on counting for the M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 693 recycling and reuse rates in addition to their relative efficiency. Following Boer et al. [38], the value of recycled content and their relative efficiency in molten salt storage varies between different materials inventory. On the other hand, with the lag of data regarding the reuse concept in the molten salt material inventory, we excluded from reuse from the initial scenario calculations. Table 4 shows a summary for the recycling rates of the TES material inventory, which are extracted from CES Selector 2018 [69], whereas their relative efficiency is following the estimated values in Verberne work [62]. Fig. 4. A molten salt TES pilot plant located in Seville (Spain) [65]. Fig. 5. A cross-section for the molten salts storage tank where:(a) roof &(b) construction base [67]. Table 1 The molten salt storage tanks characteristics [26]. Units Value Diameter m 22.4 Height m 11 Storage unit volume m 3 4335 Weight of steel t 279 Lantern material insulation thickness mm 125 Roof insulating thickness mm 125 Foamglas®thickness mm 40 Number of brick foundation e2 Number of brick vessel e1 Number of brick bottom e5 Table 2 Material inventory during the manufacturing phase in the liquid media system [65]. Component Units Material Used KNO 3 kg 3,300,000 NaNO 3 kg 2,200,000 Concrete m 3 551.71 Poor Concrete m 3 236.45 Sheet metal m 2 3360.95 Firebricks kg 1,271,756.92 Carbon steel kg 554,052.81 Ceramic fibre kg 10419.79 Aluminum sheet m 2 1548.18 Sand kg 417,726.27 Foamglas kg 4256.08 M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 694 Environmental Management - Life Cycle Assessment: Goal and Scope Definition and Inventory Analysis, 1997. [56] International Organization for Standardization (ISO), ISO 14042:Environmental Management - Life Cycle Assessment - Life Cycle Impact, 2000. [57] M. Goedkoop, R. Heijungs, M. Huijbregts, A. De Schryver, J. Struijs, R. van Zelm, ReCiPe 2008: a Life Cycle Impact Assessment Method Which Comprises Harmonised Category Indicators at the Midpoint and the Endpoint Level; First Edition Report I: Characterisation, PRe Consultants, Amersfoort, 2013, p. 126. https://www.leidenuniv.nl/cml/ssp/publications/recipe_characterisation.pdf. [58] B.P. Weidema, C. Bauer, R. Hischier, C. Mutel, T. Nemecek, J. Reinhard, C.O. Vadenbo, G. Wernet, Data Quality Guideline for the Ecoinvent Database Version 3, 2013. http://www.ecoinvent.org/database/methodology-ofecoinvent-3/methodology-of-ecoinvent-3.html. [59] M. Hauschild, M. Goedkoop, J. Guinee, R. Heijungs, M. Huijbregts, O. Jolliet, M. Margni, A. De Schryver, D. Pennington, R. Pant, S. Sala, M. Brandao, M. Wolf, Recommendations for Life Cycle Impact Assessment in the European context - based on existing environmental impact assessment models and factors (International Reference Life Cycle Data System - ILCD handbook), EUR 24571 EN, Publications Office of the European Union, Luxembourg (Luxembourg), 2011, https://doi.org/10.2788/33030. [60] M.A.J. Huijbregts, Z.J.N. Steinmann, P.M.F. Elshout, G. Stam, F. Verones, M. Vieira, M. Zijp, A. Hollander, R. van Zelm, ReCiPe2016: a harmonised life cycle impact assessment method at midpoint and endpoint level, Int. J. Life Cycle Assess. 22 (2017) 138e147, https://doi.org/10.1007/s11367-016-1246y. [61] Ellen MacArthur Foundation, Growth within: a Circular Economy Vision for a Competitive Europe, 2015. [62] J.J.H. Verberne, Building Circularity Indicators - an Approach for Measuring Circularity of a Building, 2016. https://pure.tue.nl/ws/files/46934924/8467331.pdf. [63] K. Mantalovas, G. Di Mino, Integrating circularity in the sustainability assessment of asphalt mixtures, Sustain. Times 12 (2020) 9e12, https:// doi.org/10.3390/su12020594. [64] Eunomia, Support to the waste targets review, final report to DG environment of the European commission. http://ec.europa.eu/environment/waste/pdf/ target_review/Eunomia_support_waste_targets_review.pdf, 2016. [65] L. Mir o, E. Or o, D. Boer, L.F. Cabeza, Embodied energy in thermal energy storage (TES) systems for high temperature applications, Appl. Energy 137 (2015), https://doi.org/10.1016/j.apenergy.2014.06.062. [66] R. Gabbrielli, C. Zamparelli, Optimal design of a molten salt thermal storage tank for parabolic trough solar power plants, J. Sol. Energy Eng. Trans. ASME. 131 (2009) 410011e4100110, https://doi.org/10.1115/1.3197585. [67] C. Prieto, R. Osuna, A.I. Fern andez, L.F. Cabeza, Thermal storage in a MW scale. Molten salt solar thermal pilot facility: plant description and commissioning experiences, Renew. Energy 99 (2016) 852e866, https://doi.org/10.1016/ j.renene.2016.07.053. [68] Ecoinvent, Eco invent. https://www.ecoinvent.org/home.html, 2017. (Accessed 12 March 2018). [69] Granta Design Ltd, CES Selector 2018, 2018. M.H. Abokersh, M. Norouzi, D. Boer et al. Renewable Energy 175 (2021) 686e701 701