Review Is Embodied Energy a Better Starting Point for Solving Energy Security Issues?—Based on an Overview of Embodied Energy-Related Research

Jinghan Chen 1 , Wen Zhou 2,* and Hongtao Yang 3

1 School of Economics and Management, Harbin Engineering University, Harbin 150001, China 2 College of System Engineering, National University of Defense Technology, Changsha 410073, China 3 School of Business Administration, Huaqiao University, Quanzhou 362021, China * Correspondence: [email protected]

 Received: 26 June 2019; Accepted: 5 August 2019; Published: 7 August 2019 

Abstract: Embodied energy is termed as the total (direct and indirect) energy required to produce economic or environmental goods and services. It is different from the direct energy measurement of energy consumption. Due to the importance of energy security, it has attracted increasing attention. In order to explore whether and to what extent embodied energy can provide a more innovative approach and competitive perspective to energy security issues, 2608 relevant pieces of literature from the Web of Science core collection are analyzed in this study. Results show that embodied energy has been taken seriously. Moreover, by reviewing the typical literature, this paper first summarizes the embodied energy calculation methods and models, then investigates how embodied energy provides a new perspective to energy issues, and lastly analyzes how to show value in energy security issues in its application of guiding policy-making and energy security studies. In summary, there is no doubt that embodied energy can provide a more integrated perspective on energy consumption and demand and provide a more scientific reference for policy-making to enhance energy security. However, because of data and application scope limitations, establishing a comprehensive energy security research and application system with embodied energy measurements needs hard work.

Keywords: energy security; energy policy; embodied energy; input–output analysis; application

1. Introduction Human survival and development cannot exist without energy. Energy is the lifeblood of economic development and modern society. An appropriate approach to addressing energy security has become a part of the development strategies of various countries, regions, as well as the whole world [1,2]. It also occupies an essential position in the policy agenda of many nations. In contemporary economic developments, energy tends to become a significant political, social, and economic objective [3]. In the meantime, global economic growth means that the demand for energy is increasing year by year. According to The World Energy Outlook 2018, based on current and scheduled policies, energy demand is expected to grow by more than 25% by 2040, requiring an annual investment of $2 trillion in new energy supplies [4]. However, the increase in proven energy reserves is far behind the rise in energy consumption. According to the BP Statistical Review of World Energy in 2018, the remaining recoverable reserves of coal in the world were 1,035,012 million tons by the end of 2017, with a reserve–production ratio of 134, but only 79 in the Asia-Pacific region, and the reserves–production ratios of natural gas and oil were only 54.1 and 52.5, respectively [5]. Besides, global and pollution caused by energy consumption also need to be considered by energy policymakers [6,7]. Additionally, because of the unbalanced distribution of resources, the energy security of a country

Sustainability 2019, 11, 4260; doi:10.3390/su11164260 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 4260 2 of 22 is also affected by its import and export target areas. For example, in 2017, China’s dependence on foreign oil reached 67% [8]. Therefore, it is a serious challenge for all countries to ensure energy balance through effective energy management, and then to achieve of the economy and environment, especially for large energy-consuming countries like China, the United States, and the United Kingdom. In order to ensure the long-term stability of energy supply and demand, improve energy efficiency, and reduce environmental impact, it is necessary to formulate effective energy security policies. To this end, many indicators such as energy intensity, net–import dependency, and energy per capita for measuring energy (primary) security were proposed in many articles, and many scholars and practitioners use these indicators to evaluate the safety status of industries or regions [9–13]. However, these indicators are based on direct energy use, which cannot fully describe energy consumption. Therefore, integrating indirect energy in the energy measurement system may provide a new perspective for understanding regional energy security issues and ultimately lead to a more intelligent discussion of energy security issues [2]. As early as the 1980s, Costanza [14] put forward that a critical aspect of energy analysis is to determine the total energy demanded to produce economic or environmental products and services. This total energy is called embodied energy. Compared with the analysis based on direct energy supply and demand, embodied energy can provide a more comprehensive perspective on economic and social energy-related issues, which may help to provide an empirical reference in the ecological and economic system [14,15]. At present, scholars have made many attempts to apply embodied energy to analyze energy issues in the industry or regional and international trade, such as evaluating energy policies for guiding policy-making. For example, through the embodied energy analysis in the import and export trade of the UK, Tang et al. concluded that the problem of energy security in the UK is more serious than conventional understanding [16]. By studying the role of embodied energy in the European industry, Popescu et al. [3] found that the burden of carbon tax within Europe on domestic countries and industries is unequal. Embodied energy can incorporate indirect energy consumption into conventional energy security indicators and can also change perceptions of regional energy security performance and performance compared with other regions [2]. However, although embodied energy is widely used, a comprehensive review of how embodied energy is used in different areas such as in measuring energy consumption or flow paths in industrial or regional economic systems is lacking. Therefore, the aim of this paper is to form a comprehensive understanding of embodied energy and investigate its application, and especially to explore whether it can provide a more innovative and competitive perspective on the research of energy security issues and depict its value. This paper illustrates the development of related research and reviews some typical research to summarize and identify how embodied energy measurement benefits energy issues, and especially how embodied energy can benefit energy security enhancement. In particular, this paper offers the following contributions:

(1) A comprehensive understanding of embodied energy and development of relevant research; (2) An analysis of why and how embodied energy can benefit energy security issues; (3) Future improvement and research directions.

To do so, we have organized the paper as follows. Section2 provides a historical and overall understanding of related research by elaborating on the definition of embodied energy and its characteristics and presenting development trends, research fields, and research trends of related research on it. Section3 summarizes the embodied energy calculation methods and models, how embodied energy provides a new perspective to energy issues, and how to show its worth in energy security issues through guiding policy-making and energy security studies. Section4 highlights some key insights for the application of embodied energy in energy security issues and raises some questions which cannot be completely answered here. Section5 concludes the paper. Sustainability 2019, 11, 4260 3 of 22

2. Background and Development

2.1. Definition and Characteristics of Embodied Energy

2.1.1. Definition of Embodied Energy The definition of embodied energy is not controversial. It is derived from [14,17], and its formal appearance in public was proposed by the International Federation of Advanced Research Institutions (IFIAS) in 1974 at a conference, which was being used for measuring the total energy required for the production of economic or environmental goods and services. It includes the energy consumed directly and indirectly in every stage of the process. In 1980, embodied energy appeared in an academic paper for the first time. Costanza [14] responded to some scholars’ queries about embodied energy and demonstrated how to calculate embodied energy in the economic system through the input–output method in his article “Embodied Energy and Economic Valuation” in 1980. Specifically, in the research on energy issues in trade, embodied energy is the direct and indirect energy consumed in import and export goods and services in international trade, the direct and indirect energy consumed in the transfer of products and services from one region to another, and the direct and indirect energy consumed in the flow of products and services between industries or sectors [17–22]. Furthermore, in the relative micro-field, embodied energy refers to the energy consumed by all other products and services used in the manufacturing, maintenance, and processing of products. For example, in the related research of building and construction engineering, embodied energy refers to the energy embedded in all products and services used by a building from its design, initial construction, maintenance and replacement to its final demolition, which represents the energy consumed in the whole life cycle of the building [23,24]. Moreover, it is different from operation energy, and a concept can only reflect direct energy consumed in the construction phase. In addition, based on embodied energy, to understand energy consumption and its environmental impact more deeply, concepts such as embodied coal, embodied oil, embodied solar energy, and embodied nuclear energy are extended from embodied energy. Like the definition of embodied energy, they represent a specific type of energy consumed directly and indirectly in products or services [25–28]. Moreover, the appearance of embodied CO2 and embodied emission can realize the linkage between energy consumption and environment issues [29,30].

2.1.2. Characteristics of Embodied Energy

Comprehensiveness Compared with the traditional direct energy consumption measurement, the apparent advantage of embodied energy is that it calculates not only the direct energy consumption but also the indirect energy consumption. It calculates not only the energy consumption of a particular stage but also the energy consumption of the whole life cycle. Embodied energy analysis can integrate history and off-site energy consumption related to on-site production, which can provide a more systematic view of energy demand. Additionally, it can give us a more comprehensive perspective on evaluating all the energy demand for the development of a product or a country. [31,32].

Flowability Because of the transaction characteristics of products and services, their transfer in different subjects makes embodied energy have the characteristics of flowability [33]. Especially in related research on energy issues in trade, the import and export of products and services between different regions, through embodied energy, can help explore the energy flow path and amount embedded in these trades; the flow of goods and services is the flow of energy. Sustainability 2019, 11, 4260 4 of 22

Separability In the previous discussion of the definition of embodied energy, it was stated that embodied energy is a comprehensive concept of energy. According to this, there are different types of energy, like coal, fossil oil, nature gas, hydro-energy, nuclear energy, and wind. Embodied energy can also be divided into different categories like embodied fossil oil, embodied coal, and embodied water. At present, more research is focused on the resources which occupy a relatively large proportion of the total energy consumption, such as embodied coal [25], embodied oil [26], and embodied water energy [34].

Scalability The scalability of embodied energy is mainly reflected in two levels. First, it can extend from measuring all the energy consumed by a single product or service to measuring the energy consumption of the whole supply chain from upstream to downstream [35]. Second, in order to better reflect the impact of energy consumption on the environment, it can be gradually extended to the research field of emissions, and the terms “embodied carbon” [36,37], “embodied emissions”, and “embodied CO2” the authors in [38–41] have been using to measure the emitted by a product or service in the whole production process.

2.2. Development of Relevant Embodied Energy Studies

2.2.1. Information on Relevant Embodied Energy Literature

Literature Selection Criteria and Procedure The dataset of this study was built based on the search results from the Web of Science core collection. The Web of Science database is the most widely used for scientific literature, and it contains more than 6000 scientific and technological journals, more than 1700 social sciences journals, and more than 1100 art and humanities journals [42,43]. Moreover, it provides a statistical analysis of the search results and supports various retrieval methods such as terms, keywords, journals, and titles. In consideration of that, this study aimed to explore whether embodied energy could benefit energy security issues based on a comprehensive understanding of relevant embodied energy research; we first used “embodied energy” as a search term and found 4438 documents published in the journal up to 24 April 2019. Then, we filtered the initial dataset by using the following rules. First, we divided the dataset according to the type of literature; peer-reviewed journal articles, proceedings, and reviews were retained, while other types like abstracts, letters, case reports, and editorials were excluded. Second, we based our selection on research direction; we excluded those articles in totally irrelevant fields like PSYCHOLOGY, CELL BIOLOGY, SPORT SCIENCES, MICROBIOLOGY, ZOOLOG and others. Lastly, we excluded totally unrelated literature by skimming through the titles and abstracts. Then, we finally got 2608 documents for further analysis.

Annual Volume Analysis Figure1 is about the number of papers published in the past years. Research on embodied energy exhibited an ascending trend. In addition, during the development phase, the research was discrete before 1991, and the ascending trend became stable from 1991. This is due to the caused by the Persian Gulf War in 1990, when many countries started to pay more attention to energy issues. Moreover, according to Price’s theory of the growth stage of scientific and technological literature, embodied energy-related research was in the embryonic stage before 1995. Then, it entered the stage of exponential growth in 1996, and this stage continued until 2015. After 2015, it showed a linear growth trend. In conclusion, although there was a slight decrease in 2018, the relevant studies of embodied energy are still in the stage of rapid development. Furthermore, it needs to be noted that the data are Sustainability 2019, 11, 4260 5 of 22

based on the search results of April, so the data of 2019 cannot reflect the number of papers throughout Sustainability 2019, 11, x FOR PEER REVIEW 5 of 21 the year, but we believe more related articles would emerge until the end of the year.

450

400 398 350 360 319 300 267 250 192 200 176 150 149150 129 100 113 80 50 424945 17 1416 21 2424 0 0 1 2 1 3 2 1 4 5 11 1211 10 1980 1982 1984 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

Figure 1. Annual Publications. Figure 1. Annual Publications. Journal Distribution Journal Distribution Table1 shows the top 10 journals with the most embodied energy publications. Among them, the JournalTable of Cleaner1 shows Productionthe top 10 journals and Energy with and the Buildingsmost embodied rank as energy the champion publications. and runner-up Among them, with the211 Journal documents of Cleaner and 200 Production documents, and respectively. Energy and Buildings rank as the champion and runner-up with 211 documents and 200 documents, respectively. Table 1. Journals with the most embodied energy publications. Table 1. Journals with the most embodied energy publications. Journals Publications Research Categories Journals Publications Research Categories Science & Technology—Other Journal of Cleaner Production 211 Science Topics;& Technology—Other Engineering; Topics; Journal of Cleaner Production 211 Engineering;Environmental Environmental Sciences & Ecology Sciences & ConstructionEcology & Building Energy and Buildings 200 ConstructionTechnology; & Energy Building & Fuels; Technology; Energy and Buildings 200 EnergyEngineering & Fuels; Engineering BusinessBusiness & & Economics; Economics; Energy && Fuels; Energy Policy 118 Energy Policy 118 EnvironmentalFuels; Environmental Sciences Sciences & Ecology & Ecology Applied Energy 113 Energy & Fuels; Engineering Applied Energy 113 Energy & Fuels; Engineering Construction & Building Technology; Building and Environment 95 Construction & Building Building and Environment 95 Engineering Technology; Engineering Science & Technology—Other Topics RenewableRenewable Sustainable Sustainable Energy Energy Reviews 87 Science & Technology—Other 87 Energy & Fuels Reviews Topics Energy & Fuels Engineering; Environmental Sciences & Environmental Science Technology 52 Engineering; Environmental Environmental Science Technology 52 Ecology Sciences & Ecology Science & Technology—Other Topics; Science & Technology—Other Journal of Industrial Ecology 43 Engineering; Environmental Sciences & Journal of Industrial Ecology 43 Topics; Engineering; Environmental SciencesEcology & Ecology Engineering; Environmental Sciences & Resources Conservation and Engineering; Environmental Resources Conservation and Recycling 4141 Recycling SciencesEcology & Ecology Building ResearchBuilding and Research Information and 36 ConstructionConstruction & Building & Building Technology 36 Information Technology Moreover, these journals all belong to district 1 of JCR, which illustrates the importance of embodied energy-related research. Furthermore, based on the research area of these journals, it can be seen that the related research of embodied energy may be distributed in fields like Science and Technology—Other Topics, Engineering, Environmental Sciences and Ecology, Construction and Building Technology, and Energy and Fuels. Sustainability 2019, 11, 4260 6 of 22

Moreover, these journals all belong to district 1 of JCR, which illustrates the importance of embodied energy-related research. Furthermore, based on the research area of these journals, it can be seen that the related research of embodied energy may be distributed in fields like Science and Technology—Other Topics, Engineering, Environmental Sciences and Ecology, Construction and Building Technology, and Energy and Fuels. Sustainability 2019, 11, x FOR PEER REVIEW 6 of 21 2.2.2. Research Field and Trend Analysis 2.2.2. Research Field and Trend Analysis Considering that this study aimed to explore the value of embodied energy on economic and environmentalConsidering issues, that and this especially study aimed on energy to explore security the issues, value of this embodied paper carried energy out on a economic scientometric and analysisenvironmental by CitespaceV. issues, Citespace and especially is a javaon energy application security for issues, scientific this paper literature carried analysis out a scientometric which was developedanalysis by by Dr. CitespaceV. Chen Chaomei Citespace from is thea java School application of Information for scientific Science literature and Technology,analysis which Redsell was Universitydeveloped and by WISE Dr. Laboratory,Chen Chaomei jointly. from Based the School on the of algorithm Information and Science time, Citespace and Technology, can identify Redsell the researchUniversity frontier and terminology WISE Laboratory, in a specific jointly. knowledge Based on the field algorithm [44]. Specifically, and time, thisCitespace study can used identify CiteSpace the research frontier terminology in a specific knowledge field [44]. Specifically, this study used to conduct a co-occurrence analysis of terms and keywords. CiteSpace to conduct a co-occurrence analysis of terms and keywords. Citespace’s co-occurrence analysis of keywords and terminology is an analysis of the keywords Citespace’s co-occurrence analysis of keywords and terminology is an analysis of the keywords provided by authors in the dataset. Cluster analysis uses keywords with distinct characteristics such provided by authors in the dataset. Cluster analysis uses keywords with distinct characteristics such as the clustering object, so as to find popular words that have existed in the research field for many as the clustering object, so as to find popular words that have existed in the research field for many years,years, and and which which can can depict depict the the distribution distribution of of research research fields. fields. Figure Figure2 2shows shows the the largest largest networknetwork clusteringclustering map map for thefor the co-occurrence co-occurrence of keywordsof keywords in in embodied embodied energy-related energy-relatedliterature. literature. The nodes representrepresent the terms the terms in the in bibliography, the bibliography, the relationship the relationship between between the nodes the represents nodes represents the co-occurrence the co- relationshipoccurrence between relationship them, between and the them, label and with the “#” label is the with name “#” ofis the the name term of and the keyword term and cluster. keyword In Figurecluster. 5, the In number Figure of5, the nodes number is 226, of the nodes number is 226, of the edges number is 1780, of edges the network is 1780, densitythe network is 0.07, density and the is maximum0.07, and network the maximum clustering network spectrum clustering accounts spectrum for 93% accounts of the overall for 93% network, of the overall which network, is significantly which representative.is significantly The representative. module value The (Modularity module value Q) (Modularity is 0.3578, and Q) theis 0.3578, mean and contour the mean value contour (Mean Silhouette)value (Mean is 0.5616, Silhouette) which means is 0.5616, the dividedwhich means cluster th structuree divided is cluster obvious, structure and the is clustering obvious, and result the is credible.clustering As can result be seen is credible. from the As figure, can thebe seen important from andthe figure, popular the areas important of embodied and popular energy areas research of are mainlyembodied distributed energy research in areas ofare buildings, mainly distributed input–output in areas analysis, of buildings, ecological input–output footprint accounting, analysis, and regionalecological consumption footprint accounting, activities. and regional consumption activities.

Figure 2. Node network of articles. Figure 2. Node network of articles. In addition, in order to further understand the research focus of embodied energy, this paper In addition, in order to further understand the research focus of embodied energy, this paper exported the results to Excel and organized them. Table2 shows the top 50 keywords in the frequency exported the results to Excel and organized them. Table 2 shows the top 50 keywords in the frequency ranking. The 50 keywords show a total frequency of 7435 times, which accounts for 86.2% of the total keyword frequency, 8624.

Sustainability 2019, 11, 4260 7 of 22 ranking. The 50 keywords show a total frequency of 7435 times, which accounts for 86.2% of the total keyword frequency, 8624.

Table 2. High-frequency keywords for embodied energy related research (Top 50).

No. Keyword Freq No. Keyword Freq 1 embodied energy 761 26 climate change 104 2 life cycle assessment 454 27 100 3 energy 378 28 embodied carbon 98 4 consumption 310 29 efficiency 93 5 co2 emission 281 30 input–output analysis 90 6 construction 236 31 energy efficiency 89 7 international trade 223 32 86 8 China 221 33 trade 81 9 sustainability 220 34 footprint 77 10 emission 215 35 sector 77 11 greenhouse gas emission 213 36 76 12 system 211 37 life cycle energy 75 13 performance 201 38 house 72 14 building 197 39 optimization 70 15 impact 196 40 cost 58 16 LCA 167 41 energy use 56 17 environmental impact 165 42 technology 51 18 input–output analysis 164 43 office building 50 19 residential building 159 44 framework 49 20 model 151 45 simulation 46 21 design 131 46 inventory 45 22 energy consumption 130 47 policy 44 23 carbon 123 48 Embodied energy 41 24 life cycle 114 49 management 39 25 carbon emission 109 50 economy 38

From the research dimension reflected by the data, the high-frequency keywords can be divided into categories as follows: One is related to research methods, such as “life cycle assessment”, “input–output analysis”, and “simulation”; the other is related to research objects, such as “international trade”, “China”, “construction”, and “housing”; the third is related to research topics, such as “carbon dioxide emissions”, “”, “sustainability”, “energy efficiency”, “policy”, “cost”, “energy consumption”, and so on. It can be seen that since embodied energy has been used in academic research, issues related to China, trade, and construction have been holding a high level of attention, and “sustainability,” “energy performance,” and “emissions” are the central issues. Furthermore, in order to investigate the latest research trend on embodied energy, this study also conducted a co-occurrence analysis of terms and keywords from 2016 to 2019. Table3 shows the 2019 part of the 2016–2019 co-occurrence results. Twenty-two co-occurrence keywords emerged in these years, such as “waste”, “transfer”, “supply chain”, and “strength”. However, compared with the analysis results based on the whole documents, it is worth noting that there were new high-frequency keywords that emerged such as “network,” “flow,” “supply chain,” “driving force,” and “ analysis,” which reflects the fact that embodied energy measurement has been applied in various fields of research and that some new research questions have emerged. At the same time, network and decomposition analysis have been applied in many articles. Moreover, to explore the changes in the research highlights of embodied energy from the time span covered by the dataset and to determine the frontier trends, this paper also used Citespace to detect burst words. Burst words refer to words that appear more frequently or frequently in a certain period and can effectively portray the evolution of a research field according to the trend of word frequency and its time cover situation. Therefore, they are a supplement to the co-occurrence analysis of keywords and can help researchers explore the emerging frontier trend of relevant research. Sustainability 2019, 11, 4260 8 of 22

Table4 shows the results of the analysis. It can be seen that most frontier burst words were about Sustainability 2019, 11, x FOR PEER REVIEW 8 of 22 sustainability and the environment, such as sustainability and sustainable development which were the main research4 frontiersstrength between12 1996–2007input and output 2004–2013, 20 respectively. built environment Moreover, embodied emissions5 and structuralresource decomposition 13 analysisflow were the latest21 research highlightsassessment andlca trends. 6 requirement 14 economic growth 22 air pollution 7 renewableTable 3. energyResults of keyword15 co-occurrencedurability analysis for 2016–2019 (2019). 8 power generation 16 driving force No. Keywords No. Keywords No. Keywords Moreover, to explore the changes in the research highlights of embodied energydecomposition from the time 1 waste 9 operational energy 17 span covered by the dataset and to determine the frontier trends, this paper also useanalysisd Citespace to detect bu2rst words. transferBurst words refer to 10 words that network appear more frequently 18 or circularfrequently economy in a certain input–output period 3and can effectively supply chain portray the 11 evolution of a research field according19 to the cement trend of word model frequency and its time cover situation. Therefore, they are a supplement to the co-occurrence analysis 4 strength 12 input output 20 built environment of keywords and can help researchers explore the emerging frontier trend of relevant research. Table 5 resource 13 flow 21 assessment lca 4 shows the results of the analysis. It can be seen that most frontier burst words were about 6 requirement 14 economic growth 22 air pollution sustainability7 and renewable the environment, energy such 15 as sustainability durability and sustainable development which were the main8 research power fron generationtiers between 161996–2007 drivingand 2004 force–2013, respectively. Moreover, embodied emissions and structural decomposition analysis were the latest research highlights and trends.

Table 4. Top 34 keywords with the strongest citation bursts. Table 4. Top 34 keywords with the strongest citation bursts.

Keywords Year Strength Begin End 1980–2019 ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▃▂ sustainability 1980 5.1744 1996 2007 ▂▂▂▂▂▂▂▂▂▂▂ embodied ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂ 1980 16.9196 1997 2005 energy ▂▂▂▂▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃ import 1980 8.6156 1998 2010 ▃▃▂▂▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃ united states 1980 10.8901 2001 2013 ▃▃▃▃▃▂▂▂▂▂▂ sustainable ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃ 1980 6.7481 2004 2013 development ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃ 1980 8.1758 2006 2013 ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ index 1980 4.5023 2007 2012 ▃▃▃▃▂▂▂▂▂▂▂ input–output ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ 1980 5.9576 2007 2010 approach ▃▃▂▂▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ 1980 5.6802 2007 2013 ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ responsibility 1980 5.1164 2007 2015 ▃▃▃▃▃▃▃▂▂▂▂ ecological ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ 1980 9.1417 2007 2014 footprint ▃▃▃▃▃▃▂▂▂▂▂ emergy ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ 1980 5.2424 2007 2013 analysis ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃ wood 1980 8.1945 2008 2013 ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ biomas 1980 4.253 2009 2012 ▃▃▃▃▂▂▂▂▂▂▂ energy ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 4.253 2009 2012 analysis ▃▃▃▃▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ energy use 1980 7.9758 2010 2012 ▂▃▃▃▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ methodology 1980 5.9936 2010 2013 ▂▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ resources use 1980 4.1693 2011 2014 ▂▂▃▃▃▃▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ oil 1980 3.745 2011 2013 ▂▂▃▃▃▂▂▂▂▂▂ environmental ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 5.3527 2011 2013 assessment ▂▂▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ environment 1980 5.4648 2012 2014 ▂▂▂▃▃▃▂▂▂▂▂ Sustainability 2019, 11, 4260 9 of 22

Sustainability 2019, 11, x FOR PEER REVIEW Table 4. Cont. 9 of 22

Keywords Year Strength Begin End 1980–2019 greenhouse ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 4.0218 2012 2013 gas ▂▂▂▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ cost 1980 4.8938 2012 2015 ▂▂▂▃▃▃▃▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ dwelling 1980 5.2936 2012 2015 ▂▂▂▃▃▃▃▂▂▂▂ environmental ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 5.3633 2012 2016 performance ▂▂▂▃▃▃▃▃▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ climate 1980 4.0194 2012 2016 ▂▂▂▃▃▃▃▃▂▂▂ renewable ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 6.2598 2013 2015 energy ▂▂▂▂▃▃▃▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ requirement 1980 3.9291 2013 2014 ▂▂▂▂▃▃▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ need 1980 6.1473 2013 2014 ▂▂▂▂▃▃▂▂▂▂▂ greenhouse ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 4.4685 2013 2014 gas ▂▂▂▂▃▃▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ aggregation 1980 3.9093 2013 2014 ▂▂▂▂▃▃▂▂▂▂▂ embodied ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ 1980 4.4425 2015 2019 emission ▂▂▂▂▂▂▃▃▃▃▃ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ perspective 1980 5.2075 2016 2017 ▂▂▂▂▂▂▂▃▃▂▂ structural ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂ decomposition 1980 7.881 2017 2019 ▂▂▂▂▂▂▂▂▃▃▃ analysis

3. Review Results 3. Review Results 3.1. Calculation Methods of Embodied Energy 3.1. Calculation Methods of Embodied Energy The input–output model is the basic method of calculating embodied energy [45]. It was founded Theby Wassily input–output Leontief. model According is the to basicthe authors method in of[16 calculating,46], in the input embodied–output energy model, [the45]. total It was output founded by Wassilyof an economy,Leontief. According can be expressed to the authors as the in sum[16,46 of], in intermediate the input–output consumption, model, the, total and finaloutput of an economy,consumption,X can be, where expressed is the as total the sumoutput of vector, intermediate is the consumption, final consumptionAX, vector, and final and consumption, A is the Y, wheredirectX inputis the coefficients total output matrix vector, that isY isshow then finalin Equation consumption (2). In this vector, matrix, and A is is the the technical direct input coefficoefficient.cients matrix It describes that is shown the relationship in Equation between (2). In all this sectors matrix, of theα is economy. the technical coefficient. It describes the relationship between all sectors of the economy.= + = ( − ) (1)

⋯ …1 X =⎡ AX + Y = (I A)− Y ⎤ (1) ⋯ − ⋯  ⎢ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⎥   =α11⎢ α12 α1j α1n⎥  (2)  ···⋯ ···⋯   α ⎢ α α α ⎥   21 ⋯ 22⋯ ⋯ 2⋯j ⋯ ⋯2n   ⎢ ··· ··· ⎥   ⎣ ⋯ ⋯ ⎦  A =  ··················  (2)  α α α α  In addition, in Equation (1),  is ithe1 identityi2 matrix,ij and thein matrix ( − ) is the Leontief  ··· ···    inverse matrix.  ··················  α αn α αnn denotes the intermediate inputn1 vector2 which··· cannj be ···obtained by multiplying the direct input coefficient matrix by the total output vector. Turning to embodied energy,1 such as the embodied In addition, in Equation (1), I is the identity matrix, and the matrix (I A)− is the Leontief inverse matrix. energy flow in global trade. For example, besides the technical coefficient− , a complete consumption AXcoefficientdenotes is also the intermediatewidely used ininput input- vectoroutput whichmodeling. can Its be matrix obtained can be by expressed multiplying as follows: the direct input coefficient matrix by the total output vector. Turning to embodied energy, such as the embodied Sustainability 2019, 11, 4260 10 of 22 energy flow in global trade. For example, besides the technical coefficient α, a complete consumption coefficient is also widely used in input-output modeling. Its matrix can be expressed as follows:    b11 b12 b1j b1n   ··· ···   b b b b   21 22 2j 2n   ··· ···    B =  ·················· . (3)  b b b b   i1 i2 ij in   ··· ···     ··················  b bn b bnn n1 2 ··· nj ··· Sustainability 2019, 11, x FOR PEER REVIEW 10 of 21 B is the complete consumption coefficient matrix and can be calculated as follows:

B is the complete consumption coefficient matrix and1 can be calculated as follows: B = (I A)− I (4) 𝐵=(𝐼−− 𝐴)−−𝐼 (4) In matrix B, the parameter bij measures how much direct and indirect output from sector i will In matrix 𝐵, the parameter b measures how much direct and indirect output from sector 𝑖 be used given each output increase in sector j. Then embodied energy in international trade can be will be used given each output increase in sector 𝑗. Then embodied energy in international trade can calculated as follows: be calculated as follows: n C X E = M b I E j ij. (5) 𝐸=Y + Y Y ∑ 𝑀 ×𝑏× . (5) − j=1 InIn Equation Equation (5), (5),E denotesE denotes embodied embodied energy; energy;C denotes𝐶 denotes the the energy energy consumption consumption of of a countrya country or a or a region; 𝑌 denotes the output of the energy sector;I 𝑌 denotes the energy sector’s imports from region; Y denotes the output of the energy sector; Y denotes the energy sector’s imports from other other countries;E 𝑌 denotes the energy sector’s exports to other countries; 𝑀 denotes exports in countries; Y denotes the energy sector’s exports to other countries; Mj denotes exports in sector j. sector 𝑗. In the above model, the consumption coefficient B followed a simple extension of monetary In the above model,P the consumption coefficient B followed a simple extension of monetary input–output balance, εiXij = εiXi, which has been criticized for it cannot reflect energy conversation input–output balance,i=1 ∑ 𝜀𝑋 =𝜀𝑋 , which has been criticized for it cannot reflect energy conditions.conversation Therefore, conditions. the Therefore, authors in the [31 authors,33] proposed in [31,33] another proposed model. another In their model. study, In their the study, principle the of input–outputprinciple of balanceinput–output for Producer balance fori is Producer shown in 𝑖 Figure is shown3. in Figure 3.

FigureFigure 3. 3.The The principle principle ofof input–outputinput–output balance balance of of embodied embodied energy. energy.

It illustratesIt illustrates that that the the physical physical balancebalance in terms of of embodied embodied energy energy flow flow for for producer producer i cani can be be expressedexpressed as as follows: follows: 𝑑 + ∑ 𝜀 ×𝑥 =𝜀 ×∑ 𝑥+ 𝑓(𝑖 = 1,2, ⋯ . 𝑛; 𝑗 =1,2,⋯,𝑛). (6) Xn Xn    In this equation,di + 𝑑 ε denotesj xji = theεi direct energyxij + fi input(i = 1,of2, sector.n ;𝑖j; =𝜀 1, denotes 2, , n the). indirect energy (6) × ×   ··· ··· intensity of sector 𝑗; j=𝑥1 denotes the intermediatej=1 input 𝑗 to produce product 𝑖 ; 𝑓 denotes the sum of final consumption. It can also be expressed as a matrix as follows: 𝐷 +𝑋𝑍 =𝑌𝑍. (7)

𝑑 𝜀 𝑥 ⋯𝑥 𝑑 𝜀 In Equation (7), 𝐷 = ; 𝑋 = ⋮⋱⋮ ; 𝑍 = ; and 𝑌= ⋮ ⋮ 𝑥 ⋯𝑥 𝑑 𝜀 ∑ 𝑥 +𝑓 0⋯ 0 0⋱00 ⎛ ⎞ ; 𝑍=𝐷×(𝑌−𝑋), and it is the embodied energy intensity ⋮0⋱⋮ ⎝ 00⋯∑ 𝑥 +𝑓⎠ vector. Thus, embodied energy 𝐸 can be calculated by the following equation: 𝐸=𝑍×𝑋. (8) Sustainability 2019, 11, 4260 11 of 22

In this equation, di denotes the direct energy input of sector i; εj denotes the indirect energy intensity of sector j; xij denotes the intermediate input j to produce product i ; fi denotes the sum of final consumption. It can also be expressed as a matrix as follows:

DT + XTZT = YZT. (7)      d1     ε1     x11 xn1     d2   ···   ε2  T   T  . . .  T   In Equation (7), D =  .  ; X =  . .. .  ; Z =  .  ;  .   . .   .   .     .    x1n xnn   dn ··· εn  n   P   x1j + f1 0 0   j=1 ···       ..   0 . 0 0  1 and Y =   ; Z = D (Y X)− , and it is the embodied energy  . . .  × −  . 0 .. .     n   P +   0 0 xnj fn  ··· j=1 intensity vector. Thus, embodied energy E can be calculated by the following equation:

E = Z X. (8) × In addition, different input–output models have been developed to satisfy different research contexts. Specifically, the single-region input–output (SRIO) model is popular in academic research; it uses data from a region or a country to calculate the embodied energy in a system [47]. For example, Tang et al. (2013) used this method to calculate the embodied energy sources in British international trade [16]. Lin et al. (2017) [47] used this method to calculate the embodied energy consumption of China’s construction industry. It is also widely used in the calculation of implicit carbon emissions. Typically, Yan and Yang (2010), Xu et al. (2011), and Minx et al. (2011) [48–50] use a single regional input–output (SRIO) approach to evaluate China’s embodied emissions in different periods. However, the assumption of this approach is that the same technology is used to produce the same products in all countries and regions. The results may be biased as it does not distinguish between imports and domestic production, nor can it reflect the impact of different intermediate inputs [51]. Therefore, in order to more accurately reflect the relationship between energy consumption and flow in different regions, multi-region input–output models (MRIO) appeared. Generally, the Global Trade Analysis Project (GTAP) and OECD input–output databases are the two most prominent sources for MRIO models [41,45,52,53]. In this way, the input–output data of various countries are based on unified statistics, and more parties can better approve bilateral trade flow data. For example, Bortolamedi (2015) [2] used this method to calculate the embodied energy in trade and further integrated it into a new energy security index to better guide the assessment of regional security performance and the comparative analysis of the regional security situation. Zhang et al. (2016) [54] and Gao et al. (2018) [55] used this method to calculate the embodied energy for exploring how the embodied energy is transferred between regions through China’s domestic trade. However, MRIO is often criticized for its extensive data requirements [51]. At the same time, in order to better integrate energy issues with environmental impacts, the Environment Input–Output (EIO) method emerged. If the division of SRIO and RIO is based on the data foundation difference of the embodied energy measurement, EIO is based on the calculation ideology of embodied energy. It is an extension of the standard Leontief input–output (I-O) model and describes the total energy consumption (embodied energy) required by the economic output (goods or services) of a production unit driven by final demand. EIO can calculate the direct energy consumption of a sector’s final demand and all indirect energy consumption of other sectors in the supply chain [56]. It is also usually combined with Life Circle Analysis (LCA), called the EIO–LCA model. New and typical research like that of Tao et al. (2018) [57] used this Sustainability 2019, 11, 4260 12 of 22 method to calculate and decompose the embodied energy in manufacturing trade between China and the European Union (EU) from 1995 to 2011. Furthermore, because EIO–LCA can provide a life cycle perspective, this method is used in urban planning. It can provide a linkage between urban system activities and their lifecycle materials and energy input and can reveal the relationship between specific urban areas and supply areas. It then provides a foundation for further investigation of the ecological, social, and political impact of a series of activities in one place on another [58].

3.2. A New Perspective of Demand and Consumption of Energy Generally, the greatest contribution of embodied energy is providing a new perspective for observing energy demand and consumption. Firstly, in related research on energy issues, one can get different research results through embodied energy measurement, which can be a meaningful comparison and compensation of results based on direct energy measurement. Therefore, it can enhance understanding of energy consumption of a region, country, or industry [36,56]. As Figure4 shows, when considering indirect energy consumption, there are significant changes in sector energy consumptionSustainability 2019 compositions, 11, x FOR PEER [33 REVIEW]. 12 of 21

FigureFigure 4.4. A comparison example of energyenergy consumptionconsumption compositionscompositions withwith twotwo didifferentfferent measurements.measurements. [ 33[33].].

Moreover, in the study of energy supply diversification, most studies are based on the perspective of energy sources or energy suppliers and only consider direct energy imports. Integrating embodied energy into relevant studies is an effective supplement for the vulnerability of conclusions drawn by considering direct energy supply only [59]. Moreover, through embodied energy measurement, Li et al. (2016) [60] found that energy embodied in domestic trade dominated Beijing’s energy consumption, while local direct energy accounted for less than 1/3 of Beijing’s total embodied energy consumption. Zhang et al. (2016) [54] found that the directions of energy flow between regions in China were different from the directions of direct energy flows. Similarly, in the research of energy imports and exports, because embodied energy covers both direct energy and indirect energy demand, energy relations between countries were different in comparison with relations only considering direct energy demand. For example, some indicators such as energy intensity, energy net import dependency, and primary dependency all changed when considering embodied energy [2]. Cui et al. (2015) [61] found that the energy embodied in China’s global trade increased rapidly during 2001–2007 and faster than the total direct energy exported in the same period; China is shown as an energy exporter in terms of embodied energy. Tang et al. (2013) [16] found that alongside the more obvious direct energy imports, if net embodied fossil energy imports are considered, the gap between energy consumption and production in the UK is much larger than commonly perceived. Sustainability 2019, 11, 4260 13 of 22

Moreover, in the study of energy supply diversification, most studies are based on the perspective of energy sources or energy suppliers and only consider direct energy imports. Integrating embodied energy into relevant studies is an effective supplement for the vulnerability of conclusions drawn by considering direct energy supply only [59]. Moreover, through embodied energy measurement, Li et al. (2016) [60] found that energy embodied in domestic trade dominated Beijing’s energy consumption, while local direct energy accounted for less than 1/3 of Beijing’s total embodied energy consumption. Zhang et al. (2016) [54] found that the directions of energy flow between regions in China were different from the directions of direct energy flows. Similarly, in the research of energy imports and exports, because embodied energy covers both direct energy and indirect energy demand, energy relations between countries were different in comparison with relations only considering direct energy demand. For example, some indicators such as energy intensity, energy net import dependency, and carrier dependency all changed when considering embodied energy [2]. Cui et al. (2015) [61] found that the energy embodied in China’s global trade increased rapidly during 2001–2007 and faster than the total direct energy exported in the same period; China is shown as an energy exporter in terms of embodied energy. Tang et al. (2013) [16] found that alongside the more obvious direct energy imports, if net embodied fossil energy imports are considered, the gap between energy consumption and production in the UK is much larger than commonly perceived. In addition, economic activities cannot be independent of energy supply limitations. However, it is easy to overlook this limitation when the analysis covers only a small part of the economic system, while embodied energy can help understand how the consumption of one activity shifts to another part of the system when analyzing economic activities from a system-wide perspective [14]. Furthermore, because embodied energy reflects not only the direct energy consumption of economic or environmental products but also the indirect energy consumption throughout their life cycle, it can effectively link the energy consumption relationships of different sectors. In particular, by applying complex network analysis techniques, scholars have a deeper understanding of the entire embodied energy flow network. For instance, Liu et al. (2010) [18] identified energy-intensive industries from an embodied energy perspective by measuring energy consumption in 29 industrial sectors with embodied energy in China and comparing the differences of production energy use. Furthermore, An et al. (2015) [15] depicted the embodied energy flow mode between industries in China and Shi et al. (2017) [22] revealed the energy flow pattern of global trade by embodied energy flow networks. A summary of related research can be seen in Table5.

Table 5. Typical research by using complex network analysis.

Author Year Study Target Key Results The basic network features change little during the research period; An et al. [15] 2015 Chinese Industries industries that have most embodied energy flows change from oil-related industries to coal-related industries. 80% of flows are between different countries; Shi et al. [22] 2017 global sectors the network is sensitive; The network presents an obvious clustering feature. At the global level, small-world nature has been found; Chen et al. [17] 2018 Global the economies are highly connected through embodied energy transfer. Heterogeneity distribution of different Gao et al. [57] 2018 Interprovincial in China types of energy flow. Sustainability 2019, 11, 4260 14 of 22

Table 5. Cont.

Author Year Study Target Key Results World embodied rare earths link network is clearly divided into two communities, and Global and China the network can reveal the small world Wang et al. [62] 2019 embodied rare earths nature characteristics; China, Germany, and the USA are the three most important economies. There is significant small-world property in the global embodied mineral flow network; The global embodied other Non-Metallic Mineral in China is the Jiang et al. [63] 2018 mineral flow between most important consumer of embodied industrial sectors minerals; the embodied mineral flows have strong directionality. Sectoral investigation indicates that Regional and sectoral Tang et al. [64] 2019 infrastructure construction remained energy network of China dominant in the current Chinese economy. The small world nature identified key Sun et al. [33] 2016 Sectors in China sectors of IEFNs (indirect energy flow networks; most of the embodied energy convergence and transmission is concentrated in a few industries; Internal and external preferential selections is an important Feng et al. [65] 2019 industries of mechanism; the embodied energy flow manufacturing in China patterns of the internal network of manufacturing mainly include two-focus and multi-focus convergence patterns.

Beyond using embodied energy to assess energy consumption in the macroeconomic system, embodied energy is also widely used in the microeconomic system, especially in energy-related research of building and construction projects [66,67]. For example, Held et al. (2012) [68] compared the total embodied energy of the water produced by eight interventions used in different areas to evaluate their energy efficiency and the role and status of people and materials in the process. The embodied energy of buildings has always been considered very important, and its combination with LCA links the energy consumption results of embodied energy with the environmental impact [69]. For example, the studies of Asdrubali et al. (2013) [70] and Basbagill et al. (2013) [71] on traditional building optimization and environmental impact optimization in the design stage used embodied energy as the measurement tool. Hashemi et al. (2015) [72] evaluated the current conditions of Ugandan low-income tropical housing by embodied energy with a focus on construction methods and materials in order to identify the key areas for improvement. Additionally, it can refer to relevant review articles such as Chastas’ s review article on embodied energy in residential buildings in 2016 [73].

3.3. Embodied Energy and Energy Security Energy security is a critical issue for sustainable development [5]. The value of embodied energy measurement for energy security issues is reflected in two aspects at least. On the one hand, embodied energy can provide additional insights for appropriate energy policy to enhance energy security [17]. On the other hand, it can directly provide a helpful perspective for research on energy security issues. Like the previous statement suggests, embodied energy can provide a new perspective to investigate energy consumption and energy flows in economic systems, which can be a basis for energy policymaking and implementation and help decision-makers make more appropriate policies [30,56,74,75]. For example, Shi et al. (2017) [22] studied the flow of global embodied energy Sustainability 2019, 11, 4260 15 of 22 between global sectors and believe that these results can provide new insight into the formulation of energy-related policies. Tang et al. (2019) [64] studied the embodied energy in China’s economy, which can help formulate fair and reasonable energy-saving policies for suppliers and consumers from regional and sectoral perspectives. Additionally, with the development of research, besides using embodied energy as a measurement tool to measure energy consumption and flow in trade to provide a meaningful perspective, scholars also use more methods to deeply explore energy-related issues on the basis of embodied energy measurement, which can be a clearer reference for policy-making. By applying structural decomposition analysis (SDA), scholars can investigate not only energy consumption but also reasons for changes, which would benefit the specification of policy-making recommendations. Typical studies are combined with the structural deconstruction analysis method. Liu et al. (2018) [76] analyzed the reasons for the change of coal consumption in China based on embodied energy measurement. Their results indicated that coal consumption variations during 1997–2014 in China can be divided into four phases, and their reasons can be explained by four factors, including economic scale effect, industrial structure effect, energy intensity effect, and energy mix effect. Moreover, they identified the sectors’ roles in each factor based on SDA. Other similar research includes Liu et al. (2010) [18], who identified the driving forces of embodied energy changes in exports through structural decomposition analysis (SDA); they suggested that the energy embodied in trade should receive special attention in energy policies and that environmental factors should be considered in policy-making, which can help harmonize the country’s economic development targets with its environmental priorities. Based on the decomposition analysis of the change trend of embodied energy flows in China and EU trade, Tao (2018) [57] analyzed the structural changes in the industrial sector, changes in energy structure, and changes in trade members. The author then pointed out that these results can help China better evaluate China–EU trade from a global and a clearer perspective and stated that it is significant for China to develop its own energy policy from a global perspective and to deal with subsequent trade and climate negotiations. In addition, as described in the previous part, combined with complex network analysis, the related research of embodied energy can form a deeper understanding of energy issues from the perspective of the network, which can put forward more targeted recommendations based on network characteristics. For example, Gao et al. (2018) [55] used complex network technology to analyze the inter-provincial embodied energy transfer in China and explain in detail how the results of the analysis can be applied to policy development at all levels. Sun et al. (2016) [33] found that 20% of the energy flow edges carry approximately 80% of the total indirect energy flow volume based on the characteristics of embodied energy flow networks between China’s industrial sectors. Then, they stated that policy-making should consider not only key sectors but also key energy flow paths. Feng et al. (2019) [65] analyzed the embodied energy flow patterns in the internal and external industries of China’s manufacturing industry. They stated that China’s energy supply policy should consider the embodied energy convergence and transmission between internal and external industries of the manufacturing industry, energy-related industrial clusters, and key industries. Then, the transformation, upgrading, and sustainable development of China’s manufacturing industry and breaking energy and environmental constraints would enjoy the benefit. Meanwhile, similar research has been extended to a broader and subdivided field because of its separability and scalability. For instance, Wang et al. [62] (2019) identified different countries’ roles in the global rare earth resources flow network, and they suggested that China should develop policies based on its role in the rare earth flow network to enhance its influence in the global rare earth related industries. In addition, in some literature, embodied energy can also be used to evaluate relevant policies. For example, Li et al. (2016) [59] found that in China’s 11th and 12th five-year plan, its energy policy cannot help reduce total energy consumption. Cui et al. (2015) [61] used embodied energy to analyze the impact of China’s export policy adjustment on China’s sustainable development. In summary, embodied energy-related research can provide a reference for more appropriate policy development and ultimately help enhance energy security. Sustainability 2019, 11, 4260 16 of 22

Furthermore, the conventional energy security framework is usually confined in the context of direct energy commodity trade [77]. In contrast to direct energy input, embodied energy as a conception provides a complete perspective on energy analysis, and it can provide additional insights to energy security issues [17,78]. For instance, Tang (2013) [16] calculated the total amount of embodied fossil energy in the UK’s import and export, analyzed its industry and national distribution, and evaluated the amount of net embodied fossil energy import of the UK. From the results of those analyses, he argued thatSustainability the energy 2019, 11 security, x FOR PEER problem REVIEW in the UK is more serious than the conclusion based on direct energy16 of 21 import and that the energy security policy should be reconsidered. Bortolamedi (2015) [2] argued that the incorporation ofTable embodied 6. An evaluation energy into system the energywith embodied security energy assessment as an index system [80]. is a wise choice and canMetrics/KPIs provide guidance forUnit formulating better energy security policies. Description Chen et al. (2018) [17] argued that internationalPower organizationskW such as the International Energy Instant Agency power should load extend their definition of energyEnergy security from kWh, an embodiment kJ perspective. Energy Moreau consumed and in Vuille a specific (2018) time [79 period] believe that although theProcess traditional energy transfer kWh, of energy-intensivekJ Total energy industries consumed abroad by can a process improve in a energyspecific performancetime period in the traditionalTheoretical sense,energy it might kWh, kJ lead Energy to energy consumed use andonly securityby manufacturing deterioration processes through in a specific embodied time period energy analysis.Auxiliary Therefore, energy kWh,they arguedkJ Energy that consumed energy security by subsyste indicatorsms of a machine should betool adjusted in a specific in combinationtime period with embodied energy to avoidAllocated conflicts energy between consumed energy by services intensity used and to energymaintain trade the environment security. Sato for Indirect energy kWh, kJ et al. (2017) [59] argued that when the diversityproduction of direct activities energy such importsas heating, is lighting, limited, etc. the diversity of Energy consumed by transportation equipment such as robots, conveyors, etc. embodiedHandling energyenergy imports kWh, kJ helps to improve energy security. In addition, although the application of embodied energyEnergy at the macro-level is still at a theoretical Feature energy kWh, kJ Energy used to manufacture a feature stage, its application at the relative micro-level to enhance energy security is more feasible. Typical Embodied energy kWh, kJ Energy used to manufacture a part studies include Uluer et al. (2016) [80], who integrated embodied energy into the performance Energy cost Currency The monetary cost of energy used for a specific time period evaluation system of energy reduction in the manufacturing process chain (E-MPC); as Table6 shows, Value-added energy Energy consumed by a machine during actual process activities as a % embodied(VAE) per machine energyis one of the key performancepercentage indicators. of all Moreover,activities, including they gave idling an implementation program,Value-added as Figureenergy5 shows andEnergy conducted consumed a pilot by casea part study during to all prove actual its process practicability activities and as a advantage.percentage % From(VAE) the per chart, part we can see that embodiedof all activities, energy estimation including waitin and identificationg, handling, and is an indirect important shares process.

Figure 5. An implementation system [[80].80].

Other similar literature includes Mo et al. (2010) [81], who evaluated the embodied energy consumption in drinking water supply systems. Through the case study of a municipal water system, they proposed developing a more comprehensive understanding of embodied energy and drinking water and stated that water consumption could be affected by supply- and demand-side policies. Then, they proposed some practical recommendations like the actual water price being determined based on the energy price calculated by a more integrated system. Goe and Gaustad (2014) [82] argued that physical material constraints threaten energy security, but traditional approaches often lead to command-and-control policies and a broader definition of criticality that goes beyond physical scarcity to include sustainability indicators like embodied energy. Then, they took solar photovoltaic materials as an example and gave a targeted policy reference. Sustainability 2019, 11, 4260 17 of 22

Table 6. An evaluation system with embodied energy as an index [80].

Metrics/KPIs Unit Description Power kW Instant power load Energy kWh, kJ Energy consumed in a specific time period Total energy consumed by a process in a specific Process energy kWh, kJ time period Energy consumed only by manufacturing Theoretical energy kWh, kJ processes in a specific time period Energy consumed by subsystems of a machine tool Auxiliary energy kWh, kJ in a specific time period Allocated energy consumed by services used to Indirect energy kWh, kJ maintain the environment for production activities such as heating, lighting, etc. Energy consumed by transportation equipment Handling energy kWh, kJ such as robots, conveyors, etc. Energy Feature energy kWh, kJ Energy used to manufacture a feature Embodied energy kWh, kJ Energy used to manufacture a part The monetary cost of energy used for a specific Energy cost Currency time period Energy consumed by a machine during actual Value-added energy (VAE) % process activities as a percentage of all activities, per machine including idling Energy consumed by a part during all actual Value-added energy (VAE) % process activities as a percentage of all activities, per part including waiting, handling, and indirect shares

Other similar literature includes Mo et al. (2010) [81], who evaluated the embodied energy consumption in drinking water supply systems. Through the case study of a municipal water system, they proposed developing a more comprehensive understanding of embodied energy and drinking water and stated that water consumption could be affected by supply- and demand-side policies. Then, they proposed some practical recommendations like the actual water price being determined based on the energy price calculated by a more integrated system. Goe and Gaustad (2014) [82] argued that physical material constraints threaten energy security, but traditional approaches often lead to command-and-control policies and a broader definition of criticality that goes beyond physical scarcity to include sustainability indicators like embodied energy. Then, they took solar photovoltaic materials as an example and gave a targeted policy reference.

4. Discussion Embodied energy is essentially an indicator of energy consumption in the econometric system relative to direct energy or operation energy. Because it reflects the direct and indirect energy consumption in the economic system, it can provide a new perspective for the study of energy-related issues. It enables us to analyze energy-related issues from the perspective of the whole supply chain and the whole energy flow network, instead of based on single energy supply or a single stage of energy consumption [14,74]. Moreover, due to the scalability of the concept of embodied energy and the application of more analytical methods like SDA and complex network analysis, embodied energy can be used in more fields and help us understand energy issues better. For example, it can be used to identify the linkage effects of policies and contribute to the entire industry and finally to solve energy security problems from the entire economic system. According to the literature, embodied energy is directly applied to the research of energy security issues in only a few studies. However, embodied Sustainability 2019, 11, 4260 18 of 22 energy can be used to evaluate the energy consumption of a building in its whole life cycle at the micro-level and help understand the energy flow between various regions or industries at the macro level. It can provide a systematic perspective rather than a separate approach for the improvement of energy security and energy sustainability issues. At the same time, it is worth noting that although embodied energy can provide a more comprehensive perspective on energy consumption, its widespread application and revealing its real value in practice still face some challenges. As we proposed at the beginning of this study, to what extent embodied energy measurement can provide innovative ideas for energy security issues remains largely open.

4.1. How to Solve the Problem of Timeliness and Authority of Data? At present, embodied energy calculation is based on input–output analysis and the data foundation are the input–output table. However, up to now, the newest input–output data can be found in the available databases like WIOD and Eora et al., which have lagged behind for at least three years. The limitation of data leads to a discussion on whether the conclusions based on these data can effectively reflect the current energy status, thus providing a valuable reference for energy policy development and other issues. For example, China’s input–output table has only been updated up to 2015, and it has been delayed for nearly four years. In 2017, China issued the “Several Opinions on Further Deepen the Reform of the Oil and Gas System,” which changed the energy management system and had a profound influence on energy supply and demand and the economy. Therefore, to what extent the research results based on data from 2015 can provide advisable and practical reference needs further discussion.

4.2. How to Develop More Practical Recommendations or Plans for Energy Policy-Making to Improve Energy Security? Most of the existing research focuses on embodied energy analysis, such as the consumption of embodied energy, flow paths, and driving factors in different systems. Although some scholars proposed some recommendations for policy-making, most of them were just theoretical and gave directions. Hard work is still needed to transfer the suggestions to a feasibly specific policy or implementation plan. Moreover, from the research of policy evaluation to evaluate the value of embodied energy, because of the lack of sufficient research and uniform standards, further research is necessary.

4.3. To What Extent Can Help Solve Energy Security Issues? We believe that the application of embodied energy allows us to have a different and more scientific understanding of existing energy security issues; For example, some studies mention that embodied energy should be integrated into energy security research systems [2,66]. However, building a complete set of energy research and practical systems based on embodied energy still requires hard work. For example, are existing energy security indicators also applicable to the embodied energy measurement framework? In the meantime, according to the definition of energy security issued by the International Energy Agency (IEA), the long-term energy security is the availability of a regular supply of energy at an affordable price [83], which means sufficient energy supply, stable price, and energy supply channel security. Embodied energy analysis cannot solve channel security. Moreover, with the increasing attention to environmental issues, the concept of energy security also extends to ecological security. In this context, whether embodied energy is better than other analyses such as embodied carbon emissions or emerge analysis needs further exploration.

5. Conclusions This paper made a comprehensive description of the development and application of embodied energy in academia. Specifically, this paper firstly introduced the definition and characteristics of Sustainability 2019, 11, 4260 19 of 22 embodied energy and depicted the development of previous studies. Then, based on a review of the typical literature, this paper investigated how embodied energy can provide a meaningful reference for studies on energy, and especially on energy security. Specifically, from the analysis of the literature, embodied energy has attracted enough attention, and it has been widely used in various fields. Among these fields, the investigation of embodied energy in trade and in buildings are two important fields. The input–output model is the calculation foundation of embodied energy, but it would be used in different models according to different contexts. Additionally, it can provide a new perspective on understanding energy consumption, which is useful to understanding energy demand and consumption based on direct energy consumption. At the same time, through a comprehensive understanding of energy consumption, it can be a useful and more explicit reference for policy-making to enhance energy security. Moreover, it is an advisable choice to integrate embodied energy into the study of energy security issues. In summary, as an energy measurement tool, embodied energy can provide more comprehensive results of energy consumption, which contains all indirect and direct energy consumption. It can benefit the understanding of how to improve the energy efficiency and energy sustainability of the economic system, then provide recommendations and directions for policy-making to enhance energy security. Indeed, because of the limitations of the data foundation, research foundation, and methodology, we still face many challenges in exploring more of the value of embodied energy in the study and practice of energy security issues. Therefore, related topics are still worth developing in the future.

Author Contributions: Conceptualization, methodology, formal analysis, writing—original draft preparation, writing—review and editing, and visualization, J.C. and W.Z.; conceptualization and supervision, H.Y. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Chow, J. Energy Resources and Global Development. Science 2003, 302, 1528–1531. [CrossRef][PubMed] 2. Bortolamedi, M. Accounting for hidden energy dependency: The impact of energy embodied in traded goods on cross-country energy security assessments. Energy 2015, 93, 1361–1372. [CrossRef] 3. Popescu, G.H.; Mieila, M.; Nica, E.; Andrei, J.V. The emergence of the effects and determinants of the energy paradigm changes on European Union economy. Renew. Sustain. Energy Rev. 2018, 81, 768–774. [CrossRef] 4. World Energy Outlook 2018. Available online: https://www.iea.org/weo2018/ (accessed on 2 April 2019). 5. BP Statistical Review of World Energy. Available online: https://www.bp.com/content/dam/bp/business-sites/ en/global/corporate/pdfs/energy-economics/statistical-review/bp-stats-review-2019-full-report.pdf (accessed on 26 June 2019). 6. Turton, H.; Barreto, L. Long-term security of energy supply and climate change. Energy Policy 2006, 34, 2232–2250. [CrossRef] 7. Chow, G.C. China’s Energy and Environmental Problems and Policies. Asia Pac. J. Account. Econ. 2008, 15, 57–70. [CrossRef] 8. Qian, X.; Liu, C.; Jiang, X.; Xu, J. Overview of international oil and gas industry developments in 2017 and outlook for 2018. Int. Pet. Econ. 2018, 26, 32–38. 9. Barrett, M.; Lowe, R.; Oreszczyn, T.; Steadman, P.; Oreszczyn, T. How to support growth with less energy. Energy Policy 2008, 36, 4592–4599. [CrossRef] 10. Bashmakov, I.; Myshak, A. Russian energy efficiency accounting system. Energy Effic. 2014, 7, 743–759. [CrossRef] 11. Erdal, L.; Pekta¸s,A.O.; Ozkan, O.; Özkan, F. Security of energy supply in Japan: A key strategy and solutions. Int. J. Glob. Warm. 2015, 7, 128. [CrossRef] 12. Aguayo, F.; Gallagher, K.P. Economic reform, energy, and development: The case of Mexican manufacturing. Energy Policy 2005, 33, 829–837. [CrossRef] 13. Leung, G.C. China’s oil use, 1990–2008. Energy Policy 2010, 38, 932–944. [CrossRef] 14. Costanza, R. Embodied Energy and Economic Valuation. Science 1980, 210, 1219–1224. [CrossRef][PubMed] Sustainability 2019, 11, 4260 20 of 22

15. An, Q.; An, H.; Wang, L.; Gao, X.; Lv, N. Analysis of embodied exergy flow between Chinese industries based on network theory. Ecol. Model. 2015, 318, 26–35. [CrossRef] 16. Tang, X.; Snowden, S.; Höök, M. Analysis of energy embodied in the international trade of UK. Energy Policy 2013, 57, 418–428. [CrossRef] 17. Chen, B.; Li, J.; Wu, X.; Han, M.; Zeng, L.; Li, Z.; Chen, G. Global energy flows embodied in international trade: A combination of environmentally extended input-output analysis and complex network analysis. Appl. Energy 2018, 210, 98–107. [CrossRef] 18. Liu, H.; Xi, Y.; Guo, J.; Li, X. Energy embodied in the international trade of China: An energy input–output analysis. Energy Policy 2010, 38, 3957–3964. [CrossRef] 19. Sun, C.; Ma, T.; Xu, M. Exploring the prospects of cooperation in the manufacturing industries between India and China: A perspective of embodied energy in India-China trade. Energy Policy 2018, 113, 643–650. [CrossRef] 20. Yang, R.; Long, R.; Yue, T.; Shi, H. Calculation of embodied energy in Sino-USA trade: 1997–2011. Energy Policy 2014, 72, 110–119. [CrossRef] 21. Zhang, B.; Chen, Z.; Xia, X.; Xu, X.; Chen, Y. The impact of domestic trade on China’s regional energy uses: A multi-regional input–output modeling. Energy Policy 2013, 63, 1169–1181. [CrossRef] 22. Shi, J.; Li, H.; Guan, J.; Sun, X.; Guan, Q.; Liu, X. Evolutionary features of global embodied energy flow between sectors: A complex network approach. Energy 2017, 140, 395–405. [CrossRef] 23. Dixit, M.K. Life cycle recurrent embodied energy calculation of buildings: A review. J. Clean. Prod. 2018. [CrossRef] 24. Dixit, M.K.; Culp, C.H.; Fernandez-Solis, J.L. System boundary for embodied energy in buildings: A conceptual model for definition. Renew. Sustain. Energy Rev. 2013, 21, 153–164. [CrossRef] 25. Wu, X.; Chen, G. Coal use embodied in globalized world economy: From source to sink through supply chain. Renew. Sustain. Energy Rev. 2018, 81, 978–993. [CrossRef] 26. Tang, X.; Zhang, B.; Feng, L.; Snowden, S.; Höök, M. Net oil exports embodied in China’s international trade: An input–output analysis. Energy 2012, 48, 464–471. [CrossRef] 27. Cortés-Borda, D.; Guillén-Gosálbez, G.; Jiménez, L. Solar energy embodied in international trade of goods and services: A multi-regional input–output approach. Energy 2015, 82, 578–588. [CrossRef] 28. Cortés-Borda, D.; Guillén-Gosálbez, G.; Jiménez, L. Assessment of nuclear energy embodied in international trade following a world multi-regional input–output approach. Energy 2015, 91, 91–101. [CrossRef] 29. Wiedmann, T.; Lenzen, M.; Turner, K.; Barrett, J. Examining the global environmental impact of regional consumption activities—Part 2: Review of input–output models for the assessment of environmental impacts embodied in trade. Ecol. Econ. 2007, 61, 15–26. [CrossRef] 30. Lindner, S.; Guan, D. A Hybrid-Unit Energy Input-Output Model to Evaluate Embodied Energy and Life Cycle Emissions for China’s Economy. J. Ind. Ecol. 2014, 18, 201–211. [CrossRef] 31. Chen, Z.-M.; Chen, G.; Chen, G.; Chen, G. Demand-driven energy requirement of world economy 2007: A multi-region input–output network simulation. Commun. Nonlinear Sci. Numer. Simul. 2013, 18, 1757–1774. [CrossRef] 32. Wu, X.; Xia, X.; Chen, G.; Wu, X.; Chen, B. Embodied energy analysis for coal-based power generation system-highlighting the role of indirect energy cost. Appl. Energy 2016, 184, 936–950. [CrossRef] 33. Sun, X.; An, H.; Gao, X.; Jia, X.; Liu, X. Indirect energy flow between industrial sectors in China: A complex network approach. Energy 2016, 94, 195–205. [CrossRef] 34. Shao, L.; Chen, G.; Chen, G. Embodied water accounting and renewability assessment for ecological wastewater treatment. J. Clean. Prod. 2016, 112, 4628–4635. [CrossRef] 35. Kara, S.; Ibbotson, S. Embodied energy of manufacturing supply chains. CIRP J. Manuf. Sci. Technol. 2011, 4, 317–323. [CrossRef] 36. Machado, G.; Schaeffer, R.; Worrell, E. Energy and carbon embodied in the international trade of Brazil: An input–output approach. Ecol. Econ. 2001, 39, 409–424. [CrossRef] 37. Monahan, J.; Powell, J. An embodied carbon and energy analysis of modern methods of construction in housing: A case study using a lifecycle assessment framework. Energy Build. 2011, 43, 179–188. [CrossRef] 38. Matthews, H.S. Embodied Environmental Emissions in U.S. International Trade, 1997 2004. Environ. Sci. − Technol. 2007, 41, 4875–4881. [CrossRef] Sustainability 2019, 11, 4260 21 of 22

39. Davis, S.J.; Peters, G.P.; Caldeira, K. The supply chain of CO2 emissions. Proc. Natl. Acad. Sci. USA 2011, 108, 18554–18559. [CrossRef] 40. Wiedmann, T. A review of recent multi-region input–output models used for consumption-based emission and resource accounting. Ecol. Econ. 2009, 69, 211–222. [CrossRef] 41. Davis, S.J.; Caldeira, K. Consumption-based accounting of CO2 emissions. Proc. Natl. Acad. Sci. USA 2010, 107, 5687–5692. [CrossRef] 42. Merigó, J.M.; Miranda, J.; Modak, N.M.; Boustras, G.; De La Sotta, C. Forty years of Safety Science: A bibliometric overview. Saf. Sci. 2019, 115, 66–88. [CrossRef] 43. Norris, M.; Oppenheim, C. Comparing alternatives to the Web of Science for coverage of the social sciences’ literature. J. Inf. 2007, 1, 161–169. [CrossRef] 44. Chen, C.M. Searching for intellectual turning points: Progressive knowledge domain visualization. Proc. Natl. Acad. Sci. USA 2004, 101, 5303–5310. [CrossRef] 45. Bordigoni, M.; Hita, A.; Le Blanc, G. Role of embodied energy in the European manufacturing industry: Application to short-term impacts of a carbon tax. Energy Policy 2012, 43, 335–350. [CrossRef] 46. Leontief, W. Environmental Repercussions and the Economic Structure: An Input-Output Approach. Rev. Econ. Stat. 1970, 52, 262. [CrossRef] 47. Lin, L.; Fan, Y.; Xu, M.; Sun, C. A Decomposition Analysis of Embodied Energy Consumption in China’s Construction Industry. Sustainability 2017, 9, 1583. [CrossRef]

48. Yunfeng, Y.; Laike, Y. China’s foreign trade and climate change: A case study of CO2 emissions. Energy Policy 2010, 38, 350–356. [CrossRef] 49. Xu, M.; Li, R.; Crittenden, J.C.; Chen, Y. CO2 emissions embodied in China’s exports from 2002 to 2008: A structural decomposition analysis. Energy Policy 2011, 39, 7381–7388. [CrossRef] 50. Minx, J.C.; Baiocchi, G.; Peters, G.P.; Weber, C.L.; Guan, D.; Hubacek, K. A “Carbonizing Dragon”: China’s Fast Growing CO2Emissions Revisited. Environ. Sci. Technol. 2011, 45, 9144–9153. [CrossRef] 51. Qi, T.; Winchester, N.; Karplus, V.J.; Zhang, X. Will economic restructuring in China reduce trade-embodied CO2 emissions? Energy Econ. 2014, 42, 204–212. [CrossRef] 52. Atkinson, G.; Hamilton, K.; Ruta, G.; Van Der Mensbrugghe, D. Trade in ‘virtual carbon’: Empirical results and implications for policy. Glob. Environ. Chang. 2011, 21, 563–574. [CrossRef] 53. Hertwich, E.G.; Peters, G.P. Carbon footprint of nations: A global, trade-linked analysis. Environ. Sci. Technol. 2009, 43, 6414–6420. [CrossRef] 54. Zhang, B.; Qiao, H.; Chen, Z.; Chen, B. Growth in embodied energy transfers via China’s domestic trade: Evidence from multi-regional input–output analysis. Appl. Energy 2016, 184, 1093–1105. [CrossRef] 55. Gao, C.; Su, B.; Sun, M.; Zhang, X.; Zhang, Z. Interprovincial transfer of embodied primary energy in China: A complex network approach. Appl. Energy 2018, 215, 792–807. [CrossRef] 56. Liu, Z.; Geng, Y.; Lindner, S.; Zhao, H.; Fujita, T.; Guan, D. Embodied energy use in China’s industrial sectors. Energy Policy 2012, 49, 751–758. [CrossRef] 57. Tao, F.; Xu, Z.; Duncan, A.A.; Xia, X.; Wu, X.; Li, J. Driving forces of energy embodied in China-EU manufacturing trade from 1995 to 2011. Resour. Conserv. Recycl. 2018, 136, 324–334. [CrossRef] 58. Pincetl, S.; Bunje, P.; Holmes, T. An expanded urban metabolism method: Toward a systems approach for assessing urban energy processes and causes. Landsc. Urban Plan. 2012, 107, 193–202. [CrossRef] 59. Sato, M.; Kharrazi, A.; Nakayama, H.; Kraines, S.; Yarime, M. Quantifying the supplier-portfolio diversity of embodied energy: Strategic implications for strengthening energy resilience. Energy Policy 2017, 105, 41–52. [CrossRef] 60. Li, J.; Xia, X.; Chen, G.; Alsaedi, A.; Hayat, T. Optimal embodied energy abatement strategy for Beijing economy: Based on a three-scale input-output analysis. Renew. Sustain. Energy Rev. 2016, 53, 1602–1610. [CrossRef] 61. Cui, L.-B.; Peng, P.; Zhu, L. Embodied energy, export policy adjustment and China’s sustainable development: A multi-regional input-output analysis. Energy 2015, 82, 457–467. [CrossRef] 62. Wang, X.; Yao, M.; Li, J.; Ge, J.; Wei, W.; Wu, B.; Zhang, M. Global embodied rare earths flows and the outflow paths of China’s embodied rare earths: Combining multi-regional input-output analysis with the complex network approach. J. Clean. Prod. 2019, 216, 435–445. [CrossRef] 63. Jiang, M.; An, H.; Guan, Q.; Sun, X. Global embodied mineral flow between industrial sectors: A network perspective. Resour. Policy 2018, 58, 192–201. [CrossRef] Sustainability 2019, 11, 4260 22 of 22

64. Tang, M.; Hong, J.; Liu, G.; Shen, G.Q. Exploring energy flows embodied in China’s economy from the regional and sectoral perspectives via combination of multi-regional input–output analysis and a complex network approach. Energy 2019, 170, 1191–1201. [CrossRef] 65. Feng, Z.; Zhou, W.; Ming, Q. Embodied Energy Flow Patterns of the Internal and External Industries of Manufacturing in China. Sustainability 2019, 11, 438. [CrossRef] 66. Fay, R.; Treloar, G.; Iyer-Raniga, U. Life-cycle energy analysis of buildings: A case study. Build. Res. Inf. 2000, 28, 31–41. [CrossRef] 67. Bribián, I.Z.; Capilla, A.V.; Usón, A.A. Life cycle assessment of building materials: Comparative analysis of energy and environmental impacts and evaluation of the eco-efficiency improvement potential. Build. Environ. 2011, 46, 1133–1140. [CrossRef] 68. Held, R.B.; Zhang, Q.; Mihelcic, J.R. Quantification of human and embodied energy of improved water provided by source and household interventions. J. Clean. Prod. 2013, 60, 83–92. [CrossRef] 69. Suh, S.; Lenzen, M.; Treloar, G.J.; Hondo, H.; Horvath, A.; Huppes, G.; Jolliet, O.; Klann, U.; Krewitt, W.; Moriguchi, Y.; et al. System Boundary Selection in Life-Cycle Inventories Using Hybrid Approaches. Environ. Sci. Technol. 2004, 38, 657–664. [CrossRef] 70. Asdrubali, F.; Baldassarri, C.; Fthenakis, V. Life cycle analysis in the construction sector: Guiding the optimization of conventional Italian buildings. Energy Build. 2013, 64, 73–89. [CrossRef] 71. Basbagill, J.; Flager, F.; Lepech, M.; Fischer, M. Application of life-cycle assessment to early stage building design for reduced embodied environmental impacts. Build. Environ. 2013, 60, 81–92. [CrossRef] 72. Hashemi, A.; Cruickshank, H.; Cheshmehzangi, A. Environmental Impacts and Embodied Energy of Construction Methods and Materials in Low-Income Tropical Housing. Sustainability 2015, 7, 7866–7883. [CrossRef] 73. Chastas, P.; Theodosiou, T.; Bikas, D. Embodied energy in residential buildings-towards the nearly zero energy building: A literature review. Build. Environ. 2016, 105, 267–282. [CrossRef] 74. Lenzen, M. Primary energy and greenhouse gases embodied in Australian final consumption: An input-output analysis. Energy Policy 1998, 26, 495–506. [CrossRef] 75. Hong, J.; Shen, G.Q.; Guo, S.; Xue, F.; Zheng, W. Energy use embodied in China’s construction industry: A multi-regional input–output analysis. Renew. Sustain. Energy Rev. 2016, 53, 1303–1312. [CrossRef] 76. Tang, X.; Jin, Y.; McLellan, B.C.; Wang, J.; Li, S. China’s coal consumption declining—Impermanent or permanent? Resour. Conserv. Recycl. 2018, 129, 307–313. [CrossRef] 77. Kruyt, B.; Van Vuuren, D.; De Vries, H.; Groenenberg, H.; Van Vuuren, D. Indicators for energy security. Energy Policy 2009, 37, 2166–2181. [CrossRef] 78. Li, J.; Chen, G.; Wu, X.; Hayat, T.; Alsaedi, A.; Ahmad, B.; Chen, G. Embodied energy assessment for Macao’s external trade. Renew. Sustain. Energy Rev. 2014, 34, 642–653. [CrossRef] 79. Moreau, V.; Vuille, F. Decoupling energy use and economic growth: Counter evidence from structural effects and embodied energy in trade. Appl. Energy 2018, 215, 54–62. [CrossRef] 80. Uluer, M.U.; Unver, H.O.; Gok, G.; Fescioglu-Unver, N.; Kilic, S.E. A framework for energy reduction in manufacturing process chains (E-MPC) and a case study from the Turkish household appliance industry. J. Clean. Prod. 2016, 112, 3342–3360. [CrossRef] 81. Mo, W.; Nasiri, F.; Eckelman, M.J.; Zhang, Q.; Zimmerman, J.B. Measuring the Embodied Energy in Drinking Water Supply Systems: A Case Study in The Great Lakes Region. Environ. Sci. Technol. 2010, 44, 9516–9521. [CrossRef] 82. Goe, M.; Gaustad, G. Identifying critical materials for photovoltaics in the US: A multi-metric approach. Appl. Energy 2014, 123, 387–396. [CrossRef] 83. Proskuryakova, L. Updating energy security and environmental policy: Energy security theories revisited. J. Environ. Manag. 2018, 223, 203–214. [CrossRef]

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