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 climate change 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 supply and demand balance through effective energy management, and then to achieve sustainable development 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 manufacturing 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 systems ecology [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 carbon dioxide 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 energy crisis 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 carbon footprint 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 concrete 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 building material 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”, “greenhouse gas 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 “decomposition 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 ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃ exergy 1980 8.1758 2006 2013 ▃▃▃▃▃▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ index 1980 4.5023 2007 2012 ▃▃▃▃▂▂▂▂▂▂▂ input–output ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ 1980 5.9576 2007 2010 approach ▃▃▂▂▂▂▂▂▂▂▂ ▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃ emergy 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)