## 2 Concepts Review  
### Energy Sustainability  
According to the World Energy Council (WEC), the definition of energy sustainability is based on three core dimensions: energy security, energy equity, and environmental sustainability. Together, they make up a \"trilemma\" and achieving high performance in all three dimensions entails complex, interwoven links between public and private actors, governments and regulators, economic and social factors, national resources, environmental concerns, and individual behaviors [11]. In order to evaluate the performance of this energy sustainability trilemma for countries, the WEC uses the Energy Trilemma Index.  
The index is a complete, rigorous, and widely-recognized indicator for decision-making on energy policy [12]. In its calculation, 75% corresponds to the analysis of the energy performance of the countries, where energy security, energy equity, and environmental sustainability are evaluated. The remaining 25% corresponds to the analysis of contextual performance. Generally speaking, a country is awarded a better rating and position in the ranking when it demonstrates a better energy and contextual performance. From now on, to talk about energy sustainability in this paper, we will only refer to energy performance, and the following acronyms will be used: ETI to talk about the Energy Trilemma Index; ESR to talk about energy security; EQR to talk about energy equity; and EVR to talk about environmental sustainability.  
Energy security is the effective management of primary energy supply from domestic and external sources, reliability of energy infrastructure, and ability of energy providers to meet current and future demand; energy equity is the accessibility and affordability of energy supply across the entire population; and environmental sustainability is achievement of supply- and demand-side energy efficiencies and development of energy supply from renewable and other low-carbon sources [11]. The contextual indicators consider the broader circumstances of energy performance, including a country's ability to provide coherent, predictable and stable policy and regulatory frameworks, initiate R&D and innovation, and attract investment [12].  
ETI studies 125 countries according to region and their gross domestic product per capita (group GDP or Group-Roman_number). The regions are Europe (E), Sub-Saharan Africa (Sub-Saharan S), Asia (A), Latin America and the Caribbean (LAC, L), the Middle East (MENA, M), and North Africa (North-America, N). The GDP is divided into Group-I (more than 33,500 USD), Group-II (between 14,300 and 33,500 USD), Group-III (between 6000 and 14,300 USD), and Group-IV (less than 6000 USD). In the context of this paper, the results in 2017 of this index were organized in Figure 2. Group-I does not contain any countries form the LAC or Sub-Saharan regions and Group-IV does not contain countries in MENA, North America. Group-II is the only group with has countries from all six regions of the study.  
The ETI also has an interactive tool, the pathway calculator, that can be used to determine what is necessary to improve the ranking position and understand the impact of policymaking on achieving a sustainable energy future [13]. Figure 3 presents the indicators of this virtual tool. According to this tool, basically a country obtains a better energy sustainability by diversifying its mix of primary energy sources, diversifying its mix of electricity generation, reducing energy imports, and reducing its CO\({}_{2}\) and other GHG emissions.