**1. Introduction**

The increase in population and social growth has accelerated energy consumption, bringing concerns over energy security [1]. Most countries have set their energy policies and strategies to ensure energy security for sustainable development and growth [2–4]. However, because of the geographical inequality of energy resources and resource depletion, many countries have difficulty in lowering their dependence on energy imports and ensuring their energy supply security. Moreover, energy supply security is exposed by exterior risks, such as energy price fluctuations, supply disruption, and geopolitical uncertainty.

To reduce the risk of energy supply security related to energy import dependence, Månsson et al. [5] listed solutions such as improving diversity, financial portfolios, and reliable supply and transit routes. Chalvatzis and Ioannidis [6] also focused on energy dependence and diversity as two main paradigms of energy supply security and insisted that energy dependence has raised the importance of diversity in energy security paradigms. They categorized the primary energy import dependence of different countries and analyzed their primary energy diversity to provide reference benchmarks among countries. Matsumoto et al. [7] also classified the overall improvement of primary energy diversity in EU countries using a clustering method.

Moreover, countries that are highly dependent on energy imports such as Singapore, Taiwan, Japan, and Korea have also put great effort into diversifying energy resources to reduce their energy security vulnerability. These countries have perceived the importance of energy diversification and formulated policy concepts in the direction of diversifying energy resources [8,9]. Chuang and Ma [10] evaluated the diversity of primary energy in the energy supply structure of Taiwan through the most commonly used diversification

indices: the Shannon-Wiener Index (*SWI*) and the Herfindahl-Hirschman Index (*HHI*). In another case study, Matsumoto conducted a socioeconomic study of the primary energy security performance of Japan considering the diversity of primary energy based on the *SWI* [11]. These research works provided quantifiable indices and helped to compare the given values to that of other countries.

However, as the *SWI* and *HHI* only deal with the number and proportion of energy resources, there is no information on the origin of energy resources. The results have thus led to demonstrate the diversity of already available energy resources in the county. To express the diversity and other factors related to energy resources simultaneously, a few researchers have proposed a diversity index weighted by the dependence on energy suppliers and their political stability [5,7,10,11], the diversity and reliability index [10], and the diversity and reliability variance index [10] based on the *SWI* and the *HHI*. However, these indices mostly require more specific data that are not easily accessible, nor available to the public. In addition, there has been a lack of study on the direct relationship between energy dependence and diversity.

This study thus suggests modified indices of energy diversity based on the *SWI* and *HHI*, which are weighted by import dependence-related parameters, as supplementary data to assess the energy supply security of a country. As these indices basically stem from the preliminarily obtained import dependence and primary energy diversity, the results are relatively easily obtained and compared with the reference values. The suggested indices are applied to evaluate the energy supply security of any country. In order to validate the effectiveness of the indices, this study adapted the indices to the case of Korea, a country highly dependent on energy import and making an effort to ensure their energy supply security, and compared the results with that of the conventional metrics.

The main contributions of this study are summarized as follows:


The remainder of this paper is organized as follows: Section 2 introduces a literature review on global, regional, and national analysis of energy security. Section 3 presents the analysis methodology of energy supply security, including indices of energy dependence and energy diversity, proposed energy diversity indices, and data for evaluating the energy supply security of Korea. Section 4 describes the results and discussion on the case study of Korea. Section 5 provides the main conclusions and suggestions.

## **2. Literature Review**

## *2.1. Global Level Analysis of Energy Supply Security*

The main themes of energy supply security have changed slightly as the interests of society, technology trends, and geopolitical and economic situations change. The definition of energy supply security has been represented by a number of factors such as availability [12,13], infrastructure [13], energy price [12,14,15], environment [15–17], efficiency [15,18,19], social effects [17], and governance [19]. These definitions have even

varied depending on the target application domains and their detailed subjects. While "reliable supplies of energy at reasonable prices to support the economy and industry" has been used broadly in the literature as the definition of energy supply security [14], other researchers have extended the definition to social welfare and socio-cultural effects [15,17]. Therefore, researchers, policymakers, and stakeholders have faced difficulty in establishing a global definition of energy security and its measuring method.

A number of studies have tried to integrate these concepts of energy supply security. Winzer [20] listed the definition of energy supply security by reviewing papers published in different domains in order to map the conceptual boundaries of energy security. Kruyt et al. [19] classified energy security indicators incorporating regionalization, globalization, economic efficiency, and environmental acceptability. Based on these factors, several frameworks for defining and methodologies for evaluating energy security have been suggested at the international, regional, and national levels. The analyses at the international level have provided the position, direction, and strengths and weaknesses of energy security in different countries [9,21–23]. These investigations have recommended policy prescriptions based on the evaluation results of energy security and have served as a reference for energy security among countries with differing conditions.
