**5. Conclusions**

This study presented a methodology to evaluate the energy supply security of a country by using different indices of energy dependence and energy diversity. The considered indicators were energy import ratio and energy import dependence as the indices of energy dependence and the *SWI* and *HHI* as those of energy diversity. This study also proposed two indices, the *D\_SWI* and *D\_HHI*, that reflect the correlation between a country's energy diversity and energy import dependence.

The presented methodology and indices were applied to quantitatively assess Korea's energy supply security. Korea's energy imports were over 14% of total imports during the period between 1991 and 2018. The energy import dependence increased from 91.20% to 97.79% with the economic development of the country for the period from 1991 to 1997. After the liquidity crisis in 1997, the dependence gradually decreased and reached 94.49% in 2018. The energy diversity of Korea in terms of the *SWI* and *HHI* was also measured. The primary energy sources considered were anthracite, bituminous, petroleum, LPG, LNG, general hydro, pumped hydro, nuclear, and renewables. From the measurement of energy diversity, it was found that the energy transition from petroleum to nuclear and renewable energy contributed to increasing the diversity of primary energy.

From the calculation results of the *SWI* and *HHI*, similar diversity trends were obtained in the given periods from 1991 to 2010 and from 2013 to 2017. Diversity gradually decreased because of the concentration of oil consumption until 1995. Since then, diversity has globally improved with fluctuations. In addition, unmatched trends between the *SWI* and *HHI* appeared for 2010–2011 and 2011–2012 because of the emphasis of the *SWI* on rare resources and the *HHI* on abundant resources in the shares of energy.

However, a country highly dependent on energy imports cannot be evaluated as a secure country in terms of energy supply security, even if it obtains higher energy diversity. As Korea is highly dependent on energy imports because of its high energy consumption and insufficient energy resources, the analysis of the energy supply security of Korea should consider the types and the proportion of available energy resources and their import dependence. By focusing on the correlation between Korea's energy diversity and import dependence, this study also suggested modified indices of energy diversity based on the *SWI* and *HHI*, which are weighted by import dependence-related parameters. As a result, the *D\_SWI* showed lower values of energy diversity, even in recent years, because of higher energy import dependence. The *D\_HHI* almost doubled compared with the value of the *HHI* because of the higher energy import dependence of the country and followed the trend of the *HHI* which emphasizes abundant resources. This result implies that the correlation of energy dependence and energy diversity of a country should also be considered as an important factor to evaluate energy supply security and to make energy policy strategies.

In focusing on the correlation between energy diversity and import dependence, the national energy policy should promote renewable energy because its use could mitigate the environmental effects and energy dependence. It might permit more diversified energy options in the transition of electricity-driven transport and industry. In practice, the automotive industry and R&D of electric vehicles and renewable hydrogen vehicles could be boosted from several perspectives. Policy-supported R&D investment and infrastructure in renewable energy and energy-efficient technologies with demand-side management should be implemented to stabilize the energy supply security of the country in the long term. Comparative study on energy dependence and diversity at the international level should be conducted to further analyze the characteristics of energy security of each country and classify them to suggest referable policies as well.

**Author Contributions:** All the authors have been involved in the preparation of the manuscript. All the authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, grant numbers 2017R1D1A3B03035693 and 2020R1I1A1A01073797. This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE), grant number 20192010107050.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
