*4.3. Energy Diversity Weighted by Energy Import Dependence*

The energy diversity of Korea was quantitatively evaluated by using the proposed energy dependence-related indices, the *D\_SWI* and *D\_HHI*. Figure 8 describes the *D\_SWI*. The theoretical maximum value of the *D\_SWI* is approximately 2.302. As expected, a different trend is shown in the *D\_SWI* compared with that of the *SWI*. The maximum value of the *D\_SWI* is found in 1991 when the import dependence reaches the lowest level. Then, the *D\_SWI* drops continuously by 0.122 until 1997 when the country strongly relies on energy imports. The global trend of the *D\_SWI* from 1998 to 2010 tends to recover with fluctuation. Then, the value continuously increases from 2011 and finally reaches 0.181 in 2018. This increasing trend is linked to the country's energy dependence that has decreased since 2011 as shown in Figure 4.

**Figure 8.** The *D\_SWI* of Korea from 1991 to 2018.

However, this diversity index does not improve significantly compared with the past. The latest obtained value is even lower than that of the early 1990s (1991–1993) when the import dependence of the country was lower than 95%. This result shows the important role of the use of indigenous energy resources to improve energy supply security evaluated by the given index. This result implies the need to use indigenous energy resources and the challenges to promote self-sufficient energy such as anthracite coal and LNG. In addition, R&D on energy-efficient systems and energy systems driven by renewable energy should be supported to improve the energy supply security of the country in the long term.

Figure 9 illustrates the *D\_HHI*. The theoretical minimum value of the *D\_HHI* is 1000 when the energy import dependence is the minimum, which is zero. However, because the import dependence of Korea is far from zero, rather close to one, the *D\_HHI* is nearly

doubled by the *HHI*. This value follows the trend of *HHI*. As the share of oil imports has been over 40% for the period from 1991 to 1997, the effect of oil import on the *D\_HHI* as well as on the *HHI* is comparatively high, whereas its effect on the *SWI* and the *D\_SWI* is reduced. This means that the *D\_HHI* and *HHI* still retain the importance of abundant resources despite the high weight of energy import dependence. It indicates that avoiding any abundant energy resources could lower the *D\_HHI* more effectively than lowering their import dependence.

**Figure 9.** The *D\_HHI* of Korea from 1991 to 2018.

In summary, the *SWI* and *HHI* showed a similar trend of energy diversity of the country as referring to other research on energy supply security [6,7,10,11,45]. In more detail, the *SWI* focuses on rare energy resources, while the *HHI* highlights the abundance of any energy resources. However, these two indices overlooked the energy import dependence of the target country. As introducing the energy import dependence as a weighting factor in the *D\_SWI* and *D\_HHI*, the correlation between the country's energy diversity and energy import dependence could be accounted for in the evaluation of energy supply security. Based on the import ratio of each energy source, the *SWI* is scaled from 0 to 1 for the *D\_SWI*, whereas the *HHI* is multiplied by the value from 1 to 2 for the *D\_HHI*.

Moreover, the overall evaluation of the energy supply security of Korea infers that the country should attempt to balance the use of different types of primary energy resources because they cannot liberalize its energy imports. To reduce the concentration of abundant import resources such as oil and coal, and increase the use of rare resources, electrification in the energy sector and a higher proportion of renewable energy in electricity generation could be alternative solutions to improve its energy supply security regarding the scenario 2030. Green financing and investment in renewable energy projects are recommended as practical solutions to promote a higher proportion of renewable energy and sustainable technologies, including hydrogen and electric vehicles [46,47]. Besides, the energy policy focusing on demand-side management should strongly be applied to the country for effective and efficient use of limited resources [24].
