Energy systems are changing as a result of the transition to renewable energy from fossil fuels and the increased deployment and integration of fluctuating renewable energy sources. However, global supply chain challenges, energy crises, and geopolitical conflicts have underlined the need for not only reducing carbon emissions and optimising costs in energy systems but also ensuring energy security and resilience. This adds new dimensions to energy system modelling and to the evaluation of energy system scenarios as an integral part of energy strategy and energy policy development. This study reviews existing energy security and resilience indicators and proposes a decentralisation adjusted diversity index approach for assessing the diversity of energy systems. The proposed diversity index method can be applied alongside different energy system modelling frameworks and can be applied ex post to modelling outputs, as demonstrated by its use with EnergyPLAN and OSeMOSYS for two different national cases, Denmark and Greece. Both cases highlight energy systems transitioning from fossil fuel-based energy systems towards 100% renewable energy systems. Hence the study uses the decentralisation adjusted diversity index to evaluate how the dynamics of energy system diversity changes with the transition to variable renewable energy resources. The phase-out of fossil fuels reduces traditional energy fuel type diversity; however, this is offset by the implementation of geographically dispersed renewable energy sources. This results in an overall increased energy diversity index value for renewable energy systems and thereby increased energy system security and resilience.
