Combined utilization of electricity and thermal storages in a highly renewable energy system within an island society

With the increasing share of intermittent renewable energy resources, the demand for different forms of energy storage increases. In this paper, a comparative analysis was performed on two energy storage solutions: smallscale underground pumped hydro storage (PHS) and high -temperature thermal energy storage (HTTES). Using the PLEXOS energy and power system modeling software, the study analyzed the operation and performance of these storage systems in & Aring;land Island, an autonomous region of Finland. The novelty of the paper is in evaluating the business cases of these storage solutions when operating separately and combined. Business cases are studied by comparing annual energies, operating hours, price paid, and price received of the storage types in different scenarios. The results showed that both storages promoted electrification and renewable energy integration, with the HTTES reducing curtailment by 77 % and the PHS by 4 %. The storage systems were also able to take advantage of lower -cost hours of external electricity markets, with the PHS increasing electricity imports by 0.2 % and the HTTES by 1.5 %. Moreover, the PHS enabled the sale of excess electricity to external markets, leading to an increase in exports by 0.3 %. In contrast, the HTTES increased local consumption of excess production, reducing exports by 1.1 %. Results showed that integrating both storage systems in the same energy system could achieve significant benefits, such as reduced curtailment of renewable energy. However, the study also highlighted that the two systems could compete for the same resources in an open energy market, leading to a decrease in annual energy and net profits for both systems. A PHS exhibits an 11 % reduction in annual energies, when compared with operating alone, whereas for an HTTES, the reduction is 1 %. Similarly, the net profit of the PHS is reduced by 25.2 % and that of the HTTES by 4.9 %. Concluding the business case analysis, changes in annual energy and operation hours are not linearly correlated with revenue and charging cost, and the dynamics of the electricity market and the studied energy system must also be considered. Finally, energy storage can be considered a key element in supporting the energy transition; however, the importance of careful planning must be emphasized when integrating energy storage systems into an energy market.