1. Introduction
Island network energy supply is characterized by increased generation costs, mainly due to the use of thermal generators using imported fossil fuels [1].Importing depleting energy resources, coupled with volatile fuel prices, removes the island’s sense of self-sufficiency and security of supply [2].However, the islands exhibit excellent wind and solar potential that is rarely realized on the mainland [2,3]. For example, Greece has more than 100 inhabited islands, average annual wind speed is about 9.5 m/s, and solar irradiance is about 1800 kWh/m2 [3]. In contrast, the winds in the North Sea are excellent, with an average wind speed of about 10.5 m/s [4]; Germany has a large installed photovoltaic capacity, with an average annual solar irradiance of approximately 1,100 kWh/m2.2 [5]. Excellent renewable potential contributes to the island’s self-sustainability.
Due to rapidly decreasing costs, renewable energy sources (RES) can be used in island systems to reduce generation costs, dependence on imported fossil fuels, and CO2 emissions [6]. However, the intermittency of renewable energy does not guarantee uninterrupted power supply to island loads throughout the year.To overcome the shortcomings caused by the variability of renewable energy generation, several solutions have been proposed in the literature, such as (a) Curtailment of very large renewable energy sources [7](b) Island-mainland interconnection [8]and (c) incorporating energy storage systems (ESS) [1,2,3,6].The first solution requires an oversized RES, operating at a sub-optimal operating point (power curtailment) to provide available power after the load rises [7]. This solution is not cost-effective as it requires the installation of much higher renewable energy capacity than the system needs, resulting in huge renewable energy curtailments.The second solution could unlock additional renewable energy penetration on the island and reduce curtailment by leveraging the power of interconnection links to balance the renewable energy deficit (or surplus) [8,9].At present, many countries such as Greece [8] Plans are underway to interconnect its islands with the mainland to establish an efficient energy transition towards decarbonization.However, islanded networks powered only by interconnecting cables are less secure; for example, multiple interconnecting cable failures have been reported in previous years, resulting in total outage duration ranging from 1 day to several weeks [10].A third solution requires integrating ESS by storing renewable energy surplus and releasing it in times of renewable energy shortages [1,2,3].Declining ESS costs over the past few years have made renewable energy storage competitive [11,12]achieving total annual penetration of renewable energy up to 90% in a sustainable manner [2,3,6]. ESS can also be used in conjunction with interconnection links to improve the economic viability of RES and the island’s security of supply [13].
According to the running time scale, ESS is divided into short-term storage, long-term storage and hybrid storage. [14].Short-duration ESS has a short discharge cycle and is used for primary and secondary response reserves, black start and power quality services [11].refer to [11] It is expected that after 2030, lithium-ion batteries will constitute the most cost-effective short-term ESS.Long-duration ESS can provide full power for days or weeks and facilitate energy arbitrage, load shifting, tertiary response reserves, and covering long-term renewable energy shortfalls [15]. Hunter et al. [12] It is stated that for a storage time of 12 hours, pumped hydro storage (PHS) and compressed air energy storage (CAES) systems have the lowest levelized cost of energy. However, PHS requires a favorable geographical location to accommodate the reservoir, while CAES requires the presence of deep underground structures such as salt domes.refer to [12,16] It is estimated that hydrogen will become the most competitive for long-term storage in the near future, especially for discharge times exceeding 24 hours. Corbetaldo et al. [17] Proving that for a fully renewable California electricity system, hydrogen storage is inherently less expensive than a system with battery storage.reached similar conclusions [18]the authors concluded that hydrogen could become the fuel of the future because it is inexhaustible, environmentally friendly, and not affected by external factors.
Multiple studies have shown that hybrid storage (a combination of short-term and long-term) is the most cost-effective solution [19,20,21,22,23,24,25,26,27,28], because it combines the advantages of both technologies.refer to [26,28,29,30] Hybrid hydrogen battery storage proved to be beneficial for island networks and remote communities due to the complementary properties of hydrogen and batteries in terms of efficiency, self-discharge and storage costs. Daoud et al. [20] Different storage mixes for remote communities were studied and it was concluded that batteries and hydrogen were the most cost-effective. Praveen et al. [31] and Morocco etc. [24] It turns out that in off-grid applications, hydrogen can be used to reduce the size of batteries, thereby significantly reducing electricity costs. Karel et al. [25] It was further proposed that electricity costs could be further reduced if excess hydrogen was used in the transport sector and heat lost from fuel cells was recovered for heating purposes. Morocco et al. [23] Four remote, independent systems with different climate conditions were studied, concluding that hybrid hydrogen and battery storage could significantly reduce or even eliminate fossil fuel consumption in a cost-effective manner.exist [22]The authors looked at different battery and hydrogen technologies, such as alkaline electrolysis and proton exchange membrane electrolysis, lead-acid and lithium-ion batteries, and concluded that hybrid alkaline electrolysis and lithium-ion battery storage exhibits the lowest LCOE.
Other research focuses on hybrid hydrogen battery storage in small and medium-sized commercial/industrial facilities (e.g. homes, universities, offices, mines, etc.). For example, Kalantari et al. [27] The economic advantages of hybrid hydrogen battery storage in remote mining systems are highlighted. Nguyen et al. [32] A case study of a fuel cell with heat recovery system is conducted for long-range telecommunications applications in the context of an isolated hybrid hydrogen battery system. Duerson et al. [33] It is inferred that powering university campuses through solar and hybrid hydrogen battery storage could provide important environmental benefits. Peppers et al. [34] An office building in Greece has adopted a self-sustaining trigeneration (power + heat + cooling) system based on renewable hydrogen that significantly reduces environmental pollution. Lokar et al. [35] A pilot household in Slovenia was studied and it was concluded that hybrid hydrogen battery storage could become a completely self-sufficient solution and that from an economic point of view such systems could be used commercially. Jaafari et al. [36] A thermal economic analysis was performed in a residence powered by photovoltaics and hybrid hydrogen battery storage. They reasoned that from an economic perspective, 8 days of autonomy was optimal, with an average electricity cost of 0.286 €/kWh.
This paper approaches hybrid hydrogen battery storage from an investor’s perspective, studying hybrid hydrogen battery storage from a different angle than is typically adopted in the literature. It attempts to clarify under what conditions and to what extent Crete can achieve an energy transition with 80% renewable energy penetration (Greece plans to achieve an energy transition with 80% renewable energy penetration by 2030) [37]) using hybrid hydrogen and battery storage to generate profits for investors and residents on the island. In our analysis, current costs of renewable energy, hydrogen and batteries are applied, while a sensitivity analysis is performed to account for uncertainty in hydrogen prices. Additionally, different funding schemes, such as with or without subsidies, were reviewed to make the research as realistic as possible. Crete is used as a case study (650 MW peak demand) because the island has excellent wind and solar potential, favoring the incorporation of new renewable energy sources, [10] and newly installed interconnectors to the mainland, unlocking additional renewable energy capacity. Finally, to make the study as realistic and detailed as possible, real wind, solar and load data for Crete were used and analyzed hourly for the entire year. The rest of the paper is structured as follows: Section 2 presents information about Crete. Section 3 describes the operation of the power system in Crete. Part 4 optimizes the installation of new RES and hybrid hydrogen battery storage to maximize investor profits. Section 5 presents the results and sensitivity analysis of this study, and Section 6 concludes the paper.