1. Introduction
Although the concept of pumped hydro energy storage (PHES) is not recent, there has been a renewed interest over the last years in systems of this type as a means of enabling better use of the fluctuating energy production from renewable and clean sources, while ensuring grid stability and securing energy supply [
1]. The review study in [
2] reports that PHES is the most suitable technology for small autonomous island grids and massive energy storage, however, the intermittent nature of renewable energy sources, such as wind, solar, wave or tidal, imposes new challenges on increasing their penetration in electricity systems (note: in the context of this paper, penetration of renewable energy sources refers to the percentage of demand covered by renewable energy in a certain region, normally on an annual basis) [
3]. The commercial and technical maturity of PHES technology provides a cost-effective solution for large scale energy storage (>100 MW) [
1], bringing the estimated total installed capacity worldwide to 150 GW at the end of 2016, increased by approximately 6.4 GW in that year [
4].
PHES systems typically operate with low-cost off-peak or excess electricity to pump water from a lower to an upper reservoir (either natural or artificial) in order to use the stored potential energy of the water for electricity generation upon system demand [
5]. These systems are characterized by round-trip efficiency in the range of 70–80%, while figures reaching 87% are also reported in the literature [
6]. Moreover, key features of the PHES systems include fast start-up time and high ramp rate. In Korea, for example, the PHES start-up time is 4 min and the ramp rate is ten to twenty times higher compared to that of combined cycle units, as reported in [
7]. Hence, the introduction of PHES is particularly suitable for non-interconnected insular systems not only to increase the penetration of renewable energy sources, but also to tackle with the high cost of electricity production [
2,
8]. Indicatively, in a European context, the first hybrid systems that integrate pumped-storage with wind power were planned for construction in the islands of Ikaria, Greece, and El Hierro, Spain [
9,
10]. Considering that the operation of such systems in isolated islands may suffer from the lack of the required quantity of water, e.g., due to low annual rainfalls, the use of seawater pumped-storage (SPS) systems, where the sea plays the role of the lower reservoir, is proposed as the only feasible alternative to tackle with this issue [
11,
12].
Compared to traditional PHES systems that use fresh water, the storage medium in the SPS counterparts is seawater [
12]. In detail, SPS systems can pump seawater directly from the sea, thus the construction of a lower reservoir is avoided [
11], which further implies less land use and lower construction costs in this case. However, the use of seawater (instead of fresh water as in PHES systems) results in increased costs related to corrosion protection and the prevention of ground contamination from salt water [
13]. In this regard, the costs of avoiding the construction of a lower reservoir in SPS systems compensate for the additional costs incurred to effectively deal with the corrosion and leakage effects [
11]. Therefore, SPS systems comprise a solution for areas characterized by a lack of abundant natural fresh water [
14].
Along these lines, a real-world implementation is the 30 MW plant in Okinawa, Japan [
15], while there are plans for larger facilities in Ireland [
2,
9,
16] and Hawaii [
16,
17]. A review of the relevant literature reveals that rather limited research efforts have focused on systems of this type. The authors in [
18] examine the case of implementing an SPS system in São Miguel considering scenarios that combine different fuel prices and growth rates of electricity consumption. In this context, a stochastic model is proposed in [
19] for optimally sizing a pumped-storage power plant in São Miguel. On the one hand, the benefits of increasing the integration of renewable energy resources in the power system of São Miguel are presented in [
20], while on the other hand, the integration of electric drive vehicles (EDVs) in this insular power system is discussed in relevant energy modelling studies [
21,
22,
23,
24]. Specifically, the work in [
21] discusses the potential revenues for EV owners in São Miguel from providing vehicle to grid (V2G) power, while the study in [
22] examines the possibility of using the batteries of EDVs as an energy storage system for the island and the impact on the energy generation from renewable sources. The authors in [
23] analyze EDV deployment scenarios for reducing the CO
2 emissions and energy costs in isolated regions using São Miguel as a case study, while the authors in [
24] consider additional scenarios with respect to the deployment of EDVs and the increase of electricity demand in the local energy system of São Miguel. Building upon these contributions, the present work employs The Integrated MARKAL-EFOM System (TIMES) to examine scenarios that combine the introduction of a SPS system in São Miguel with the deployment of EDVs. Hence, this work presents the results obtained from different scenarios with respect to the evolution of the local energy system compared to the aforementioned published works.
The rest of the paper is organized as follows:
Section 2 provides an overview of the characteristics of São Miguel, along with the existing situation and future evolution of its energy system.
Section 3 describes in detail the methodological framework for representing the energy system of São Miguel using the TIMES model generator, while the subsequent
Section 4 presents the scenario results and discusses their significance. The last section summarizes and concludes the paper.
4. Results and Discussion
Considering that the SPS system is installed in 2013, the results obtained from the TIMES model indicate that the use of this technology is at its peak in the first year of its introduction to the local energy system, as shown in
Figure 7. Moreover, there is a slightly higher use of the storage system when there are no EDVs (Scenario 4) compared to the case of deploying EDVs in the local vehicle fleet (Scenario 6). The electricity supply from the SPS system is gradually decreasing in both cases, yet with different slopes (and thus percentages) as denoted by the different trends observed in
Figure 7 until 2017. The use of the storage system is at a minimum level in 2017, after which it starts rising again. The reason is that, in this year, there is a shutdown of one of the fuel power plants, resulting in less excess electricity to be fed in the SPS system, while there is a necessity to use the storage system once again to a greater extent in the following years. Once this power plant is off, the use of energy storage in both scenarios fluctuates to a lesser extent and finally converges to the same value with that of the year 2020.
Then, a comparison between the six scenarios is performed, in order to evaluate the most favorable conditions for the use of SPS. At this point, it is noted that all scenarios in this work assume a constant fuel price, even though previous studies have shown the significance of this factor [
18].
Based on the load profiles in
Figure 3, the results obtained from Scenarios 1, 2, 3, and 4 are compared in
Figure 8 and
Figure 9. As expected and has been shown also in earlier relevant studies [
21,
24], the introduction of EDVs as a G2V energy consumption source in 2013, along with the instantaneous penetration of the new geothermal power plant, results in significant fuel reduction of around 10%, although in the present study this depends on the percentage of the EDV penetration (4% or 32%) yet without presenting great differences in their rates. This significant reduction is accompanied by an increase in the percentage (58%) of renewable energy sources in the electricity supply as also expected. In Scenario 4, which refers to the introduction of the SPS system without the presence of EDVs, the storage provided by this system reduces further (~3%) the fuel consumption, and as also observed in Scenarios 2 and 3, the key finding is that it increases significantly the renewable energy sources penetration in the electricity mix, reaching the percentage of 71%. Moreover, the energy that is pumped and then discharged directly in the system (energy mix) reaches a high value of 72.3% in Scenario 4.
A first indication by the comparison of these scenarios is that the penetration of EDVs can be enhanced by the introduction of the new geothermal plant, which acts as a baseload, while reducing the fuel significantly and also bringing the corresponding environmental benefits. At this point, it is noted that if the EDVs were applied as a V2G source in the energy system, it would be interesting to examine how the energy storage from the EDV batteries would perform in terms of providing the necessary electricity back to the grid, while a higher penetration rate would be expected for the renewables. The storage capacity provided by the SPS acts also very efficiently and has a significant impact on the reduction of fuel consumption (−12.6%) with increased penetration rates of the new renewable energy technologies used, ranging from 58% to 71%.
Regarding the use of renewable energy in the island in 2020,
Figure 10 and
Figure 11 show that Scenarios 4, 5, and 6 once again enable a high penetration percentage (ranging from 38% to 48%). However, in this case it is well below compared to the year 2013 and it is after that year that a continuous decline is observed for the scenarios. In a first instance, it might be observed that SPS and EDVs behave competitively against each other, but this is clearly due to the increase in demand, which is not accompanied by an increase in fuel prices that would enable the use of more renewable energy technologies, as well as that of the storage system. Therefore, the two systems can coexist without creating any problems to the grid. The comparison of the direct deployment of EDVs in Scenario 5 with the gradual penetration of EDVs from 2013 to 2020 (Scenario 6) shows that both scenarios result in the same penetration level of renewable energy sources in 2020. However, it should be noted that the small percentage (4%) of EDVs in the year 2013 for scenario 6 makes feasible a higher percentage of renewable energy sources compared to scenario 5.
This work examines different cases, i.e., with or without energy storage from an SPS system under different levels and strategies of EDVs deployment, assuming a constant growth rate of electricity demand and a fixed fuel price. The effect of these two parameters for an SPS system in São Miguel (without the presence of EDVs) has been analyzed by the authors in [
18]. In the light of this study, it is noted that the increase of fuel prices favors the use of renewable energy sources, i.e., increases the penetration of renewable energy sources, as well as the use of the SPS system. For example, high fuel prices make expensive wind turbines more competitive. On the other hand, it is also observed that the higher growth rate of electricity demand does not necessarily mean higher use of the SPS system under low fuel prices.
5. Conclusions
This work presents a number of scenarios for the island of São Miguel with the aim to provide some useful insight into the potential impact of an SPS system on the local energy system. The analysis of the scenarios indicates that the implementation of the storage system would have multiple benefits for the island. In detail, it would allow the increase of renewable energy use, thus lowering the imports of fuels, as intermittent energy sources could be used more reliably. Furthermore, given the large differences between peak and off-peak electricity consumptions, combined with the large geothermal and wind potential in the island, energy could be stored in times of low demand for later use. At a first glance, three possible storage cycles are available: seasonal, weekly, and daily. The two most important ones are the weekly and daily cycles. The weekly cycles would store energy during the weekends to use it later on during the week, while the daily cycles would store during the nights to use during peak hours.
Moreover, the scenario results showed that with the fuel prices under study an SPS system would benefit the further penetration of renewables. Even in the presence of EDVs under a G2V scheme, the system would still permit to have a high penetration of renewables though not as high as before, but the environmental benefits would be particularly significant, while substituting a large number of internal combustion engine vehicles and without presenting any problems in the grid.
Concluding, this paper analyzes the electricity dynamics and builds upon the expected demand growth rates and fixed fuel prices to establish the scenarios under study. No assumption or further analyses were made on a possible future increase of fuel prices that would certainly benefit further the use of the SPS system. Thus, future work could study the impact of fluctuations in fuel prices or how the halt of consumption growth would influence the use of energy storage, as well as on sensitivity analyses to verify the robustness of the results.