15
Energy storage for offshore wind farms
Abstract
In this chapter the basic grid-scale storage technologies, capable of storing large amounts of electricity produced from offshore wind parks, are presented. These are the pumped storage systems (PSS) and the compressed air energy storage systems. Fundamental technical and economic features are presented, as well as the basic specifications of the already-constructed plants from both technologies in the world.
The chapter is completed with the presentation of two case studies of seawater PSS, of small and large size, cooperating with an onshore and an offshore wind park, respectively. Technical details regarding the construction, the siting and the dimensioning of the two systems are explained. The presentation is integrated with the calculation of the annual energy production and storing of the systems and the economic evaluation. The presented case studies reveal the technical and economic feasibility of storing energy from wind parks with seawater PSS.
Keywords
Adiabatic compressed air energy storage systems; Cost-effective storage; Isolated autonomous non-interconnected insular power systems; Offshore wind parks; Seawater pumped storage systems; Wind power energy penetration maximization15.1. Introduction
15.1.1. The necessity of energy storage
Storing goods is a common practice applied in several human and natural activities. It helps towards the optimum management of the required supplies for the implementation of a specific action or for regular daily activities. For example, the invention and the use of refrigerators enabled the storage and conservation of foods for longer periods, contributing considerably to the time and economic savings of our daily living. Energy storage is inherently present in animal and human bodies, which is critical for survival in harsh conditions. Energy storage is designed in manmade systems as well. The filling of the tanks of vehicles or central heaters enables the covering of long distances and the heating up of buildings for long time periods.
The simplified examples reveal the importance of energy storage in technical and natural worlds. In electrical systems, the hugely important role of storage is predominantly provided in the form of reserves in power plants. The storage of electricity constitutes a significant procedure, especially in cases of high power production from renewable energy source (RES) power plants, such as wind parks. The non-guaranteed power production from wind parks and the wind turbines' technical specifications can introduce significant problems (contingencies), especially in small autonomous systems or those of high wind power penetration. The wind turbines' low tolerances in system events often leads to their tripping (power production cut) in case of a slight variation of the system's frequency or voltage amplitude. This vulnerability can be the beginning of further generators' sequential tripping, leading ultimately to a total blackout, especially in cases of weak and non-interconnected systems. To avoid this negative effect of wind parks in systems' dynamic security, the wind power penetration is restricted up to a maximum percentage of the power demand. This percentage depends on the size of the system, the available spinning reserve of the thermal generators, the weather conditions and it is usually configured around 30% of the power demand [1–6]. The excess wind power is rejected.
Moreover, the stochastic power production from wind parks cannot follow the power demand variation by itself adequately. This stochastic nature prevents the maximization of wind parks' power production in small autonomous power systems, such as the insular ones. In these cases, although the low power demand can be totally covered from wind parks, this possibility is not feasible due to the non-guaranteed wind power availability.
The above-mentioned inadequacies can be handled with the introduction of storage power plants and their cooperation with the wind parks and other RES power plants. It has been proved through a number of electrical system simulations, that the dynamic security of the system is ensured with the support of a storage device, such as conventional electrochemical batteries, hydro turbines of a pumped storage system or the air turbines of a compressed air energy storage system [3,6–9]. Additionally, with the support of storage power plants, the stochastic power production from the wind parks can be adapted to the power demand, through the sequential charging and discharging procedure, whenever there are wind power surpluses or shortage exhibits, respectively. Actually, the combined operation of an RES power plant and a storage device, commonly described as a ‘hybrid power plant’, converts the stochastic power availability from the RES into guaranteed power production, enabling the power plant to follow adequately the varying power demands and approach high annual energy production penetration [10–14].
Even in large interconnected systems, the installation of high wind parks and the corresponding penetration of high non-guaranteed power can cause serious dynamic security problems. With the wind parks being the leading RES technology regarding the development of RES power projects in interconnected systems, the total installed RES power increases rapidly [15]. The importance of the introduction of electricity storage power plants in interconnected systems in turn rises. With the continuously reducing reserves of conventional fossil fuels and the simultaneous increase in the global energy consumption, the shift to the alternative RES seems to be an inevitable necessity. The combined introduction of wind parks and storage power plants in interconnected systems features as the unique solution to the target of the secure maximization of RES penetration in electrical systems and the eventual total substitution of fossil fuel consumption for power production.
15.1.2. A quick glance at former studies
A large number of former studies investigate the issue of the combined operation of RES power plants with storage power plants. The most famous among the investigated technologies seems to be the wind-powered pumped storage systems (WP-PSS). These systems aim to exploit local, renewable and environmentally friendly wind energy by improving the stability of the system and reducing the use of thermal power plants, minimising the consumption of fossil fuels, reducing the cost of electricity and boosting local economies.
The most popular topic examined in existing papers is the introduction of a PSS in remote islands, to recover otherwise-rejected wind energy due to restrictions imposed for the systems' stability and dynamic security [16–20]. The PSS, using a single penstock, produces power only during the power demand peak hours and helps to reduce amount of wind power rejected.
A second approach extensively examined in the past combines the operation of wind parks and a PSS to produce guaranteed power during the power demand peak hours [10–14,21]. The guaranteed power is produced exclusively by the hydro turbines. To maximise the wind energy stored by the PSS, a double penstock is used. The economic feasibility of these WP-PSS is strongly dependent on the feed-in tariff.
A more revolutionary evolution of the above approaches is the introduction of WP-PSS in insular systems with high wind potential, aiming to maximise wind energy penetration [9,14,22,23]. Power production from these WP-PSS is not restricted to power demand peak hours but it is extended for the whole day. The high wind potential leads to annual wind energy penetration that can exceed 80%. Due to the large quantities of the produced electricity, the corresponding investments are very attractive, with lower sensitivity to the feed-in tariff.
The introduction of WP-PSS has also been studied for interconnected power systems in Greece [24], Ireland [25] and Turkey [26]. A common conclusion of these studies is that these systems are necessary to achieve high wind energy penetration. Their operation can also be combined with base thermal generators leading to significant power demand peak shaving. The economic feasibility of such systems is more likely to be guaranteed, given the large quantities of the electricity demand in the interconnected systems.
Finally, there were a number of studies that examined the contribution of a PSS to the improvement of the dynamic security of electrical systems [3,6–9]. Indicatively, the results from the simulation of the dynamic behaviour of the autonomous electricity system of Crete are presented in Figs 15.1 and 15.2. In these figures, the frequency variations after the loss of 80 MW of wind power are presented. The power demand at the time of the contingency is assumed to be 250 MW. In Fig. 15.1, the power production is supported with adequate spinning reserve from thermal generators, while in Fig. 15.2 the system is supported by the synchronized hydro turbines of a PSS. It can be seen that with the support of the hydro turbines the system recovers faster and exhibits lower permanent frequency deviation after the contingency. Consequently it clearly exhibits improved behaviour compared to the one with support from the thermal generators' spinning reserve.
Figure 15.1 Frequency variation after the loss of 80 MW of wind power in the existing power production system in Crete, with adequate spinning reserve from thermal generators.
Figure 15.2 Frequency variation after the loss of 80 MW of wind power in the power production system in Crete, with the support of the synchronized hydro turbines of a pumped storage system.
In addition to WP-PSS, other proposed technologies include wind–photovoltaic-battery systems and wind–compressed air systems (WCAES). The wind–photovoltaic systems are usually proposed for small- or medium-sized decentralized production (eg small settlements [27–29]).
The conventional lead batteries are generally not proposed for large-sized systems (power demand higher than 1 MW), taking into account their short life, the low storage capacities that can be achieved and the important environmental impacts from their disposal. Alternative battery technologies, such as the floating batteries, although they exhibit strongly improved technical features compared to the lead batteries, in terms of capacity and life time period, they still exhibit quite high procurement costs.
The introduction of WCAES has also been widely studied. Several alternatives have been examined, such as the introduction of conventional [30,31] or adiabatic [32] CAES, the cooperation with flywheel [31] or pumped storage systems [33], etc. The thermodynamic simulation of such systems, the accurate calculation of their efficiency is another favourite issue of research [32]. Another research objective often met is the optimization of the WCAES in order to achieve the highest possible wind energy penetration [34,35] with the lowest possible cost [36]. A major drawback of the WCAES is the consumption of fossil fuels, which affects considerably both the environmental aspect of these systems and the economic efficiency of the required investments. For the second reason, the introduction of WCAES seems to be feasible in places with low-price fossil fuels (eg Canada [37], Australia [38] or the USA). In such cases the WCAES exhibit quite attractive economic features.
15.2. The storage technologies
Offshore wind parks are always power plants of some tens or hundreds of MWs of installed power. The installation of high nominal power is the only way to compensate for the increased set-up cost of the offshore wind parks, compared to onshore installations. The storage power plants required for such electricity quantities must exhibit a charging/discharging ability approximately equal to the wind park's nominal power and a total energy capacity which can be between 1% and 3% of the total annual electricity production of the wind park, depending on the size of the wind park and the system that it is connected to, as well as the operational algorithm of the wind park–storage plant station. This means that for an offshore wind park with a nominal power of 50 MW and a capacity factor of 30%, a storage capacity of about 1300 MWh is required. This, in turn, implies an effective capacity for the upper reservoir of a pumped storage system of about 1,700,000 m3 with a net head of 300 m. Although theoretically there may be several different storage technologies, the alternatives for such large storage power plants are rather restricted. Practically there are two available storage technologies suitable to manage the large electricity quantities produced from offshore wind parks:
• Compressed air energy storage systems (CAESs)
• Pumped storage systems (PSSs).
These technologies are presented in the next subsections.
15.2.1. Compressed air energy storage systems
CAESs are a method of energy storage through the compression of air. CAES are distinguished into two alternatives, conventional and adiabatic. For the time being, there are two conventional CAES systems operating, one in Neuen Huntorf, Germany and one in McIntosh, USA [39]. As far as adiabatic compressed air energy storage systems (AA-CAESs) are concerned, industrial applications were expected in approximately 2015 [40].
15.2.1.1. Conventional CAES
The operation of a conventional compressed air energy storage system is presented in Fig. 15.3. Specifically, in this figure the operating algorithm of the existing CAES storage plant in Neuen Huntorf, Germany [41] is presented. Any potential electricity surplus is provided for a two-stage compressor with intercooling, that compresses ambient air up to 40–70 bar. The compressed air is then led to an after-cooler to keep its temperature close to ambient. Finally, the compressed and cool air is stored in an underground storage reservoir. When power is needed, the compressed air is heated-up by a combustion chamber in order to obtain increased power during the expansion process (expansion with reheating).
In new CAES systems, the stored compressed air is preheated by a recuperator before it enters the combustion chamber. With the use of the recuperator, the total efficiency of the storage–production cycle can be increased by 10%. On the other hand, a significant disadvantage of this alternative integration is the large size of the recuperator, which implies a considerable increase in investment [42]. The operating principle of this alternative, presented in Fig. 15.4, has been applied in the second existing CAES storage plant in McIntosh, Alabama, USA.
Figure 15.3 The structure of the existing conventional CAES storage plant in Neuen Huntorf, Germany.
Table 15.1
Fundamental technical specifications of the existing conventional CAES power plants
| Technical/economic data | Neuen Huntorf, Germany | McIntosh, Alabama, USA |
| Power (MW) | 321 | 110 |
| Storage capacity (MWh) | 1160 | 2640 |
| Cavern volume (m3) | 310,000 (2 caverns) | 560,000 |
| Storage maximum pressure (bar) | 70 | 75 |
| Turbines' mass flow (kg/s) | 416 | 154 |
| Compressors' mass flow (kg/s) | 104 | 96 |
| Set-up cost ($) | 167,000.000 | 65,000,000 |
| Set-up specific cost ($/kWh) | 143.966 | 24.621 |
| Set-up specific cost ($/kW) | 520.25 | 590.91 |
In Table 15.1 the fundamental technical specifications of the existing two CAES systems are presented.
For the compression of the incoming air, either axial compressors, achieving a pressure ratio of about 20 and a flow rate of 1.4 Mm3/h, or radial compressors, with flow rates up to 100,000 m3/h and a maximum compression pressure up to 1000 bar, can be used. With the currently available technology, air compression is executed in two stages with intercooling at temperatures from 40 to 200°C [43]. The arisen high-pressure air–fuel mixture is expanded in air turbines with pressure ratios up to 22 and with a maximum inlet temperature of 1230°C.
The storing of the compressed air at near-ambient temperature conditions allows higher density of the stored medium, reducing the required size of the storage reservoirs. For the storage of the compressed air, the aquifer, underground caverns made of high-quality rocks, depleted natural gas storage caves and salt domes are most commonly used, with storage capacities from 300,000 to 600,000 m3. Another feasible alternative is storage in underground, high-pressure pipes (20–100 bar).
15.2.1.2. Adiabatic CAES
In an AA-CAES the heat released during air compression is stored in a separate heat storage reservoir. This is the main difference from a conventional CAES. With an AA-CAES the consumption of fossil fuels for the compresses is eliminated and this is one of the main reasons for the development of AA-CAES. The structure of an AA-CAES system is shown in Fig. 15.5.
When a power surplus exists, air is compressed without intercooling and releases its heat in a separate heat storage reservoir before being stored. At discharge periods, compressed air is heated up to the appropriate turbine inlet temperature (600°C) by regaining the heat from the heat storage reservoir. Overall efficiency rates of adiabatic compressed air storage plants are expected to reach values of up to 70% [40,41,43], approaching the corresponding efficiency of a PSS.
In two-stage AA-CAES systems, heat released in the low-pressure (LP) and high-pressure (HP) compressors is stored in separate heat tanks. At discharge periods, heat from the HP and LP heat tanks is regained before the inlet to the HP and LP turbines, respectively. Two-stage AA-CAES systems achieve higher energy storage density, which compensates for the increased complexity of the plant (two heat storage tanks and piping).
Important advantages of the AA-CAES technology are the elimination of the fuel added before expansion in the turbine and of the concomitant CO2 emissions, as well as the compression of air without intercooling that allows for higher outlet temperatures from the compressor and, thus, higher amounts of heat stored in the heat tank. However, major components of the plant need to be redesigned as conventional components cannot be utilized. Specifically, heat storage tanks with capacities of 120–1800 MWhth need special design to achieve sufficiently high heat transfer rates and constant outlet temperature. Minimization of heat losses during charging and discharging of the heat reservoir is another point of consideration [44–46]. Regarding the compressor of the plant, in AA-CAES systems adiabatic compression is preferred to isothermal, which are adopted in conventional CAES plants. However, conventional compressors cannot reach the high pressures and temperatures required for adiabatic compression (100 bar/620°C for single-stage and 160 bar/450°C for two-stage AA-CAES plants) and, along with the need for low response times and high isentropic efficiency, the design of novel compressors for AA-CAES systems becomes a necessity. Recent studies converge that to illustrate that meeting these requirements is best achieved by constructing a compressor consisting of three parts: (1) an axial or a radial compressor, as a low-pressure compressor in case of high or low air flow rates, respectively, and single-shaft radial compressors for the (2) intermediate- and (3) high-pressure sectors. The turbine sector needs to be redesigned to achieve increased turbine inlet temperatures, air flow rates and efficiency. In order to satisfy these requirements, a novel non-conventional regulation stage with lower losses should be designed for improved handling of pressure and flow rate fluctuations. Preheating of the turbine is also proved to be desirable in order to achieve temperature profiles, which will enable low response times [44–46].
15.2.2. Pumped storage systems
15.2.2.1. Basic concepts
PSSs are the technically most mature and economically most competitive electricity storage technology for large power plants. With tens of PSS projects already constructed and operating worldwide under considerably different conditions, covering a power production range from 5 MW to 2 GW, huge experience has been gained regarding their technical specifications and operating procedures.
The fundamental operating principle of a PSS is presented in Fig. 15.6. Two water reservoirs are constructed in two neighbouring geographical positions, with adequate altitude difference between them, usually some hundreds of metres. The water reservoir capacities can vary from some hundreds of thousands of cubic metres to some millions of cubic metres. Water can be transferred between the two reservoirs with either a single or a double penstock. The lower edges of the penstocks are connected to a pump station and a hydro power plant. When there is a power generation surplus that must be stored, water is pumped from the lower reservoir and stored in the upper reservoir. In this way the available energy surplus is stored in the form of the gravitational energy. When power is needed, water is released from the upper reservoir and passes through the hydro power plant, thus providing the requested power.
For the time being, tens of PSSs have been installed worldwide, combined with large thermal power plants, aiming at the so-called ‘power peak shaving’. For power peak shaving, power is stored during low power demand periods (usually night-time periods) in order to be available during power demand peak periods. In this way, cheap electricity produced during low power demand periods is stored, instead of being rejected, while the use of expensive generators (most commonly gas turbines) during power demand peak periods is avoided. Power peak shaving with the use of PSSs is usually applied in systems with large thermal or nuclear power plants, where the reduction of the total produced power during low demand periods from the large operating steam turbines is often not possible. For power peak shaving applications, the employed PSS is equipped with a single penstock, since simultaneous water pumping and falling, namely simultaneous power storage and production, is not sensible.
15.2.2.2. Wind-powered pumped storage systems
The combined operation of a PSS exclusively with a wind park, although widely studied in earlier articles, has been applied in practice in only two cases. The first was on the island of El Hierro, in Canaria Archipelago, while the second one was on the Greek island of Ikaria, in the Aegean Sea. The WP-PSS is introduced in both cases aiming at the penetration maximization of the primary energy source, namely the wind energy, into the annual electricity production. The fundamental technical specifications of the introduced WP-PSS in both islands are presented in Table 15.2.
The main scope of WP-PSS stations, usually named ‘hybrid power plants’, which is to maximize the wind energy annual penetration, introduces several peculiarities in the system's design and operation. A major design innovation is the installation of a double penstock, enabling simultaneous water pumping and falling [23]. The necessity for the use of a double penstock arises from the fact that there is a maximum direct penetration percentage for the wind energy, for dynamic security reasons. The installation of a double penstock enables the storage of the wind power that cannot penetrate directly in the network, while the rest of the power demand can be covered by the hydro turbines at the same time, employing the water falling penstock. The double penstock also improves the system's flexibility and its ability to react in cases of system contingencies. For example, in case of a sudden loss of wind power production, the exclusive falling penstock enables direct power production from hydro turbines, even if water was pumped when the contingency occurred.
Table 15.2
Fundamental technical specifications of the wind-powered pumped storage systems in El Hierro and Ikaria
| Technical/economic data | El Hierro, Spain | Ikaria, Greece |
| Power demand annual peak (MW) | 13.3 | 7.8 |
| Annual electricity consumption (MWh) | 41,000 | 27,600 |
| Wind park (units/power) | 5 × 2.3 MW = 11.5 MW | 4 × 600 kW = 2.4 MW |
| Upper reservoir effective capacity (m3) | 380,000 | 900,000 (1st tank) 80,000 (2nd tank) |
| Lower reservoir effective capacity (m3) | 150,000 | 80,000 |
| Gross head (m) | 655 | 724 (1st tank) 555 (2nd tank) |
| Storage capacity (MWh) | 580 | 1500 |
| Pump station (units/power) | 2 × 1500 kW + 6 × 500 kW = 6 MW | 8 × 250 kW = 2 MW |
| Hydro power plant (units/power) | 4 Pelton × 2830 kW = 11,32 MW | 2 × 1550 kW + 1050 kW = 4.15 MW |
| Total set-up cost (€) | 64,700,000 | 26,000,000 |
| PSS set-up cost (€) | 50,000,000 | 23,000,000 |
| PSS set-up specific cost (€/kWh) | 86.21 | 15.33 |
| PSS storage–production cycle efficiency (%) | 65 | 69 |
| Annual wind energy penetration percentage (%) | 80.0 | 50.0 |
The operating algorithm of a WP-PSS hybrid power plant is also completely different from that of a conventional PSS employed for power peak shaving. This philosophy is presented in Fig. 15.7 [23].
The power demand Pd is provided with power Pw by the wind park, at a certain time point. The wind park direct penetration is always restricted to a maximum value Pwp = a·Pd (0 < a < 1), in order to ensure the system's dynamic security. This is achieved with the introduction of pump loads for the excess wind power.
Two cases are distinguished:
1. If the PSS upper reservoir is empty, the remaining power demand is covered by the existing diesel engine generators, that produce power equal to Pt = Pd − Pwp. The hydro turbines do not produce power, Ph = 0. The PSS pumps are provided with the wind power surplus Pp = Pw − Pwp, in order for water to be stored in the PSS upper reservoir.
2. When the PSS upper reservoir isn't empty, power demand is covered by the hydro turbines (Ph = Pd − Pwp). At the same time, any possible wind power surplus Pw – Pwp is stored through the water pumping penstock on the condition that the upper reservoir isn't full. If this is not the case, no more wind power can be stored; this energy can be used for other applications such as hydrogen production or desalination, etc. Power from the diesel generators is null: Pt = 0.
As it is revealed from the above analysis, thermal generators are only used as reserve units. The main power production unit is the wind park. The PSS is the storage unit.
In case the scope of the WP-PSS is the power demand peak shaving, the guaranteed power production is restricted to the demand peak hours. The rest of the above operational algorithm remains the same.
The electricity storage from offshore wind parks has not yet been studied widely. However, the basic principles do not differ from the ones met in onshore wind parks. A major difference between the two cases is the higher procurement and installation cost of the underwater connection cable in offshore installations. But this only affects the total efficiency of the overall investment, without any effect on the storage power plant technical considerations.
15.2.2.3. Seawater-pumped storage systems
An alternative of distinguished importance is the use of seawater directly in the PSS and the utilization of sea as the lower reservoir. At the time of writing there was only one commercial S-PSS (Seawater PSS) constructed worldwide – in Okinawa, Japan – used for power peak shaving. Already operating for more than 10 years, the Okinawa S-PSS is an important source of experience for similar stations [47–49]. Seawater PSSs provide a valuable solution in cases of geographical territories with low annual rainfalls, since it guarantees the adequacy of the working means in the PSS without affecting the freshwater reserves. These cases are often met in small islands or in geographical regions close to the equator.
Since sea water is utilized as the PSS lower reservoir, such storage power plants must be installed on the coastline [50]. The land morphology close to the coastline highly affects the technical feasibility of the S-PSS, as well as the total set-up cost of the project. Small mountains or hills with absolute altitudes higher than 200 m and lower than 600 m are considered ideal for the installation of the PSS upper reservoir. A mild land morphology on the coastline (lack of cliffs) helps towards the elimination of the required earth works and minimization of the set-up cost for the hydro power plant and the pump station installation. Finally, the intensive slopes of the mountains or hills can perhaps impose the underground installation of the penstocks, with the construction of underground tunnels. Such works raise the total set-up cost significantly and can negatively affect the economic feasibility of the investment, especially in cases of small S-PSS power plants. In such cases, the existence of mountains or hills that are not steep also constitutes a fundamental prerequisite to ensure the economic feasibility of the hybrid power plant.
The proximity of the S-PSS installation sites to the sea makes them an excellent prospect for storing electricity produced from offshore wind parks. The two power plants (offshore wind park and S-PSS) can be connected to a common substation, built near the coastline, constraining, on the one hand, the network connection cost and enabling, on the other hand, flexible operation of the overall hybrid station, without any interference to the rest of the utility network or other existing power plants.
In the following section, two characteristic case studies of wind-powered S-PSSs will be presented, one large and one small.
15.3. Indicative case studies: S-PSSs in Rhodes and Astypalaia
In this section the studies of two WP-PSSs on the islands of Rhodes and Astypalaia are presented. Each presented system aims at the maximization of wind power penetration in isolated insular power systems.
Each one of the presented power production systems consists of a wind park (an offshore and an onshore one) and a PSS. The PSS reservoirs are connected to each other with a double penstock. The construction of two penstocks, one exclusively for water fall and one exclusively for pumping water, enables flexible operation of the PSS and maximizes the station's contribution to the maximization of the wind energy penetration, as well as to the system's stability and dynamic security.
The presented PSSs utilize the sea as the lower reservoir and work on seawater so that the water supply is guaranteed even for areas with relatively low annual rainfalls. The proximity of the PSSs to the sea seems also to be an ideal choice for storing electricity from offshore wind parks.
15.3.1. Siting of the S-PSSs
The installation sites for the examined S-PSSs in Rhodes and Astypalaia were selected after thorough examinations in several areas that should satisfy certain morphological prerequisites, such as:
• Proximity to the coastline;
• An appropriate area for the upper reservoir, namely a flat area or physical cavities with adequate land (depending on the reservoirs' capacity) and at least 150-m altitude above sea level;
• A gradual slope from the upper reservoir position to the coastline where the penstock will run (no cliffs, gaps, gorges or canyons must be met along the penstock routes);
• The ratio of the penstock length (L) over the absolute altitude difference between the penstock's ends (H) must not exceed the value of 5;
• Adequate land (about 30,000 m2) next to the coast for the installation of the hydrodynamic stations;
• The absence of other activities in the area (eg tourism) that could raise negative reactions from the local community against the construction and the operation of the S-PSS;
• Site accessibility by land and sea.
Satisfying certain criteria in the above areas will help to justify the technical and economic feasibility of the S-PSS as the technical works required are minimized and, consequently, the investment for set-up is lower.
Apart from the above, the selection of the appropriate sites also depends on the size of the S-PSS which, in turn, depends on the size of the non-interconnected electricity system. In Table 15.3, the characteristic power demands on the islands of Rhodes and Astypalaia are presented.
From Table 15.3 it can be seen that Rhodes is a large isolated power system, while the system on Astypalaia is a small one. The proposed S-PSSs will cooperate with wind parks, one offshore and one onshore, aiming at maximizing wind power penetration. Therefore, the size of the S-PSS is defined by the power demand scale. The size of the S-PSS is also defined by the necessity to improve the economic feasibility of the project (the investment's economic indices improve with the size of the S-PSS).
The required energy-storing capacity arises from the daily power production required and it determines the capacity of the upper reservoir (taking into account the absolute altitude of the reservoir). An initial estimate, based on experience, is an upper reservoir of 4,500,000 m3 at 160 m altitude and of 300,000 m3 at 300 m altitude for the S-PSS in Rhodes and Astypalaia, respectively.
Table 15.3
Main electricity demand features in the islands of Rhodes and Astypalaia
| Rhodes (2011) | Astypalaia (2013) | |
| Maximum annual power demand (MW) | 176.40 | 2.25 |
| Minimum annual power demand (MW) | 38.70 | 0.31 |
| Total annual energy consumption (MWh) | 789,168.37 | 6670.11 |
| Mean daily energy consumption (MWh) | 2162.11 | 18.27 |
Following the above parameters, the overall positioning of the S-PSS, including the upper reservoir, the penstock route and the hydrodynamic stations (pump station and hydro power-plant), is presented in Figs 15.8 and 15.9.
On the right-hand side of Fig. 15.8, particularly, the position of the PSS upper reservoir in the southwest part of the island of Rhodes is presented. On the same map, the siting of the wind turbines of an offshore wind parks is also presented. However, thorough description of this task is beyond the scope of this chapter. On the left-hand side of Fig. 15.8, detailed siting and design of the upper reservoir is depicted. Analytical description of this particular drawing is provided in Section 15.3.2.
The S-PSS characteristic features, defined by the land morphology at the installation sites, are presented in Table 15.4. Observing Figs 15.8 and 15.9 and the data presented in Table 15.4, leads to the fundamental prerequisites set for the selection of the installation sites being fulfilled on both occasions.
Once the sites for the S-PSS components have been selected, the positioning of the components and the calculation of the required works are performed. They are presented in the following sections.
15.3.2. The design of the reservoirs
Some important issues regarding the two reservoirs of the S-PSS stations in Rhodes and Astypalaia are presented below:
1. The reservoir site in Astypalaia is a hilltop, while the reservoir site in Rhodes is a physical cavity. In both areas there is enough land available to satisfy the reservoirs' required capacities.
2. The reservoir basin in Astypalaia will be created with excavation works that lend the form of an inverted truncated cone. The limestone formations are tectonic with joint and fissures systems, karstified at places, making the excavations in the upper area of the artificial reservoir relatively easy, through the use of pile drivers and/or explosives.
3. In Rhodes, the area selected for the PSS upper tank is a valley which can be made into a reservoir by constructing two dams. No additional digging will be required.
4. A disadvantage of the site selected in Rhodes is its low absolute altitude (160 m). The PSS's head height is maximized by using the sea as the PSS's lower reservoir.
5. The reservoirs in Rhodes and Astypalaia follow the contours of 160 and 340 m, respectively, above sea level, which is the outer contour found in the flat terrain of the site; the maximum available flat area is occupied. The incline dip is set at 3:1, following the relevant bibliography for artificial dam and tank construction [51,52].
Table 15.4
Characteristic features of the S-PSS, defined by the land morphology
| Island | Rhodes | Astypalaia |
| Upper reservoirs' sites absolute altitude H (m) | 160 | 350 |
| Available land for the upper reservoir (m2) | 450,000 | 40,000 |
| Distance of upper reservoir from the coast L (m) | 750 | 1110 |
| Ratio L/H | 4.69 | 3.17 |
| Average inclination of penstock route (°) | 22.68 | 20.48 |
Topographic maps of the reservoir installation sites, provided by the Hellenic Military Geographical Service, are digitized and the reservoirs are designed thoroughly on a digitized terrain of contours with 4-m height difference. The volume of the required excavation work, the reservoir capacities and depth, etc., are calculated and the results are presented in Table 15.5.
It can be seen from Table 15.5 that the physical cavity found in Rhodes reduces the volume of excavation works required. This is not the case for the reservoir in Astypalaia, where the reservoir will be formed by excavating, raising the S-PSS set-up cost. In both occasions the required reservoir capacities are achieved.
In fact, the capacity of the resulting reservoir in Astypalaia is much higher than that initially required. This was later found to be a major benefit since it allows for the contribution of the WP-PSS in controlling power production and the improvement of the system's security [1,2,6,7,9]. Furthermore, the excess energy storage capacity enables the combined operation of the WP-PSS with a desalination plant for potable water production. This perspective is of crucial importance for areas with low annual rainfalls, such as the islands under examination. In both cases, higher reservoir capacities will be justified by the impending interconnection of the islands with the mainland power system. In that case, the hybrid performance of the projects will not be used; the wind farms will be directly connected to the grid, while the hydro part will perform as a simple pumped storage system.
Table 15.5
Characteristic features of the PSSs' upper tanks
| Island | Rhodes | Astypalaia |
| Reservoirs' overall characteristics | ||
| Gross capacity (m3) | 5,107,924 | 269,238 |
| Effective capacity (m3) | 4,554,257 | 255,648 |
| Energy storage capacity (MWh) | 1787 | 219 |
| Minimum water volume in reservoir (m3) | 553,667 | 13,590 |
| Upper surface area (m2) | 414,997 | 30,503 |
| Bottom area (m2) | 424,540 | 31,676 |
| Upper surface altitude (m) | 160 | 348 |
| Bottom altitude (m) | 145 | 333 |
| Maximum depth (m) | 15 | 15 |
| Inner incline slope | 1:3 | 1:3 |
| Total digging volume (m3) | 0 | 484,980 |
| Dams | ||
| NW dam's volume (m3) | 81,975 | – |
| SE dam's volume (m3) | 304,064 | – |
| NW dam's total length (m) | 220 | – |
| SE dam's length (m) | 387 | – |
| NW dam's maximum height (m) | 20 | – |
| SE dam's maximum height (m) | 40 | – |
To prevent leakage of seawater from the upper reservoir, the technology applied in the Okinawa S-PSS was adopted [47–49]. This technology is presented in Fig. 15.10.
An ethylene propylene diene monomer (EPDM) rubber sheet has been adopted for the lining of the upper pond. The EPDM has been proven, via a number of tests, to exhibit excellent material properties and weather-resistance characteristics.
For the lining structure, a drainage layer will be constructed using gravel materials (20 mm or less). The layer will be 50 cm thick across the entire surface of the pond. On this layer, a cushioning material to prevent damage from the angular parts of gravels will be laid using a non-woven spun bonded fabric of polyester. Then, an EPDM sheet of 2.0-mm thick will be installed as a surface material. The span of sheet anchors was set at a standard of 8.5 m (on slopes).
If damage to the sheet occurs, seawater leakage will be detected by seawater sensors and pressure gauges, both of which are installed in the pipes connected to the drainage layer in each zone. The detector will emit an alarm to indicate seawater leakage, and at the same time the pump will recharge leaked seawater to the upper pond. This system prevents seawater from leaking into the neighbouring environment. Furthermore, since the rubber sheet is the top layer of the lining structure, it can be repaired easily.
The water intake from the reservoir will flow through an intake tower at the bottom of the reservoir, as shown in Fig. 15.11. The dimensions shown in Fig. 15.11 refer to the upper reservoir in Astypalaia. The tower's entrance will be covered with a filter grid, to prevent debris from entering the penstock. The tower's height is selected to ensure that a minimum water volume will always be stored in the upper reservoir. This is to prolong the lifetime of the reservoir's bottom sealing, by avoiding the direct exposure of the sealing materials to the solar radiation.
Finally, the soil and dredging spoil from the excavation works will be used to raise an embankment around the reservoirs, to prevent the spread of seawater from the wind to the surrounding environment. The height of the embankment must be at least 2 m.
15.3.3. Construction of the penstock
The routes of the penstocks were chosen with the following criteria:
• the minimization of the penstock lengths;
• the avoidance of intensive slopes and cliffs;
• the land's morphology where the penstock reaches the sea must be gradual.
For both S-PSSs, an underground tunnel from the water intake position in the reservoir to the other side must be dug. This necessity arises from the morphological conditions created by the construction of the upper reservoir in flat or concave areas and the formation of the reservoir with excavation works.
The above can be seen in Fig. 15.12, where the sectional vertical view of the penstock's route from the reservoir to the sea is presented for the S-PSS in Astypalaia. In Fig. 15.12, the penstock follows the underground tunnel for, approximately, the first 164 m from the reservoir. Excluding that first part, the rest of the penstock is positioned on the surface of the mountain-side. The same configuration applied for the Rhodes S-PSS, where the length of the tunnel is 237 m.
The major issue with the penstock construction is the selection of a corrosion-resistant material, suitable for the transportation of seawater. An excellent material is glass reinforced polyester (GRP). The chemical structure of this material is not affected by seawater. Moreover, it exhibits a very low flow losses coefficient (approximately 0.030). It is lighter and cheaper than steel, hence it can be transported and installed more easily than steel pipes, significantly reducing the project set-up cost. On the other hand, the ability of GRP to withstand hydrostatic pressure restricts the use of GRP tubes (nominal pressure of GRP tubes decreases with increasing diameter).
Figure 15.12 Sectional vertical view of the penstock's route from the reservoir to the sea for the S-PPS in Astypalaia.
In the case of Astypalaia, the required nominal diameter is 1.50 m. For that diameter, GRP tubes are constructed with nominal pressures lower than 32 bar. As the head height is 348 m, it implies that the penstock can be constructed with GRP tubes for altitudes between 40 and 348 m. For altitudes below 40 m the penstock will be constructed from St52, exhibiting a yield point of 330 MPa. In order to protect the steel tubes from seawater corrosion, a thick film of mixed phenol and epoxy resins, without dissolver, will be applied in the inner surface of the tubes.
In Rhodes, the water flow required for the PSS's operation was calculated as 111.23 m3/s for water fall and 66.69 m3/s for water pumping. Given that the maximum diameter of commercially available steel pipes in Greece is 2540 mm (100 in.), two sets of 20 parallel pipelines are used for the water fall and pumping penstocks. The use of GRP tubes in this case is not possible, because of the large required diameters.
The maximum pressure in the penstocks is 27.59 bars (16.1 bars hydrostatic pressure plus 11.49 bars) due to the hydraulic hammer effect for instant flow cut. The minimum wall thickness of ×70 steel tubes/100 in. diameter is given as 12.70 mm with a corresponding nominal pressure of 44 bars, adequate for the specific installation.
The construction of the penstock is designed as presented in Tables 15.6 and 15.7 for Rhodes and Astypalaia, respectively. The selection of the different pipe thicknesses (except for the necessity to withstand hydrostatic pressure and hydraulic hammer) also aims at the minimization of cost.
Special digging works and techniques (eg cut and cover) are required in cases of abrupt changes in the landscape along the route of the penstock as well as close to the coast line, aiming at creating secure passage and grounding of the penstock and its protection against the corrosive conditions caused by the combination of seawater and high winds.
Presented in Fig. 15.13 is a 3D view of the mountain slope with the penstock route for Astypalaia S-PSS. The figure shows the length of the different penstock sections, namely different nominal pressures. The underwater suction pipeline and the position of the hydrodynamic stations are also shown.
Table 15.6
The analysis of the construction of the PSS penstocks in Astypalaia
| Absolute altitude (m) | Material | Maximum hydrostatic pressure (bar) | Nominal pressure (bar) | Route's length (m) | Total tubes' length (m) |
| 348–300 | GRP | 4.8 | 6 | 323 | 646 |
| 300–240 | GRP | 10.8 | 12 | 382 | 764 |
| 240–200 | GRP | 14.8 | 16 | 188 | 376 |
| 200–160 | GRP | 18.8 | 20 | 157 | 315 |
| 160–120 | GRP | 22.8 | 25 | 138 | 276 |
| 120–40 | GRP | 30.8 | 32 | 201 | 402 |
| 40–0 | Steel X70 | 34.8 | 44 | 95 | 190 |
| Total route's and tube's length (m) | 1485 | 2969 | |||
A similar 3D view of the penstock route in Rhodes is presented in Fig. 15.14.
The ratio of the head height, H, over the penstock length, L, is calculated at 4.24 for Astypalaia and 5.48. These values (lower than or close to 5) are quite satisfactory, positively affecting several parameters regarding the construction of the penstock and the operation of the PSS, such as the pipeline diameter and thickness required, the water flow linear losses, the PSS total efficiency and finally the penstock set-up cost and economic indices of the investment.
15.3.4. The hydrodynamic machine stations and the suction pipeline
The hydrodynamic stations, namely the pump station and the hydro power plant are going to be built onshore, next to the sea. The fundamental prerequisites that the sites must fulfil are:
• buildings must be protected against the sea as waves may reach several metres height during the winter period, taking into account the strong winds blowing in the Aegean Sea;
• the absolute altitude of the hydro power plant must be as low as possible, to maximise the head height;
• the pump suction level must be below sea level to allow natural water flow from the sea to the pump suction side.
To meet the above requirements, the hydro turbines and the pumps will be installed in two different buildings. Three-dimensional views of the final positioning in the islands of Astypalaia and Rhodes are shown in Figs 15.13 and 15.14, respectively.
Table 15.7
The analysis of the construction of the PSS penstocks in Rhodes
| Penstock | Absolute altitude (m) | Material | Nominal (external) diameter (mm) | Wall thickness (mm) | Nominal pressure (bar) | Route's length (m) | Total tubes' length (m) |
| Falling penstock | 144–8 | Steel X70 | 2540 | 12.70 | 44 | 856 | 17,120 |
| Pumping penstock | 144–1 | Steel X70 | 2540 | 12.70 | 44 | 877 | 17,540 |
In Rhodes, a flat coastal area of adequate land is found where the penstock reaches the coast (Fig. 15.14). The construction of the pump station and hydro power house including the accompanied works for the coastline is straightforward.
On the contrary, in Astypalaia, where the penstock reaches the coastline, the land is steep and is exposed to the erosive marine environment so it is more susceptible to collapses and land slips (see Fig. 15.13).
Water suction from the sea to the pump station can be accomplished in two ways:
• An alternative is the installation of a long pipeline along the sea bed, starting from the pump station and ending where the sea depth is 15–20 m. The pump station is constructed below sea level to ensure natural water flow through the underwater pipeline. This technique exhibits much lower set-up cost than the first and the visible changes to the natural landscape are minimal.
The second method was selected for both S-PSS. The underwater pipeline extends into the sea until depths greater than 15 m are reached (see Fig. 15.15 for Astypalaia). At these depths, stresses to the water suction structure attributed to waves on the surface are negligible. Moreover, the seawater remains relatively clear, free of any underwater debris or waste (eg sand, algae, small stones) as these are swept away by the underwater streams, reducing the probability of such objects entering the pipeline.
The underwater suction pipelines will be buried 0.5–1.0 m under the sea bed. The pipeline water inlet will be covered with filter grids, to prevent objects from entering the water inflow. In both S-PSSs examined, GRP tubes with nominal pressure of 6 bars will be used for the pipeline. In Astypalaia's S-PSS, a single suction pipeline of 1.50-m inner diameter is required, while in Rhodes' S-PSS, 20 parallel suction pipelines of 2.00-m inner diameter are required. The length of the underwater pipelines is determined by the seabed morphology to ensure that water suction takes places at depths greater than 15 m for the reasons mentioned above. In Astypalaia's S-PSS, the 20-m isobath is found at 92 m from the coast, while in Rhodes, the 20-m isobath is found at 350 m from the coast.
As mentioned earlier, the pump suction level must be below sea level to ensure the natural inflow of water from the sea. By applying Bernoulli's law and taking into account the pipelines' length and inner diameter, the suction geostatic height (−20 m in both cases), the required water flows (3.33 m3/s for each pipeline in Rhodes and 0.69 m3/s in Astypalaia) and the GRP material flow losses coefficient (f = 0.029), the suction level in both pump stations is calculated to be 1 m below sea level. A sectional view of the pump station in Astypalaia is shown in Fig. 15.16. The pump station building will be 15 m from the coastline, to protect it from the waves.
The hydro power plant building will be constructed next to the pump station. A sectional view from the hydro power plant building in Astypalaia is shown in Fig. 15.17. At both sites, the positioning of the power plant is 10 m from the coastline to protect the building from the waves. This determines the absolute altitude above sea level of the hydro turbines and, consequently, the total geostatic head height for power production from the S-PSS. A water disposal canal of reinforced concrete will lead the water to the sea after its passage through the hydro turbines.
15.3.5. Hydrodynamic machines
Pelton hydro turbines and multistage pump models are selected for the operation of the examined S-PSS. The Pelton model is selected because it exhibits constant and high efficiency for 90% of the output power range, low cost, robust construction and it allows an increase in power production within a few seconds. The last feature is very important regarding the power system's stability and dynamic security.
Single-staged pump models were selected for both PSSs. The pump's power regulation will be performed with a cyclo-converter based on thyristors, to follow the available variable power production from the wind park, as well as to avoid the weak system's security and stability events caused by abrupt variation of the pumps' load. Modern voltage source converters (VSC) based on IGBTs can also be used in some other systems in the MW range, and they provide the additional capability of reactive power control for the grid network, in addition to the control functionalities for starting and variable speed operation.
Two parallel horizontal-axis Pelton hydro turbines, each of nominal 2 MW, will be installed in Astypalaia's S-PSS, giving a total hydro power of 4 MW. Taking into account that the maximum power demand in 2013 was 2.55 MW, there will always be a hydro power surplus of 1.45 MW. This power surplus can be exploited as power production spinning reserve, contributing to the system's frequency regulation and to the improvement of the dynamic security. The guaranteed power will be produced 24 h/day.
Similarly in Rhodes, 20 parallel horizontal-axis Pelton hydro turbines, each of nominal power 8 MW, will be installed, giving a total hydro power of 160 MW. Taking into account that the maximum power demanded from the hydro turbine generators is 113.90 MW, there will be a hydro power surplus of 46.1 MW. This surplus capacity can also be used as a power production spinning reserve, contributing to the system's frequency regulation and to an improvement of the dynamic security. The maximum water flow of each hydro turbine is 5.56 m3/s.
For the S-PSS in Astypalaia, the required pumped water flow can be provided by a combination of four parallel pump units, each of 842 kW nominal axis power. The selected pump model is a single-stage pump with horizontal axis.
The total pumped water flow required (66.69 m3/s) can be provided by combining 134 parallel single-stage pump units, each of 1074 kW nominal motor power (absorbed power 934 kW). The total maximum demand for electric power is 143.9 MW. The nominal hydrodynamic efficiency is given 85.0%.
The runners of both Pelton models are constructed of stainless steel grade G-X5CrNi13.4Mo. The needle tips and the nozzle tip wearing rings are replaceable, and are also constructed from stainless steel.
In both pump stations, the selected pump models are developed for reverse osmosis desalination plants. The pump's shaft, the impeller, the suction stage, the casing, the diffuser and the pressure enclosure are constructed from duplex steel.
15.3.6. Wind parks
A peculiarity observed in Rhodes is the relatively low wind potential met onshore, giving a capacity factor usually lower than 25% while the capacity factor in other Aegean islands is higher (>40%). Contrary to the above, higher wind potential is found offshore from Rhodes, especially off the southwest coast of the island (capacity factors of ∼35%). Although an offshore wind park requires higher set-up costs, the wind conditions are improved and planning restrictions are reduced, and as a result the payback time for the investment can be reduced. For these reasons, in Rhodes the wind park will be installed offshore.
A wind turbine of high nominal power needs to be selected due to the following reasons:
• limited space available for the installation of the wind park due to high sea depths in Rhodes;
• approximately 150–200 MW of wind power is required, as predicted by the maximum annual power demand in 2011, presented in Table 15.3, and the capacity factor at the installation site (the final value just lower than 35%).
A wind turbine model of 5 MW nominal power has been selected.
The positioning of the wind turbines (rotor diameter of 126 m) southwest of Rhodes is presented in Fig. 15.8. The wind turbines are positioned in-line perpendicular to the main wind direction. Downwind turbines are placed half way between the two wind turbines in front of it. The distance between two wind turbines on the same line is 5D = 630 m. The distance between two lines of wind turbines is 7D = 882 m. The minimum distance from the coast is 300 m. The maximum depth of installation is 60 m, to avoid higher installation costs related to increasing foundation costs. Following these guidelines, 35 wind turbines are positioned, providing 175 MW.
With the restrictions discussed above, the limited offshore area available and the requirement for power defined the density of the wind turbine installation. This results in significant shading losses which vary between 1.36% and 15.56%.
In Astypalaia the small size of the power demand decreases the requirements for the installed wind power. A small wind park of four 900-kW nominal power wind turbines, giving a total nominal power of 3.6 MW, is proved to be adequate for the introduced WP-PSS. The wind park is installed onshore, on a hill close to the S-PSS installation site. The siting of the wind turbines, presented in Fig. 15.9, is almost perpendicular to the predominant wind direction, giving low shading losses (from 0.11% to 2.11%).
15.3.7. Annual energy productions and storage
The dimensioning of the examined systems is presented in Table 15.8. Given the size of the WP-PSS, as presented in Table 15.8 and by simulating the systems' annual operation, the annual energy produced and stored can be calculated; the results are shown in Table 15.9.
Table 15.8
Results of the systems' dimensioning
| Rhodes | Astypalaia | |
| Wind park nominal power (MW) | 175.00 | 3.60 |
| Hydro turbines nominal power (MW) | 160.00 | 4.00 |
| Pumps nominal power (MW) | 143.92 | 3.54 |
| Maximum falling flow (m3/s) | 111.23 | 0.85 |
| Maximum pumping flow (m3/s) | 66.69 | 0.77 |
| Falling penstock minimum diameter (m) | 7.20 | 1.50 |
| Pumping penstock minimum diameter (m) | 5.60 | 1.50 |
| Number of parallel falling pipelines/nominal diameter (mm) | 20/2540 | 1/1500 |
| Number of parallel pumping pipelines/nominal diameter (mm) | 20/2540 | 1/1500 |
The percentage of the direct wind power penetration to the guaranteed power production, defined in Section 15.2.2.2 is set equal to 0 in Astypalaia and 50% in Rhodes.
Table 15.9
Annual energy production and storing – wind park capacity factors
| Rhodes | Astypalaia | |
| Wind park energy penetration (MWh) | 223,501.56 | 0.00 |
| Hydro turbines energy production (MWh) | 177,886.96 | 6076.08 |
| Total RES energy production (MWh) | 401,388.52 | 6076.08 |
| Thermal generators energy production (MWh) | 387,779.88 | 594.03 |
| Total stored energy (MWh) | 271,880.75 | 11,551.22 |
| Wind park rejected energy (MWh) | 24,250.67 | 530.22 |
| Wind park total produced energy (MWh) | 519,632.98 | 12,081.44 |
| Wind park rejected energy percentage (%) | 4.67 | 4.39 |
| Wind park's final capacity factor (%) | 33.90 | 38.30 |
| PSS total annual efficiency (%) | 65.43 | 57.56 |
| Annual wind energy penetration (%) | 50.86 | 91.09 |
Furthermore, Table 15.9 presents additional information from the operation of the WP-PSS as follows:
• percentage of wind energy rejected Ewr over the total annual wind energy production Ew
[15.1]
• wind park capacity factor, calculated as the annual wind energy penetration Ewp over Pw·T where Pw is the nominal power of the wind park and T the annual time period
[15.2]
• PSS annual efficiency, calculated as Eh the annual electricity production from the hydro turbines over Est the annual energy stored
[15.3]
• the annual RES penetration to the electricity production, calculated as the total wind energy penetration plus hydro energy generation over Ed the total annual electricity consumption.
[15.4]
The small size of the power demand in Astypalaia and the remarkable available wind potential enables an annual RES penetration to the electricity production of higher than 91%. In Rhodes, the high power demand, compared to the lower wind potential and the limited available space for the wind park installation, gives an annual RES penetration of up to 50%.
15.3.8. Economic results
This section presents some fundamental economic results regarding the corresponding investments for the construction of the S-PSS in Rhodes and Astypalaia. The S-PSS construction cost, presented in Table 15.10, is calculated based on the systems' dimensioning and site-specific positioning. The total construction cost per kilowatt is calculated over the PSS guaranteed power (160 MW in Rhodes and 4 MW in Astypalaia). The effect of the project's size on the cost per kilowatt is clearly shown.
Following the Greek legislation regarding the operation of WP-PSSs, the investments' economic evaluation is performed, giving the economic indices over the equities as presented in Table 15.11. The annual revenues of the projects are based on the vending of the produced electricity from the WP-PSS, namely either the hydro turbines or the wind energy direct penetration. The electricity vending prices, according to the existing legislation, are configured on the basis of the electricity production cost of existing thermal power plants of the insular systems. These prices are set as equal to 0.25 €/kWh for Rhodes (existing electricity production cost 0.26 €/kWh) and 0.37 in Astypalaia (existing electricity production cost 0.40 €/kWh).
Table 15.10
WP-PSS set-up cost calculation
| No | Set-up cost component | Set-up cost (€) | |
| Rhodes | Astypalaia | ||
| 1 | Wind park | 350,000,000 | 3,960,000 |
| 2 | Hydro power plant | 60,000,000 | 2,200,000 |
| 3 | Pumps station | 90,000,000 | 1,700,000 |
| 4 | Upper reservoir | 18,500,000 | 3,500,000 |
| 5 | Penstocks | 45,000,000 | 1,300,000 |
| 6 | New roads construction | 1,000,000 | 2,100,000 |
| 7 | New utility network | 30,000,000 | 400,000 |
| 8 | Several infrastructure works | 5,000,000 | 500,000 |
| 9 | Secondary electromechanical equipment | 5,000,000 | 500,000 |
| 10 | Consultants fees | 2,000,000 | 500,000 |
| 11 | Several other costs | 5,000,000 | 500,000 |
| Total set-up cost | 611,500,000 | 17,160,000 | |
| Total PSS set-up cost | 213,500,000 | 8,700,000 | |
| Total set-up specific PSS cost (€/kW) | 1334 | 2175 | |
| Total set-up specific PSS cost (€/kWh) | 119.47 | 39.73 | |
Table 15.11
Economic indices calculated over the investments' equity
| Rhodes | Astypalaia | |
| Net present value – N.P.V. (€) | 356,052,917 | 10,298,045 |
| Internal rate of return – I.R.R. (%) | 13.73 | 31.42 |
| Payback period (years) | 6.35 | 3.10 |
| Discounted payback period (years) | 7.89 | 3.65 |
| Return on equity – R.O.E. (%) | 83.23 | 441.05 |
| Return on investment – R.O.I. (%) | 332.90 | 110.26 |
The indices presented in Table 15.11 prove the economic feasibility of the examined investments.
15.4. Conclusions
The storing of energy is a procedure of ultimate importance for the normal and cost-effective operation of power systems. In the case of non-guaranteed power plants, such as wind parks, the importance of storage increases further due to the necessity to adapt the stochastic nature of the power production to the inelastic power demand. The necessity of wind power storage becomes more intensive in cases of non-interconnected systems, imposed by the sensitive dynamic security requirements and the restricted wind penetration possibilities often found in such systems.
Offshore wind parks constitute a special category of wind park, characterized of increased set-up cost compared to the onshore installations. To compensate this disadvantage, offshore wind parks with high nominal power are usually designed and developed, in order to increase the annual electricity production and the corresponding revenues of the investment. The large quantities of electricity production from offshore wind parks imply the introduction of respectively adequate storage power plants. The available technologies for large power storage plants are the PSSs and the CAESs.
PSSs are the only power storage technology with tens of different installations around the world. The operation of such power plants since the early 1960s provides considerable experience regarding their construction and operation. PSSs are already considered a technically matured and well-established technology. PSSs exhibit high efficiency of the storage–production cycle, usually higher than 65%. The specific set-up cost of PSSs varies from 1300 to 2500 €/kW of guaranteed power or from 40 to 120 €/kWh of storage capacity, depending on the size of the storage plant. In most cases PSSs constitute an economically competitive technology, capable of providing the initially stored electricity with prices lower than the existing specific power production system.
CAES is a rather new technology, although the first CAES plant was installed in Germany in the late 1970s (1978). However, since then only one more CAES plant has been installed in the USA in the early 1990s (1991), leaving considerably less experience about the operation of such systems. Most probably, the alternative storage technology, ie PSS, with better economic and efficiency characteristics, was the main obstacle to the development of more CAES storage plants. CAES power plants exhibit lower efficiency than the PSSs, and the efficiency is currently around 40–50%. The set-up cost of the two existing CAES plants is estimated at around 750–800 €/kW of guaranteed power or from 30–200 €/kWh of storage capacity.
The alternative of adiabatic CAES (AA-CAES) is estimated to cause a 10% increase to the overall efficiency of the CAES, with a related increase in the set-up cost.
A special category of PSS is the ones that use seawater. At time of writing there is only one seawater PSS (S-PSS) constructed worldwide, which is in Okinawa, Japan, used for power peak shaving. Already operating for more than 10 years, the Okinawa S-PSS is an important source of experience for similar stations. Two case studies were presented, one small and one large, for the cooperation of seawater PSS with an offshore and an onshore wind park. Seawater can be pumped directly from the sea, thus construction of a lower reservoir is avoided, compensating for the higher costs arising from the use of corrosion-resistant materials for certain components.
The proximity of the S-PSS installation sites to the sea makes them an excellent prospect for storing electricity produced from offshore wind parks. The two power plants (offshore wind park and S-PSS) can be connected to a common onshore substation, reducing on the one hand the network connection cost and enabling, and on the other hand allowing more flexible operation of the overall hybrid station, without any interference of the rest of the utility network or other existing power plants.
Special issues regarding the use of seawater in PSSs, such as the use of materials for the construction of the penstock, the construction of the upper reservoir, placing the pump station and the hydro power plant on the coast and the selection of pump and hydro turbine models are presented thoroughly.
The presented cases studies proved the economic and technical feasibility of seawater PSSs cooperating with wind parks, from small power plants (power demand in the range of 2.5 MW) to large ones. A fundamental parameter affecting the economic feasibility of the storage plant is the price of the produced electricity, which must be defined according to the existing specific electricity production cost of the existing power production system. If this prerequisite is fulfilled, the economic feasibility of the PSS is usually guaranteed.
Abbreviations
| AA-CAES | Adiabatic compressed air energy storage systems |
| CAES | Compressed air energy storage systems |
| EPDM | Ethylene propylene diene monomer |
| GRP | Glass reinforced polyester |
| HP | High pressure |
| LP | Low pressure |
| PSS | Pumped storage systems |
| RES | Renewable energy sources |
| S-PSS | Seawater pumped storage systems |
| WCAES | Wind-compressed air systems |
| WP-PSS | Wind-powered pumped storage systems |
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