Seawater pumped storage systems and offshore wind parks in islands with low onshore wind potential. A fundamental case study

Energy 66 (2014) 470e486

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Seawater pumped storage systems and offshore wind parks in islands with low onshore wind potential. A fundamental case study Dimitris Al. Katsaprakakis*, Dimitris G. Christakis Wind Energy and Power Plants Synthesis Laboratory, Technological Educational Institute of Crete, Estavromenos, Heraklion Crete, Greece, 710 04

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 May 2013 Received in revised form 2 January 2014 Accepted 6 January 2014 Available online 6 February 2014

The scope of this article is to investigate the effects of introducing a WP-PSS (Wind Powered Pumped Storage System) in isolated electricity systems assuming unfavourable conditions such as low onshore wind potential and low PSS head height. These disadvantages can be compensated with the installation of offshore wind parks, larger reservoirs and double penstocks to allow simultaneous water fall and pumping using pipes of the greatest diameter that are currently commercially available. With the above modifications, the energy efficiency of the WP-PSS improves while the installation costs rise. A new operation algorithm for the WP-PSS is created to fully utilize the capacity of the double penstock and ultimately maximise wind energy penetration. A case study for a WP-PSS on the island of Rhodes is presented in this paper. Despite unfavourable conditions, the WP-PSS model leads to the following results:

Keywords: Hydro power plants Wind powered seawater pumped storage Isolated insular systems Offshore wind parks Renewable energy sources Wind energy penetration


Annual wind energy penetration exceeds 50% of the annual electricity consumption.
The WP-PSS exhibits attractive financial induces without including any subsidies. The WP-PSS presented in this paper proved to be technically and economically feasible and revealed that WP-PSSs are a guaranteed choice for large scale penetration of R.E.S. in electrical systems. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction 1.1. Accomplished research The introduction of WP-PSSs (wind powered pumped storage systems) in isolated electricity systems has been widely studied in other articles. These systems aim to exploit the local, renewable and environmentally friendly wind energy by improving the stability of the system and reducing the use of thermal power plants; minimizing 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 (pumped storage system) in remote islands, to recover otherwise-rejected wind energy due to restrictions imposed for the systems’ stability and dynamic security [1e5]. The PSS, using a single penstock, produces power only during the power demand peak hours and helps to reduce the wind power rejected.

* Corresponding author. Tel.: þ30 2810 379220; fax: þ30 2810 319478. E-mail addresses: [email protected] (D.Al. Katsaprakakis), dimitris@wel. teicrete.gr (D.G. Christakis). 0360-5442/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2014.01.021

Financial factors which determine the scale of the PSS’s parts lead to a system that rejected a considerable amount of wind energy and a need to consume energy during the night which originated from thermal units. 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 [6e10]. The guaranteed power is produced exclusively by the hydro turbines. To maximize the wind energy stored by the PSS, a double penstock is used. The economic feasibility of these WP-PSSs is strongly dependant 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 maximize wind energy penetration [11e13]. Power production from these WP-PSSs 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 PSS in large insular systems, operating exclusively with thermal power plants has been also studied,

D.Al. Katsaprakakis, D.G. Christakis / Energy 66 (2014) 470e486

Notation nP nT

pumps total mean efficiency hydro turbines total mean efficiency r water density g acceleration of gravity g water specific weight QP pumps nominal flow QT hydro turbines nominal flow HTgeo natural geostatic height between upper reservoir and hydro turbines HPgeo natural geostatic height between upper reservoir and pumps HT ¼ HTgeo  dhfT total head height for hydro turbines production HP ¼ HPgeo þ dhfP total head height for pumps operation dhfT flow losses for hydro turbines production dhfP flow losses for pumps operation t time calculation step (1 h) L penstock length d penstock inner diameter f dimensionless coefficient of penstock linear flow losses EP provided energy for pumps to be stored in PSS upper tank during one time step ET produced energy from PSS hydro turbines during one time step VP stored water volume in PSS upper tank during one time step

leading to significant primary energy saving and production cost reduction [14]. A similar study, applied in interconnected systems, is presented in Ref. [15]. The introduction of WP-PSS has also been studied for interconnected power systems in Greece [16], Ireland [17] and Turkey [18]. A common conclusion of these articles 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 saving. The economic feasibility of such systems is guaranteed, given the large quantities of the electricity demand in the interconnected systems. Finally, a number of articles examine very specific issues in designing and optimizating operation of a PSS or a WP-PSS, such as introducing variable speed pump unit which significantly reduces the wind energy rejected [19]. Ref. [20] proves a PSS contributes to the dynamic security and stability of remote insular systems. An interesting effort is presented in Ref. [21], with the introduction of a computer software for locating potential sites for PSS, while a relative work is presented in Ref. [22], with the development of a GIS-based model to calculate the potential for transforming conventional hydro power schemes and non-hydro reservoirs to PSS. A totally novel approach is presented in Ref. [23], with the contribution of PSS to improve the operation of constant-pressure compressed air energy storage. Lastly, in Ref. [24] a mathematical model for the optimisation of the dimensioning of a WP-PSS is presented and applied for several case studies.

VT

removed water volume from PSS upper tank during one time step p the current hydrostatic water pressure in penstock t pipelines wall thickness E ¼ 2.2 GPa modulus of elasticity of water EB ¼ 2.2 GPa modulus of elasticity of steel c velocity of pressure wave from hydraulic hammer dp pressure wave from hydraulic hammer 3 s ¼ 0.10 mm absolute roughness of welded steel tubes Ewp wind park’s annual energy penetration Ewr wind park’s annual rejected energy Ew wind park’s total annual produced energy Eh hydro turbines annual energy production Est total annually stored energy in PSS wr wind park’s rejected energy percentage cfw wind park’s final capacity factor nPSS PSS total annual efficiency Pw wind park’s nominal power T annual time period Vst water volume stored in the PSS upper tank from the previous calculation time step Vrem water volume removed from the PSS upper tank during the current calculation time step Vadd water volume added in the PSS upper tank during the current calculation time step Vmin minimum water volume that cannot be removed from the PSS upper tank Vmax maximum water volume that can be stored in the PSS upper tank

capacity factor, higher offshore wind potential, small size and relatively high power demand, mild land morphology and low annual rainfall. The investigation of interconnecting Rhodes’ grid with the Turkish one, since this opportunity exhibits only for this specific island would restrict the universality of the article, has not been examined. 1.3. The proposed system e operation algorithm The proposed power production system consists of a wind park and a pumped storage system (PSS). It is presented graphically in Fig. 1. 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 maximises the station’s contribution to the maximisation of the wind energy penetration, as well as to the system’s stability and dynamic security.

1.2. Aim of this work The scope of this paper is to investigate the technical and economic feasibility of a WP-PSS under adverse conditions. Island of Rhodes is selected as a case study owing to: low wind energy

471

Fig. 1. Annual electricity power demand of Rhodes in 2011.

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The introduced system aims at maximizing wind power penetration. Without the support of a PSS, the wind power penetration in non-interconnected systems is low due to restrictions for the system’s security [25e30]. The proposed PSS utilizes the sea as the lower reservoir and seawater so that the water supply is guaranteed even for areas with relatively low annual rainfalls (eastern Mediterranean). At time of writing there is only one S-PSS (Seawater PSS) constructed worldwide in Okinawa, Japan, used for power peak saving. Already operating for more than ten years, the Okinawa S-PSS is an important source of experience for similar stations [31e33]. The operation algorithm of the introduced WP-PSS is presented in Fig. 1. 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 ¼ a$Pd (0 < a < 1), in order to ensure the system’s stability. Two cases are distinguished: 1. If the PSS upper tank is empty, the remaining power demand is covered by the existing thermal generators, that produce power equal to Pth ¼ Pd  Pwp. The hydro turbines do not produce power, Pht ¼ 0. The PSS pumps are provided with the wind power surplus Pp ¼ Pw  Pwp, in order to be stored in the PSS upper tank. 2. When the PSS upper reservoir isn’t empty, power demand is covered by the hydro turbines (Pht ¼ 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 happens, no more wind power can be stored; this energy can be used for other applications such as hydrogen production or desalination etc. Power from thermal generators is null: Pth ¼ 0. With the proposed system, thermal generators are only used as reserve units. The main power production unit is the wind park. The PSS is the storage unit.

1.4. Novelty of the article The combined operation of a PSS and an offshore wind park has not been studied previously, an interesting research issue and an innovative topic of the present article. Additionally, the examined case study can be considered as a worst case scenario for introducing a WP-PSS in remote islands, configured by the following adverse conditions: a. the construction of an offshore wind park increases set-up costs

b. the offshore wind capacity factor (w33%) is relatively low and so electricity production from the wind park will be low c. higher upper reservoir’s capacity due to the low head height of the PSS (150 m) also increases set-up cost d. the storage capacity of the PSS is relatively low when considering the project’s size and the island’s power demand e. although a double penstock provides higher operational flexibility, the set-up cost increases further. These adverse design conditions create a negative environment for the introduction of WP-PSS in the specific island, further emphasizing the importance for studying this system. The ultimate scope of the paper is to prove that even in this unfavourable case, the introduction of a WP-PSS can be technically and economically feasible. Another novelty of this article is the algorithm created to simulate the operation of the WP-PSS with two independent pipelines, one exclusively for water fall and one exclusively for pumping water. This feature enables simultaneous energy storage and energy production, maximizing wind energy penetration without affecting the system’s stability and dynamic security. A new operational algorithm is proposed, presented in Section 6. Slight variations of the investigated system’s technical and financial parameters to accommodate conditions in other isolated systems worldwide, are not expected to greatly affect the results of the present article. 2. The existing isolated power production system of Rhodes Rhodes is one of the most popular tourist destinations in Greece. The local economy is mainly tourism during the summer. This causes intensive seasonable variations in the electricity demand, as observed in Fig. 2. The island of Rhodes is an isolated electricity system, powered by a thermal power plant and a wind park. The electricity production is mainly based on thermal generators burning fossil fuels (see the thermal generators synthesis in Table 2). The cost of energy generation is therefore heavily dependant on the price of fossil fuels. The main electricity demand features of the island for the years 2005 and 2011 are presented in Table 1 [34]. The wind power penetration is restricted to ensure the stability of the system [25]. Thermal spinning reserve is usually kept in order to improve the dynamic security of the system, increasing the cost of electricity production. A simulation of the system’s operation in 2011 provides the results presented in Table 3. The annual fuel consumption specific cost is calculated as 0.1348 V/kWh. Averaged prices of 2011 for

Fig. 2. Wind park and pumped storage system co-operating with the existing thermal power plant.

D.Al. Katsaprakakis, D.G. Christakis / Energy 66 (2014) 470e486 Table 1 Main electricity demand features in the island of Rhodes in 2005 and 2011.

Maximum annual power demand (MW) Minimum annual power demand (MW) Total annual energy consumption (MWh) Mean daily energy consumption (MWh)

2005

2011

Reduction percentage (%)

218.72

176.40

19.35

38.94

38.70

0.62

875,581.37

789,168.37

9.87

2398.85

2162.11

9.87

Table 2 The synthesis of the existing electricity production system in Rhodes at the end of 2011.

Wind potential measurements were captured for a time period of six months (October 2009eMarch 2010). The wind atlas of the selected area was developed, based on the recorded measurements. The WaSP software was employed in this task (RISOE, National Laboratory for Sustainable Energy, Technical University of Denmark). The results of the executed calculations are presented in Figs. 3 and 4. The availability of onshore wind potential measurements in central Rhodes (mountain of Emponas), together with the offshore measurements, allow us to simulate the wind velocity times series at the southwest coast of the island. This is achieved by applying a simple linear interpolation procedure between the available wind velocity time-series, as expressed mathematically with the following relationship:

Generator description

Installed power (MW)

uoff=apresep ðiÞ ¼

Steam turbines Diesel engines Gas turbines Total thermal installed power Wind parks

30.0 94.0 68.0 192.0 11.7

where:

heavy fuel oil and diesel oil, equal to 0.62679 V/kg and 1.09767 V/lt respectively, were adopted. The thermal capacity of heavy fuel and diesel oil were assumed equal to 11.36 kWh/kg and 9.90 kWh/lt. Finally, the thermal generators’ efficiency variation curves versus the power production, as they are provided by the manufacturer, were considered. Apart from fuel, the total specific cost of electricity production also includes emissions of gases, maintenance cost, labour cost etc. All these are not affected by the power demand, hence they are called “constant specific cost”. For Rhodes, this cost is 0.1035 V/ kWh, confirmed by the Greek Power Production Corporation (P.P.C.). The total specific cost of electricity production is the total of the above presented costs, namely 0.2461 V/kWh. The characteristics of the existing electricity production system:





the the the the

high electricity production specific cost high dependence on imported energy sources high quantity of the CO2 annual emissions dynamic security and stability problems

set the conditions for introducing a new power production system, based on local R.E.S. in Rhodes. 3. The wind potential evaluation The wind potential offshore from Rhodes is evaluated with a meteorological mast, installed in a small islet (islet of “Karavolas”), located at the wind park’s installation site.

473

uoff=octemar $u ðiÞ uon=octemar on=apresep

(1)

uoff/apresep, the missing offshore wind velocity from April to September uon/apresep, the measured onshore wind velocity from April to September i, the current time calculation hourly step from April to September uoff =octemar , the averaged wind velocity from October to March, measured offshore uon=octemar the averaged wind velocity from October to March, measured onshore. The constructed wind velocity time series is employed in the computational simulation of the operation of the WP-PSS, required for optimising the system design. 4. The micro-sitting of the offshore wind park 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 150e200 MW of wind power is required, as predicted by the maximum annual power demand in 2011, presented in Table 1, and the capacity factor at the installation site (final value 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. 5. The wind

Table 3 Results from the simulation of the operation of the existing electricity production system in Rhodes in 2011. Energy production (MWh) Steam turbines 240,840 Diesel engines 525,958 Gas turbines 12 Wind parks 23,434 Thermal generators start up procedure e Total/average 790,244 Total annual fossil fuel cost (MV): Total annual fossil fuel specific cost (V/kWh): Total annual wind energy purchase cost (MV): Total annual fossil fuel and wind energy cost (MV): Total annual CO2 emissions (tn):

Production specific cost (V/kWh)

Efficiency (%)

Heavy fuel oil consumption (tn)

Diesel oil consumption (klt)

Heavy fuel oil cost (MV)

Diesel oil cost (MV)

0.1599 0.1230 1.0897 0.0920 e 0.1335

34.50 44.86 10.18 e e 41.61

61,451 103,200 e e 185 164,836

e e 12 e 5 17

38.517 64.685 e e 0.116 103.318

e e 0.013 e 0.005 0.018 103.340 0.1348 2.160 105.490 523,398

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Fig. 3. Wind rose and Weibull distribution at the installation position of the meteorological mast.

turbines are positioned in-line perpendicular to the main wind direction. Down-wind 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 ¼ 630 m. The distance between two lines of wind turbines is 7D ¼ 882 m. Minimum distance from the coast is 300 m. 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, as dictated by the optimization procedure, presented analytically in a following chapter. 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%.

5. The sitting of the pumped storage system 5.1. The upper reservoir After a thorough investigation of the area near the wind park (including field research), an optimum position for the PSS upper tank was selected. This position is presented in Fig. 6. 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. Characteristics of the PSS upper reservoir construction are presented in Table 4. A 3D digitized drawing of the PSS upper tank is presented in Fig. 7. A disadvantage of the site selected is its low absolute altitude (160 m). To maximise the PSS’s head height, the sea is used as the PSS’s lower reservoir.

Fig. 4. The wind atlas of the southwest coast of Rhodes with the wind mast installation position.

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Fig. 5. The micro-siting of the offshore wind park in the southwest of the island of Rhodes.

The low PSS’s head height (145e160 m) needs to be offset by increasing the upper tank’s capacity which is feasible considering the morphology of the selected site. The use of seawater also guarantees availability of the large quantity of water required. To prevent leakage from the upper tank, the technology applied in Okinawa S-PSS was adopted. Further details regarding the construction of the upper reservoir can be found in Ref. [35]. 5.2. The penstock The route of the penstock was chosen with the following criteria:


the minimization of the penstock length
the avoidance of intensive slopes and cliffs
the land’s morphology where the penstock reaches the sea must be mild. Apart from a small underground section (237 m), the penstock runs above ground. The proposed route is shown in Fig. 8 and some technical details are presented in Table 5. The water fall 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 twenty parallel pipelines are used for the water fall and pumping penstocks.

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Fig. 6. The PSS upper tank position and the penstock route on the Greek Geodetic Coordinate Reference System.

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, calculated using relationships (2) and (3) [36,37]). The minimum wall thickness of X70 steel tubes/100 in diameter is given as 12.70 mm with a corresponding nominal pressure of 44 bars, adequate for the specific installation.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u E u i c ¼ t h r$ 1 þ EEB $dt

(2)

dp ¼ r$c$du

(3)

The ratio of the penstock length (L) to head height (H) is 5.8, acceptable, although it is better that the ratio is lower than 5, to reduce water flow losses head and minimise pipe diameter [27,36,37]. Additional technical details regarding materials and installation of penstocks for sea-water can be found in Ref. [35]. 5.3. The hydrodynamic machines stations The hydrodynamic machines stations, namely the Pump Station and the Hydro-Power Plant will be positioned onshore, on the coast. The site must fulfil the following conditions:


buildings must be safe from rough sea weather
the absolute altitude of the hydro power-plant must be as low as possible, to maximize the system head height
the pump suction level needs to be below sea level to achieve natural intake of seawater into the pump station. To meet these conditions, the hydro power-plant and the pump station will be in two different buildings. A 3D view of the selected positioning is presented in Fig. 8. Sea-water intake to the pump station can be accomplished with the construction of a breakwater structure using pre-cast concrete blocks [31,32]. The pump suction level needs to be below sea level to ensure the natural inflow of water. By applying the Bernoulli’s law, given that the suction pipelines’ length is too short (20 m), the suction level is calculated to be lower than 1 m below sea level. The positioning of the hydro power plant is 10 m from the coastline to protect the building from rough sea weather (8 m absolute altitude). After passing through the hydro turbines, a water disposal canal from reinforced concrete will lead the water back to the sea. 6. Fundamental points of the computational procedure 6.1. Aim of the computational procedure

Table 4 Characteristic features of the PSS upper tank. Reservoir’s overall characteristic Gross capacity (m3) Effective capacity (m3) Minimum water volume in reservoir (m3) Upper surface area (m2) Bottom area (m2) Upper surface altitude (m) Bottom altitude (m) Maximum depth (m) Inner incline slope Total digging volume (m3)

5,107,924 4,554,257 553,667 414,997 424,540 160 145 15 1:3 0

Dams NW dam’s volume (m3) SE dam’s volume (m3) NW dam’s total length (m) SE dam’s length (m) NW dam’s maximum height (m) SE dam’s maximum height (m)

81,975 304,064 220 387 20 40

The computational procedure aims at optimizing the dimensions of the WP-PSS. The optimization criteria are selected to

Fig. 7. 3-D digitized representation of the PSS upper tank.

D.Al. Katsaprakakis, D.G. Christakis / Energy 66 (2014) 470e486

477

Fig. 8. 3-D digitized representation of the PSS upper tank.

be the corresponding investment’s financial indexes, specifically, the investment’s Internal Rate of Return and Payback Period.

6.4. Calculation procedure The basic steps of the computational procedure are listed below:

6.2. Independent variable The independent variable of the calculation procedure is the wind park’s nominal power. Given that the selected wind turbine has nominal power of 5 MW, the computational optimization procedure is executed iteratively with an increment step for the independent variable of 5 MW. As the quantity of the wind turbines increases, annual wind energy penetration Ewp and storage Ews increases. Rejected wind energy Ewr increases as well, either because the PSS upper tank is full with no room for energy storage or due to wind power penetration restrictions. The optimum wind park power can be found to maximise wind energy penetration (Ewp þ Ews) without significantly increasing the wind energy rejected (Ewr). 6.3. Data The information required to run the optimisation procedure is:
the annual time series of mean hourly values for power demand
the annual time series of mean hourly values for wind energy production
the upper tank capacity
the PSS head height of water fall and water pumping
the hydro turbines’ and pumps’ efficiencies
the penstock length.

i. Following the system’s operational algorithm presented in Section 1.3, wind power penetration, power generation from the hydro turbines and the thermal plants and pumps’ operation power were calculated for each time step. The operational algorithm for the WP-PSS is summarized by the following relationships: a. Wind power penetration If Pw  a$Pd, then Pwp ¼ a$Pd, else Pwp ¼ Pw. b. Hydro turbines and thermal generators power production If Vst  Vrem  Vmin, then Pht ¼ 0 and Pth ¼ Pd  Pwp, else Pht ¼ Pd  Pwp and Pth ¼ 0,where Vrem ¼ t(Pd  Pwp)/nTrgHT is the required water volume that must be provided for the hydro turbines to produce power equal to Pd  Pwp for a time period t (for each calculation time step t equals to 1 h). c. Pumps operational power If Vst þ Vadd  Vmax, then Pp ¼ 0, else Pp ¼ Pw  Pwp,where Vadd ¼ npt(Pw  Pwp)/rgHp is the water volume stored in the PSS upper reservoir if the pumps are provided with power equal to Pw  Pwp for a time period t.

Table 5 The analysis of the construction of the PSS penstocks. 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 Pumping penstock

144e8 144 to 1

Steel X70 Steel X70

2540 2540

12.70 12.70

44 44

856 877

17,120 17,540

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Given the results for the dynamic security of power systems supported with PSS [20,25,26], the maximum wind energy penetration permitted (a) is set to 50% of power demand.

The financial analysis of the WP-PS and the calculation of its financial indexes, along with all the assumptions and parameters, are presented in Section 9.

ii. The water volume stored and let out from the PSS upper tank is calculated for every time step using the following relationships (4) [36,37]:

7. Results

VT ¼

tPht n tP &V ¼ P P rgHP nT rgHT P

(4)

iii. The stored volume in the PSS upper tank after each time step is:

Vst ðiÞ ¼ Vst ði  1Þ  Vrem ðiÞ þ Vadd ðiÞ

(5)

iv. The penstocks’ diameter is selected in order to minimize penstock flow losses and flow velocity. The penstock flow losses are given by the relationships (6) [36,37]. The penstock’s diameter is selected so that flow losses are not higher than 5% of the PSS head height.

dhfT ¼ f

L 8$QT2 L 8$QP2 &dhfP ¼ f 5 5 2 d gp d g p2

(6)

The flow losses coefficient f is given by the empirical relationship (7) of Nikuradse [36], for steel welded penstock and turbulent flow.

  1 1 pffiffiffi ¼ 2$log þ 1:14 3s f

(7)

The water falling and pumping flows inside the pipelines are calculated for every time step once the hydro turbines and pumps operation power have been calculated with the well-known relationships [36,37]:

QT ¼

Pht n P &Q ¼ P P rgHP nT rgHT P

(8)

v. The calculations presented above are executed for a time period of one year in hourly time steps. From the simulation of the yearly operation, we get the following results: a. the annual maximum hydro turbine and pump power define the corresponding nominal power of the hydrodynamic machines b. the penstock diameter is defined by the maximum annual water flow c. with the integration of the annual time series for the wind power penetration, the hydro turbines and thermal generators production and the pumps operation, the corresponding annual energy quantities are calculated d. using the financial model presented later in this paper, the WP-PSS set-up cost, annual operational and maintenance costs and annual revenues are calculated, along with the corresponding financial indexes. vi. The procedure above is executed iteratively with increasing wind power. The optimum dimensioning is derived as the one with best financial indexes. The procedure described above is shown in the flow chart shown in Fig. 9.

The calculation data are summarized in Table 6. The absolute roughness of the penstock material selected was that of welded steel [36]. To simplify the calculations and to preserve the generality of the study, without affecting their accuracy [37], total characteristic mean efficiencies were used for the hydro turbines and pumps annual operation. Since a large number of hydro turbines and pumps units will be installed (see next section), their operation at maximum efficiency will be enabled. So this approximation is accurate. The penstock length and the head height are given by the land morphology at the PSS installation site. Fig. 10 presents the relationship between the independent variable and the optimisation criteria. As shown by these figures, the project’s financial indexes are optimised when 175 MW of wind power is installed, hence 35 wind turbines of 5 MW. The dimensioning of the other WP-PSS components is presented in Table 7. Given the size of the WP-PSS, as presented in Table 7, and by simulating the system’s annual operation under the conditions presented in Section 1.3, the annual energy produced and stored can be calculated; the results are shown in Table 8. For comparison, Table 8 also shows the annual energy produced and stored by a single penstock WP-PSS of the same dimensions. The operation algorithm of the single penstock WP-PSS we adopted has been presented and examined in previous articles [11e13]. This comparison presents how a double penstock allows greater R.E.S. penetration. Furthermore, Table 8 presents additional information from the operation of the WP-PSS as follows:
percentage of wind energy rejected

wr ¼

Ewr Ew

(9)


wind park capacity factor

cfw ¼

Ewp Pw $T

(10)


PSS annual efficiency

nPSS ¼

Eh Est

(11)

From Tables 7 and 8, the following conclusions can be mentioned:
The resulting hydro and pump power requires a high water flow through the PSS pipelines which can only occur using many parallel pipelines.
The chosen quantity and diameter of parallel pipelines keeps losses low. In fact, there is a slight over-dimensioning of the PSS pipelines to allow a future increase of hydro turbine or pump power.
Installing a double penstock allows 9.3% increase in annual wind energy penetration (compared to a similar system with single penstock) resulting in a corresponding decrease of energy production from thermal generators.

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Fig. 9. Flow chart depicting the introduced WP-PSS dimensioning procedure.

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Table 6 The calculation data. Falling flow head height (m) Pumped flow head height (m) Falling flow penstock length (m) Pumped flow penstock length (m) Hydro turbines total mean efficiency Pumps total mean efficiency PSS upper tank effective capacity (m3) Penstock roughness (mm)

Table 7 Fundamental results of the dimensioning optimisation. 135.85e150.85 146.66e161.66 856 877 0.90 0.78 4,554,257 0.10


If a single penstock is installed, the wind energy rejected annually would be around 18.96%. This falls to 4.67% with the double penstock. The synthesis of the annual power production in Rhodes is shown in Fig. 11. Fig. 12 presents the power production synthesis from 17/4 until 28/4. Finally, Fig. 13 shows the variation of water storage in PSS upper tank. The following remarks arise from Figs. 11e13:
Fig. 12: wind energy penetration is limited to 50% of current power demand, regardless of availability. Hydro turbines or thermal generators are used to supply the remaining power required.
Figs. 11 and 13: water storage in the PSS’s upper tank is relatively low during the summer period as more power production from

Wind park nominal power (MW) Hydro turbines maximum annual power production (MW) Pumps maximum annual power storing (MW) Maximum falling flow (m3/s) Maximum pumping flow (m3/s) Falling penstock minimum diameter (m) Pumping penstock minimum diameter (m) Number of parallel falling pipelines/nominal diameter (mm) Number of parallel pumping pipelines/nominal diameter (mm) Maximum velocity during falling flow (m/s) Maximum velocity during pumping flow (m/s) Falling flow maximum linear losses head (m) Pumping flow maximum linear losses head (m)

175.00 113.90 126.67 111.23 66.69 7.20 5.60 20/2540 20/2540 1.57 1.12 1.15 0.66

the WP-PSS is required. During this period, PSS storage is often not enough for the hydro power production required and the thermal generators are used. The contribution of all power production units is shown in Fig. 14 depending on the use of a double or a single penstock. Wind energy penetration with the use of a double penstock is higher than 50% (including direct wind energy penetration and production from the hydro turbines) and with the use of a single penstock this is below 42%. In Fig. 15 it is seen that the limit we have set for direct wind power penetration (50% of power demand) restricts the wind

Fig. 10. Variation of the investment’s economical indexes versus the installed wind power.

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the simulation of the current power system), we come to the following conclusions:

Table 8 Annual energy production and storing e wind park’s capacity factor.

Wind park energy penetration (MWh) Hydro turbines energy production (MWh) Total R.E.S. energy production (MWh) Thermal generators energy production (MWh) Total stored energy (MWh) Wind park rejected energy (MWh) Wind park total produced energy (MWh) Wind park rejected energy percentage (%) Wind park’s final capacity factor (%) PSS total annual efficiency (%)

481

Double penstock

Single penstock

223,501.56 177,886.96 401,388.52 387,779.88 271,880.75 24,250.67 519,632.98 4.67 33.90 65.43

180,474.48 147,496.60 327,971.08 461,197.32 225,285.73 94,951.27 519,632.98 18.27 33.90 65.47


50% reduction of energy annually produced by the diesel engines and 33% reduction of energy produced by the steam turbines are achieved.
The thermal generators were operating below nominal power with lower efficiency resulting in a higher average specific production cost.
Fuel consumption decreases leading to lower fuel costs by more than 40%. Similarly, CO2 emissions decrease more than 40%.

8. Hydrodynamic machines energy used directly by the system. As a result, more than 50% of the wind energy produced is stored by the PSS. Optimizing the dimensioning of the WP-PSS, using a double penstock, is successful as there is low wind energy rejection (less than 5%). The thermal power plants’ annual operation is simulated and the results are presented in Table 9, with the introduction of the WP-PSS. By comparing Tables 9 and 3 (which shows results from

Pelton hydro turbines and a single stage pump model are selected for the S-PSS. The Pelton model is selected because it exhibits constant and high efficiency for 90% of the output power range, relatively low cost, robust construction and 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.

Fig. 11. Annual power production synthesis graph.

Fig. 12. Power production synthesis graph from 5-4-2008 to 25-4-2008.

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Fig. 13. Stored water volume in PSS upper tank annual variation graph.

Fig. 14. Energy production percentage contribution.

Fig. 15. Exploitation of the wind park produced energy.

The use of a single stage pump model is possible due to the low head height. The regulation of pump power will be performed using a cyclo-converter, so that it follows the variable available power production from the wind park and to avoid the weak

system’s security and stability events caused by abrupt variations in the pumps’ load. Twenty parallel horizontal axis Pelton hydro turbines, each of nominal power 8 MW, will be installed, giving a total hydro power

Table 9 Results from the simulation of the operation of the thermal power plant in Rhodes with the introduction of the proposed power system.

Steam turbines Diesel engines Gas turbines Thermal generators start up procedure Total/average Total annual fossil fuel cost (MV): Total annual CO2 emissions (tn):

Energy production (MWh)

Production specific cost (V/kWh)

Efficiency (%)

Heavy fuel oil consumption (tn)

Diesel oil consumption (klt)

Heavy fuel oil cost (MV)

Diesel oil cost (MV)

164,752 261,586 7 e 426,345

0.1656 0.1229 1.0740 e 0.1399

33.31 44.91 10.32 e 40.43

43,535 51,277 e 342 95,154

e e 7 4 11

27.288 32.140 e 0.214 59.641

e e 0.007 0.004 0.011 59.641 302,141

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of 160 MW. Taking into account that the maximum power produced by the hydro turbines is calculated at 113.90 MW, there will be a hydro power surplus of 46.1 MW. This power surplus can be used as power production spinning reserve, contributing to the system’s frequency regulation and to the improvement of the dynamic security. The maximum water flow of each hydro turbine is calculated as 5.56 m3/s. The runners of the Pelton model are constructed of stainless steel grade G-X5CrNi13.4Mo. The needle tips and the nozzle tip wearing rings are replaceable, also constructed from stainless steel. 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 selected pump model is 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. The selected pump model’s characteristic curves are presented in Fig. 16. 9. Economic evaluation 9.1. Set-up cost and funding scheme The proposed power production system is evaluated financially. The estimation of the systems set-up cost was based on market research and on bibliography [38e44]. Table 10 shows the estimated set-up cost for each component, the project’s total set-up cost along with the funding scheme.

483

Table 10 Power production system set-up cost analysis and funding scheme. Power production system initial cost component

Specific cost

Total cost (V)

Offshore wind park (with underwater cables) (V/MW) Hydro power plant (V/MW) Pump station (V/MW) Upper reservoir (V) Penstock (V) Roads construction (V/km) Onshore network connection (V) SCADA/secondary electromechanical equipement (V) Concrete block breakwater at penstock’ lower intake (V) Several infrastructure works (V) Several/Unpredictable costs (V) Consultants’ fees (V) Total cost:

2,000,000

350,000,000

Funding scheme Private capitals 25% (V) Loans 75% (V) Subsidy 0% (V)

400,000 600,000

50,000

60,000,000 90,000,000 18,500,000a 45,000,000b 1,000,000c 30,000,000d 5,000,000 3,000,000 2,000,000 5,000,000 2,000,000 611,500,000

152,875,000 458,625,000 0,00

a It consists of the bottom sealing and configuration works and the dams’ construction. b The required steel X70 total mass is calculated 27,435,207 kg. A steel price of 1.2 V/kg was provided by the manufacturer. The installation cost was estimated equal to the 30% of the tubes’ cost. c It is estimated that 20 km of new roads will be constructed. d The length of the required 66 kV voltage new utility network is estimated equal to 50 km. 11 parallel N2XS(F)2Y 38/66 kV cables will be installed. A new voltage substation of 20/66 kV is required.

9.2. Annual revenues The project’s annual revenues come from selling the energy produced according to the Greek legislation [45,46]. The price for electricity produced by a wind powered pumped storage system is set according to the specific cost of electricity production of the substituted thermal generators. This cost has been calculated as 0.2461 V/kWh. For the purpose of this study, we have assumed a price of 0.25 V/kWh.

All the energy produced by the WP-PSS, 401,388.52 MWh as shown in Table 8, will be sold to the system. The annual revenue will be 100,347,130 V. 9.3. Operational and maintenance cost The system’s annual operational & maintenance costs are presented below:

Fig. 16. Characteristic curves of the selected pump model.

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Table 11 Investment’s financial ratios.

Equity’s net present value e N.P.V. (V) Equity’s internal rate of return e I.R.R. (%) Equity’s payback period (years) Equity’s discounted payback period (years) Return on investment e R.O.I. (%) Return on equity e R.O.E. (%) Energy production specific cost (V/kWh)

-

-

-

Double penstock

Single penstock

356,052,917.46 13.73 6.35 7.89 83.23 332.90 0.1349

181,097,090.85 6.87 11.06 12.54 55.62 222.47 0.1530

Public contribution set by the Greek state, 3% of annual revenue. Wind turbine offshore maintenance 0.020 V/kWh. PSS maintenance 200,000 V. Loan payments, with a payback period of 10 years and 5.5% interest rate. Labour cost 500,000 V. Insurance 4& of the project’s total set-up cost. Several other costs 100,000 V. State taxes 20% of profit.

The project’s life period was assumed to be 20 years (according to the current Greek legislation) and the discount rate was set equal to 5.0%. The equity’s amortizations are calculated following the reduced remain coefficient method, according to the Greek legislation [45,46]. Following the above mentioned assumptions the investment’s annual cash flows for 20 years are calculated. 9.4. Financial ratios and sensitivity analysis The investment’s financial indexes are calculated using the estimated set-up cost, the operational and maintenance costs and the annual revenue; the results are presented in Table 11. Table 11 also presents the same indexes for a single penstock WP-PSS. In this case, the penstock’s installation cost is assumed to be 25,000,000 V. As we can see, a single penstock WP-PSS with lower energy penetration from RES (Renewable Energy Sources) sources results in significantly worse financial indexes. The double penstock allows almost 10% more renewable energy and improves the financial outcome of the investment. Figs. 17 and 18 present sensitivity analysis for the equity’s internal rate of return and payback period in terms of various investment parameters.

Fig. 17. Sensitivity analysis graph of equity’s internal rate of return in terms of the investment’s various parameters.

D.Al. Katsaprakakis, D.G. Christakis / Energy 66 (2014) 470e486

10. Conclusions The introduction of a WP-PSS is investigated for an insular system with low onshore wind potential and relatively mild land morphology (low head height). To overcome these unfavourable conditions:
the wind park is installed offshore which increased the set-up cost
an upper reservoir is constructed with maximum capacity allowed according to the site specific land morphology
seawater is used as the storage medium
the sea is used as the PSS’s lower reservoir. The WP-PSS investigated aims to maximize the penetration of wind energy. A new WP-PSS operation algorithm is proposed, which allows continuous power production from the WP-PSS (not restricted to power demand peak hours only) and the use of a double penstock which allows simultaneous power production from the hydro turbines and power storage by the pumps. Despite the unfavourable design conditions, the dimensioning, the sitting and the financial evaluation of the proposed system produced satisfactory results:

485


Electricity production by the wind park and the PSS exceed 50% of the annual electricity consumption on the island.
The proposed WP-PSS appears to have attractive financial indexes.
Installing a double penstock increases the annual R.E.S. penetration by more than 10% (in comparison to using a single penstock), and significantly improves the financial indexes of the investment.
Environmental benefits that arise from reduction of fossil fuel consumption are not taken into account in the present study but they raise the attractiveness of the proposed power system. In the majority of isolated power production systems worldwide, electricity production is based on fossil fuels. Peculiarities found in these systems result in expensive and unstable electricity production. Increasing fossil fuel prices recorded during the last years raise the cost of electricity production and negatively affect the local economies. Moreover, the sensitive ecosystems frequently found in these areas are harmed by the pollution related to thermal power plants. Remarkable renewable energy potential is frequently recorded in non interconnected islands. The exploitation of these sources can provide the solution for power production combined with advantages for the local communities.

Fig. 18. Sensitivity analysis graph of equity’s payback period in terms of the investment’s various parameters.

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The WP-PSS investigated in this article, examined under the most unfavourable conditions, proved to be technically and financially feasible. Therefore WP-PSSs constitute a guaranteed choice, based on technically and economically mature technologies, for large scale penetration of R.E.S. in electrical systems.

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