Analysis of the combined use of wind and pumped storage systems in autonomous Greek islands

Analysis of the combined use of wind and pumpedstorage systems in autonomous Greek islands

G. Caralis and A. Zervos

Abstract: In autonomous islands, the wind penetration is restricted due to technical reasons relatedwith the safe operation of the electrical systems. The combined use of wind power with pumpedstorage (WPS) systems is considered as a means to exploit the abundant wind potential, increasethe wind installed capacity and substitute conventional peak supply. An approach for the simulationof the autonomous electrical systems is proposed and applied in three islands. The simulation isbased on the non-dynamic analysis of the electrical system, in order to calculate the energy contri-bution of the different power units. The aim is to analyse the prospects of WPS systems to decreasethe electrical system’s cost. The results show that the integration of WPS in autonomous islandsmay decrease the system’s electricity production cost.

List of symbols

a Upper part of the demand to be covered by thehydro-turbine

d Allowed instantaneous wind penetration

BEW Benefit from the wind energy absorbed directlyby the grid

CEC Cost of conventional energy used for pumping

D Electricity demand

DP Diameter of the penstock

EC Electricity production of conventional unit

EPC Electricity Production Cost

EPCC EPC of conventional units

EPCS EPC of whole electrical system

EPCW EPC of wind power

EP:available Total energy available for pumping (conven-tional and wind energy surplus)

EP:final Cumulative final energy used for pumping

EP:C Conventional energy used for pumping

EP:W,h Wind energy surplus used for pumping

ESO Electricity system operator

ESP_T Desirable hydro-turbine production based onthe set-points

ESP_P Annual conventional electricity available forpumping

ET Electricity production of hydro-turbine

EW Annual energy production of wind farms out ofthe WPS

EW,h Annual energy production of wind farms in theWPS

# The Institution of Engineering and Technology 2007

doi:10.1049/iet-rpg:20060010

Paper first received 28th November 2006 and in revised form 11th January 2007

The authors are with the National Technical University of Athens (NTUA),PO Box 64514, Zografou Campus, 15704 Athens , Greece

E-mail: zervos@fluid.mech.ntua.gr

IET Renew Power Gener., 2007, 1, (1), pp. 49–60

EW,Total Cumulative annual energy production of windfarms

EW!A Wind energy absorbed by the grid directly fromthe wind farms out of the WPS

EW!R Curtailment of surplus wind by the wind farmsout of the WPS

EW,h!A Wind energy absorbed by the grid directly fromthe wind farms in the WPS

EW,h!R Curtailment of surplus wind by the wind farmsin the WPS

FC Fuel cost (per kWh produced)

g Gravity

HFO Heavy Fuel Oil

i Discount rate (i ¼ 5%)

LFCU Average load factor of the committed conven-tional units

LA Predefined level of demand upon which thehydro-turbine is to be set into operation

LH Level of the water in the higher reservoir in thebeginning of the simulations’ step

LP Length of the penstock (distance between thehigher and the lower reservoir)

L Level of the water in the lower reservoir in thebeginning of the simulations’ step

LH,END Level of the water in the higher reservoir in theend of the simulations’ step

LL,END Level of the water in the lower reservoir in theend of the simulations’ step

MAC Maximum conventional ability of the com-mitted conventional units

N Lifetime of the investment (N ¼ 20 years forthe wind farms and N ¼ 30 years for theconventional units)

NC Number of conventional units committed

NP Number of pumps installed

OMCC Operation and Maintenance cost of the conven-tional units

OMCWPS Operation and maintenance cost of the WPS

PAbsorbed Grid ability to directly absorb wind power

PC Conventional unit production

Peak Last 24-hours peak demand

pm Market price for the conventional electricityused for pumping

pW Fixed market price for the wind power directlyabsorbed by the grid

PP:Avail Cumulative available power for pumping

PP:C Conventional power finally used for pumping

PP:Final Cumulative power finally used for pumping

PP,R Rated power of each pump

PP:W Wind power finally used for pumping

PSU Pumped Storage Unit

PT,R Rated power of the hydro turbine

PT,Produced Turbine power production

PW,R Wind installed capacity out of the WPS

PW,h,R Wind installed capacity in the WPS

PW Wind power production of the wind farms outof the WPS

PW,h Wind power production of the wind farms in theWPS

PW!A Wind power absorbed directly from the windfarms out of the WPS

PW!R Curtailment of surplus wind power by the windfarms out of the WPS

PW,h!A Wind power absorbed directly from the windfarms in the WPS

PW,h!R Curtailment of surplus wind power by the windfarms in the WPS

R Annuity factor

RAE Regulatory Authority for Energy

SRC Rest capacity of the committed conventionalunits

SRT Rest capacity of the hydro-turbine

SPC Set-point of the conventional units

SPP Set-point for the available conventional powerfor pumping

SPT Set-point of the turbine

TICC Total investment cost of the conventional units

TICW Total investment cost of wind applications

TMC Technical minimums of conventional units

TMT Technical minimums of the hydro turbine

U Wind velocity

VH Capacity of the higher reservoir

VH!L Water flow from the higher to the lowerreservoir

VL Capacity of the lower reservoir

VL!H Water flow from the lower to the higherreservoir

WPS Wind and Pumped Storage System

WPSO WPS operator

1 Introduction

The autonomous electrical systems in Greek islands arebased almost entirely on oil. They are characterised byhigh wind potential and there is a high investor’s interest

for wind applications. Given the current infrastructureand the technical constraints, the prospects of wind powerto decrease both the system’s electricity production cost(EPCS) and the dependence on the oil are limited [1]. Thereason is that wind farms operating in autonomoussystems are subject to output power limitations, relatedwith technical constraints of the conventional generatingunits, namely the minimum loading levels of the thermalunits (technical minimum) and a dynamic penetrationlimit, applied for stability purposes [1–4]. Analysis ofdifferent island systems has shown that the increased windpenetration without storage could reduce the EPCS onlyup to a maximum of 7% (unpublished).

Conventional units and existing wind farms comprise thecurrent infrastructure of the autonomous electrical systemsto meet the demand. A wind and pumped storage (WPS)system, comprising new wind farms, two reservoirs forthe recycling of water, hydro turbines, pumps and pen-stocks, is proposed as a mean to increase the wind installedcapacity, substitute expensive fuel oil and reduce therequired conventional installed capacity. The later is poss-ible because the variable output of wind power ismanaged and transformed into a guaranteed power supply.Recently, the idea of pumped storage units (PSU) hasbeen analysed by the scientific community for variousautonomous islands [5–10]. Inconsistency on the assump-tions always makes difficult the comparison betweendifferent approaches and islands.

In the current paper, the prospects of the WPS todecrease the EPCS, to increase the renewable energysupply and the renewable capacity credit, are examinedcomparatively in three islands different in size and charac-teristics, using the same assumptions and approach. Threerepresentative Greek islands have been selected as casestudies.

† Crete is the largest Greek island and represents highcurrent EPCS due to the extensive use of gas turbines tocover medium loads. The electrical system consists of 6steam units with 107 MW installed capacity, 1 combinedcycle with 36 MW, 13 gas turbines with 403 MW and 8diesel with 344 MW.† Lesvos is a medium-sized island with relatively lowcurrent EPCS due to the use of heavy fuel oil and thesmooth demand distribution. The electrical system consistsof 15 Heavy Fuel Oil (HFO) units with 106 MW installedcapacity.† Serifos is a typical small island with very high EPCS

and misdistribution of the demand between winter andsummer due to tourism. The electrical system consists ofseven diesel units with 5.8 MW installed capacity.

The simulation is based on the convolution of the demandand the wind time series. For the application in each island,the hourly data of the electricity demand, the conventionalunits and their features (maximum ability, technical mini-mums, order of commitment, fuel consumption, installationcost, operation and maintenance cost, fuel cost) and thewind hourly data are required.

2 Operation and architecture of the WPS

Different options related with the design and the operationof the WPS have been analysed, compared and evaluated[7]. As a result the features that are used in this paperinclude connection of the Wind farms with the pumpingstation through the central grid, peak demand supply ofthe hydro-turbine, consideration of the hydro-turbine as a

IET Renew Power Gener., Vol. 1, No. 1, March 2007

spinning reserve to increase the direct wind powerabsorbed, double penstock and complementary pumpingusing conventional power given the amount of conventionalunits’ spinning reserve.

2.1 Connection of the WPS with the electricalsystem

The connection of the wind farms with the PSU is proposedthrough the central grid, under the condition that thepumping loads are considered as deferrable loads. Thismeans that in case of wind loss or other stability problem,pumps are disconnected.

Additionally, wind power from the WPS can be directlyabsorbed by the electrical system, according to the technicalconstraints imposed by the electrical system operator(ESO). The amount of wind power that can be absorbeddirectly by the grid is dependent on the wind installedcapacity outside of the WPS, and on the allowed instan-taneous wind power penetration ‘d’. As a result, the windinstalled capacity outside of the WPS should be defined,before going on with the analysis of the WPS. The windpower absorbed in priority from these wind farms isdefined by the technical constraints described by theRegulatory Authority for Energy [3] and the WPS inte-gration will not effect their operation.

The pumping station should also be directly connected tothe main grid, in order to use surplus wind power in priorityand sometimes conventional power for complementarypumping.

In Fig. 1, the proposed structure and the interconnectionsof the whole electrical system after the WPS integration ispresented.

2.2 Operational target of the hydro-turbine

The objective of the hydro-turbine is to provide peakdemand supply. Since there are seasonal and daily upsand downs of the demand, and such an operational cycleof the hydro-turbine is needed to provide the financial feasi-bility, the daily (and not the weekly or the seasonal) peakdemand supply is used. So, the hydro-turbine is settinginto operation when the demand exceeds a predefinedlevel which is not stable during the year, but follows thedaily peak of the demand

LA ¼ ð1 � aÞ � Peak ð1Þ

where Peak is the last 24-h peak and a is the upper part ofthe demand to be covered by the hydro-turbine. The ESO,who monitors the electricity demand, is proposed to beresponsible for the assignment of the turbine’s set-pointto the WPS’s operator (WPSO), when the demand Dexceeds the LA. By this way the set-point of the turbine isdefined as

SPT ¼ D � LA ð2Þ

WPS (Hybrid system)

Conventional Units

Wind Farms

Pumped Storage/ Hydro turbine

LoadWind Farms out of the WPS

Fig. 1 Structure of the electrical system after the WPSintegration

The reliability of the WPS is measured by the hydro-turbine’s ability to correspond at the set-points. Therequired rated power of the turbine is defined by the aand the annual peak demand.

2.3 Conventional power for pumping

Conventional power is proposed for complementarypumping in order to provide the unfailing weekday oper-ation of the hydro-turbine and avoid the over-dimensioningof the reservoir. The use of conventional power for pumpingis going to increase the total demand and the requiredconventional production, so the available conventionalpower for pumping should be defined by the ESO. Forthis purpose, the assignment of a set-point by the ESO issuggested. The use of the available conventional electricityfor pumping is then decided by the WPSO considering thelevel of the water in the higher reservoir and the availabilityof the surplus wind power.

The preferable definition of the set-point for conventionalpumping takes into consideration the spinning reserve of thecommitted conventional units [7]. This definition permitsthe use of conventional power for pumping only in caseof conventional power surplus, so further conventionalunits are not committed to cover pumping loads. The advan-tage of this proposal is that the conventional units operationis improved and the available conventional power forpumping is widely distributed during the day. (Conventionalunits operate more efficiently in full load.)

2.4 Wind penetration

The ability of the conventional power stations to balance outboth the variability of the demand and the wind power,defines the wind power to be directly absorbed by thegrid. Wind power is absorbed in priority by the windfarms outside of the WPS system and then, if there is amargin for further wind power absorption, by the windfarms in the WPS system.

Before the simulation of the electrical system, the windpenetration which is allowed should be defined. A stablemaximum instantaneous wind penetration, as a percentageof the load demand is used (i.e. d ¼ 30%), while the windpenetration is further increased considering the hydro-turbine as spinning reserve. This operation presupposesthe two-sided communication between ESO and WPSO.Specifically, the ESO should know the amount of the hydro-turbine’s spinning reserve, which is dependent both on theturbine’s capacity and on the water availability in thehigher reservoir, in order to allow equal increase of windpenetration. Finally, the turbine should be committed,otherwise the required time between the possible windloss and the hydro-turbine response may cause a stabilityproblem in the autonomous electrical system.

2.5 Double penstock

Double penstock is used providing operational flexibilityand the direct quick response of the turbine when it isneeded. The same time that the stochastic and variablewind power production is curtailed due to the technicalconstraints imposed for the safe operation of the electricalsystem and pumps should be set into operation, thesystem may need the uninterrupted and scheduledproduction of the hydro-turbine. This can be achievedonly with the two penstocks.

3 Simulation

For the simulation, the design parameters of the plantshould be known: hydraulic head H, length LP and diameterDP of the penstock, capacity of the higher VH and lowerreservoir VL, rated power of the wind farms PW,R, PW,h,R

(with the index h the wind farms inside the WPS are distin-guished from the wind farms outside the WPS), rated powerof the turbine PT,R, number of pumps NP and rated power ofeach pump PP,R and the part of the peak to be supplied bythe turbine a. An overview of the basic calculations is pre-sented in the Table 1.

The main steps of the calculations are:

† For every time-step (i.e. 1 h) the water level in the higherLH and lower reservoir LL, the electricity demand D and thewind velocity U are known. The wind power production ofthe wind farms PW,h and PW is calculated given the windinstalled capacity PW,h,R and PW,R. For the first step ofthe simulation, an initial water level in the two reservoirsis needed. The independence of the results is proved usingdifferent initial water levels.† The hydro-turbine’s set-point SPT [(1) and (2)] and thenumber of conventional units committed NC are calculated

Table 1: Overview of the basic calculations

Input Calculation/assumptions Output

1 U a common power curve for the wind farms is used. PW,h, PW

2 D Equations (1) and (2)

constraint: the SPT cannot be lower than the technical minimums of the turbine,

i.e. TMT ¼ 30%PT,R. So if D2 LA , TMT then SPT ¼ 0.

SPT, LH, LH, VH, VL SRT ¼PT;R � SPT; if SPT . 0

0; if SPT ¼ 0

� �ð3Þ

constraints considered: -water availability in the higher reservoir / level of the

water in the lower reservoir

3 D, SPT a strict order of commitment for the conventional units is considered.

MAC � D � SPT ð4Þ

NC, TMC, MAC

4 D, TMC, d PAbsorbed ¼SRT þminfd �D;TMCg ð5Þ

PW!A ¼PAbsorbed; if PAbsorbed , PW

PW; if PAbsorbed � PW

� �ð6aÞ

PW!C ¼ PW �PW!A ð6bÞ

PW;h!A

PAbsorbed�PW!A ; if PAbsorbed�PW!A , PW;h

PW;h; if PAbsorbed �PW!A � PW;h

( ); if PAbsorbed . PW

0; if PAbsorbed , PW

9>=>; ð7aÞ

PW;h!C ¼ PW;h �PW;h!A ð7bÞ

PW,h!A, PW!A,

PW,h!C, PW!C

5 D, PW,h!A, PW!A, SPT SPC ¼ D � SPT � PW!A � PW;h!A ð8Þ SPCU

6 PW,h!C, SPC, MAC, SPP ¼ MAC � SPC ð9Þ

PP:Avail ¼ PW;h!C þ SPP ð10Þ

PP:Avail

7 LH, LL, VH, VL, PP,R, NP constraints considered: number of pumps and rated power of the pumping

station; level of the water in the higher reservoir/water availability in the

lower reservoir

PP:Final

8 PP:Final PP:W ¼ PW;h!R; PP:C ¼ PP:Final � PW;h!R;

PP:W ¼ PP:Fianl; PP:C ¼ 0;

�if ðPP:Final � PW;h!RÞ

if ðPP:Final , PW;h!RÞ

)ð11Þ

PP:W, PP:C

9 PP:Final operational curves of the pumps and the penstock. VL!H

10 SPT, LH, LL, VH, VL constraints considered: technical minimum of the hydro-turbine; water

availability in the higher reservoir/level of the water in the lower reservoir

PT,Produced

11 PT,Produced operational curves of the turbine and penstock. VH!L

12 LH, LL, VL!H, VH!L LH;END ¼ LH þ VL!H � VH!L ð12Þ

LL;END ¼ LL � VL!H þ VH!L ð13Þ

LH,END, LL,END

Note: In the case that SPTþ PW,h!Aþ PW!A . D, which could happen when aþ d . 1, an iterative is used before the required SPT andthe NCU are defined

by the ESO (4). The cumulative maximum ability of thecommitted conventional units MAC and the hydro-turbineshould be able to meet the demand even if all the windpower is lost, providing the safe operation of the electricalsystem.† Given the turbine’s set-point, the amount of the hydro-turbine’s spinning reserve SRT (3) is taken into consider-ation to increase the wind penetration. The SRT is calculatedgiven the hydro-turbine’s rated capacity PT,R, the turbine’sproduction SPT and the available water in the higher reser-voir LH. The ability of the electrical system to absorb windpower is increased by the SRT (5).† The wind power absorbed (PW,h!A, PW!A) andcurtailed (PW,h!C, PW!C) are calculated [(6a), (6b), (7a),and (7b)].† The conventional units set-point SPC (8) and the avail-able conventional power for pumping SPP (9) are calcu-lated. So, the total available power for pumping PP:Avail isderived (10).† The final power used for pumping PP:Final is constrainedby the rated power of pumps (PP,R, NP) the capacity ofthe reservoirs (VH, VL) and the available water. The partof the PP:Final which is derived from the wind farms or thegrid (PP:W, PP:C) is calculated (11). Then, the waterflow from the lower to the higher reservoir VL!H iscalculated given the operational curves of the pumps andthe penstock.† The desirable hydro-turbine production is defined by theset-points sent by the ESO. The final turbine’s power pro-duction PT,Produced depends on the availability of the waterin the upper reservoir. Then the water flow from thehigher to the lower reservoir VH!L is calculated given theoperational curves of the turbine and penstock.† Finally, at the end of each step, the level of the higherand lower reservoirs (LH,END, LL,END) are calculated [(12)and (13)].

When all the above calculations for all the time-steps(8760 h) are completed, then the following annual energyamounts are derived:

† desirable hydro-turbine’s production ESP_T;† final hydro-turbine’s production ET;† production of wind farms (EW, EW,h, EW,Total);† wind energy absorbed by the grid directly (EW!A,EW,h!A);† wind energy curtailed (EW!C, EW,h!C);† conventional energy available for pumping ESP_P;† total energy available for pumping (conventional andcurtailed wind energy) EP:available;† energy used for pumping: cumulative, conventional andwind energy (EP:final, EP:C and EP:W,h).

4 Evaluation criteria

The contribution of the WPS, together with economical andreliability indexes, are used to describe the performance ofthe electrical system after the WPS integration. The conven-tional units’ EPC EPCC, the EPCS and the EPCT are used todescribe the economic impact of the WPS to the electricalsystem. The most critical is the EPCS, when it is comparedwith the current cost, the resulting benefit, if any, from theWPS integration is defined. The EPCT is important for theprivate investor, indicating a first estimation of the requiredprice for the turbine’s electricity production which providesthe feasibility of the investment. Finally, the modificationof the EPCC due to the WPS integration is critical for the

ESO in order to accept this price. The autonomous Greekislands are excluded from the market liberalization andthe system operator remains the owner of the local powerstations.

The EPC of the turbine EPCT is defined under theassumption that the whole investment is considered as amean to provide guaranteed electricity supply duringpeak demand, so the wind energy sold in a fixed price isconsidered as inflow

EPCT ¼TICWPS � R þ OMCWPS þ CEC � BEW

ð14Þ

where TICWPS is the total investment cost, OMCWPS theoperation and maintenance cost of the WPS, CEC the costof conventional energy used for pumping and BEW thebenefit from the wind energy directly absorbed by thegrid. If the market price is pm, then

CEC ¼ pm � EP:C ð15Þ

and if the fixed price for wind power is pw, then

BEW ¼ pW � EW;h!A ð16Þ

The degree of accomplishment of hydro-turbine’s set-pointDA_SPT is the index to measure the reliability of the WPS.This index is highly dependent on the design of the WPS;namely the wind installed capacity, the capacity of thereservoir and the operational target of the turbine. It isdefined as the ratio of the actual energy production ET tothe desirable production defined by the hydro-turbine’s set-points (ESP_T)

DA SPT ¼ET

ð17Þ

The EPCC is defined as

EPCC ¼TICC � R þ OMCC

ð18Þ

where TICC is the total investment cost of the essentialconventional units, OMCC the operation and maintenancecost and EC the conventional energy production. TheOMCC has a fixed cost part, a variable cost part andthe fuel cost.

OMCC ¼ FixedCost þ VariableCost þ FuelCost ð19Þ

The EPCS is calculated, assuming that the redundant unitsare uninstalled

EPCS ¼TIC � R þ OMC

ETotal

ð20Þ

where TIC includes the cumulative investment cost of all thepower plants (essential conventional units, WPS and windfarms outside the WPS); the OMC includes the fixed cost,the variable cost and the fuel cost for the operation and main-tenance of the system; and ETotal is the total electricity demand.

The annuity factor R is defined as

1 � ð1 þ iÞ�Nð21Þ

where i is the discount rate and N the lifetime of theinvestment.

5 Dimensioning procedure

In fact, given the wind potential, the wind installed capacityis the parameter which defines the available for exploitationenergy. The part of this energy which can be directlyabsorbed by the grid, depends on the technical constraintsof the electrical system. The part of the surplus windenergy which can be finally exploited depends on the archi-tecture the design and the size of the WPS. For a given windinstalled capacity, there is a direct relation between the sizeof the reservoirs, and the maximum operational target thatcan be covered by the hydro-turbine with reliability reach-ing the 100%. The capacity of the hydro-turbine is thendefined by this target.

The aim of the proposed approach is to take comparableresults for the three islands, so the basic parameters in issueare introduced dimensionless. The wind installed capacity isdefined as percentage of the mean annual load demand,which is a fair way to levelize wind installed capacity indifferent in size systems [1]. Twenty different values forwind installed capacity are examined from 50 to 430% ofthe mean annual load demand by step 20% (20 cases).The capacity of the reservoir is defined from 10 to 150 bystep 20, times the maximum hourly water pumpingability (8 cases), namely as a function of the wind installedcapacity.

For each wind installed capacity and reservoirs capacity,the maximum operational target that can be achievedby 100% is calculated using an iterative procedure.That is to say, that the maximum turbine size which isjustified by the wind installed capacity and the reservoirscapacity should be defined. It is obvious that a biggertarget could be set, but it would be achieved in less than100% of the year. The target of the dimensioning is toguarantee the reliability of the turbine operation (achieveDA_SPT ¼ 100%), and conventional units are supposed tobe removed. So, the conventional installed capacity isredefined by the peak demand, the supply of the turbineand a safe margin of conventional installed capacity backup (20%).

For each different value of wind installed capacity, thereis an optimum value for the reservoir which provides thelower EPCT. By the investor’s point of view the target isthe maximum contribution with the least cost. So, from160 cases resulted – all of them provide the reliability ofthe turbine operation – the 20 cases with the maximumtarget achievement and the least cost comprise theoptimum solution (lower envelope curve of the EPCT’scurves) for the current island and scenario. An example isgiven for the island of Crete in Fig. 2, where the 20 differentcurves represent different wind installed capacity inside theWPS (153–1314 MW).

6 Assumptions

The following assumptions are considered.

† Hydraulic head between the two reservoirs H ¼ 300 m,and distance between them LP ¼ 3000 m.† Annual mean wind velocity: 8.1 m/s at the hub-height.(The same wind time-series are used in the three islands).† Financial evaluation without any subsidy.† Reference price for the oil 54$/b, which is the annualmean value for 2005.† Wind installed capacity out of the WPS is assumed to be70% of the mean annual load in Crete, 40% in Lesvos and60% in Serifos. This is the wind installed capacity whichprovides the minimization of the EPCS without storage

(unpublished). The WPS is introduced as a means tofurther increase the development of wind energy.† The diameter of the two penstocks is calculated given therated capacity of the pumping station and the hydro-turbine(Section 10).† Ten centrifugal pumps connected in parallel are used toprovide operational flexibility [7]. Given the availablepower for pumping the numbed of committed pumps iscalculated (Section 10).† The efficiency of the pumping station and hydrogeneration are analytically calculated for the differentpoints of operation (annex 2). The results show that thewhole efficiency of the plants (the ratio of the annualenergy produced by the hydro-turbine compared to theannual energy consumed for pumping) in the examinedcases varies between 55 and 69%.† For the estimation of the WPS investment cost, empiricalformulas have been used [7]. For the wind farms aninvestment cost of 1000 E/kW has been considered [11].An overview of the WPS’s cost estimation is presented inTable 3.† The calculation of the conventional EPC is based on theassumptions (Table 2) related with the installation cost, thefixed OMC, the variable OMC and the fuel consumption inthe various operational points.† The discount rate is considered i ¼ 5% and the lifetime Nof the investment 20 years for the wind applications and 30years for the conventional units.† Market price for the conventional electricity used forpumping pm is 0.035 E/kWh (low night tariff) or 0.09 E/kWh (regular tariff).† Fixed price for the wind production pw is 0.08 E/kWh.

20000000

40000000

60000000

80000000

100000000

120000000

140000000

160000000

0% 20% 40% 60% 80%

WPS share of the peak

153214275336

397458519580641703764825886947100810691130119112521314

0% 20% 40% 60% 80%WPS share of the peak

C (€

153214275

336397

458519580641

703764

88694710081069

11301191

12521314

Fig. 2 Example of the dimensioning procedure (Crete)

a Reservoir’s capacityb Turbine’s EPC

IET Renew Po

Table 2: Overview of the formulas and assumptions for the WPS cost estimation

Equipment, cost symbol Data/formula for cost estimation, E

Wind farms, CW 1000 /kW

Pumps, CP CP ¼ NP. C0,P

. (PP,rated/H P0.3)0.82, C0,P ¼ 1814

Hydro-turbine, CT CT ¼ C0,T. (PT,rated/H T

0.3)0.82, C0,T ¼ 4687

Reservoir, CR CR ¼ 420 . V 0.7

Penstock (CPenstock)

1:25 �XI

ðWM � pDI � eI � LÞ � CM|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}Material Cost

þðppI � LÞ � CI|fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl}Insulation Cost

1:5 �pD2

4� L

!� CE|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}

Excavation Cost

26666664

37777775

8>>>>>><>>>>>>:

9>>>>>>=>>>>>>;

Grid connection, CGC 4%�(CPþ CTþ CRþ CPenstock)

Control system, CCS 1.6%�(CPþ CTþ CRþ CPenstock)

Transportation of equipment, CT 2.4%�(CPþ CTþ CRþ CPenstock)

Personal, CP 30%�(CPþ CTþ CRþ CPenstock)

Others, CO 2%�(CPþ CTþ CRþ CPenstock)

Operation and maintenance, OMCWPS 2%�(CPþ CTþ CRþ CPenstockþ CW)

7 Results

7.1 Comparison between islands

The earlier described indexes are presented for the threerepresentative islands (Crete, Serifos and Lesvos) in Fig. 3.

† The Turbine’s EPC in Fig. 3a.† The Conventional units’ EPC in Fig. 3b.† The hybrid’s energy supply (%) in Fig. 3c.† The electrical system’s EPC (EPCC - E/kWh) in Fig. 3d.

Additionally, the EPCC in Figs. 3a and b and the EPCS inFig. 3d before the WPS integration are presented withstraight lines.

wer Gener., Vol. 1, No. 1, March 2007

The EPCT is expected higher in Serifos than in the otherislands due to the small size of the system. Far from that, thecurrent EPCS is higher in Serifos, providing higher competi-tiveness for the WPS. In Crete and Serifos, the system’sEPCS is decreased, thanks to the introduction of the WPSup to 40 and 20%, respectively, whereas in Lesvos it isincreased. In parallel, the EPCC is increased in Serifosand Lesvos, whereas in Crete is decreased or almostremains the same. This is justified because in Crete thenumber of conventional units is larger than in the otherislands and so the system is more flexible to meet thedemand more efficiently and to remove the redundantunits. Even in cases where the EPCC is expected to behigher, the public utility is going to earn the differencebetween its current cost (presented with straight lines in

Lesvos

Serifos

0.000.050.100.150.200.250.300.350.400.450.50

0% 20% 40% 60% 80% 100%WPS share of the peak

C (€

CreteLesvosSerifos

Lesvos

Serifos

0.000.050.100.150.200.250.300.350.400.450.50

PC (€

CreteLesvosSerifos

Serifos

Lesvos

0.000.05

0.100.15

0.200.250.30

0.350.40

0.450.50

(€/k

CreteLesvosSerifos

Fig. 3 Comparison of the three islands Crete, Lesvos and Serifos

a Turbine’s EPCb Conventional units’ EPCc Hybrid’s energy supplyd Electrical system’s EPC

Table 3: Assumptions of the conventional EPC’s calculation

Type of unit Installation cost,

Fixed OMC,

Variable OMC,

Fuel consumption in the

nominal point, g/kWh

Steam 1345 16 0.0011 221

Comb.cycle 662 22 0.0008 410

Gas 321 25 0.0007 310

Diesel 1272 9.4 0.0016 240

HFO 1345 9.4 0.0016 240

Diesel (small units) 2544 9.4 0.0016 225

Figs. 3a and b) and the price which has to pay to the WPSinvestor, which should be at least the EPCT. This means thatthe public utility will have the opportunity to buy cheaperelectricity instead of producing it with higher cost.Finally, the higher contribution of the WPS can be achievedin the case of Serifos.

In Fig. 4, the required cost per kilo watt of guaranteedpower and per kilo watt of installed capacity (wind andhydro-turbine’s capacity) is presented. The different sizeof the islands, the different size of the plants and the differ-ent cost of the various subsystems as it is introduced by theformulas in Table 2, give reasons for the differencesbetween the three islands. As a result the plant is expectedmore expensive in Serifos due to the small size (Figs. 4band 3a). Additionally, the different duration curve of thedemand in the three islands may effect the dimensioningof the plant and its cost per kilo watt of guaranteed power(Fig. 4a). For example in Serifos there is a very shortperiod of peak demand in summer and much lowerdemand in the rest of the year. Seasonal strong north

ty (€

Cretelesvosserifos

(€/k

Cretelesvosserifos

Fig. 4 Comparison of the cost of the WPS plants in the threeislands Crete, Lesvos and Serifos

a Per kilowatt of guaranteed powerb Per kilowatt of installed capacity (wind and hydro-turbine)

winds called ‘meltemi’ during summer justifies a biggerturbine – in relation to the rest components (wind capacityand reservoir) – and higher energy and peak supplyachievement occurs.

In Table 4 the required wind installed capacity, thedimensioning of the WPS (reservoir, turbine) and the peakand energy supply achieved are presented. Regionalfeatures – for example, the existence of a reservoir or afavourite site of installation with bigger hydraulic head orhigher wind potential – may further improve the feasibilityof the plant. Additionally, the geographical distribution ofwind farms in a large system may decrease the requiredcapacity of the reservoir for the same target achievement.

7.2 Sensitivity on the allowed instantaneouswind penetration

In this paragraph, using the reference values for the windpotential (mean annual wind velocity 8.1 m/s) and for theoil price (54$/b), the sensitivity of the results on theallowed instantaneous wind penetration is examined. Twodifferent values of the allowed instantaneous wind pen-etration (40 and 50%) are introduced in comparison withthe basic value (30%).

The effect of the allowed instantaneous wind penetrationon the results is not so important, as it is in case withoutstorage (unpublished). This means that the feasibility ofthe WPS is proved even if the technical constraints of thesystem remain strict (Figs. 5–7). The reason is that theconsideration of the turbine as a spinning reserve allowswhether or not the increase of the directly absorbed windpower.

7.3 Sensitivity on the oil price

The EPCS with (straight lines) and without the WPS inte-gration are compared for three different oil prices (54, 75and 100$/b). As it is expected, in Crete and Serifos(Figs. 8 and 9) the highest the WPS penetration, thelowest is the dependence of the EPCS on the oil price. Inthe case of Lesvos (Fig. 10) although with the current oilprice the integration of the WPS may not decrease thesystem’s EPC, the situation is changed as soon as the oilprice is increased. This means that even in cases, wherethe integration of the WPS does not provide a lower costtoday, it reduces the risk to deal with higher costs in thenear future due to a probable increase of the oil price.Consequently, the dependence of the EPC on the oil pricecan be significantly reduced with the WPS integration.

7.4 Financial evaluation

The price for the electricity produced by the turbine is notstrictly defined, so there is an uncertainty in the financialevaluation of the investment. The new law 3468/2006 in

Table 4: Proposed dimensioning in the three case-studies islands

Island Peak, MW Wind capacity, MW Reservoir, †106 m3 Turbine, MW % Peak supply % Energy supply

Crete 563 825–1314 60–120 310–405 55–75% 46–72%

Lesvos 58 84–145 4.4–14 30–46 57–81% 51–79%

Serifos 2.9 2.2–3.6 0.2–0.3 2.3–2.9 75–100% 68–83%

0.000.02

0.040.06

0.080.100.12

0.140.16

0.180.20

(€/k

withoutWPS

30%40%50%

Fig. 5 Sensitivity of the results in Crete on the allowed instan-taneous wind penetration (8.1 m/s, 54$/b)

a Hybrid’s energy supply (%)b Electrical system’s EPC (E/kWh)

Lesvos

(€/k

withoutWPS

Lesvos

0%10%20%

30%40%50%60%

70%80%90%

30%40%50%

Fig. 6 Sensitivity of the results in Lesvos on the allowed instan-taneous wind penetration (8.1 m/s, 54$/b)

Greece proposes that this price should be defined, respect-ively, to the average variable cost of the conventional elec-tricity production in the autonomous electrical system in theperiod of the license issue. In Figs. 11–13, the requiredprices to provide an Internal Rate of Return (IRR) 8, 12and 16% to the investors in the three islands are presentedunder the following assumptions.

† Own capitals: 50%.† Subsidy: 0%.† Loan: 50% with discount rate 6% and 10 years paymentperiod.† Lifetime of the project 20 years and residual value 35%.† Tax 25%.

Serifos

h) 30%

withoutWPS

Serifos

20%30%40%

50%60%70%

80%90%

30%40%50%

Fig. 7 Sensitivity of the results in Serifos on the allowed instan-taneous wind penetration (8.1 m/s, 54$/b)

100$/b

h) 54$/b

100$/b

withoutWPS

Fig. 8 Sensitivity of the electrical system’s EPC in Crete on theoil price with and without WPS integration (8.1 m/s, d ¼ 30%)

Lesvos

100$/b

0.000.020.040.060.080.100.120.140.160.180.20

h)54$/b

100$/b

withoutWPS

Fig. 9 Sensitivity of the electrical system’s EPC in Lesvos on theoil price with and without WPS integration (8.1 m/s, d ¼ 30%)

Serifos54$/b

100$/b

0.000.05

0.100.150.200.25

0.300.350.40

0.450.50

(€/k

Wh) 54$/b

100$/b

withoutWPS

Fig. 10 Sensitivity of the electrical system’s EPC in Serifos onthe oil price with and without the WPS integration (8.1 m/s,d ¼ 30%)

Fig. 11 Required price for the electricity produced by the hydro-turbine to provide IRR ¼ 8, 12 and 16% in Crete (8.1 m/s,d ¼ 30%)

Fig. 12 Required price for the electricity produced by the hydro-turbine to provide IRR ¼ 8, 12 and 16% in Lesvos (8.1 m/s,d ¼ 30%)

The results show that in Crete and Serifos the investmentsare feasible, since the required price is competitive to theEPCS and the fuel cost of the system before the WPS inte-gration. The situation is different for Lesvos, where thefeasibility of the investment is marginal.

8 Conclusions

With the introduction of the WPS, the wind penetration inautonomous systems can be increased, simultaneouslydecreasing the system’s EPC. As a consequence, the oper-ation of conventional units and their required installedcapacity can be significantly reduced.

The financial benefit from the introduction of the WPSshould be shared between the ESO and the investor, bythe definition of a suitable price. The main parameterswhich should be taken into consideration for the definitionof the suitable price are the size of the plant, the size ofthe island, the current cost of the system and the durationcurve of the demand.

Another important advantage of the WPS integration isthat the production cost is to a large extent known inadvance, contrary to the current cost which dependsstrongly on the oil price. Thus the installation of WPS canprovide both financial and environmental benefits and isstrongly recommended.

9 References

1 Caralis, G., and Zervos, A.: ‘Assessment of the wind penetration inAutonomous Greek islands’. 3rd National Conf. for RES, RENES,February 2005, Athens, pp. 23–25 (in Greek)

2 Papathanassiou, S., and Boulaxis, N.: ‘Power limitations and energyyield evaluation for wind farms operating in island systems’, Renew.Energy, 2006, 31, (4), pp. 457–479

3 Regulatory Authority of Energy: ‘Methodology for the assessmentof wind penetration in non-interconnected islands’ Availableat: http://www.rae.gr/k2/ape-penetration.pdf, http://www.rae.gr/K2/deliberation-ape.html (in Greek), accessed February 2003

4 Katsaprakakis, D., and Christakis, D.: ‘The Wind power penetration inthe island of Crete’. RES & RUE for Islands Int. Conf., Cyprus, 30–31August 2004

5 Zervos, A., Caralis, G., Gorgoulis, M., and Zografakis, N.:‘Implementation plan for the large scale deployment of RES inCrete-Greece’. Altener 2000 Conf., 2000, Toulouse-France

6 Boulaxis, N.G., and Papadopoulos, M.P.: ‘Assessment of thecontribution of hybrid systems in renewable energy penetration inislands’. ISAP 2003, Lemnos Greece, 31 September, PaperISAP03/141

7 Caralis, G., and Zervos, A.: ‘Prospects of wind and pumped storagesystems’ integration in Greek islands’. EWEC 2006, Athens, GreeceAvailable at http://www.ewec2006proceedings.info/allfiles2/457_Ewec2006fullpaper.pdf

Fig. 13 Required price for the electricity produced by the hydro-turbine to provide IRR ¼ 8, 12 and 16% in Serifos (8.1 m/s,d ¼ 30%)

8 Kaldellis, J., Kavvadias, K., and Vlachou, D.: ‘Electricity loadmanagement of APS using wind-hydro solution’. Proc. MedPower,2002, Athens, Greece

9 Katsaprakakis, D., Betzios, G., and Christakis, D.: ‘combinedmanagement of water sources and wind energy in Lesvos – Is thetarget of 100% renewable electricity supply in Lesvos possible?’.17th Conf. Pan-Hellenic Network of Ecological Organizations,Lesvos-Greece, 7–9 October 2005 (in Greek)

10 Theodoropoulos, P., Mantas, Z., Zervos, A., and Betzios, G.:‘Integrated power system of Serifos Island with high RESpenetration using pump-storage’. Int. Conf., RES for Island Tourismand Water, 2003, EREC, Crete-Greece

11 EWEA. ‘wind energy the facts: an analysis of wind energy in theEU-25’ February 2004

12 Manolakos, D., Papadakis, G., Papantonis, D., and Kyritsis, S.: ‘Asimulation-optimisation programme for designing hybrid energysystems for supplying electricity and fresh water throughdesalination to remote areas. Case study: the Merssini village,Donoussa island, Aegean Sea, Greece’, Energy, 2001, 26,pp. 679–704

10 Appendix

10.1 Pumps–hydro-turbine simulation

The problem of the simulation of the pumping station can bedescribed as ‘Which is the required power to commit ipumps and which is the flow of the water’, and in thecase of the hydro-turbine’s simulation as ‘Which is therequired volume of the water to produce the requiredpower’.

The pump and turbine characteristics (Head-flow curveand efficiency-flow curve) are introduced dimensionless(Figs. 14, 15). The presented curves were assumed as repre-sentative for a wide range of centrifugal pumps and Peltonturbines with different nominal characteristics [12].

The operation point (Fig. 16) of the pumping station iscalculated as the intersection of the pumps characteristic(H, Qi), where i is the number of pumps in operation andthe pipeline characteristic HP(Q).

HPðQÞ ¼ H þ z � Q2

ð22Þ

where z is the loss coefficient of the penstock, H the hydrau-lic head and Q the water flow.

The operation point of the hydro-turbine is calculated asthe intersection of the hydro-turbine characteristic (H, Q),and the pipeline characteristic HP(Q).

HTðQÞ ¼ H � z � Q2

ð23Þ

The loss coefficient is calculated

z ¼ l �L

2gA2ð24Þ

where, l (The loss coefficient and the dimensionless losscoefficient are calculated for the different water flows.) isthe dimensionless loss coefficient, LP the length of the pen-stock, DP the diameter of the penstock and A the area of thepenstock. For the calculation of the l, the formula of theColebrook and White is used

1ffiffiffil

p ¼ �2 log2; 51

Re �ffiffiffil

p þ1s

� �ð25Þ

where 1s is the relative roughness of the penstock’s wall1s ¼ 1/D and 1 the absolute roughness.

A short iterative procedure is used to solve the abovecomplex formula. The Reynolds is defined as Re ¼ c . D/v,where c is the water velocity in the penstock c ¼ Q/Aand v is the cinematic viscosity of the waterv ¼ 1.31 � 1026 (m2 . s21) for water temperature 108C.

0% 50% 100% 150% 200%Qdimensionless

η dimensionless

0% 50% 100% 150% 200%Qdimensionless

Hdimensionless

Fig. 14 Dimensionless efficiency-flow and head-flow curve for arepresentative centrifugal pump

0% 20% 40% 60% 80% 100% 120%Qdimensionless

ηdimensionless

Fig. 15 Dimensionless efficiency-flow curve for a representativePelton turbine

Fig. 16 Definition of the operation point of the pumping station

Definition of the Penstock’s diameter: The diameter ofthe two penstocks for the turbine DP,T, and the pumpingstation DP,P is calculated, assuming a maximum permittedwater velocity Vmax, which occur for the maximum waterflow Qmax. The following equations are applied for the pen-stocks of the pumping station and the hydro turbine.

DP ¼4Qmax

ðVmax � pÞ1=2

ð26Þ

Vmax ¼ 0:125 � ð2gHÞ0:5

ð27Þ

In the case of the pumping station, the maximum water flowQmax,P is defined by the number of pumps and the water

flow of each pump in the nominal point of operation QP,R

Qmax;P ¼ NP � QP;R ¼ NP �r � g � hP � NP;R

Hð28Þ

where hP ¼ 0.85 is the efficiency of the pump in thenominal point.

In the case of the hydro-turbine, the maximum water flowQmax,T is defined by the hydro-turbine

Qmax;T ¼ 1:3 � QT ;R ¼ 1:3 �r � g � NT;R

hT � Hð29Þ

where hT ¼ 0.90 is the efficiency of the hydro-turbine in thenominal point.

Analysis of the combined use of wind and pumped storage systems in autonomous Greek islands

Documents

Transcript of Analysis of the combined use of wind and pumped storage systems in autonomous Greek islands

ST.JOSEPH'S COLLEGE (AUTONOMOUS) IRINJALAKUDA

Solor Islands

Hosts as islands

Dual-wavelength bidirectional pumped high-power Raman ...

SOLOMON ISLANDS - GFDRR

The Philippine Islands

Installation and operating instructions pumped electric shower

Study on the impact of Kundah Pumped Storage Hydro ...

National Workshop on Pumped Storage Hydropower Projects

The Problem with Islands; Comparing Mysticisms in Iceland and Faroe Islands

Challenges for pumped hydropower plants in the future grid ...

Spatiotemporal structures in the internally pumped optical parametric oscillator

Numerical and Experimental Analysis of Remotely Pumped Dual-Function EDFA

Untitled - Maritime Cook Islands

OGASAWARA ISLANDS

A.V.V.M. SRIPUSHPAM COLLEGE (AUTONOMOUS)

autonomous - audisankara.ac

Encyclopedia of the Islands

SEVEN ISLANDS - SEBI

ANDHRA LOYOLA COLLEGE (AUTONOMOUS ...