Performance and Economic Study of Interconnected PV System With

8/14/2019 Performance and Economic Study of Interconnected PV System With

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Faculty of Engineering, Elminia University,

Elminia, Egypt

Performance and Economic Study ofInterconnected PV System with Electric

Utility Accompanied with Battery Storage


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This paper presents a complete study from economic and

reliability point of view to determine the impact of

interconnecting PVPS with electric utility, EU taking into

consideration battery storage, BS, all radiation and variation of

the load demand during the day. Three different configurations of

PVPS have been investigated. The first PVPS is stand-alone

PVPS accompanied with BS. The second PVPS is PVPS

interconnected with EU without BS. Grid-connected PVPS

accompanied with BS represents the third alternative.

Object of this paper


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A comparative study between these three different configurations

has been carried out from economic and reliability points of view

with the main goal of selecting suitable one, to be installed atZafarna site to feed the load requirement. A simulation is carried

out over one year using the hourly data of the load demand,

insolation and temperature at Zafarna site, Egypt as a case study.

System ConfigurationThe electrical power generated and terminal voltage of PV module

depends on solar radiation and ambient temperature. The

equivalent electrical circuit describing the solar cells array used inthe analysis is shown in Fig. 1.


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Fig.1 Equivalent circuit of PV solar cells array.

The mathematical equation describing the I-V

characteristics of a PV solar cells array is given by:


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where, I(t)- is the output current, Amp, V(t) is the output voltage,

Volt, A is the ideality factor for p-n junction, T(t) is the

temperature, Kelvin, K is the Boltzman's constant in Joules per

Kelvin, and, q is the charge of the electron in Coulombs. Rsh is

the internal shunt resistance, and Rs is the series resistance.

Io(t) : The hourly reverse saturation current, Amp. This current

varies with temperature as follows:

shR

sR*I(t))t(V

)t(+

+

=

1

T(t)KA

sR*I(t)V(t)q

expoIph(t)II(t)

( ) ( )

= 1/T

r1/T

I/K

goE*qexp

3

rT(t)/TorI)t(o

I


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Iph(t) : The hourly generated current of solar cells module. This

current varies with temperature according to the following equation:

where, Tr is the reference temperature, oK, Ego is the band-gapenergy of the semiconductor used in solar cell array, KI is the

short circuit current temperature coefficient, and, Ior is the

saturation current at Tr, Amp. (t) is the average hourly

radiation on the tilted surface, kW/m2. Isc is the PV cell short-

circuit current at 25o C and 100 mW/cm2.

The output of the solar cell module can be calculated by the

following equation:

100/)t(T

H*298)))t(T(I

Ksc(I)t(

phI +=


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)t(I*)t(V)t(out,pv

P =


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The energy balance between the load and the output of PV system must be

carried out to compute the optimum number of PV modules, Npv. The

output power from PV system must satisfy the load power demand. The

hourly generated power, Ppv,out(t), and hourly load power, PLoad(t), are

compared with each other. If Ppv,out(t) is larger than the load power

demand then there is an hourly surplus power, but if Ppv,out(t) is smaller

than the load power demand then there is an hourly deficit power. At any

value of Npv, if the summation of hourly surplus power equals to thesummation of hourly deficit power then this value of Npv represents the

optimum number of PV modules. The following equations have been used to

get the optimum number of PV modules [7].

Calculation of Optimum Number of PV Modules


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08760t

1t

)]t(Load

P)t(out,pv

P*pv

N[=

=

>IF

Then, number of PV modules must be decreased by one module

and repeating the foregoing process[7]:

08760t

1t

)]t(LoadP)t(

out,pvP*

pvN[=

=


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Then, Npv is the optimum number of PV modules satisfies the

energy balance condition.2-2 Sizing of BS

The size of battery bank in a stand-alone PVPS, PVPS

interconnected with EU accompanied with or without BS is

dependent on three main factors [8], [9]:-

The number of hours that the battery bank should provide the

critical load without input from PV system or EU.

The depth of discharge of the battery strongly depends on the

applications, and can vary from a few percent to as much as

80%. i.e. the energy can be taken from BS equal to 80% of itscharge.

The total ampere of the critical load current.


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For stand-alone PVPS, the battery storage capacity can be

found as follows:

( ) )*bat

/()deficit(Min)Surplus(MaxPV

BS =

For PVPS interconnected with EU, the size of battery bank can

be expressed by the following Equation:

)*bat

Tcri/(*criticalL,

PEU

BS =Where;

PL,critical : The maximum critical load in the period Tcri. Tcri:The number of hours that the battery bank should provide the

critical load without input from PV system or EU. :The depthof discharge of the battery in percent. bat: The efficiency ofBS.


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The major concern in the design of an electric power system that

utilizes renewable energy sources is the accurate selection of

system components that can economically satisfy the load

demand. The system's components are found subject to [10]:

1. Minimize the cost of electricity production ($/kWh).2- Ensure that the load is served according to a certain reliability

criteria.

3- Minimize the power purchased from the grid.

To evaluate the economics of the PV system, a Life Cycle Cost,

LCC methodology has been applied to obtain the ECF over life

span of PV system.

Calculation of Energy Cost Figure


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The ECF is derived as the ratio of annual payment and annual

energy output of PVPS and it can be written as follows:

PVPSofoutputEnergy/APECF =Calculation of Reliability

Reliability issue has been considered as an important step in any

system design process. Generally, reliability is defined as the

probability that an item of power system component will performits specified function without failure for a specific period of time or

number of cycles [12].


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Figure 2-a represents the layout block diagram of

PV/EU accompanied with BS, while Fig. 2-b shows

the reliability block diagram of grid-connected PVPS

system accompanied with BS. From Fig. 2-a and Fig.2-b the reliability of each configuration under study

can be calculated as follows:


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U t i l i t yI n t e r f a c e

z

PVarray

(A)

T.L.(d)

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C.B.(a)

Trans..

(b) C

.B

.(c)

Generation

Station(G)11

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C.B.(e)

Trans..

(t)C.B.(f)C.B.(h)

Trans..

(g)

Subsyste

m(1)

Subsyste

m(1)

Radiation

(I)

Inverter

(D)

C.B

.(E)

Trans..

(F)

Trans..

(O)

0.4/11

Distrib

Panal(L o a d

C.B.

(M)

C.B.(N)

C.B.(J)C.B.(K)

T.L.(

L)

Volatge

Regulator(V)

C.B.(B)

(S)

ControlUnit

(C)

K

L

M

N

O

H

L o a d

G

a

b

c

d

e

t

h

z

y

REU

I

A

B

C

D

E

F

J

V

S

P o w e rS w i t c h

y

Fig. 2

a : Block Diagram of

PVPS/EU accompanied

with BS.

b : System Reliability Block

Diagram of PVPS/EU

accompanied with BS.


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Reliability of Stand-alone PVPS.

From Fig. 2-a & Fig. 2-b the reliability of PVPS accompanied with

BS can be calculated as follows:-

[ ]3

R)2

R1)(1

R1(1BS/PV

R =


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Where;

R1=RI RA RB , R2=RI RA RB RV RS , R3= RC RD RE RF RJ

RK RL RM RN RO RH,RI : Reliability of the Irradiance= Probability(I Imin), RA :Reliability of the PVPS array, RB : Reliability of circuit breaker=

RE=RJ= RK= RM=RN= Ra= Rc= Re= Rf= Rh , RV : Reliability

of the Regulator, RS : Reliability of the Battery storage, RC :Reliability of the DC switch, RD : Reliability of the inverter, RF :

Reliability of the transformer= RO= Rb = Rt= Rg, RL :

Reliability of the transmission line, RH : Reliability of the

distributed panel.


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Reliability of PVPS interconnected with EU.

From Fig. 2-a and Fig. 2-b the reliability of PVPS interconnectedwith EU can be calculated as follows:-

g3

R)

g2

R1)(

g1

R1(1

EU/PV

R

=

Where;

R1g= RG Ra Rb Rc Rd Re Rt Rh Rz Ry

R2g = RI RA RB RC RD RE RF RJR3g = RK RL RM RN RO RH

Rz: Reliability of power switch, Ry: Reliability of utility

interface.


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Reliability of grid-connected PVPS accompanied withBS.

Also, from Fig. 2-a and Fig. 2-b the reliability of PVPS

interconnected with EU accompanied with BS can be

calculated as follows:-

[ ] 4R)g1R1)(PVBSR1(1BS/EU/PVR =


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1.Applications and Results

A new proposed computer program has been used based on the

above methodology for calculating optimum design of stand-

alone PVPS with BS, PVPS interconnected with EU without BS,and PVPS interconnected with EU accompanied with BS.

The input data of this program are:

1. Hourly radiation, kW/m2.The average radiation of the horizontal surface and the ambient

temperature for the selected site has obtained from the Egyptian

Metrological Authority for Zafarna site.

2- Characteristics of PV module ASE-300-DGF/17 type [14].

3-Site latitude, Degrees.

Zafarna site is located on the western coast of the Seuz Gulf,

latitude 29.07o N and longitude 31.36o E, Egypt.

4- Hourly load demand, kW.


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The price of selected components.

Price of PV solar module =$2/Wp, Price of Batteries=$120/kWh,

Price of Inverter= $400/kW, Price of Regulator= $100/kW, Price

of electrical Equipment = 10% of PV modules cost, Price of

Civil work = 10% of PV modules cost, Annual fixed cost of

O&M of PV=1% of Pv system cost [11].

Economic assumptions as follows:

Interest rate of 8%, General inflation 3%, Real discount

rate=interest rate-General inflation=5% [11].


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1- Optimum number of PV modules.

The outputs of this program are:

The output of the proposed computer program is the optimum

total number of PV modules, number of series modules per string,

and finally number of inverter units. Total Number o

Modules=14760, Number of series Modules/string=20, Number

of parallel strings/subsystem=18, Number of Modules

/subsystem=360, Number of inverter units=41 with its rating o

100kW.


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Battery Sizing

A battery with capacity of 1312 Ah and nominal voltage of 24V

was used in this study.BS capacity for stand-alone PVPS

The BS capacity for stand-alone PVPS can be found by using Eq.

(8) as follows:

( ) Mwh53.70.8)*/(0.8)948.14(421.19PVBS ==BS capacity for PVPS Interconnected with EU

The size of BS for PVPS interconnected with EU can be found

from Eq. (9) by changing the critical load PL,critical during one

hour from 0% to 100% of maximum load demand. (ba=80% ,

=80%). Table (1) shows the relation between BS capacity for

PVPS interconnected with EU and critical load.


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Table (1) Economic of PVPS/EU Under Different Values of BS

ECF Cent/kWhCost of BS, $BS capacity, kWhP

L,critical=

% Pmax-LoadkW

9.35050000%

9.380637500312.520010%

9.404667500562.536018%

9.41067500062540020%9.4407112500937.560030%

9.4707150000125080040%

9.50071875001562.5100050%

9.53082250001875120060%

9.56082625002187.5140070%

9.59083000002500160080%

9.62093375002812.5180090%

9.650937500031252000100%


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3- Cost of kWh generated, $/kWh.The generation cost of the system is estimated based on the

methodology of LCC. The PVPS output annual energy is equal to

9,370,269 kWh. As a result of the estimation, the ECF is calculatedas 15.7967 cent/kWh for stand-alone PVPS, 9.3505 Cent/kWh for

PVPS interconnected with EU without BS. Table (1) shows the

relation between ECF and BS capacity.

From this Table, it can be seen that the ECF increases linearly

against BS capacity. Also, BS capacity is increased linearly with

critical load. Grid failure and low radiation produces 1.10 %

losses of annual energy from PVPS. This value has been obtained

from a new computer program. If we convert these losses to

money we will buy a BS by this amount of money.

The cost of 1.10% losses of annual energy from PVPS can be

found as follows:


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Cost_loss=0.011*9,370,269*0.093505*7= $67465.20

The BS capacity purchased by this money =67465.20/120=562.21 kWh. Assuming life span of BS is 7 year.

So, the optimum BS accompanied with PVPS interconnected

with EU equal to 18 % of maximum load demand or 30% of

average load demand.

4- Reliability Calculation

As a result of computer program, the reliability of eachcomponent of the system under study is calculated and tabulated

in Table (2).


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Table (2) The Calculated Reliability of Each Component

of PVPS/EU/BS

Reliabilityaverage=10-6Component

0.987383562716RA

0.9999100043R

B= R

E= R

J= R

K= R

M

= RN

= Ra= R

c= R

e= R

h

0.999971RC

= RD

0.9999850.5R

F= R

O= R

b= R

t

RL=R

d

0.9994001799200RH

0.99790220370RV

0.9999760.8RS

0.99850112450RG

0.999971Rz

0.9999940.2Ry


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Comparative Study Table (3) reveals the comparative study between stand-alone

PVPS, PVPS interconnected with EU without BS and Grid-

connected PVPS with BS combinations. From this table, it can

be seen that the reliability of the grid-connected PVPS with

BS not only is higher than that of stand-alone PVPS with

1.30222905% but also is higher than that of PVPSinterconnected with EU without BS with 0.0191590% w.r.t.

the reliability of the later. On the other hand, the ECF o

stand-alone PVPS not only is higher that of PVPS/EU with

68.939% but also is higher that of PVPS/EU/BS with67.967%w.r.t. the ECF of the later. Then, it can be concluded

that the most techno-economical combination is PVPS

interconnected with EU for Zafarna site.


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Table (3) Comparative Study between Stand-alonePVPS, PV/EU and PV/EU/BS

PVPS /EU/BSPVPS/EUStand-alone

PVPSCombinations

0.999075450.998884070.986232446Reliability

9.40469.350515.7967

ECF,

Cent/kWh


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Conclusions

This computer program can be applied in any site of the world. In

this paper, the computer program has been applied to study

different configurations of PVPS to feed a load of 2MW located at

Zafarna site, Egypt as a case study. The following are the salient

discussions and conclusions that can be drawn from this study:-

This paper presents a new complete study from economic and

reliability point of view to determine the impact of interconnecting

PVPS with EU taking into consideration BS all radiation and

variation of the load demand during the day. Three different

configurations of PVPS have been investigated. A new computerprogram based on Matlab software introduces in this paper


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1- Using of BS with the configuration of PVPS/EU has saved

about 1.1% of the system energy losses.

2-The optimum size of BS accompanied with PVPSinterconnected with EU equal to 18 % of maximum loaddemand or 30% of average load demand.3-The ECF of stand-alone PVPS was 15.7967 Cent/kWh, 9.3505

Cent/kWh for grid-connected PVPS and 9.4046 Cent/kWh for

PVPS/EU accompanied with BS.

4-The PVPS/EU interconnected with BS has a higher reliability

level than that of stand-alone PVPS or Grid-connected PVPS

without BS.5-The price of kWh produced from PVPS has been decreased by

full utilization of PVPS energy through the year using BS.

Performance and Economic Study of Interconnected PV System With

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