Photovoltaic energy system

International Conference on Renewable Energies and Power Quality (ICREPQ’14)
Cordoba (Spain), 8th to 10th April, 2014
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ISSN 2172-038 X, No.12, April 2014
Photovoltaic energy system
A. Mohamed CHERFI, F.Z Zerhouni
1
Department of Electrical Engineering
University of Sciences and Technology Mohamed Boudiaf (U.S.T.M.B.O)
BP 1505, Oran El M'Naouer, Oran, Algeria
e-mail: [email protected],
[email protected]
Abstract.Studies proved that photovoltaic generators
cannot deliver maximum power constantly.Therefore, it is
necessary to insert an adaptation stage between the generator
and the load.In this paper we studied the various influential
conditions on the electric characteristics of the photovoltaic
generator. We also proposed an adaptation stage: a static
converter of a step-up structure (boost).
The voltage generatedmay dependon the materialusedfor
manufacturingthe cell and on various factors;
temperature,
insolation....The
combination
in
severalPVcells
gives
a
serial/parallel
of
photovoltaicgenerator (GPV).
3.
Modellingof a photovoltaic cell
Keywords
APVcell canbe modelledfromequation (1) definingits
staticbehaviour.
Photovoltaic, converters, boost, maximum power point.
This equation considers the short-circuit current strongly
varied withirradiance and different resistances
modelling losses due to connections.
1. Introduction
Nowadays, the energy consumption worldwide is
increasing to the point that our modern society would be
hard if it should go without.
Most of this power consumption comes from fossil
fuels.The massive use can lead.The rarefaction and the
exhaustion of these reserves is a reality. There is an
imminent danger induced by the changes mainly
through pollution and global warningby the rejections of
gases with greenhouse effect [1].
Indeed, today, the principal problem is the reduction of
the conventional energyconsumption for the adaptation
of renewable energy.
2.
Photovoltaic energy
Photovoltaic (PV) solarenergy isthedirect conversion
ofaportion of solar radiationto electrical energy.
This energy conversion is carried out through a
photovoltaic (PV) cell based on a physical phenomenon
called the photovoltaic effect to produce an
electromotive force when the surface of the cell is
exposed to light.
Thus, in static, the behaviour of a PV cell consisting of a
PN junction based on silicon can be described by the
following equation [2]:
Vcell+Icell.Rs
Vcell+Icell.Rs
Icell=Icc-I0[exp⁡(
A.Vt
)-1]-(
Rp
)
(1)
Where I0 (A) is the saturation current, Vt (V) the
thermodynamic potential, A is the ideality factor, ICELL
(A) isthe cell terminal current, VCELL (V) is the terminal
voltage (V), ICC (A) is the short-circuit current of the
cell, Rp (Ω)is the shunt resistance characterizing the
leakage currents of the junction and RSis (Ω) is the
series resistance representing various resistances of
contacts and connections.
Figure 1 illustrates the electric equivalent sheme of a PV
real cell.
Fig.1 The equivalent circuit of a real PV cell
Figures 2 and 3 show current-voltage and power-voltage
characteristics obtained at STC.
Equation (1) is the expression ofthe cell voltage
according to the current:
Vcell=-Rs.Icell + A.Vt.Ln[
Icc-Icell+I0
I0
](2)
4. Electric characteristics of a photovoltaic
module
For modelling,we chose PV KyoceraK51LA361 cells.
ThetableI showsthe various parameters of this module at
reference conditions"STC" (Standard Test Conditions).
Table I. –Module parameters
5.
Influence
of
temperature
and
irradiance on the GPV’s characteristics
Temperature and sun lightare the keyparametersfor
determining
the
generator
photovoltaic
cells
GPV’selectrical characteristics.
To see theinfluence of these twoparameters on theGPV
characteristics,we plotted the GPV’selectrical curves
fordifferentirradiances
(T=
25°
C)and
differenttemperatures(E=1000w/m2) using the following
equations [3]:
E2
Parameters
Values
Voc(v)
21
Icc(A)
3.25
Vopt(v)
17
Iopt(A)
3.02
Ns
36
Pm(w)
51
I2=I1+Icc. -1 +T2-T1.α0(3)
E1
V2=V1+T2-T1.β-Rs.I2-I1-Kc.I2(T2-T1)
(4)
V2, I2, E2, T2 arerespectivelyoutput voltage, output
current,
solar
radiation,
and
temperature
atdesiredconditions.
V1, I1, E1 and T1 arerespectivelyvoltage, output
current, solar radiation, and temperature at
measuredconditions.
Iccis theshort circuit currentat referenceconditions.β is a
coefficient taking into accountthe temperature
effectonthe voltage(β =-7.8* 10-2V / °C).
α0 is a coefficient taking into accountthe temperature
effect onthe current (α0 =1.6 *10-3A / °C). Rs is
theseriesresistance.
Kcis thecorrection factorcurve(Kc=5.5 *10-3Ω / °C).
A. The irradiance effect
Results obtained for the GPV’s electric characteristics,
versus the irradiance are:
Fig.2 Current-voltage characteristics
Fig.4 irradiance effect on the current-voltage characteristics
Fig.3 Power-voltage characteristic.
6. The maximum power point
There is aparticular point PPM on the I(V) curve, At this
point, power P is maximum. It iscalled optimalPower .
The coordinatesofthis point areVopt andIopt (Figure 8)[4].
Fig. 5irradianceeffect on the power-voltage characteristics.
When we vary irradiance at constanttemperature, the
short-circuit current and the maximum power vary
proportionally with irradiance. Open circuit voltage
remains almost constant.
B. The temperatureeffect
The temperaturehas amajor influence onthe electrical
characteristics of the GPV.
Figures 6and 7 present the GPV variations according to
the temperature at constant irradiance (E=1000W/m2).
Fig.8 photovoltaic module PPM
A. The irradiance effect on the PPM,
The following figures represent the irradiance effect on
the PPM coordinates:
Fig.6 temperatureeffect on the current-voltage characteristics
Fig.9the irradiance effect on the optimal voltage (Vopt).
Fig.7 the temperature effect on the power-voltage
characteristics.
The open circuit voltage is inversely proportional to the
temperature. The short-circuit current increases with the
temperature. This increase is less than the voltage drop.
Fig.10the irradiance effect on the optimal current(Iopt).
the temperature. On the other hand, the optimal current
remains almost identical.
7. Influence
ofseries
résistanceon
theelectrical characteristic
To see the effect of the series resistance on the electrical
characteristic of the GPV, we plotted the curves for
Rs1= 0Ω, Rs2=0.4 Ω and Rs3 = 1Ω.
Fig.11 The irradiance effect on the PPMpower .
The results obtained are represented on the figure (15):
B. The temperature InfluenceonPPM, Vopt, and Iopt
The temperature effect on the PPM parameters
isrepresented by the following figures:
Fig.15 Effect of series resistance on I–V behaviour
From the figure (15), we notice that when Rs decreases the
slope of curve I-V increases.
Fig.12 The temperature effect on the optimal voltage(Vopt).
8. Structure of the static converter of
energy
The choice of the conversion structure is carried out
according to the load. We chose the boost conversion
for our load: a battery of 24V.
Thecircuit diagram isshown in the following figure:
Fig.13 The temperature effect on the optimal current (Iopt).
Fig.16 Boost converter equivalent circuit
The application of the Kirchhoff’s(see figure 17) gives
two operation phases [5]:
Fig.14 The temperature effect on the PPM power.
When we vary the temperature, the optimal voltage and
the maximum power point are inversely proportional to
Fig.17 two operation phases
A. First time interval (switch closed)
Is=Ipv(1-α).
The mosfet is closed on the interval of Tclosed= αT (α: the
converter cyclic ratio ).
di
Vl=Vpv=v=L. →di=v.
dt
dt
(5)
L
A.
The ripple current
The ripple current is defined by the following equation:
∆IL=Imax-Imin=
By solving this differential equation, weobtain the
following formula, which expresses the evolution of the
inductorcurrent:
Vpv
→i=
.t+Imin
L
∆IL=Imax-Imin=
Vpv
L.f
.α(10)
This
expression
shows
thattheripple
current
decreaseswhenthe switching frequencyforthe value of
theinductanceL increases.
At t=α.t, we have:
i=Imax
B. The ripplevoltage
we obtain the Following equation:
Imax=
B.
Vpv
.αT→
L
Vpv
L
.αt+Imin(6)
Second time interval(switch open)
To determine the expression of the ripple voltage ∆VS,
we assume that the current Isperfectly constant [6]. We
have the following relation:
1
ic
Vc=Vs= ) i.dt →Us= .t+cst(11)
C
c
With cst: a constant.
The switch is open on the intervaloftopen = (1-α) *T, the
inductor voltage is then:
di
dt
VL=Vpv-Vs=L. →di=(Vpv-Vs). (7)
dt
L
By solving this differential equation, we obtain the
following formula, which expresses the evolution of
inductor current:
i=
Vpv-Vs
L
(t-αT)+Imax(8)
•
cst=Vmax
-→
0 ≤ t ≤ αt : +
àt=α.t→Vs=Vsmin
Ic
Vsmin= .αt+Vmax(12)
C
We have:Ic=-Is, we obtain the ripple voltage expression
on the condenser output:
Is
∆Vs = Vmax − Vmin = C.f .α(13)
C. Simulation of the boost converter undermatlab
At t=T, we have:
i=Imin
The following figure represents the block diagram of the
boost converter:
we obtain the following equation :
Imin=
Vpv-Vs
L
.T(1-α)+Imax(9)
The value of average voltage at the inductor terminals is
defined by the following formula:
Vs=
Vpv
1-α
If we consider that the converter is ideal (output 100%),
we can write:
Pe=Ps→Vpv.Ipv=Vs.Is
We obtain then the average value of current Is crossing
the load:
Fig.18 boost converter block diagram
The simulation results of the boost converter are
represented by the following figures:
References
[1] MedKhaled KAHALERRAS & Mohamed CHERFI,
Conversion de l’énergieéolienne en énergieélectrique par
ungénérateur, Badji-Mokhtar university, Annaba (2010), PP.1.
Fig.19 Inductor current, voltage
[2]
Jean-François
REYNAUD,
Recherchesd’optimumsd’énergies
pour
charge/décharged'unebatterie à technologieavancéedédiée à
des applic
ations photovoltaïques », Toulouse University III - Paul
Sabatier, Toulouse (2011), PP.43.
[3]FZ zerhouni, Adaptation optimale d’une charge à un
générateur photovoltaïque,University of Sciences and
Technology Mohamed Boudiaf, Oran(1996).
[4]M.BOUKLI-HACENE Omar , Conception et
réalisation d’un générateurphotovoltaïque muni d’un
convertisseur
MPPT
pour
unemeilleuregestionénergétique ,AbouBakrBelkaiduniv
ersity , Tlemcen (2011),PP.14-15.
Fig.20 MOSFET current, voltage
[5]RedaMerahi, Modélisation d’un dispositif MPPT pour
l’étude de l’augmentation de la puissance produite par les
générateursphotovoltaïque, Badji-Mokhtar university, Annaba
(2010),
[6] BenoîtIssartel , Conception d’un convertisseur DC/DC de
type boost ,Polytech’Clermont-Ferrand, France, PP.8.
Fig.21 Diode current, voltage
Fig.22 load voltage
8. Conclusion
In this paper, we show the principal characteristics of a
photovoltaic generator (GPV) and the effect of
irradiance and temperature on these characteristics.
We also studied the adaptation of the DC-DC boost
converter. The use of this structure is an economic
solution compared to the pull-down structure as the nonreturn current from the battery to the GPV’s protection
can be provided directly by the diode acting as a
freewheel in the structure.

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