8th Power Electronics, Drive Systems & Technologies Conference (PEDSTC 2017) 14-16 Feb. 2017, Ferdowsi University of Mashhad, Mashhad, Iran
Application of High Output Voltage DC-DC Converters Along with Using Battery to Extract Maximum Power from the Solar Cell Farzad Mohammadzadeh Shahir, Student Member,IEEE Department of Electrical Engineering, Urmia Branch, Islamic Azad University, Urmia, Iran E-mail:
[email protected]
Abstract-In system
this paper, the performance of a new hybrid
consists
photovoltaic
Ebrahim Babaei, Senior Member, IEEE Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran E-mail:
[email protected]
(PV)
of
a
new
structure
boost
dc-dc
11. DESCRIPTION OF THE STUDEID STRUCTURE
converter,
system, battery bank and bidirectional dc-dc
According to Fig. 1, a dc-dc boost converter is used between solar cell and dc link due to low solar energy. Also, the battery bank is used with bidirectional dc-dc converter. When PV energy is more than load, the surplus energy will be stored on the battery and in the range that solar cell production can be decreased due to changes in temperature and radiation, the energy in the battery will be used to supply the load.
converter is described by introducing new control scheme. Before, the modeling of each parts of proposed hybrid system is explained. Then, the proposed control scheme for proposed hybrid system is defined based on incremental conductance
(IC )
algorithm for
achieving maximum power point tracking (MPPT) . The validity of proposed hybrid system performance is examined by simulation results in
MATLAB/SIMULINK.
Keywords-De-Dc converter; pllOtovo/taic system; MPPT; solar cell
I.
INTRODUCTION
With increasing prices and due to finiteness of fossil fuel and partly because of environmental issues, the use of renewable energies, especially solar energy is now increasing. For commercial use of solar energy, reducing costs and system size and improving efficiency of power output are of the main challenges [1-2]. The use of maximum power obtainable from photovoltaic is one of the tasks of DC-DC converters [3]. PV module has a nonlinear V-I characteristic and in different weather conditions and temperature only at one point the maximum power is extracted from the PV module. This point is called the maximum power point (MPP). Different methods for finding the MPP has been presented in the literature, including methods of perturb and observe (P&O) [4], IC [5-7], methods based on neural network and fuzzy logic [8-9], open circuit voltage deficit, fractional short circuit current, and RCC method.
Step-up Converter .
Fig. I. The structure of solar cells system connected to the load with battery bank
The most common structure for PV module connection is too use a dc-dc converter. Dc-dc converter is a buck or boost voltage converter that tracks maximum power point. In [10-12] to increase photovoltaic voltage level a conventional boost converter is used. Since the voltage level cannot be increased by this converter more than 4-5 times, thus using high voltage gain converters is essential.
Fig. 2 shows the equivalent circuit of a PV module [12]. Practical modules are consisting of several arrays, which each array was created from a combination of series and parallel solar cell. V-I equation of PV module is expressed as follows:
I Ipv =
In this paper, in PV systems high voltage gain dc-dc converter with battery bank is used. IC method was used for MPPT, the proposed structure is described in the second part and the PV models, MPPT method, the battery model and the high voltage gain dc-dc converter and bidirectional converter will be presented respectively. The control system is explained in third section and in section four, the simulation results are presented.
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Solar Cell Model and Algorithm ofMaximum Power
A.
-10
where
[
exp(
1"1"
V+RI
]
-I) a V') t
and
10
N
.
43
N,
Rp
(I)
.1
are the PV current and saturation current
of so1ar ce11, respectlve1y. V, module with
V+R./
series cells.
s
kT .
= __
q
q
IS
therma1 vo1tage
0
f
is charge of the electron; k is
Boltzmann's constant; T is p-n bond temperature (Kelvin) and a is consonant of idealness of the diode. In (1), Rp and R s , respectively, are equivalent parallel and series resistance. Equation (1) is extracted from I-V curve of Fig. (3). Normally PV module manufacturer provides three-point short circuit current (0, I,e) , voltage and current at the maximum power point (VMPP,IMP P) and open circuit voltage (Voc' 0) of V-I modules. PV modules manufacturers provide some laboratory information to consumers about electrical and thermal features of module. These include nominal short-circuit current (Isc,n)'
=
Io,n
(4)
exp (VoV,n /'a Vt,n ) - 1
where V:,n is thermal voltage of
N,
cells in series at
temperature of T, . In this paper, Io,n can be achieved based on information provided by the manufacturer and from (4). a is fixed amount diode that is chosen in different articles in the range of 1 � a � l.5 In this paper, to extract maximum power, IC algorithm was used [5-7] that is illustrated in Fig. (4), This MPPT method has rapid response and high dynamic compared with other methods and experimental results show the high efficiency of this method compared with other methods. In IC method, conductivity change (di/ dv) is used to determine (dp / dv) . So, we get:
voltage and current at the maximum power point (VMl'l" 1Ml'l' ) and nominal open-circuit voltage (Voe,n)' open circuit voltage
temperature coefficient (Kv), the thennal coefficient of short circuit (K1) and the experimental maximum output power
(Pmax,e) ,
dP/ dV =0,
at MPP
dP/ dV> 0.
left of MPP
dP/ dV 160 0.4
Given that the DC link. was 700V and load resistor was 200n so the amount of power consumption is 2450W . In the first scenario, at 500 W/m2 and a temperature of 25°C maximum production capacity at voltage of 52V and current of 26mA was l365W (Fig. 9(d)). Since, it is less than the power consumption so the difference (1085W) will be supplied by battery which is shown in Fig. 9(a). At the moment 0.8s, at a constant temperature, amount of radiation changes to 1000 W/m2 , at the voltage of 53V and current of to 52.7A (Fig. 9(d)) and production capacity would be 2794W . So it is higher than the power consumption and 344W is stored in battery. Fig. 9(b), shows the current, voltage and state of charge (SOC). At earlier than 0.8s, because PV power is lower than load it causes the battery to discharge (SOC reduction), in other words, current of battery is positive and power flow from the battery to the load. At a later time by increasing the productivity of PV, battery charges (absorption of current) and the SOC increases. The control system was able to reference
0.6
0.8 timers)
: � : �
1.2
(b)
800 750 -
.; 700 u
650 600 0.4
0.6
0.8
timers) (c)
46
1.2
8oo r-----�----�---,
750 u
>�700 �------------�
--------------�
__
650 600 �----��----�---L1.2 0.4 0.8 0.6 timers) (c)
0.6
0.8 timeM
1
1.2
(d)
Fig. 9. Simulation results for scenario I. (a) power flow response. (b) battery. (c) dc link. (d) PV voltage and current
B. Results o{Scenario 2 In this scenario, at a temperature of 25°C and radiation of 900 W/ m2 , PV production capacity at a voltage of 55V and current of 49A (Fig. 10(d)), was 2695W . Because PV production capacity is more than consumption then 245W of power will be stored in battery as shown in the Fig. lO(a). At 0.8s, with constant radiation temperature changes to 55°C, and voltage and current values are respectively 47.4V and 49A (Fig. 10(d)). In this cases production capacity is 2322W that is lower than power consumption. Thus 128W will be supplied by battery. Current, voltage and battery SOC is shown in Fig. lOeb). At earlier than 0.8s, because PV power is more than load it cause the battery to charge (increasing SOC) and at later times by reduction in the productivity of PV, battery discharge and SOC decreases. In this scenario, control system was properly able to stabilize dc linl( voltage in the reference value of 700V , which is shown in Fig. 10(c).
3000
.-.-.-.-.-.-.---.-.
2000 looo ..
;
=..
p pv ....... Pbat
_._. •
1000
l
�
(d)
V.
j
.
. . . .. . .. ........................................
-1000 0.6
0.4
Go
33.753 33.752
rJ) 33.751
] >
200 180 160
�
1.2
REFERENCES [I]
: � :!'-:
t
0.4
time(5j
(a)
yH -2
0.8
0.6
0.8
timers)
CONCLUSION
In this paper, a new structure for boost dc-dc converter was applied with new hybrid system. It was obvious that the new boost dc-dc converter structure had proper performance. The IC algorithm was used for MPPT with a battery bank and bidirectional dc-dc converter to increase stability and reliability of load characterizes. This algorithm was considered under two different scenarios. Firstly, by considering radiations variations and constant value for its temperature, it was shown how the voltage and current of the load was controlled by application of a new dc-dc boost converter, a battery bank and a bidirectional dc-dc converter. Also, the simulation results was obtained in radiations constant and temperature variations state were shown how load characterizes could be constant. Meanwhile, it was shown that the dc link voltage was controlled at 700V due to appropriate performance of proposed control scheme.
,....... -._._._._._._._._.-
o .............................................,
1.2
Fig. 10. Simulation results for scenario 2, (a) power flow response,(b) battery, (c) dc link, (d) PV voltage and current
·V
-PLoad
0.8 timers)
0.6
[2]
: �
[3]
: j
[4]
1.2
[5]
(b)
47
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