Maximum Power Point Tracking Based on Look up Table Approach
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Advanced Materials Research Vol. 768 (2013) pp 124-130
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.768.124
Maximum Power Point Tracking Based on Look up Table Approach
S. Malathy1, a, R. Ramaprabha2, b
1
Assistant Professor, Department of EEE, SSN College of Engineering, Chennai, Tamilnadu, India
2
Associate Professor, Department of EEE, SSN College of Engineering, Chennai, Tamilnadu, India
a
malathys@ssn.edu.in, b ramaprabhar@ssn.edu
Keywords: PV, maximum power point tracking, P&O, Look up table.
Abstract. This work proposes a lookup table based approach to track the maximum power from a
solar photovoltaic (PV) module. The performance of the solar PV module is greatly influenced by
various environmental factors and it is therefore necessary to operate the PV module at its optimal
point so as to ensure that maximum power is extracted from the PV source. Several fixed step and
variable step maximum power point tracking (MPPT) algorithms have been proposed in the
literature. In this paper a simple and fast maximum power tracking method based on lookup table
approach is proposed. The maximum power point voltages for various insolation levels are obtained
from the experimental setup and are fed to the look up table. This look up table thus formulated can
then provide the reference voltage for various insolation conditions without many computations.
The performance of the proposed method is compared with that of the conventional MPPT methods
like perturb and Observe (P&O), Incremental Conductance (INC) and Fuzzy logic (FLC) based
MPPT. The simulation results show that the lookup table (LUT) approach tracks the maximum
power point faster than the conventional algorithms under changing illumination conditions and
reduces simulation time.
Introduction
At present, the country is severely affected by frequent power outages and scheduled power cuts
and this has opened up great opportunity for solar based products. The total energy received from
the Sun is far more than our energy demand. Most parts of the country have a sunny weather for
about 200 to 300 days of a year and receive solar radiation of about 4 kWh/ sq. m. If this available
energy could be trapped and converted to useful electric power, it is possible to meet the energy
demand to a greater extent.
A photovoltaic cell converts solar energy directly into electrical energy. The performance of the
PV module depends on various factors which includes insolation level, temperature, wind speed and
shading by near by structures or passing clouds. Since the efficiency is less in the commercially
used mono/poly crystalline solar PV modules and the capital cost involved is high, it becomes
necessary to utilize the available PV modules to its maximum capacity. One way to maximize the
utilization is by mechanical tracking [1]. The panel is adjusted according to the direction of Sun so
that maximum solar energy is received by the panel. The second way is to reconfigure the solar PV
arrays so as operate near the maximum power point. The third way is to introduce an electric
tracking method where in a maximum power point tracker is placed between the panel and the load.
The load resistance is adjusted till it is equal to that of the source resistance and thus the maximum
power transfer is ensured. Several MPPT algorithms have been proposed in literature [2]. This
paper proposes a simple technique to track the maximum power point under changing illumination
conditions based on LUT approach.
Maximum Power Point Tracking Methods
The efficiency of the PV cell depends not only on the material and technology but also on the
environmental factors like temperature and insolation. The PV current is directly proportional to
insolation and the open circuit voltage of the PV cell is inversely proportional to the working
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Advanced Materials Research Vol. 768 125
temperature. The output voltage of the PV cell is very less and usually 36 cells are connected in
series to form a PV module. The modules are then connected in different ways; series, parallel or
series –parallel based on the voltage and current requirements. The characteristics of the PV module
are highly non-linear. The operating point of the PV module is also dependent on loading conditions.
When the load draws a current that is very much lesser or greater than the optimum current of the PV
module, the efficiency of the system reduces to great extent and in this case the PV system is said to
be under utilized. [1]- [2]. The overall efficiency of the interconnected PV system can be improved
by properly choosing PV modules and operating them at their respective optimal power points.
MPPT is used for this purpose. The block diagram of a typical PV system with MPPT is shown in
Fig 1.
Fig 1. Block diagram of a PV system with MPPT
Matlab/Simulink model of PV module is developed based on the mathematical equations [3]-[6]
for SOLKAR make PV module and is shown in the Fig 2. The specifications of the of the PV
module at STC (G=1000 W/m2, T=250C) are,
Max. Power (P max) : 37.08 W
Voltage at Max. power(Vmpp) : 16.56 V
Current at Max. power (Impp) : 2.25 A
Open circuit voltage (Voc) : 21.24 V
Short circuit current (Isc) : 2.55 A
No. of Series Cells (Ns) : 36
Fig 2. Matlab/Simulink model of a PV module
126 Energy Efficient Technologies for Sustainability
The characteristics of the PV module are presented in Fig 3 for various insolation and
temperature conditions. It is evident that the short circuit current and the open circuit voltage of the
PV module vary with insolation and temperature in turn varies the power. The photon current
depends on insolation whereas the open circuit voltage depends on the working temperature [6]-[8].
The maximum power points for various insolation levels are plotted on the V-P characteristics of
the PV module as shown in Fig 4.
Fig 3. Characteristics at different insolation levels Fig 4. Maximum power point at various
and Temperature insolation levels
The conventional methods [8]-[9] like load matching, Perturb and Observe (P&O), Incremental
conductance (INC), Fuzzy logic are considered in this work and their performance is compared with
that of the proposed method.
Load matching Method
The maximum operating point for various insolation levels are obtained and based on this data a
matching load is identified. This method is very simple and it does not require extra circuits/
algorithm to track the maximum power point. The load is so designed that the average load voltage
is very close to maximum power point voltage. This method is not suited for changing load
conditions.
Perturb and Observe Method
In this method, the MPPT controller perturbs the operating voltage by a small amount and the
power is measured. If there is an increase in power the perturbation is continued in the same
direction till there is no increase in power [10] – [12]. The drawback in this method is, when the
maximum power point is reached; V will oscillate about the Vmp.
Incremental Conductance Method
It is based on the fact that dP/dV = 0 at maximum power point. The value of dP/dV is greater
than 0 when the operating voltage V is lesser than Vmp and dP/dV < 0 when the operating voltage
V is greater than Vmp. Since, P=V.I; dP/dV = (I+V)*(dI/dV). The MPP condition can be obtained
from these conditions as, dI/dV= -I/V. In this method the incremental conductance of the PV
module is compared with the instantaneous conductance and this information is used to adjust the
reference voltage further [13]-[14].
Advanced Materials Research Vol. 768 127
Fuzzy Logic based Method
The step size is fixed in the case of Incremental conductance method. To speed up the tracking it
is necessary to vary the step size. For large step size, tracking speed increases but, accuracy is lost.
If step size is reduced to a small value, accuracy increases at the cost of increased tracking time.
The step size can be chosen to be more initially and the size can be reduced as the maximum power
point is approached. This variable step size can be obtained by Fuzzy logic based MPPT controller.
The performance of FLC based MPPT is better than the other conventional techniques [15]-[16].
Look up Table Method
The proposed method requires real time data or data obtained from more accurate model which
mimic the behavior of the actual PV module. In this paper, for the proposed MPPT method, a look
up table is formulated based on the experimental data obtained. The experimental set up with
electronic load is shown in Fig 5. The electronic load is used to trace the V-I characteristics of the
PV module and the V-P characteristics are then derived from the stored data in digital storage
oscilloscope. The maximum power point at various insolation levels are obtained from the V-P
curve. The variation in maximum power point voltage Vmp with respect to insolation level is shown
in Fig 6.
Fig 5. Experimental set up to trace V-I curve Fig 6. Vmp versus insolation
The values of Vmp at various insolation levels are then fed to the lookup table thus enabling it to
compute the Vmp that corresponds to the given insolation level without much computation. A pilot
panel, which has similar characteristics as that of the other panels in the system can be used to
measure the open circuit voltage and short circuit current. From these data, the insolation level can
be determined and corresponding Vmp can be obtained from the lookup table. This approach reduces
the complexity and tracks the Vmp faster than the other algorithms. The reference voltage provided
by the lookup table is compared with the actual PV module voltage and the error thus obtained is
given to a PI controller and a comparator to generate the gating pulses required for the DC-DC
converter. The duty cycle of the pulse is adjusted till the load impedance is matched with that of the
source. The Matlab/ Simulink model of the overall system is shown in the Fig 7.
Fig 7. Matlab/Simulink based model with LUT based MPPT method
128 Energy Efficient Technologies for Sustainability
Results and Discussions
MPPT algorithms should quickly track the maximum power point at changing environmental
conditions. The change in insolation is mimicked by a signal builder where the insolation is
changed form 1000 W/m2 to 500 W/m2 and then to 800 W/m2 at a uniform interval of 0.1 Sec. The
methods considered are P& O, INC, FLC based MPPT and the proposed LUT method. All the
methods tracked the maximum power point successfully and are shown in Fig 8. It is evident from
Fig 8 that the lookup table based approach tracks the change faster than the other methods due to
less computations involved. FLC performs better than INC and P& O algorithm because of variable
step size. INC algorithm in turn is faster than the P& O algorithm. The change in the output voltage
of the converter at varying insolation condition is shown in Fig 9. The input power and the output
power of the PV system with maximum power point tracker are depicted in Fig 10 and Fig 11.
Fig 8. Tracking by various MPPT methods Fig 9. Output voltage of converter
Fig 10. Power delivered by the Module Fig 11. Output power of the converter
The efficiency of the proposed MPPT method along with the conventional methods is shown in
Fig 12 and comparison is given in Table 1. All the methods are equally efficient. The Incremental
conductance method has slightly higher efficiency than the other methods. The LUT based method
has very slight decrease in efficiency as it involves approximations.
Fig 12. Efficiency of MPPT methods at changing insolation condition
The comparison has been made for different test conditions and from the results; it is evident that
the LUT based MPPT responds faster i.e. optimal point tracking time is lesser than the conventional
model. For complex interfacing systems like grid connected PV system, the proposed LUT MPPT
not only reduces the complexity of the overall system but also reduces the tracking time and
simulation time.
Advanced Materials Research Vol. 768 129
Table 1. Comparison of MPPT Methods
MPPT Method
Parameters
LUT P&O INC Fuzzy
Tracking
0.015 0.06 0.035 0.03
time (s)
Tracking
Efficiency 95.4 96.18 96.3 95.8
(%)
Conclusion
The work proposes look up table based MPPT for faster tracking of maximum power point under
changing illumination conditions. Besides, it also reduces the simulation complexities of the PV
interfacing system as it requires fewer computations. For LUT method, the required data are
extracted from the practical characteristics using electronic load. Look up table is developed in
Matlab and is tested for varying illumination conditions using proper curve fitting method for in
between data. The conventional MPPT algorithms like P& O, INC and FLC based variable step
algorithm are also simulated in Matlab and tested for changing illumination. The performance of the
proposed LUT based MPPT is compared with these methods in terms of the tracking time and
tracking efficiency. It is found that the LUT based approach responds quicker to the change in
environmental conditions and tracks the maximum power point faster. This method of
implementing MPPT is very much useful for simulation studies of larger PV fed system but LUT
requires real time data to get proper results.
Acknowledgment
The authors wish to thank the management of SSN College of Engineering, Chennai for providing
all the computational facilities to carry out this work.
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