DYNAMIC POWER OPTIMIZATION OF CONTOURED FLEXIBLE PV ARRAY UNDER
NON-UNIFORM ILLUMINATION CONDITIONS
P. Sharma, B. Patnaik, S.P. Duttagupta, V. Agarwal,
Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, India-400 076
Lightweight, flexible PV (FPV) modules are suitable for
BIPV and stand-alone applications where contoured
layouts are required and system weight is a constraint.
This report focuses on optimization of a contoured FPV
array which operates under Non Uniform Illumination
(NUl) conditions. NUl conditions can occur at low solar
angle for contoured layouts and also due to unexpected
partial shading events. The output Power Voltage (P-V)
characteristics under contouring and partial shading
demonstrate severe oscillatory behavior around the
maximum power region, which results in inefficient
implementation of established tracking algorithms that
seek to detect a dominant global maxima. An algorithm for
MPP tracking has been proposed that takes into account
both static and dynamic parameters. An integrated test
system has been developed which can accurately monitor
solar irradiance (range 100-1000 Watts/m2) and solar
angle and load dependent instantaneous output voltage
and current. The system has been designed for maximum
current and voltage rating of 3A and 25V respectively.
While an output power penalty results due to contouring,
our results show a very small energy differential up to a
maximum array contour value of 10°. This analysis may
lead to development of novel, flexible PV array based
designs.
The P-V characteristics of C-FPV modules depend on
many factors such as solar irradiance, module
temperature, solar angle, contour angle and nature of
shading. Thus, there is a requirement for a monitoring
system which analyzes how input parameters such as
solar irradiance (G), solar angle U and module layout will
influence output parameters such as (load dependent)
instantaneous current and voltage. The test system
integrates measurement of array output voltage and
current and a Distributed Sensor Unit (DSU) which
monitors instantaneous solar irradiance and solar angle,
and is also able to identify partial shading events. The goal
is to determine the impact of non-uniform illumination
(NUl). While the NUl impact is primarily measured in terms
of reduction in output power, the test system is also able
to monitor subtle irregularities in P-V characteristics. An
important determination for C-FPV modules is the
threshold solar angle parameter. At incident solar angle
values lower than the threshold level, the P-V
characteristics are highly irregular and the optimization
process is complex.
The comprehensive testing of C-FPV modules with
varying degrees of contour angle (6°_22°) has shown that
low contour angles (10°) result in only a small decrease
(3%) in energy output (as compared to a flat array layout).
These results can enable formulation of new layout
designs for BIPV as well as stand-alone PV systems.
INTRODUCTION
SYSTEM AND EXPERIMENTAL DETAILS
Amorphous silicon (a-Si) based flexible PV (FPV) modules
have certain advantages such as high Watt-peak/kg (1215) value and the ability to integrate on to contoured
surfaces (for example, a tent), which are useful for various
portable as well as building-integrated applications [1]. A
contoured FPV (C-FPV) array with varying contour angles
has been deployed in the outdoors and the power output
has been optimized. There is a significant impact of Non­
Uniform Illumination (NUl) conditions on Power Voltage
(P-V) characteristics, due to contouring as well as
temporary shading events. Previously, the power
optimization of a partially shaded crystalline Si PV array
has involved detection of a global maximum power point
while rejecting local maxima points [2]. In contrast to the
above scenario, for the a-Si based FPV array under NUl
conditions, the P-V characteristics display a highly
oscillatory profile around the peak power region. The
existing power tracking or optimization techniques (e.g.
perturb & observe) [3, 4] do not work efficiently under
these conditions. A more effective approach is proposed
based on a dynamic scanning window that is responsive
to NUl conditions.
The test system is shown in Fig. 1(a) and comprises of a
variable load, power conditioning circuitry, distributed
sensor unit (DSU), and micro-controller. The DSU
monitors solar irradiance and solar angle and also
identifies partial shading events. The module temperature
is also recorded. Fig. 1(b) shows the experimental set up.
The load circuitry consists of a relay and resistor array
arranged in an 8x8 matrix [5]. The microcontroller
switches the relays such that a step-wise changing load is
realized at equal intervals of time. Thus the complete I-V
characteristics are captured over a time interval of 35
seconds. The present limitation is due to the large relay
settling time (O.5s). The instantaneous output voltage and
current data is sampled and digitized before being
analyzed. The module temperature is measured using
LM35 sensors mounted on the FPV module. The voltage
signal from LM35 is also amplified and digitized prior to
transmission. The DSU measures solar irradiance and
solar angle.
Solar irradiance measurement was performed by
calibrating light sensors with c-Si modules that have well
defined and stable characteristics [6]. However, the
A BSTRACT
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IEEE
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principal challenges here are to track non-uniform
illumination (NUl) events and solar angle. An NUl event is
reported for the first time for hybrid a-Si FPV modules at
low solar angles. At a certain threshold solar angle there is
a larger than predicted change in power output
(decreasing in early morning, increasing in late afternoon).
Also in the NUl regime, the P-V characteristics become
significantly noisy and irregular in nature.
15 ,----,
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Sensor data
12
200
400
600
800
Solar irradiance (W/m')
(a)
Remote
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Sensor Unit
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20
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(b)
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.
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(b)
Figure 1 (a) Block diagram of measurement system (b)
Experimental set up
For contoured layouts, the NUl influence is observed
through is displayed by the presence and frequency of
multiple local maxima points. The coordinated
measurement of solar angle and P-V characteristics help
pinpoint the threshold solar angle (as explained in the next
section) which is a function of the module contour angle.
MEASUREMENT SYSTEM RESULTS
The solar irradiance calibration is performed by
benchmarking against well established and stable short­
circuit current vs. irradiance (/sc-G) relationship for c-Si
modules [6]. The Isc of the photodiode sensors utilized in
the DSU vary linearly with irradiance as shown in Fig. 2(a)
and is expressed as
Isensor (G)
=
al x
G
(1)
where, Isenso,{G) (in M) is short circuit current of
photodiodes used in sensors, G is solar irradiance and al
is the coefficient with value 2e-OB.
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5
- 22° Contour plus shading
;; 4
�
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... 3
2
.
o
15
10
20
25
Voltage (V)
(C)
Figure 2 (a) DSU calibration: solar irradiance
dependence of light sensors, (b) P-V characteristics of
contoured FPV modules (c) P-V characteristics of
contoured modules subjected to partial shading
Fig 2 (b) shows P-V characteristics for i) flat FPV module
(00 contour) and ii) various contoured FPV modules (at G450 W/m2). An increase in contour angle (Fig. 2b) coupled
with shading of P-V characteristics unexpected events
(Fig. 2c), leads to an extended oscillatory behavior near
the peak power point.
Threshold angle determination
The function of the DSU is to estimate the instantaneous
solar angle from solar irradiance measurements. The
module mounted sensors tracks direct solar radiation from
000969
7AM to 7PM (March 7-21, 2009), corresponding to a solar
angle value of 0°_180°. Figure 3(a) shows an experimental
pattern for (normalized) direct solar irradiance vs. solar
angle with a superimposed theoretical irradiance pattern.
Aside from the saturation region observed for the 60°-120°
arc, Fig.3a shows that solar angle can be estimated in
reverse from the direct irradiance measurements. The
threshold region for NUl is observed to exist in the 0°_30°
arc and 150°-180° arc.
(C1) or contoured at 15° (C2). A normal P-V trace is
recorded corresponding to 33° solar Angle (C1), as
indicated by the monotonically decreasing dPldV vs. V
characteristics in Fig. 3(c). In contrast, at 31° solar angle
(C1) characteristics show presence of multiple local
maxima points, with corresponding dPldV zero-crossings
in the plot range (lsc to Maximum Power Point or MPP).
There is a greater NUl impact for large contoured layout
as compared to small contoured case, whereby multiple
peaks are observed for 35° (C2) but not for 37°(C2). As
expected, the threshold solar angle is higher for large
contoured case.
Power output variation with contour angle
30
90
60
ISO
120
180
Solar AneIe
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The contour layout influence on power-voltage
characteristics can be detected as multiple local maxima
points for a-Si FPV modules. The coordinated
measurement of solar angle and power-voltage
characteristics helps pinpoint the threshold solar angle for
contoured modules discussed above. The threshold angle
is expected to change with module contour angle. It has
been observed that the penalty for contouring the FPV
modules exists only for the low solar angles during early
morning and late afternoon hours (lower irradiance
values). The maximum power difference at peak
irradiance level (10am-3pm) between flat layout modules
and for 6° contour angle is 0.8% and for 10° contour angle
is 3%. This leads to an important conclusion that at
low contouring levels the power penalty will not be
significant.
SCANNING WINDOW DETERMINATION
8
to
12
Voltage (V)
14
16
18
(b)
2.5
-35"(C2)
-37"(C2)
---31° (el)
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VoUage (V)
(C)
Figure 3 (a) Direct solar irradiance variation vs. solar
angle. Tracking device: centrally mounted light sensor
(b) P-V characteristics for a-Si FPV module contoured
at SO (C1) and 15° (C2), at the non uniform illumination
threshold, (c) dP/dV-V characteristics for a-Si module,
contoured layouts C1 and C2 before and after
crossing of threshold solar angle
The NUl impact on P-V characteristics of a-Si modules is
shown in Fig. 3(b). The module layout is contoured at 6°
978-1-4244-5892-9/10/$26.00 ©2010
IEEE
The concept of scanning window was derived from the fact
that the maximum power point occurs at different
percentage of open circuit voltages for a specific module
technology (a-Si), and is also sensitive to NUl events
(shading, contouring or both). In the case of contouring,
occurrence of multiple peaks depends upon the solar
angle and there is a threshold angle which differentiates
the single peak and the multiple peak regimes. The
constant voltage strategy [7] for maximum power point
tracking cannot be used under this scenario. The scanning
window is a function of contour angles and various
environmental conditions. The scanning window size
depends upon the contour angle and solar irradiance. With
the application of scanning window for MPPT, the
efficiency of MPPT tracker is expected to increase.
From Table1 it is clear that the scanning window width
varies with respect to solar irradiance bands as well as
with contour angles. As expected the scan width is
smallest in case of flat or low (6°) contoured modules. The
maximum size of scanning window for whole range of
solar irradiance and contour angle is from 0.55Voc to
0.85Voc as clear from Table1. For each solar irradiance
band the scanning window has different values for
different contour angles. Lower and upper limit for
scanning window is determined from the observations and
analysis of the data obtained from measurement test
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system. The dP/dV characteristic gives the lower and
upper limits of scanning window as clear from Fig 3c.
MPPT scheme without scanning window is referred as
technique T1 and MPPT scheme with window is referred
as technique T2.
Table1. Scanning window range for different solar
.
I
Irrad·lance bands and contour angles
Scanning window (%Voc) for
Solar irradiance
different contour angles
(W/m2)
0° _6°
7° _15° 15° - 22°
Band1
65-75
70-85
55-75
G < 500
67-77
Band2 500<G<700
66-72
67-77
Band3
65-70
64-69
64-70
G > 700
Table2. Tracking performance comparison of techniques
l
I erent contour angles
T1 and T2 �or d·ff
Tracking Time (in
Average Steady State
MPPT
Power (vi!)
steps)
Algorithm
C2
C2
C3
C3
C1
C1
Without
9.35
1
Scanning
7.65
14.95
20
20
Window
With
2
1
Scanning
14.95
9.55
0
8.08
Window
POWER OPTIMIZATION
The impact of NUl conditions on P-V characteristics
depends upon the contour angle and threshold angle.
Above the threshold angle (single peak regime) and in
absence of partial shading events, conventional MPPT
strategies will work satisfactorily. In all other scenarios, P­
V characteristics demonstrate strong OSCillatory behavior
around the peak power point, and conventional algorithms
will fail to converge.
The MPPT scheme proposed in the present work first
monitors NUl conditions by checking for solar irradiance,
solar angle and dynamic change in solar irradiance as
shown in Fig 4(a). The DSU informs the system about
instantaneous solar angle values and solar irradiance from
where the condition for different irradiance bands and
threshold angles are evaluated. After evaluating threshold
conditions, the algorithm decides whether to adopt a
scanning window strategy or conventional MPPT strategy.
The algorithm will take the mean value of the scanning
window which is quite close to maximum power point
voltage and then begins with conventional perturb and
observe technique. It has been found from the
experimentation and observations that the temperature
effect on Voc is very small on the considered FPV module
[8].
Figure 4 (b) shows the comparison of the average daily
energy output of FPV modules as a function of contouring.
As compared to a module with a flat layout, the decrease
in energy output is 3% (6°), 13% (15°), 25% (22°). The
reason for the smaller than expected energy penalty is
that at high solar angles the impact of contouring on non­
uniform illumination is minimized.
Solar angle and solar irradiance values will decide the
lower and upper limits of scanning window. Three different
contour angles have been considered to demonstrate the
proposed scheme. The algorithm is verified for three
different contour angles which are changed after certain
interval. P-V characteristics for different contoured angles
considered are given in Fig 5(a). The irradiance and
temperature conditions are 400 W/m2 and 40°C. The
maximum power output for three contoured angles C1
(22°), C2 (15°) and C3 (0°) is 8.1, 9.6 and 15.0 W
respectively. The tracked power with and without scanning
window technique is given in Table2.
978-1-4244-5892-9/10/$26.00 ©2010
IEEE
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system
Solar
Solar Partial shading
irradiance angle
event
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Check threshold
angle
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t
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(a)
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.0 Degree
120
80
40
0
Module layout
(b)
Figure 4 (a) Flow chart of power optimization
technique (b) Optimized average energy output from
contoured and flat FPV array
It is clear from Fig 5(b) that T1 is slow as compared to T2.
The tracked power differs by approx 6% from maximum
power as clear from Table2. At 100th step, the contour
angle conditions were changed and the response time is
reasonably good for T1 as compared to T2. However, the
power difference is almost 3%. This is because the
scanning for the T1 starts from voltage equal to 0.8Voc in
000971
every case and in some cases maximum power point
voltage is close to this value and for other it is not.
Therefore, it does not work efficiently for all the conditions.
The third step change is at 200th steps, when the contour
angle was changed to 0°. At 0° the response time for T1 is
poor but the power tracked is accurate. The present
demonstration considered here has the maximum power
point voltage ranging from 0.6Voc to O.BVoc and
demonstrate the high efficiency of the proposed technique.
In all three cases the tracking efficiency of proposed
technique is near to 99.7% and is faster as compared to
technique T1.
16
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t
[2] H. Patel and V. Agarwal, "MATLAB-Based Modeling
to Study the Effects of Partial Shading on PV Array
Characteristics", IEEE Transactions on Energy
Conversion, vol. 23, pp. 302-310, 200B.
"
S-
9.6W
t
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10
8.1W
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o
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20 steps
50
9.3W
7.6SW
100
150
200
250
[4] S. Jain and V. Agarwal, "Comparison of the
performance of maximum power point tracking
schemes applied to single stage grid connected
photovoltaic systems, " lET Electr. Power Appl., vol. 1
no 5, pp. 753-762, 2007.
[6] P.
Sharma,
S.P.
Duttagupta,
V. Agarwal,
"Characterization
and
Modeling
of
Flexible
Photovoltaic
Modules
for
Portable
Power
Applications," 1st International Conference on
Sustainable Power Generation and Supply, Nanjing,
China, April 6, 2009.
1S.0W
� 12
::i!
[1] P. Sharma, S.P. Duttagupta, V. Agarwal, "Parameter
Extraction for Flexible Photovoltaic Modules to
Determine the Impact of FPV Technologies on High
Insolation Module Performance" in SPIE Proceedings,
vol. 7331, 2009.
[5] Prasad V. Joshi, "Design of Mobile Integrated Power
Plant," M. Tech. Dissertation, Indian Institute of
Technology, Bombay, India, July 200B.
220 steps
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::;
REFERENCES
[3] T. Esram and P.L. Chapman, "Comparison of
photovoltaic array maximum power point tracking
techniques," IEEE Trans. on energy conversion, vol.
22 no. 2, pp. 439-449, 2007.
6
O"
optimization algorithm that is sensitive to NUl conditions
has been proposed and evaluated. This analysis can lead
to novel, optimized FPV integrated designs.
300
No. of steps
[7] K. Kobayashi, H. Matsuo, and Y. Sekine, "An
excellent operating point tracker of the solar cell
power supply system, " IEEE Transaction Ind.
Electron., vol. 53 no 2, pp. 495 - 499, 2006.
[B] http://www.powerupco.com/panels/unisolar/flx.pdf
(b)
Figure 5 (a) P-V characteristics considered for
demonstration of scanning window technique with
different contour angles (b) Variation of output power
with time for both the techniques
CONCLUSIONS
Flexible, light-weight PV modules have potential
application as part of BIPV or stand-alone systems where
system weight is a constraint and where the layout may
require contouring. Power optimization for contoured
modules requires remote monitoring of non-uniform
illumination conditions. A test system has been developed
which monitors instantaneous array voltage and current as
well as distributed solar irradiance and solar angle. An
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