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Journal ArticleDOI

Experimental observations on hot-spots and derived acceptance/rejection criteria

01 Aug 2015-Solar Energy (Pergamon)-Vol. 118, pp 28-40

TL;DR: The IES–UPM observations on 200 affected photovoltaic modules are presented, as well as electroluminescence, peak power rating and operating voltage tests have been carried out, and hot-spots temperature gradients larger than 20 °C are proposed as rejecting conditions for routine inspections under contractual frameworks.
Abstract: The hot-spot phenomenon is a relatively frequent problem occurring in current photovoltaic generators. It entails both a risk for the photovoltaic module’s lifetime and a decrease in its operational efficiency. Nevertheless, there is still a lack of widely accepted procedures for dealing with them in practice. This paper presents the IES–UPM observations on 200 affected photovoltaic modules. Visual and infrared inspection, as well as electroluminescence, peak power rating and operating voltage tests have been carried out. Thermography under steady state conditions and photovoltaic module operating voltage, both at normal photovoltaic system operating conditions, are the selected methods to deal in practice with hot-spots. The temperature difference between the hot-spot and its surroundings, and the operating voltage differences between affected and non-affected photovoltaic modules are the base for establishing defective criteria, at the lights of both lifetime and operating efficiency considerations. Hot-spots temperature gradients larger than 20 °C, in any case, and larger than 10 °C when, at the same time, voltage operating losses are larger than the allowable power losses fixed at the photovoltaic module warranties, are proposed as rejecting conditions for routine inspections under contractual frameworks. The upper threshold of 20 °C is deduced for temperate climates from the basic criterion of keeping absolute hot-spot temperatures below 20 °C.

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Experimental observations on hot-spots and derived
acceptance/rejection criteria
R. Moretón , E. Lorenzo, L. Narvarte
Abstract
The hot-spot phenomenon is a relatively frequent problem occurring in current photovoltaic generators. It entails both a risk for the
photovoltaic module's lifetime and a decrease in its operational efficiency. Nevertheless, there is still a lack of widely accepted procedures
for dealing with them in practice. This paper presents the IES-UPM observations on 200 affected photovoltaic modules. Visual and
infrared inspection, as well as electroluminescence, peak power rating and operating voltage tests have been carried out.
Thermography under steady state conditions and photovoltaic module operating voltage, both at normal photovoltaic system operating
conditions, are the selected methods to deal in practice with hot-spots. The temperature difference between the hot-spot and its surround-
ings,
and the operating voltage differences between affected and non-affected photovoltaic modules are the base for establishing defective
criteria, at the lights of both lifetime and operating efficiency considerations. Hot-spots temperature gradients larger than 20 °C, in any
case,
and larger than 10 °C when, at the same time, voltage operating losses are larger than the allowable power losses fixed at the pho-
tovoltaic module warranties, are proposed as rejecting conditions for routine inspections under contractual frameworks. The upper
threshold of 20 °C is deduced for temperate climates from the basic criterion of keeping absolute hot-spot temperatures below 20 °C.
Keywords:
Hot-spot; PV module lifetime; PV module efficiency; Operation losses
1.
Introduction
A hot-spot consists of a localized overheating in a pho-
tovoltaic (PV) module. It appears when, due to some
anomaly, the short circuit current of the affected cell
becomes lower than the operating current of the whole, giv-
ing rise to reverse biasing, thus dissipating the power gen-
erated by other cells as heat. Fig. 1 shows two infrared
(IR) images of hot-spots. The anomalies that cause
hot-spots can be external to the PV module: shading
(Alonso-García et al.,
2003;
Herrmann et al., 1997;
Molenbroek et al, 1991) or dust (Lorenzo et al, 2014);
or internal: micro-cracks (Brun and Melkote, 2009;
Buerhop et al., 2012; Garcia et al., 2013; Grunow et al.,
2005;
Paggi and Sapora, 2013; Paggi et al., 2013), defective
soldering (Buerhop et al., 2012; Chaturvedi et al., 2013;
Gabor et al, 2006; Garcia et al, 2013; Muñoz et al,
2008),
potential induced degradation (Berghold et al.,
2013;
Hacke et al., 2010), material imperfections (Vasko
et al., 2014). In general, a hot-spot entails a decrease of
the operational efficiency of the PV module. Moreover,
when a hot-spot persists over time, it entails a risk for
the PV module's lifetime (Buerhop et al., 2012; Garcia
et al., 2013; Muñoz et al., 2011; Osterwald and
McMahon, 2009; Radziemska,
2003;
Simon and Meyer,
2010;
Solórzano and Egido, 2014).

Fig. 1. IR images of hot-spots, (a) General view of a PV array with
hot-spots caused by potential induced degradation (suggested by the
appearance of a regular pattern), (b) Hot-spot in a PV module caused by
micro-cracks. The operating temperature at the hot-spot is 87 °C while the
mean temperature of the rest of the module is 53 °C, which represents a
temperature difference of 34 °C.
Hot-spots are relatively frequent in current PV genera-
tors and this situation will likely persist as the PV module
technology is evolving to thinner wafers, which are prone
to developing micro-cracks during the manipulation pro-
cesses (manufacturing, transport, installation, etc.)
(Chaturvedi et al., 2013; Gabor et al., 2006; Grunow
et al., 2005; Kajari-Schroder et al., 2011; Kontges et al.,
2011).
Fortunately, they can be easily detected through
IR inspection, which has become a common practice in
current PV installations (Auer et al., 2007; Botsaris and
Tsanakas, 2010; Buerhop et al., 2011a,b,2012; Hoyer and
Buerhop, 2008; Kasemann et al., 2009; King et al., 2000;
Muñoz et al., 2011). However, the impacts of hot-spots
on operational efficiency and lifetime have been scarcely
addressed, which helps to explain why there is a lack of
widely accepted procedures for dealing with hot-spots in
practice as well as specific criteria referring to the accep-
tance or rejection of affected PV modules in commercial
frameworks. For example, the hot-spot resistance test
included in IEC-61215 (Crystalline silicon terrestrial photo-
voltaic modules. Design qualification and type approval) is
successfully passed if the module resists the hot-spot condi-
tion for a period of 5 h, which suggests that this standard
addresses transitory hot-spots, as those caused by also
transitory shading, but not permanent ones, caused by
internal module defects (IEC, 1995). Along the same lines,
the IEC-62446 (Grid connected photovoltaic systems.
Minimum requirements for system documentation, com-
missioning tests and inspection) only states: "A hot-spot
elsewhere in a module usually indicates an electric problem
[...] In any case investigate the performance of all modules
that show significant hot-spots" (IEC, 2009). Furthermore,
a draft of the IES-60904-12 (Photovoltaic devices: infrared
thermography of photovoltaic modules) clearly establishes
how to capture, process and analyze the IR images, but still
does not set out any PV module acceptance/rejection crite-
ria (IEC, 2014). The Instituto de Energía Solar -
Universidad Politécnica de Madrid (IES-UPM) experience
includes many cases of actors in the PV sector, mainly
module manufacturers and engineering, procurement and
construction companies (EPCC), requesting advice on
how to proceed with collections of IR images of affected
modules, and whose corresponding contracts lacked the
foresight to pose a relevant question: which ones of the
affected PV modules should be replaced under the PV man-
ufacturer's responsibility?
The purpose of this paper is to suggest a protocol for the
effective treatment of hot-spots in the field, addressing both
the lifetime and the operational efficiency of the PV mod-
ules.
Starting from the observations of 200 affected mod-
ules (at two PV plants at Cuenca and Cáceres, Spain) as
experimental support, hot-spot observation procedures
and well defined acceptance/rejection criteria are proposed,
-T
|
^V
k
J
/
1
i
\
.
V
\
\
V
>
9
Non defective cells
i
lc \,
hc,D \
\ Defective j \
\
cdl
\ \
Y
\
VND V
w
0>]
Fig. 2. (a) Electrical connection of n originally identical cells protected by
a by-pass diode. One of the cells is affected by dust, shading or any internal
defect that limits its short-circuit current, (b) /- V curves of the affected cell
and a non-affected one. The current imposed by the non-defective cells
makes the defective cell work in the second quadrant, thus dissipating
power and generating a hot-spot.

oriented
to
their possible application
in
contractual
frameworks.
2.
Fundamentals
of
hot-spots
For explanation purposes,
we
first consider
the
case
of a
group
of
n identical solar cells, associated
in
series
and
pro-
tected
by a
by-pass diode
(Fig. 2a). The
operating condi-
tions:
incident irradiance,
G,
operating cell temperature,
T&
and
polarization voltage,
V, are
such that
a
certain cur-
rent, I &
is
circulating through these cells.
A
hot-spot
appears
in a
cell
(Fig. 2b)
when some defect
(micro-crack, shade,
etc.)
reduces
its
corresponding short
circuit current, hc,T>,
SO
that
/SC,D
<I
C
(1)
which forces
the
cell
to
operate
at a
negative voltage,
Vn
= -(/i -
1)KND
+ V (2)
where subscripts
"D" and "ND"
refer, respectively,
to
defective
and
non-defective cells. Consequent power dissi-
pation heats
the
defective cell, giving rise
to a
hot-spot,
characterized
by the
temperature increase
of
this cell
in
relation
to the
non-defective ones,
AT
HS
. In
what follows,
we will refer
to
this value
as the
"hot-spot temperature".
The by-pass diode assures
V ^ 0,
thus limiting
the
negative
biasing
and the
power dissipation
in
this cell. Obviously,
the maximum hot-spot temperature
is
then attained when
the group
is
short-circuited
or,
which
is
nearly
the
same,
when
the
bypass-diode
is ON.
Note that
AT
HS
is
directly
related
to the
product
I
c
x r
D
. In
other words, hot-spot
temperature mainly depends
on the
operating voltage
and
incident irradiance (which modulates
I
c
), on the
defect
gravity (which determines 7SC,D)
and on the
second quad-
rant
I-V
characteristic
of the
defective cell (which modu-
lates V-D).
AS
this characteristic
can
substantially differ
Voltage (V)
Fig.
3.
Second quadrant
/-
V characteristics
of
7
individual cells
of a
same
PV module (Alonso-García
and
Ruiz, 2006).
The
great dispersion
in the
second quadrant behavior
is
notorious.
If
any
of
these cells were defective,
and considering
the
value indicated
by the
horizontal continuous line
as
the operating current (imposed
by the
non-defective cells),
it can be
observed that
the
derived dissipated power varies about
one
order
of
magnitude depending
on the
particular affected cell.
from
one
cell
to
another, even within
the
same
PV
module
(Alonso-García
and
Ruiz, 2006),
the
hot-spot temperature
also depends
on the
particular defective cell.
As a
represen-
tative example,
Fig. 3
shows
the
second quadrant
I-V
curves
of
7
individual solar cells
of a
same
PV
module mea-
sured
by
Alonso-García
and
Ruiz (2006).
It can be
observed that power dissipation
at a
hot-spot
can
vary
nearly
an
order
of
magnitude depending
on the
defective
cell (Alonso-García
and
Ruiz, 2006; Alonso-García
et al.,
2003;
Herrmann
et al.,
1997,1998; Muñoz
et al.,
2011).
The hot-spot temperature
can be
easily estimated
on the
assumptions that
n >
1
and
that heat
is
homogeneously
dissipated over
the
solar cell surface. Then,
it is
rather
straightforward
to
deduce that
where
S is the
solar cell surface
and C
T
a
thermal dissipa-
tion coefficient that
can be
estimated from
the
Nominal
Operation Cell Temperature (NOCT):
_ NOCT
(°C) - 20
1
'~ 800
W/m
2
For example, NOCT
= 47 °C, S =
225 cm
2
,
I
c
= 6 A and
V
D
=
10
V
lead
to C
T
=
0.036 °C/($)
and AT
HS
= 96 °C.
However, this
is of
scarce practical value because heat
dissipation
is
rarely homogeneous over
the
cell surface,
as
can
be
clearly observed
in Fig. lb.
Obviously,
any
non-homogeneity translates into
AT
HS
increase, thus wors-
ening
the
case. More detailed explanations
on the
relation
between power
and
temperature
are
found
in the
literature
(Buerhop
et al.,
2011b; Hoyer
et al.,
2009).
Now,
let us
consider
the
case
of a PV
module made
up
of three series associated groups, each made
up of n
cells
and
a
bypass diode
(Fig. 4a).
Note that many currently
commercial
PV
modules respond
to
this configuration,
with
n
ranging typically from
18 to 24. A
defective cell like
the
one
described above does
not
reduce
now the PV
mod-
ule sort-circuit current
but
becomes
an
anomalous step
in
the first quadrant
of the I-V and P-V
curves
(Fig. 4b).
Again,
AT
HS
depends
on the
operating voltage
of the
concerned group, which,
in
turn, depends
on the
operating
voltage
of the PV
module.
The
voltage
at the
step marks
the bypass diode turning
ON, and AT
HS
reaches
its
maxi-
mum
for the
voltage range below this step.
Fig. 5
shows
examples
of I-V
curves
of
real modules affected
by
hot-spots.
It is
worth noting that current
at the
maximum
power point
of the
defective module,
7M,D,
is
always lower
than that corresponding
to the
non-defective ones, 7M,ND:
^M,D
<
^M,ND
(3)
Furthermore,
if a
module like these
is
connected
in
series
with many other modules (often between
20 and 30
mod-
ules)
and the
resulting string
is
connected
to an
inverter
able
to
impose
the
Maximum Power Point (MPP),
the
operating current
of the
group must range from 7
M
,ND
and
7M,D-
Then,
the
larger
the
number
of
modules
in the

Fig.
4. (a)
Electrical scheme of a PV module with 3 groups, each
of
them made
up
of n cells
and a
by-pass diode, (b) I-Vand
P-V
curves
of
a non-defective
(blue)
and a
defective (red) module.
The
difference
in the
current
at the
maximum power point between
the
defective module,
7M,D,
and the
non-defective
module,
J
M
,ND,
can be
observed.
(For
interpretation
of
the references
to
colour
in
this figure legend,
the
reader
is
referred
to
the web version
of
this article.)
series,
the
closer
the
operating current will
be to
7M,ND-
In
this situation,
the
operating voltage
of
the defective module
is well below that corresponding
to its
MPP. The important
thing
to
remember
is
that
the
power loss
of a
defective
PV
module
is
much larger when
it
works associated
to
other
non-defective modules than when
it
works alone.
A
practi-
cal consequence
of
the latter
is
that this module could pass
the standard warranty conditions (referring
to the
maxi-
mum power
of the
module alone) while failing
to
deliver
the power
in
practice.
Fig.
6
helps
us to
understand
a
hot-spot derived phe-
nomenon related
to the
operating voltage
of the PV
mod-
ules.
This fact results from
the
typical slight current
excursion caused
by the
inverter
MPP
tracking algorithm,
when
a
defective
PV
module
is
integrated into
a
string with
a large number
of
non-defective ones. Associated voltage
excursions
in the
defective module
are
much larger than
that corresponding
to the
non-defective ones. Note that
the operating voltage differences between defective
and
non-defective modules,
AV
HS
,
also fluctuate following
the
MPP search.
It is
worth mentioning that such voltage
dif-
ferences
can be
easily understood
as
direct power losses,
as
the
current
is
common
to all the
modules
in the
string.
In turn, these voltage fluctuations translate into
AT
HS
fluctuations. This
is
visible
in Fig. 7,
which shows
the
records, every
5 s, of
AV
H
s, versus
AT
HS
for one
particular
defective module
at the
Cáceres
PV
plant (measurement
details
are
explained later) over
a
period
of one day.
Black dots represent
the
moments with high
and
stable
irradiance, while gray diamonds refer
to
unstable
or low
irradiance periods. Large instability
is
observed during
low
and
variable irradiance moments (which
is
obviously
also associated
to the low AT
HS
region, below
20 °C)
when
the
MPP
algorithms
are
prone
to
instability. However,
the
relationship between
AV
HS
, and AT
HS
becomes essentially
stable
in the
high irradiance
(and so
high AT
HS
) region,
where most
of the
energy
is
generated.
These phenomena
can
also
be
observed
in
Fig.
8,
which
shows
the
simultaneous records
of the
in-plane irradiance
(black line)
and the
operating voltages
of 3
modules
of
the same string
(one
non-defective, blue dots;
and two
defective,
red and
yellow dots). Large voltage excursions
in
the
defective modules become evident. This observation
was made
at the
Cáceres
PV
plant,
at a
system with
one-axis azimuthal tracking affected
by
clouds, what
explains
the
evolution
of the
incident irradiance.
The
con-
sidered string
was
composed
of 22 PV
modules associated
in series.
Two of
these modules were affected
by
hot-spots while
the
others were
not.
Finally,
not
only defective cells
but
also defective
by-pass diodes
can
bring about hot-spots.
In the
latter case,
short-circuited diodes give rise
to an
easily recognizable
thermal pattern, consisting
of an
anomalous hotter band,
somewhat like
a
brushstroke extended over
the
cells pro-
tected
by the
affected diode, with several cells exhibiting
temperature differences
of
about
5 °C (as
these cells
are
short-circuited, they
do not
deliver
any
electrical power,
having
to
dissipate
the
corresponding power
as
heat)
(Buerhop
et al.,
2011b). Furthermore, temperature disper-
sion between these cells
is
also expected. This behavior
occurs because
the
solar cells that make
up
real
PV
mod-
ules
are not
completely identical,
but
have
a
certain electri-
cal characteristic mismatch that becomes
a
dispersion
of
voltage.
At the
short-circuit condition imposed
by the

20
Voltage
(V)
20
30
Voltage
(V)
Voltage
(V)
Fig.
6. I-V
curves
of a
non-defective (blue)
and a
defective
(red)
module.
The maximum power points
are
marked
for
each module.
It is
assumed
that
the
defective module
is
connected
in
series with
a
large number
of
non-defective ones
and the
resulting string
is
connected
to an
inverter able
to impose
the
maximum power point. Because
of the
disproportion
between
the
number
of
defective
and
non-defective modules,
the
operation
current
of
the whole string
is
close
to
that corresponding
to the
maximum
power
of a
non-defective module. Sinusoidal signal
(1)
represents
the
current oscillations around this value
due to the MPP
tracking strategy
of
the inverter. Sinusoidal signal
(2) and (3)
represent
the
corresponding
voltage oscillations
at a
non-defective
and at the
defective modules.
Voltage excursions
are
clearly greater
in the
defective module.
The
defective module does
not
only produce less power
but
also fluctuates
more
in
operating voltage.
(For
interpretation
of the
references
to
colour
in this figure legend,
the
reader
is
referred
to the web
version
of
this
article.)
Fig.
5. (a) I-V
and
P-V
curves
of
a non-defective (blue
and
green) module
and
of a
defective module
(red and
black) affected
by a
fill-factor loss
(b)
/- V
and P-
V curves
of a
non-defective (blue
and
green) module
and of a
defective
(red and
black) module with
a
"step" anomaly.
(For
interpre-
tation
of
the references
to
colour
in
this figure legend,
the
reader
is
referred
to
the web
version
of
this article.)
defective diode,
the sum of
the voltage
of all the
cells pro-
tected
by it is
null, leading some cells becoming positive
biased
and
others becoming negative biased.
In
this situa-
tion,
the
latter
are
slightly hotter than
the
former.
Fig. 9
shows
an
example
of a PV
module with
a
conducting
by-pass diode measured
at the
IES-UPM facilities.
Obviously, despite
the
temperature difference remaining
low, such
a
module loses effective power,
at a
ratio equal
to
the
number
of
defective diodes divided
by the
total num-
ber
of
diodes.
50
45
40
a 35
H
bJD
=
B
30
2
01
a
o
25
20
-J&*v
10
15
20
dT
HS
(°C)
25
30
3.
Experimental data
Fig.
7.
Operating voltage losses
of a
defective
PV
module respect
to a
non-defective one,
AV
HS
,
versus corresponding hot-spot temperature,
AT
HS
.
The
MPP
tracking algorithm makes
AV
HS
fluctuate
at low
irradiance.
In this work,
we
have analyzed
a
sample
of
200 defective
PV modules from
two PV
plants located
at
Cuenca
and
Cáceres (Spain): respectively,
122
poly-crystalline silicon
modules from
one
single manufacturer (p-Sil)
and 78
mono
and
poly-crystalline silicon modules from
two
man-
ufacturers
(m-Si and
p-Si2). These defective modules were
selected
on the
basis
of a
previous
IR
report made
by the
maintenance personnel
of
the
PV
plants. Then,
we
carried
out
the
following tests: visual inspection,
IR
inspection,
electroluminescence
(EL),
peak power
and
operating volt-
age.
The
Cuenca
PV
plant
(12
MW)
has
been
in
operation
since September 2011. Hot-spots soon appeared,
but the
module manufacturer agreed
to
substitute
all the
modules
exhibiting
a
hot-spot temperature greater than
30 °C on
March
2013. The IR
inspection that
led to the
selection
of
the
sample
of
defective modules analyzed
in
this article

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Abstract: A new bypass strategy for monocrystalline and polycrystalline solar panels allowing significant hot spot temperature reduction in both partial and full shading conditions is presented. The approach relies on a series-connected power MOSFET that subtracts part of the reverse voltage from the shaded solar cell, thereby acting as a voltage divider. Differently from other active bypass circuits, the proposed solution does not require either a control logic or power supply and can be easily substituted to the standard bypass diode. The operation of the new circuit is described with reference to the shading condition prescribed by the EN 61215 qualification procedure. Experiments performed on two commercial solar panels have shown that the shaded cell can be cooled up to 24 °C with respect to the case in which the standard bypass diode is adopted.

75 citations


Journal ArticleDOI
Abstract: Hot spotting is a reliability problem in photovoltaic (PV) panels where a mismatched cell heats up significantly and degrades PV panel output power performance. High PV cell temperature due to hot spotting can damage the cell encapsulate and lead to second breakdown, where both cause permanent damage to the PV panel. Therefore, the design and development of a hot spot mitigation technique is proposed using a simple, low-cost and reliable hot spot activation technique. The hot spots in the examined PV system is detected using FLIR i5 thermal imaging camera. Several experiments have been studied during various environmental conditions, where the PV module P-V curve was evaluated in each observed test to analyze the output power performance before and after the activation of the proposed hot spot mitigation technique. One PV module affected by hot spot was tested. The output power increased by approximate to 3.6 W after the activation of the hot spot mitigation technique. Additional test has been carried out while connecting the hot spot PV module in series with two other PV panels. The results indicate that there is an increase of 3.57 W in the output power after activating the hot spot mitigation technique.

50 citations


Journal ArticleDOI
Abstract: Towards tackling the evident practical challenges of fault detection and diagnosis for PV modules, especially in large-scale installations, this paper proposes two different techniques for advanced inspection mapping of PV plants; aerial triangulation and terrestrial georeferencing. The former uses data of aerial thermal/visual imagery of operating PV modules, obtained by an unmanned aerial vehicle (UAV), to generate static “inspection maps”, in the form of true orthophoto mosaics. On the other hand, georeferencing is used to associate terrestrial thermal/visual imagery, obtained at distinct positions in a PV plant, with geographic data. By such way, inspection is based on a dynamic virtual map of the installation. Both mapping techniques were tested in two grid-connected PV systems, of a total installed power of 70.2 KWp. Several defective modules were easily and accurately detected, typically as abnormal temperature profiles, in the infrared (IR) spectrum. In addition, specific thermal image patterns of operating modules, were validated and quantified by additional diagnostic measurements, and were assigned to possible fault types. On the basis of the experience feedback, the potential of the proposed techniques and their limitations, for further application to PV plants of larger scale, are also discussed.

44 citations


Journal ArticleDOI
Mahmoud Dhimish1, Violeta Holmes1, Bruce Mehrdadi1, Mark Dales1  +1 moreInstitutions (1)
Abstract: Hot spotting is a reliability problem in photovoltaic (PV) panels where a mismatched cell heats up significantly and degrades PV panel output-power performance. High PV cell temperature due to hot spotting can damage the cell encapsulate and lead to second breakdown, where both cause permanent damage to the PV panel. Therefore, the development of two hot-spot mitigation techniques is proposed using a simple and reliable method. PV hot spots in the examined PV system were inspected using the FLIR i5 thermal imaging camera. Multiple experiments have been tested during various environmental conditions, where the PV module $I - V$ curve was evaluated in each observed test to analyze the output-power performance before and after the activation of the proposed hot-spot mitigation techniques. One PV module affected by the hot spot was tested. The output power during high irradiance levels is increased to approximately 1.26 W after the activation of the first hot-spot mitigation technique. However, the second mitigation technique guarantees an increase in the power up to 3.97 W. An additional test has been examined during the partial shading condition. Both proposed techniques ensure a decrease in the shaded PV cell temperature; thus, there is an increase in PV output power.

41 citations


References
More filters

Journal ArticleDOI
E. Radziemska1Institutions (1)
Abstract: The influence of temperature and wavelength on electrical parameters of crystalline silicon solar cell and a solar module are presented. At the experimental stand a thick copper plate protected the solar cell from overheating, the plate working as a radiation heat sink, or also as the cell temperature stabilizer during heating it up to 80°C. A decrease of the output power (−0.65%/K), of the fill-factor (−0.2%/K) and of the conversion efficiency (−0.08%/K) of the PV module with the temperature increase has been observed. The spectral characteristic of the open-circuit voltage of the single-crystalline silicon solar cell is also presented. It is shown that the radiation-rate coefficient of the short-circuit current-limit of the solar cell at 28°C is 1.2%/(mW/cm2).

421 citations


Journal ArticleDOI
M.C. Alonso-García1, J.M. Ruiz, F. Chenlo1Institutions (1)
Abstract: A conventional photovoltaic module has been prepared with the purpose of accessing its cells either individually or associated. Measurements of every cell and of the whole module have been performed in direct and reverse bias, with the objective of documenting the scattering in cell parameters, working point of the cells and shading effects. Several shading profiles have been tested, and the influence of the reverse characteristic of the shaded cell in module output is stressed.

328 citations


Journal ArticleDOI
TL;DR: Findings of PV plant evaluations carried out during last years are presented, showing the possible degradation of PV modules and hidden manufacturing defects.
Abstract: The fast growth of PV installed capacity in Spain has led to an increase in the demand for analysis of installed PV modules. One of the topics that manufacturers, promoters, and owners of the plants are more interested in is the possible degradation of PV modules. This paper presents some findings of PV plant evaluations carried out during last years. This evaluation usually consists of visual inspections, I–V curve field measurements (the whole plant or selected areas), thermal evaluations by IR imaging and, in some cases, measurements of the I–V characteristics and thermal behaviours of selected modules in the plant, chosen by the laboratory. Electroluminescence technique is also used as a method for detecting defects in PV modules. It must be noted that new defects that arise when the module is in operation may appear in modules initially defect-free (called hidden manufacturing defects). Some of these hidden defects that only appear in normal operation are rarely detected in reliability tests (IEC61215 or IEC61646) due to the different operational conditions of the module in the standard tests and in the field (serial-parallel connection of many PV modules, power inverter influence, overvoltage on wires, etc.).

277 citations


"Experimental observations on hot-sp..." refers background in this paper

  • ...Fortunately, they can be easily detected through IR inspection, which has become a common practice in current PV installations (Auer et al., 2007; Botsaris and Tsanakas, 2010; Buerhop et al., 2011a,b,2012; Hoyer and Buerhop, 2008; Kasemann et al., 2009; King et al., 2000; Muñoz et al., 2011)....

    [...]

  • ...It can be observed that power dissipation at a hot-spot can vary nearly an order of magnitude depending on the defective cell (Alonso-García and Ruiz, 2006; Alonso-García et al., 2003; Herrmann et al., 1997,1998; Muñoz et al., 2011)....

    [...]


01 Jan 2014
TL;DR: The international Task 13 expert team has summarized the literature as well as their knowledge and personal experiences on actual failures of PV modules, and introduces a signal transition method for the detection of defective circuits in installed PV modules.
Abstract: One key factor of reducing the costs of photovoltaic systems is to increase the reliability and the service life time of the PV modules. Today's statistics show degradation rates of the rated power for crystalline silicon PV modules of 0.8%/year Jordan11. To increase the reliability and the service life of PV modules one has to understand the challenges involved. For this reason, the international Task 13 expert team has summarized the literature as well as their knowledge and personal experiences on actual failures of PV modules. The target audience of this work is PV module designers, PV industry, engineering lines, test equipment developers, testing companies, technological research laboratories, standardisation committees, as well as national and regional planning authorities. In the first part, this document reports on the measurement methods which allow the identification and analysis of PV module failures. Currently, a great number of methods are available to characterise PV module failures outdoors and in labs. As well as using I-V characteristics as a diagnostic tool, we explain image based methods and visual inspection. For each method we explain the basis, indicate current best practice, and explain how to interpret the images. Three thermography methods are explained: thermography under steady state conditions, pulse thermography and lock-in thermography. The most commonly used of these methods is thermography under steady state conditions. Furthermore electroluminescence methods have become an increasingly popular standard lab approach for detecting failures in PV modules. 2A less common but easier to use method is UV fluorescence. This method can be used to detect module failures similar to those detected with thermography and electroluminescence techniques; however, the PV modules must be sited outdoors for at least one and a half years for the method to be effective. For visual documentation of module conditions in the field, we set up a standard which is now accepted and used by all authors documenting such tests. This standard format allows the documentation of visible module failures in standardised way and makes the data accessible for statistical evaluation. Furthermore we introduce a signal transition method for the detection of defective circuits in installed PV modules. All methods are linked to the PV module failures which are able to be found with these methods. In the second part, the most common failures of PV modules are described in detail. In particular these failures are: delamination, back sheet adhesion loss, junction box failure, frame breakage, EVA discolouration, cell cracks, snail tracks, burn marks, potential induced degradation, disconnected cell and string interconnect ribbons, defective bypass diodes; and special failures of thin-film modules, such as micro arcs at glued connectors, shunt hot spots, front glass breakage, and back contact degradation. Where possible, the origin of the failure is explained. A reference to the characterisation method is given to identify the failure. If available, statistics of the failure type in the field and from accelerating aging tests are shown. For each failure, a description of safety issues and the influence on the power loss is given, including typical follow-up failure modes. In the third part, new test methods are proposed for detection of PV module failures in the field. A special focus is made on mechanical tests because many problems have arisen in the last few years from the mechanical loading of modules. These mechanical loads occur during transportation and from snow loads on modules mounted on an incline. Furthermore, testing for UV degradation of PV modules, ammonia corrosion (sometimes found in roofs of stock breeding buildings) and potential induced degradation are described. The latter method caused some controversy within the international standardization committee until the finalization of this report because many alternative suggestions from different countries were proposed. The test methods are explained in detail, linked to failure descriptions and the results are compared to real failure occurrences, where possible. During a past Task 13 project phase, we recognised that the topic �3.2 Characterising and Classifying Failures of PV Modules� is an important on-going subject in the field of PV research. The current review of failure mechanisms shows that the origin and the power loss associated with some important PV module failures is not yet clear (e.g. snail tracks and cell cracks). There are also still some questions as to how best to test for some types of failure (e.g. potential induced degradation and cell cracks). Furthermore, despite the fact that a defective bypass diode or cell interconnect ribbon in a PV module may possibly lead to a fire, very little work has been done to detect these defects in an easy and reliable way once installed in a PV system. However, there are research groups currently working on those topics in order to overcome these challenges. Therefore, it is planed to continue our in-depth review of failures of photovoltaic modules in an extension of the TASK 13 project.

271 citations


Journal ArticleDOI
Carl R. Osterwald1, T. J. McMahon1Institutions (1)
TL;DR: The intent and history of these qualification tests, provided in this review, shows that standard module qualification test results cannot be used to obtain or infer a product lifetime.
Abstract: We review published literature from 1975 to the present for accelerated stress testing of flat-plate terrestrial photovoltaic (PV) modules. An important facet of this subject is the standard module test sequences that have been adopted by national and international standards organizations, especially those of the International Electrotechnical Commission (IEC). The intent and history of these qualification tests, provided in this review, shows that standard module qualification test results cannot be used to obtain or infer a product lifetime. Closely related subjects also discussed include: other limitations of qualification testing, definitions of module lifetime, module product certification, and accelerated life testing. Copyright © 2008 John Wiley & Sons, Ltd.

255 citations


"Experimental observations on hot-sp..." refers background or methods in this paper

  • ...However, previous experiences do not allow a clear relation between module temperature and lifetime to be established (Osterwald and McMahon, 2009)....

    [...]

  • ...We propose to consider 85 °C, which is the maximum temperature of the thermal cycling tests described in the IEC-61215, as the maximum absolute PV module temperature for acceptance/rejection purposes (Osterwald and McMahon, 2009)....

    [...]


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