Renewable Energy 29 (2004) 1997–2010
www.elsevier.com/locate/renene
Technical note
Estimation of photovoltaic module yearly
temperature and performance based on
Nominal Operation Cell Temperature
calculations
M.C. Alonso Garcı́a , J.L. Balenzategui
Departamento de Energı́as Renovables, CIEMAT, Av. Complutense 22, E-28040 Madrid, Spain
Received 10 November 2003; accepted 23 March 2004
Abstract
The simulation of module temperature from Nominal Operation Cell Temperature
(NOCT) is widely used to easily estimate module performance along the year. In this context, it is important to determine this parameter in a reliable way, as it is used to compare
the performance of different module designs and can influence system predictions. At present
there are several international standards that indicate the method to calculate NOCT in
crystalline and thin-film terrestrial photovoltaic modules. This work presents the results
obtained when applying these standards to different types of PV modules, including glass–
glass and glass–tedlar structures, crystalline and thin-film technologies, and some special
module designs for building integration applications. NOCT values so calculated have been
used to estimate the yearly module temperature and performance for different orientations
and tilted angles, analysing temperature influence in these estimations. Possible error sources
that could bring about erroneous values of NOCT are also analysed.
# 2004 Elsevier Ltd. All rights reserved.
Keywords: Qualification and testing; PV module; Yearly performance
1. Introduction
Module temperature is a parameter that has great influence in the behaviour of a
PV system, as it modifies system efficiency and output energy [1]. It depends on the
Corresponding author. Tel.: +34-91-346-6360; fax: +34-91-346-6037.
E-mail address: carmen.alonso@ciemat.es (M.C. Alonso Garcı́a).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.renene.2004.03.010
1998
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
module encapsulating material, its thermal dissipation and absorption properties,
the working point of the module, the atmospheric parameters such as irradiance
level, ambient temperature and wind speed [2] and the particular installing conditions.
It is common to use NOCT as an indicative of module temperature, in fact,
manufacturers usually include this parameter in module data sheets. It is defined as
the mean solar cell junction temperature within an open-rack mounted module in
Standard Reference Environment (SRE): tilt angle at normal incidence to the
direct solar beam at local solar noon; total irradiance of 800 W/m2; ambient temv
perature of 20 C; wind speed of 1 m/s and nil electrical load. It is an important
parameter in module characterisation, since it is a reference of how the module will
work when operating in real conditions. Furthermore, in PV system design and
simulation programs, many of the calculations are based on the determination of
module temperature from ambient temperature and NOCT.
There are several international standards that include the means to calculate
NOCT in crystalline PV modules (EN-61215) [3], thin-film PV modules (EN-61646)
[4] or both (non-concentrator terrestrial PV modules and arrays, ASTM E1036M)
[5]. All of them are based on the fact that difference between module temperature
minus ambient temperature is largely independent of air temperature and is essentially linearly proportional to irradiance level. Besides, some methods have been
proposed to determine NOCT from indoor measurements with solar simulator if
NOCT of a reference module is known [6] although they have been also questioned
in some publications [7].
Work presented in this paper is addressed to two general objectives:
– To verify the suitability of the procedure to determine NOCT according to the
international standards in a country with a Mediterranean climate such as Spain,
with elevated summer temperatures. This has been performed in different types
of modules, including special designs for building integration application and
thin-film modules.
– To apply NOCT values in yearly simulations of module temperature and energy
yields. This has been calculated for PV modules with different tilt angles and
orientations, analysing the influence of temperature and NOCT measured values
on the obtained results.
2. Procedure
The procedure to determine the NOCT of a PV module included in the standards EN-61215 and EN-61646 is based on the fact that the difference between the
module temperature Tj and the ambient temperature Tamb can be considered independent of the ambient temperature and linearly proportional to the irradiance at
levels above 400 W/m2. The procedure calls for plotting (Tj Tamb ) against
irradiance for a period when wind conditions are favourable. All data points taken
during the following conditions must be rejected: irradiance below 400 W/m2,
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
1999
wind speed outside the range 1 0:75 m=s, ambient temperatures outside the range
v
20 15 or varying by more than 5 C, a 10 min. interval after a wind gust of more
v
than 4 m/s and wind direction within 20 of east or west. From the linear
regression of the plotting of the difference between module minus ambient temperature against the irradiation, a preliminary value of NOCT is obtained. This
v
value will be corrected to 800 W/m2 and 20 C depending on the average values of
ambient temperature and wind speed during the test. The procedure will be
repeated at least in another day.
2.1. Experimental details
An outdoor facility with a data acquisition system has been set-up at Ciemat following the indications of the above mentioned standards. It consists of an open
rack to place the modules similar to the ones used in PV power plants and the
necessary instrumentation to record the parameters used in NOCT determination.
Irradiance, ambient temperature, wind speed, wind direction and module temperature have been recorded every 30 s during the testing periods. For the special
cases of modules with an air camera between the front and rear side, several thermocouples have been adhered in both sides to evaluate temperature differences.
2.2. Sample description
NOCT facility was used to measure conventional PV modules and some special
designs for building integration. Table 1 lists the main characteristics of the conventional modules tested. Samples m-1 to m-6 are m-Si modules belonging to the
same manufacturer whose main difference is the cell area and the number of cells
per module. Front cover is glass, back cover is white tedlar, the encapsulant is
EVA and the frame anodised aluminium. Besides, a-1 is an a-Si module encapsulated in glass–EVA–white tedlar and a-2 is another a-Si module from a different
manufacturer encapsulated in glass–glass.
Table 2 summarized the main characteristic of the second group of modules tested, denoted from now S-1 to S-9. They are modules specially designed for building
Table 1
First group of modules subjected to the test. Nc is the number of cells in the module and P.F. the
packing factor, indicated only for the modules with the same type of cell from the same manufacturer
Name
Nc
Module area (m2)
Cell area (cm2)
Cell type
P.F.
m-1
m-2
m-3
m-4
m-5
m-6
a-1
a-2
36
36
36
36
36
36
14
57 110
0.96989
0.47601
0.63120
0.47601
0.335
0.513
0.191
0.600
235.8
117.9
147.5
117.9
73.75
117.9
0.911
133.4
m-Si
m-Si
m-S
m-Si
m-Si
m-Si
a-Si
a-Si
0.875
0.892
0.841
0.892
0.792
0.820
–
–
Nc
72
72
36
72
72
30
30
30
57
Name
S-1
S-2
S-3
S-4
S-5
S-6
S-7
S-8
S-9
0.8996
0.8996
0.4400
0.8996
0.8996
0.6000
0.6000
0.6656
0.6656
Module area (m2)
100
100
104.5
100
100
174.6
174.6
174.6
104.4
Cell area (cm2)
LI (TG)
CG (TG)
LI (TG)
CG (TG)
LI (TG)
TCO
TCO
HG/PVB/TCO
HG/PVB/TCO
Front cover
FG (TG)
FG (CT)
Transp. tedlar
FG (CT)
FG (TG)
TG
TG
FG/air/TG
FG/air/TG
Rear cover
EFG/multi
EFG/multi
Mono-Si
EFG
EFG
ASITHRU
a-Si
ASITHRU
ASITHRU
Cell
Resin
EVA
EVA
EVA
Resin
EVA
EVA
PVB
Resin
Encapsulant
2 mm tape
None
Anodised Al frame
None
Hot melt
None
None
Sealing material
Sealing material
Edge seal
Table 2
Second group of PV modules under test. ASITHRU are amorphous silicon semitransparent cells, while standard a-Si cells are opaque. EFG/multi are
multicrystalline-Si substrate cells processed as EFG technology. Used key: LI, low iron glass; CG, cerium-doped low iron glass; CT, chemically tempered
glass; TG, tempered glass; TCO, glass with transparent conductor oxide; FG, float glass; PVB, poly-vinil-butilen adhesive; Nc, number of cells
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M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
2001
integration under an EC funded project, and they include m-Si, multi-Si and a-Si
cells with different transparency degrees and encapsulated with different materials.
3. Results on NOCT determination
3.1. Consideration on the method
When applying the procedure to the modules listed in Tables 1 and 2, first thing
observed was that different periods along one day that could fulfil the conditions
expressed in the standard were possible to find. Fig. 1 shows an example of a typical plot of Tj Tamb versus irradiance for a clear sunny day. It can be observed
that, even though at high irradiances the two lines seems to come near, for irradiances below 700 or even 800 W/m2 the lines separate with differences that can
v
reach more than 5 C at the same irradiance. This fact has also been observed previously by some authors [8]. One of the specifications of the standard is that ambiv
ent temperatures outside the range 20 15 or varying by more than 5 C should be
rejected. Paying attention to that, one might think that the temperature should not
v
change in more than 5 C in the period of testing. If we apply that to the graph of
Fig. 1, only a portion of it would be valid, furthermore, several different portions
of it would be valid depending on the temperature range. Calculations on NOCT
were made for different temperature intervals along the day in all the samples here
tested. It was found that when NOCT is calculated with data measured only in the
Fig. 1. Differences in module temperature for the same irradiance depending on the period of the day.
Ambient temperature is also plotted.
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M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
v
morning, their values were around 3 C lower than when it is calculated with data
measured only in the afternoon, after midday. This is probably due to a combined
effect of different parameters affecting the final module temperature, as ambient
and sky temperature, spectral irradiance distribution, the own module thermal lag,
and infrared radiation reflexion and emission from the floor and surrounding
objects. For example, with reference to the irradiance distribution, it has been
demonstrated that, at least in the case of Madrid (location where this work has
been developed), afternoon spectra have relative higher contents on infrared radiation than in the morning [9]. If all data points are taken into account for NOCT
computation, the value of NOCT obtained is an average between morning and
afternoon ones, and is very close to the value obtained when only the portion of
the graph with higher irradiances is observed (notice that the irradiance range
should be of at least 300 W/m2 according to the standard).
This difference is less strong for the case of modules whose back cover is tedlar,
when compared to glass–glass modules as it can be seen in Fig. 1. This is an expected behaviour derived form the worse thermal cooling of a glass–glass structure
when compared with a glass–tedlar arrangement, conducting to different temperature gradients between cell and rear covers. For the temperatures measured in the
front cover of sample S-8, the effect gets stronger, obtaining differences in Tj v
Tamb of more than 10 C for the same level of irradiance measured in the morning
and in the evening.
The values that have been considered as the most representative of the module
behaviour along the day have been those obtained taking into account all possible
acceptable data points, including morning and afternoon, as they give an averaged
value of NOCT. Nevertheless, special care must be taken when only a portion
of the day is taken to calculate NOCT, i.e. for example, when only half of the day
has stable irradiance conditions, as it could result in NOCT values over or under
estimated.
3.2. Temperature measurements on special module designs
For modules listed in Table 2, a problem arose when determining module temperature. Samples S-8 and S-9 are specially designed for the integration into buildings, and count on an air camera in their back side. In these cases, the temperature
measured with a thermocouple in the back side serves only as a rough indicative of
the insulating capability of the module structure, but not to estimate cell temperature since there is no thermal coupling between the cells and the sensors placed in
the rear side. Several thermocouples were attached in the front and rear side of one
v
of these modules, obtaining temperature differences of more than 15 C. An
example is shown in Fig. 2, where the temperature of thermocouples attached in
the front and rear side of a module with air camera (S8), and the temperature of a
thermocouple in the rear side of a quite similar module (S6) without air camera
during one day is plotted. It can be noticed that this last temperature is in between
the two measurements of sample S8. In these cases, or in general in non-conventional PV modules in which the temperature of the rear side of the module is not
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
2003
Fig. 2. Differences in module temperatures along the day depending on the position of the thermocouples.
indicative of the cell temperature, it would be helpful to count on internal temperature sensors provided by manufacturers inside the encapsulation or to state alternative methods to correctly determine module temperature.
3.3. NOCT values
The values of NOCT obtained for the two groups of samples are presented in
the following. For the second group of samples, NOCT values of S-8 and S-9 are
not presented because, as it has been analysed in the preceding section, the measured module temperatures are not indicatives of the cell temperature, and the
obtained values of NOCT would be erroneous.
All the calculations of NOCT have been performed in at least three days per
sample, and the values presented are the average of the different values encountered.
In Table 3 are presented NOCT values obtained for the first group of samples.
Samples m-1 to m-6, that belong to the same manufacturer, have NOCT in a range
Table 3
NOCT values of samples in Table 1
Sample
m-1
v
48.1
NOCT ( C)
v
Mean Ta ( C) 33.0
Test period
September
m-2
m-3
m-4
m-5
m6
a-1
a-2
49.0
18.8
March
50.3
18.8
March
49.1
18.8
March
47.5
18.4
April
47.8
18.4
April
49.2
48.8
25
25
April–June
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M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
Table 4
NOCT values of samples in Table 2
Sample
S-1
NOCT
49.4
v
Mean Ta ( C) 18.5
Test period
S-2
S-3
S-4
S-5
S-6
S-7
50.2
18.5
46.8
18.5
April–May
49.1
18.5
50.4
18.5
46.5
28.7
July
48.5
28.7
July
v
of 2.8 C of difference between maximum and minimum. Samples m-2 and m-4,
that correspond to the same module type, with the same cell size but different
power class, present almost the same NOCT value as it can be observed in Table 3.
Besides, sample m-5, with the lowest cell area, has also the lowest NOCT. Nevertheless, no conclusion related to cell size can be inferred from these results as it is
not a general tendency.
It has been pointed out elsewhere [7] that more than cell size is the packing factor which influences NOCT due to the reduced absorption of module compound
and high emissivity of tedlar sheet. In our case the sample with the lowest packing
factor has also the lowest NOCT, in accordance to Ref. [7], but it is not a clear
trend among the rest of the samples. Other aspects related either to module structure or to the environmental factors in the period of NOCT determination could
influence the measurements.
In Table 4 are presented the results on the second group of samples, excluding
S-8 and S-9. First thing that can be observed in Table 4 are the lower values of
NOCT for the a-Si modules when compared to the other glass–glass samples.
Besides, the semitransparent a-Si module presents a specially low value of NOCT.
Glass–tedlar crystalline Si modules presented in Table 4 show lower values of
NOCT than glass–glass modules. Notice that this result is based on measurements
carried out in the rear cover of the module. The thermal effects above mentioned
could call for a modification in the application of NOCT standards paying
attention to module structure in order to measure module temperatures as close as
possible to cell temperatures.
Despite the special structure of sample S-8 made it unsuitable for the application
of the standard, the same calculations were carried out from the measurements of
the thermocouple in its front and rear side. The average values so obtained were
v
v
36.20 C for the thermocouple adhered in the back side, and 50.20 C for the
thermocouple in the front side.
4. Estimation of yearly module temperature and performance
An example of application of NOCT determination is the calculation of module
temperature from ambient temperature, available solar irradiance and NOCT
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
2005
following the known equation:
Tm ¼ Tamb þ ðNOCT 20Þ
E
800
ð1Þ
where Tm is the module temperature, Tamb is the ambient temperature and E the
irradiance in W/m2. Expression (1) is widely used to estimate in a simple way module temperature along the year [7,10], although it is only valid for open rack systems. The use of the Installed Nominal Operation Cell Temperature (INOCT) has
also been reported [11], that is the cell temperature of an installed array taking into
account the mounting configuration of the module and that it is connected to a
load. For the case of an open rack, it is recommended that one use a value of
v
INOCT 3 C less than the NOCT value [11], or calculate it based on NOCT and
the PV module efficiency, which gives similar results [8,12]. Roof integrated modules operate at temperatures higher than those in open rack, having INOCT values
v
from 10 to 20 C above depending on the convective cooling [11,13].
A simulation of the yearly module generation and module temperature has been
performed using Eq. (1) and a calculation program developed by Ciemat based on
the SRADLIB library [14] for module in-plane irradiance computation. Typical
Meteorological Year of Madrid has been used as source of ambient temperature
and global radiation values as a site that could present average weather data for
many Spanish locations. The values of direct and diffuse irradiance on horizontal
and tilted surfaces have been then calculated in an hourly basis by means of the
models proposed by Macagnan [15] and Perez [16]. Ground radiation is calculated
from an isotropic radiation model [17]. The module rating methodology is based
on the experimental I–V curves measured for the reported samples at standard test
conditions and the correction procedures suggested by the standard EN-60891 [18]
for different working conditions. Although other rating procedures that have been
proposed could give more refined results [12], we have used those included in the
standards as a simple way to evaluate the relative influence of the module temperature and NOCT values on yearly performance estimations.
Simulation has been performed for PV modules placed at different tiled angles
and orientations. In Fig. 3 is shown an example of the calculated maximum daily
v
temperatures that can reach a module with NOCT ¼ 47 C along the year when it
v
is located on a south oriented vertical or tilted 35 surface. Notice that these temperature estimated values have been obtained despite the specific structure of the
module (glass–glass or glass–tedlar), because neither the NOCT determination procedure nor Eq. (1) are taking into account a different thermal behaviour of different structures as other refined models does (for example model presented in Ref.
[11]).
Another example of NOCT influence is shown in Fig. 4, where the differences in
the maximum module temperature have been calculated, in a monthly average
basis, when the module orientation is changed from south-east to south-west, for
different tilted angles. In general, it must be noticed the higher module temperatures in the south-west orientation when main insolation of the module takes place
during the afternoon, specially for modules placed far from the optimum orien-
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M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
Fig. 3. Calculated maximum module daily temperature for a module placed in south oriented vertical
v
v
and 35 tilted surface in Madrid (NOCT ¼ 47 C).
Fig. 4. Estimated monthly average difference in maximum module temperature between south-west and
v
south-east oriented surfaces at different tilted angles (NOCT ¼ 47 C).
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
2007
tation. This is in accordance with the results obtained previously of higher temperatures of the module in the evening than in the morning for the same level of
irradiance.
The same simulations were calculated with modules with different NOCT values,
obtaining differences of the same order of the differences in input NOCT. For
v
example, when increasing NOCT from 47 to 50 C, the module temperature
v
increased during the year a mean of ~3 C for south oriented surfaces tilted 20 and
v
v
50 , and ~2 C for vertical south-west and south-east surfaces. Maximum differv
ences obtained when NOCT is so changed in calculations were of ~4 C for the
v
two south oriented surfaces and ~5 C for the two vertical surfaces. An example of
these results is shown in Fig. 5.
However, besides these temperature differences so calculated, it becomes of
higher importance their influence on the estimation of the yearly module performance. We have simulated the yearly energy production of the samples S3 and S5
for all orientations (from east to west) and slopes (from horizontal to vertical). For
both samples, we have calculated the difference between module yearly performv
ance when NOCT values are increased or decreased in 3 C from its measured
value, taking into account that this could be a range of suitable values that could
have been obtained with the indeterminations mentioned for the standard procedure. Calculation results, presented in Fig. 6, show that estimated differences as
high as 1.3% for S3 sample and as high as 1.5% for S5 have been obtained in module output only when NOCT values are so changed. This is in reasonable agreement with previous results with more refined rating procedures which estimated
Fig. 5. Calculated differences in maximum daily temperature for a south oriented module at two
v
different sloped angles, only by changing its NOCT value from 50 to 47 C.
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M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
Fig. 6. Relative estimated increase (in percentage) in yearly energy output for a module with (a) glass–
v
v
tedler and (b) glass–glass structure when NOCT values are varied in 6 C (from NOCT þ3 C to
v
NOCT 3 C).
v
differences as high as 3% in energy output for NOCT values differing in 9 C [7].
Notice also that obtained differences are higher for orientations and slopes close to
v
optimum values (south oriented, 35 inclination) for modules placed in Madrid.
M.C. Alonso Garcı́a, J.L. Balenzategui / Renewable Energy 29 (2004) 1997–2010
2009
5. Conclusions
Main conclusions inferred from the application of the International Standards
EN 61215 and EN-61646 to determine the NOCT of a PV device are the following:
– When we consider the temperature of a PV module during the day, a heating of
the module is observed during the afternoon–evening. This makes differences of
Tmod Tamb higher in the afternoon than in the morning for the same values of
v
irradiance, causing differences of about 2–3 C in final NOCT values. NOCT
calculated using the maximum possible data points in the morning and afternoon gives averaged values that seems to be more representative of module
behaviour during the day.
v
– Despite these uncertainties in NOCT calculation, inaccuracies of about 3 C in
NOCT value do not introduce excessive errors (about 1.5%) on yearly performance estimations, as temperature has a second order influence on module
energy output.
– The method as it is proposed in EN-61215 and EN-61646 is not the most
adequate for modules integrated into buildings (modules located in vertical or
horizontal surfaces, modules closed to walls or isolated in their back side, or
window modules that count on air cameras between the two external glasses), as
their working conditions will be very different to the those expressed in the standard. Improvements in relation to particular module location and installation
could be considered.
– A method for the correct determination of module temperature taking into
account different encapsulations or structures (glass–glass, glass–tedlar, window
thermally isolated modules) should be defined when it is necessary to obtain
accurate predictions of temperature and performance.
Acknowledgements
Part of this work has been supported by the European funded Project JOR3CT97-0182. Authors wish to thank Mr. Faustino Chenlo for his valuable discussions and Mr. Miguel Angel Ariza for his collaboration in NOCT measurements.
Typical Meteorological Year of Madrid was developed by Mr. Luis Zarzalejo at
Ciemat.
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