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Analysis of the configuration and the location of thermographic equipment for the inspection in photovoltaic systems
Accepted Manuscript ANALYSIS OF THE CONFIGURATION AND THE LOCATION OF THERMOGRAPHIC EQUIPMENT FOR THE INSPECTION IN PHOTOVOLTAIC SYSTEMS G. Álvarez-Tey, R. Jiménez-Castañeda, J. Carpio PII: DOI: Reference:
S1350-4495(17)30443-7 https://doi.org/10.1016/j.infrared.2017.09.022 INFPHY 2393
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
25 July 2017 22 September 2017 30 September 2017
Please cite this article as: G. Álvarez-Tey, R. Jiménez-Castañeda, J. Carpio, ANALYSIS OF THE CONFIGURATION AND THE LOCATION OF THERMOGRAPHIC EQUIPMENT FOR THE INSPECTION IN PHOTOVOLTAIC SYSTEMS, Infrared Physics & Technology (2017), doi: https://doi.org/10.1016/j.infrared. 2017.09.022
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ANALYSIS OF THE CONFIGURATION AND THE LOCATION OF THERMOGRAPHIC EQUIPMENT FOR THE INSPECTION IN PHOTOVOLTAIC SYSTEMS G. Álvarez-Tey
(1)
, R. Jiménez-Castañeda(1), J. Carpio
(2)
(1)
(2)
Departamento de Ingeniería Eléctrica, Escuela Superior de Ingeniería, Universidad de Cádiz, Campus Universitario de Puerto Real, 11519 Puerto Real (Cádiz), SPAIN, tlf.: + 34 956 483326, e-mail: [email protected] Departamento de Ingeniería Eléctrica, Electrónica y Control, E.T.S. Ingenieros Industriales, U.N.E.D., C/ Juan del Rosal 12, 28040 Madrid, SPAIN
Highlights
The infrared thermography is used to carry out maintenance in photovoltaic systems. Analysis of the influence of the reflected temperature in outdoor IR inspections. Value of the emissivity in PV modules of diverse types is determined. Study of the proper location in order to minimize reflections of the sun and the sky. Abstract The infrared (IR) thermography is a non-destructive technique (NDT) which is used to carry out maintenance quickly and easily in photovoltaic (PV) systems. IR imaging with thermographic cameras under steady state conditions is a usual method for quality control of PV modules and plants in operation. For the proper IR inspection which determines the severity or the importance of the detected findings, it is necessary to consider different aspects of the configuration and the location of the thermographic equipment which allow reducing measuring errors. This paper considers some elements which contribute to the accurate configuration of the thermographic equipment. The influence of the reflected apparent temperature in outdoor IR inspections is analysed and it is proposed a simple method for obtaining it. Besides, the importance of the emissivity in IR thermography is analysed. For that, the value of the emissivity in PV modules of various types both front and rear shape is determined experimentally. It is also studied the proper location of the thermographic equipment in order to minimize reflections of the sun and the sky. For this objective, it is studied the ideal and minimum height of inspection according to the layout of the PV system. In a particular case, it is also analysed the influence of the horizontal angle of thermographic inspection and the reflected radiation. Keywords: Infrared thermography, PV system maintenance, Glass emissivity, Reflected radiation
characterization may be of qualitative type if it is expected to determinate partial defects (hot spots, dirt, decoupling cells, etc.) without bearing in mind the severity of these defects. The characterization may also be of quantitative type if it is expected to determinate accurately the temperature what makes possible to consider the importance of the detected incidents and the consequent maintenance operations [5], [6], [7].
1. Introduction In the last few years, an important increase has been producing worldwide in the number of photovoltaic (PV) systems, in the amount of installed power and also energy provided [1]. This growth has made suitable maintenance techniques necessary. Carrying out a thermographic inspection in a PV system in outdoor it is possible to characterize its behaviour [2], [3], [4]. The 1
However, the thermographic inspection in PV systems in outdoor is complex and requires certain knowledge about thermic science and to be specialized in its application. An inaccurate configuration and an unsuitable location of the thermographic camera may cause inaccurate results which may lead to a misinterpretation thereof.
2. Implementation and results 2.1. Determination of the reflected apparent temperature in IR inspections in outdoor exposures The infrared radiation which is sent out from a clear or overcast sky has an impact on PV system modules and it is necessary to bear in mind. Such radiation comes from all directions and it has to be differentiated from isolated radiation which is sent out by the sun. If the sky is clear, and due to the high transmissivity values of atmosphere in 7-14 μm wavelength range, the reflected apparent temperature will drop at very low values. Values which range from -50 °C to -60 °C may even be reached on clear days. The area of the sky seen from the surface of the earth is superior to the area of the sun, so the apparent temperature reflected in the outdoor IR thermography may drop below 0 °C. When IR inspections are carried out in outdoor, it is necessary to know the equivalent value of the sky temperature with the aim of compensating its effect over the inspected PV modules. This balance will be carried out by configuring properly the value of the reflected apparent temperature in the thermographic equipment. An unsuitable configuration of this parameter for outdoor IR inspections, or mistaking such parameter with the atmospheric temperature may lead to errors in the determination of frontal temperature from up to 10 ºC in PV modules. In some equipment, this parameter is delimitated to a specific value and a configuration of cooler values will not be possible.
Thermographic infrared images taken of PV modules in operation are influenced by several parameters as ambient temperature, wind speed, irradiation and cloud coverage. The possible occurrence of hot spots in PV modules can be easily detected by IR thermography. However, the severity or the importance of these will depend on the reached temperature value. For this, it is necessary to minimize measurement errors and to have specific maintenance criteria. A protocol of acceptance/ rejection criterion of hot spots in PV systems by using IR thermography have been published [8]. Recently, the standard IEC 62446-3 [9] which allow IR inspections to be carried out in this kind of systems have been published. This international standard defines outdoor thermography on photovoltaic modules and Balance-of-system (BOS) components of PV power plants in operation. Standard IEC 62446-3 considers the requirements for the measurement equipment, ambient conditions, inspection procedure, inspection report, personnel qualification and a matrix for thermal abnormalities as a guideline for the inspection. This standard aims to harmonize and homogenize the inspection methodology in order to facilitate the comparison of results.
In Fig. 1 in the graphic, it is possible to observe the effect of the temperature variation of an object at 30 °C under certain conditions (emissivity 0.9, reflected apparent temperature 0 ºC, distance 5 m, atmospheric temperature 20 ºC and relative humidity 50%) when the variation of the reflected apparent temperature and different emissivity values are taken into account. The values taken into consideration for reflected apparent temperature from -50 ºC up to 30 ºC are those which could be given in an inspection in an
This paper considers some aspects which contribute to the accurate configuration of the thermographic equipment. It is also studied the proper location of the thermographic equipment in order to minimize reflections of the sun and the sky.
2
outdoor exposure. The values taken into consideration for the emissivity are the typical ones for glass in PV modules.
unsuitable configuration of such parameters in the thermographic equipment. In order to determinate experimentally the reflected apparent temperature of the sky, according to reference [10], a thermography camera and a Lambert radiator may be used. A Lambert radiator is an object which reflects the incidental radiation with optimum diffusion, that is, with the same intensity in all directions. A Lambert radiator, as shown in Fig. 3, may be made easily with crumpled aluminium foil and then smoothed over a cardboard as a support.
Temperature variation of object at 30 °C with reflected apparent temperature and emissivity Temperature reading (°C)
40 38 Emissivity
36 34
0,8
32
0,85
30
0,9
28
0,95
26 -50
-40
-30
-20
-10
0
10
20
30
Reflected apparent temperature (°C)
Fig. 1. Effect of the temperature reading variation of an object at 30 °C with reflected apparent temperature for different emissivity values
37,4 °C
30
In Fig. 2 in the graphic, it is possible to observe the effect of the temperature variation of an object at 18ºC in the same conditions as those quoted above, when both the variation of the reflected apparent temperature and the different emissivity values are considered.
20
10
0 - 5,6 °C
Fig. 3. Experimental test with aluminium foil for measuring the reflected apparent temperature of the sky
For outdoor applications in clear sky conditions, depending on the incident angle, this method can be ineffective because the camera can present under-load saturation as consequence of the atmospheric transmittance window [11]. In this case is possible to measure the effective sky temperature using an infrared reflector with reflectance properties as close as possible to those of the specimen for to reduce errors [12]. For this purpose, a PV module of the same type must be used.
Temperature variation of object at 18 °C with reflected apparent temperature and emissivity Temperature reading (°C)
28 26 Emissivity
24 22
0,8
20
0,85
18 16
0,9
14
0,95
12 -50
-40
-30
-20
-10
0
10
20
30
Reflected apparent temperature (°C)
Fig. 2. Effect of the temperature reading variation of an object at 18 °C with reflected apparent temperature for different emissivity values
To calculate the reflected apparent temperature of the sky, the PV module will be placed in the shade, near the object to be measured, in a parallel position to specimen, in such a way, that the object is directly exposed to the sky. The thermography camera is configured with a distance to object=0 and emissivity=1. The temperature measured with the camera will be close to the actual reflected apparent temperature consequently such value will have to be configured in the camera in order to obtain subsequently balanced measuring values with this effect.
The temperature values of the object in the Fig. 1 and Fig. 2 have been obtained by software of FLIR thermographic equipment. It´s necessary take into account the fact that the correct temperature reading is obtained only if the correct emissivity (here 0.9) and reflected apparent temperature (here 0°C) is entered to the system. This situation is indicated in Fig. 1 and Fig. 2 by a mark. These graphics allow estimate the error that may be made as a consequence of an
3
Regarding the place of inspection in order to determine the emissivity, the following considerations should be taken:
2.2. Experimental determination of the emissivity in PV modules The emissivity value is a fundamental parameter to carry out quantitative thermography [13]. As a first approximation, or in those cases where a high accuracy level is not required, it is possible to consult in tables the emissivity values for different materials. Concerning the glass used for the photovoltaic industry, the references consulted [14], [11], [15] indicate emissivity values ranging from 0.85 a 0.92. Concerning the Tedlar, which is a common polymeric material which covers the back side of the PV modules, the tables indicate an emissivity value about 0.9. However, in situations where one needs to know the temperature with high accuracy, the emissivity must be obtained experimentally. Thus, in order to obtain the emissivity of a particular material, it is possible to carry it out in two ways [16], [17], [18],[19]:
The experimental emissivity obtained in the frontal part shows some difficulties in its determination due to the existence of reflections (sun, sky, buildings, etc.) which implies a disturbance for measuring the temperature. The experimental emissivity obtained at the back area is determined more easily because there are fewer disturbances of external reflections. However, it is shown a higher difficulty in order to access to sloping systems and with a more limited vision field which makes physically difficult to carry out the measurement. In order to determinate experimentally the emissivity, both frontal and rear shape, some inspections are carried out for the PV modules of 3 different technologies (monolayer Si-a, trilayer Si-a and crystalline Si). It is followed the previous procedure established, by using three contact sensors, which are placed on frontal and back sides of the PV module under study. The test is carried out without an electrical charge of the module, that is, at open circuit. The inspection on the front side is carried out at a short distance in such a way that the solar reflection or other nearby elements is avoided. The inspection at the back side is carried at a distance which allows covering the biggest vision field. The necessary minimum irradiance on the plane of the modules for this test must be greater than 600 W/m2 while the wind speed must be less than 1 m/s [2], [9], [20].
1. By using a contact sensor: The touch sensor is placed on a reference point and the temperature is measured. Subsequently, the emissivity value in the thermographic equipment is adjusted until the temperature value matches the reading of the contact sensor. 2. By using an auxiliary element of known emissivity: It is necessary to place an auxiliary element (a tape or paint) on the reference object whose emissivity is known. The temperature of the tape or paint is measured by adjusting the emissivity to a correct value. Once known the actual temperature of the object and taking an adjacent reference point, the emissivity value of such point will be adjusted until the measured temperature value is provided.
The thermographic images carried out are shown in Fig. 4, while a summary of the results obtained of emissivity and conditions are shown in Table 1.
4
Monolayer Si-a Inclination = 90° AR01
Trilayer Si-a Inclination = 15° 35,7 °C
Crystalline Si Inclination = 15° 48,2 °C
46,0 °C 40
AR01
value decreases while the reflectivity value increases. Both parameters have complementary values. In the case of the glass used in PV modules, shown in Fig. 5, we are able to observe this type of behaviour. The ideal situation which the emissivity value can be considered constant is carrying out the inspection with an angle slightly greater than 0º in order to avoid self-reflection but never exceeding an angle 40 ° as maximum limit to carry out the inspection.
45 AR01
Frontal
30
40
20
35
10
30 0
25 20,7 °C
AR01
- 5,5 °C
24,2 °C
44,1 °C
34,4 °C
42,1 °C
AR01
40
40 AR01
35
Rear
35 30 30
25 20
25 18,5 °C
16,4 °C
24,2 °C
Fig. 4. Thermographic images obtained for experimental determination of emissivity
Monolayer Si-a
Reflected Apparent Temperature (°C) Atmospheric Temperature (°C) Relative Humidity (%) Temperature Measured (°C) Distance (m) Calculated Emissivity
Trilayer Si-a
Crystalline Si
Frontal
Rear
Frontal
Rear
Frontal
Rear
0
0
-45
30
-45
30
27,6
27,6
26,6
26,6
27,1
27,1
41
41
41
41
41
41
32,9
33,1
36,7
36,7
38,0
38,2
2
1
2
1
2
1
0,861
0,916
0,879
0,960
0,788
0,985
Fig. 5. Varying the emissivity and reflectivity in the glass with the angle of incidence [17]
It can be considered that the emissivity value remains unchanged between these values. An inspection angle greater than 40° implies both a strong decrease in the emissivity and a significant increase in the reflectivity and therefore an increase in the effect of the reflected radiation.
Table 1. Results of emissivity obtained and conditions for obtaining
The emissivity results obtained are approximated to the values provided by tables. In any case, the experimental determination by using the contact sensor, as described above, allows by means of IR thermography to obtain accurately the temperature that can reach the PV modules on both sides.
2.3. Determination of the height for the front thermographic inspection The value of the emissivity of a material depends on the angle of the thermographic inspection [2], [9], [14]. In the case of PV modules, which are plane elements and with specular reflection, the inspection with a 0º angle should be avoided because a frontal or perpendicular inspection on the surface it could mean the operator reflection. As the inspection angle increases, the emissivity
Fig. 6. Experimental arrangement to determine the emissivity with IR radiation source and thermographic camera according to angle of inspection on PV module
The limit value of 40° has been verified experimentally in laboratory according to the layout indicated in Fig. 6. The emissivity and reflectivity on glass-coated PV module has been obtained for different inspection angles 5
inspection, angle of inclination (α) of PV modules and the total length (L) of them. The equation 1 allows, considering the established rectangular triangle, to calculate the ideal height (h) once the remaining parameters (α, L and d) are known.
with the use an IR radiation source and a thermographic camera. In the case of PV systems in outdoor exposures, and taking into consideration that their modules are normally arranged with a certain inclination, it will be necessary to determinate the height for the thermographic inspection from an inspection angle near to 0º, that is, perpendicular to the target.
It is also possible to determine the minimum height for the thermographic inspection. From this position of the camera, the whole length of the module with an angle less than 40 ° must be inspected. Such minimum height is reached, as seen in Fig. 8, when the limit value of 40 ° at the upper end of the PV modules is reached as it corresponds to the most unfavourable situation. For the calculation of such minimum height (hm), the following parameters, shown in Fig. 8, are taking into consideration: horizontal distance (d) from the place of inspection, angle of inclination (α) of PV modules and total length (L) of PV modules arranged. The equation 2, which has been obtained by application of sine theorem, allows to calculate the minimum height (hm) once the remaining parameters (α, L and d) are known.
h
. L
α
hm
40°
d
Fig. 7. Arrangement of elements and parameters to calculate the inspection ideal height
0
L
(1)
α d
Equation 1. Equation to calculate the inspection ideal height
Fig. 8. Arrangement of elements and parameters to calculate the inspection minimum height
We refer to such height as the ideal one, although the inspection must be carried out from a position that avoids the reflection itself. Concerning the calculation of the ideal height (h), the following parameters, shown in Fig. 7, are taking into consideration: horizontal distance (d) from the place of
0
(2) Equation 2. Equation to calculate the inspection minimum height
6
It is necessary to take into consideration, as shown in Fig. 7 and Fig. 8, both the inspection ideal height (h) and the inspection minimum height (hm) have been established taking as a reference the lower end of PV modules. If such a lower end were higher than the ground level such height would be added to the calculated one in order to place the thermography equipment.
LI01
LI02
35,6 °C
26,4 °C
It is summarized in Table 2, the values of the ideal height (h) and minimum height (hm) calculated by applying the equations 1 and 2 for the cases in which the inclination (α) takes values of 30 ° and 40 °, the length (L) takes values of 2 and 4 m, and the distance (d) takes values of 2 and 4 m. From the results obtained, the difficulty of carrying out the inspection from the ground level is checked, therefore it might be necessary to use either ladders or aerial means (drone or helicopter) [21], [22]. α
L (m)
d (m)
h (m)
hm (m)
30°
2
2
5,5
2,4
30°
2
4
7,5
4,0
30°
4
2
8,9
3,1
30°
4
4
10,9
4,7
40°
2
2
3,9
1,9
40°
2
4
5,5
3,5
40°
4
2
6,3
2,3
40°
4
4
7,9
3,8
°C 36 34 32 30 28 26
Fig. 9. Effect of the reflection of the sky to the front of the PV module
In the thermography in Fig. 10, it is observed the effect of the sky reflection on the PV system located at the front side which has an inclination of 15 ° while the thermal solar system located the back side has an inclination of 45 °. If we consider the same emissivity value and the same environmental conditions, it is obtained a difference of temperature of about 7 ° C less for frontal system as a result of the reflection of the sky.
Table 2. Ideal height and minimum height values which are calculated for the indicated values of α, L and d
56,7 °C 55
50
In the thermography in Fig. 9 it is observed the effect of the reflection of the sky at the top of the PV module due to gradual change of the emissivity. Moreover, real temperature gradient from bottom to top it is caused by air circulation at the top. The module shows an inclination of 40º, d= 2 m and an inspection height of 1.5m. The analysis is carried out with the straight lines located on each module which indicate a gradual variation in temperature as a result of reflection of the sky and the effect of air circulation.
45
40
35 32,5 °C
Fig. 10. Solar reflection effect due to different angles of inclination of the systems
7
point inspection and the thermal imaging camera. Above an inspection angle of 40 °, as seen in Fig. 5, the emissivity value decreases while the reflectivity value increases. Due to this situation, the effect of reflections coming from both the sun and the sky increase with such angle.
2.4. Analysis of the influence of the horizontal angle of thermographic inspection In IR inspections in outdoor, it will be necessary to consider the influence of reflections which may alter the temperature reading of the measured object. These reflections will normally depend on both solar radiation as an isolated source and the sky, whether it is clear or there is a presence of clouds. In the case of PV modules, due to their plane form, these reflections will be of specular type. For that reason, it will be necessary to analyse both the thermographic inspection angle influence according to the sun location at the moment of the inspection and the sky conditions. Likewise, we need to consider, as it has been discussed in the previous section, that the value of the calculated emissivity will remain constant up to a maximum angle of about 40° measured between the normal to the plane of the modules and the direction which connects
With the objective of analysing the effect of the horizontal angle of the thermographic inspection, due to the reflected radiation of the sun and the sky, it is carried out a test in outdoor. The horizontal angle of the thermographic inspection is established as the angle measured on the horizontal plane which forms the inspection direction of the thermographic equipment and the direction south. This definition is analogous to definition of solar azimuth, in such a way that, may be possible to draw conclusions depending on the solar position.
38,0 °C
38,0 °C
AR02 AR01
AR01
AR02
18,1 °C
75°
18,1 °C 38,0 °C
38,0 °C
AR02
-75°
AR01
60°
AR01
-60°
AR02
45° AR02
18,1 °C
AR01
38,0 °C 18,1 °C
38,0 °C
-45°
AR01
30° 38,0 °C
AR02
15°
0°
AR01
-30°
-15°
AR01
AR02
38,0 °C
AR02
18,1 °C 18,1 °C
AR01
38,0 °C
AR02
AR02
38,0 °C
AR01
38,0 °C
AR02
AR01
18,1 °C
18,1 °C
18,1 °C
18,1 °C
18,1 °C
Fig. 11. Thermographic images taken from different horizontal angles of inspection
8
Azimuth =-19,70° Elevation = 42,61°
Two PV modules of polycrystalline silicon at open circuit are used in this test. These modules are exposed to sunlight, orientated to the south and with an inclination angle of 40°. The thermographic images are carried out from different horizontal angles of inspection (75°, 60°, 45°, 30°, 15°, 0°, -15°, -30°, -45°, 60° and -75°) and at a distance of 2.5 m. An emissivity of 0.92, a reflected apparent temperature of -42° C and an atmospheric temperature of 20.0° C it is considered. The irradiance measured on the plane of the modules is greater than 600 W/m2. The solar position at the time of test is of an elevation of 43.12° and an azimuth of -17.09°. The test lasts a few minutes; therefore, the variation of the solar position is not taken into consideration.
The situation of the maximum temperatures reading is located in the lower part of the modules according to isotherms in green in Fig. 11. The situation of the minimum temperatures reading is located in the upper part of the modules according to the isotherms in orange colour as shown in Fig. 11. However the values of maximum and minimum temperatures reading vary as seen in Fig. 12 as a consequence of the reflections coming from both the sun and the sky. It is verified that at the furthest upper parts of the inspection site it is produced a predominance of reflections of the sky (cooler temperatures). This is due to the angle of incidence increases over 40° for such areas and the reflectivity is increased. As for the obtained thermographies in the solar reflection areas (angles 15º, 30º and 45º) it is also verified that reflections of the sun increase. This is due to the specular behaviour of the modules according to the position of the sun during the inspection (hotter temperatures). As for position 75°, the reflectivity is high so the effect of solar reflections is greater. As for positions -60° and -75°, the reflectivity is high but the reflections of the sky predominate.
The different IR thermographies and the analysis tools used can be observed in Fig. 11. Concretely, a rectangular area on the surface of each PV module has been used, in such a way, that the aluminium frame and the protective diode box located at the top back side can be avoided. It has also been used an isothermal in colour green which indicates for each image the zones of maximum temperature for the modules. Likewise, it has been used an isothermal in colour orange which indicates for each image the zones of minimum temperature for the modules. The values of maximum and minimum temperature reading according to the horizontal angle of thermographic inspection are shown graphically in Fig. 12.
Therefore, the ideal horizontal angle of thermographic inspection must be the angle which minimizes the solar reflection and does not increase the reflectivity. A suitable position may be to locate the thermographic camera behind the sun, and with an inspection angle less than -45°. Besides, in order to minimize reflections of the sky will be necessary to carry out the inspection at a given height.
Temerature reading (°C)
Maximum and minimum temperatures according to the horizontal angle of the inspection 38,0 33,0 28,0
3. Conclusions The IR thermography applied to PV systems is a technique that must be applied suitably in order to obtain accurate temperature values. These temperature values may be associated with incidents so that measuring errors should be reduced for their evaluation. It is necessary to consider the following aspects to carry out the inspection:
Maximum Minimum
23,0
18,0 75° 60° 45° 30° 15° 0° -15° -30° -45° -60° -75°
Angles of inspection (°)
Fig. 12. Maximum and minimum temperature reading according to the horizontal angle of inspection
9
1. - Consider the importance of the apparent reflected temperature in inspections in outdoor and obtain it by using an infrared reflector. 2. - Determine the value of the emissivity of the PV modules experimentally according to the place of inspection. 3. - Determine the ideal height and minimum height to carry out the inspection according to the layout of the PV system. 4. - Consider the inspection with a horizontal angle in such a way that the solar reflectance is minimized. Following the previous recommendations, the effectiveness of maintenance operations of these systems is increased.
[7]
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[9]
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(2005)e1, Standard test methods for measuring and compensating for emissivity using infrared imaging radiometers, no. Standard E 1933–99a. 2006, pp. 5–7. FLIR Systems, “The Ultimate Infrared Handbook for R&D Professionals,” p. 44, 2012. W. Minkina and S. Dudzik, “Infrared Thermography: Errors and Uncertainties.” John Wiley & Sons, Ltd., 2009. M. Vollmer and K. P. Möllmann, “Infrared Thermal Imaging: Fundamentals, Research and Applications.” Wiley, 2011. FLIR Systems, “Thermal Imaging Guidebook for building and renewable energy applications.” p. 68, 2011. C. Buerhop and H. Scheuerpflug, “Comparison of IR-Images and Module Performance under Standard and Field Conditions,” 2015, vol. 1, pp. 3260–3264. V. P. and R. G. Jorge Coello, Leonardo Perez, “IR Thermography Inspection of PV modules in large PV Plants with UAV,” in 31st European Photovoltaic Solar Energy Conference and Exhibition, 2015.
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S1350-4495(17)30443-7 https://doi.org/10.1016/j.infrared.2017.09.022 INFPHY 2393
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
25 July 2017 22 September 2017 30 September 2017
Please cite this article as: G. Álvarez-Tey, R. Jiménez-Castañeda, J. Carpio, ANALYSIS OF THE CONFIGURATION AND THE LOCATION OF THERMOGRAPHIC EQUIPMENT FOR THE INSPECTION IN PHOTOVOLTAIC SYSTEMS, Infrared Physics & Technology (2017), doi: https://doi.org/10.1016/j.infrared. 2017.09.022
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ANALYSIS OF THE CONFIGURATION AND THE LOCATION OF THERMOGRAPHIC EQUIPMENT FOR THE INSPECTION IN PHOTOVOLTAIC SYSTEMS G. Álvarez-Tey
(1)
, R. Jiménez-Castañeda(1), J. Carpio
(2)
(1)
(2)
Departamento de Ingeniería Eléctrica, Escuela Superior de Ingeniería, Universidad de Cádiz, Campus Universitario de Puerto Real, 11519 Puerto Real (Cádiz), SPAIN, tlf.: + 34 956 483326, e-mail: [email protected] Departamento de Ingeniería Eléctrica, Electrónica y Control, E.T.S. Ingenieros Industriales, U.N.E.D., C/ Juan del Rosal 12, 28040 Madrid, SPAIN
Highlights
The infrared thermography is used to carry out maintenance in photovoltaic systems. Analysis of the influence of the reflected temperature in outdoor IR inspections. Value of the emissivity in PV modules of diverse types is determined. Study of the proper location in order to minimize reflections of the sun and the sky. Abstract The infrared (IR) thermography is a non-destructive technique (NDT) which is used to carry out maintenance quickly and easily in photovoltaic (PV) systems. IR imaging with thermographic cameras under steady state conditions is a usual method for quality control of PV modules and plants in operation. For the proper IR inspection which determines the severity or the importance of the detected findings, it is necessary to consider different aspects of the configuration and the location of the thermographic equipment which allow reducing measuring errors. This paper considers some elements which contribute to the accurate configuration of the thermographic equipment. The influence of the reflected apparent temperature in outdoor IR inspections is analysed and it is proposed a simple method for obtaining it. Besides, the importance of the emissivity in IR thermography is analysed. For that, the value of the emissivity in PV modules of various types both front and rear shape is determined experimentally. It is also studied the proper location of the thermographic equipment in order to minimize reflections of the sun and the sky. For this objective, it is studied the ideal and minimum height of inspection according to the layout of the PV system. In a particular case, it is also analysed the influence of the horizontal angle of thermographic inspection and the reflected radiation. Keywords: Infrared thermography, PV system maintenance, Glass emissivity, Reflected radiation
characterization may be of qualitative type if it is expected to determinate partial defects (hot spots, dirt, decoupling cells, etc.) without bearing in mind the severity of these defects. The characterization may also be of quantitative type if it is expected to determinate accurately the temperature what makes possible to consider the importance of the detected incidents and the consequent maintenance operations [5], [6], [7].
1. Introduction In the last few years, an important increase has been producing worldwide in the number of photovoltaic (PV) systems, in the amount of installed power and also energy provided [1]. This growth has made suitable maintenance techniques necessary. Carrying out a thermographic inspection in a PV system in outdoor it is possible to characterize its behaviour [2], [3], [4]. The 1
However, the thermographic inspection in PV systems in outdoor is complex and requires certain knowledge about thermic science and to be specialized in its application. An inaccurate configuration and an unsuitable location of the thermographic camera may cause inaccurate results which may lead to a misinterpretation thereof.
2. Implementation and results 2.1. Determination of the reflected apparent temperature in IR inspections in outdoor exposures The infrared radiation which is sent out from a clear or overcast sky has an impact on PV system modules and it is necessary to bear in mind. Such radiation comes from all directions and it has to be differentiated from isolated radiation which is sent out by the sun. If the sky is clear, and due to the high transmissivity values of atmosphere in 7-14 μm wavelength range, the reflected apparent temperature will drop at very low values. Values which range from -50 °C to -60 °C may even be reached on clear days. The area of the sky seen from the surface of the earth is superior to the area of the sun, so the apparent temperature reflected in the outdoor IR thermography may drop below 0 °C. When IR inspections are carried out in outdoor, it is necessary to know the equivalent value of the sky temperature with the aim of compensating its effect over the inspected PV modules. This balance will be carried out by configuring properly the value of the reflected apparent temperature in the thermographic equipment. An unsuitable configuration of this parameter for outdoor IR inspections, or mistaking such parameter with the atmospheric temperature may lead to errors in the determination of frontal temperature from up to 10 ºC in PV modules. In some equipment, this parameter is delimitated to a specific value and a configuration of cooler values will not be possible.
Thermographic infrared images taken of PV modules in operation are influenced by several parameters as ambient temperature, wind speed, irradiation and cloud coverage. The possible occurrence of hot spots in PV modules can be easily detected by IR thermography. However, the severity or the importance of these will depend on the reached temperature value. For this, it is necessary to minimize measurement errors and to have specific maintenance criteria. A protocol of acceptance/ rejection criterion of hot spots in PV systems by using IR thermography have been published [8]. Recently, the standard IEC 62446-3 [9] which allow IR inspections to be carried out in this kind of systems have been published. This international standard defines outdoor thermography on photovoltaic modules and Balance-of-system (BOS) components of PV power plants in operation. Standard IEC 62446-3 considers the requirements for the measurement equipment, ambient conditions, inspection procedure, inspection report, personnel qualification and a matrix for thermal abnormalities as a guideline for the inspection. This standard aims to harmonize and homogenize the inspection methodology in order to facilitate the comparison of results.
In Fig. 1 in the graphic, it is possible to observe the effect of the temperature variation of an object at 30 °C under certain conditions (emissivity 0.9, reflected apparent temperature 0 ºC, distance 5 m, atmospheric temperature 20 ºC and relative humidity 50%) when the variation of the reflected apparent temperature and different emissivity values are taken into account. The values taken into consideration for reflected apparent temperature from -50 ºC up to 30 ºC are those which could be given in an inspection in an
This paper considers some aspects which contribute to the accurate configuration of the thermographic equipment. It is also studied the proper location of the thermographic equipment in order to minimize reflections of the sun and the sky.
2
outdoor exposure. The values taken into consideration for the emissivity are the typical ones for glass in PV modules.
unsuitable configuration of such parameters in the thermographic equipment. In order to determinate experimentally the reflected apparent temperature of the sky, according to reference [10], a thermography camera and a Lambert radiator may be used. A Lambert radiator is an object which reflects the incidental radiation with optimum diffusion, that is, with the same intensity in all directions. A Lambert radiator, as shown in Fig. 3, may be made easily with crumpled aluminium foil and then smoothed over a cardboard as a support.
Temperature variation of object at 30 °C with reflected apparent temperature and emissivity Temperature reading (°C)
40 38 Emissivity
36 34
0,8
32
0,85
30
0,9
28
0,95
26 -50
-40
-30
-20
-10
0
10
20
30
Reflected apparent temperature (°C)
Fig. 1. Effect of the temperature reading variation of an object at 30 °C with reflected apparent temperature for different emissivity values
37,4 °C
30
In Fig. 2 in the graphic, it is possible to observe the effect of the temperature variation of an object at 18ºC in the same conditions as those quoted above, when both the variation of the reflected apparent temperature and the different emissivity values are considered.
20
10
0 - 5,6 °C
Fig. 3. Experimental test with aluminium foil for measuring the reflected apparent temperature of the sky
For outdoor applications in clear sky conditions, depending on the incident angle, this method can be ineffective because the camera can present under-load saturation as consequence of the atmospheric transmittance window [11]. In this case is possible to measure the effective sky temperature using an infrared reflector with reflectance properties as close as possible to those of the specimen for to reduce errors [12]. For this purpose, a PV module of the same type must be used.
Temperature variation of object at 18 °C with reflected apparent temperature and emissivity Temperature reading (°C)
28 26 Emissivity
24 22
0,8
20
0,85
18 16
0,9
14
0,95
12 -50
-40
-30
-20
-10
0
10
20
30
Reflected apparent temperature (°C)
Fig. 2. Effect of the temperature reading variation of an object at 18 °C with reflected apparent temperature for different emissivity values
To calculate the reflected apparent temperature of the sky, the PV module will be placed in the shade, near the object to be measured, in a parallel position to specimen, in such a way, that the object is directly exposed to the sky. The thermography camera is configured with a distance to object=0 and emissivity=1. The temperature measured with the camera will be close to the actual reflected apparent temperature consequently such value will have to be configured in the camera in order to obtain subsequently balanced measuring values with this effect.
The temperature values of the object in the Fig. 1 and Fig. 2 have been obtained by software of FLIR thermographic equipment. It´s necessary take into account the fact that the correct temperature reading is obtained only if the correct emissivity (here 0.9) and reflected apparent temperature (here 0°C) is entered to the system. This situation is indicated in Fig. 1 and Fig. 2 by a mark. These graphics allow estimate the error that may be made as a consequence of an
3
Regarding the place of inspection in order to determine the emissivity, the following considerations should be taken:
2.2. Experimental determination of the emissivity in PV modules The emissivity value is a fundamental parameter to carry out quantitative thermography [13]. As a first approximation, or in those cases where a high accuracy level is not required, it is possible to consult in tables the emissivity values for different materials. Concerning the glass used for the photovoltaic industry, the references consulted [14], [11], [15] indicate emissivity values ranging from 0.85 a 0.92. Concerning the Tedlar, which is a common polymeric material which covers the back side of the PV modules, the tables indicate an emissivity value about 0.9. However, in situations where one needs to know the temperature with high accuracy, the emissivity must be obtained experimentally. Thus, in order to obtain the emissivity of a particular material, it is possible to carry it out in two ways [16], [17], [18],[19]:
The experimental emissivity obtained in the frontal part shows some difficulties in its determination due to the existence of reflections (sun, sky, buildings, etc.) which implies a disturbance for measuring the temperature. The experimental emissivity obtained at the back area is determined more easily because there are fewer disturbances of external reflections. However, it is shown a higher difficulty in order to access to sloping systems and with a more limited vision field which makes physically difficult to carry out the measurement. In order to determinate experimentally the emissivity, both frontal and rear shape, some inspections are carried out for the PV modules of 3 different technologies (monolayer Si-a, trilayer Si-a and crystalline Si). It is followed the previous procedure established, by using three contact sensors, which are placed on frontal and back sides of the PV module under study. The test is carried out without an electrical charge of the module, that is, at open circuit. The inspection on the front side is carried out at a short distance in such a way that the solar reflection or other nearby elements is avoided. The inspection at the back side is carried at a distance which allows covering the biggest vision field. The necessary minimum irradiance on the plane of the modules for this test must be greater than 600 W/m2 while the wind speed must be less than 1 m/s [2], [9], [20].
1. By using a contact sensor: The touch sensor is placed on a reference point and the temperature is measured. Subsequently, the emissivity value in the thermographic equipment is adjusted until the temperature value matches the reading of the contact sensor. 2. By using an auxiliary element of known emissivity: It is necessary to place an auxiliary element (a tape or paint) on the reference object whose emissivity is known. The temperature of the tape or paint is measured by adjusting the emissivity to a correct value. Once known the actual temperature of the object and taking an adjacent reference point, the emissivity value of such point will be adjusted until the measured temperature value is provided.
The thermographic images carried out are shown in Fig. 4, while a summary of the results obtained of emissivity and conditions are shown in Table 1.
4
Monolayer Si-a Inclination = 90° AR01
Trilayer Si-a Inclination = 15° 35,7 °C
Crystalline Si Inclination = 15° 48,2 °C
46,0 °C 40
AR01
value decreases while the reflectivity value increases. Both parameters have complementary values. In the case of the glass used in PV modules, shown in Fig. 5, we are able to observe this type of behaviour. The ideal situation which the emissivity value can be considered constant is carrying out the inspection with an angle slightly greater than 0º in order to avoid self-reflection but never exceeding an angle 40 ° as maximum limit to carry out the inspection.
45 AR01
Frontal
30
40
20
35
10
30 0
25 20,7 °C
AR01
- 5,5 °C
24,2 °C
44,1 °C
34,4 °C
42,1 °C
AR01
40
40 AR01
35
Rear
35 30 30
25 20
25 18,5 °C
16,4 °C
24,2 °C
Fig. 4. Thermographic images obtained for experimental determination of emissivity
Monolayer Si-a
Reflected Apparent Temperature (°C) Atmospheric Temperature (°C) Relative Humidity (%) Temperature Measured (°C) Distance (m) Calculated Emissivity
Trilayer Si-a
Crystalline Si
Frontal
Rear
Frontal
Rear
Frontal
Rear
0
0
-45
30
-45
30
27,6
27,6
26,6
26,6
27,1
27,1
41
41
41
41
41
41
32,9
33,1
36,7
36,7
38,0
38,2
2
1
2
1
2
1
0,861
0,916
0,879
0,960
0,788
0,985
Fig. 5. Varying the emissivity and reflectivity in the glass with the angle of incidence [17]
It can be considered that the emissivity value remains unchanged between these values. An inspection angle greater than 40° implies both a strong decrease in the emissivity and a significant increase in the reflectivity and therefore an increase in the effect of the reflected radiation.
Table 1. Results of emissivity obtained and conditions for obtaining
The emissivity results obtained are approximated to the values provided by tables. In any case, the experimental determination by using the contact sensor, as described above, allows by means of IR thermography to obtain accurately the temperature that can reach the PV modules on both sides.
2.3. Determination of the height for the front thermographic inspection The value of the emissivity of a material depends on the angle of the thermographic inspection [2], [9], [14]. In the case of PV modules, which are plane elements and with specular reflection, the inspection with a 0º angle should be avoided because a frontal or perpendicular inspection on the surface it could mean the operator reflection. As the inspection angle increases, the emissivity
Fig. 6. Experimental arrangement to determine the emissivity with IR radiation source and thermographic camera according to angle of inspection on PV module
The limit value of 40° has been verified experimentally in laboratory according to the layout indicated in Fig. 6. The emissivity and reflectivity on glass-coated PV module has been obtained for different inspection angles 5
inspection, angle of inclination (α) of PV modules and the total length (L) of them. The equation 1 allows, considering the established rectangular triangle, to calculate the ideal height (h) once the remaining parameters (α, L and d) are known.
with the use an IR radiation source and a thermographic camera. In the case of PV systems in outdoor exposures, and taking into consideration that their modules are normally arranged with a certain inclination, it will be necessary to determinate the height for the thermographic inspection from an inspection angle near to 0º, that is, perpendicular to the target.
It is also possible to determine the minimum height for the thermographic inspection. From this position of the camera, the whole length of the module with an angle less than 40 ° must be inspected. Such minimum height is reached, as seen in Fig. 8, when the limit value of 40 ° at the upper end of the PV modules is reached as it corresponds to the most unfavourable situation. For the calculation of such minimum height (hm), the following parameters, shown in Fig. 8, are taking into consideration: horizontal distance (d) from the place of inspection, angle of inclination (α) of PV modules and total length (L) of PV modules arranged. The equation 2, which has been obtained by application of sine theorem, allows to calculate the minimum height (hm) once the remaining parameters (α, L and d) are known.
h
. L
α
hm
40°
d
Fig. 7. Arrangement of elements and parameters to calculate the inspection ideal height
0
L
(1)
α d
Equation 1. Equation to calculate the inspection ideal height
Fig. 8. Arrangement of elements and parameters to calculate the inspection minimum height
We refer to such height as the ideal one, although the inspection must be carried out from a position that avoids the reflection itself. Concerning the calculation of the ideal height (h), the following parameters, shown in Fig. 7, are taking into consideration: horizontal distance (d) from the place of
0
(2) Equation 2. Equation to calculate the inspection minimum height
6
It is necessary to take into consideration, as shown in Fig. 7 and Fig. 8, both the inspection ideal height (h) and the inspection minimum height (hm) have been established taking as a reference the lower end of PV modules. If such a lower end were higher than the ground level such height would be added to the calculated one in order to place the thermography equipment.
LI01
LI02
35,6 °C
26,4 °C
It is summarized in Table 2, the values of the ideal height (h) and minimum height (hm) calculated by applying the equations 1 and 2 for the cases in which the inclination (α) takes values of 30 ° and 40 °, the length (L) takes values of 2 and 4 m, and the distance (d) takes values of 2 and 4 m. From the results obtained, the difficulty of carrying out the inspection from the ground level is checked, therefore it might be necessary to use either ladders or aerial means (drone or helicopter) [21], [22]. α
L (m)
d (m)
h (m)
hm (m)
30°
2
2
5,5
2,4
30°
2
4
7,5
4,0
30°
4
2
8,9
3,1
30°
4
4
10,9
4,7
40°
2
2
3,9
1,9
40°
2
4
5,5
3,5
40°
4
2
6,3
2,3
40°
4
4
7,9
3,8
°C 36 34 32 30 28 26
Fig. 9. Effect of the reflection of the sky to the front of the PV module
In the thermography in Fig. 10, it is observed the effect of the sky reflection on the PV system located at the front side which has an inclination of 15 ° while the thermal solar system located the back side has an inclination of 45 °. If we consider the same emissivity value and the same environmental conditions, it is obtained a difference of temperature of about 7 ° C less for frontal system as a result of the reflection of the sky.
Table 2. Ideal height and minimum height values which are calculated for the indicated values of α, L and d
56,7 °C 55
50
In the thermography in Fig. 9 it is observed the effect of the reflection of the sky at the top of the PV module due to gradual change of the emissivity. Moreover, real temperature gradient from bottom to top it is caused by air circulation at the top. The module shows an inclination of 40º, d= 2 m and an inspection height of 1.5m. The analysis is carried out with the straight lines located on each module which indicate a gradual variation in temperature as a result of reflection of the sky and the effect of air circulation.
45
40
35 32,5 °C
Fig. 10. Solar reflection effect due to different angles of inclination of the systems
7
point inspection and the thermal imaging camera. Above an inspection angle of 40 °, as seen in Fig. 5, the emissivity value decreases while the reflectivity value increases. Due to this situation, the effect of reflections coming from both the sun and the sky increase with such angle.
2.4. Analysis of the influence of the horizontal angle of thermographic inspection In IR inspections in outdoor, it will be necessary to consider the influence of reflections which may alter the temperature reading of the measured object. These reflections will normally depend on both solar radiation as an isolated source and the sky, whether it is clear or there is a presence of clouds. In the case of PV modules, due to their plane form, these reflections will be of specular type. For that reason, it will be necessary to analyse both the thermographic inspection angle influence according to the sun location at the moment of the inspection and the sky conditions. Likewise, we need to consider, as it has been discussed in the previous section, that the value of the calculated emissivity will remain constant up to a maximum angle of about 40° measured between the normal to the plane of the modules and the direction which connects
With the objective of analysing the effect of the horizontal angle of the thermographic inspection, due to the reflected radiation of the sun and the sky, it is carried out a test in outdoor. The horizontal angle of the thermographic inspection is established as the angle measured on the horizontal plane which forms the inspection direction of the thermographic equipment and the direction south. This definition is analogous to definition of solar azimuth, in such a way that, may be possible to draw conclusions depending on the solar position.
38,0 °C
38,0 °C
AR02 AR01
AR01
AR02
18,1 °C
75°
18,1 °C 38,0 °C
38,0 °C
AR02
-75°
AR01
60°
AR01
-60°
AR02
45° AR02
18,1 °C
AR01
38,0 °C 18,1 °C
38,0 °C
-45°
AR01
30° 38,0 °C
AR02
15°
0°
AR01
-30°
-15°
AR01
AR02
38,0 °C
AR02
18,1 °C 18,1 °C
AR01
38,0 °C
AR02
AR02
38,0 °C
AR01
38,0 °C
AR02
AR01
18,1 °C
18,1 °C
18,1 °C
18,1 °C
18,1 °C
Fig. 11. Thermographic images taken from different horizontal angles of inspection
8
Azimuth =-19,70° Elevation = 42,61°
Two PV modules of polycrystalline silicon at open circuit are used in this test. These modules are exposed to sunlight, orientated to the south and with an inclination angle of 40°. The thermographic images are carried out from different horizontal angles of inspection (75°, 60°, 45°, 30°, 15°, 0°, -15°, -30°, -45°, 60° and -75°) and at a distance of 2.5 m. An emissivity of 0.92, a reflected apparent temperature of -42° C and an atmospheric temperature of 20.0° C it is considered. The irradiance measured on the plane of the modules is greater than 600 W/m2. The solar position at the time of test is of an elevation of 43.12° and an azimuth of -17.09°. The test lasts a few minutes; therefore, the variation of the solar position is not taken into consideration.
The situation of the maximum temperatures reading is located in the lower part of the modules according to isotherms in green in Fig. 11. The situation of the minimum temperatures reading is located in the upper part of the modules according to the isotherms in orange colour as shown in Fig. 11. However the values of maximum and minimum temperatures reading vary as seen in Fig. 12 as a consequence of the reflections coming from both the sun and the sky. It is verified that at the furthest upper parts of the inspection site it is produced a predominance of reflections of the sky (cooler temperatures). This is due to the angle of incidence increases over 40° for such areas and the reflectivity is increased. As for the obtained thermographies in the solar reflection areas (angles 15º, 30º and 45º) it is also verified that reflections of the sun increase. This is due to the specular behaviour of the modules according to the position of the sun during the inspection (hotter temperatures). As for position 75°, the reflectivity is high so the effect of solar reflections is greater. As for positions -60° and -75°, the reflectivity is high but the reflections of the sky predominate.
The different IR thermographies and the analysis tools used can be observed in Fig. 11. Concretely, a rectangular area on the surface of each PV module has been used, in such a way, that the aluminium frame and the protective diode box located at the top back side can be avoided. It has also been used an isothermal in colour green which indicates for each image the zones of maximum temperature for the modules. Likewise, it has been used an isothermal in colour orange which indicates for each image the zones of minimum temperature for the modules. The values of maximum and minimum temperature reading according to the horizontal angle of thermographic inspection are shown graphically in Fig. 12.
Therefore, the ideal horizontal angle of thermographic inspection must be the angle which minimizes the solar reflection and does not increase the reflectivity. A suitable position may be to locate the thermographic camera behind the sun, and with an inspection angle less than -45°. Besides, in order to minimize reflections of the sky will be necessary to carry out the inspection at a given height.
Temerature reading (°C)
Maximum and minimum temperatures according to the horizontal angle of the inspection 38,0 33,0 28,0
3. Conclusions The IR thermography applied to PV systems is a technique that must be applied suitably in order to obtain accurate temperature values. These temperature values may be associated with incidents so that measuring errors should be reduced for their evaluation. It is necessary to consider the following aspects to carry out the inspection:
Maximum Minimum
23,0
18,0 75° 60° 45° 30° 15° 0° -15° -30° -45° -60° -75°
Angles of inspection (°)
Fig. 12. Maximum and minimum temperature reading according to the horizontal angle of inspection
9
1. - Consider the importance of the apparent reflected temperature in inspections in outdoor and obtain it by using an infrared reflector. 2. - Determine the value of the emissivity of the PV modules experimentally according to the place of inspection. 3. - Determine the ideal height and minimum height to carry out the inspection according to the layout of the PV system. 4. - Consider the inspection with a horizontal angle in such a way that the solar reflectance is minimized. Following the previous recommendations, the effectiveness of maintenance operations of these systems is increased.
[7]
[8]
[9]
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