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Quantifying the uniformity of solar cells using thermal imaging diagnosis
Accepted Manuscript Quantifying the uniformity of solar cells using thermal imaging diagnosis Buntoon Wiengmoon , Krissanapong Kirtikara , Chaya Jivacate , Dhirayut Chenvidhya , Roongrojana Songprakorp PII:
S1359-4311(14)00384-6
DOI:
10.1016/j.applthermaleng.2014.05.023
Reference:
ATE 5622
To appear in:
Applied Thermal Engineering
Received Date: 6 December 2013 Revised Date:
24 April 2014
Accepted Date: 6 May 2014
Please cite this article as: B. Wiengmoon, K. Kirtikara, C. Jivacate, D. Chenvidhya, R. Songprakorp, Quantifying the uniformity of solar cells using thermal imaging diagnosis, Applied Thermal Engineering (2014), doi: 10.1016/j.applthermaleng.2014.05.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Quantifying the uniformity of solar cells using thermal imaging diagnosis Buntoon Wiengmoona, Krissanapong Kirtikaraa,b, Chaya Jivacateb, Dhirayut Chenvidhyab* and Roongrojana Songprakorpa Division of Energy Technology, School of Energy, Environment and Materials, b CES Solar Cells Testing Center, Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand * Corresponding Author: Email [email protected]; [email protected] Tel./Fax: +662 470 7445
Abstract
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Local hot spots and localized heating in solar cell can be detected by Infrared (IR) thermography via thermal images of the surface temperature distribution. This paper presents the method to quantify the quality of solar cells using thermal imaging analysis. By the infrared thermal imaging method, the local parallel resistances of solar cell have been analyzed, and the indices for qualifying localized heating in solar cell are proposed. The indicators, H-index, A-index and L-index, are derived using a cone shape model based on statistical analysis of thermal images. The experiment was carried out with 20 unused and no-encapsulated single crystalline solar cells, 10 cm x 10 cm in size. Among the test cells, eight of those cells were selected and compared with four different values of parallel resistances. From a comparison, one cell has a uniform temperature profile while the others exhibit local hot spots. The electrical performance measurements, such as performance at Standard Test Condition, dark IV characteristics, were also done under reverse bias conditions for comparison. The IR thermal images were taken while the cells were being tested under reverse bias in the dark and the air temperature was controlled at 25 °C. The analysis results can be summarized as follows (1) L-index relates to Tmax closely, (2) the lower the L-index, the better the temperature uniformity (3) the proposed indicators and statistical values of IR thermal analysis are in line with the resulting parallel resistances, and (4) IR thermal analysis in conjunction with the dark IV characteristic can be used to examine local structural defects in a whole cell. Moreover, the electrical parallel resistance can be calculated and verified using the proposed indicators and statistical values obtained by IR thermography.
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Keywords: Infrared thermography, solar cell, hot spot, local parallel resistances
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1. Introduction
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In photovoltaic (PV) applications and PV industry, current-voltage characteristics are usually measured either under illumination according IEC60904-1 or in the dark. The bulk parameters of the whole cell, such as series resistance, parallel resistance or a combination of shunt resistance and diode resistance, can be obtained by I-V measurements. However, the non-uniformity or inhomogeneity and local hot spots heating of a solar cell cannot be revealed or determined by the IV characteristic measurement. The hot-spot heating occurs in a PV module when its operating current exceeds the reduced short circuit current of a shaded or faulty cell or a group of cell. The local hot-spot of a cell has been less emphasized in use although the hot-spot endurance test of PV module and its effect is featured in the IEC61215:2005 standard. The infrared images are used for determining the hottest cell in a module under illumination as the cell temperature is related to its infrared emissivity. However, there are many methods that can be used for detecting failure of a PV module or a cell [1-11]. The methods include infrared images (IR), lock-in thermography (LIT), resonance ultrasonic vibrations (RUV) technique, electro-luminescence (EL) and photoluminescence (PL).
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There are various investigations regarding the local shunt heating or hot spots regions of a PV cell. In multi-crystalline solar cells, shunt defects can be detected by using lock-in thermography (LIT) and identified by scanning electron microscopy (SEM), electron beam induced current (EBIC), transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). For PV cell under reverse voltage biasing, shunt defects can locally increase the temperature of the junction [1]. The hot spot risk depends on wafer raw materials, cell sorting and fabrication process and string length in the module. The hot spots with smaller or higher leakage current density are more critical for a hot spot effect in a partially shaded PV module [2]. The non-ideal characteristic of solar cell can have large recombination current, ohmic characteristic at low reverse bias and pre-breakdown at the reverse bias far below the expected breakdown voltage [3]. Several techniques have been proposed to locate recombination defects and shunts in solar cells [7-9]. The electron beam induced current (EBIC), light beam induced current (LBIC) are typically used to measure a variation of the induced current under the local electron and photon beam excitation at the surface of the solar cell. LBIC and LIT provide complementary information to inhomogeneous solar cells. LBIC mainly displays inhomogeneities that reduce the short circuit current due to recombination-active defects. LIT has been employed to investigate inhomogeneities of the dark I–V characteristics, which mainly affects the fill factor and the open circuit voltage of the cell [8]. There are also other combination techniques such as local current-voltage curves (LIVT) and dynamic precision contact thermography (DPCT) [7-11]. LIT techniques are operating in the dark with forward bias called DLIT and Illuminated LIT (ILIT) under load condition or open-circuit voltage. Lock-in thermography provides useful investigation techniques, which allow to image not only shunts in solar cells but also inhomogeneities of the lifetime, the series resistance, and the distribution of Joule losses under realistic operation of the cell. Lock-in thermography is also a base of IR lifetime mapping techniques, which allow very efficient investigations for not only of the lifetime distribution, but also of its dependence on experimental parameters like the temperature or the illumination intensity [11]. The LIT has become a universal tool for characterizing solar cells. In our previous paper [13], the method to determine local parallel resistances of a solar cell has been developed and verified by using thermal images. The electrical-thermal model was derived. The parallel resistance of a cell can be determined from thermal measurement as well as electrical measurement, but local parallel resistances cannot be obtained from the current voltage measurement. Thermal imaging analysis can provide non-destructive assessment of local hot spot of solar cells and their local parallel resistances with basic instruments. In this paper, the method to quantify the quality of solar cells is presented by using thermal imaging analysis. Unused and non-encapsulated cells are performed in the dark with reverse bias. The indicators are derived and validated through comparison with temperature profiles and data set of electrical measurement. The focus of this paper is the new idea and indices to
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indicate the quality of solar cell by using basic instrument. By the infrared thermal imaging analysis, the quality of a solar cell is not only visual observation, but local parallel resistances and quantitative indicators can also represent the quality of solar cells.
2. Method to estimate local parallel resistances [13]
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In our previous work [13], the local parallel resistances of a solar cell can be derived from an IR thermal image in the dark and the electrical – thermal model. The spatial local parallel resistances were calculated from the spatial surface temperature distribution. The best case calculation was based on the method to estimate local parallel resistances with 64 elements or sub-cells/one cell. The correlation of temperature distribution, the electrical bulk resistance and local parallel resistances were derived using thermal image analysis and the verification was done against the experimental results. It is found in our previous publication that the difference in calculation results from thermal imaging analysis and electrical bulk resistance is less than 2% for the spatial calculation approach. An example of estimation of local parallel resistances is shown in Fig. 1. The method derived the local parallel resistances of each solar cell from its IR thermograph of a cell under reverse biasing in the dark. The local parallel resistances are calculated by the electrical – thermal model.
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(a) (b) Fig.1 Example of estimation of local parallel resistances of a solar cell (a) An IR thermal image and (b) local parallel resistances obtained by calculation
2.1 Electrical – thermal model of a solar cell
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The internal parameters of a solar cell consist of a series resistance (Rs), a shunt resistance (Rsh) and equivalent diode resistance (Rd) as shown in Fig. 2. Rs and Rsh can generally be determined either by IV characteristic at standard test condition (STC) according to IEC60904-1 or by dark IV characteristic measurement. -
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Fig. 3 Electrical and thermal power balance in a cell with flowing current
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An experimented cell is vertically hung in the temperature controlled chamber as shown in Fig.3. The cell is subjected to a reverse bias voltage in the dark. By the geometry of solar cell, the thickness of cell (300 µm) is less than one-hundredth of the length or the width of cell (10 cm). Therefore, the convection heat transfer on the area of edge of cell can be negligible when comparing with the surface area of cell. The equal heat transfer on both sides of cell can be assumed by vertically hung in the temperature controlled chamber. The power balance equation can be written as (1) Power in = Q& storage + 2 Q& convection + 2 Q& radiation From the equation (1), the thermal model can be derived for a laminar regime under free convection. The first term, Q& storage, is derived from the rate of change of temperature as
cell area (m2) – for this case 0.0099 m2 density of silicon ( 2330kg/m3) specific heat of silicon (712 J/kg-K) thickness of cell (m) – for this case 3x10-4 m temperature of wall chamber the gradient of a temperature of cell (Tcell) / time (t)
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Q& storage = DlAcell c p (dTcell dt )
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The convection heat transfer in the second term of the equation (1) is due to the temperature difference between cell’s surface and the environment, air. Q& convection = Acell hcell (Tcell − Tair ) (3) Where hcell convection heat transfer coefficient (W/m2 · K) Tcell cell temperature Tair temperature of air in the chamber This gives the convection heat transfer coefficient, hcell = Nucell k
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Where Nucell Nusselt number k thermal conductivity of air (W/m · K) L length of cell (m)
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In the laminar regime under free convection heat transfer, the Nusselt number for a cell hung vertically can be derived as follows [12, 13]: 1
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Where Grashof number, GrL = g β (Tcell − Tair ) L3 / ν 2 β coefficient of volume expansion (1/K) g gravitational acceleration (m/s2) Pr Prandtl number ν kinematic viscosity of air (m2/s) The physical and thermal properties of air are listed in Table 1
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Table 1 Physical properties of air at atmospheric pressure T, k, Pr ν, β, K W/(m.K) m2/s × 106 (1/K) 250 10.53 0.02227 0.00400 0.722 300 16.84 0.02624 0.00333 0.708 350 20.76 0.03003 0.00286 0.697 The radiation heat transfer of a cell and walls of dark chamber can be calculated by 4 4 σ (Tcell − Twall ) (5) Q& cell − wall = 1 − ε cell 1 − ε walll 1 + + Acell ε cell Awall ε wall Acell Fcell − wall Where Acell cell area total inside surface area of the dark chamber – for this case 0.45 m2 Awall Tcell cell temperature temperature of wall chamber Twall σ Stefan–Boltzmann constant (5.67 x 10-8 W/m2 · K4) ε emissivity (1) F view factor is the proportion of the radiation (1)
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Table 2 Values of physical properties and values in experimental setup
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density of silicon specific heat of silicon Stefan–Boltzmann constant Values in experimental setup emissivity view factor is the proportion of the radiation cell area thickness of cell total inside surface area of the dark chamber
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By assuming energies in the cell are balanced when the electrical energy and the thermal energy are taken into account, the bulk resistance of the cell can accordingly be determined using Eq. (6):
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P=V2/R = Q& storage + 2 Q& convection + 2 Q& radiation
4 4 σ Tcell − Twall R = V 2 DlAcell c p (dT dt ) + 2 Acell hcell (Tcell − Tair ) + 2 1 − ε cell 1 − ε wall 1 + + Acell ε cell Awall ε wall Acell Fcell − wall
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Additionally, we assume that one whole cell consists of number of constituent cells having small areas, and the equation (6) is also valid for each constituent cell. Each constituent cell is formed between the top and bottom surfaces where the conductive heat exchange across such cell’s contact surface is assumed to be small compared to convective and radiative heat transfer due to nearly equal temperature of surrounding constituent cells. In such case, R is the local parallel resistance of the constituent cell and T is the representative temperature of the constituent cell. If we assume that the bias voltage across the cell is constant, the resistance R is related to the cell material resistivity (ρ) through the relation ρ = RA/ℓ; ( ℓ, thickness of cell (m)). So, the equivalent subcell resistance is determined from the equation (6).
3. Experimental
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The electrical and thermal infrared measurements were made on twenty 10 cm × 10 cm unused and non-encapsulated crystalline cells. The effective area of each cell is 0.0099 m2. The IV characteristics under illumination at standard test condition (STC) were measured according to IEC60904-1 (1000 W/m2, AM 1.5G and cell temperature at 25° C) by the Pasan Sun Simulator 3b, AAA class according to IEC60904-9. In the dark, each cell under measurement was vertically hung in the temperature controlled chamber (0.3 m × 0.3 m × 0.3 m) at 25 °C of cell temperature. Dark IV characteristics were measured by both forward and reverse biasing. For Infrared thermal images, the IV characteristic measurements were carried out in the dark chamber with controlled temperature at 25°C of air temperature. Figure 4 illustrates a schematic diagram of the experimental setup. After reverse biasing a cell for 600 seconds, allowing the steadystate cell temperature to be reached, infrared images were taken. The infrared images are obtained by a high quality image portable IR camera (Thermo Tracer TH770 of NEC San-ei Instruments, Ltd.). Each picture has 320 pixels ×240 pixels. One pixel can represent a temperature ranging from -20 to 100 °C with resolution 0.1 °C. The Viewer Software TH78-719 was used to download thermal images from the internal memory to a PC. A programmable current source (KEITHLEY Model 224) supplied a reverse bias at 100 mA to the cells. It is equal to current density 1 mA/cm2. Three temperature values (midcell surface temperature, air temperature and wall temperature), current and voltage of cells are recorded by a data acquisition unit (Yokogawa MX100). The measurement was set up as similar as the experiment to verify the method to estimate local parallel resistances of a solar cell in our previous work [13], but this paper aims to propose the method to quantify the quality due to localized heating or hot spot on a solar cell. The experimental setup consists of (1) Performance measurements at standard test condition (STC) (2) dark IV measurements under both forward biasing and reverse biasing (this paper focuses only on reverse biasing) and (3) infrared thermal imaging under reverse biasing in the dark and analyzing at steady state cell temperature condition. The analysis is based on infrared thermal imaging. The measurement uncertainty of infrared thermal imaging is ±2.48 °C. The uncertainty analysis is based on the accuracy, the resolution and the measurement uncertainty of calibration certificate. The stated measurement uncertainties are the expanded measurement uncertainties obtained from the combined standard measurement uncertainties multiplied by the coverage factor k=2. The
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measurement uncertainty is calculated according to JCGM100:2008 where the JCGM is Joint Committee for Guides in Metrology. Solar cell
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Fig. 4 the measurement system setup
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4. Results and discussion 4.1 Performance parameters at STC
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From the experimental results at STC, the statistical values of cells parameters, such as average values and standard deviations, can be calculated from the IV characteristics of 20 cells and shown in Table 3. The parameters consist of the short circuit current (Isc), the open circuit voltage (Voc), the current at maximum power (Im), the voltage at maximum power (Vm), the fill factor (FF), the series resistance (Rs) and the shunt resistance (Rsh). The parameters can be obtained from the Pasan Sun Simulator. According to the standard deviations, each electrical parameters obtained from IV characteristics of the 20 cells listed in Table 3 is quite similar, except the shunt resistance. However, these parameters are bulk parameters which represent the whole cell and do not present significant difference values in some local areas including local irregularities, for example, hot spots on the cell.
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Table 3 Average values and standard deviations of cells parameters at STC of 20 cells Rs ISC VOC Im Vm Pm FF (A) (V) (A) (V) (W) (%) (Ω) Average 3.40 0.59 2.96 0.44 1.30 64.8 0.036 values Standard 0.05 0.01 0.07 0.01 0.06 2.7 0.0046 deviations
Rsh (Ω) 7.98 3.34
4.2 Infrared Thermal images in the dark
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The current flowing into the ideal cell, in the dark with reverse biasing, is limited by the shunt resistance (Rsh). For non-ideal cell, the current flowing in the dark is determined by the combination of Rs, Rsh and Rd. In this paper, the Rsh in parallel with Rd, is called the parallel resistance, RRD. From the IR thermography results of 20 cells, the eight cells are selected to analyze with different manners. The selection is based on values of RRD and temperature distribution profile (Tave and Tmax) as shown in Table 4. The thermal images of selected cells are shown in Fig. 5. The 4 selected cells consisting of cell No. 5, 7, 8 and 17 are considered as uniform temperature distribution that are shown in the left-hand side of Fig. 5. The other 4 selected cells consisting of cell No. 6, 13, 15 and 19 are considered as cells with localized heating or local hot-spot that are shown in righthand side of Fig. 5. By thermal images, the cells with localized heating and cells with uniform temperature can be manifested by visual observation. It is noted that (a) For the cells with uniform temperature distribution, the higher the average temperature, the higher the parallel resistance.
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(b) For the cells with localized heating, the localize heating on a cell cannot be demonstrated by the parallel resistance and average temperature. It can only be observed by the maximum temperature and visual observation.
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Fig. 5 Thermal images of selected solar cells
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Table 4 Temperature distribution and RRD of selected solar cells Tave (oC) Cell T max(oC) RRD (Ω) No. 5 61.1 26.3 28.6 No. 7 65.0 27.4 29.9 No. 8 75.7 27.1 29.4 No. 17 140.2 28.7 32.4 No. 6 64.9 28.4 41.3 No. 13 98.5 28.1 40.6 No. 15 120.5 29.9 46.4 No. 19 148.7 30.1 40.7
4.3 IR thermograph versus IV characteristics
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In Fig. 6, the IV characteristics at STC of the selected cells are shown. It is found that cells with localized heating cannot obviously be demonstrated by IV curves under illumination. IV characteristics can exhibit only the bulk electrical parameters of cells.
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Fig. 6 IV characteristic at STC of the selected solar cells (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19 In Fig. 7, the results of dark IV characteristics can be separated in two groups, although IV characteristics in the dark cannot reveal the local hot-spot of cells. The 8 selected cells are given by visual observation on IR thermal images. For the 4 cells with uniform temperature distribution, dark IV curves are shown in Fig. 7(a). The other 4 selected cells are shown in Fig. 7(b). The dark IV characteristics of cell No. 5, 7, 8 and 17 with reverse biasing are as similar as exponential relationship.
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For cell No. 6, 13, 15 and 19, the dark IV characteristics are quit linear relationship as similar as ohmic effects. It is found that the lower the slope of IV curve, the higher the RRD. It means that the dark IV characteristics measurement with reverse biasing can describe which cell having localized heating or local hot-spot manner, but it cannot quantify the quality or revealed severity level of localized heating. 0 Cell 7
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Fig. 7 Dark IV characteristics of solar cells with reverse biasing, (a) a uniformity of the surface temperature of cells, (b) localized heating or hot spot on the cells.
4.4 local parallel resistances
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Local parallel resistances of a solar cell can be calculated and derived from IR thermal image and the electrical-thermal model [13]. The local parallel resistances of the 8 selected cells are calculated as shown in Fig. 8. The 4 selected cells with uniform temperature distribution consist of Cell No. 5, 7, 8 and 17. The uniformity and values of local parallel resistances are validated the previous section of measurement. The other 4 selected cells with localized heating consist of Cell No. 6, 13, 15 and 19. The areas of local hot-spot demonstrated by thermal images are correspondent with local parallel resistances in Fig. 8. For example, in Cell No. 19, there are two local hot-spots in IR image and two low values of local parallel resistances.
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4.5 Histogram of temperature profile
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Fig. 8 The local parallel resistances of each cell which the cell had a uniformity of the surface temperature and the cell had localized heating or hot spot.
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The method validation of local parallel resistances by using IR thermal images are derived and verified in previous section. This section is the application how to quantify the quality of a solar cell by using IR thermograph. The histogram of temperature distribution is plotted and compared in Fig. 9. The results are summarized in table 5.
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Fig.9 Comparison of histogram of the number of pixel and temperature (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19 Table 5 The parallel resistances (RRD) of cells and results analysis T max/Tave Tave(oC) T max(oC) SD No. RRD (Ω) 61.1 65.0 75.7 140.2
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Temperature distribution uniform uniform uniform uniform local hotspot local hotspot local hotspot local hotspot
4.6 Histogram of local parallel resistance In this section, the histogram of local parallel resistances and the value of resistance of the selected cells is analyzed and plotted in Fig. 10.
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Fig.10 Comparison of histogram of the number of elements and local resistance (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19
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From the experimental results shown in Fig. 5 to Fig.7 and the analytical results shown in Fig. 8 to Fig.10, it can be summarized that (1) There are 4 pairs of cells (8 selected cells from 20 cells under measurement) to be compared. In each pair of cell, one is quite uniform temperature distribution, but another one is cell with localized hot spot. (2) Fig.5 demonstrates temperature distribution of each cell and shows the cells having localized hot spot (cell No. 6, 13, 15 and 19). The values average temperature of each cells correlate with values of shunt resistances given by IV characteristics at STC (in Fig.6) and dark IV characteristics (in Fig.7). (3) IV characteristics at STC (in Fig.6) are limited to describe the localized hot spot on the cells, but dark IV characteristics (in Fig.7) can demonstrate the cells having localized hot spot defects with as ohmic effect (in Fig.7 (b)). However, dark IV characteristics cannot reveal or quantify the severity level of localized heating. It is confirmed by thermal images in Fig.5. (4) Fig.8 is to confirm the calculation results of local parallel resistances of solar cell obtained by the equation (6). The locations on cells having local hot spot demonstrated by thermal images in Fig. 5 are conformed to local parallel resistances in Fig.8.
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(5) Fig.9 and Fig.10 show histogram of surface temperature and histogram of calculated local resistances by our method, respectively. Cells having localized hot spot and cells with uniform surface temperature can be demonstrated and comparable in both location plots (Fig. 5 and 9) and in histogram plots (Fig.10 and 11). Histograms are useful in statistical analysis and quantifying the quality of cells.
4.7 Method to quantify the local quality of a solar cell
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The experimental results were obtained from current-voltage measurements both at standard test condition (STC) and in the dark (dark IV characteristics) and IR thermal images. In Fig.8, local parallel resistances are derived and calculated by the method presented in our previous report [13]. The four pairs of cells (totaling 8 cells) are selected from 20 cells and grouped by values of parallel resistances (RRD) in the dark with reverse bias and thermal images as shown in table 4. In each pair of cells with similar RRD values, one is uniform surface temperature distribution, and another one is cell with localized hot spot. Uniformity is described by the ratio of Tmax/Tave of IR thermograph. Cells having Tmax/Tave over than 1.1 are cells with local hot spot. By IV characteristics in the dark with reverse biasing, it is observed that IV curves are quite linear relationship as similar as ohmic effect cells for localized hot spot cells. Slopes of linear curves depend on RRD. The slope is higher while the RRD is lower. In section 4.5 and 4.6, histograms of surface temperature and histograms of local parallel resistances of cells are plotted and compared in Fig. 9 and 10. Both histograms are validated and comparable. By thermal imaging analysis, it is not only visual observation for hot spot regions on a cell, but we also propose the indicators to quantify the local quality of a cell in this paper. The localized heating may be due to the effect of local structural defects. In this work, the proposed indicators consisting of height index (H-index), area index (A-index) and angle index (L-index), are derived by using the cone shape model as shown in Fig. 11. Statistical values of an IR thermograph, consisting of average temperature (Tave), maximum temperature (Tmax) and standard deviation (SD), are simply calculated from captured data.
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Fig. 11 Derivation of indicators obtained by thermal imaging data In Table 6, the proposed indicators and statistical values are analyzed for the eight selected cells. The RRD is calculated from the electrical measurement for benchmarking. Table 6 The proposed indicators of thermal imaging analysis No.w cell 5 cell 7 cell 8 cell 17 cell 6 cell 13 cell 15
R RD (Ω) 61.1 65.0 75.7 140.2 64.9 98.5 120.5
Tave (oC) 26.2 27.3 27.3 29.9 28.4 28.1 29.9
SD 0.4 0.9 0.7 1.3 1.4 1.7 2.0
T max 28.6 29.9 29.4 32.4 41.3 40.6 46.4
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H-Index 9.2 9.5 7.7 8.4 45.2 44.6 55.1
A-index 8.7 8.5 8.7 7.7 6.5 5.8 4.9
L-index 74.0 71.9 76.7 71.8 13.1 13.9 8.1
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From the results shown in Table 6 and Fig. 11, it is noted that - H-index reveals and represents the temperature uniformity of a cell that uniformity or hot spot (Hindex Less than10% the cell is uniformity). - L-index closely relates to Tmax, H-index and A-index - L-index reveals and represents the shape of the local heating or hot of a cell. - For L-index less than 20 degree, the localized heating is dominant, such as cell 6, 13, 15 and 19, the maximum temperature is high value.
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In Table 7, the correlations of parallel resistance and thermal imaging indicators are analyzed by using Pearson correlation. It is found that - L-index correlates with Tmax, H-index and A-index more than 90% - Tave correlates with RRD more than 92.7%
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Table 7 Correlations of resistance and thermal imaging indices RRD Tave SD Tmax Tave 0.907 SD 0.629 0.675 Tmax 0.46 0.626 0.908 H-index 0.226 0.392 0.836 0.962 A-index -0.582 -0.723 -0.904 -0.972 L-index -0.267 -0.448 -0.772 -0.92
H-index
A-index
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Moreover, the RRD can be correspondingly calculated by the regression equation from the proposed indicators and statistical values of an IR thermograph as follows:-
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RRD = 1 – 12.3 Tave + 46.9 SD + 20.4 Tmax – 8.52 H-index – 15.9 A-index – 0.248 L-index
(7)
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It means that series and parallel resistances of a solar cell can be calculated from IR thermal image analysis. This paper firstly aims to derive and to verify the method to quantify the quality of solar cell in thermal equilibrium or steady state conditions, although the equation (6) is based on transient phenomenon of electrical – thermal model of solar cell. In this paper, it is limited by the experimental setup and measuring instrument. Furthermore, this method is possible to extend for time dependent analysis, but it needs to verify. Regarding the optimization operation point, (1) Time to thermal equilibrium condition is more than 300 second, so we selected 600 second to ensure the equilibrium. (2) The measurement is made under reverse bias condition in the dark. The parallel resistances are dominant. (3) The equations (6 and 7) are derived and verified for general use, but the experimental parameters are particular for cells under measurement.
5. Conclusion
The localized heating and the surface temperature profile can generally be visualized via an IR thermal image under dark reverse biasing condition. This paper presents the method to quantify the quality of solar cell using IR thermal image analysis. The experiments were carried out on the 20 unused and non-encapsulated single crystalline cells (10 cm x 10 cm). Each cell was measured IV characteristic at standard test condition according to IEC60904-1, dark IV characteristic and infrared thermal image under reverse biasing in the dark. Eight cells, in four pairs, were selected to describe in different
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manners. The selection was based on values of parallel resistance (RRD) and temperature distribution profile. The comparison is made on each pair of cells with close values of parallel resistances. While one cell is visual uniform temperature profile, another is cell with localize heating or hot spot region. By the experimental results, the local parallel resistances are calculated from IR thermal image and the electricalthermal model detailed in our previous work [13]. On local hot spot cells, the weak areas of hot spot regions appeared by visual observation are corresponding with the local parallel resistances analysis. Histograms of temperature distribution and local parallel resistances are analyzed. Localize heating or local hot spot area represents the effect of local structural defects described by avalanche breakdown or thermal breakdown of a cell. Cells having localized heating or local hot spot can be visually observed by apparent high temperature area. In other way, it can be observed by the ohmic behavior in the dark IV characteristic under reverse bias condition. However, this paper presents that cells having local hot spot can be quantified by local parallel resistances and three proposed indices. The method to calculate local parallel resistance was presented in our previous work [13], but it is applied to analyze quality of cells. The indicators proposed in this paper consist of H-index, A-index and L-index. The indices are derived by using cone shape model. It can be summarized that (1) the L-index closely relates to Tmax, H-index and A-index (2) the lower the L-index, the better the temperature uniformity (3) the proposed indicators and statistical values of IR thermograph can be made linear regression with resultant parallel resistance, and (4) IR thermal analysis can evaluate local structural defects corresponding to the dark IV characteristic interpretations in bulk parameters.
Acknowledgements
References
3. 4. 5. 6.
7. 8.
9.
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A. Simo, S. Martinuzzi, Hot Spots and Heavily Dislocated Regions in Multicrystalline Silicon Cells, 21st IEEE Photovoltaic Specialists Conference, 1(1990) 800-805. S. Wendlandt, A. Drobisch, T. Buseth, S. Krauter, P. Grunow, Hot Spot Risk Analysis on Silicon Cell Modules, 25th EU PVSEC and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain, 4002-4006. O. Breitenstein, J. Bauer, A. Lotnyk, J.M. Wagner, Defect induced non-ideal dark I-V characteristics of solar cells, Superlattice Microst. 45 (2009) 182-189. M. Danner, K. Büchner, Reverse Characteristics of Commercial Silicon Solar Cells - Impact on Hot Spot Temperatures and Module Integrity, 26th IEEE PVSC, Anaheim (1997) 1137. M.C. Alonso-Garcıa , J.M. Ruız, Analysis and modelling the reverse characteristic of photovoltaic cells, Sol. Energ. Mat. Sol. C., 90 (2006) 1105-1120. A. Kaminski, J. Jouglar, M. Mergui, Y. Jourlin, A. Bouille, P.L. Vuillermoz, A. Laugier, Infrared characterization of hot spots in solar cells with high precision due to signal treatment processing, Sol. Energ. Mat. Sol. C., 51 (1998) 233-242. I.E. Konovalov, O. Breitenstein, K. Iwig, Local current-voltage curves measured thermally (LIVT): A new technique of characterizing PV cells, Sol. Energ. Mat. Sol. C., 48 (1997) 53-60. A. Kaminski, O Breitenstein, J.P. Boyeaux, P. Rakotoniaina, A. Laugier, Light Beam Induced Current and Infrared Thermography Studies of Multicrystalline Silicon Solar Cell, J. Phys.: Condens., 16 (2004) S9-S18. O. Breitenstein, M. Langenkamp, J. P. Rakotoniaina, EBIC investigation of a 3-dimensional network of inversion channels in solar cells on silicon ribbons, J. Solid State Phenom., 78-79 (2001) 29-38.
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The authors are grateful to the School of Energy, Environment and Materials, the CES Solar Cells Testing Center (CSSC), the Pilot Plant Development and Training Institute (PDTI) of the King Mongkut’s University of Technology Thonburi for the facilities and supporting. We are grateful to The Office of Commission on Higher Education, Ministry of Education under the Ministry Staff Development Project through Naresuan University for the financial support. We would like to thank Dr. Sirichai Thepa, Dr.Tanokkorn Chenvidhya and the staff of CSSC for their valuable comments and discussion.
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10. O. Breitenstein, W. Eberhardt, K. Iwig, Imaging The Local Forward Current Density of Solar Cells By Dynamical Precision Contact Thermography, 1994 IEEE Photovoltaic Specialists Conference , 2 (1994) 1633 – 1636. 11. O. Breitenstein, J.P. Rakotoniaina, M. Kaes, S. Seren, G. Hahn, W. Warta, J. Isenberg, T. Pernau, Lock-in Thermography - A Universal Tool for Local Analysis of Solar Cells, 15th PVSEC, (2005) 1279-1280. 12. H. Straube, O. Breitenstein, Estimation of heat loss in thermal wave experiments, J. Appl. Phys., 109, 064515 (2011) 13. B. Wiengmoon, K. Kirtikara, C. Jivacate, D. Chenvidhya, Local parallel resistances of solar cell derived by the thermal image analysis, Renew. Energ., 55 (2013) 49-54.
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Highlights on the Manuscript: ‘Quantifying the uniformity of solar cells using thermal imaging diagnosis’ Buntoon Wiengmoon, Krissanapong Kirtikara, Chaya Jivacate, Dhirayut Chenvidhya and Roongrojana Songprakorp
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- The method to quantify the quality of solar cells is presented by using IR images. - Local parallel resistances can be estimated by a thermal image. - The focus is the new idea with proposed indices to indicate quality of solar cells. - Correlations of parallel resistances and the proposed indices are presented. - Surface temperature distribution of a solar cell is quantified by the indices.
S1359-4311(14)00384-6
DOI:
10.1016/j.applthermaleng.2014.05.023
Reference:
ATE 5622
To appear in:
Applied Thermal Engineering
Received Date: 6 December 2013 Revised Date:
24 April 2014
Accepted Date: 6 May 2014
Please cite this article as: B. Wiengmoon, K. Kirtikara, C. Jivacate, D. Chenvidhya, R. Songprakorp, Quantifying the uniformity of solar cells using thermal imaging diagnosis, Applied Thermal Engineering (2014), doi: 10.1016/j.applthermaleng.2014.05.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Quantifying the uniformity of solar cells using thermal imaging diagnosis Buntoon Wiengmoona, Krissanapong Kirtikaraa,b, Chaya Jivacateb, Dhirayut Chenvidhyab* and Roongrojana Songprakorpa Division of Energy Technology, School of Energy, Environment and Materials, b CES Solar Cells Testing Center, Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi (KMUTT), 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand * Corresponding Author: Email [email protected]; [email protected] Tel./Fax: +662 470 7445
Abstract
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Local hot spots and localized heating in solar cell can be detected by Infrared (IR) thermography via thermal images of the surface temperature distribution. This paper presents the method to quantify the quality of solar cells using thermal imaging analysis. By the infrared thermal imaging method, the local parallel resistances of solar cell have been analyzed, and the indices for qualifying localized heating in solar cell are proposed. The indicators, H-index, A-index and L-index, are derived using a cone shape model based on statistical analysis of thermal images. The experiment was carried out with 20 unused and no-encapsulated single crystalline solar cells, 10 cm x 10 cm in size. Among the test cells, eight of those cells were selected and compared with four different values of parallel resistances. From a comparison, one cell has a uniform temperature profile while the others exhibit local hot spots. The electrical performance measurements, such as performance at Standard Test Condition, dark IV characteristics, were also done under reverse bias conditions for comparison. The IR thermal images were taken while the cells were being tested under reverse bias in the dark and the air temperature was controlled at 25 °C. The analysis results can be summarized as follows (1) L-index relates to Tmax closely, (2) the lower the L-index, the better the temperature uniformity (3) the proposed indicators and statistical values of IR thermal analysis are in line with the resulting parallel resistances, and (4) IR thermal analysis in conjunction with the dark IV characteristic can be used to examine local structural defects in a whole cell. Moreover, the electrical parallel resistance can be calculated and verified using the proposed indicators and statistical values obtained by IR thermography.
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Keywords: Infrared thermography, solar cell, hot spot, local parallel resistances
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1. Introduction
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In photovoltaic (PV) applications and PV industry, current-voltage characteristics are usually measured either under illumination according IEC60904-1 or in the dark. The bulk parameters of the whole cell, such as series resistance, parallel resistance or a combination of shunt resistance and diode resistance, can be obtained by I-V measurements. However, the non-uniformity or inhomogeneity and local hot spots heating of a solar cell cannot be revealed or determined by the IV characteristic measurement. The hot-spot heating occurs in a PV module when its operating current exceeds the reduced short circuit current of a shaded or faulty cell or a group of cell. The local hot-spot of a cell has been less emphasized in use although the hot-spot endurance test of PV module and its effect is featured in the IEC61215:2005 standard. The infrared images are used for determining the hottest cell in a module under illumination as the cell temperature is related to its infrared emissivity. However, there are many methods that can be used for detecting failure of a PV module or a cell [1-11]. The methods include infrared images (IR), lock-in thermography (LIT), resonance ultrasonic vibrations (RUV) technique, electro-luminescence (EL) and photoluminescence (PL).
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There are various investigations regarding the local shunt heating or hot spots regions of a PV cell. In multi-crystalline solar cells, shunt defects can be detected by using lock-in thermography (LIT) and identified by scanning electron microscopy (SEM), electron beam induced current (EBIC), transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). For PV cell under reverse voltage biasing, shunt defects can locally increase the temperature of the junction [1]. The hot spot risk depends on wafer raw materials, cell sorting and fabrication process and string length in the module. The hot spots with smaller or higher leakage current density are more critical for a hot spot effect in a partially shaded PV module [2]. The non-ideal characteristic of solar cell can have large recombination current, ohmic characteristic at low reverse bias and pre-breakdown at the reverse bias far below the expected breakdown voltage [3]. Several techniques have been proposed to locate recombination defects and shunts in solar cells [7-9]. The electron beam induced current (EBIC), light beam induced current (LBIC) are typically used to measure a variation of the induced current under the local electron and photon beam excitation at the surface of the solar cell. LBIC and LIT provide complementary information to inhomogeneous solar cells. LBIC mainly displays inhomogeneities that reduce the short circuit current due to recombination-active defects. LIT has been employed to investigate inhomogeneities of the dark I–V characteristics, which mainly affects the fill factor and the open circuit voltage of the cell [8]. There are also other combination techniques such as local current-voltage curves (LIVT) and dynamic precision contact thermography (DPCT) [7-11]. LIT techniques are operating in the dark with forward bias called DLIT and Illuminated LIT (ILIT) under load condition or open-circuit voltage. Lock-in thermography provides useful investigation techniques, which allow to image not only shunts in solar cells but also inhomogeneities of the lifetime, the series resistance, and the distribution of Joule losses under realistic operation of the cell. Lock-in thermography is also a base of IR lifetime mapping techniques, which allow very efficient investigations for not only of the lifetime distribution, but also of its dependence on experimental parameters like the temperature or the illumination intensity [11]. The LIT has become a universal tool for characterizing solar cells. In our previous paper [13], the method to determine local parallel resistances of a solar cell has been developed and verified by using thermal images. The electrical-thermal model was derived. The parallel resistance of a cell can be determined from thermal measurement as well as electrical measurement, but local parallel resistances cannot be obtained from the current voltage measurement. Thermal imaging analysis can provide non-destructive assessment of local hot spot of solar cells and their local parallel resistances with basic instruments. In this paper, the method to quantify the quality of solar cells is presented by using thermal imaging analysis. Unused and non-encapsulated cells are performed in the dark with reverse bias. The indicators are derived and validated through comparison with temperature profiles and data set of electrical measurement. The focus of this paper is the new idea and indices to
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indicate the quality of solar cell by using basic instrument. By the infrared thermal imaging analysis, the quality of a solar cell is not only visual observation, but local parallel resistances and quantitative indicators can also represent the quality of solar cells.
2. Method to estimate local parallel resistances [13]
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In our previous work [13], the local parallel resistances of a solar cell can be derived from an IR thermal image in the dark and the electrical – thermal model. The spatial local parallel resistances were calculated from the spatial surface temperature distribution. The best case calculation was based on the method to estimate local parallel resistances with 64 elements or sub-cells/one cell. The correlation of temperature distribution, the electrical bulk resistance and local parallel resistances were derived using thermal image analysis and the verification was done against the experimental results. It is found in our previous publication that the difference in calculation results from thermal imaging analysis and electrical bulk resistance is less than 2% for the spatial calculation approach. An example of estimation of local parallel resistances is shown in Fig. 1. The method derived the local parallel resistances of each solar cell from its IR thermograph of a cell under reverse biasing in the dark. The local parallel resistances are calculated by the electrical – thermal model.
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(a) (b) Fig.1 Example of estimation of local parallel resistances of a solar cell (a) An IR thermal image and (b) local parallel resistances obtained by calculation
2.1 Electrical – thermal model of a solar cell
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The internal parameters of a solar cell consist of a series resistance (Rs), a shunt resistance (Rsh) and equivalent diode resistance (Rd) as shown in Fig. 2. Rs and Rsh can generally be determined either by IV characteristic at standard test condition (STC) according to IEC60904-1 or by dark IV characteristic measurement. -
Rs Rd
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Fig. 2 Equivalent circuit of a solar cell with reverse biasing in the dark
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Fig. 3 Electrical and thermal power balance in a cell with flowing current
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An experimented cell is vertically hung in the temperature controlled chamber as shown in Fig.3. The cell is subjected to a reverse bias voltage in the dark. By the geometry of solar cell, the thickness of cell (300 µm) is less than one-hundredth of the length or the width of cell (10 cm). Therefore, the convection heat transfer on the area of edge of cell can be negligible when comparing with the surface area of cell. The equal heat transfer on both sides of cell can be assumed by vertically hung in the temperature controlled chamber. The power balance equation can be written as (1) Power in = Q& storage + 2 Q& convection + 2 Q& radiation From the equation (1), the thermal model can be derived for a laminar regime under free convection. The first term, Q& storage, is derived from the rate of change of temperature as
cell area (m2) – for this case 0.0099 m2 density of silicon ( 2330kg/m3) specific heat of silicon (712 J/kg-K) thickness of cell (m) – for this case 3x10-4 m temperature of wall chamber the gradient of a temperature of cell (Tcell) / time (t)
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Q& storage = DlAcell c p (dTcell dt )
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The convection heat transfer in the second term of the equation (1) is due to the temperature difference between cell’s surface and the environment, air. Q& convection = Acell hcell (Tcell − Tair ) (3) Where hcell convection heat transfer coefficient (W/m2 · K) Tcell cell temperature Tair temperature of air in the chamber This gives the convection heat transfer coefficient, hcell = Nucell k
L
Where Nucell Nusselt number k thermal conductivity of air (W/m · K) L length of cell (m)
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In the laminar regime under free convection heat transfer, the Nusselt number for a cell hung vertically can be derived as follows [12, 13]: 1
Nu cell = 4
GrL 0.718 Pr 2 4 (0.952 + Pr) 14
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Where Grashof number, GrL = g β (Tcell − Tair ) L3 / ν 2 β coefficient of volume expansion (1/K) g gravitational acceleration (m/s2) Pr Prandtl number ν kinematic viscosity of air (m2/s) The physical and thermal properties of air are listed in Table 1
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Table 1 Physical properties of air at atmospheric pressure T, k, Pr ν, β, K W/(m.K) m2/s × 106 (1/K) 250 10.53 0.02227 0.00400 0.722 300 16.84 0.02624 0.00333 0.708 350 20.76 0.03003 0.00286 0.697 The radiation heat transfer of a cell and walls of dark chamber can be calculated by 4 4 σ (Tcell − Twall ) (5) Q& cell − wall = 1 − ε cell 1 − ε walll 1 + + Acell ε cell Awall ε wall Acell Fcell − wall Where Acell cell area total inside surface area of the dark chamber – for this case 0.45 m2 Awall Tcell cell temperature temperature of wall chamber Twall σ Stefan–Boltzmann constant (5.67 x 10-8 W/m2 · K4) ε emissivity (1) F view factor is the proportion of the radiation (1)
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Table 2 Values of physical properties and values in experimental setup
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density of silicon specific heat of silicon Stefan–Boltzmann constant Values in experimental setup emissivity view factor is the proportion of the radiation cell area thickness of cell total inside surface area of the dark chamber
D cp σ
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By assuming energies in the cell are balanced when the electrical energy and the thermal energy are taken into account, the bulk resistance of the cell can accordingly be determined using Eq. (6):
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P=V2/R = Q& storage + 2 Q& convection + 2 Q& radiation
4 4 σ Tcell − Twall R = V 2 DlAcell c p (dT dt ) + 2 Acell hcell (Tcell − Tair ) + 2 1 − ε cell 1 − ε wall 1 + + Acell ε cell Awall ε wall Acell Fcell − wall
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Additionally, we assume that one whole cell consists of number of constituent cells having small areas, and the equation (6) is also valid for each constituent cell. Each constituent cell is formed between the top and bottom surfaces where the conductive heat exchange across such cell’s contact surface is assumed to be small compared to convective and radiative heat transfer due to nearly equal temperature of surrounding constituent cells. In such case, R is the local parallel resistance of the constituent cell and T is the representative temperature of the constituent cell. If we assume that the bias voltage across the cell is constant, the resistance R is related to the cell material resistivity (ρ) through the relation ρ = RA/ℓ; ( ℓ, thickness of cell (m)). So, the equivalent subcell resistance is determined from the equation (6).
3. Experimental
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The electrical and thermal infrared measurements were made on twenty 10 cm × 10 cm unused and non-encapsulated crystalline cells. The effective area of each cell is 0.0099 m2. The IV characteristics under illumination at standard test condition (STC) were measured according to IEC60904-1 (1000 W/m2, AM 1.5G and cell temperature at 25° C) by the Pasan Sun Simulator 3b, AAA class according to IEC60904-9. In the dark, each cell under measurement was vertically hung in the temperature controlled chamber (0.3 m × 0.3 m × 0.3 m) at 25 °C of cell temperature. Dark IV characteristics were measured by both forward and reverse biasing. For Infrared thermal images, the IV characteristic measurements were carried out in the dark chamber with controlled temperature at 25°C of air temperature. Figure 4 illustrates a schematic diagram of the experimental setup. After reverse biasing a cell for 600 seconds, allowing the steadystate cell temperature to be reached, infrared images were taken. The infrared images are obtained by a high quality image portable IR camera (Thermo Tracer TH770 of NEC San-ei Instruments, Ltd.). Each picture has 320 pixels ×240 pixels. One pixel can represent a temperature ranging from -20 to 100 °C with resolution 0.1 °C. The Viewer Software TH78-719 was used to download thermal images from the internal memory to a PC. A programmable current source (KEITHLEY Model 224) supplied a reverse bias at 100 mA to the cells. It is equal to current density 1 mA/cm2. Three temperature values (midcell surface temperature, air temperature and wall temperature), current and voltage of cells are recorded by a data acquisition unit (Yokogawa MX100). The measurement was set up as similar as the experiment to verify the method to estimate local parallel resistances of a solar cell in our previous work [13], but this paper aims to propose the method to quantify the quality due to localized heating or hot spot on a solar cell. The experimental setup consists of (1) Performance measurements at standard test condition (STC) (2) dark IV measurements under both forward biasing and reverse biasing (this paper focuses only on reverse biasing) and (3) infrared thermal imaging under reverse biasing in the dark and analyzing at steady state cell temperature condition. The analysis is based on infrared thermal imaging. The measurement uncertainty of infrared thermal imaging is ±2.48 °C. The uncertainty analysis is based on the accuracy, the resolution and the measurement uncertainty of calibration certificate. The stated measurement uncertainties are the expanded measurement uncertainties obtained from the combined standard measurement uncertainties multiplied by the coverage factor k=2. The
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measurement uncertainty is calculated according to JCGM100:2008 where the JCGM is Joint Committee for Guides in Metrology. Solar cell
V Tcell
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Fig. 4 the measurement system setup
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4. Results and discussion 4.1 Performance parameters at STC
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Table 3 Average values and standard deviations of cells parameters at STC of 20 cells Rs ISC VOC Im Vm Pm FF (A) (V) (A) (V) (W) (%) (Ω) Average 3.40 0.59 2.96 0.44 1.30 64.8 0.036 values Standard 0.05 0.01 0.07 0.01 0.06 2.7 0.0046 deviations
Rsh (Ω) 7.98 3.34
4.2 Infrared Thermal images in the dark
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The current flowing into the ideal cell, in the dark with reverse biasing, is limited by the shunt resistance (Rsh). For non-ideal cell, the current flowing in the dark is determined by the combination of Rs, Rsh and Rd. In this paper, the Rsh in parallel with Rd, is called the parallel resistance, RRD. From the IR thermography results of 20 cells, the eight cells are selected to analyze with different manners. The selection is based on values of RRD and temperature distribution profile (Tave and Tmax) as shown in Table 4. The thermal images of selected cells are shown in Fig. 5. The 4 selected cells consisting of cell No. 5, 7, 8 and 17 are considered as uniform temperature distribution that are shown in the left-hand side of Fig. 5. The other 4 selected cells consisting of cell No. 6, 13, 15 and 19 are considered as cells with localized heating or local hot-spot that are shown in righthand side of Fig. 5. By thermal images, the cells with localized heating and cells with uniform temperature can be manifested by visual observation. It is noted that (a) For the cells with uniform temperature distribution, the higher the average temperature, the higher the parallel resistance.
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Cell 6
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(b) For the cells with localized heating, the localize heating on a cell cannot be demonstrated by the parallel resistance and average temperature. It can only be observed by the maximum temperature and visual observation.
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Fig. 5 Thermal images of selected solar cells
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Table 4 Temperature distribution and RRD of selected solar cells Tave (oC) Cell T max(oC) RRD (Ω) No. 5 61.1 26.3 28.6 No. 7 65.0 27.4 29.9 No. 8 75.7 27.1 29.4 No. 17 140.2 28.7 32.4 No. 6 64.9 28.4 41.3 No. 13 98.5 28.1 40.6 No. 15 120.5 29.9 46.4 No. 19 148.7 30.1 40.7
4.3 IR thermograph versus IV characteristics
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Fig. 6 IV characteristic at STC of the selected solar cells (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19 In Fig. 7, the results of dark IV characteristics can be separated in two groups, although IV characteristics in the dark cannot reveal the local hot-spot of cells. The 8 selected cells are given by visual observation on IR thermal images. For the 4 cells with uniform temperature distribution, dark IV curves are shown in Fig. 7(a). The other 4 selected cells are shown in Fig. 7(b). The dark IV characteristics of cell No. 5, 7, 8 and 17 with reverse biasing are as similar as exponential relationship.
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For cell No. 6, 13, 15 and 19, the dark IV characteristics are quit linear relationship as similar as ohmic effects. It is found that the lower the slope of IV curve, the higher the RRD. It means that the dark IV characteristics measurement with reverse biasing can describe which cell having localized heating or local hot-spot manner, but it cannot quantify the quality or revealed severity level of localized heating. 0 Cell 7
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Fig. 7 Dark IV characteristics of solar cells with reverse biasing, (a) a uniformity of the surface temperature of cells, (b) localized heating or hot spot on the cells.
4.4 local parallel resistances
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Local parallel resistances of a solar cell can be calculated and derived from IR thermal image and the electrical-thermal model [13]. The local parallel resistances of the 8 selected cells are calculated as shown in Fig. 8. The 4 selected cells with uniform temperature distribution consist of Cell No. 5, 7, 8 and 17. The uniformity and values of local parallel resistances are validated the previous section of measurement. The other 4 selected cells with localized heating consist of Cell No. 6, 13, 15 and 19. The areas of local hot-spot demonstrated by thermal images are correspondent with local parallel resistances in Fig. 8. For example, in Cell No. 19, there are two local hot-spots in IR image and two low values of local parallel resistances.
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Fig. 8 The local parallel resistances of each cell which the cell had a uniformity of the surface temperature and the cell had localized heating or hot spot.
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Fig.9 Comparison of histogram of the number of pixel and temperature (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19 Table 5 The parallel resistances (RRD) of cells and results analysis T max/Tave Tave(oC) T max(oC) SD No. RRD (Ω) 61.1 65.0 75.7 140.2
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Temperature distribution uniform uniform uniform uniform local hotspot local hotspot local hotspot local hotspot
4.6 Histogram of local parallel resistance In this section, the histogram of local parallel resistances and the value of resistance of the selected cells is analyzed and plotted in Fig. 10.
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Fig.10 Comparison of histogram of the number of elements and local resistance (a) cell 5 and cell 6 (b) cell 7 and cell 13 (c) cell 8 and cell 15 (d) cell 17 and cell 19
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From the experimental results shown in Fig. 5 to Fig.7 and the analytical results shown in Fig. 8 to Fig.10, it can be summarized that (1) There are 4 pairs of cells (8 selected cells from 20 cells under measurement) to be compared. In each pair of cell, one is quite uniform temperature distribution, but another one is cell with localized hot spot. (2) Fig.5 demonstrates temperature distribution of each cell and shows the cells having localized hot spot (cell No. 6, 13, 15 and 19). The values average temperature of each cells correlate with values of shunt resistances given by IV characteristics at STC (in Fig.6) and dark IV characteristics (in Fig.7). (3) IV characteristics at STC (in Fig.6) are limited to describe the localized hot spot on the cells, but dark IV characteristics (in Fig.7) can demonstrate the cells having localized hot spot defects with as ohmic effect (in Fig.7 (b)). However, dark IV characteristics cannot reveal or quantify the severity level of localized heating. It is confirmed by thermal images in Fig.5. (4) Fig.8 is to confirm the calculation results of local parallel resistances of solar cell obtained by the equation (6). The locations on cells having local hot spot demonstrated by thermal images in Fig. 5 are conformed to local parallel resistances in Fig.8.
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(5) Fig.9 and Fig.10 show histogram of surface temperature and histogram of calculated local resistances by our method, respectively. Cells having localized hot spot and cells with uniform surface temperature can be demonstrated and comparable in both location plots (Fig. 5 and 9) and in histogram plots (Fig.10 and 11). Histograms are useful in statistical analysis and quantifying the quality of cells.
4.7 Method to quantify the local quality of a solar cell
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The experimental results were obtained from current-voltage measurements both at standard test condition (STC) and in the dark (dark IV characteristics) and IR thermal images. In Fig.8, local parallel resistances are derived and calculated by the method presented in our previous report [13]. The four pairs of cells (totaling 8 cells) are selected from 20 cells and grouped by values of parallel resistances (RRD) in the dark with reverse bias and thermal images as shown in table 4. In each pair of cells with similar RRD values, one is uniform surface temperature distribution, and another one is cell with localized hot spot. Uniformity is described by the ratio of Tmax/Tave of IR thermograph. Cells having Tmax/Tave over than 1.1 are cells with local hot spot. By IV characteristics in the dark with reverse biasing, it is observed that IV curves are quite linear relationship as similar as ohmic effect cells for localized hot spot cells. Slopes of linear curves depend on RRD. The slope is higher while the RRD is lower. In section 4.5 and 4.6, histograms of surface temperature and histograms of local parallel resistances of cells are plotted and compared in Fig. 9 and 10. Both histograms are validated and comparable. By thermal imaging analysis, it is not only visual observation for hot spot regions on a cell, but we also propose the indicators to quantify the local quality of a cell in this paper. The localized heating may be due to the effect of local structural defects. In this work, the proposed indicators consisting of height index (H-index), area index (A-index) and angle index (L-index), are derived by using the cone shape model as shown in Fig. 11. Statistical values of an IR thermograph, consisting of average temperature (Tave), maximum temperature (Tmax) and standard deviation (SD), are simply calculated from captured data.
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Fig. 11 Derivation of indicators obtained by thermal imaging data In Table 6, the proposed indicators and statistical values are analyzed for the eight selected cells. The RRD is calculated from the electrical measurement for benchmarking. Table 6 The proposed indicators of thermal imaging analysis No.w cell 5 cell 7 cell 8 cell 17 cell 6 cell 13 cell 15
R RD (Ω) 61.1 65.0 75.7 140.2 64.9 98.5 120.5
Tave (oC) 26.2 27.3 27.3 29.9 28.4 28.1 29.9
SD 0.4 0.9 0.7 1.3 1.4 1.7 2.0
T max 28.6 29.9 29.4 32.4 41.3 40.6 46.4
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L-index 74.0 71.9 76.7 71.8 13.1 13.9 8.1
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From the results shown in Table 6 and Fig. 11, it is noted that - H-index reveals and represents the temperature uniformity of a cell that uniformity or hot spot (Hindex Less than10% the cell is uniformity). - L-index closely relates to Tmax, H-index and A-index - L-index reveals and represents the shape of the local heating or hot of a cell. - For L-index less than 20 degree, the localized heating is dominant, such as cell 6, 13, 15 and 19, the maximum temperature is high value.
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In Table 7, the correlations of parallel resistance and thermal imaging indicators are analyzed by using Pearson correlation. It is found that - L-index correlates with Tmax, H-index and A-index more than 90% - Tave correlates with RRD more than 92.7%
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Table 7 Correlations of resistance and thermal imaging indices RRD Tave SD Tmax Tave 0.907 SD 0.629 0.675 Tmax 0.46 0.626 0.908 H-index 0.226 0.392 0.836 0.962 A-index -0.582 -0.723 -0.904 -0.972 L-index -0.267 -0.448 -0.772 -0.92
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It means that series and parallel resistances of a solar cell can be calculated from IR thermal image analysis. This paper firstly aims to derive and to verify the method to quantify the quality of solar cell in thermal equilibrium or steady state conditions, although the equation (6) is based on transient phenomenon of electrical – thermal model of solar cell. In this paper, it is limited by the experimental setup and measuring instrument. Furthermore, this method is possible to extend for time dependent analysis, but it needs to verify. Regarding the optimization operation point, (1) Time to thermal equilibrium condition is more than 300 second, so we selected 600 second to ensure the equilibrium. (2) The measurement is made under reverse bias condition in the dark. The parallel resistances are dominant. (3) The equations (6 and 7) are derived and verified for general use, but the experimental parameters are particular for cells under measurement.
5. Conclusion
The localized heating and the surface temperature profile can generally be visualized via an IR thermal image under dark reverse biasing condition. This paper presents the method to quantify the quality of solar cell using IR thermal image analysis. The experiments were carried out on the 20 unused and non-encapsulated single crystalline cells (10 cm x 10 cm). Each cell was measured IV characteristic at standard test condition according to IEC60904-1, dark IV characteristic and infrared thermal image under reverse biasing in the dark. Eight cells, in four pairs, were selected to describe in different
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manners. The selection was based on values of parallel resistance (RRD) and temperature distribution profile. The comparison is made on each pair of cells with close values of parallel resistances. While one cell is visual uniform temperature profile, another is cell with localize heating or hot spot region. By the experimental results, the local parallel resistances are calculated from IR thermal image and the electricalthermal model detailed in our previous work [13]. On local hot spot cells, the weak areas of hot spot regions appeared by visual observation are corresponding with the local parallel resistances analysis. Histograms of temperature distribution and local parallel resistances are analyzed. Localize heating or local hot spot area represents the effect of local structural defects described by avalanche breakdown or thermal breakdown of a cell. Cells having localized heating or local hot spot can be visually observed by apparent high temperature area. In other way, it can be observed by the ohmic behavior in the dark IV characteristic under reverse bias condition. However, this paper presents that cells having local hot spot can be quantified by local parallel resistances and three proposed indices. The method to calculate local parallel resistance was presented in our previous work [13], but it is applied to analyze quality of cells. The indicators proposed in this paper consist of H-index, A-index and L-index. The indices are derived by using cone shape model. It can be summarized that (1) the L-index closely relates to Tmax, H-index and A-index (2) the lower the L-index, the better the temperature uniformity (3) the proposed indicators and statistical values of IR thermograph can be made linear regression with resultant parallel resistance, and (4) IR thermal analysis can evaluate local structural defects corresponding to the dark IV characteristic interpretations in bulk parameters.
Acknowledgements
References
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A. Simo, S. Martinuzzi, Hot Spots and Heavily Dislocated Regions in Multicrystalline Silicon Cells, 21st IEEE Photovoltaic Specialists Conference, 1(1990) 800-805. S. Wendlandt, A. Drobisch, T. Buseth, S. Krauter, P. Grunow, Hot Spot Risk Analysis on Silicon Cell Modules, 25th EU PVSEC and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain, 4002-4006. O. Breitenstein, J. Bauer, A. Lotnyk, J.M. Wagner, Defect induced non-ideal dark I-V characteristics of solar cells, Superlattice Microst. 45 (2009) 182-189. M. Danner, K. Büchner, Reverse Characteristics of Commercial Silicon Solar Cells - Impact on Hot Spot Temperatures and Module Integrity, 26th IEEE PVSC, Anaheim (1997) 1137. M.C. Alonso-Garcıa , J.M. Ruız, Analysis and modelling the reverse characteristic of photovoltaic cells, Sol. Energ. Mat. Sol. C., 90 (2006) 1105-1120. A. Kaminski, J. Jouglar, M. Mergui, Y. Jourlin, A. Bouille, P.L. Vuillermoz, A. Laugier, Infrared characterization of hot spots in solar cells with high precision due to signal treatment processing, Sol. Energ. Mat. Sol. C., 51 (1998) 233-242. I.E. Konovalov, O. Breitenstein, K. Iwig, Local current-voltage curves measured thermally (LIVT): A new technique of characterizing PV cells, Sol. Energ. Mat. Sol. C., 48 (1997) 53-60. A. Kaminski, O Breitenstein, J.P. Boyeaux, P. Rakotoniaina, A. Laugier, Light Beam Induced Current and Infrared Thermography Studies of Multicrystalline Silicon Solar Cell, J. Phys.: Condens., 16 (2004) S9-S18. O. Breitenstein, M. Langenkamp, J. P. Rakotoniaina, EBIC investigation of a 3-dimensional network of inversion channels in solar cells on silicon ribbons, J. Solid State Phenom., 78-79 (2001) 29-38.
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The authors are grateful to the School of Energy, Environment and Materials, the CES Solar Cells Testing Center (CSSC), the Pilot Plant Development and Training Institute (PDTI) of the King Mongkut’s University of Technology Thonburi for the facilities and supporting. We are grateful to The Office of Commission on Higher Education, Ministry of Education under the Ministry Staff Development Project through Naresuan University for the financial support. We would like to thank Dr. Sirichai Thepa, Dr.Tanokkorn Chenvidhya and the staff of CSSC for their valuable comments and discussion.
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10. O. Breitenstein, W. Eberhardt, K. Iwig, Imaging The Local Forward Current Density of Solar Cells By Dynamical Precision Contact Thermography, 1994 IEEE Photovoltaic Specialists Conference , 2 (1994) 1633 – 1636. 11. O. Breitenstein, J.P. Rakotoniaina, M. Kaes, S. Seren, G. Hahn, W. Warta, J. Isenberg, T. Pernau, Lock-in Thermography - A Universal Tool for Local Analysis of Solar Cells, 15th PVSEC, (2005) 1279-1280. 12. H. Straube, O. Breitenstein, Estimation of heat loss in thermal wave experiments, J. Appl. Phys., 109, 064515 (2011) 13. B. Wiengmoon, K. Kirtikara, C. Jivacate, D. Chenvidhya, Local parallel resistances of solar cell derived by the thermal image analysis, Renew. Energ., 55 (2013) 49-54.
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Highlights on the Manuscript: ‘Quantifying the uniformity of solar cells using thermal imaging diagnosis’ Buntoon Wiengmoon, Krissanapong Kirtikara, Chaya Jivacate, Dhirayut Chenvidhya and Roongrojana Songprakorp
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- The method to quantify the quality of solar cells is presented by using IR images. - Local parallel resistances can be estimated by a thermal image. - The focus is the new idea with proposed indices to indicate quality of solar cells. - Correlations of parallel resistances and the proposed indices are presented. - Surface temperature distribution of a solar cell is quantified by the indices.