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Determination of solar cell electrical parameters and resistances using color and white LED-based solar simulators with high amplitude pulse input voltages
Renewable Energy 54 (2013) 131e137
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Determination of solar cell electrical parameters and resistances using color and white LED-based solar simulators with high amplitude pulse input voltages Anon Namin a, c, *, Chaya Jivacate b, Dhirayut Chenvidhya b, Krissanapong Kirtikara b, Jutturit Thongpron c a
Division of Energy Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand Clean Energy System Solar Cell Testing Centre (CSSC), King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand c Division of Electrical Engineering, Faculty of Engineering, Rajamangala University of Technology Lanna, Chiangmai 50300, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2012 Accepted 13 August 2012 Available online 13 September 2012
Advances in LEDs allow construction of LED solar simulators for characterization of solar cells at moderate costs. Four single-color LED simulators and one multi-color LED simulators are constructed. High irradiance is obtained by employing high pulsing voltages to LEDs. Using the simulators electrical parameters and resistances of solar cell are made. Applying correction methods recommended in the IEC 891 Standard, results from the red and blue LED simulators and the Class AAA simulator are in good agreement. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Solar cell IeV characteristics Solar cell resistance LED Solar simulator Pulse LED
1. Introduction Advances in high intensity light emitting diodes (LEDs) recently lead to many works on LED based solar simulators and IeV characterization of cells. S. Kohraku and K. Kurokawa, for the first time, report a measurement of spectral responses of solar cells using 4color and 6-color LED simulators [1]. The light intensity obtained is low, about 10 Wm2, with 3% uniformity. Low cost and simplicity of LED-based simulators are pointed out. This is followed by their later work on determination of spectral responses and IeV characteristics of Si solar cells using 4-color LEDs (blue, red, infrared and white) at higher irradiance of 110 Wm2. A compensation method to convert test results at low light intensity is proposed [2]. The method is termed a “Two-curve method” and is different from the IEC 891 Standard for IeV characterization at non-STC. R. Grischke et al. undertake a computer simulation to calculate deviations of values of open circuit, short circuit current density, efficiency and fill-factor of Si solar cell using a LED Flasher ArrayLFA from those values obtained from measurements under air mass (AM) 1.5 illumination [3]. In the construction of the LFA twocolor (888 nm and 470 nm) LEDs and three-color (940 nm, 700 nm * Corresponding author. Division of Electrical Engineering, Faculty of Engineering, Rajamangala University of Technology Lanna, Chiangmai 50300, Thailand. Tel.: þ66 5372 3979; fax: þ66 5372 3978. E-mail addresses: [email protected], [email protected] (A. Namin).
and 470 nm) LEDs are used. J. Koyanagi and K. Kurokawa make comparison of IeV curves of Si solar cell obtained from arrays of white LEDs and 4-color LEDs (blue, red, infrared and white), and dark current [4]. Later on, a white LED simulator is used to measure IeV curves of Si solar cell under two different illumination values of dark and 420 Wm2 by Y. Tsuno, K. Kamisako, and K. Kurokawa [5]. From this result, measured at about 40% of irradiance at STC, an IeV curve at STC is estimated and compared with that directly obtained from one-sun irradiance. M. Bliss, T. R. Betts, and R. Gottshalg develop an LED-based solar similar prototype producing light at variable flash speeds and pulse shapes [6]. Color and UV LEDs and halogen light sources are employed. Their simulator can operate as a continuous light source for long term measurements, and achieve 1-sun intensity with closely matched spectrum. S. H. Jang and M. Shin discuss a fabrication of an LED simulator and its thermal characteristics [7]. IeV curves of a solar cell tested under the simulator at 500 and 900 Wm2 are compared with that obtained from 1-sun Xenon lamp. After that, F. C. Krebs, K. O. Sylvester-Havid, and M. Jorgensen synthesized an approximate AM 1.5 G spectral distribution by independent tuning of the 18 different wavelengths (390e940 nm) LEDs for IeV characterization small laboratory solar cells. The 743 Wm2 (scalable to 2230 Wm2) of 390e940 nm synthesized AM 1.5 G has been presented [8]. Then, D. Kolberg et al. present achievements in development of close match to AM 1.5 G with already given the temporal stability of LED solar simulator [9]. Recently, A. M. Bazzi et al. present a prototype of six colors,
0960-1481/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.08.046
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A. Namin et al. / Renewable Energy 54 (2013) 131e137
Class C uniformity on 5.0*10.0 cm2 area, and 12% temporal stability of LED-based solar simulator with current-mode control [10]. And, B. H. Hamadani et al. discuss the development and fabrication of a scalable large-area LED-based solar simulator [11]. An AM 1.5 spectrum in the range of 400e970 nm was synthesized. Irradiance of 28.9 Wm2 of Class CCA solar simulator on the 25*50 cm2 test plane is achieved. Salient points of past research work on LED simulators, described above, are summarized in Table 1. Different single-color LEDs are used in fabrication of simulators. Single-color LED simulators cannot provide the required AM 1.5 spectrum. Moreover, to prevent build-up of high temperature of LEDs under continuous operations, the supply voltage of LEDs is generally kept low, resulting in low intensity. Two approaches have emerged. The first approach is to combine multi-color LEDs to provide irradiance close to AM 1.5 spectrum. The second approach is correction of measurements at non-STC conditions (low irradiance values) to the STC (1000 Wm2) using a “Two-curve method” [2]. In this research work, we fabricate solar simulators employing single-color red, green, blue and white LEDs that provide higher irradiance (1000 Wm2), using the pulse input voltage of high amplitude. High irradiance is achieved applying high amplitude voltage pulses to LEDs. This results in pulse operations, in contrast to low voltage continuous operations of past research. Two positive features are simultaneously realized, namely, more intense light output from LEDs and lower temperature rise of LEDs. IeV curves of a solar cell under different color LEDS are undertaken. Calculations are made on solar cell resistance (series resistance, shunt resistance and internal dynamic resistance) and solar cell parameters (open
circuit voltage, short circuit current, maximum power, efficiency and fill factor). 2. Series, shunt and internal dynamic resistances A DC equivalent circuit of a solar cell, shown in Fig. 1, consists of three resistances, namely, series resistance (RS), shunt resistance (Rsh) and internal dynamic resistance (Rd). These resistances are normally determined from IeV characteristics obtained from solar simulators based on flash xenon lamps or halogen lamps. Series resistance is determined by contacts and bulk resistances whereas shunt resistance by defects and traps in a solar cell. From a graphical representation of IeV curves shown in Fig. 2, a series resistance (RS) is a slope of the dark IeV curve in the first quadrant, a shunt resistance (Rsh) is a slope at near-zero voltage, the internal dynamic resistance (Rd) is a slope at of the IeV curve (dark and illumination) at an operating point. In addition, the external dynamic resistance is the ratio of the voltage and current at terminals of a solar cell, and, therefore, is equal to series resistance in series with the internal dynamic resistance [12]. 3. Experiments 3.1. LED solar simulator Four color LED are used, namely, red-R at 632 nm (LTL2F3VEKNT), green-G at 525 nm, TOL-50bUGdCTa-M4), blue-B at 468 nm (TOL-50aUBdCEa-ETB6) and white-W (LTW-2S3D7. Five
Table 1 Outlines salient points of past research work on LED simulators. Research works
Objectives
Sizes of LED array and test planes
LEDs and components
Operation mode
Simulator characteristics
Kohraku S. and Kurokawa K. [1]
To measure spectral responses
21*21 cm2 array, 10*10 cm2 test plane
B, R, IR, W
Modulation on bias pulsed light
Kohraku S. and Kurokawa K. [2] Grischke R. et al [3]
To measure spectral responses and IeV curves 1.To study solar cell parameters 2. To develop simulation program
21*21 cm2 array, 10*10 cm2 test plane 8.0*8.0 cm2 array
B, R, IR, W
Continuous
2 colors (880/470 nm), 3 colors (940, 700, 470 nm)
Flash
Koyanagi J. and Kurokawa K. [4]
To measure spectral responses and IeV curves and compare with Xe lamps To measure spectral responses and IeV curves To measure solar simulator performances
21*21 cm2 array, 10*10 cm2 test plane
B, R, IR, W
Continuous
3% uniformity Low intensity (10 Wm2) 5% uniformity 100 Wm2 Intensity ratios of 30.6%, 44.1%, and 25.3% from 940, 700 and 470 nm LEDs 110 Wm2
34*34 cm2 array, 10*10 cm2 test plane 38*38 cm2 array, 20*20 cm2 test plane (Class A uniformity at 6.0*6.0 cm2) N/A
2304 LEDs (B, R, 2 IR)
Continuous
8 color LEDs, halogen lamps
Continuous, flash
96 package of 1.5 W LEDs, collimated lens, AM 1.5D filter 18 color LEDs Synthesized by independent tuning z800 single dies, z70 packages LEDs (350e1100 nm) 59 LEDs, 6 colors, (3 UV, 7 blue, 7 cyan, 7 green, 21 neutral white, and 14 warm white) 34 of 15 W LEDs 5 colors (405, 518, 590, 627, nm and white 5500 K)
Continuous, flash
500 Wm2 900 Wm2
Continuous, pulse
Synthesized AM 1.5 G 743 Wm2
Continuous
Close to match AM 1.5 G 0.3% stability
Continuous
Class C uniformity, 12% uniformity
Continuous, pulse
28.9 Wm2 (continuous), 52.3 Wm2 (pulse) Class CCA
Tsuno Y., Kamisako K. and Kurokawa K. [5] Bliss M., Betts T.R., and Gottshalg R. [6]
Jang S. H. and Shin M. [7]
To study thermal characteristics of simulator
Krebs F. C., Sylvester-Havid K. and Jorgensen M. [8]
To measure solar simulator performances and IeV curves
1.7*4.7 cm2 array 2.5*5.0 cm2 test plane
Kolberg D., Schubert F., Lontke N., et al. [9]
To development a spectral tunable LED-based solar simulator
N/A
Bazzi A. M. et al [10]
To development a prototype LED-based solar simulator
5.0*10.0 cm2 test plane
Hamadani B. H. et al [11]
To development and fabrication a scalable large-area LED-based solar simulator
4 of 7.8*7.8 cm2 array, 5-m-long light guides, 25*50 cm2 test plane
2% uniformity 420 Wm2 590 Wm2 Class BAA
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the test plane, a solar cell and calibrated photo-detector (13DAS003) are placed. A pulse power supply for an array is constructed from a high voltage DC supply (Xantrex XDC20-600) and a pulsing circuit. Amplitudes of pulse signals can be continuously varied from 0 to 150 V with a pulse width of 10 ms and a period of 1 s. Study is made on power from pulsed operation (at different amplitudes, pulse widths and pulse periods) and compared with continuous operation of LEDs. This is to obtain the maximum power that LEDs can withstand before being damaged due to temperature rise. For pulse operation, we find that 2e3 times of LED rated voltages and 8e10 times of rated currents are feasible with forced air-cooling. Fig. 1. Shows DC equivalent circuit of a solar cell.
3.2. Measurement and calculations LED arrays, each at 227.5 mm 227.5 mm, are constructed and serve as light sources of simulators, namely (Fig. 3), (a) Four single-color LED arrays. Each array consisting of 1024 LEDs has 64 parallel strings, each string has 16 LEDs in series with a current limiting resistor. (b) One combined RGB array. It has 1024 LEDs. The array has 128 parallel strings, each string has 8 LEDs. Percentages of irradiance from red, green and blue LEDs to total irradiance are 35%, 33% and 32%, respectively. Each LED array is placed 3 mm above a glass diffuser which is 30 mm over the test plane. LED heat sinks and forced air cooling (between heat sinks and a glass diffuser) provide heat removal and maintain a constant temperature heat sink temperature of 25 C. On
We undertake the following measurements and calculations (a) Study on thermal characteristics of LED arrays under continuous and pulse operations. (b) Irradiance characteristics from different LED arrays. Two features are studied. The first feature is irradiance stability under a pulse operation with 10 ms pulse duration. This pulse duration is chosen as it is longer than the cycling time of an electronic load in IeV measurement. The second feature is relationship between the LED current and irradiance on the test plane. The evaluation procedures follow the IEC 60904-9 standard [13].
IeV characteristics of a 12.5 12.5 cm2 XeSi solar cell using the four single-color arrays (R, B, G, W) and one RGB array, under pulse operation. The IeV characterization method follows the IEC 609041 Standards [14]. (c) From measurements using LED simulators corrections recommended by the IEC 891 Standard are carried out [15]. Calculations are made on solar cell electrical parameters (ISC, VOC, Pmp, efficiency, fill factor) and resistances (RS, Rsh, Rd) (Fig. 4). 4. Results and discussion 4.1. Thermal characteristics of LED arrays
Fig. 2. Illustrates typical dark and illuminated IeV curves of a solar cell.
The five LED arrays are at first supplied with respective rated voltages on a continuous basis for 300 s without forced cooling. Infrared pictures of heat sinks at the rear of each LED array are then taken. Subsequently, a pulse voltage signal, at 3e5 times respective LED rated voltages, having 10 ms pulse duration and 1 s period, is
Fig. 3. Shows a schematic structure of the LED solar simulator constructed in this work.
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Fig. 4. Shows a schematic concept of measurement and calculations.
applied for 300 s. Infrared pictures are similarly taken. Comparison is made on a normal picture and the two infrared pictures. As an example, only results of the red array are shown, Fig. 5. In a continuous voltage operation, a marked temperature rise of the heat sinks is clearly seen (63 C) in a continuous operation. However, the array remains at room temperature (27 C) under pulse operation. This demonstrates the merit of the pulse operation mode.
4.2. Stability of irradiance from LED arrays under pulse operations Irradiance stability of the four single color LED arrays and one RGB array under 10 ms pulse operations are made. The nature of time-dependent irradiance achieved form five arrays is similar. For illustration, only results of the red array are shown, Fig. 6. Owing to the current recovery nature of the employed high voltage power
Fig. 5. Shows pictures of heat sinks at the rear of the red LED array under continuous and pulse operations.
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Table 2 Shows temporal stability and corresponding simulator classes operating at irradiance between 400e1000 Wm2 obtained from the 5 LED simulators.
Fig. 6. Shows irradiance on the test plan obtained from the red LED simulator and temporal stability.
supply, current outputs during the first 2 ms of operation are slightly fluctuating. So on the time axis we show the plots having 8 ms duration, between the 2 ms and 10 ms points. We see that amplitudes of pulses applied to LEDs can be increased to at least twice the rated voltage, and the array still provides a stable light output. From Table 2, temporal stability of 8 out of 9 irradiance levels is better than 3%, one level is over 3% but still under 5%. Therefore, the 5 simulators are Class B. In the IEC 891 Standard the term “instability” is used for temporal stability of irradiance. Spatial uniformity of irradiance on the 17*17 cm2 test plane is also measured, but not reported here, and is better than 5%. The 5 simulators are Class B.
4.3. Relationship between the LED current and irradiance on the test plane LED light outputs, and, subsequently, irradiance decrease with increasing LED current, due to temperature rises. Only red and blue arrays provide irradiance higher than 1000 Wm2 while less than this irradiance value is available from green, white and RGB arrays. LEDs become less efficient as their light outputs rapidly fall with increasing current (Fig. 7).
LED arrays
Irradiance (Wm2)
Temporal stability %
Class
Red Blue Red Blue RGB Red Green Blue White
1010 1008 820 812 810 430 408 408 420
1.47 1.41 1.82 2.61 3.10 1.75 2.40 2.45 2.85
A A A B B A B B B
To permit higher light outputs from the green, white and RGB arrays, LED current should be increased without accompanying large increase in temperature. This would be possible if chilled air, instead of room temperature air, is used in the cooling. But this would be complex and more expensive. 4.4. Uncorrected IeV characteristics obtained from the five arrays at non STC Comparison in made on solar cell IeV curves using four singlecolor simulators, one three-color simulator and a Class AAA solar simulator (PASAN Sun Simulator IIIc), and plotted in Fig. 8. LED voltages and current are varied to provide irradiance in the range of 400e1000 Wm2, whenever possible. Out of the 5 LED simulators, only the red and blue LED simulators provide an irradiance of 1000 Wm2. In addition, all 5 LED simulators have light spectra different from AM 1.5 whereas the Class AAA simulator has. At a comparable irradiance level of 1000 Wm2, Fig. 8(a), IeV curves of the red LED simulator (at 925 and 1000 Wm2) and the blue LED simulator (at 1000 Wm2) show slight deviations from the IeV curve of the Class AAA simulator. Larger deviations are observed at lower irradiance levels, 800 Wm2 in Fig. 8(b) and 400 Wm2 in Fig. 8(c). 4.5. Corrected IeV characteristics at STC The IEC 891 Standard provides methods of correction for irradiance and temperature at non STC conditions. The Standard recommends that such correction method should be applied to Class B simulator (spectrum, uniformity and temporal stability/ instability) or better. From the above results, all the 5 LED simulators constructed are Class B, considering their uniformity and temporal stability. The Standard further recommends that non-STC measurements should be made at an irradiance level of and over 800 Wm2. Adopting the correction method outlined in the IEC 891 Standard, all non-STC IeV curves are subsequently corrected. Corrected IeV curves are plotted in Fig. 9. Small deviations compared to the Class AAA curve are significant. This is the first instance that the IEC 891 correction method is applicable for LED simulators. Works of Kohraku, S. and Kurokawa, K. described above use a different correction method, called the Two-curve method. Other research works give less explicit description on how non-STC results are corrected. 4.6. Electrical parameters and resistances of solar cell at STC
Fig. 7. Shows relationship between irradiance on the test plane and LED currents from the five arrays.
From the corrected IeV curves of the red and blue simulators at 1000 Wm2 we calculate internal dynamic resistance (Rd) and electrical parameters (ISC, VOC, Pmp, efficiency, fill factor) at the STC.
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Fig. 8. Shows uncorrected IeV curves from LED simulators.
The series resistance and shunt resistance are measured under dark, so no corrections are needed. The results are shown in Table 3. Comparing values of electrical parameters and resistances using the red and blue simulators with those of the Class AAA simulator, both the red and blue simulator results are in very good agreement with the Class AAA simulator. Differences in resistances and electrical parameters in most cases are about 2% or less. The exception is the maximum power measured by the blue LED simulator, the deviation is 4.1%. Accuracy of results is important in using red and blue LEDs, with confidence, as light sources of solar simulators. This should pave ways for employing red and blue LEDs in constructions of less expensive solar simulators [16]. 4.7. Internal dynamic resistance Internal dynamic resistance of the solar cell is calculated from corrected IeV measurements for red and blue LED simulators. Table 3 Internal dynamic resistance resistances and parameters at STC and series resistance and shunt resistance under dark. Parameters
Fig. 9. Shows the corrected IeV curves at STC.
Values measured Values measured with LED simulators with Class AAA (corrected with methods of IEC 891 Standard) simulator Blue LED Red LED simulator simulator and error (%) and error (%)
1000 G (Wm2) T (C) 25 0.58 Voc (V) 4.63 Isc (A) 1.72 Pmp (W) 0.42 Vmp (V) Imp (A) 4.06 FF (%) 63.5 h (%) 11.7 0.03 RS (U) 6.40 Rsh (U) 0.08 Rd @ Vmp (U)
1000 25 0.57 (1.7%) 4.63 (0.0%) 1.65 (4.1%) 0.41 (2.4%) 4.03 (0.7%) 62.1 (1.4%) 11.1 (0.6% 0.03 (0.0%) 6.40 (0.0%) 0.08 (0.0%)
1000 25 0.59 (1.7%) 4.63 (0.0%) 1.69 (1.7%) 0.41 (2.4%) 4.11 (1.2%) 61.9 (1.6%) 11.5 (0.2%) 0.03 (0.0%) 6.40 (0.0%) 0.08 (0.0%)
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and Dr. Veerapon Monyakul. Research funds for A. Namin are made available from the Rajamangala University of Technology Lanna.
References
Fig. 10. Dynamic resistance of the solar cell using red and blue LED simulators.
Comparison is made with values under dark and calculated from the Class AAA simulator. Results are plotted as Fig. 10. Good agreements are found. 5. Conclusion In this paper, four single-color (red, green, blue, white) LED solar simulators and one red-green-blue simulator, all having high irradiance, are constructed. High irradiance is achieved by pulsing LEDs with high input voltages. Irradiance uniformity and instability qualify the five simulators as Class B. Solar cell electrical parameters and resistances using these simulators are made. Applying correction methods recommended in the IEC 891 Standard, for IeV characterization at non-STC, results on electrical parameters and resistances measured with the red and blue LED simulators and the Class AAA simulator are in good agreement. Acknowledgments The authors wish to acknowledge supports from the CSSC for excellent research facilities. Valuable advices have been given by Assistant Professor Proapran Plienpoo, Dr. Koarakot Wattanavichean
[1] Kohraku S, Kurokawa K. New method for solar cell measurement by LED solar simulator. In: 3rd World conference on photovoltaic energy conference, May 11e18, Osaka, Japan; 2003. p. 1977e80. [2] Kohraku S, Kurokawa K. A fundamental experiment for discrete-wavelength LED solar simulator. Solar Energy Materials & Solar Cells 2006;90:3364e70. [3] Grischke R, Schmidt J, Albert H, Laux A, Metz A, Hilsenberg U, et al. LED fasher arrays (FLA) for an improved quality control in solar cell production lines, EU PVSEC19th, vol. 2; 2004. p. 2519e94. [4] Koyanagi J, Kurokawa K. A fundamental experiment of solar cell’s IeV characteristics measurement using LED solar simulator. Renewable Energy 2006 Proceedings, p. 270e3. [5] Tsuno Y, Kamisako K, Kurokawa K. New generation of PV module rating by LED solar simulatorea noval approach and its capacities. IEEE 10.1109/ PVSC.2008.4922582. [6] Bliss M, Betts TR, Gottshalg R. An LED-based photovoltaic measurement system with variable spectrum and flash speed. Solar Energy Materials & Solar Cell 2009;93:825e30. [7] Jang SH, Shin MW. Fabrication and thermal optimization of LED solar cell simulator. Current Applied Physics 2010;10:S537e9. [8] Kerbs FC, Sylvester-Havid KO, Jorgensen M. A self-calibrating led-based solar test platform. Progress in Photovoltaics: Research and Applications 2011;19: 97e112. [9] Kolberg D, Schubert F, Lontke N, Zwigart A, Spinner DM. Development of tunable close match LED solar simulator with extended spectral range to UV and IR. Energy Procedia 2011;8:100e5. [10] Bazzi AM, Klein Z, Sweeney M, Kroeger K, Shenoy P, Krein PT. Solid-state light simulator with current-mode control. IEEE; 2011. 978-1-4244-8085-2/11. [11] Hamadani BH, Chua K, Roller J, Bennahmias MJ, Campbell B, Yoon HW, et al. Towards realization of a large-area light-emitting diode-based solar simulator. Progress in Photovoltaics: Research and Applications 2011. http:// dx.doi.org/10.1002/pip.1231. [12] Thongpron J, Kirtikara K, Jivacate C. A method for the determination of dynamic resistance of photovoltaic modules under illumination. Solar Energy Materials & Solar Cells 2006;90:3078e84. [13] IEC, IEC 60904e9. Photovoltaic devicesepart 9: solar simulator performance requirement. ed.2; 2007. [14] IEC, IEC 60904e1. Part 1: measurement of photovoltaic current-voltage characteristics, characteristics of crystalline silicon photovoltaic devices; 2006. [15] IEC, IEC 891. Procedures for temperature and irradiance corrections to measured IeV characteristics of crystalline silicon photovoltaic devices; 1987. [16] Namin A, Jivacate C, Chenvidhya D, Kirtikara K, Thongpron J. Construction of tungsten halogen, pulsed LED, and combined tungsten halogen-LED solar simulators for solar cell IeV characterization and electrical parameters determination. International Journal of Photoenergy 2012;2012:9. http://dx. doi.org/10.1155/2012/527820. Article ID 527820.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Determination of solar cell electrical parameters and resistances using color and white LED-based solar simulators with high amplitude pulse input voltages Anon Namin a, c, *, Chaya Jivacate b, Dhirayut Chenvidhya b, Krissanapong Kirtikara b, Jutturit Thongpron c a
Division of Energy Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand Clean Energy System Solar Cell Testing Centre (CSSC), King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand c Division of Electrical Engineering, Faculty of Engineering, Rajamangala University of Technology Lanna, Chiangmai 50300, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2012 Accepted 13 August 2012 Available online 13 September 2012
Advances in LEDs allow construction of LED solar simulators for characterization of solar cells at moderate costs. Four single-color LED simulators and one multi-color LED simulators are constructed. High irradiance is obtained by employing high pulsing voltages to LEDs. Using the simulators electrical parameters and resistances of solar cell are made. Applying correction methods recommended in the IEC 891 Standard, results from the red and blue LED simulators and the Class AAA simulator are in good agreement. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Solar cell IeV characteristics Solar cell resistance LED Solar simulator Pulse LED
1. Introduction Advances in high intensity light emitting diodes (LEDs) recently lead to many works on LED based solar simulators and IeV characterization of cells. S. Kohraku and K. Kurokawa, for the first time, report a measurement of spectral responses of solar cells using 4color and 6-color LED simulators [1]. The light intensity obtained is low, about 10 Wm2, with 3% uniformity. Low cost and simplicity of LED-based simulators are pointed out. This is followed by their later work on determination of spectral responses and IeV characteristics of Si solar cells using 4-color LEDs (blue, red, infrared and white) at higher irradiance of 110 Wm2. A compensation method to convert test results at low light intensity is proposed [2]. The method is termed a “Two-curve method” and is different from the IEC 891 Standard for IeV characterization at non-STC. R. Grischke et al. undertake a computer simulation to calculate deviations of values of open circuit, short circuit current density, efficiency and fill-factor of Si solar cell using a LED Flasher ArrayLFA from those values obtained from measurements under air mass (AM) 1.5 illumination [3]. In the construction of the LFA twocolor (888 nm and 470 nm) LEDs and three-color (940 nm, 700 nm * Corresponding author. Division of Electrical Engineering, Faculty of Engineering, Rajamangala University of Technology Lanna, Chiangmai 50300, Thailand. Tel.: þ66 5372 3979; fax: þ66 5372 3978. E-mail addresses: [email protected], [email protected] (A. Namin).
and 470 nm) LEDs are used. J. Koyanagi and K. Kurokawa make comparison of IeV curves of Si solar cell obtained from arrays of white LEDs and 4-color LEDs (blue, red, infrared and white), and dark current [4]. Later on, a white LED simulator is used to measure IeV curves of Si solar cell under two different illumination values of dark and 420 Wm2 by Y. Tsuno, K. Kamisako, and K. Kurokawa [5]. From this result, measured at about 40% of irradiance at STC, an IeV curve at STC is estimated and compared with that directly obtained from one-sun irradiance. M. Bliss, T. R. Betts, and R. Gottshalg develop an LED-based solar similar prototype producing light at variable flash speeds and pulse shapes [6]. Color and UV LEDs and halogen light sources are employed. Their simulator can operate as a continuous light source for long term measurements, and achieve 1-sun intensity with closely matched spectrum. S. H. Jang and M. Shin discuss a fabrication of an LED simulator and its thermal characteristics [7]. IeV curves of a solar cell tested under the simulator at 500 and 900 Wm2 are compared with that obtained from 1-sun Xenon lamp. After that, F. C. Krebs, K. O. Sylvester-Havid, and M. Jorgensen synthesized an approximate AM 1.5 G spectral distribution by independent tuning of the 18 different wavelengths (390e940 nm) LEDs for IeV characterization small laboratory solar cells. The 743 Wm2 (scalable to 2230 Wm2) of 390e940 nm synthesized AM 1.5 G has been presented [8]. Then, D. Kolberg et al. present achievements in development of close match to AM 1.5 G with already given the temporal stability of LED solar simulator [9]. Recently, A. M. Bazzi et al. present a prototype of six colors,
0960-1481/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.08.046
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Class C uniformity on 5.0*10.0 cm2 area, and 12% temporal stability of LED-based solar simulator with current-mode control [10]. And, B. H. Hamadani et al. discuss the development and fabrication of a scalable large-area LED-based solar simulator [11]. An AM 1.5 spectrum in the range of 400e970 nm was synthesized. Irradiance of 28.9 Wm2 of Class CCA solar simulator on the 25*50 cm2 test plane is achieved. Salient points of past research work on LED simulators, described above, are summarized in Table 1. Different single-color LEDs are used in fabrication of simulators. Single-color LED simulators cannot provide the required AM 1.5 spectrum. Moreover, to prevent build-up of high temperature of LEDs under continuous operations, the supply voltage of LEDs is generally kept low, resulting in low intensity. Two approaches have emerged. The first approach is to combine multi-color LEDs to provide irradiance close to AM 1.5 spectrum. The second approach is correction of measurements at non-STC conditions (low irradiance values) to the STC (1000 Wm2) using a “Two-curve method” [2]. In this research work, we fabricate solar simulators employing single-color red, green, blue and white LEDs that provide higher irradiance (1000 Wm2), using the pulse input voltage of high amplitude. High irradiance is achieved applying high amplitude voltage pulses to LEDs. This results in pulse operations, in contrast to low voltage continuous operations of past research. Two positive features are simultaneously realized, namely, more intense light output from LEDs and lower temperature rise of LEDs. IeV curves of a solar cell under different color LEDS are undertaken. Calculations are made on solar cell resistance (series resistance, shunt resistance and internal dynamic resistance) and solar cell parameters (open
circuit voltage, short circuit current, maximum power, efficiency and fill factor). 2. Series, shunt and internal dynamic resistances A DC equivalent circuit of a solar cell, shown in Fig. 1, consists of three resistances, namely, series resistance (RS), shunt resistance (Rsh) and internal dynamic resistance (Rd). These resistances are normally determined from IeV characteristics obtained from solar simulators based on flash xenon lamps or halogen lamps. Series resistance is determined by contacts and bulk resistances whereas shunt resistance by defects and traps in a solar cell. From a graphical representation of IeV curves shown in Fig. 2, a series resistance (RS) is a slope of the dark IeV curve in the first quadrant, a shunt resistance (Rsh) is a slope at near-zero voltage, the internal dynamic resistance (Rd) is a slope at of the IeV curve (dark and illumination) at an operating point. In addition, the external dynamic resistance is the ratio of the voltage and current at terminals of a solar cell, and, therefore, is equal to series resistance in series with the internal dynamic resistance [12]. 3. Experiments 3.1. LED solar simulator Four color LED are used, namely, red-R at 632 nm (LTL2F3VEKNT), green-G at 525 nm, TOL-50bUGdCTa-M4), blue-B at 468 nm (TOL-50aUBdCEa-ETB6) and white-W (LTW-2S3D7. Five
Table 1 Outlines salient points of past research work on LED simulators. Research works
Objectives
Sizes of LED array and test planes
LEDs and components
Operation mode
Simulator characteristics
Kohraku S. and Kurokawa K. [1]
To measure spectral responses
21*21 cm2 array, 10*10 cm2 test plane
B, R, IR, W
Modulation on bias pulsed light
Kohraku S. and Kurokawa K. [2] Grischke R. et al [3]
To measure spectral responses and IeV curves 1.To study solar cell parameters 2. To develop simulation program
21*21 cm2 array, 10*10 cm2 test plane 8.0*8.0 cm2 array
B, R, IR, W
Continuous
2 colors (880/470 nm), 3 colors (940, 700, 470 nm)
Flash
Koyanagi J. and Kurokawa K. [4]
To measure spectral responses and IeV curves and compare with Xe lamps To measure spectral responses and IeV curves To measure solar simulator performances
21*21 cm2 array, 10*10 cm2 test plane
B, R, IR, W
Continuous
3% uniformity Low intensity (10 Wm2) 5% uniformity 100 Wm2 Intensity ratios of 30.6%, 44.1%, and 25.3% from 940, 700 and 470 nm LEDs 110 Wm2
34*34 cm2 array, 10*10 cm2 test plane 38*38 cm2 array, 20*20 cm2 test plane (Class A uniformity at 6.0*6.0 cm2) N/A
2304 LEDs (B, R, 2 IR)
Continuous
8 color LEDs, halogen lamps
Continuous, flash
96 package of 1.5 W LEDs, collimated lens, AM 1.5D filter 18 color LEDs Synthesized by independent tuning z800 single dies, z70 packages LEDs (350e1100 nm) 59 LEDs, 6 colors, (3 UV, 7 blue, 7 cyan, 7 green, 21 neutral white, and 14 warm white) 34 of 15 W LEDs 5 colors (405, 518, 590, 627, nm and white 5500 K)
Continuous, flash
500 Wm2 900 Wm2
Continuous, pulse
Synthesized AM 1.5 G 743 Wm2
Continuous
Close to match AM 1.5 G 0.3% stability
Continuous
Class C uniformity, 12% uniformity
Continuous, pulse
28.9 Wm2 (continuous), 52.3 Wm2 (pulse) Class CCA
Tsuno Y., Kamisako K. and Kurokawa K. [5] Bliss M., Betts T.R., and Gottshalg R. [6]
Jang S. H. and Shin M. [7]
To study thermal characteristics of simulator
Krebs F. C., Sylvester-Havid K. and Jorgensen M. [8]
To measure solar simulator performances and IeV curves
1.7*4.7 cm2 array 2.5*5.0 cm2 test plane
Kolberg D., Schubert F., Lontke N., et al. [9]
To development a spectral tunable LED-based solar simulator
N/A
Bazzi A. M. et al [10]
To development a prototype LED-based solar simulator
5.0*10.0 cm2 test plane
Hamadani B. H. et al [11]
To development and fabrication a scalable large-area LED-based solar simulator
4 of 7.8*7.8 cm2 array, 5-m-long light guides, 25*50 cm2 test plane
2% uniformity 420 Wm2 590 Wm2 Class BAA
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the test plane, a solar cell and calibrated photo-detector (13DAS003) are placed. A pulse power supply for an array is constructed from a high voltage DC supply (Xantrex XDC20-600) and a pulsing circuit. Amplitudes of pulse signals can be continuously varied from 0 to 150 V with a pulse width of 10 ms and a period of 1 s. Study is made on power from pulsed operation (at different amplitudes, pulse widths and pulse periods) and compared with continuous operation of LEDs. This is to obtain the maximum power that LEDs can withstand before being damaged due to temperature rise. For pulse operation, we find that 2e3 times of LED rated voltages and 8e10 times of rated currents are feasible with forced air-cooling. Fig. 1. Shows DC equivalent circuit of a solar cell.
3.2. Measurement and calculations LED arrays, each at 227.5 mm 227.5 mm, are constructed and serve as light sources of simulators, namely (Fig. 3), (a) Four single-color LED arrays. Each array consisting of 1024 LEDs has 64 parallel strings, each string has 16 LEDs in series with a current limiting resistor. (b) One combined RGB array. It has 1024 LEDs. The array has 128 parallel strings, each string has 8 LEDs. Percentages of irradiance from red, green and blue LEDs to total irradiance are 35%, 33% and 32%, respectively. Each LED array is placed 3 mm above a glass diffuser which is 30 mm over the test plane. LED heat sinks and forced air cooling (between heat sinks and a glass diffuser) provide heat removal and maintain a constant temperature heat sink temperature of 25 C. On
We undertake the following measurements and calculations (a) Study on thermal characteristics of LED arrays under continuous and pulse operations. (b) Irradiance characteristics from different LED arrays. Two features are studied. The first feature is irradiance stability under a pulse operation with 10 ms pulse duration. This pulse duration is chosen as it is longer than the cycling time of an electronic load in IeV measurement. The second feature is relationship between the LED current and irradiance on the test plane. The evaluation procedures follow the IEC 60904-9 standard [13].
IeV characteristics of a 12.5 12.5 cm2 XeSi solar cell using the four single-color arrays (R, B, G, W) and one RGB array, under pulse operation. The IeV characterization method follows the IEC 609041 Standards [14]. (c) From measurements using LED simulators corrections recommended by the IEC 891 Standard are carried out [15]. Calculations are made on solar cell electrical parameters (ISC, VOC, Pmp, efficiency, fill factor) and resistances (RS, Rsh, Rd) (Fig. 4). 4. Results and discussion 4.1. Thermal characteristics of LED arrays
Fig. 2. Illustrates typical dark and illuminated IeV curves of a solar cell.
The five LED arrays are at first supplied with respective rated voltages on a continuous basis for 300 s without forced cooling. Infrared pictures of heat sinks at the rear of each LED array are then taken. Subsequently, a pulse voltage signal, at 3e5 times respective LED rated voltages, having 10 ms pulse duration and 1 s period, is
Fig. 3. Shows a schematic structure of the LED solar simulator constructed in this work.
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Fig. 4. Shows a schematic concept of measurement and calculations.
applied for 300 s. Infrared pictures are similarly taken. Comparison is made on a normal picture and the two infrared pictures. As an example, only results of the red array are shown, Fig. 5. In a continuous voltage operation, a marked temperature rise of the heat sinks is clearly seen (63 C) in a continuous operation. However, the array remains at room temperature (27 C) under pulse operation. This demonstrates the merit of the pulse operation mode.
4.2. Stability of irradiance from LED arrays under pulse operations Irradiance stability of the four single color LED arrays and one RGB array under 10 ms pulse operations are made. The nature of time-dependent irradiance achieved form five arrays is similar. For illustration, only results of the red array are shown, Fig. 6. Owing to the current recovery nature of the employed high voltage power
Fig. 5. Shows pictures of heat sinks at the rear of the red LED array under continuous and pulse operations.
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Table 2 Shows temporal stability and corresponding simulator classes operating at irradiance between 400e1000 Wm2 obtained from the 5 LED simulators.
Fig. 6. Shows irradiance on the test plan obtained from the red LED simulator and temporal stability.
supply, current outputs during the first 2 ms of operation are slightly fluctuating. So on the time axis we show the plots having 8 ms duration, between the 2 ms and 10 ms points. We see that amplitudes of pulses applied to LEDs can be increased to at least twice the rated voltage, and the array still provides a stable light output. From Table 2, temporal stability of 8 out of 9 irradiance levels is better than 3%, one level is over 3% but still under 5%. Therefore, the 5 simulators are Class B. In the IEC 891 Standard the term “instability” is used for temporal stability of irradiance. Spatial uniformity of irradiance on the 17*17 cm2 test plane is also measured, but not reported here, and is better than 5%. The 5 simulators are Class B.
4.3. Relationship between the LED current and irradiance on the test plane LED light outputs, and, subsequently, irradiance decrease with increasing LED current, due to temperature rises. Only red and blue arrays provide irradiance higher than 1000 Wm2 while less than this irradiance value is available from green, white and RGB arrays. LEDs become less efficient as their light outputs rapidly fall with increasing current (Fig. 7).
LED arrays
Irradiance (Wm2)
Temporal stability %
Class
Red Blue Red Blue RGB Red Green Blue White
1010 1008 820 812 810 430 408 408 420
1.47 1.41 1.82 2.61 3.10 1.75 2.40 2.45 2.85
A A A B B A B B B
To permit higher light outputs from the green, white and RGB arrays, LED current should be increased without accompanying large increase in temperature. This would be possible if chilled air, instead of room temperature air, is used in the cooling. But this would be complex and more expensive. 4.4. Uncorrected IeV characteristics obtained from the five arrays at non STC Comparison in made on solar cell IeV curves using four singlecolor simulators, one three-color simulator and a Class AAA solar simulator (PASAN Sun Simulator IIIc), and plotted in Fig. 8. LED voltages and current are varied to provide irradiance in the range of 400e1000 Wm2, whenever possible. Out of the 5 LED simulators, only the red and blue LED simulators provide an irradiance of 1000 Wm2. In addition, all 5 LED simulators have light spectra different from AM 1.5 whereas the Class AAA simulator has. At a comparable irradiance level of 1000 Wm2, Fig. 8(a), IeV curves of the red LED simulator (at 925 and 1000 Wm2) and the blue LED simulator (at 1000 Wm2) show slight deviations from the IeV curve of the Class AAA simulator. Larger deviations are observed at lower irradiance levels, 800 Wm2 in Fig. 8(b) and 400 Wm2 in Fig. 8(c). 4.5. Corrected IeV characteristics at STC The IEC 891 Standard provides methods of correction for irradiance and temperature at non STC conditions. The Standard recommends that such correction method should be applied to Class B simulator (spectrum, uniformity and temporal stability/ instability) or better. From the above results, all the 5 LED simulators constructed are Class B, considering their uniformity and temporal stability. The Standard further recommends that non-STC measurements should be made at an irradiance level of and over 800 Wm2. Adopting the correction method outlined in the IEC 891 Standard, all non-STC IeV curves are subsequently corrected. Corrected IeV curves are plotted in Fig. 9. Small deviations compared to the Class AAA curve are significant. This is the first instance that the IEC 891 correction method is applicable for LED simulators. Works of Kohraku, S. and Kurokawa, K. described above use a different correction method, called the Two-curve method. Other research works give less explicit description on how non-STC results are corrected. 4.6. Electrical parameters and resistances of solar cell at STC
Fig. 7. Shows relationship between irradiance on the test plane and LED currents from the five arrays.
From the corrected IeV curves of the red and blue simulators at 1000 Wm2 we calculate internal dynamic resistance (Rd) and electrical parameters (ISC, VOC, Pmp, efficiency, fill factor) at the STC.
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Fig. 8. Shows uncorrected IeV curves from LED simulators.
The series resistance and shunt resistance are measured under dark, so no corrections are needed. The results are shown in Table 3. Comparing values of electrical parameters and resistances using the red and blue simulators with those of the Class AAA simulator, both the red and blue simulator results are in very good agreement with the Class AAA simulator. Differences in resistances and electrical parameters in most cases are about 2% or less. The exception is the maximum power measured by the blue LED simulator, the deviation is 4.1%. Accuracy of results is important in using red and blue LEDs, with confidence, as light sources of solar simulators. This should pave ways for employing red and blue LEDs in constructions of less expensive solar simulators [16]. 4.7. Internal dynamic resistance Internal dynamic resistance of the solar cell is calculated from corrected IeV measurements for red and blue LED simulators. Table 3 Internal dynamic resistance resistances and parameters at STC and series resistance and shunt resistance under dark. Parameters
Fig. 9. Shows the corrected IeV curves at STC.
Values measured Values measured with LED simulators with Class AAA (corrected with methods of IEC 891 Standard) simulator Blue LED Red LED simulator simulator and error (%) and error (%)
1000 G (Wm2) T (C) 25 0.58 Voc (V) 4.63 Isc (A) 1.72 Pmp (W) 0.42 Vmp (V) Imp (A) 4.06 FF (%) 63.5 h (%) 11.7 0.03 RS (U) 6.40 Rsh (U) 0.08 Rd @ Vmp (U)
1000 25 0.57 (1.7%) 4.63 (0.0%) 1.65 (4.1%) 0.41 (2.4%) 4.03 (0.7%) 62.1 (1.4%) 11.1 (0.6% 0.03 (0.0%) 6.40 (0.0%) 0.08 (0.0%)
1000 25 0.59 (1.7%) 4.63 (0.0%) 1.69 (1.7%) 0.41 (2.4%) 4.11 (1.2%) 61.9 (1.6%) 11.5 (0.2%) 0.03 (0.0%) 6.40 (0.0%) 0.08 (0.0%)
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and Dr. Veerapon Monyakul. Research funds for A. Namin are made available from the Rajamangala University of Technology Lanna.
References
Fig. 10. Dynamic resistance of the solar cell using red and blue LED simulators.
Comparison is made with values under dark and calculated from the Class AAA simulator. Results are plotted as Fig. 10. Good agreements are found. 5. Conclusion In this paper, four single-color (red, green, blue, white) LED solar simulators and one red-green-blue simulator, all having high irradiance, are constructed. High irradiance is achieved by pulsing LEDs with high input voltages. Irradiance uniformity and instability qualify the five simulators as Class B. Solar cell electrical parameters and resistances using these simulators are made. Applying correction methods recommended in the IEC 891 Standard, for IeV characterization at non-STC, results on electrical parameters and resistances measured with the red and blue LED simulators and the Class AAA simulator are in good agreement. Acknowledgments The authors wish to acknowledge supports from the CSSC for excellent research facilities. Valuable advices have been given by Assistant Professor Proapran Plienpoo, Dr. Koarakot Wattanavichean
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