Design Construct and Evaluation of Six-Spectral LEDs-Based Solar Simulator Based on IEC 60904-9

This article presents the optical design of a six-spectral LED-based solar simulator. The study focuses on the measurement methods of spectral match, the nonuniformity and the temporal instability on the test plane. The proposed six-spectral LED solar simulator has an illuminated area of 900 cm and can characterize medium size photovoltaic devices under variable irradiance. The spectral range covered is between 400 nm and 1100 nm which offer the capability of characterizing silicon PV technologies. The evaluation method is in accordance with IEC 60904-9. The Solar Power meter was used to measure the irradiance and a Compact Array Spectrometer and Fiber optic spectrometer were applied for the spectral match test. The irradiance is controlled via LabVIEW which can adjust the power to drive all LEDs in the constant current mode. The irradiance of 1000 W/m was applied during testing. The results found that the spectral match classification is of class B. The non-uniformity of irradiance is of A-class across the illuminated area. The temporal instability of irradiance is capable of reaching class A. This idea is perfectly appropriate for the application of the testing of I-V characteristic of the silicon solar cell. For future work, the author has to improve the spectral match to reach an A Class.


II. LITERATURE REVIEW
Kolberg. [7]was reported on a fully LED-based solar simulator for the PVindustry. The spectrum of a solar simulator was from 350 to 1100 nm. Several spectrums of LEDs were applied to be the light source which includes visible light, Ultraviolet spectrum and Near Infrared spectrum. The temporal stability of the light intensity was ± 0.3%. Rumyantsev [8] developed a solar simulator that was used for the testing of the IV-characteristic of the 0.5×1.0 m 2 HCPV module (InGaP/GaAs/Ge), and silicone-on-glass Fresnel lens panels. This study used the light emitting diode as a light source and control in flash mode. The testing area was about 50x100 cm 2 . Solar irradiance was 835 W/m 2 . Light intensity was under flash illumination. The light source had a spectral uniformity within ±4%, with the light stability of ±2%. Photong [9] was presented the Halogen-LED hybrid solar simulator that was used for studying the IVcharacteristic of the solar cell. Two 50-watts halogen lamps and 1024 LEDs with 0.5W each (wavelength 450 -480 n.m.) was applied. This study was able to generate 1040 W/m 2 as maximum irradiance flux and match with the spectral match in class B (according with IEC 40904-9). The study could be applied to the 17 × 17cm 2 of the test areas. Bazzi. [3] reported the solid state solar simulator with six colors of the LEDs. The six-LEDs with difference light spectrum (between 400 to 1100 n.m.) were chosen to be the light source. The maximum irradiance was equal to 759 W/m 2 and the solar simulator operated on the test plane of about 10×10 cm 2 . The ASTM standard was applied to test the quality of the solar simulator. Grandi. [5]developed the hybrid LEDs-Halogen based solar simulator prototype which was used for solar cell characteristic tests. The spectral match is in class B (IEC 60904-9). The luminance flux maximum is 755 W/m 2 on the test area of about 20×50 cm 2 . Namin. [4] presented the LED solar simulators for the 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. Low cost LED simulators operated under pulse operations gave high irradiance close to 1000 Wm −2 . Irradiance uniformity and temporal instability quantify the five simulators as Class B. The solar simulator can test the I-V curves of on 12.5×12.5 cm 2 of the Solar cell. Saadaoui. [10] reported the LEDs-halogen based solar simulator which was used to test the characteristic of Dye Sensitized solar cell. Their application is for the laboratory use. Grandi. [11] presented the LEDs-Halogen-based Solar simulator with seven spectrums of LED. This simulator was used for the testing of the I-V characteristic of solar cell. The luminance was adjusted by using the voltage control technique. The maximumluminance is 590 W/m 2 and the test plane is about 12.5 × 12.5 cm 2 . The quality of this solar simulator matched with Class ABB (IEC 60904-9).
Stuckelberger. [12] presented the class AAA LEDs-based solar simulator. The simulator is comprised of 11 colors of 19 LEDs (399-728 n.m.). The system is designed specifically for light soaking and current-voltage measurements of amorphous silicon solar cells. The reference spectrum was AM1.5g-with a spectral match corresponding to class A+ or better. The concept of modular LED blocks and electronics guarantees good uniformity and easy up-scalability. Instead of cost-intensive LED drivers, low-cost power supplies were used with current control, including a feedback loop on in-house developed electronics. This prototype satisfies the highest classifications (better than AAA from 400 to 750 nm) with an illuminated area of 18 cm × 18 cm with the maximum irradiance of about 2,435 W/m 2 Novickovas. [1] reported on an efficient design of a light-emitting diode (LED)-based AAA class solar simulator, employing only 19 high-power emitters for a usable illuminated area of at least 5 cm in diameter with 1 sun irradiance. The number of LEDs used in this design was very low and was able to include several wavelengths of the LED groups (400-950 n.m.) and taking advantage of wide emission angle for others. The socalled A class spectrum was also achieved for the larger area of more than 6×6 cm 2 , covering zones with B and C class irradiance non-uniformity.
Schubert. [13] reported a comparative study of the three type solar simulator. There are 19-spectral LEDs solar simulator, Halogen solar simulator, and Xenon+Halogen solar simulator. The result was that a 19-LED solar simulator is comparable to an industry standard-filtered Xenon solar simulator. By improving the LED spectral tunability, better performance can be expected.
Leary [14] studied the performance of two different types of the solar simulators by comparing between the xenon lamp-based (Class : AAA) and led-based solar (Class :AAA) simulators. The I-V characteristics of the solar cells were tested in this study. The results demonstrated that the LED-based simulator produced a more stable, flexible, and accurate match to AM1.5G than the xenon lamp-based simulator with similar marks in the quality of PV cell response.
Watjanatepin [15] presented the Chip-On-Board LEDs solar simulator. Eighteen 50-Watt COB LEDs (CCT: 3000 K, 6000K) were applied as light source. The non-uniformity of irradiance was approximately of Class: C on the test area of 224 cm 2 (distance from the light source is about 25 cm). The irradiance can be linearly adjusted from 0 to 769 W/m 2 via the LabVIEW. The solar simulator was applied for the laboratory use. The LEDs were installed on an aluminum anodize heat sink. There are thirty fins with the electric fan ventilation system at the back of the heat sink. (Figure.1) The luminance area is 295 mm x 210 mm. There are three sets of high quality switching power supply to feed power to the LEDs. The diagram of the six-spectrums of LEDs (light source) is as shown in Figure 2. The mirror wall was installed inside of the test room as shown in Figure  2.

D.The IEC 60904-9 Standard
This study reference from the IEC 60904-9 (Photovoltaic device -Part 9: Solar Simulator Performance requirements). The IEC 60904-9 is the testing standard for testing the solar simulator for the indoor case. That is used for solar cell V-I characteristic test. This standard needs to test in the three categories. There is (a) The spectral match (b) The Non -uniformity of irradiance, and (c) The temporal instability of irradiance. [16] 1) The spectral match The spectral match can measure by using the Spectroradiometer. That should measure the irradiance in each wave range of the band width of the light. (Band width, nm) The measured results would compare with the standard value of AM 1.5 G in each wave range band. Find out the percentage of the spectral match. The percentage of total irradiance is specified in Table I.

2) The Non -uniformity of irradiance
The non-uniformity of irradiance on the test plane is the percentage of the difference between the maximum irradiance and the minimum irradiance over the sum of maximum irradiance and minimum irradiance.According to the IEC 60904-9 standard [16] the test plane will bedivided into 64 equal size test positions (8x8). Then the irradiance is measured from each position and the minimum and maximum value of irradiance is taken to calculate the percentage of non-uniformity by using equation (1).
Q max is the measured value of the maximum irradiance (W/m 2 ) Q min is the measured value of a minimum irradiance (W/m 2 )

3) The temporal instability of irradiance
The temporal instability of irradiance is distinguished into two types which are the short-term instability, (STI) and long-term instability, (LTI) which will depend on the type of the solar simulator. This study will be focused on the steady state solar simulator for I-V measurement. [16] The STI will be related with the data sampling time. The LTI will be related with the time of data acquisition that is measured for the I-V characteristic of the solar cell under test. The equation of temporal stability is as shown in equation (2). % 100% (2) The IEC 60904-9 classified the standard of a solar simulator into three quality classes. There are Class A, Class B, and Class C. Each class will define the percentage of the spectral match, non-uniformity of irradiance, and temporal instability of irradiance as shown in Table II.

E. Non-uniformity of Irradiance and Temporal instability test procedure
The non-uniformity testing procedure is based on IEC-60904-9 Edition 2.0 2007-10on page [10][11]. The flowchart of the step by step procedure is shown in Figure 4.The temporal instability of the solar simulator used for the testing procedure is based on the IEC 60904-9 on page 11-13. The author tested two schemes which were (1) Short-term instability (STI) test and (2) Long-term instability (LTI) test. The LTI in this study is tested for 10 minute, and STI is tested on the pulse mode with t on =t off =10s. The sampling rate is 1 second.To measure the irradiance by using the Solar Power Meter model TES1333-R.

A. Non-uniformity of irradiance
The measured results of the irradiance from position A1 to H8 (64 positions in total) showed that the highest irradiance on the test plane is about 1013 W/m 2 at the position E5. The lowest is equal to 993 W/m 2 at the position A1. The average value of irradiance over the test plane is approximately 1003 W/m 2 .The calculation results of non-uniformity on the test area of 32.5cm x 28cmis equal to 1.915 % which meets the IEC 60904-9 standard in Class: A. (Figure 6) The uniformity of the light source was very good because the author designed a symmetrical installing position for all the LEDs. According with the study of Stuckelberger. [12] Moreover, the author kept the distance between the light source and the test area to about 45 cm which is well positioned for an excellent non-uniformity. That conforms to the study of Watjanatepin [15] Shatat [17]and Mohan. [18] The mirror wall also contributed to the good results in non-uniformity of irradiance which, as mentioned previously, met class A.

B. Temporal instability 1) Long-term instability test
The temporal stability of irradiance from the Six-Spectrums LEDs-Based Solar Simulator. This study using the Long-term instability testing method.(IEC 60904 -9, 2007)To turn on the power supply continue to 10 min. and give a worm-up time of LED is about 20 seconds. The control of irradiance constant at 1000 W/m 2 .The results found that the LTI is equal to 1.038 %. (Table III) This mean that the temporal stability of the irradiance by the continuous running. It is meet the Class: A of the IEC 60904-9. The measured results as shown in figure7.

2) Short-term instability test
The Short-term instability testing method. (IEC 60904 -9, 2007) To turn on the power supply and set up the solar simulator generated the luminance in pulse mode (turn-on time10 s, duty cycle 50%). No need to worm-up time of the LED before test. The control of irradiance constant at 1000 W/m 2 .The measured pulse of the irradiance as show in figure 8. The calculated results (by equation 2) found that the STI is equal to 0.36 %. 0.29 % and 0.32%. This means that the short-term temporal stability of the irradiance meets the Class: A of the IEC 60904-9. The technical design achieved satisfactory results of having a temporal instability. This is a good quality of a power supply. The author used low ripple power supply with the constant current control as the LED driver. This is in accordance with the study results of Bazzi [3] and the design of Novickovas [1] and Stuckelberger [12] whose results showed that the uniformity measurements were also in class A.

C. Spectral match
The spectral match test results as analysis by the software (Specwin-Pro and Spectra-Wiz) then plot the relative spectral irradiance of the 6-spectral LEDs based solar simulator prototype as shown in figure 9. The calculation of the spectral match factor of a solar simulator prototype compare with the AM 1.5G was show in Table IV. The result show that the spectral match meet in Class B(IEC 60904-9). It is possible to improve the spectral match to reach an A-Class. From Table IV, the percentage of irradiance is too much in the range of 500-600 nm. The author could decrease the percentage of irradiance from 27.8 to 24.8 and the spectral match factor will be less than 1.25 or meet A-class. On the other hand, the author has to increase the irradiance of LED 940 nm from 9.96% to 11.96%, which will also be able to make the spectral match reach Class A. A good solution is to separate the power supply to be 6 sets. Each power supply will control each LED type, this is in accordance with the idea of Bazzi. [3]In this case the authors can dependently control the percentage of irradiance in each range. This means that the author can increase or decrease the irradiance in 500-600 nm and 900-1100 nm to meet class A. Affect to the cost of solar simulator will increase.

V. CONCLUSION AND OUT LOOK
The six-spectral LEDs solar simulator is effective for the generation of high irradiance about 1000 W/m 2 on the test plane. There are six different LED types in this study (cool white, warm white, UV 410nm, IR 740nm, IR 850nm, and IR 940nm). This solar simulator reaches Class BAA on 400 to 1100 nm spectral range (IEC 60904-9 standard). This idea can reduce the number of the LED chips and the inactivation of complex optics was achieved. The prototype is a compact system with simplified LED circuit design. The prototype was demonstrated using six types of high-power LEDs. Only 98 LEDs are needed for a usable illuminated area of 900 cm 2 . This solar simulator is perfectly appropriate for testing the I-V characteristic of the medium size silicon solar cells.
The author has to improve the spectral match to reach A Class. Moreover, the author has innovating ideas for the design of the AAA Class of LED solar simulator by using five-spectral of LED and separate individual power supply for controlling the irradiance of each type of LEDs for future work. Relative spectral irradiance of 6-spectral LED based solar simulator