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Spectral mismatch correction factor indicated by average photon energy for precise outdoor performance measurements of different-type photovoltaic modules
Renewable Energy 114 (2017) 567e573
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Spectral mismatch correction factor indicated by average photon energy for precise outdoor performance measurements of differenttype photovoltaic modules Jakapan Chantana a, *, Hiroyuki Mano a, Yuhei Horio a, Yoshihiro Hishikawa b, Takashi Minemoto a a b
Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan National Institute of Advance Industrial Science and Technology (AIST), Research Center for Photovoltaic Technologies, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2017 Received in revised form 10 July 2017 Accepted 13 July 2017 Available online 17 July 2017
Outdoor performance of different-type photovoltaic (PV) modules ((1) heterostructure-with-intrinsicthin-layer, (2) single-crystalline silicon back-contact, (3) multi-crystalline silicon, (4) CuInSe2, and (5) CdTe), which is short-circuit current (ISC), is measured using common single-crystalline silicon (sc-Si) PV module as PV module irradiance sensor (PVMS) with fast measurement time (below 200 ms). Errors between corrected ISC (outdoor measurement) and ISC (indoor measurement) under standard test condition (STC; AM 1.5G, 1 kW m2 and 25 C) of the PV modules are investigated. The corrected ISC of outdoor PV modules in the whole year is derived by correcting their measured ISC with a ratio of ISC of PVMS under STC to the measured ISC of PVMS for irradiance correction, thereby yielding the decrease in the error. Moreover, spectral mismatch correction factor (MMF) is also utilized for the correction of the measured ISC of different-type PV modules for the precisely corrected ISC. The average photon energy, indexing spectral irradiance, is proposed as an indicator of the MMF for the first time. Ultimately, median (0.87%) and interquartile range (1.36%) of error for CdTe PV module are achieved by correcting the measured ISC using PVMS and MMF, although spectral response of CdTe PV module very different from that of PVMS. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Spectral mismatch correction factor Spectral response PV module irradiance sensor Average photon energy Short-circuit current
1. Introduction Electricity power greatly contributing to the quality of people life is generally generated by burning fossil fuels, which have detrimental impacts on the environment and will be depleted. Sustainable energy has attracted much attention as it can solve the foreseeable energy crisis by providing a clean and eternal energy source [1,2]. One of the most promising technologies for the sustainable energy is photovoltaics (PV), a process of converting light energy into electricity. Since outdoor PV module installations have significantly increased as sustainable energy, the highly accurate evaluation of the PV modules at the installation site (outdoor measurement) compared with the indoor evaluations is of importance. The indoor evaluation is performed through the controlled
* Corresponding author. E-mail addresses: [email protected] (J. Chantana), re0025pi@ed. ritsumei.ac.jp (H. Mano), [email protected] (Y. Horio), y-hishikawa@aist. go.jp (Y. Hishikawa), [email protected] (T. Minemoto). http://dx.doi.org/10.1016/j.renene.2017.07.061 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
laboratory testing under standard test condition (STC), which is denoted from here on as indoor measurement. Although the outdoor measurement of the PV modules with comparable accuracy to the indoor measurement of the PV modules is feasible, the outdoor measurement opportunity is normally limited to clear sunny days with the stable spectral (or solar) irradiance [3,4], which is not frequently available in many installation sites. Therefore, various precise PV measurements are generally performed indoor using solar simulator [5,6]. However, the evaluation of outdoor PV modules using the solar simulator in the indoor site is the difficult task. The evaluation methods in the outdoor location (installation site) under several irradiance and temperature conditions such as varying cloud cover, and aerosol optical depth should be consequently developed for precise measurement of the outdoor PV module performances as compared with the indoor performances. It was recently reported that the precise outdoor PV measurement in comparison to the indoor measurement with the precision of ±1-±2% has been attained under unstable outdoor weather conditions by utilizing
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the outdoor measurement procedure consisting of fast currentvoltage measurement, simultaneous irradiance (Irr) measurement by PV reference device, and correct attachment method of temperature sensors [7e9]. It is noted that the simultaneous Irr measurement by PV reference device is essential since the spectral irradiance (E) always changes. However, the PV reference solar cell for simultaneous Irr measurement must have the similar spectral response with the outdoor test PV modules for highly precise outdoor PV measurement. In other words, the PV reference device should be the same type as the test PV modules. It is well known that spectral mismatch correction factor (MMF) is used to correct the performances, especially short-circuit current (ISC), of the solar cells under the real spectral power distribution of the light source and their spectrum response (SR) different from reference spectral power distribution (AM 1.5G, IEC 60904-4) and the SR of the PV reference device, respectively [10e12]. In this work, a PV reference device, named PV module irradiance sensor (PVMS), for simultaneous Irr measurement is utilized to precisely evaluate the outdoor performance of various-type test PV modules, especially the ISC. This is because ISC is strongly affected by the E. For the utilization of the PV reference device to evaluate the various-type test PV modules, MMF normally utilized for Irr correction is therefore used to the correct ISC of the test PV modules under outdoor measurement for high accuracy as compared with the ISC under indoor measurement. To achieve the goal, the relation between the E shape and average photon energy (APE) is first investigated at the installation site of the test PV modules. The APE is consequently used as an index of solar spectral irradiance distribution. Then, the correlations of the APE with ISC and MMF are examined. As a result, the APE is utilized as an indicator of MMF of the PV test modules for the first time for easier and cheaper measurement. Ultimately, the measured ISC (ISC-meas) of the outdoor test PV modules at outdoor location is corrected using the MMF for the corrected ISC (ISC-corrected) with highly precise evaluation, which is very close to ISC of the test PV modules, measured indoor under STC (IEC 60904-4) composed of incident solar irradiance of 1 kW/m2, AM 1.5G solar spectrum distribution, and module temperature (Tmod) of 25 C [12,13].
2. Experimental details
Table 1 Their properties of test PV modules ((1) multi-crystalline silicon (mc-Si), (2) heterostructure-with-intrinsic-thin-layer (HIT), (3) single-crystalline silicon backcontact (BC), (4) CuInSe2 (CIS), and (5) CdTe), and the properties of singlecrystalline silicon (sc-Si) PV module as PV module irradiance sensor (PVMS). Module
Area of PV module (m2)
Power (W)
Eg (eV)
a (%/ C)
ISC-STC (A)
1. 2. 3. 4. 5. 6.
0.265 1.280 1.630 1.220 0.720 0.634
33.2 244.4 323.0 161.5 97.8 87.7
1.13 1.09 1.10 1.21 1.47 1.12
0.039 0.034 0.035 0.006 0.045 0.020
2.005 5.980 6.380 2.066 1.534 5.370
mc-Si HIT BC CIS CdTe sc-Si (PVMS)
and east longitude 135 570 ). It is known that ISC is most sensitive to the E. The ISC of the PVMS (sc-Si PV module) is 5.37 A, measured under STC (IEC 60904-4 with Class-A) [9,12] by indoor precision measurement at National Institute of Advanced Industrial Science and Technology (AIST), where this ISC is named ISC-PVMS-STC. Class-A means the deviations within 25% in all spectral bands produced by a solar simulator from those of the AM 1.5 reference spectrum. The ISC-PVMS-STC is used to correct the measured ISC (ISC-meas) of the test PV modules under outdoor measurement for high accuracy as compared with the ISC under the indoor measurement under STC. In addition, the Irr (or the integrated spectral irradiance) was observed by pyranometer (EKO MS-402) from 285 to 2800 nm, which was recorded every 1 min, where it takes approximately 8 s for each measurement to obtain one stabilizing Irr. This could not lead to the precise outdoor PV measurement since the E always changes. Spectral irradiance was observed from 350 to 1050 nm by spectro-radiometer (EKO MS-700). If light is less than 1 sun (AM 1.5G solar spectrum distribution), it takes about 5 s for one E by EKO MS-700, while if that is close to 1 sun, it takes less than 5 s. The pyranometer and spectroradiometer were placed next to the five different types of the above test PV modules in the same orientation as the modules. Tmod was measured using the thermocouple attached to the back side of all PV modules. For the investigation of the relation between the E shape and APE at the installation site of the test PV modules, a value of APE was calculated as the integrated spectral irradiance divided by the integrated photon flux density, as demonstrated:
2.1. PV modules and measurement setup Five different types of the test PV modules were investigated for the accurate outdoor performances at the installation site (north latitude 34 580 , and east longitude 135 570 ). Namely, (1) multicrystalline silicon (mc-Si) PV module (Kyocera KC32T-02) composed of 57.33 cm2 sized wafers, (2) heterostructure-withintrinsic-thin-layer (HIT) PV module (Panasonic N244a) consisting of 156.25 cm2 sized wafers, (3) single-crystalline silicon backcontact (BC) PV module (SunPower E20/327) constituting 156.25 cm2 sized wafers, (4) CuInSe2 (CIS) PV module (Solar Frontier SF-160S) with the solar cell stripe of 61.3 cm2, and (5) CdTe PV module (First Solar FS-4100) with the solar cell stripe of 58.60 cm2 were installed with capacities of 33.2, 244.4, 323, 161.5, and 97.8 W in Table 1, respectively. In this work, single-crystalline Si (sc-Si) PV module (SHARP NT-84L5H) with capacity of 87.7 W in Table 1, situated by the five different types of the above test PV modules, was utilized as PV reference device or PVMS for Irr measurement with fast measurement time (below 200 ms) for high accuracy of outdoor measurement of the test PV modules. The sc-Si PV module is composed of 156.25 cm2 sized wafers. All PV modules facing due south with a tilt angle of 15.3 are located at Ritsumeikan University, Kusatsu-city, Shiga-prefecture in Japan (north latitude 34 580 ,
Z
y
Zx
APExy ðeVÞ ¼ q
x
EðlÞdl
y
and
FðlÞdl
FðlÞ ¼
EðlÞ Ephoton ðlÞ
(1)
where q (C) is elementary charge, F (s1 m2 nm1) is the spectral photon flux density, and Ephoton (eV) is the energy of single photon [14e18]. Unit of E is W m2 nm1. The l denotes wavelength. Owing to the limitation of the measuring instrument, x and y in Equation (1) are set to be 350 and 1050 nm, respectively. The APE is utilized as an index to indicate solar spectral irradiance distribution, where its value of AM 1.5G reference spectrum estimated in the wavelength range of 350e1050 nm is 1.88 eV [15,19].
2.2. Methodology for calculations of short-circuit current and spectral mismatch correction factor After calculating APE, the ISC of the test PV modules and MMF as a function of APE at the installation site of the test PV modules are investigated to comprehend the impact of APE (E shape) on the ISC and MMF. The ISC is estimated by the following formula:
J. Chantana et al. / Renewable Energy 114 (2017) 567e573
Z ISC ¼ S
EðlÞSRðlÞdl
(2)
where S (m2) is the area of PV modules in Table 1, SR (A W1 nm1) is the spectral response of the test PV modules. It is noted that the ISC-PVMS-STC (indoor measurement) for PVMS (sc-Si PV module) is 5.37 A under STC, which is used for the correction of the ISC-meas. In this work, to use the PVMS for the outdoor evaluation of differenttype PV modules, MMF is also used to correct the ISC-meas of the test PV modules under outdoor measurement for high accuracy, which is close to the ISC under the indoor measurement under STC. MMF is estimated using the following formula [10,11]:
Z MMF ¼ Z
Eref ðlÞSRref ðlÞdl
Z
Emeas ðlÞSRsample ðlÞdl Z Emeas ðlÞSRref ðlÞdl Eref ðlÞSRsample ðlÞdl
(3)
569
each spectral band was calculated with different APE values. More detail for the calculation of the PTI was discussed in our previous work [15,17]. Fig. 1(a) consequently shows PTIs of the Es with APE values in a range of 1.85e1.96 eV with 0.01 eV intervals. It is noted that the standard deviation of the PTI of all spectral bands is less than 0.16% (absolute %) as depicted in Fig. 1(b). As depicted in Fig. 1(a), the PTI is well proportional to the value of the APE. The PTI in a short wavelength range of about 350e600 nm increases with enhancing the APE value, whereas the PTI in the long wavelength region of approximately 600e1050 nm decreases with increasing the APE value. The Irr of AM 1.5G in wavelength range of 350e1050 nm is 765 W/m2. The Es with various APE values under a constant Irr of 765 W/m2 as depicted in Fig. 2 are thus obtained by multiplying the PTI in Fig. 1(a) with 765 W/m2. According to Fig. 2, the relationship between the E shape and APE is obviously demonstrated at the installation site of the test PV modules. The E in short and long
where Eref and SRref are the spectral irradiance (AM 1.5G) and spectral response of PVMS (the sc-Si PV module). The SRref and SRsample are measured at 25 C. Emeas is the spectral irradiance incident on the outdoor test PV modules at the outdoor installation site. SRsample is spectral response of the outdoor test PV modules. 3. Results and discussion 3.1. Average photon energy as an index of spectral irradiance and an indicator of spectral mismatch correction factor First, the relationship between the E shape and APE was scrutinized at the installation site of the test PV module. The outdoor E (solar spectrum) at the installation site was consequently accumulated by the spectro-radiometer from June 2015 to May 2016 with data points greater than 32000. The E with low Irr (below 1.0 W m2) was excluded to avoid the scattered data of low irradiance spectra. The measured Es with the wavelength range of 350e1050 nm were indexed as APE ¼ 1.849e1.851 (1.850 ± 0.001), 1.859e1.861 (1.860 ± 0.001), …, 1.949e1.951 (1.950 ± 0.001) and 1.959e1.961 (1.960 ± 0.001) eV. The measured E for different APE values was then divided into 71 spectral bands with 10 nm intervals, and the mean of percentage to the total irradiance (PTI) of
Fig. 2. Spectral irradiances (E) with APE values from 1.85 to 1.96 eV with 0.01 eV intervals at the installation site of the test PV modules. The figure was constructed with approximately 32000 data points from June 2015 to May 2016.
Fig. 1. (a) Mean and (b) standard deviation of percentage to the total irradiance (PTI) of each spectral band (10 nm intervals) with the APE range of 1.85e1.96 eV with 0.01 eV intervals. The figures were constructed with approximately 32000 data points from June 2015 to May 2016.
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wavelength areas increases and decreases, respectively, when the APE value is enhanced from 1.85 to 1.96 eV. In other words, the E changes from red-rich to blue-rich spectra with enhancing APE. The results suggested that the APE value uniquely yields the E and can be used as an index to indicate solar spectral irradiance distribution. To calculate the ISC of the PV modules using Equation (2), their SRs needed to be known. Fig. 3 subsequently demonstrates relative SRs of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe) in this work, which were measured at 25 C. The SRs of the outdoor test PV modules (HIT, BC, mc-Si, CIS, and CdTe) are denoted as SRsample, while the SR of sc-Si PV module as PVMS is expressed as SRref. According to the figure, bandgap-energies (Eg) of HIT, BC, scSi, mc-Si, CIS, and CdTe modules are about 1.09, 1.10, 1.12, 1.13, 1.21, and 1.47 eV in Table 1, respectively. The ISC of all PV modules was estimated using Equation (2) with the Es with different APE values in a range of 1.85e1.96 eV in Fig. 2 to investigate the effect of APE on the ISC, where the E can be well indexed by APE. The ISC was then
normalized by ISC@STC. The ISC@STC was the ISC of all PV modules calculated using Equation (2) under STC. Fig. 4 consequently illustrates ISC/ISC@STC of all PV modules as a function of APE. In Fig. 4, the ISC/ISC@STC of CdTe PV module is slightly influent by the variation of APE. This is because the Es, indicated by APE in Fig. 2, change to blue-rich spectra with increasing APE, which is suitable for the large Eg (1.47 eV) of CdTe for high generated current; however, the number of photon is decreased with high APE. As a result, the ISC/ISC@STC of CdTe PV module is not affected very much by APE. On the other hand, the ISC/ISC@STC of HIT, BC, sc-Si, mc-Si, and CIS PV modules is reduced with enhancing APE from 1.85 to 1.96 because of the decrease in the number of photon. It is revealed that the APE, indexing the E, differently affects the ISC of the PV modules depending on their SR, especially in the case between PVMS (sc-Si PV module) and CdTe PV module owing to their very different SRs in Fig. 3. In addition, since the Es are previously obtained by multiplying PTI with the Irr, Equation (3) for MMF calculation is simplified by replacing Emeas by PTI as shown in the following equation:
Z
Z Eref ðlÞSRref ðlÞdl PTIðlÞSRsample ðlÞdl Z MMF ¼ Z PTIðlÞSRref ðlÞdl Eref ðlÞSRsample ðlÞdl
(4)
As previously demonstrated, the PTI and Es at the installation site of the outdoor test PV modules in Figs. 1 and 2 can be well indexed by APE. The correlation between MMF and APE is therefore investigated using Equation (4). It is noted that due to the limitation of the measurement instrument, the l range for the MMF calculation in Equation (4) is from 350 to 1050 nm. Fig. 5 consequently presents MMF of the outdoor test PV modules (HIT, BC, mc-Si, CIS and CdTe) as a function of APE, where sc-Si PV module is utilized as PVMS. In Fig. 5, MMF is most close to 1 with varying APE from 1.85 to 1.96 eV in case of BC, HIT, mc-Si PV modules since the SRs among sc-Si (PVMS), BC, HIT, and mc-Si modules are almost similar in Fig. 3. On the other hand, MMF is most deviated from 1 in case of CdTe PV module because SRs between sc-Si (PVMS) and CdTe PV modules are most different as seen in Fig. 3. The highest MMF is approximately 1.06 at APE of 1.96 eV for CdTe PV module, thus yielding the most different ISC/ISC@STC between CdTe PV module and Fig. 3. Relative spectral responses of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe).
Fig. 4. ISC/ISC@SCT of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe) as a function of APE.
Fig. 5. Spectral mismatch correction factor (MMF) as a function of APE for the PV modules (HIT, BC, mc-Si, CIS, and CdTe) at the installation site, where sc-Si module was used as PVMS. The range of wavelength (l) for the MMF calculation in Equation (4) is from 350 to 1050 nm.
J. Chantana et al. / Renewable Energy 114 (2017) 567e573
PVMS (sc-Si PV module) at APE of 1.96 eV as seen in Fig. 4. It is consequently suggested that the ISC-meas of the various-type test PV modules needs to be corrected by not only the ISC-PVMS-STC of PVMS but also MMF for the corrected ISC (ISC-corrected) with high accuracy, especially in the case the CdTe PV module with its SR very different from that of the PVMS. In Fig. 5, it is additionally proposed that APE can be used as an indicator of MMF of the outdoor test PV modules for the first time for easier measurement.
For the utilization of PVMS (sc-Si PV module) to evaluate the various-type test PV modules at their installation site, the ISC-meas of the test PV modules under outdoor measurement is corrected by MMF for the corrected ISC (ISC-corrected) with high accuracy, close to their ISC under the indoor measurement under STC, where the MMF is predicted by APE as shown in Fig. 5. For comparison, three ISC correction methods are estimated. First, ISC-corrected (1) of the outdoor test PV modules, called (1) Tmod þ pyranometer, was calculated by correcting the ISC-meas using Tmod and Irr, obtained by the pyranometer, as shown in the following formula:
1
1 þ a Tmodsample 25
1 Irrpyranometer IrrSTC
(5)
where Irrpyranometer (kW m2) is the Irr of the spectral irradiance incident on the outdoor test PV modules measured by pyranometer. Room temperature is 25 C. IrrSTC is the Irr under STC condition (1 kW m2). ISC-meas (A) is the measured ISC of the outdoor test PV modules at the outdoor location. The a is temperature coefficient of Isc of test PV modules. The a is 0.020, 0.039, 0.034, 0.035, 0.006, and 0.045%/ C for sc-Si (PVMS), mc-Si, HIT, BC, CIS, and CdTe in Table 1, respectively. Tmod-sample is the temperature of the outdoor test PV module and added for the effect of module temperature. Second, ISC-corrected (2) of the outdoor test PV modules, named (2) Tmod þ PVMS, was estimated by correcting the ISC-meas using Tmod ( C) and IrrPVMS. The IrrPVMS is calculated from ISC-PVMS-meas and ISC-PVMS-STC of PVMS (sc-Si PV module). ISC-corrected (2) is estimated using the following formula:
ISCcorrected ð2Þ ¼ ISCmeas
1 1 IrrPVMS 1 þ a Tmodsample 25 (6)
where IrrPVMS is expressed as follow.
IrrPVMS ¼ ISCPVMSmeas
1 ISCPVMSSTC
module) at the outdoor location, and ISC-PVMS-STC of 5.37 A for PVMS (indoor measurement) was obtained under STC by the indoor precision measurement. Third, ISC-corrected (3) of the outdoor test PV modules, called (3) Tmod þ PVMS þ MMF, was evaluated by correcting the ISC-meas using Tmod ( C), IrrPVMS, obtained by PVMS, and MMF for high accuracy, as shown in the following formula:
1 1 ISCcorrected ð3Þ ¼ ISCmeas Irr PVMS 1 þ a Tmodsample 25
3.2. Spectral mismatch correction factor for precise outdoor performance measurements
ISCcorrected ð1Þ ¼ ISCmeas
571
1 ð1 þ aðTmodPVMS 25ÞÞ (7)
IrrPVMS for the Irr correction is derived from PVMS (sc-Si PV module) with much faster measurement time (below 200 ms) than Irrpyranometer (about 8 s) for high accuracy since the spectral irradiance always changes. It is noted that the measurement time is the time for one measurement to obtain the stabilizing data, and the IrrPVMS from PVMS and Irrpyranometer from pyranometer were measured every 1 min. Tmod-PVMS is the temperature of PVMS (sc-Si PV module). ISC-PVMS-meas is the measured ISC of PVMS (sc-Si
1 MMF (8)
In this work, the MMF of the test PV modules is readily obtained when APE, calculated from the E in a range of 350e1050, at the installation site is known as shown in Fig. 5. It is reported that APE, estimated from the E in two limited bands of 450e500 nm and 800e850 nm, can precisely describe the APE, calculated from the E in a range of 350e1050 nm [14]. Therefore, the MMF in Equation (8) could be predicted by APE with two limited bands of 450e500 nm and 800e850 nm in the future, making it much easier and faster. After calculating the ISC-corrected using three different methods, the ISC-corrected of the outdoor test PV modules at the installation site (outdoor measurements) is then compared with their ISC-STC (indoor measurements) by using error (%). The ISC-STCs are ISCs of the outdoor test PV modules (mc-Si, HIT, BC, CIS, and CdTe), which were measured under STC by indoor precision measurements at AIST. ISCSTCs of the outdoor test PV modules are 2.005, 5.980, 6.380, 2.066, and 1.534 A for mc-Si, HIT, BC, CIS, and CdTe modules in Table 1, respectively. Error (%) for the comparison between outdoor and indoor measurements is subsequently demonstrated in the following formula:
Errorð%Þ ¼
ISCcorrected ISCSTC 100 ISCSTC
(9)
In this work, for the estimations of the ISC-corrected and error (%), ISC-meas of the outdoor test PV modules, ISC-PVMS-meas of PVMS, Tmod, Irr by pyranometer, and E by spectro-radiometer for APE to predict MMF at the installation site were accumulated every 1 min from 4 a.m. to 8 p.m. from December 2015 to January 2017 with data points greater than 56,000. Consequently, Fig. 6 illustrates histograms of errors (%) of the outdoor test PV modules ((a) mc-Si and (b) CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC-STC. It is noted that the SR of mc-Si is most identical to that of PVMS, while SR of CdTe is most different from that of PVMS as depicted in Fig. 3. The histogram is a graphical representation of the data distribution. It is disclosed in Fig. 6 that the error distributions of both mc-Si and CdTe PV modules obtained by (2) Tmod þ PVMS and (3) Tmod þ PVMS þ MMF become narrower than those by (1) Tmod þ pyranometer. For mc-Si PV module, the error distribution and median, which were estimated by (2) Tmod þ PVMS and (3) Tmod þ PVMS þ MMF, are not changed very much. However, for CdTe PV module, the error distribution becomes further narrower and the median is closer to 0 (0.87%), when the error calculated by (3) Tmod þ PVMS þ MMF. The result implies the most precise evaluation of the ISC-corrected of the CdTe PV module corrected by (3) Tmod þ PVMS þ MMF. In addition, Fig. 7 depicts errors (%) of the outdoor test PV modules (a whole year data) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparisons to the ISC-STC, where median and interquartile range of error (%) are expressed as median and IQR. IQR is an indicator of
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Fig. 6. Histograms of error (%) of the outdoor test PV modules ((a) mc-Si and (b) CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC under STC condition (ISC-STC), measured in indoor by AIST. The figures were constructed with approximately 56000 data points from December 2015 to January 2017.
statistical dispersion, equal to the difference between 75th and 25th percentiles. It is clearly revealed that the median and IQR of error (%) for all outdoor test PV modules (mc-Si, HIT, BC, CIS, and CdTe modules) by (2) Tmod þ PVMS are much lower than those by (1) Tmod þ pyranometer. This implied that the ISC-corrected (2) of the outdoor test PV modules using Equation (6) is more precise than the ISC-corrected (1) using Equation (5). This is because the ISC-corrected (2) was derived from Tmod ( C) and the IrrPVMS, calculated from ISCPVMS-meas and ISC-PVMS-STC of PVMS with short measurement time (below 200 ms) and the similar angle-of-incidence dependence of
Fig. 7. Error (%) of the outdoor test PV modules (HIT, BC, mc-Si, CIS, and CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC under STC condition (ISC-STC), measured in indoor by AIST. Median and interquartile range of error (%) are expressed as median and IQR. The figures were constructed with approximately 56000 data points from December 2015 to January 2017.
PVMS and the outdoor test PV modules. On the other hand, the ISC(1) was obtained from Tmod ( C) and the Irrpyranometer, taken by the pyranometer with longer measurement time (about 8 s) and the different angle-of-incidence dependence of the pyranometer from the outdoor test PV modules. It is noted that the fast measurement time is needed for Irr since the outdoor spectral irradiance always changes. Additionally, the median and IQR of error (%) for mc-Si, HIT, CIS, and BC test PV modules by (2) Tmod þ PVMS are not different very much from those by (3) Tmod þ PVMS þ MMF. This is because the MMF predicted by APE in Fig. 5 is estimated in the wavelength range from 350 to 1050 nm, limited by the instrument ability, which do not cover all wavelengths of SRs of mc-Si, HIT, CIS, and BC test PV modules in Fig. 3. On the other hand, the wavelength range from 350 to 1050 nm for the MMF indicated by APE in Fig. 5 most covers the wavelength of SR of CdTe PV module, when comparing with the mc-Si, HIT, CIS, and BC test PV modules in Fig. 3. As a result, IQR and median of errors (%) by (3) Tmod þ PVMS þ MMF are obviously reduced in the case of CdTe PV module when compared with those of the error by (2) Tmod þ PVMS, where the MMF is most deviated from 1 in the case of CdTe PV module in Fig. 5 owing to the SR of CdTe PV module very different from that of PVMS (sc-Si PV module) seen in Fig. 3. Ultimately, the median (0.87%) and IQR (1.36%) of error for CdTe module are achieved using (3) Tmod þ PVMS þ MMF. The results suggest that although the SR of the CdTe PV module is very different from that of PVMS (sc-Si), the ISC-corrected with high accuracy for the CdTe PV module was attained by correcting its ISC-meas (outdoor) with not only the ratio of ISCPVMS-STC to ISC-PVMS-meas for the irradiance correction (fast measurement time below 200 ms) but also MMF for spectral mismatch correction. It is noted that the MMF is readily indicated by APE for faster and cheaper measurement. Based on the results, for more precise evaluation of outdoor ISC of the various-type PV test modules using MMF indicated by APE, the wavelength range for investigation of the correlation between MMF and APE should cover all wavelength of SR of the outdoor test corrected
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PV module by using new system with novel spectro-radiometer. 4. Conclusions
[4] [5]
PVMS (sc-Si PV module) for IrrPVMS is utilized to precisely evaluate the outdoor ISC of the various-type test PV modules. To accomplish the goal, the relation between the E shape and APE is first studied. It is disclosed that the APE value uniquely yields the E and can be used as an index of solar spectral irradiance distribution. The ISC of the test PV module and MMF as a function of APE are then examined. It is revealed that the APE is utilized as an indicator of MMF of the outdoor test PV modules for the first time for easier measurement. It is moreover shown that the APE has the different influence on the ISC and MMF of the test PV modules depending on their SR, especially in the case between PVMS (sc-Si PV module) and CdTe PV module owing to their very different SRs. Therefore, the ISC-meas (outdoor) of the various-type test PV modules needs to be corrected by not only the ratio of ISC-PVMS-STC to ISC-PVMS-meas but also MMF for the ISC-corrected with high accuracy. Ultimately, the median (0.87%) and IQR (1.36%) of error for CdTe module are achieved using (3) Tmod þ PVMS þ MMF although the SR of CdTe PV modules very different from that of PVMS (sc-Si PV module). Acknowledgement This work is partly supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] Y. Hamakawa, Background and motivation for thin-film solar-cell development, in: Y. Hamakawa (Ed.), Thin-film Solar Cells Next Generation Photovoltaics and its Applications, Springer, Heidelberg, 2004, pp. 1e14. [2] E. Vallat-Sauvain, A. Shah, J. Bailat, Epitaxial thin film crystalline silicon solar cells on low cost silicon carriers, in: J. Poortmans, V. Arkhipov (Eds.), Thin Film Solar Cells, Fabrication, Characterization and Applications, Wiley, Chichester, 2006, pp. 1e32. [3] European Commission Joint Research Center, Guidelines for PV Power
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Spectral mismatch correction factor indicated by average photon energy for precise outdoor performance measurements of differenttype photovoltaic modules Jakapan Chantana a, *, Hiroyuki Mano a, Yuhei Horio a, Yoshihiro Hishikawa b, Takashi Minemoto a a b
Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan National Institute of Advance Industrial Science and Technology (AIST), Research Center for Photovoltaic Technologies, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2017 Received in revised form 10 July 2017 Accepted 13 July 2017 Available online 17 July 2017
Outdoor performance of different-type photovoltaic (PV) modules ((1) heterostructure-with-intrinsicthin-layer, (2) single-crystalline silicon back-contact, (3) multi-crystalline silicon, (4) CuInSe2, and (5) CdTe), which is short-circuit current (ISC), is measured using common single-crystalline silicon (sc-Si) PV module as PV module irradiance sensor (PVMS) with fast measurement time (below 200 ms). Errors between corrected ISC (outdoor measurement) and ISC (indoor measurement) under standard test condition (STC; AM 1.5G, 1 kW m2 and 25 C) of the PV modules are investigated. The corrected ISC of outdoor PV modules in the whole year is derived by correcting their measured ISC with a ratio of ISC of PVMS under STC to the measured ISC of PVMS for irradiance correction, thereby yielding the decrease in the error. Moreover, spectral mismatch correction factor (MMF) is also utilized for the correction of the measured ISC of different-type PV modules for the precisely corrected ISC. The average photon energy, indexing spectral irradiance, is proposed as an indicator of the MMF for the first time. Ultimately, median (0.87%) and interquartile range (1.36%) of error for CdTe PV module are achieved by correcting the measured ISC using PVMS and MMF, although spectral response of CdTe PV module very different from that of PVMS. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Spectral mismatch correction factor Spectral response PV module irradiance sensor Average photon energy Short-circuit current
1. Introduction Electricity power greatly contributing to the quality of people life is generally generated by burning fossil fuels, which have detrimental impacts on the environment and will be depleted. Sustainable energy has attracted much attention as it can solve the foreseeable energy crisis by providing a clean and eternal energy source [1,2]. One of the most promising technologies for the sustainable energy is photovoltaics (PV), a process of converting light energy into electricity. Since outdoor PV module installations have significantly increased as sustainable energy, the highly accurate evaluation of the PV modules at the installation site (outdoor measurement) compared with the indoor evaluations is of importance. The indoor evaluation is performed through the controlled
* Corresponding author. E-mail addresses: [email protected] (J. Chantana), re0025pi@ed. ritsumei.ac.jp (H. Mano), [email protected] (Y. Horio), y-hishikawa@aist. go.jp (Y. Hishikawa), [email protected] (T. Minemoto). http://dx.doi.org/10.1016/j.renene.2017.07.061 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
laboratory testing under standard test condition (STC), which is denoted from here on as indoor measurement. Although the outdoor measurement of the PV modules with comparable accuracy to the indoor measurement of the PV modules is feasible, the outdoor measurement opportunity is normally limited to clear sunny days with the stable spectral (or solar) irradiance [3,4], which is not frequently available in many installation sites. Therefore, various precise PV measurements are generally performed indoor using solar simulator [5,6]. However, the evaluation of outdoor PV modules using the solar simulator in the indoor site is the difficult task. The evaluation methods in the outdoor location (installation site) under several irradiance and temperature conditions such as varying cloud cover, and aerosol optical depth should be consequently developed for precise measurement of the outdoor PV module performances as compared with the indoor performances. It was recently reported that the precise outdoor PV measurement in comparison to the indoor measurement with the precision of ±1-±2% has been attained under unstable outdoor weather conditions by utilizing
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the outdoor measurement procedure consisting of fast currentvoltage measurement, simultaneous irradiance (Irr) measurement by PV reference device, and correct attachment method of temperature sensors [7e9]. It is noted that the simultaneous Irr measurement by PV reference device is essential since the spectral irradiance (E) always changes. However, the PV reference solar cell for simultaneous Irr measurement must have the similar spectral response with the outdoor test PV modules for highly precise outdoor PV measurement. In other words, the PV reference device should be the same type as the test PV modules. It is well known that spectral mismatch correction factor (MMF) is used to correct the performances, especially short-circuit current (ISC), of the solar cells under the real spectral power distribution of the light source and their spectrum response (SR) different from reference spectral power distribution (AM 1.5G, IEC 60904-4) and the SR of the PV reference device, respectively [10e12]. In this work, a PV reference device, named PV module irradiance sensor (PVMS), for simultaneous Irr measurement is utilized to precisely evaluate the outdoor performance of various-type test PV modules, especially the ISC. This is because ISC is strongly affected by the E. For the utilization of the PV reference device to evaluate the various-type test PV modules, MMF normally utilized for Irr correction is therefore used to the correct ISC of the test PV modules under outdoor measurement for high accuracy as compared with the ISC under indoor measurement. To achieve the goal, the relation between the E shape and average photon energy (APE) is first investigated at the installation site of the test PV modules. The APE is consequently used as an index of solar spectral irradiance distribution. Then, the correlations of the APE with ISC and MMF are examined. As a result, the APE is utilized as an indicator of MMF of the PV test modules for the first time for easier and cheaper measurement. Ultimately, the measured ISC (ISC-meas) of the outdoor test PV modules at outdoor location is corrected using the MMF for the corrected ISC (ISC-corrected) with highly precise evaluation, which is very close to ISC of the test PV modules, measured indoor under STC (IEC 60904-4) composed of incident solar irradiance of 1 kW/m2, AM 1.5G solar spectrum distribution, and module temperature (Tmod) of 25 C [12,13].
2. Experimental details
Table 1 Their properties of test PV modules ((1) multi-crystalline silicon (mc-Si), (2) heterostructure-with-intrinsic-thin-layer (HIT), (3) single-crystalline silicon backcontact (BC), (4) CuInSe2 (CIS), and (5) CdTe), and the properties of singlecrystalline silicon (sc-Si) PV module as PV module irradiance sensor (PVMS). Module
Area of PV module (m2)
Power (W)
Eg (eV)
a (%/ C)
ISC-STC (A)
1. 2. 3. 4. 5. 6.
0.265 1.280 1.630 1.220 0.720 0.634
33.2 244.4 323.0 161.5 97.8 87.7
1.13 1.09 1.10 1.21 1.47 1.12
0.039 0.034 0.035 0.006 0.045 0.020
2.005 5.980 6.380 2.066 1.534 5.370
mc-Si HIT BC CIS CdTe sc-Si (PVMS)
and east longitude 135 570 ). It is known that ISC is most sensitive to the E. The ISC of the PVMS (sc-Si PV module) is 5.37 A, measured under STC (IEC 60904-4 with Class-A) [9,12] by indoor precision measurement at National Institute of Advanced Industrial Science and Technology (AIST), where this ISC is named ISC-PVMS-STC. Class-A means the deviations within 25% in all spectral bands produced by a solar simulator from those of the AM 1.5 reference spectrum. The ISC-PVMS-STC is used to correct the measured ISC (ISC-meas) of the test PV modules under outdoor measurement for high accuracy as compared with the ISC under the indoor measurement under STC. In addition, the Irr (or the integrated spectral irradiance) was observed by pyranometer (EKO MS-402) from 285 to 2800 nm, which was recorded every 1 min, where it takes approximately 8 s for each measurement to obtain one stabilizing Irr. This could not lead to the precise outdoor PV measurement since the E always changes. Spectral irradiance was observed from 350 to 1050 nm by spectro-radiometer (EKO MS-700). If light is less than 1 sun (AM 1.5G solar spectrum distribution), it takes about 5 s for one E by EKO MS-700, while if that is close to 1 sun, it takes less than 5 s. The pyranometer and spectroradiometer were placed next to the five different types of the above test PV modules in the same orientation as the modules. Tmod was measured using the thermocouple attached to the back side of all PV modules. For the investigation of the relation between the E shape and APE at the installation site of the test PV modules, a value of APE was calculated as the integrated spectral irradiance divided by the integrated photon flux density, as demonstrated:
2.1. PV modules and measurement setup Five different types of the test PV modules were investigated for the accurate outdoor performances at the installation site (north latitude 34 580 , and east longitude 135 570 ). Namely, (1) multicrystalline silicon (mc-Si) PV module (Kyocera KC32T-02) composed of 57.33 cm2 sized wafers, (2) heterostructure-withintrinsic-thin-layer (HIT) PV module (Panasonic N244a) consisting of 156.25 cm2 sized wafers, (3) single-crystalline silicon backcontact (BC) PV module (SunPower E20/327) constituting 156.25 cm2 sized wafers, (4) CuInSe2 (CIS) PV module (Solar Frontier SF-160S) with the solar cell stripe of 61.3 cm2, and (5) CdTe PV module (First Solar FS-4100) with the solar cell stripe of 58.60 cm2 were installed with capacities of 33.2, 244.4, 323, 161.5, and 97.8 W in Table 1, respectively. In this work, single-crystalline Si (sc-Si) PV module (SHARP NT-84L5H) with capacity of 87.7 W in Table 1, situated by the five different types of the above test PV modules, was utilized as PV reference device or PVMS for Irr measurement with fast measurement time (below 200 ms) for high accuracy of outdoor measurement of the test PV modules. The sc-Si PV module is composed of 156.25 cm2 sized wafers. All PV modules facing due south with a tilt angle of 15.3 are located at Ritsumeikan University, Kusatsu-city, Shiga-prefecture in Japan (north latitude 34 580 ,
Z
y
Zx
APExy ðeVÞ ¼ q
x
EðlÞdl
y
and
FðlÞdl
FðlÞ ¼
EðlÞ Ephoton ðlÞ
(1)
where q (C) is elementary charge, F (s1 m2 nm1) is the spectral photon flux density, and Ephoton (eV) is the energy of single photon [14e18]. Unit of E is W m2 nm1. The l denotes wavelength. Owing to the limitation of the measuring instrument, x and y in Equation (1) are set to be 350 and 1050 nm, respectively. The APE is utilized as an index to indicate solar spectral irradiance distribution, where its value of AM 1.5G reference spectrum estimated in the wavelength range of 350e1050 nm is 1.88 eV [15,19].
2.2. Methodology for calculations of short-circuit current and spectral mismatch correction factor After calculating APE, the ISC of the test PV modules and MMF as a function of APE at the installation site of the test PV modules are investigated to comprehend the impact of APE (E shape) on the ISC and MMF. The ISC is estimated by the following formula:
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Z ISC ¼ S
EðlÞSRðlÞdl
(2)
where S (m2) is the area of PV modules in Table 1, SR (A W1 nm1) is the spectral response of the test PV modules. It is noted that the ISC-PVMS-STC (indoor measurement) for PVMS (sc-Si PV module) is 5.37 A under STC, which is used for the correction of the ISC-meas. In this work, to use the PVMS for the outdoor evaluation of differenttype PV modules, MMF is also used to correct the ISC-meas of the test PV modules under outdoor measurement for high accuracy, which is close to the ISC under the indoor measurement under STC. MMF is estimated using the following formula [10,11]:
Z MMF ¼ Z
Eref ðlÞSRref ðlÞdl
Z
Emeas ðlÞSRsample ðlÞdl Z Emeas ðlÞSRref ðlÞdl Eref ðlÞSRsample ðlÞdl
(3)
569
each spectral band was calculated with different APE values. More detail for the calculation of the PTI was discussed in our previous work [15,17]. Fig. 1(a) consequently shows PTIs of the Es with APE values in a range of 1.85e1.96 eV with 0.01 eV intervals. It is noted that the standard deviation of the PTI of all spectral bands is less than 0.16% (absolute %) as depicted in Fig. 1(b). As depicted in Fig. 1(a), the PTI is well proportional to the value of the APE. The PTI in a short wavelength range of about 350e600 nm increases with enhancing the APE value, whereas the PTI in the long wavelength region of approximately 600e1050 nm decreases with increasing the APE value. The Irr of AM 1.5G in wavelength range of 350e1050 nm is 765 W/m2. The Es with various APE values under a constant Irr of 765 W/m2 as depicted in Fig. 2 are thus obtained by multiplying the PTI in Fig. 1(a) with 765 W/m2. According to Fig. 2, the relationship between the E shape and APE is obviously demonstrated at the installation site of the test PV modules. The E in short and long
where Eref and SRref are the spectral irradiance (AM 1.5G) and spectral response of PVMS (the sc-Si PV module). The SRref and SRsample are measured at 25 C. Emeas is the spectral irradiance incident on the outdoor test PV modules at the outdoor installation site. SRsample is spectral response of the outdoor test PV modules. 3. Results and discussion 3.1. Average photon energy as an index of spectral irradiance and an indicator of spectral mismatch correction factor First, the relationship between the E shape and APE was scrutinized at the installation site of the test PV module. The outdoor E (solar spectrum) at the installation site was consequently accumulated by the spectro-radiometer from June 2015 to May 2016 with data points greater than 32000. The E with low Irr (below 1.0 W m2) was excluded to avoid the scattered data of low irradiance spectra. The measured Es with the wavelength range of 350e1050 nm were indexed as APE ¼ 1.849e1.851 (1.850 ± 0.001), 1.859e1.861 (1.860 ± 0.001), …, 1.949e1.951 (1.950 ± 0.001) and 1.959e1.961 (1.960 ± 0.001) eV. The measured E for different APE values was then divided into 71 spectral bands with 10 nm intervals, and the mean of percentage to the total irradiance (PTI) of
Fig. 2. Spectral irradiances (E) with APE values from 1.85 to 1.96 eV with 0.01 eV intervals at the installation site of the test PV modules. The figure was constructed with approximately 32000 data points from June 2015 to May 2016.
Fig. 1. (a) Mean and (b) standard deviation of percentage to the total irradiance (PTI) of each spectral band (10 nm intervals) with the APE range of 1.85e1.96 eV with 0.01 eV intervals. The figures were constructed with approximately 32000 data points from June 2015 to May 2016.
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wavelength areas increases and decreases, respectively, when the APE value is enhanced from 1.85 to 1.96 eV. In other words, the E changes from red-rich to blue-rich spectra with enhancing APE. The results suggested that the APE value uniquely yields the E and can be used as an index to indicate solar spectral irradiance distribution. To calculate the ISC of the PV modules using Equation (2), their SRs needed to be known. Fig. 3 subsequently demonstrates relative SRs of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe) in this work, which were measured at 25 C. The SRs of the outdoor test PV modules (HIT, BC, mc-Si, CIS, and CdTe) are denoted as SRsample, while the SR of sc-Si PV module as PVMS is expressed as SRref. According to the figure, bandgap-energies (Eg) of HIT, BC, scSi, mc-Si, CIS, and CdTe modules are about 1.09, 1.10, 1.12, 1.13, 1.21, and 1.47 eV in Table 1, respectively. The ISC of all PV modules was estimated using Equation (2) with the Es with different APE values in a range of 1.85e1.96 eV in Fig. 2 to investigate the effect of APE on the ISC, where the E can be well indexed by APE. The ISC was then
normalized by ISC@STC. The ISC@STC was the ISC of all PV modules calculated using Equation (2) under STC. Fig. 4 consequently illustrates ISC/ISC@STC of all PV modules as a function of APE. In Fig. 4, the ISC/ISC@STC of CdTe PV module is slightly influent by the variation of APE. This is because the Es, indicated by APE in Fig. 2, change to blue-rich spectra with increasing APE, which is suitable for the large Eg (1.47 eV) of CdTe for high generated current; however, the number of photon is decreased with high APE. As a result, the ISC/ISC@STC of CdTe PV module is not affected very much by APE. On the other hand, the ISC/ISC@STC of HIT, BC, sc-Si, mc-Si, and CIS PV modules is reduced with enhancing APE from 1.85 to 1.96 because of the decrease in the number of photon. It is revealed that the APE, indexing the E, differently affects the ISC of the PV modules depending on their SR, especially in the case between PVMS (sc-Si PV module) and CdTe PV module owing to their very different SRs in Fig. 3. In addition, since the Es are previously obtained by multiplying PTI with the Irr, Equation (3) for MMF calculation is simplified by replacing Emeas by PTI as shown in the following equation:
Z
Z Eref ðlÞSRref ðlÞdl PTIðlÞSRsample ðlÞdl Z MMF ¼ Z PTIðlÞSRref ðlÞdl Eref ðlÞSRsample ðlÞdl
(4)
As previously demonstrated, the PTI and Es at the installation site of the outdoor test PV modules in Figs. 1 and 2 can be well indexed by APE. The correlation between MMF and APE is therefore investigated using Equation (4). It is noted that due to the limitation of the measurement instrument, the l range for the MMF calculation in Equation (4) is from 350 to 1050 nm. Fig. 5 consequently presents MMF of the outdoor test PV modules (HIT, BC, mc-Si, CIS and CdTe) as a function of APE, where sc-Si PV module is utilized as PVMS. In Fig. 5, MMF is most close to 1 with varying APE from 1.85 to 1.96 eV in case of BC, HIT, mc-Si PV modules since the SRs among sc-Si (PVMS), BC, HIT, and mc-Si modules are almost similar in Fig. 3. On the other hand, MMF is most deviated from 1 in case of CdTe PV module because SRs between sc-Si (PVMS) and CdTe PV modules are most different as seen in Fig. 3. The highest MMF is approximately 1.06 at APE of 1.96 eV for CdTe PV module, thus yielding the most different ISC/ISC@STC between CdTe PV module and Fig. 3. Relative spectral responses of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe).
Fig. 4. ISC/ISC@SCT of the PV modules (HIT, BC, sc-Si (PVMS), mc-Si, CIS, and CdTe) as a function of APE.
Fig. 5. Spectral mismatch correction factor (MMF) as a function of APE for the PV modules (HIT, BC, mc-Si, CIS, and CdTe) at the installation site, where sc-Si module was used as PVMS. The range of wavelength (l) for the MMF calculation in Equation (4) is from 350 to 1050 nm.
J. Chantana et al. / Renewable Energy 114 (2017) 567e573
PVMS (sc-Si PV module) at APE of 1.96 eV as seen in Fig. 4. It is consequently suggested that the ISC-meas of the various-type test PV modules needs to be corrected by not only the ISC-PVMS-STC of PVMS but also MMF for the corrected ISC (ISC-corrected) with high accuracy, especially in the case the CdTe PV module with its SR very different from that of the PVMS. In Fig. 5, it is additionally proposed that APE can be used as an indicator of MMF of the outdoor test PV modules for the first time for easier measurement.
For the utilization of PVMS (sc-Si PV module) to evaluate the various-type test PV modules at their installation site, the ISC-meas of the test PV modules under outdoor measurement is corrected by MMF for the corrected ISC (ISC-corrected) with high accuracy, close to their ISC under the indoor measurement under STC, where the MMF is predicted by APE as shown in Fig. 5. For comparison, three ISC correction methods are estimated. First, ISC-corrected (1) of the outdoor test PV modules, called (1) Tmod þ pyranometer, was calculated by correcting the ISC-meas using Tmod and Irr, obtained by the pyranometer, as shown in the following formula:
1
1 þ a Tmodsample 25
1 Irrpyranometer IrrSTC
(5)
where Irrpyranometer (kW m2) is the Irr of the spectral irradiance incident on the outdoor test PV modules measured by pyranometer. Room temperature is 25 C. IrrSTC is the Irr under STC condition (1 kW m2). ISC-meas (A) is the measured ISC of the outdoor test PV modules at the outdoor location. The a is temperature coefficient of Isc of test PV modules. The a is 0.020, 0.039, 0.034, 0.035, 0.006, and 0.045%/ C for sc-Si (PVMS), mc-Si, HIT, BC, CIS, and CdTe in Table 1, respectively. Tmod-sample is the temperature of the outdoor test PV module and added for the effect of module temperature. Second, ISC-corrected (2) of the outdoor test PV modules, named (2) Tmod þ PVMS, was estimated by correcting the ISC-meas using Tmod ( C) and IrrPVMS. The IrrPVMS is calculated from ISC-PVMS-meas and ISC-PVMS-STC of PVMS (sc-Si PV module). ISC-corrected (2) is estimated using the following formula:
ISCcorrected ð2Þ ¼ ISCmeas
1 1 IrrPVMS 1 þ a Tmodsample 25 (6)
where IrrPVMS is expressed as follow.
IrrPVMS ¼ ISCPVMSmeas
1 ISCPVMSSTC
module) at the outdoor location, and ISC-PVMS-STC of 5.37 A for PVMS (indoor measurement) was obtained under STC by the indoor precision measurement. Third, ISC-corrected (3) of the outdoor test PV modules, called (3) Tmod þ PVMS þ MMF, was evaluated by correcting the ISC-meas using Tmod ( C), IrrPVMS, obtained by PVMS, and MMF for high accuracy, as shown in the following formula:
1 1 ISCcorrected ð3Þ ¼ ISCmeas Irr PVMS 1 þ a Tmodsample 25
3.2. Spectral mismatch correction factor for precise outdoor performance measurements
ISCcorrected ð1Þ ¼ ISCmeas
571
1 ð1 þ aðTmodPVMS 25ÞÞ (7)
IrrPVMS for the Irr correction is derived from PVMS (sc-Si PV module) with much faster measurement time (below 200 ms) than Irrpyranometer (about 8 s) for high accuracy since the spectral irradiance always changes. It is noted that the measurement time is the time for one measurement to obtain the stabilizing data, and the IrrPVMS from PVMS and Irrpyranometer from pyranometer were measured every 1 min. Tmod-PVMS is the temperature of PVMS (sc-Si PV module). ISC-PVMS-meas is the measured ISC of PVMS (sc-Si
1 MMF (8)
In this work, the MMF of the test PV modules is readily obtained when APE, calculated from the E in a range of 350e1050, at the installation site is known as shown in Fig. 5. It is reported that APE, estimated from the E in two limited bands of 450e500 nm and 800e850 nm, can precisely describe the APE, calculated from the E in a range of 350e1050 nm [14]. Therefore, the MMF in Equation (8) could be predicted by APE with two limited bands of 450e500 nm and 800e850 nm in the future, making it much easier and faster. After calculating the ISC-corrected using three different methods, the ISC-corrected of the outdoor test PV modules at the installation site (outdoor measurements) is then compared with their ISC-STC (indoor measurements) by using error (%). The ISC-STCs are ISCs of the outdoor test PV modules (mc-Si, HIT, BC, CIS, and CdTe), which were measured under STC by indoor precision measurements at AIST. ISCSTCs of the outdoor test PV modules are 2.005, 5.980, 6.380, 2.066, and 1.534 A for mc-Si, HIT, BC, CIS, and CdTe modules in Table 1, respectively. Error (%) for the comparison between outdoor and indoor measurements is subsequently demonstrated in the following formula:
Errorð%Þ ¼
ISCcorrected ISCSTC 100 ISCSTC
(9)
In this work, for the estimations of the ISC-corrected and error (%), ISC-meas of the outdoor test PV modules, ISC-PVMS-meas of PVMS, Tmod, Irr by pyranometer, and E by spectro-radiometer for APE to predict MMF at the installation site were accumulated every 1 min from 4 a.m. to 8 p.m. from December 2015 to January 2017 with data points greater than 56,000. Consequently, Fig. 6 illustrates histograms of errors (%) of the outdoor test PV modules ((a) mc-Si and (b) CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC-STC. It is noted that the SR of mc-Si is most identical to that of PVMS, while SR of CdTe is most different from that of PVMS as depicted in Fig. 3. The histogram is a graphical representation of the data distribution. It is disclosed in Fig. 6 that the error distributions of both mc-Si and CdTe PV modules obtained by (2) Tmod þ PVMS and (3) Tmod þ PVMS þ MMF become narrower than those by (1) Tmod þ pyranometer. For mc-Si PV module, the error distribution and median, which were estimated by (2) Tmod þ PVMS and (3) Tmod þ PVMS þ MMF, are not changed very much. However, for CdTe PV module, the error distribution becomes further narrower and the median is closer to 0 (0.87%), when the error calculated by (3) Tmod þ PVMS þ MMF. The result implies the most precise evaluation of the ISC-corrected of the CdTe PV module corrected by (3) Tmod þ PVMS þ MMF. In addition, Fig. 7 depicts errors (%) of the outdoor test PV modules (a whole year data) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparisons to the ISC-STC, where median and interquartile range of error (%) are expressed as median and IQR. IQR is an indicator of
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Fig. 6. Histograms of error (%) of the outdoor test PV modules ((a) mc-Si and (b) CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC under STC condition (ISC-STC), measured in indoor by AIST. The figures were constructed with approximately 56000 data points from December 2015 to January 2017.
statistical dispersion, equal to the difference between 75th and 25th percentiles. It is clearly revealed that the median and IQR of error (%) for all outdoor test PV modules (mc-Si, HIT, BC, CIS, and CdTe modules) by (2) Tmod þ PVMS are much lower than those by (1) Tmod þ pyranometer. This implied that the ISC-corrected (2) of the outdoor test PV modules using Equation (6) is more precise than the ISC-corrected (1) using Equation (5). This is because the ISC-corrected (2) was derived from Tmod ( C) and the IrrPVMS, calculated from ISCPVMS-meas and ISC-PVMS-STC of PVMS with short measurement time (below 200 ms) and the similar angle-of-incidence dependence of
Fig. 7. Error (%) of the outdoor test PV modules (HIT, BC, mc-Si, CIS, and CdTe) for the ISC-corrected obtained by (1) Tmod þ pyranometer, (2) Tmod þ PVMS, and (3) Tmod þ PVMS þ MMF in comparison to the ISC under STC condition (ISC-STC), measured in indoor by AIST. Median and interquartile range of error (%) are expressed as median and IQR. The figures were constructed with approximately 56000 data points from December 2015 to January 2017.
PVMS and the outdoor test PV modules. On the other hand, the ISC(1) was obtained from Tmod ( C) and the Irrpyranometer, taken by the pyranometer with longer measurement time (about 8 s) and the different angle-of-incidence dependence of the pyranometer from the outdoor test PV modules. It is noted that the fast measurement time is needed for Irr since the outdoor spectral irradiance always changes. Additionally, the median and IQR of error (%) for mc-Si, HIT, CIS, and BC test PV modules by (2) Tmod þ PVMS are not different very much from those by (3) Tmod þ PVMS þ MMF. This is because the MMF predicted by APE in Fig. 5 is estimated in the wavelength range from 350 to 1050 nm, limited by the instrument ability, which do not cover all wavelengths of SRs of mc-Si, HIT, CIS, and BC test PV modules in Fig. 3. On the other hand, the wavelength range from 350 to 1050 nm for the MMF indicated by APE in Fig. 5 most covers the wavelength of SR of CdTe PV module, when comparing with the mc-Si, HIT, CIS, and BC test PV modules in Fig. 3. As a result, IQR and median of errors (%) by (3) Tmod þ PVMS þ MMF are obviously reduced in the case of CdTe PV module when compared with those of the error by (2) Tmod þ PVMS, where the MMF is most deviated from 1 in the case of CdTe PV module in Fig. 5 owing to the SR of CdTe PV module very different from that of PVMS (sc-Si PV module) seen in Fig. 3. Ultimately, the median (0.87%) and IQR (1.36%) of error for CdTe module are achieved using (3) Tmod þ PVMS þ MMF. The results suggest that although the SR of the CdTe PV module is very different from that of PVMS (sc-Si), the ISC-corrected with high accuracy for the CdTe PV module was attained by correcting its ISC-meas (outdoor) with not only the ratio of ISCPVMS-STC to ISC-PVMS-meas for the irradiance correction (fast measurement time below 200 ms) but also MMF for spectral mismatch correction. It is noted that the MMF is readily indicated by APE for faster and cheaper measurement. Based on the results, for more precise evaluation of outdoor ISC of the various-type PV test modules using MMF indicated by APE, the wavelength range for investigation of the correlation between MMF and APE should cover all wavelength of SR of the outdoor test corrected
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PV module by using new system with novel spectro-radiometer. 4. Conclusions
[4] [5]
PVMS (sc-Si PV module) for IrrPVMS is utilized to precisely evaluate the outdoor ISC of the various-type test PV modules. To accomplish the goal, the relation between the E shape and APE is first studied. It is disclosed that the APE value uniquely yields the E and can be used as an index of solar spectral irradiance distribution. The ISC of the test PV module and MMF as a function of APE are then examined. It is revealed that the APE is utilized as an indicator of MMF of the outdoor test PV modules for the first time for easier measurement. It is moreover shown that the APE has the different influence on the ISC and MMF of the test PV modules depending on their SR, especially in the case between PVMS (sc-Si PV module) and CdTe PV module owing to their very different SRs. Therefore, the ISC-meas (outdoor) of the various-type test PV modules needs to be corrected by not only the ratio of ISC-PVMS-STC to ISC-PVMS-meas but also MMF for the ISC-corrected with high accuracy. Ultimately, the median (0.87%) and IQR (1.36%) of error for CdTe module are achieved using (3) Tmod þ PVMS þ MMF although the SR of CdTe PV modules very different from that of PVMS (sc-Si PV module). Acknowledgement This work is partly supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] Y. Hamakawa, Background and motivation for thin-film solar-cell development, in: Y. Hamakawa (Ed.), Thin-film Solar Cells Next Generation Photovoltaics and its Applications, Springer, Heidelberg, 2004, pp. 1e14. [2] E. Vallat-Sauvain, A. Shah, J. Bailat, Epitaxial thin film crystalline silicon solar cells on low cost silicon carriers, in: J. Poortmans, V. Arkhipov (Eds.), Thin Film Solar Cells, Fabrication, Characterization and Applications, Wiley, Chichester, 2006, pp. 1e32. [3] European Commission Joint Research Center, Guidelines for PV Power
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