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Outdoor performance evaluation of photovoltaic modules using contour plots
Current Applied Physics 10 (2010) S257–S260
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Outdoor performance evaluation of photovoltaic modules using contour plots Takashi Minemoto *, Hiroaki Takahashi, Yasuhito Nakada, Hideyuki Takakura College of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu 525-8577, Japan
a r t i c l e
i n f o
Article history: Received 23 December 2008 Accepted 3 June 2009 Available online 11 November 2009 Keywords: Photovoltaic module Outdoor performance Silicon Solar spectrum
a b s t r a c t The impact of environmental parameters on different types of Si-based photovoltaic (PV) modules (single crystalline Si (sc-Si), amorphous Si (a-Si) and a-Si/ microcrystalline Si (lc-Si)) which have different spectral responses were characterized using contour plots. The contour plots of PV performance as a function of module temperature and spectral irradiance distribution were created to separate the impact of the two environmental parameters. The performance of the sc-Si PV module was dominated by the module temperature while those of a-Si and a-Si/lc-Si ones were mainly influenced by the spectral irradiance distribution. Furthermore, the frequency of outdoor conditions and the reliability of the contour plots of the PV performance were discussed for the evaluation of PV modules by means of energy production. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, new types of photovoltaic (PV) modules which are based on thin-film semiconductors are emerging in the PV market. These PV modules such as amorphous Si (a-Si)/microcrystalline Si (lc-Si) tandem and CuInSe2 related compound show different spectral responses compared to that of conventional crystalline Si PV modules. Thus, the influence of spectral irradiance distribution on the outdoor performance of the PV modules is not fully understood yet. Also, module temperature varies from hour to hour as well as the spectral distribution. The effects of these two environmental parameters have to be separated to understand the effect of each parameter on the PV output. In this study, the impacts of module temperature and spectral irradiance distribution on the outdoor performance of Si-based PV modules are analyzed by contour plots. Also, the reliability of the contour plots is discussed statistically.
2. Experiments The single crystalline Si (sc-Si), a-Si and a-Si/lc-Si PV modules with capacities of 5, 2 and 1 kW, respectively, facing due south with a tilt angle of 15.3° are installed at Kusatsu-city, Shiga-prefecture in Japan (34°580 N, 135°570 E). Solar spectra of the wavelength range of 350–1050 nm were recorded by a spectroradiometer (MS-700, EKO, Japan). Also, the temperature of the back side of the PV modules (Tmod) and tilt irradiance (Irr) were * Corresponding author. Fax: +81 77 561 3065. E-mail address: [email protected] (T. Minemoto). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.025
measured by thermocouple and pyranometer (MS-62, EKO, Japan), respectively. The measurement equipments experience the same exposure condition as the PV modules and obtain the data every 1 min. As an index of spectral irradiance distribution, average photon energy (APE) is used. APE is calculated by dividing the integrated irradiance with the integrated photon flux density, yielding the average energy per photon [1]. The APE value for the AM 1.5 reference solar spectrum [2] calculated with the 350–1050 nm wavelength range is 1.88 eV. As the index of the outdoor performance of PV modules, performance ratio (PR) is used. PR indicates PV module efficiency without the effect of the irradiance intensity, which is defined as the actual output energy divided by the nominal output energy calculated from the PV module performance under the standard test condition (STC). The contour plots of PR as a function of Tmod and APE were created to separate the influence of the two environmental parameters. In the plots, the ranges for Tmod and APE are 0–90 °C (5 °C step) and 1.85–2.03 eV (0.01 eV step), respectively. Instantaneous PR values measured every 1 min were collected for each environmental conditions and then the contour plots of the average PR value and standard deviation (r) were created. Here, the data points with the irradiance higher than 0.2 kW/m2 were used to omit the lower performance of the PV modules and the uncertainness of the operation timing of the inverter for maximum power point tracking. Using the same method, the contour plots of data points for Irr were created to show the distribution and frequency of the environmental conditions. In this study, the data period of single year from January 2005 through December 2005 is used. The total number of data points is greater than 115,000.
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shown in Fig. 2a, the PR of the sc-Si PV module decreases with increasing Tmod while there is no apparent APE dependence. The behavior is reasonable because the spectral response of sc-Si PV modules widely covers the solar spectrum with the wavelength range of 350–1050 nm. On the other hand, the r value for the scSi PV modules (Fig. 2d) shows the smallest values of approximately 5% (abs.) at around Tmod = 50 °C and APE = 1.92 eV and that shows high values towards the edge parts of the plots. The distribution of r is quite similar to that of the data points for Irr shown in Fig 1. Thus, the high r values would be attributed to the small number of data points. The contour plot will be reliable to use for the estimation of the output energy of sc-Si PV modules installed at locations where the distribution of the outdoor environmental condition is similar to this measurement site. However, if the condition is drastically different, the contour plot will become unreliable. To make the contour plots universally reliable, field tests at various measurement sites with different outdoor conditions have to be carried out. As shown in Fig. 2b, the a-Si PV module shows both Tmod and APE dependences. The PR of the a-Si PV modules increases with increasing APE and that increases with increasing Tmod at APE > 1.95 eV. However, if the contour plot is restricted by the high irradiance area, the PR is almost dominated by APE. This is because a-Si PV modules have a spectral response in short wavelength region of 300–750 nm, and if the Irr is fixed, blue rich spectra include higher photon flux density in the short wavelength region compared to redder spectra. The high PR value in high Tmod at APE > 1.95 eV seems graphically reasonable because of its gradual change. However, considered from the operation mechanism of solar cells, the performance of PV modules generally decreases with increasing Tmod. In a-Si solar cells, there are two complex mecha-
3. Results and discussion 3.1. Irradiance distribution Fig. 1 shows the contour plots of data points of Irr with different Irr range of (a) all, (b) 0–0.2, (c) 0.2–0.4, (d) 0.4–0.6, (e) 0.6–0.8 and (f) >0.8 kW/m2 as a function of Tmod and APE for the a-Si PV module. As shown in Fig. 1a, the data points for Irr has two peaks around Tmod = 45 °C and APE = 1.93 eV (Peak 1), and Tmod = 30 °C and APE = 1.96 eV (Peak 2). Fig. 1b–f indicate that the data points distribution shifts from low Tmod and high APE region to high Tmod and low APE region with increasing Irr. The Peak 1 and Peak 2 consist of the data points of Irr with higher and lower than 0.4 kW/m2, respectively. The similar data points for different Irr levels means that the contribution to PV output of Peak 1 is greater than that of Peak 2. Also, the environmental condition corresponding to STC, i.e., Tmod = 25 °C and APE = 1.88 eV is not frequent in this measurement site. From the view points of actual energy production by PV modules, the PR of PV modules should be designed to show high values at around Peak 1. The outdoor performance of PV modules is dominated by Irr, Tmod and APE. Thus, the contour plots for these environmental parameters to reveal the frequent condition at outdoor as shown in this paper is important for designing or choosing PV modules to produce high energy. 3.2. Tmod and APE impacts on PV output Fig. 2 shows the contour plots of the average of PR for (a) sc-Si, (b) a-Si and (c) a-Si/lc-Si PV modules and r of PR for (d) sc-Si, (e) a-Si and (f) a-Si/lc-Si PV modules as a function of Tmod and APE. As
Data points for Irradiance
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o
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Tmod ( C)
Data points for Irradiance 0 500 1000 1500 2000 2500 3000 3500
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0 1.85
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APE (eV)
Fig. 1. Contour plots of data points for Irr with different Irr range for a-Si PV module.
1.91
APE (eV)
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Not determined 90
(a) sc-Si, PR
(b) a-Si, PR
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o
o
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(e) a-Si, σ
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(f) a-Si/μc-Si, σ
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σ (abs. %) 90
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APE (eV)
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PR (%)
1.94
1.97
2.00
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1.91
APE (eV)
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2.03
0 1.85
10 12.5
1.88
APE (eV)
1.91
1.94
1.97
2.00
2.03
APE (eV)
Fig. 2. Contour plots of average and r of PR for sc-Si, a-Si and a-Si/lc-Si PV modules.
nisms of the annealing effect and light induced degradation (LID) to decrease and increase the dangling bonds density in the a-Si layer, respectively. The reasons of the behavior of PR for the a-Si PV module will be that the annealing effect of a-Si is greater than LID in this year. It should be noted that the high r value of >10% indicates the uncertainness of the PR values in this region (Fig. 2e). Also, there is temperature history in the performance of a-Si PV modules, the PR values in the region with small number of data points can be changed, meaning that the contour plot of PR may be changed year by year. Additional attention should be paid in analyzing the behavior of a-Si PV modules. As shown in Fig. 2c, the PR of the a-Si/lc-Si PV module is influenced by both Tmod and APE, which is the similar behavior to the aSi PV module. If the contour plot is restricted by the high irradiance area, the PR is almost dominated by APE; however the impact of APE on PR is smaller than that of the a-Si PV module. The a-Si/ lc-Si PV module is the stack of the a-Si top cell and lc-Si bottom cell which have different spectral responses, i.e., shorter and longer wavelength light for a-Si and lc-Si, respectively. The top and bottom cells are series connected so that the total current is limited by the smaller current from either of the cells. Thus, the current matching which directly related to APE should affect the PV output. However, considering a current–voltage characteristic of two different diodes series connected, current mismatching increase a fill factor of solar cells [3]. This should be the reason why the APE dependence of the a-Si/lc-Si PV module is smaller than that of the a-Si PV module. There are two regions with high PR in APE > 1.95 eV. However, the r values are also high in these regions and the complex behavior of a-Si should affect the performance of
the a-Si/lc-Si PV module. Thus, the reliability of these regions is not high enough especially in PV modules including the a-Si layer. The outdoor performance of PV modules should be rated by energy production which can be estimated by considering the distribution of outdoor environmental conditions, the corresponding PR plots of PV modules, and also the corresponding r plots to assure data reliability.
4. Conclusions The impacts of environmental parameters (Tmod and APE) on the sc-Si, a-Si and a-Si/lc-Si PV modules were analyzed. The PR of the sc-Si PV module was mainly dominated by Tmod and not by APE. In contrast, the PRs of a-Si and a-Si/lc-Si PV modules were mainly dominated by APE. The frequent conditions at outdoor is significantly different to the condition corresponding to STC. The outdoor performance of PV modules should be characterized by considering the distribution of outdoor environmental conditions, the corresponding PR plots of PV modules, and also the corresponding r plots to assure data reliability.
Acknowledgments The authors would like to thank A. Nakajima of Kaneka Corporation for useful discussion. This work was partly supported by the Incorporated Administrative Agency and New Energy and Industrial Technology Development Organization (NEDO).
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References [1] S. Williams, T. Betts, T. Helf, R. Gottschalg, H. Beyer, D. Infield, in: Conference Records of 3rd World Conference on Photovoltaic Energy Conversion, 2003, p. 1908.
[2] International Electrotechnical Commission, IEC 60904-3, 1989. [3] A. Nakajima, M. Ichikawa, T. Sawada, M. Yoshimi, K. Yamamoto, Jpn. J. Appl. Phys. 43 (2004) L1162.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Outdoor performance evaluation of photovoltaic modules using contour plots Takashi Minemoto *, Hiroaki Takahashi, Yasuhito Nakada, Hideyuki Takakura College of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu 525-8577, Japan
a r t i c l e
i n f o
Article history: Received 23 December 2008 Accepted 3 June 2009 Available online 11 November 2009 Keywords: Photovoltaic module Outdoor performance Silicon Solar spectrum
a b s t r a c t The impact of environmental parameters on different types of Si-based photovoltaic (PV) modules (single crystalline Si (sc-Si), amorphous Si (a-Si) and a-Si/ microcrystalline Si (lc-Si)) which have different spectral responses were characterized using contour plots. The contour plots of PV performance as a function of module temperature and spectral irradiance distribution were created to separate the impact of the two environmental parameters. The performance of the sc-Si PV module was dominated by the module temperature while those of a-Si and a-Si/lc-Si ones were mainly influenced by the spectral irradiance distribution. Furthermore, the frequency of outdoor conditions and the reliability of the contour plots of the PV performance were discussed for the evaluation of PV modules by means of energy production. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, new types of photovoltaic (PV) modules which are based on thin-film semiconductors are emerging in the PV market. These PV modules such as amorphous Si (a-Si)/microcrystalline Si (lc-Si) tandem and CuInSe2 related compound show different spectral responses compared to that of conventional crystalline Si PV modules. Thus, the influence of spectral irradiance distribution on the outdoor performance of the PV modules is not fully understood yet. Also, module temperature varies from hour to hour as well as the spectral distribution. The effects of these two environmental parameters have to be separated to understand the effect of each parameter on the PV output. In this study, the impacts of module temperature and spectral irradiance distribution on the outdoor performance of Si-based PV modules are analyzed by contour plots. Also, the reliability of the contour plots is discussed statistically.
2. Experiments The single crystalline Si (sc-Si), a-Si and a-Si/lc-Si PV modules with capacities of 5, 2 and 1 kW, respectively, facing due south with a tilt angle of 15.3° are installed at Kusatsu-city, Shiga-prefecture in Japan (34°580 N, 135°570 E). Solar spectra of the wavelength range of 350–1050 nm were recorded by a spectroradiometer (MS-700, EKO, Japan). Also, the temperature of the back side of the PV modules (Tmod) and tilt irradiance (Irr) were * Corresponding author. Fax: +81 77 561 3065. E-mail address: [email protected] (T. Minemoto). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.025
measured by thermocouple and pyranometer (MS-62, EKO, Japan), respectively. The measurement equipments experience the same exposure condition as the PV modules and obtain the data every 1 min. As an index of spectral irradiance distribution, average photon energy (APE) is used. APE is calculated by dividing the integrated irradiance with the integrated photon flux density, yielding the average energy per photon [1]. The APE value for the AM 1.5 reference solar spectrum [2] calculated with the 350–1050 nm wavelength range is 1.88 eV. As the index of the outdoor performance of PV modules, performance ratio (PR) is used. PR indicates PV module efficiency without the effect of the irradiance intensity, which is defined as the actual output energy divided by the nominal output energy calculated from the PV module performance under the standard test condition (STC). The contour plots of PR as a function of Tmod and APE were created to separate the influence of the two environmental parameters. In the plots, the ranges for Tmod and APE are 0–90 °C (5 °C step) and 1.85–2.03 eV (0.01 eV step), respectively. Instantaneous PR values measured every 1 min were collected for each environmental conditions and then the contour plots of the average PR value and standard deviation (r) were created. Here, the data points with the irradiance higher than 0.2 kW/m2 were used to omit the lower performance of the PV modules and the uncertainness of the operation timing of the inverter for maximum power point tracking. Using the same method, the contour plots of data points for Irr were created to show the distribution and frequency of the environmental conditions. In this study, the data period of single year from January 2005 through December 2005 is used. The total number of data points is greater than 115,000.
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T. Minemoto et al. / Current Applied Physics 10 (2010) S257–S260
shown in Fig. 2a, the PR of the sc-Si PV module decreases with increasing Tmod while there is no apparent APE dependence. The behavior is reasonable because the spectral response of sc-Si PV modules widely covers the solar spectrum with the wavelength range of 350–1050 nm. On the other hand, the r value for the scSi PV modules (Fig. 2d) shows the smallest values of approximately 5% (abs.) at around Tmod = 50 °C and APE = 1.92 eV and that shows high values towards the edge parts of the plots. The distribution of r is quite similar to that of the data points for Irr shown in Fig 1. Thus, the high r values would be attributed to the small number of data points. The contour plot will be reliable to use for the estimation of the output energy of sc-Si PV modules installed at locations where the distribution of the outdoor environmental condition is similar to this measurement site. However, if the condition is drastically different, the contour plot will become unreliable. To make the contour plots universally reliable, field tests at various measurement sites with different outdoor conditions have to be carried out. As shown in Fig. 2b, the a-Si PV module shows both Tmod and APE dependences. The PR of the a-Si PV modules increases with increasing APE and that increases with increasing Tmod at APE > 1.95 eV. However, if the contour plot is restricted by the high irradiance area, the PR is almost dominated by APE. This is because a-Si PV modules have a spectral response in short wavelength region of 300–750 nm, and if the Irr is fixed, blue rich spectra include higher photon flux density in the short wavelength region compared to redder spectra. The high PR value in high Tmod at APE > 1.95 eV seems graphically reasonable because of its gradual change. However, considered from the operation mechanism of solar cells, the performance of PV modules generally decreases with increasing Tmod. In a-Si solar cells, there are two complex mecha-
3. Results and discussion 3.1. Irradiance distribution Fig. 1 shows the contour plots of data points of Irr with different Irr range of (a) all, (b) 0–0.2, (c) 0.2–0.4, (d) 0.4–0.6, (e) 0.6–0.8 and (f) >0.8 kW/m2 as a function of Tmod and APE for the a-Si PV module. As shown in Fig. 1a, the data points for Irr has two peaks around Tmod = 45 °C and APE = 1.93 eV (Peak 1), and Tmod = 30 °C and APE = 1.96 eV (Peak 2). Fig. 1b–f indicate that the data points distribution shifts from low Tmod and high APE region to high Tmod and low APE region with increasing Irr. The Peak 1 and Peak 2 consist of the data points of Irr with higher and lower than 0.4 kW/m2, respectively. The similar data points for different Irr levels means that the contribution to PV output of Peak 1 is greater than that of Peak 2. Also, the environmental condition corresponding to STC, i.e., Tmod = 25 °C and APE = 1.88 eV is not frequent in this measurement site. From the view points of actual energy production by PV modules, the PR of PV modules should be designed to show high values at around Peak 1. The outdoor performance of PV modules is dominated by Irr, Tmod and APE. Thus, the contour plots for these environmental parameters to reveal the frequent condition at outdoor as shown in this paper is important for designing or choosing PV modules to produce high energy. 3.2. Tmod and APE impacts on PV output Fig. 2 shows the contour plots of the average of PR for (a) sc-Si, (b) a-Si and (c) a-Si/lc-Si PV modules and r of PR for (d) sc-Si, (e) a-Si and (f) a-Si/lc-Si PV modules as a function of Tmod and APE. As
Data points for Irradiance
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Data points for Irradiance
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Tmod ( C)
Data points for Irradiance 0 500 1000 1500 2000 2500 3000 3500
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0 1.85
1.88
APE (eV)
Fig. 1. Contour plots of data points for Irr with different Irr range for a-Si PV module.
1.91
APE (eV)
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Not determined 90
(a) sc-Si, PR
(b) a-Si, PR
75
o
o
Tmod ( C)
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40
(c) a-Si/μc-Si, PR
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(e) a-Si, σ
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σ (abs. %) 90
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PR (%)
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APE (eV)
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1.97
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2.03
0 1.85
10 12.5
1.88
APE (eV)
1.91
1.94
1.97
2.00
2.03
APE (eV)
Fig. 2. Contour plots of average and r of PR for sc-Si, a-Si and a-Si/lc-Si PV modules.
nisms of the annealing effect and light induced degradation (LID) to decrease and increase the dangling bonds density in the a-Si layer, respectively. The reasons of the behavior of PR for the a-Si PV module will be that the annealing effect of a-Si is greater than LID in this year. It should be noted that the high r value of >10% indicates the uncertainness of the PR values in this region (Fig. 2e). Also, there is temperature history in the performance of a-Si PV modules, the PR values in the region with small number of data points can be changed, meaning that the contour plot of PR may be changed year by year. Additional attention should be paid in analyzing the behavior of a-Si PV modules. As shown in Fig. 2c, the PR of the a-Si/lc-Si PV module is influenced by both Tmod and APE, which is the similar behavior to the aSi PV module. If the contour plot is restricted by the high irradiance area, the PR is almost dominated by APE; however the impact of APE on PR is smaller than that of the a-Si PV module. The a-Si/ lc-Si PV module is the stack of the a-Si top cell and lc-Si bottom cell which have different spectral responses, i.e., shorter and longer wavelength light for a-Si and lc-Si, respectively. The top and bottom cells are series connected so that the total current is limited by the smaller current from either of the cells. Thus, the current matching which directly related to APE should affect the PV output. However, considering a current–voltage characteristic of two different diodes series connected, current mismatching increase a fill factor of solar cells [3]. This should be the reason why the APE dependence of the a-Si/lc-Si PV module is smaller than that of the a-Si PV module. There are two regions with high PR in APE > 1.95 eV. However, the r values are also high in these regions and the complex behavior of a-Si should affect the performance of
the a-Si/lc-Si PV module. Thus, the reliability of these regions is not high enough especially in PV modules including the a-Si layer. The outdoor performance of PV modules should be rated by energy production which can be estimated by considering the distribution of outdoor environmental conditions, the corresponding PR plots of PV modules, and also the corresponding r plots to assure data reliability.
4. Conclusions The impacts of environmental parameters (Tmod and APE) on the sc-Si, a-Si and a-Si/lc-Si PV modules were analyzed. The PR of the sc-Si PV module was mainly dominated by Tmod and not by APE. In contrast, the PRs of a-Si and a-Si/lc-Si PV modules were mainly dominated by APE. The frequent conditions at outdoor is significantly different to the condition corresponding to STC. The outdoor performance of PV modules should be characterized by considering the distribution of outdoor environmental conditions, the corresponding PR plots of PV modules, and also the corresponding r plots to assure data reliability.
Acknowledgments The authors would like to thank A. Nakajima of Kaneka Corporation for useful discussion. This work was partly supported by the Incorporated Administrative Agency and New Energy and Industrial Technology Development Organization (NEDO).
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