ELSEVIER Solar Energy Materials and Solar Cells 48 (1997) 69-75

Solar Energy Materials and Solar Cells

Intercomparison of irradiance measurements based on WRR and ETL irradiance scales

Ryuichi Shimokawa a'*, Ichiro Saito a, Yukiharu Miyake b, Hirosi Ikeda b, Fumiaki Nagamine c, Sanekazu Igari c

a Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan b EKO Instruments Trading Co., Ltd., 1-1-21 Hatagaya, Shibuya-ku, Tokyo 151, Japan

c Japan Quality Assurance Organization (JQA), 2-17-22 Akasaka, Minato-ku, Tokyo 107, Japan

Abstract

We report the corrected intercomparison of the World Radiometer Reference (WRR) irra- diance scale and the Electrotechnical Laboratory (ETL) spectral irradiance scale. In addition, we confirm the intercomparison precision using the test facility where the irradiance of ETL 500 W standard lamp can be measured directly with the cavity radiometer. The results showed that the irradiance based on the WRR scale was 0.5-0.7% lower than the one based on the ETL scale.

Keywords: Solar cells; Irradiance measurement; Cavity radiometer

1. Introduction

The photovoltaic performance measurement is made by comparing the test sample with a calibrated reference cell which has essentially the same relative spectral response as the test sample, and the irradiance of solar simulator or natural sunlight is measured with the reference cell which has been calibrated against the standard reporting conditions for terrestrial measurements [1]. For the calibration of reference solar cells, "Solar Simulator Method" based on the ETL (Electrotechnical Laborat- ory) spectral irradiance scale is employed in Japan [2], while the WRR (World Radiometer Reference) scale is required internationally for the irradiance measure- ment. We previously performed the preliminary intercomparison of the WRR and

* Corresponding author.

0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 7 1- 8

70 R. Shimokaua el a/ Solar Em'~?,,.' A,la:erials a ,d b'ol,r CelLs 4,~ (1997) 69 75

ETL scales using our developed intermediary radiometer ACR-1 [3]. Consequently, we found that the irradiance based on the working radiometer PMO-6 (Compagnie Industrielle Radioelectrique, WRR scale) was about 1.0% lower than the integrated irradiance based on the working standard lamp {ETL scale) [3].

In this paper, we report the corrected intercomparison of the WRR and ETL scales. In addition, we confirm the intercomparison precision using the test facility where the irradiance of ETL 500 W standard lamp can be measured directly with the radiometer ACR-1.

2. Experimental

Fig. 1 shows the procedures for the intercomparison of irradiance measurement based on the WRR scale and integrated irradiance measurement based on the ETL scale (see National Institute of Standards and Technology report of the international intercomparison [4]). Previously, simultaneous measurements of direct solar radi- ation were performed with the radiometer ACR-I and the working radiometer PMO-6. Also, solar simulator radiation is simultaneously measured with the radio-

Solar ~ Direct Simulator Sunlight

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Fig. 1. lntercomparison of irradiance measurcnlents based on the WRR and ETL scales.

R. Shimokawa et al./Solar Energy. Materials and Solar Cells 48 (1997) 69- 75 71

Photometric ~ ~/ Standard 40 degree F.O.V Lamp (500W) Radiometer ACR-1

Fig. 2. Test facility for the irradiance measurement of the ETL 500 W standard lamp in combination with glass filters.

meter ACR-1 and spectroradiometer calibrated relative to the working standard lamp. The intermediary radiometer ACR-1 having a 40 ° full-angle field-of-view has the construction by which we can measure direct solar radiation with a 5 ° full-angle field-of-view when the collimation tube is mounted and has been confirmed to have the uncertainty of 0.495% for the direct sunlight and solar simulator irradiance measurements [3].

This time, we calibrated the ACR-1 and the working PMO-6 with the JMA (Japan Meteorology Agency) national standard radiometer PMO-6 which is one of the instruments used to maintain the traceability to the WRR scale in Japan. Simulta- neous measurements of direct solar radiation were performed with these three radiometers. Calibration procedures are described in the International Organization for Standardization (ISO) document "ISO-9059". Also, we calibrated the working standard lamp with the ETL national standard lamp. Spectral distribution of the working standard lamp was measured by comparing spectral irradiances alternately produced on a white diffusing surface by the working standard lamp and ETL standard lamp. Details of the calibration apparatus were described pre- viously [5].

Subsequently, to confirm the intercomparison precision, the irradiances of the ETL 500 W standard lamp used in combination with glass filters (L-37 and IR-85) were measured directly with the ACR-1 as shown in Fig. 2 and were compared with the integrated irradiances which were calculated from the products of ETL spectral irradiance scale and absolutely-measured filter-transmittances. The 500 W standard lamp was placed at a distance of 120 cm from the cavity aperture of the ACR-1. The light was limited to a small cone (about 8 ° full-angle field-of-view) around the ACR-1 by using diaphragms. The color temperature of the 500 W standard lamp is about 3172 k. The spectral irradiances at the wavelength beyond 2500 nm were determined by the extrapolation method using the 3172k black body radiation.

72 R. Shimokawa et al. :Solar Energy Materials and Solar Cells 48 (1997) 69 75

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Fig. 3. Typical results of simultaneous measurenlents of direct sunlight by the JMA PMO-6 (open diamonds) and the working PMO-6 (closed squares).

3. Results and discussion

Fig. 3 shows the typical results of simultaneous measurements of direct sunlight by the JMA PMO-6 (closed diamonds) and the working PMO-6 (closed squares). We repeated the measurement many times and confirmed that the irradiance by the working PMO-6 was in good agreement with the one by the JMA PMO-6 and the readings of ACR-I irradiance should be increased by 0.3-0.5% based on the WRR scale. While, Fig. 4 shows the measured spectral irradiances of the working (dashed line) and ETL (solid line) standard lamps, and the percent differences from ETL of the working standard lamp. Percent differences from NIST of national laboratory's spectral-irradiance measurements are reported in Ref. [4]. Many problems seem to be included in the spectral irradiance measurement. As a result of the irradiance integra- tion, we found that the readings of spectroradiometer irradiance should be decreased by 0.5% based on the ETL scale. According to the above experimental results, we corrected the intercomparison data of irradiances based on the WRR and ETL scales as shown in Fig. 5. Dashed lines show the previous intercomparison data of irradian- ces measured with the ACR-1 (closed circles) and integrated irradiances measured with the spectroradiometer calibrated relative to the working standard lamp (closed squares). The former is on an average 1.5% lower than the latter. The first two points measured with the ACR-1 seemed to be a little lower than the other points due to the initial warming-up stage. Solid lines shows the corrected intercomparison data based on the WRR and ETL scales. The readings of the ACR-1 irradiance are increased by 0.3~).5% based on the WRR scale and the readings of the spectroradiometer irra- diance are decreased by 0.5% based on the ETL scale.

R. Shimokawa et al./Solar Energy Materials and Solar Cells 48 (1997) 69- 75 73

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Fig. 5. Corrected intercomparison (solid lines) of the irradiance based on the WRR irradiance scale (open circles) and the integrated irradiance based on the ETL spectral irradiance scale (open squares). Dashed lines show the previous intercomparison data of the ACR-1 (closed circles) and the spectroradiometer based on the working standard lamp (closed squares).

Subsequently, Fig. 6 shows the spectral irradiances of E T L 500 W s tandard lamp and the absolutely-measured transmittances of L-37 (solid line) and IR-85 (dashed line) filters. Transmit ted light is noth ing beyond the wavelength of 4600 nm due to the glass absorption. Table 1 shows the irradiances of filtered E T L 500W standard lamp which were measured directly with the ACR-1 and the integrated irradiances which were calculated from the produc t of E T L spectral irradiance scale and absolute

74 R. Shimokawa et al. ,'Solar Energy Materials and Solar Cells 48 (1997) 69 75

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Fig. 6. Spectral irradiances of the ETL 500 W standard lamp and absolutely-measured transmittances of L-37 {solid line} and IR-85 (dashed line} filters.

Table 1 The irradiance measured directly with the ACR-I and the integrated irradiance based on ETL spectral irradiance scale of the filtered ETL 500 W

ACR-I irradiance ETL scale irradiance Difference (W/m 2) (W/m 2) (W/m 21

(t) without Filter 28.5 (2) L-37 Filter 22.4 22.76 0.36 (3) 1R-85 Filter 16.8 16.96 0.16

filter-transmittance. The former ACR-1 irradiance are 1.6% and 0.9% lower than the latter ETL irradiance for the L-37 and IR-85 filters, respectively. This direct inter- comparison suggests that the WRR scale is lower by 0.4-1.3% than the ETL scale because the ACR-I irradiance should be increased by 0.3 0.5% based on the WRR scale, and the intercomparison results (Fig. 3) are reasonable. But, further study is necessary to obtain the precision index. In particular, it is important to develop the optical filter which can sharply protect the transmission of the long- wavelength-light.

4. Conclusion

It is concluded from these intercomparison experiments that the irradiance based on the WRR scale is 0.5-0.7% lower than the one based on the ETL scale. But, further study is necessary to obtain the precision index.

R. Shimokawa et aL /Solar Energy Materials and Solar Cells 48 (1997) 69-75 75

References

[1] International Electrotechnical Commission document "IEC 904-3". [2] R. Shimokawa, F. Nagamine, Y. Miyake, K. Fujisawa, Y. Hamakawa, Japan. J. Appl. Phys. 26 (1987)

86. [3] F. Nagamine, R. Shimokawa, T. Abe, M. Suzuki. H. Ikeda, Y. Miyake, Proc. 1st IEEE WCPEC,

Hawaii, 1994, p.867. [4] J. Walker, R. Saunders, J. Jackson, K. Mielenz, J. Res. Natl. Inst. Stand. Technol. 96 (1991) 647. [5] M. Suzuki, N. Ooba, Metrologia 12 (1976) 123.