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Analysis of generated power of ETS-VII during solar activity maximum period
Solar Energy Materials & Solar Cells 75 (2003) 319–325
Analysis of generated power of ETS-VII during solar activity maximum period Reiko Fujitaa,*, Mitsuru Imaizumib, Kazuhiro Aoyamab, Sumio Matsudab, Shinji Tokunagaa a
Space Engineering Development Co., Ltd., ISSEI Third Building, 1-12-2, Takezono, Tsukuba, Ibaraki 305-0032, Japan b National Space Development Agency of Japan (NASDA), 2-1-1, Sengen, Tsukuba, Ibaraki 305-8505, Japan
Abstract The generated power degradation of a satellite in a low earth orbit during high solar activity period has been compared with the power degradation of a satellite during low solar activity period. A degradation prediction method is developed for this study. As a result, the effect of a large solar flare on solar cell degradation is found to be negligible in a low earth orbit. This is because the effects of shield thickness and inclination are thought to be greater than that of degrees of solar activity. r 2002 Published by Elsevier Science B.V. Keywords: Solar activity; Power; Low earth orbit; Degradation; Radiation; Fluence
1. Introduction In space environment, several kinds of radiation exist. This radiation causes degradation of solar cell, which leads to decrease in electric power from solar paddles of a satellite. Therefore, it is important to analyze generated power and understand a tendency of power degradation. The year 2000 was a maximum period of solar activity. In this period, large solar flares occurred frequently and a large quantity of radiation was spouted out from the sun. This radiation may induce severe damage to solar cells of a satellite in an orbit.
*Corresponding author. Tel.: +81-298-52-1778; fax: +81-298-50-2017. E-mail address: [email protected] (R. Fujita). 0927-0248/03/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 1 7 5 - 7
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R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
We have investigated power degradation of the solar paddles of the Engineering Test Satellite-VII (ETS-VII), which has been in a low earth orbit. The level of radiation exposure of the satellite was estimated from the amount of power degradation using a prediction method. The Japanese Earth Resources Satellite (JERS-1) is an earth observation satellite, which was also in a low earth orbit during the solar activity minimum period. The level of radiation exposure for JERS-1 was estimated by the same procedure as well, and the result was compared with that for ETS-VII.
2. Fundamental information ETS-VII consists of two satellites, Chaser and Target, in order to perform docking experiment as one of its missions. Chaser has two solar cell paddles. Since the paddles equip sun-tracking systems which control the paddles to face them toward the sun, the solar incident angle is always perpendicular to the paddles. On the other hand, Target has one paddle. Since it does not equip a sun-tracking system, the paddle is fixed to the satellite. Therefore, the solar incident angle changes due to the position of the satellite in the orbit. Chaser and Target exist in the same orbit, and they utilize the same type of solar cells. As for JERS-1, it has one paddle which has a sun-tracking system. It also utilizes the same type of solar cells to ETS-VII. Table 1 shows comparison of fundamental information of ETS-VII (Chaser/Target) and JERS-1.
3. Analysis 3.1. Procedure Fig. 1 shows the flow chart of the fluence analyzing method of radiation exposure developed in this work. Table 1 Fundamental information of ETS-VII (Chaser/Target) and JERS-1 Item
Unit
ETS-VII
JERS-1
Launch day Cell type (thickness) Flight altitude Inclination Surface shield Backside shield Paddle type
M/D/Y (mm) km deg g/cm2 g/cm2 —
11/27/97 Si BSFR (200) 550 35 0.026a 0.1101b Rigid
02/11/92 Si BSFR (50) 570 98 0.024 0.0326 Semi-rigid
a
Surface shield consists of coverglass and glue. Backside shield consists of polyimide film, carbon fiber reinforced plastic, aluminum honeycomb core, silver fluoroplastics and glue. b
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
321
Fig. 1. Flow of the fluence analyzing method of radiation exposure developed in this work.
Since ETS-VII was in a low earth orbit, changes in generated power due to following factors were relatively large; (a) reflected sunlight from the earth, (b) solar insolation, and (c) temperature of solar cell. Therefore, at first, we examined the factors and the locations of the satellite both of which might affect electric power output, in order to analyze under the same conditions. Next, we eliminated these factors from raw flight data of power, so as to standardize them. The power degradation tendency was obtained from the standardized data. On the other hand, we have developed a degradation prediction method which estimates the level of radiation exposure based on ground irradiation test results. The level of radiation exposure corresponding to the amount of power degradation was estimated using this method. The same method was applied to estimate the level of radiation exposure for JERS-1. 3.2. Power degradation Fig. 2 shows trend of the actual generated power (raw flight data) and the calibrated power (standardized by solar insolation and temperature) of Chaser and Target. Some temperature sensors were attached on the backside of the paddles. In such case, the value of temperature was converted into the surface temperature by estimation from the consideration of the sensor position and temperature gradient. However, all the estimated cell temperatures were not likely to be accurate enough, thus the data with the estimated cell temperature between 65–751C for Chaser and 58–691C for Target were selected for the following evaluation.
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R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
Fig. 2. (a, b) Trend of raw flight data and standardized data of generated power of ETS-VII.
These values should include only degradation due to radiation exposure. The slope of the fitted line (negative value) confirms that there is significant decrease in the power due to the degradation of the solar cells. The quantity of the annual power decrease and its rate for Chaser and Target are summarized in Table 2. 3.3. Fluence of radiation exposure The level of radiation exposure (fluence) was calculated from the degradation shown in Fig. 2. The fluence is expressed as a value equivalent to 1 MeV electrons with consideration of shielding effect by such as cover glass.
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
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Table 2 Annual degradation and its rate
Chaser Target
Annual degradation (W)
Degradation rate (%)
Distribution (%)
15.3 2.8
0.5 0.3
3.8 4.8
Fig. 3. Fluence of radiation exposure of ETS-VII (Chaser/Target) and JERS-1. X-axis is in UT (days after launch).
The estimated fluence at the end of the mission term is 4.1 1012 (e/cm2) for Chaser and 3.9 1012 (e/cm2) for Target. The fluence for Chaser is slightly larger than that for Target. Fluence of the two satellites for 1.5 years after launch was also calculated. The result was 2.3 1012 (e/cm2) for Chaser for ETS-VII and 1.2 1013 (e/cm2) for JERS-1. Therefore, the fluence for ETS-VII is less than that for JERS-1 despite that the mission term of ETS-VII was in solar activity maximum while that of JERS-1 was solar activity minimum. Fig. 3 shows the fluence obtained from decrease in the generated power values shown in Fig. 2.
4. Results and discussion 4.1. Comparison between Chaser and Target With regard to the degradation rate, the rate of Chaser is larger than that of Target. As a result, the fluence for Chaser is slightly larger than that for Target, but
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R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
Chaser and Target showed the same order of magnitude of fluence. Cover glass thickness and the backside shield materials are the same for Chaser and Target, and the orbits of these two satellites are also the same. Therefore, the differences in the rate and the fluence are supposed to be due to the fact that the paddles of Chaser have sun-tracking systems, while the paddle of Target does not. However, distribution of the data for Target is larger than that for Chaser. Thus, it is difficult to distinguish whether this is due to the sun-tracking or not.
4.2. Comparison between Chaser of ETS-VII and JERS-1 As for the fluence for Chaser of ETS-VII and for JERS-1, the fluence for Chaser is less than that for JERS-1, despite the difference of solar activity. There are two possible reasons as follows. (1) Difference of inclination angle: Radiation environment becomes severe as inclination angle approaches 901. JERS-1 is the solar synchronous orbital satellite whose inclination angle is 981. On the other hand, ETS-VII has an inclination angle of 351. (2) Difference in paddle structure (difference in shield thickness): With regard to the surface shield thickness (cover glass thickness), ETS-VII and JERS-1 are almost the same. However, backside shield thickness is different. ETS-VII has rigid type paddles, while JERS-1 has a semi rigid type paddle. Rigid type paddle has an aluminum base, while semi rigid type has not. Their backside shield areal densities are 0.0326 (g/cm2) for JERS-1 and 0.110 (g/cm2) for ETS-VII. Therefore, the shield of ETS-VII is 3.8 times thicker than that of JERS-1. (3) Difference in cell thickness: The cell thickness of ETS-VII (200 mm) is thicker than that of JERS-1 (50 mm). Generally, thinner cell is more radiation tolerant than thicker cell. However, the degradation of the cells of ETS-VII is less than that of JERS-1. This should be due to that the effects of (1) and (2) is greater than the thickness effect.
4.3. Correlation to solar activity It was expected that the degradation of ETS-VII is greater than that of JERS-1, because the solar activity of the mission term of ETS-VII is more active than that of JERS-1. However, the result was contrary to expectation. Because of the reasons described in Section 4.2, the effect of shield thickness and inclination was considered to be greater than that of degrees of solar activity in the case in a low earth orbit. Distinct the power degradation due to the degradation of solar cells cannot be observed, even on the day when a huge flare occurred (July 14th, 2000). Therefore, in a low earth orbit, the effect of a solar flare on solar cells of a satellite was considered to be negligible.
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
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5. Summary Decrease in generated power due to degradation of solar cells of ETS-VII was analyzed by means of developed method for this study. The effect of a large solar flare was found to be negligible in a low earth orbit. This is because the effects of shield thickness and inclination angle are thought to be greater than that of degrees of solar activity.
Analysis of generated power of ETS-VII during solar activity maximum period Reiko Fujitaa,*, Mitsuru Imaizumib, Kazuhiro Aoyamab, Sumio Matsudab, Shinji Tokunagaa a
Space Engineering Development Co., Ltd., ISSEI Third Building, 1-12-2, Takezono, Tsukuba, Ibaraki 305-0032, Japan b National Space Development Agency of Japan (NASDA), 2-1-1, Sengen, Tsukuba, Ibaraki 305-8505, Japan
Abstract The generated power degradation of a satellite in a low earth orbit during high solar activity period has been compared with the power degradation of a satellite during low solar activity period. A degradation prediction method is developed for this study. As a result, the effect of a large solar flare on solar cell degradation is found to be negligible in a low earth orbit. This is because the effects of shield thickness and inclination are thought to be greater than that of degrees of solar activity. r 2002 Published by Elsevier Science B.V. Keywords: Solar activity; Power; Low earth orbit; Degradation; Radiation; Fluence
1. Introduction In space environment, several kinds of radiation exist. This radiation causes degradation of solar cell, which leads to decrease in electric power from solar paddles of a satellite. Therefore, it is important to analyze generated power and understand a tendency of power degradation. The year 2000 was a maximum period of solar activity. In this period, large solar flares occurred frequently and a large quantity of radiation was spouted out from the sun. This radiation may induce severe damage to solar cells of a satellite in an orbit.
*Corresponding author. Tel.: +81-298-52-1778; fax: +81-298-50-2017. E-mail address: [email protected] (R. Fujita). 0927-0248/03/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 1 7 5 - 7
320
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
We have investigated power degradation of the solar paddles of the Engineering Test Satellite-VII (ETS-VII), which has been in a low earth orbit. The level of radiation exposure of the satellite was estimated from the amount of power degradation using a prediction method. The Japanese Earth Resources Satellite (JERS-1) is an earth observation satellite, which was also in a low earth orbit during the solar activity minimum period. The level of radiation exposure for JERS-1 was estimated by the same procedure as well, and the result was compared with that for ETS-VII.
2. Fundamental information ETS-VII consists of two satellites, Chaser and Target, in order to perform docking experiment as one of its missions. Chaser has two solar cell paddles. Since the paddles equip sun-tracking systems which control the paddles to face them toward the sun, the solar incident angle is always perpendicular to the paddles. On the other hand, Target has one paddle. Since it does not equip a sun-tracking system, the paddle is fixed to the satellite. Therefore, the solar incident angle changes due to the position of the satellite in the orbit. Chaser and Target exist in the same orbit, and they utilize the same type of solar cells. As for JERS-1, it has one paddle which has a sun-tracking system. It also utilizes the same type of solar cells to ETS-VII. Table 1 shows comparison of fundamental information of ETS-VII (Chaser/Target) and JERS-1.
3. Analysis 3.1. Procedure Fig. 1 shows the flow chart of the fluence analyzing method of radiation exposure developed in this work. Table 1 Fundamental information of ETS-VII (Chaser/Target) and JERS-1 Item
Unit
ETS-VII
JERS-1
Launch day Cell type (thickness) Flight altitude Inclination Surface shield Backside shield Paddle type
M/D/Y (mm) km deg g/cm2 g/cm2 —
11/27/97 Si BSFR (200) 550 35 0.026a 0.1101b Rigid
02/11/92 Si BSFR (50) 570 98 0.024 0.0326 Semi-rigid
a
Surface shield consists of coverglass and glue. Backside shield consists of polyimide film, carbon fiber reinforced plastic, aluminum honeycomb core, silver fluoroplastics and glue. b
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
321
Fig. 1. Flow of the fluence analyzing method of radiation exposure developed in this work.
Since ETS-VII was in a low earth orbit, changes in generated power due to following factors were relatively large; (a) reflected sunlight from the earth, (b) solar insolation, and (c) temperature of solar cell. Therefore, at first, we examined the factors and the locations of the satellite both of which might affect electric power output, in order to analyze under the same conditions. Next, we eliminated these factors from raw flight data of power, so as to standardize them. The power degradation tendency was obtained from the standardized data. On the other hand, we have developed a degradation prediction method which estimates the level of radiation exposure based on ground irradiation test results. The level of radiation exposure corresponding to the amount of power degradation was estimated using this method. The same method was applied to estimate the level of radiation exposure for JERS-1. 3.2. Power degradation Fig. 2 shows trend of the actual generated power (raw flight data) and the calibrated power (standardized by solar insolation and temperature) of Chaser and Target. Some temperature sensors were attached on the backside of the paddles. In such case, the value of temperature was converted into the surface temperature by estimation from the consideration of the sensor position and temperature gradient. However, all the estimated cell temperatures were not likely to be accurate enough, thus the data with the estimated cell temperature between 65–751C for Chaser and 58–691C for Target were selected for the following evaluation.
322
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
Fig. 2. (a, b) Trend of raw flight data and standardized data of generated power of ETS-VII.
These values should include only degradation due to radiation exposure. The slope of the fitted line (negative value) confirms that there is significant decrease in the power due to the degradation of the solar cells. The quantity of the annual power decrease and its rate for Chaser and Target are summarized in Table 2. 3.3. Fluence of radiation exposure The level of radiation exposure (fluence) was calculated from the degradation shown in Fig. 2. The fluence is expressed as a value equivalent to 1 MeV electrons with consideration of shielding effect by such as cover glass.
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
323
Table 2 Annual degradation and its rate
Chaser Target
Annual degradation (W)
Degradation rate (%)
Distribution (%)
15.3 2.8
0.5 0.3
3.8 4.8
Fig. 3. Fluence of radiation exposure of ETS-VII (Chaser/Target) and JERS-1. X-axis is in UT (days after launch).
The estimated fluence at the end of the mission term is 4.1 1012 (e/cm2) for Chaser and 3.9 1012 (e/cm2) for Target. The fluence for Chaser is slightly larger than that for Target. Fluence of the two satellites for 1.5 years after launch was also calculated. The result was 2.3 1012 (e/cm2) for Chaser for ETS-VII and 1.2 1013 (e/cm2) for JERS-1. Therefore, the fluence for ETS-VII is less than that for JERS-1 despite that the mission term of ETS-VII was in solar activity maximum while that of JERS-1 was solar activity minimum. Fig. 3 shows the fluence obtained from decrease in the generated power values shown in Fig. 2.
4. Results and discussion 4.1. Comparison between Chaser and Target With regard to the degradation rate, the rate of Chaser is larger than that of Target. As a result, the fluence for Chaser is slightly larger than that for Target, but
324
R. Fujita et al. / Solar Energy Materials & Solar Cells 75 (2003) 319–325
Chaser and Target showed the same order of magnitude of fluence. Cover glass thickness and the backside shield materials are the same for Chaser and Target, and the orbits of these two satellites are also the same. Therefore, the differences in the rate and the fluence are supposed to be due to the fact that the paddles of Chaser have sun-tracking systems, while the paddle of Target does not. However, distribution of the data for Target is larger than that for Chaser. Thus, it is difficult to distinguish whether this is due to the sun-tracking or not.
4.2. Comparison between Chaser of ETS-VII and JERS-1 As for the fluence for Chaser of ETS-VII and for JERS-1, the fluence for Chaser is less than that for JERS-1, despite the difference of solar activity. There are two possible reasons as follows. (1) Difference of inclination angle: Radiation environment becomes severe as inclination angle approaches 901. JERS-1 is the solar synchronous orbital satellite whose inclination angle is 981. On the other hand, ETS-VII has an inclination angle of 351. (2) Difference in paddle structure (difference in shield thickness): With regard to the surface shield thickness (cover glass thickness), ETS-VII and JERS-1 are almost the same. However, backside shield thickness is different. ETS-VII has rigid type paddles, while JERS-1 has a semi rigid type paddle. Rigid type paddle has an aluminum base, while semi rigid type has not. Their backside shield areal densities are 0.0326 (g/cm2) for JERS-1 and 0.110 (g/cm2) for ETS-VII. Therefore, the shield of ETS-VII is 3.8 times thicker than that of JERS-1. (3) Difference in cell thickness: The cell thickness of ETS-VII (200 mm) is thicker than that of JERS-1 (50 mm). Generally, thinner cell is more radiation tolerant than thicker cell. However, the degradation of the cells of ETS-VII is less than that of JERS-1. This should be due to that the effects of (1) and (2) is greater than the thickness effect.
4.3. Correlation to solar activity It was expected that the degradation of ETS-VII is greater than that of JERS-1, because the solar activity of the mission term of ETS-VII is more active than that of JERS-1. However, the result was contrary to expectation. Because of the reasons described in Section 4.2, the effect of shield thickness and inclination was considered to be greater than that of degrees of solar activity in the case in a low earth orbit. Distinct the power degradation due to the degradation of solar cells cannot be observed, even on the day when a huge flare occurred (July 14th, 2000). Therefore, in a low earth orbit, the effect of a solar flare on solar cells of a satellite was considered to be negligible.
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5. Summary Decrease in generated power due to degradation of solar cells of ETS-VII was analyzed by means of developed method for this study. The effect of a large solar flare was found to be negligible in a low earth orbit. This is because the effects of shield thickness and inclination angle are thought to be greater than that of degrees of solar activity.