Characteristics and Corrections of Thermal Offset for Secondary Standard Pyranometers

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Title:	Characteristics and Corrections of Thermal Offset for Secondary Standard Pyranometers
Authors:	薛乃儒;Hsueh, Nai-Ju
Contributors:	大氣科學學系
Keywords:	全天空短波輻射計;熱偏移誤差;pyranometer;thermal offset
Date:	2018-08-20
Issue Date:	2018-08-31 11:25:20 (UTC+8)
Publisher:	國立中央大學
Abstract:	對於氣候研究以及太陽能發電系統的發展，準確地量測地表太陽輻射通量相當的重要。全天空短波輻射計(pyranometer)是在地表上量測太陽輻射通量最主要的儀器，為增進觀測的準確性，定期進行儀器校正以及分析其觀測的誤差來源是不可或缺的，熱偏移誤差(thermal offset error)是全天空短波輻射計的觀測中最主要的誤差來源之一，此誤差是由於全天空短波輻射計的玻璃圓頂(dome)與感測器(detector)的溫度差異，而造成地表輻射通量之量測有偏差的情形，通常會造成觀測之高估。為了解市面上不同型號的全天空短波輻射計之熱偏移誤差的特徵，以及不同誤差修正方法的效果，我們與美國國家海洋暨大氣總署(National Oceanic and Atmospheric Administration, NOAA)及全天空短波輻射計的製造商合作，匯集總共20台全天空短波輻射計於國立中央大學進行室外觀測實驗，實驗期間為2017年12月到2018年3月。為使每一台參與實驗的全天空短波輻射計之校正係數(sensitivity)追溯至共同的標準，吾人分別參考ISO9847與ISO9846進行室內與室外校正。校正結果顯示，室外校正係數具有較小的擴充不確定度(expanded uncertainty) (1.28%)，且相較於原廠的校正係數，室外校正係數的改變量較小(-0.34%)，而室內校正係數則具有2.33%的擴充不確定度及0.75%的改變量，因此我們選用室外校正係數於後續的分析。本研究參考Younkin and Long (2003)的熱偏移誤差修正方法，於夜晚建立單變數(detector only)與雙變數(full)的誤差修正模型，套用在白天的太陽輻射觀測進行誤差修正(detector only correction與full correction)，並與NOAA提供的標準件互相比對。分析結果顯示，三款新型的全天空短波輻射計(MS-80、SR25-T2及SR30-D1)之設計，對於熱偏移誤差皆有所改良，其夜間平均的熱偏移誤差僅為-0.28Wm-2，相較於其他輻射計之數值落於-2.08至-0.39 Wm-2之間少很多，因此對於其他沒有針對熱偏移誤差改良的儀器而言，進行誤差修正是必要的。根據評估，full correction method適用於一半以上參與實驗的全天空短波輻射計，例如型號CMP11、CMP21、CMP22、MS-80、裝有通風設備的SR-75、SPP與PSP，平均的修正改善量為0.17-2.51 Wm-2，而未通風的SR20-D2與PSP及通風的SR30-D1適用之修正方法為detector only correction，平均的修正改善量為0.10-1.04 Wm-2。此外，即使已進行熱偏移誤差修正，在太陽位在高仰角時，幾乎所有全天空短波輻射計量測到的太陽輻射通量都有高估的情形，我們認為這與太陽劇烈加熱下的玻璃圓頂溫度與環境溫度有關；相較在太陽位於低仰角時，觀測有較高的誤差百分比(percent deviation)，則主要與儀器感測器的餘旋響應(cosine response)有關。最後，分析使用通風設備與否的觀測資料發現，除了SR-75和SR20，其他型號的全天空短波輻射計在通風的情況下，都有較小的熱偏移誤差，此說明了通風對於全天空短波輻射計的觀測之重要性。此實驗結果說明全天空短波輻射計的選擇、加裝通風設備與否、及熱偏移誤差修正的使用皆會影響全天空短波輻射計的觀測結果，對於全天空短波輻射觀測的準確性相當重要。 ;Accurate measurement of solar radiation arriving at the Earth’s surface is an important information not only to the climate researches but also to the solar power systems. A pyranometer is a major instrument used for measuring solar radiation on a plane surface. To ensure the measuring accuracy, regular calibration of pyranometers and identification of the main errors are necessary. For pyranometers, the thermal offset error is one of the main errors which is due to the temperature difference between the detector and the domes, and it usually results in the overestimation of the measurements. In order to investigate the characteristic of thermal offset for different pyranometers and the performance of different correction methods, we collected 20 pyranometers and carried out an inter-comparison experiment at NCU in cooperation with NOAA and pyranometer manufactures from December 2017 to March 2018. All the participating pyranometers were calibrated with indoor and outdoor ISO calibration procedures to ensure the sensitivities are traceable to reference values. The sensitivities from outdoor calibration were chosen because their lower expanded uncertainty (1.28%) and lower percentage change in sensitivity (-0.34%) compared to the values of 2.33% and 0.75% for indoor calibration. Two thermal offset correction methods (i.e., detector only and full correction methods) were applied to daytime irradiance during the experimental period, and the values after correction were compared to the reference units provided by NOAA. Our results show that the three modern pyranometer models (MS-80, SR25-T2, and SR30-D1), which had been designed for improving the thermal offset error, have the mean magnitudes of nighttime thermal offset (-0.28 Wm-2) much lower than that of the other pyranometers (-2.08 to -0.39 Wm-2). For the pyranometers which were designed without considering the thermal offset error, the correction for the error is necessary. According to the assessment, the full correction method is suitable for more than half the pyranometers such as CMP11, CMP21, CMP22, MS-80, and ventilated SR-75, SPP, and PSP (average correction improvement: 0.17-2.51 Wm-2); however, the better method for unventilated SR20-D2 and PSP, and ventilated SR30-D1 is the detector only correction (average correction improvement: 0.10-1.04 Wm-2). Furthermore, although the thermal offset correction was applied, the daytime data still show overestimation for most of the pyranometers at small solar zenith angle. We suggest that the dome and ambient temperatures may play an important role in this measurement error of pyranometers. In contrast, at large solar zenith angle, the high percent deviation from the reference is due to the cosine response of the pyranometer sensor. Finally, except for the pyranometer model SR-75 and SR20, other models show that the mean values of the magnitudes of thermal offset are smaller when the ventilators were installed, which indicates the importance of ventilation to pyranometers. The results of the experiment show that the pyranometer measurements are affected by the choice of pyranometers, the on/off of the ventilation, and the usage of the thermal offset correction method.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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