Improving the calibration of silicon photodiode pyranometers

Abstract

Reliable measurements of global irradiance are essential for research and practical applications. Silicon photodiode pyranometers (SiPs) offer low-cost sensors to measure direct and diffuse irradiance despite their non-uniform spectral response over the 300-1000 nm spectral range. In this study, non-adjusted linear and adjusted calibrations were applied at different times of the year to determine sources of estimated errors in global irradiance due to the two calibration approaches, calibration time, and sensor age. 16 SiPs, along with two standards, measured incident global irradiance over a 5-year period under a range of sky conditions. Sensors performed best in the months in which they were calibrated when using the linear calibration approach. With the solar zenith angle adjusted calibration approach, certain calibration months provide a defendable validation for the following 12 months [ranging an average of 13.5-17.4 W m −2 standard error (SE)], while other calibration months do not provide consistent results and sometimes result in very poor validation (31.1-242.7 W m −2 SE). Older sensors (greater than 6 years) in general become more sensitive to solar zenith angle and their response drifts over time, while newer SiPs performed better than older sensors. Calibrations which accounted for solar zenith angle effects improved global irradiance estimates for older SiPs. For the Lincoln NE location, the appropriate calibration is in spring or late summer, regardless of calibration approach. These results indicate that solar zenith angle correction is not needed for largely diffuse components under cloudy conditions, so that in the future, a "smart" calibration may be possible, where diffuse radiation fractions are known. digitalcommons.unl.edu W a l t e r -S h e a i n M e t e o r o l o g y a n d A t m o s p h e r i c P h y s i c s( 2 0 1 8 ) 2

Worldwide evaluation and correction of irradiance measurements from personal weather stations under all-sky conditions

“…Overall, the performance both under clear and all-sky conditions of the proposed model in terms of bias are in alignment with those from stand-alone silicon photodiode pyranometers for GHI measurements in other studies, where the accuracies are within a ±5% band compared to thermopile pyranometers (King and Myers, 1997;Sengupta et al, 2012;Vignola et al, 2017;Walter-Shea et al, 2019).…”

“…There are a number of factors influencing the response of silicon-photodiode devices, such as: (i) angle of incidence, (ii) device temperature, (iii) tilt orientation, (iv) mechanical and optical asymmetries in the instrument, (v) irradiance magnitude, and (vi) spectral response (King and Myers, 1997). The difference in G readings between silicon-photodiode and thermopile pyranometers falls within ±5% when devices are correctly calibrated and maintained, and the data corrected (Al-Rasheedi et al, 2018;King and Myers, 1997;Sengupta et al, 2012;Vignola et al, 2017;Walter-Shea et al, 2019). Without corrections, the differences can reach ±15% (King et al, 1998).…”

“…The uncertainty of measurements produced by silicon-photodiode sensors has been a topic of research interest, mainly due to the low-cost solution these sensors offer for land-based measurements. A number of empirical methods have been proposed in the literature to estimate the broadband solar irradiance captured by silicon-photodiode sensors addressing effects related to temperature, spectral mismatch and solar angle of incidence (Augustyn et al, 2004;King et al, 1998;King and Myers, 1997;Vignola et al, 2017;Walter-Shea et al, 2019).…”

“…Existing empirical methods in the literature utilise solar zenith angle to compensate the spectral mismatch and cosine response effects, e.g. (King et al, 1998;King and Myers, 1997;Vignola et al, 2017;Walter-Shea et al, 2019). Using the solar zenith angle, Augustyn et al (2004) proposed a piece-wise, segmented function to compensate the so-called cat ears' effect at high solar zenith angles.…”

“…Although Sengupta et al (2012) concluded that uncertainties in the measurements of silicon-based devices cannot be easily quantified, empirical correction procedures have been proposed based on the absolute air mass, the solar zenith angle and the temperature of the device (King et al, 1998;King and Myers, 1997;Vignola et al, 2017;Walter-Shea et al, 2019). Such empirical models require sensor-specific data that may not be readily available for generic PWS.…”