Abstract
Abstract. Resource assessment for concentrated solar power (CSP) needs accurate direct normal irradiance (DNI) measurements. An option for such measurement campaigns is the use of thoroughly calibrated rotating shadowband irradiometers (RSIs). Calibration of RSIs and Si-sensors is complex because of the inhomogeneous spectral response of these sensors and incorporates the use of several correction functions. One calibration for a given atmospheric condition and air mass might not be suitable under different conditions. This paper covers procedures and requirements of two calibration methods for the calibration of rotating shadowband irradiometers. The necessary duration of acquisition of test measurements is examined with regard to the site-specific conditions at Plataforma Solar de Almería (PSA) in Spain. Seven data sets of long-term test measurements were collected. For each data set, calibration results of varying durations were compared to its respective long-term result. Our findings show that seasonal changes of environmental conditions are causing small but noticeable fluctuation of calibration results. Calibration results within certain periods (i.e. November to January and April to May) show a higher likelihood of deviation. These effects can partially be attenuated by including more measurements from outside these periods. Consequently, the duration of calibrations at PSA can now be selected depending on the time of year in which measurements commence.
Highlights
Concentrated solar power (CSP) projects require accurate assessment of the available direct beam resource
A first site-specific assessment of the necessary calibration duration at Plataforma Solar de Almería (PSA) has been presented in Geuder et al (2014) in which a minimum measuring period of 30 days was recommended based on data collected from a single instrument
We investigated the subject further by use of a total of seven long-term data sets ranging from 251 to 1289 day duration collected over a period of 6.5 years from five rotating shadowband irradiometers (RSIs) instruments (Table 5)
Summary
Concentrated solar power (CSP) projects require accurate assessment of the available direct beam resource. RSIs with continuous rotation use the LI-COR LI-200SA pyranometer This photodiode instrument experiences systematic errors due to cosine and temperature effects and its non-uniform spectral responsivity. In further deviation from the ISO standards for thermopile sensors, the RSI calibration methods by DLR assign more than one calibration factor for each instrument, since multiple components of solar irradiance with differing spectral composition and sensor responsivity are determined. These RSI calibration methods have been applied at the Plataforma Solar de Almería (PSA) (latitude 37.0909◦ N, longitude −2.3581◦ E, altitude 500 m a.m.s.l., semi-arid climate) for a number of years. The site dependence of the performance is discussed in Geuder et al (2016)
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