|Quality Assessment with SERI_QC|
After data are collected, quality assessment can be performed to indicate whether a data value is reasonable, too small, too large, or missing. It is not used to change data values, and should not be confused with quality control and quality assurance. Quality control and quality assurance occur before and during data collection and include procedures such as the proper selection and installation of instruments and data acquisition equipment, as well as regular maintenance and calibration. Quality assessment cannot replace quality control and quality assurance because it will not detect small changes caused by dirty or unleveled sensors or changes in calibration factors.
Quality assessment procedures for the solar radiation data assign a flag ranging from O to 99 to each of the hourly data elements. To select data for analysis purposes, the flags may be used to screen the data files for data meeting user-defined acceptance criteria.
Quality Assessment for Solar Radiation Data
The three solar radiation data elements - global horizontal, diffuse horizontal, and direct normal - are quality assessed using SERI QC, a procedural and software package developed by NREL (1993). SERI QC defines ranges of acceptable data, depending on whether one, two, or all three hourly data elements are present. Ranges are defined based on dimensionless parameters normalized with respect to extraterrestrial radiation, where
|Kt||= Clearness index or global horizontal transmittance|
|= Global horizontal radiation / extraterrestrial horizontal radiation|
|Kd||= Diffuse horizontal transmittance|
|= Diffuse horizontal radiation / extraterrestrial horizontal radiation|
|Kn||= Direct normal transmittance|
|= Direct normal radiation / extraterrestrial direct normal radiation.|
Depending on the circumstances, SERI QC performs one-element, two-element, or three-element tests. First, it performs a one-element test by defining a range of acceptable values between minimum and maximum values of Kt, Kd, or Kn, depending on the element being tested, based on three air mass regimes and the month of the year.
Second, if the zenith angle (at the middle of the hour) is less than or equal to 80°, and all three of the elements are present, SERI QC performs a three-element test by defining a range of acceptable values so that the equation Kt = Kd + Kn is satisfied within an arbitrary error limit of ± 0.03, which accounts for measurement uncertainties.
Third, if the data pass the three-element test (or only two elements passed the one-element test), SERI QC performs a two-element test by defining a range of acceptable values within boundaries such as those shown in Figure 1. The boundaries in the figure are previously determined empirically for three different air mass regimes for each month using data collected at the site. In Figure 1, the direct normal transmittance is plotted against global horizontal transmittance. The lower boundary illustrates that with increasing cloud cover, direct normal radiation decreases more rapidly than global horizontal radiation. Consequently, data for cloudy skies are grouped toward the lower boundary. Clear sky data reside near the upper boundary.
For some seldom occurring conditions, data depicting real conditions may reside outside the boundaries and be flagged as bad data. For example, if the sun is near the edge of a cloud, some of the sun's rays can be reflected off the edge of the cloud and increase the global horizontal radiation and Kt without affecting the direct normal radiation and Kn. This shifts the data point to the right in Figure 1, and it may be to the right of the l ower boundary if Kt is large enough.
Bad data can also be flagged good by SERI_QC. For overcast sky conditions, SERI_QC does not detect an improperly adjusted shadowband because Kn = 0 and Kt = Kd within its arbitrary error limit.
After all SERI QC tests are completed, flags are assigned to the data according to the convention listed in Table 1.