One of the most significant inputs to PV performance models is the measured irradiance on the plane of the array.  If the irradiance sensor is misaligned relative to the array or the array orientation is out of specification, the prediction accuracy will suffer.  This mini-study is aimed at discovering the sensitivity of predicted annual energy from a PV system to errors in the alignment of the irradiance sensor and/or the array.

Misalignment can be described with errors in the tilt and azimuth angles of the sensor relative to the PV array.  As an example, we assume a latitude-tilt PV array in Albuquerque, NM (Latitude = 35.05°) facing due South (azimuth = 180°).  Errors in the tilt angle affect the relative magnitude of the plane of the array irradiance, but not the shape of the diurnal signal (start and end of the day remain the same).  Errors in the azimuth angle affect shape of the diurnal signal. 

To demonstrate this we have generated a 1-min resolution annual clear sky irradiance signal using a clear sky irradiance model (Ineichen and Perez, 2002). From this estimate of global horizontal irradiance, we have estimated the direct normal irradiance using the DISC model (Maxwell, 1987) and calculated the diffuse horizontal using the sun position and algebra.  Then we have calculated the diffuse plane of the array irradiance using the Perez transposition model (Perez et al., 1990), and the ground reflected plane of array irradiance assuming a surface albedo of 0.2.   

References:

Ineichen, P. and R. Perez, “A New airmass independent formulation for the Linke turbidity coefficient”, Solar Energy, vol 73, pp. 151-157, 2002.

Maxwell, E. L., “A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation”, Technical Report No. SERI/TR-215-3087, Golden, CO: Solar Energy Research Institute, 1987.

Perez, R., Ineichen, P., Seals, R., Michalsky, J., Stewart, R., 1990.  Modeling daylight availability and irradiance components from direct and global irradiance. Solar Energy 44 (5), 271–289.

Contributed by Sandia National Laboratories