In this step the irradiance data is transposed to the plane of the array.  The sub-models included in this step include various array tracking algorithms, estimates for the reflectivity of the ground (albedo),  and models to calculate the diffuse irradiance on the array from the sky.

The calculation of the incident irradiance on the array is critical for modeling the performance of a PV system.  This calculation involves:

• Defining or determining the orientation of the array, which can be either fixed or variable in time (tracking arrays).
• Estimating the contributions of the beam and diffuse irradiance components (sky diffuse and ground reflected diffuse).

Alternatively, one can measure the plane of array irradiance directly with a pyranometer, reference cell, or reference module mounted in the same orientation of the array.  The analyst must understand the different characteristics of these sensors to ensure that any subsequent corrections made to the measured POA irradiance are appropriate.  For example, a matched reference cell or module probably has the same reflective properties of the modules in the array, so reflective losses are inherently included in the measured value and should not be “corrected” for again.

A fundamental step in calculating PV performance is determining the irradiance incident on the plane of the array (POA) as a function of time.  This POA irradiance is dependent upon several factors, including:

• Sun Position
• Array Orientation (fixed or tracking)
• Irradiance Components (Direct and Diffuse)
• Ground Surface Reflectivity (Albedo)
• Shading (near and far obstructions)

Mathematically POA irradiance,  is:

 

where is the POA beam component, is the POA ground-reflected component, and is the POA sky-diffuse component.

 

 

Content contributed by Sandia National Laboratories