Simulation Variable - GlobBak (Rear POA)

[Lucy]

Hello, I am trying to get a sense of how rear POA reading would be calculated using PVsyst simulation variables. The variables in question to complete this calculation are 

GlobBak - global irradiance on rear side

BackShd - Shadings loss on rear side

 

1. Is the GlobBak variable incident to the the back of the module for a single axis tracker system?

2. Would the GlobBak data be equivalent to the readings of a rear mounted POA sensor? Or, question 3

3. Does the GlobBak variable already account for any rear shading irrradiance loss (BackShd) such that the rear POA reading would be higher (GlobBack minus BackShd)?

 

[André Mermoud]

The calculation sequence is as shown on the loss diagram. For getting the values and  acronyms in EXCEL, you can open the report, Menu "Export > Loss diagram values". 

You will get a table to be pasted in EXCEL:

Here you can see the flow of the calculation of the energies. 

The GlobBak value is accounted after taking the rear shading losses into account.

Now the problem of measuring this value on-site is really difficult. The irradiance received by the backside is highly dependent on the position in the table (top to bottom in sheds, or from axis to edge in a tracker). There is no real consensus about how to measure this value on-site.

Please see the help  "Project design > Results >  Performance Ratio" for a more detailed discussion of this problem.