Confusion Regarding Irradiance Calculation

Yes.

You may want to refer to the original papers for these equations. You can find them listed in the manual's References section: De Soto (2004a), De Soto (2004b), and Duffie and Beckman (2013).

Best regards,
Paul.

Where can I find the value of 'a' in eqn 4.2 ? 

Hi David,

Maybe the original source code will help:

github.com/NREL/ssc/blob/37637c9d492e4e2e79adbf43ccb9afad7abb4ddb/shared/lib_irradproc.cpp#L681

Equation 4.2 is a representation of Line 693.

Best regards,
Paul.

Why does the source code have different formulas than what is shown in the document ? 

Hi David,

The equations in the source code are adapted from the equations in the original model descriptions and may include modifications to make them work in the context of the computer model.

Best regards,
Paul.

Question related to this stream:  

Eq. 9.11 in the tech manual calculates the sky diffuse shading factor.  It is a function with 4 variables: array tilt ( betas), length of side of the array (, Distance between arrays ( R) and number of rows. .
frac calculated manually. with tilt= 40 deg, B=4, R=12.5 nrows=16 gives  frac= 0.981326  (a constant not function of the irradiance)
frac calculated with the same parameters by SAM shown in 'data table' / 'lifetime hourly data' / "Subarray 1 Self-shading non-linear sky diffuse irradiance factor | (frac)" = 0.951244 ( also constant over the year)

Why is the value calculated by SAM different from the value calculated with Eq. 9.11 ?

frac equation equivalent attached.