Dear Miguel,
First of all, as you know, the pvwattsv1 module in the SDK is a legacy version. The current version of PVWatts in both SAM and the PVWatts website use pvwattsv5. I know you are aware of this, but I wanted to mention that for other readers.
Section 3.5 of the SSC Guide describes the Required flags. You are correct about the * and ? values. For a variable with a flag of ?="value", if you do not assign a value, the model will use the indicated default value, so I would consider it a required value from the model's perspective, but you do not need to assign it a value.
The variables you are asking about are experimental and intended to make it possible to use the pvwattsv1 module for concentrating photovoltaic collectors. Those inputs were never exposed in the SAM and online PVWatts models.
The
input is the diffuse fraction. The model uses it to calculate the POA irradiance as follows
poa = ibeam + fd * ( iskydiff + ignddiff )
. You can use it to make adjustments to the diffuse portion of the POA irradiance. For example, for a concentrating collector you could set
to remove diffuse irradiance from the POA irradiance.
The
input is not used by the model. It looks like it was intended to force the array's output to zero when the wind speed was greater than the stow speed, but that is not implemented in the model. Some trackers move the modules into a "stow" position when it the wind is strong to avoid damage to the array and tracking equipment.
The
,
,
, and
inputs are used to calculate the cell temperature
as follows:
If you assign values to U0 and U1:
pvt = wf.tdry + poa*concen/(U0+U1*wspd_corr)
, where
is the wind speed from the weather file set to zero if it is a negative value.
If U0 and U1 are not defined, then the cell temperature is calculated using the Fuentes model as described in Chapter 8 of the PVWatts Version 1 Technical Reference with the following inputs to the cell temperature function:
poa*concen, wspd_corr, wf.tdry, fhconv
.
Chapter 7 of the technical reference describes how pvwattsv1 models the module cover given the POA irradiance, whether it is calculated from the beam and diffuse irradiance or provided by the user. If you set
, the model uses the following set of b coefficients for the Sandia model polynomial equation instead of the ones shown in the manual (for a Sunpower SPR-E20-327 module):
Best regards,
Paul.