Fresnel Losses

Does SAM account for the Fresnel losses from the glass for early/late day performance? However, nowhere in SAM do they ask what the glass surface is on the module or if it is AR coated – which makes me suspicious they may not be doing it this way?

The values that you enter in the optical efficiency table on the Collector and Receiver page should include these losses.

All optical effects that are dependent on the solar position should be included in the optical efficiency table or incidence angle modifier equations.

Any other fixed losses such as reflectivity, soiling, etc., can be accounted for using the separate optical performance (Geometry Effects, Mirror Reflectivity, Mirror Soiling, etc.).

During simulations, SAM multiplies all of the optical terms that apply in each hour to determine the actual optical performance.

Best regards,
Paul.

Thanks for the reply, but this is for a PV module - where would I put those?

Oops! Sorry to have assumed you were asking about the CSP linear Fresnel model.

For the PV models, SAM accounts for incidence angle-dependent losses in the PVWatts system model, and Sandia and CEC module models, but not in the simple efficiency model. The loss parameters are built in to the models, not inputs that you can change.

The Sandia module model uses a set of six empirically determined polynomial coefficients (b0-b5) in loss factor calculations as a function of incidence angle. Because the coefficients are determined using outdoor testing of a specific module, they represent the particular glass characteristics and/or coatings present. For details, see King et al, 2004, Photovoltaic Array Performance Model ( PDF 1.8 MB )

The PVWatts model uses the same polynomial-based transmittance equations as the Sandia model, but uses values for b0-b5 that represent a "generic" glass module cover.

The CEC module model uses the Fresnel equation for non-reflected unpolarized irradiation to calculate a loss value. It assumes a typical glass cover thickness of 2 mm and the standard refractive index for glass. It calculates the transmittance for each radiation component independently using correlations from Duffie and Beckman (Solar Engineering of Thermal Processes) to calculate an effective incidence angle for diffuse and ground reflected irradiance. For details, see De Soto, 2004, Improvement and Validation of a Model for Photovoltaic Array Performance ( PDF 1.8 MB )

Best regards,
Paul.

I would like to adjust the "polynomial coefficients (b0-b5)" to reflect our empirical data. Is that possible? If not, is there software that allows changing the angle dependent losses? Thanks.

Paul Ahrens

Dear Paul,
I'm not sure which set of coefficients you mean, but will assume you mean those for the Sandia photovoltaic module model. It is always helpful to mention the name of the performance or financial model you are asking about when you post a question.
SAM reads the parameters for the Sandia module model (and for other component models) from a library file. You can use SAM's library editor to create a custom library with your own parameters. Using the library editor takes a little time to learn, but you can use the description in SAM's Help system to get started. Here's a link to the topic on the copy we posted on the SAM website: SAM Help - Libraries .
For the next version of SAM, we are changing the library file format so that it is a simple CSV format that is easier to work with than the current .samlib format.
Best regards,

Paul.