anti-reflective glass, low-light, & LID

In my experimental trials with SAM's CEC Performance Model with database (DB) and with user-entered specs (UES), I can't see where the model is taking account of anti-reflective glass, different responses to low-light, and LID (light-induced degradation.

For example, when I model any of the 4 SunPower X22-370 modules either with DB or UES, I always get Nominal POA losses of -2.796% for "reflection (IAM)." But I get exactly the same Reflection losses for Hyundi S290MI panels, which have no anti-reflective glass and much worse absorbtion of low-light (including their spectral response), and much worse LID (light-induced degradation). How can this be right?

On the other hand, the Nominal DC Energy "Module" losses for SunPower appear as -3.866, while for the Hyundais it is -6.459% (2.953% worse).

I leave the other inputs for these 2 panels identical for comparison purposes, so there is no need to consider shading, soiling, mismatch, diodes, etc.

I read the thread between Paul Gilman and pstuder of 6/13/16 and 6/14/16, which confused me further. It said the Module Loss includes IAM (but so does Reflection, according to the Losses report), spectral-related losses, temp-related losses, and "losses associated with converting light to electricity" (which sounds like everything).

If Module Loss is the place where differences of A/R glass, low-light (including IAM), and LID are being reflected, that would mean that the only difference between the best and the worst panels in this respect was 2.953%. That doesn't seem remotely accurate to me.

My reading suggests that the difference between top-quality A/R glass and plain glass is about 3%. The combination of high-transmission glass and cells that respond optimally to low-light and red-spectrum conditions is up to 1%. That differences in LID can easily be 2% - 3% from the best to the worst panels. These add up to 6% - 7% at the very least, and there are other differences in IAM and other losses between these SP and Hyundai panels as well.

Comparing that minimum of 6%-7% with the 2.953% difference in Module losses mentioned above leaves a minimal discrepancy of 3%-4%, and I'm not sure that covers all the differences.

However, when I lower the TCP on the Hyundai panel from its spec of -.45%/degree C to the -.29%/degree C of the SP panel, Hyundai's Module loss drops to BELOW the SP one: it goes down to -3.481%.

That suggests that all (and then some) of the difference between SP's & Hyundai's Module Loss is accounted for merely by the TCP, leaving no differences remaining to account for A/R glass, low light, and LID.

I'm confused.

By the way, I also checked the resulting annual energy figures as I went along, and they correspond pretty closely to these differences in Module Loss. This reassures me that the model isn't accounting for these other differences somewhere else.

My conclusion is that the CEC models, both DB and UES, do not account for these light factors at all. They should account for them in the Reflection (IAM) loss, but that is stuck at -2.796%. They do not, despite the forum thread, account for any of them in the Module Loss, which only reflects the TCP, and that is a temperature-variant factor, unlike the light factors I've been talking about.

This suggests that the SP panels are probably at least 6% - 7% better than the CEC models report, which is quite a large discrepancy.

Perhaps I've missed something. I've tried to search the instructions and forum, but haven't found any answers.

Or maybe this is a known problem that just hasn't yet been addressed.

Other than this, I very much appreciate the terrific work your team has done on both PV Watts and SAM.

Dear Dennis,

The categories shown in the loss diagram on the Losses tab of the Results page are designed to help you quickly see when there are problems with the input data in your analysis. Some of the loss categories, such as "Module mismatch" and "Diodes and connections" are inputs that you specify on the Losses input page, and others, such as "Module" and "Inverter efficiency" are calculated by the performance model.

For SAM's CEC Module performance model (a variation of the DeSoto 5-parameter single-diode model), the irradiance lost as light passes through the module cover is calculated using a fixed set of parameters as described in Equations 10.14-10.16 in the SAM Photovoltaic Reference Manual ( PDF 1.8 MB ). The CEC model does not use different module cover parameters for different types of modules because that data is not available from the CEC module database .

As of SAM 2018.11.11, SAM reports a "Reflection (IAM)" loss in the loss diagram that was included in the "Module" category in older versions of SAM. This loss depends on the angle of incidence (AOI), so it will change with different array orientations and tracking options, but not with different module types.

To update the earlier discussion on module loss that you reference, The "Module" loss category in SAM 2018.11.11 accounts for the following:

• Spectral-related losses.
• Losses associated with converting light to electricity.
• Temperature-related losses.

You can account for light-induced degradation using the Nameplate loss category on the Losses input page. For degradation of array output over the life of the system, you can use the inputs on the Lifetime input page. That page is only available when you use a financial model and the system output is modeled over a multi-year analysis period.

Best regards,
Paul.

Dear Dennis,

Your last statement is correct. The source of SAM's built-in assumptions for n and K are from Duffie and Beckman's Solar Engineering of Thermal Processes text book. In the fourth edition:

n = 1.526 is from Table 5.1.1
K = 4 is from Section 5.2

The values of n, K, and L were chosen to give reasonable results for a wide range of module cover types for estimates of the total annual electrical output of a photovoltaic system given that the CEC module database does have values for those parameters.

The "reflection (IAM)" loss is the reduction in incident irradiance that results from the application of the incidence angle modifier for beam, sky-diffuse and ground-reflected irradiance in Equation 10.10 of the SAM photovoltaic reference manual.

My definition of module loss is a bit vague because the SAM calculates the loss value in a separate step from the single-diode model calculations. You can see the calculations starting around Line 2288 of the source code at github.com/NREL/ssc/blob/develop/ssc/cmod_pvsamv1.cpp . The module loss is annual_dc_module_loss_percent, calculated in Line 2349.

SAM does not model the spectral response of photovoltaic cells to light.

I do not have an opinion about cover losses from the different modules you mention.

Best regards,
Paul.