Standard assumptions for utility-scale bifacial solar

Hello,

I often turn to NREL's ATB to find default assumptions for different technologies. However, as I work on modeling a "representative" utility-scale PV project that uses bifacial modules, it seems that the ATB has not yet really delved into these assumptions (or at least I haven't been able to find them).

Using PVWatts (specifically the PvWatts7 module in PySAM), it seems that the main assumptions that might change when using bifacial panels would be the following (listed with the standard assumption according to ATB 2021 or SAM):

• bifaciality (default = 0)
• DC-to-AC ratio (inverter loading ratio) (NREL ATB assumption: 1.34)
• ground coverage ratio (default 0.4)

I'm wondering if NREL has any publications that have more appropriate assumptions for modeling representative utility-scale PV.

With regard to bifaciality, it seems that the conservative estimate would be to use 0.65, although I've seen that there is a difference between different bifacial technologies. Is there a more reasonable estimate for today's technologies? I've looked at a few data sheets for bifacial modules, although they do not always list this parameter.

With regards to the DC-to-AC ratio, the 2021 ATB notes that the 1.34 ILR is not likely appropriate for bifacial modules: "We recognize that ILR is likely to change in the future, particularly with the adoption of bifacial modules, and to also be highly dependent on location. However, allowing for this change would require the optimization of ILR and CAPEX by resource bin and year, causing a range of prices, independent of other regional factors. We believe this would create less transparency and more confusion to the impact of technology changes to these individual LCOE categories." ( atb.nrel.gov/electricity/2021/utility-scale_pv ). Some other publications I've seen have suggested that an ILR closer to 1.1 or 1.15 eliminates most of the clipping losses for bifacial setups, but I am not sure if these ILRs are representative.

With regard to GCR, I would imagine that bifacial solar plants might have a different optimum value than 0.4, but I have not yet found much literature on the topic. I've seen some industry publications that suggest that a GCR of 0.33 - 0.4 would be appropriate for bifacial ( www.pv-magazine.com/2018/12/03/bifacial-tracker-pitch-an-important-design-factor-in-maximizing-energy-gain/ ), but these are several years old, and I'm not sure if they are representative.

Any resources that you could help point me toward for these assumptions would be helpful.

Greg

Hi Greg,

I'm not sure I have much to add to your research.

The ATB list of references is a good place to look for additional information:

atb.nrel.gov/electricity/2021/references

As you note, industry publications and websites provide insight beyond these studies from different points of view. A web search of good keywords like "bifacial" and "pv ilr" can turn those up.

Best regards,
Paul.