bifacial PV system modeling

[cwhanse]

implement NREL's rear-surface irradiance model for regular, fixed tilt arrays of bifacial PV models.

[adriesse]

Is this your practice problem, Cliff?!?

[cwhanse]

uh no this will be a big lift :) #423 is a good practice problem.

[cwhanse]

Mapping out how a fixed tilt bifacial PV system would be represented, here's a summary of my approach:

1. System description : add attributes to PVSystem for array racking geometry: number of array racks, distance between racks, height of rack above the ground, number of module rows on each rack, orientation of modules on the rack (landscape or portrait), spacing between modules (probably the rack area occupied by a module, rather than a distance between module edges), fraction of rear-surface module area 'obscured' by racking (this will be a derate on the rear irradiance), DC optimizers (module or cell level). Perhaps we should use a new class as a child of PVSystem to contain these attributes?
2. A new database for bifacial module parameters that support only PVWatts. Bifacial modules have unique characteristics that aren't present in the SAPM or CEC datasets. We have only measured a few bifacial modules so we're mostly providing a pattern to follow, not a dataset. Bifacial parameters include: bifaciality (ratio of rear to front power), temperature coefficient of power, rated power, length and width dimension, transparency factor (the fraction of front side irradiance that can pass through the module), cell array (number along length, number along width).
3. Irradiance functions. Two primary functions, one for rearside irradiance, and one for total effective irradiance. The rearside irradiance returns the rearside irradiance for each row of cells along the vertical dimension of the rack. This is a port and cleanup of the NREL bifacialvf code.
4. A total irradiance function which returns the per-cell-row irradiance, front + rear, modified for 1) bifaciality factor, 2) soiling, 3) reflection, 4) rear-side shading, and 5) spectral effects.
5. A bifacial power function which calculates per-cellrow power from total irradiance and temperature, using PVWatts - I believe this is already pvwatts_dc.
6. A string power function which operates on per-cellrow power, DC optimizer settings, and string geometry (which I haven't figured out) to return power per string of modules. I think this is in place of scale_voltage_current_power but I'm not clear on this point.

Interested in your thoughts.

[adriesse]

Getting the irradiance on different surfaces right seems to be the biggest challenge here. With the expectation of having different levels of irradiance on different cells (or rows of cells) some kind of mismatch model might be helpful too.

[wholmgren]

Regarding each numbered item:

1. Many of those racking parameters could be useful for (unimplemented) features for standard PV systems.
2. Easy to add a database to the pvlib/data directory.
3. I'm not familiar with the bifacialvf code, but that generally makes sense. Thinking dimensions... D_out = D_in x N_rows ?
4. Are the soiling and spectral effects unique to the bifacial nature of the system? I can imagine that they are.
5. Yes, it sounds like you could call pvwatts_dc on your D_in x N_rows array.
6. Maybe you need some combination of summing along the output array and using scale_voltage_current_power?

I do think a new subclass of PVSystem would be useful for standardizing the process. It would override the PVSystem.get_irradiance method and implement new methods specific to the bifacial functions.

pvsystem.py is already a big module. I wonder if we'd be better off putting most of the new stuff in e.g. bifacial.py.

ModelChain can probably accommodate this with very few changes.

[cdeline]

Thanks everyone - are you planning to work off the existing Python code in the github.com/NREL/bifacialvf repository?

[cwhanse]

Yes, but substantially refactored to integrate with the rest of pvlib, for transparency and hopefully less maintenance risk.

[wholmgren]

@cdeline thanks for sharing -- I didn't realize it was on github. Dare I ask what the license is? I am not a lawyer, but if the license is not BSD 3 compatible then I think we need to stick to working off of papers or similar instead of from code.

[adriesse]

Cool, I wasn't aware of it either. I suspect the pvlib version will look different enough that copyright will not be a problem.

[cdeline]

Will, we are using BSD-3 licenses as well, and received an open-source designation from DOE so there should be no problems with porting the software into pvlib. I don’t yet have our license document finalized and uploaded, but your question prodded me to make some progress on this! Chris Deline National Renewable Energy Laboratory (303) 384-6359 From: Will Holmgren [mailto:notifications@github.com] Sent: Thursday, February 15, 2018 2:56 PM To: pvlib/pvlib-python <pvlib-python@noreply.github.com> Cc: Deline, Chris <Chris.Deline@nrel.gov>; Mention <mention@noreply.github.com> Subject: Re: [pvlib/pvlib-python] bifacial PV system modeling (#421) @cdeline<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fcdeline&data=02%7C01%7Cchris.deline%40nrel.gov%7C3dc304a8b0ff400fc42008d574bee0a1%7Ca0f29d7e28cd4f5484427885aee7c080%7C0%7C0%7C636543285526107361&sdata=7JHfGlPgDZiQszjJDECO5DgoQbA%2B4pJjRbj8zOSJdl8%3D&reserved=0> thanks for sharing -- I didn't realize it was on github. Dare I ask what the license is? I am not a lawyer, but if the license is not BSD 3 compatible then I think we need to stick to working off of papers or similar instead of from code. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fpvlib%2Fpvlib-python%2Fissues%2F421%23issuecomment-366074666&data=02%7C01%7Cchris.deline%40nrel.gov%7C3dc304a8b0ff400fc42008d574bee0a1%7Ca0f29d7e28cd4f5484427885aee7c080%7C0%7C0%7C636543285526107361&sdata=IbkCboZxeOhf3D0PpEy4vy2y3qqbM0OCl9hRQdgOBZs%3D&reserved=0>, or mute the thread<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fnotifications%2Funsubscribe-auth%2FAWKtOHc99AFMJ-SmhyUbeyvEvqHozjJkks5tVKflgaJpZM4SCe-g&data=02%7C01%7Cchris.deline%40nrel.gov%7C3dc304a8b0ff400fc42008d574bee0a1%7Ca0f29d7e28cd4f5484427885aee7c080%7C0%7C0%7C636543285526107361&sdata=XmZMUgnNEIOVItObu7OnUOCxsrkl32%2F%2FPxyFMtQSF3I%3D&reserved=0>.

[cwhanse]

I should have asked first, thanks @wholmgren for bringing this up. @cdeline good to here. We'll keep prominent comments maintaining attribution and the supporting reference papers.

[wholmgren]

@cdeline thanks for clarifying!

[dberrian]

@cwhanse is right. Updating bifacial PV system modeling to PVlib will be a great job. I have been working on this topic recently, I can contribute and there is a lot to learn about bifacial systems and how it differs from standard PV. The issue of bifacial energy yield prediction is of interest nowadays.