pvlib & pvfactors

[herbook]

Hi,

What is the best practice for getting pvlib and pvfactors to work together? Should a specific version of pvfactors be used in order for the pvlib.bifacial.pvfactors_timeseries() to work, or does pvfactors necessitate a specific version of pvlib? Does it matter from which channel pvlib is installed? Based on the newest commit in the pvfactors repo, it seems like support for pvlib 0.9 would have been recently added.

[campanelli-sunpower]

We recently updated pvfactors master branch to support pvlib 0.9.0, which is the new minimum. However, I have not had time to release this change. I will schedule that for this week, and I will do my best to get to it.

[herbook]

Thanks @kanderso-nrel for the offer, will have to pass for now :)

[campanelli-sunpower]

My understanding is that array-like types include scalars, because they can be converted into arrays.
https://numpy.org/devdocs/reference/typing.html#numpy.typing.ArrayLike

[herbook]

I also noticed that the bifacial function seems to be sensitive for low (0 - 0.2 degrees) solar elevation angles, resulting in very large POA radiation values (even though zero DNI or a QC-controlled DNI was used as input). This would indicate that DHI can cause such results by itself.

What is a bit weird is also the fact that this behavior was only present for the fixed bifacial system (not the tracker bifacial). This might be explained by the fact AOI of the fixed system seems to be > 90 degrees during these instances (DHI>0 with AOI>90 and very low solar elevation).

Are you aware of some kind of best practices for handling/mitigating this kind of feature in the used radiation model (or is this potentially a bug)?

Here is some example input/output data from those instances (4 systems in total; 2 bifacial (_bf) and 2 monofacial, both fixed (_fixed) and tracking(_tracking)). The bifacial "POA" used here is the sum of front and backside radiation (see below for further info):

                              ghi         dni        dhi    albedo  \

2018-08-06 05:30:00+00:00 108.876663 120.166194 41.084473 0.077310
2019-04-30 05:30:00+00:00 67.985382 121.462202 49.014168 0.077686
2020-08-06 05:30:00+00:00 76.779442 113.025291 49.431770 0.077313

                           t_air    ws  apparent_elevation   aoi_fixed  \

2018-08-06 05:30:00+00:00 20.088486 3.65 0.076681 103.666160
2019-04-30 05:30:00+00:00 11.485260 0.65 0.090620 102.041153
2020-08-06 05:30:00+00:00 18.541336 2.75 0.000269 103.670848

                       aoi_tracking  poa_fixed  poa_effective_fixed  \

2018-08-06 05:30:00+00:00 47.611945 23.857340 22.731225
2019-04-30 05:30:00+00:00 45.201210 14.230449 13.555857
2020-08-06 05:30:00+00:00 47.564653 12.357039 11.756316

                       cell_temp_fixed  dc_out_fixed  \

2018-08-06 05:30:00+00:00 20.648318 0.023127
2019-04-30 05:30:00+00:00 11.893097 0.014267
2020-08-06 05:30:00+00:00 18.848631 0.012046

                       dc_out_w_losses_fixed  ac_out_fixed  **poa_fixed_bf**  \

2018-08-06 05:30:00+00:00 0.022216 0.016097 2.370895e+03
2019-04-30 05:30:00+00:00 0.013704 0.007656 4.140150e+03
2020-08-06 05:30:00+00:00 0.011571 0.005540 1.665632e+06

                       poa_effective_fixed_bf  cell_temp_fixed_bf  \

2018-08-06 05:30:00+00:00 2.252907e+03 75.573878
2019-04-30 05:30:00+00:00 3.933518e+03 129.827826
2020-08-06 05:30:00+00:00 1.582351e+06 41379.095493

                       dc_out_fixed_bf  dc_out_w_losses_fixed_bf  \

2018-08-06 05:30:00+00:00 1.797154 1.726339
2019-04-30 05:30:00+00:00 2.284149 2.194145
2020-08-06 05:30:00+00:00 -260164.440633 -249912.923616

                       ac_out_fixed_bf  poa_tracking  \

2018-08-06 05:30:00+00:00 0.97 201.975665
2019-04-30 05:30:00+00:00 0.97 351.079605
2020-08-06 05:30:00+00:00 0.00 347.756024

                       poa_effective_tracking  cell_temp_tracking  \

2018-08-06 05:30:00+00:00 195.808716 24.910933
2019-04-30 05:30:00+00:00 339.016142 21.684792
2020-08-06 05:30:00+00:00 335.341587 27.306719

                       dc_out_tracking  dc_out_w_losses_tracking  \

2018-08-06 05:30:00+00:00 0.195878 0.188160
2019-04-30 05:30:00+00:00 0.343512 0.329976
2020-08-06 05:30:00+00:00 0.332247 0.319156

                       ac_out_tracking  poa_tracking_bf  \

2018-08-06 05:30:00+00:00 0.180185 220.094967
2019-04-30 05:30:00+00:00 0.319703 364.593943
2020-08-06 05:30:00+00:00 0.309081 360.023586

                       poa_effective_tracking_bf  cell_temp_tracking_bf  \

2018-08-06 05:30:00+00:00 213.212541 25.339560
2019-04-30 05:30:00+00:00 353.429569 22.118430
2020-08-06 05:30:00+00:00 349.001703 27.663777

                       dc_out_tracking_bf  dc_out_w_losses_tracking_bf  \

2018-08-06 05:30:00+00:00 0.212923 0.204533
2019-04-30 05:30:00+00:00 0.357503 0.343416
2020-08-06 05:30:00+00:00 0.345283 0.331678

                       ac_out_tracking_bf

2018-08-06 05:30:00+00:00 0.196326
2019-04-30 05:30:00+00:00 0.332891
2020-08-06 05:30:00+00:00 0.321373

[herbook]

Other args:
slope_tilt = 36
slope_azimuth = 180
axis_azimuth = 180
gcr = 2 / 7
module_height = 1.5
pvrow_width = 1
n_pvrows = 1
index_observed_pvrow = 0
rho_front_pvrow = 0.03
rho_back_pvrow = 0.05
horizon_band_angle = 15

And here's more samples with DNI == 0. So, it seems that DHI is enough to produce those absurd values. Also here, AOI > 90 degrees. And yes, I am aware that the radiation components are not consistent with each other, however, the module does not utilize GHI. One easy solution would be just to force radiation to zero in these solar elevations, but it obviously is not an optimal solution in the long run.

BTW, what diffuse model is the bifacial script using? Perez or isotropic?

                              ghi  dni        dhi  albedo  t_air    ws  \

2018-08-06 05:30:00+00:00 108.879997 0.0 41.080002 0.08 20.09 3.65
2019-04-30 05:30:00+00:00 67.989998 0.0 49.009998 0.08 11.49 0.65
2019-08-06 05:30:00+00:00 50.680000 0.0 43.500000 0.08 19.46 4.25
2020-03-24 06:30:00+00:00 127.199997 0.0 41.320000 0.07 11.51 3.80
2020-04-29 05:30:00+00:00 32.880001 0.0 25.629999 0.08 12.82 5.85
2020-08-05 05:30:00+00:00 75.510002 0.0 47.139999 0.08 17.09 1.30
2020-08-06 05:30:00+00:00 76.779999 0.0 49.430000 0.08 18.54 2.75

                       apparent_elevation  aoi_fixed  aoi_tracking  \

2018-08-06 05:30:00+00:00 0.08 103.67 47.61
2019-04-30 05:30:00+00:00 0.09 102.04 45.20
2019-08-06 05:30:00+00:00 0.11 103.67 47.64
2020-03-24 06:30:00+00:00 0.01 91.61 31.69
2020-04-29 05:30:00+00:00 0.04 102.06 45.19
2020-08-05 05:30:00+00:00 0.15 103.66 47.66
2020-08-06 05:30:00+00:00 0.00 103.67 47.56

                       poa_fixed  poa_effective_fixed  cell_temp_fixed  \

2018-08-06 05:30:00+00:00 16.29 15.49 20.47
2019-04-30 05:30:00+00:00 14.26 13.59 11.89
2019-08-06 05:30:00+00:00 15.69 14.97 19.81
2020-03-24 06:30:00+00:00 16.48 15.66 11.89
2020-04-29 05:30:00+00:00 15.35 14.66 13.14
2020-08-05 05:30:00+00:00 15.35 14.62 17.51
2020-08-06 05:30:00+00:00 12.33 11.73 18.85

                       dc_out_fixed  dc_out_w_losses_fixed  ac_out_fixed  \

2018-08-06 05:30:00+00:00 0.02 0.02 0.01
2019-04-30 05:30:00+00:00 0.01 0.01 0.01
2019-08-06 05:30:00+00:00 0.02 0.01 0.01
2020-03-24 06:30:00+00:00 0.02 0.02 0.01
2020-04-29 05:30:00+00:00 0.02 0.01 0.01
2020-08-05 05:30:00+00:00 0.02 0.01 0.01
2020-08-06 05:30:00+00:00 0.01 0.01 0.01

                       poa_fixed_bf  poa_effective_fixed_bf  \

2018-08-06 05:30:00+00:00 4116.98 3911.47
2019-04-30 05:30:00+00:00 4406.87 4186.82
2019-08-06 05:30:00+00:00 3174.01 3015.64
2020-03-24 06:30:00+00:00 3319.58 3153.94
2020-04-29 05:30:00+00:00 2526.42 2400.44
2020-08-05 05:30:00+00:00 2680.48 2546.77
2020-08-06 05:30:00+00:00 1800648.64 1710616.46

                       cell_temp_fixed_bf  dc_out_fixed_bf  \

2018-08-06 05:30:00+00:00 116.42 2.48
2019-04-30 05:30:00+00:00 137.45 2.30
2019-08-06 05:30:00+00:00 90.86 2.22
2020-03-24 06:30:00+00:00 88.42 2.35
2020-04-29 05:30:00+00:00 64.05 2.03
2020-08-05 05:30:00+00:00 90.43 1.88
2020-08-06 05:30:00+00:00 44731.78 -304194.01

                       dc_out_w_losses_fixed_bf  ac_out_fixed_bf  \

2018-08-06 05:30:00+00:00 2.38 0.97
2019-04-30 05:30:00+00:00 2.21 0.97
2019-08-06 05:30:00+00:00 2.13 0.97
2020-03-24 06:30:00+00:00 2.26 0.97
2020-04-29 05:30:00+00:00 1.95 0.97
2020-08-05 05:30:00+00:00 1.81 0.97
2020-08-06 05:30:00+00:00 -292207.55 0.00

                       poa_tracking  poa_effective_tracking  \

2018-08-06 05:30:00+00:00 197.40 189.04
2019-04-30 05:30:00+00:00 286.39 274.31
2019-08-06 05:30:00+00:00 216.65 207.51
2020-03-24 06:30:00+00:00 247.14 236.68
2020-04-29 05:30:00+00:00 78.51 75.19
2020-08-05 05:30:00+00:00 250.72 240.13
2020-08-06 05:30:00+00:00 295.45 282.98

                       cell_temp_tracking  dc_out_tracking  \

2018-08-06 05:30:00+00:00 24.74 0.19
2019-04-30 05:30:00+00:00 19.74 0.28
2019-08-06 05:30:00+00:00 24.37 0.21
2020-03-24 06:30:00+00:00 17.28 0.24
2020-04-29 05:30:00+00:00 14.43 0.08
2020-08-05 05:30:00+00:00 24.01 0.24
2020-08-06 05:30:00+00:00 25.94 0.28

                       dc_out_w_losses_tracking  ac_out_tracking  \

2018-08-06 05:30:00+00:00 0.18 0.17
2019-04-30 05:30:00+00:00 0.27 0.26
2019-08-06 05:30:00+00:00 0.20 0.19
2020-03-24 06:30:00+00:00 0.23 0.23
2020-04-29 05:30:00+00:00 0.08 0.07
2020-08-05 05:30:00+00:00 0.23 0.22
2020-08-06 05:30:00+00:00 0.27 0.26

                       poa_tracking_bf  poa_effective_tracking_bf  \

2018-08-06 05:30:00+00:00 201.63 195.41
2019-04-30 05:30:00+00:00 288.74 279.93
2019-08-06 05:30:00+00:00 221.86 215.04
2020-03-24 06:30:00+00:00 249.19 241.55
2020-04-29 05:30:00+00:00 86.72 83.96
2020-08-05 05:30:00+00:00 254.48 246.69
2020-08-06 05:30:00+00:00 295.59 286.58

                       cell_temp_tracking_bf  dc_out_tracking_bf  \

2018-08-06 05:30:00+00:00 24.90 0.20
2019-04-30 05:30:00+00:00 19.91 0.29
2019-08-06 05:30:00+00:00 24.55 0.22
2020-03-24 06:30:00+00:00 17.40 0.25
2020-04-29 05:30:00+00:00 14.61 0.09
2020-08-05 05:30:00+00:00 24.19 0.25
2020-08-06 05:30:00+00:00 26.03 0.29

                       dc_out_w_losses_tracking_bf  ac_out_tracking_bf

2018-08-06 05:30:00+00:00 0.19 0.18
2019-04-30 05:30:00+00:00 0.27 0.27
2019-08-06 05:30:00+00:00 0.21 0.20
2020-03-24 06:30:00+00:00 0.24 0.23
2020-04-29 05:30:00+00:00 0.08 0.08
2020-08-05 05:30:00+00:00 0.24 0.23
2020-08-06 05:30:00+00:00 0.27 0.26

[herbook]

As indicated, the issue seems to be with the backside radiation calculation:

[cwhanse]

@herbook it appears this issue has evolved from "how to get pvlib and pvfactors to work together" to diagnosing weird backside irradiance from pvfactors. Has the original issue been addressed?

Because all irradiance calculations are in pvfactors (pvlib only provides an interface to pvfactors), if the original issue has been addressed, I suggest moving the discussion of weird output to the pvfactors github site.

[herbook]

@cwhanse Yes, that might be the best way to proceed from here. The reason for the original issue has been solved, however, I don't think any actions have been imposed based on that finding (not sure if any actions will be taken either).

[herbook]

I should clarify that, for the example figure shown above, a timestamp alignment issue was present (solar angles had been calculated to wrong timestamps, i.e., time shifting had been implemented incorrectly to a preprocessed dataset). Nonetheless, overestimation in specific conditions is still present (see random examples below), and should somehow be explicitly mitigated, preferably already in pvfactors. In my opinion, these kind of issues are always a potential risk when implementing implicit methods for calculating complex things.

In case interested, ERA5 data was used in these examples.

[campanelli-sunpower]

@herbook Thanks for posting these results of your investigations. Even though there is a performance cost, I have (in other code) resorted to checking the results of my implicit function calculations, e.g., using scipy.optimize.newton here.

I am still rather unfamiliar with view factors models in general, and I would prefer to hear @anomam's input before putting together a plan to address these issues.

[anomam]

hi @herbook , thank you for using pvfactors and for posting your issues.
As suggested by @cwhanse , would you mind opening an issue in https://github.com/SunPower/pvfactors and posting a small notebook to reproduce the problems you're seeing? This would help me tremendously to start investigating, as I have limited time on my weekends to work on pvfactors.