Bifacial Simulation and Effective irradiance on the rear side

[smsas]

Hi,

I am simulating bifacial production using the latest PVsyst version.

Seeing the results, I am wondering if there's any kind of validation on the results given by the model?

PVsyst consistently reports irradiance gains of 3 to 6% on the rear side of fixed systems (20-30% albedo). I have find that this is not congruent with the measured data I have read in different empirical studies:

 

• Modelling of bifacial gain for stand-alone and in-field installed bifacial (http://www.sciencedirect.com/science/article/pii/S1876610216304520)
  
  Field Example of Bifacial Gain at Sandia (https://pvpmc.sandia.gov/pv-research/bifacial-pv-project/outdoor-bifacial-pv-performance-data/field-example-of-bifacial-gain-at-sandia/)

 

I would appreciate any comment on the accuracy of the model.

Thank you very much.

Best regards,

Sebastian.

[André Mermoud]

PVsyst proposes a model, established according to well-specified hypothesis, either for the irradiance availability on the ground and for the Form factor.

It gives detailed results about the ground availability and the rear side irradiance, along the whole year according to your meteo data.

However we don't have any validation.

In fact we didn't got any results registered in good conditions with a realistic PV system up to now.

Now most of people studying this subject work with one or 2 modules on an "illimited" reflective area. This is the case of both papers you are referencing.

Even if the simulation of the Universities of Konstanz and al. are done with 5 rows, you can observe that:

- the extreme modules behave better due to the larger"seen" reflective area (on 4 edges)

- the chosen inter-row is very large (if modules are 1 m wide, the "pitch" of PVsyst is 1 * cos(25°) + 2.5 m = 3.4 m, so that the GCR is 0.29.

- the chosen albedo of 0.5 seems irrealistic in real conditions.

Their only measurements are for one pair of module only, at 1.2 m altitude.

They don't tell us how they have measured the albedo coefficient taken in the model, therefore it is difficult to have a full confidence in their validation.

The PVsyst model tries to represents a realistic PV system, i.e. with the "unlimited sheds" hypothesis.

Please see the fig 5 of the publication: with a reasonable (usual pitch (1m between rows, i.e. a GCR of 0.52), the gain is of the order of 8%, not so far from the results of PVsyst before shading and mismatch losses.

[baohua]

Hi Andre,

I also met the same problem with Sebastian, the simulated bifacial gain was significant lower than some actual test results. for example, bificality=0.9, albedo=0.7 distance from ground to module=1m, finally got the bifacial gain =11.4%. compared to the acutal test results, including utility scal system. please see http://hokkaidopvgs.jp/wp-content/uploads/2017/01/41c7223bd0545a0f4dbe8fbccbc00572.pdf , 1.25MW capacity system in japan, bifacial gain about 20%. some other test results are shown a big bifacial gain in actual test. ISC also summaried some test results, at least more than 10% bifacial gian.

measured results summary

 

I think the bifaical model should be improved a little, to make it more aline with the actual result .

Thanks,

Baohua