Consideration of Ground Reflection on Front Side in PR

[CRW]

I have a few questions concerning the way the ground reflection on the front side is considered in the loss diagram and in the calculation of the Performance Ratio:

 

• The Performance Ratio is basically the resulting specific yield divided by the irradiation on the front side. Why does an increase of the irradiation due to the “Ground reflection on front side” change the PR? For systems with steep module tilt angles the difference in PR values would be significant. I have attached an example for a vertical Agri-PV system.

 

• In the field, shouldn’t the “Ground reflection on front side” also be measured by an irradiation sensor? If yes, there would be a big difference in PR, when comparing a “real” PR with a PVsyst simulation.

 

• At the moment, ground reflection is listed after soiling and IAM factor. Shouldn’t it be considered before, as these losses affect the reflected light as well?
  For the near shading irradiance losses: are they considered for the calculation of the ground reflection? In the mentioned example, where the ground reflection is over 13%, the influence of these losses on the reflected light would change the results significantly.

 

• If find it also problematic that the results for such a system depends on the bifacial simulation and the simulation results are inconsistent with a monofacial simulation. If one would want to simulate for example a monofacial vertical system (e.g. façade), the results would differ. Do you think that in the foreseeable future it will be possible to have a better consistency in PVsyst regarding monofacial/bifacial simulation?

 

Thank you very much for your help!

[André Mermoud]

First question: In usual systems, (not vertical), the "Ground reflexion on the front side" is very low. Now the PR in a Vertical situation with bi-facial doesn't make much sense. Therefore  we don't worry about such inaccuracies.

Second question: in "normal" systems with several rows, this contribution corresponds to the reflexion of the terrain between rows.  In you particular case, you have probably one only row, or a very big pitch value.  This is a limit case for applying the vertical rows model. Don't focus on the PR (which is an artificial indicator meant for usual systems, with well normalized incident irradiaton), and take the real energy yield into consideration. By the way the PR for bifacial models is not well established and is subject to many discussions. If you want to use the PR for a contract, you should carefully define the way of defining it with your contractor. 

For the 3rd question, the IAM effect is indeed included in the "Ground reflexion on the front side" calculation, as for usual systems, this is a very sensitive correction (this reflexion occurs at very high incident angle on the front side of the collectors).

Last question: you can indeed perform a similar monofacial simulation, by setting the bifaciality factor of the PV module at a null value.