Option to Simulate Front- and Rear-side Global Plane-of-array Effective and Incident Irradiances at User-specified Point Locations in the PV Array

[kjs55]

Hello,

Every large-scale project in the USA is commissioned with an ASTM E2848 capacity test. The derived Measured Power is compared to the derived Modeled Power. Global plane-of-array POA irradiance Gpoa is essential to this multiple linear regression-based test. Measured Gpoa needs to be comparable to Modeled Gpoa (apples-to-apples).

Measured Gpoa (front-side facing):

• Sometimes installed entirely unshaded
• Sometimes installed on the (vertical) bottom of the frontmost array
• Sometimes installed on the (vertical) bottom of an array shaded by the row in front
• Sometimes installed on the (vertical) top of the frontmost array
• Sometimes installed on the (vertical) top of an array shaded by the row in front
• Sometimes installed in the (vertical) middle of the frontmost array
• Sometimes installed in the (vertical) middle of an array shaded by the row in front
• Sometimes installed on the (horizontal) edge of the frontmost array
• Sometimes installed on the (horizontal) edge of an array shaded by the row in front
• Sometimes installed in the (horizontal) middle of the frontmost array
• Sometimes installed in the (horizontal) middle of an array shaded by the row in front
• Sometimes installed at a fixed tilt
• Sometimes installed on a tracker (tracking)
• Sometimes measured with pyranometer(s)
• Sometimes measured with reference cell(s)

Measured Global POA (rear-side facing):

• Same options as above except rear-side facing
• Replace "frontmost" with "rearmost"

What is the comparable Modeled Gpoa to use for each of the above scenarios? For example, for rear-side Gpoa that is entirely unshaded, do we have to run a separate simulation w/ the structural shading loss factor removed?

My suggestion is this: Allow us to simulate the measured front- and rear-side global POA effective and incident irradiances at specific (user-specified) point locations in the array for the purposes of running the capacity test. Report it as separate columns of PVsyst other than the typical GlobEff, GlobInc, & GlobBak (e.g., Geff_SimMes, Ginc_SimMes, Geff_SimMesBak, & Ginc_SimMesBak). Other options than "_SimMes": _MesPt, _PtMes, _Sensor, etc.

Finally, sometimes one or more pyranometer(s) is used for measurements; other times, one or more reference cell(s) is used. So, I think it’s important to simulate both front- and rear-side global POA effective and incident irradiances, respectively, at the given user-specified (front- & rear-side facing) locations in the array.

Thanks.

Edited May 6, 2022 by kjs55
Add "point location" to title; underline suggestion

[kjs55]

P.S. In addition to the removal of structural shading factor SSF in the above example, and in the context of trying to use the existing PVsyst to simulate pyranometers' or reference cells' measured irradiance at one or more given point location(s) in the array, I am also wondering about the "Height above ground" input for Bifacial systems. Namely, do you simulate a different one when trying to determine the correct GlobBak to match the measured case of a rear-facing pyranometer/reference cell in the field at a given vertical (ignoring horizontal, e.g., edge effects for now) point location in the array? Are there other PVsyst inputs besides SSF and Height Above Ground to consider in the context of performing apples-to-apples, regression-based Measured vs. Modeled Power capacity tests (essentially comparing the Pmes vs. Gmes VS. Pmod vs. Gmod correlations)? (I suppose Height Above Ground also matters for front-facing, esp. if there is a row in front of the sensor, so I suppose my previous post already in some sense asked this question, just less explicitly.) Thanks again!

[itaisuez]

Hi,

I just want to re-iterate the request for what @kjs55 is proposing and at least get a response as to if this is possible and/or reasonable to implement for the developers at PVSyst.  It's an important issue. 

Specifically, we are looking for a way to execute a PVsyst simulation with a user-defined incident Global POA set of values.  We could even add the tracker angle (or fixed tilt angle) as an input if that helps, so you can use the algorithm to properly calculate Global POA effective via the amount of light that is reflected, etc.  It is impossible for the time being to take measured data and decompose it to GHI/DNI/DHI values, then re-run it through the simulation assuming that the transposition will work to re-create the POA measured values.

If the answer to the above is affirmative, the next question is can we also do it with rearside measured POA (or even hourly albedo measured values).

Kindly let us know, thanks.

Itai

 

 

[kjs55]

A useful starting point would be a clear specification of the measured GPI (see my comprehensive list of real-world options above). Namely, please instruct us clearly on how to properly measure GPI data for application/import into PVsyst.

I thought at one point I'd seen it written that the measured GPI data must be entirely unshaded (within geometrical/physical limits), but I just looked through the PVsyst Help Menu, and I can no longer (easily) find this instruction (so perhaps it never existed).

Considerations:
- Monofacial (front-side irradiance only) vs. bifacial (rear-side irradiance also)
- Pyranometer (dome-shaped) vs. reference cell (flat-glass)
- Shaded vs. unshaded

Keywords (search terms):
ASCII text file
Back-side irradiance
Bifacial
Custom file import tool
Dome-shaped
Front-side irradiance
Global Plane-of-array POA Irradiance GPI
Global Tilted Irradiance GTI
Incidence Angle Modifier IAM
Measured data
Measured GPI
Meteorological meteo weather data
Monofacial
PVsyst
Pyranometer
Rear-side irradiance
Reference cell
Shadings
Sunny-side down
Sunny-side up
Unshaded

[itaisuez]

@Administrator, please take a look at the questions/proposal and provide a response when possible.  I understand after digging deeper into this topic that a complication arises because when PVSyst has calculated global incident plane-of-array irradiance it is already decomposed into diffuse and beam.  You need this to properly account for shading losses. 

This poses a challenge in importing measured global incident POA, because you would then have to find some way to perform the decomposition in the plane-of-array instead of just using horizontal-based decomposition models.  I'm imagining your solution to this dilemma is to say "just don't import POA irradiance for any reason".  Import measured or simulated GHI/DHI (or DNI) and let PVSyst do the work.  The problem is that the actual measured POA irradiance is not typically directly reflected by calculated values using GHI/DHI measurements.  Can PVSyst please provide further guidance and clarification?

@jeffn

With best regards,

Itai

[Agnes Bridel]

Hello ,

Thank you for the suggestions. In PVsyst, POA data can be imported through " Databases -> Custom file". The conversion protocol can be found at the help page "Meteo Database > Import custom meteo files > Conversion protocol". For the moment, the measured data must be unshaded, therefore limiting to only front-side irradiance.
@itaisuez, We would suggest to import the measured data directly and the matching is then done in PVsyst with less than 1% difference (i.e. comparing the imported POA data and the PVsyst variable GlobInc. Make sure that you use the Hay model and the circumsolar in diffuse: at the time of import through the general PVsyst settings, and at the time of simulation through the project settings.

As for the suggestions, including pyranometers in the simulation scene is indeed in out long-term roadmap.

 

Best regards,

Agnes Bridel

[itaisuez]

Thanks Agnes,

Really appreciate the explanation as well as the update.  I have another comment concerning the Notes on the Meteo sources, I'll post that in the appropriate topic area.

With warm regards,

 

Itai