[BLDG-SIM] effect of partial shading on PV performance

[stvgates at pacbell.net]

eQUEST calculates the performance of a PV system assuming NO shading on the array.  It also outputs a shading report to give you an idea of how much shading the array receives over the course of the year, so that you can make a judgment regarding the impact of shading on PV performance.  eQUEST does not attempt to assess the impact of shading on PV performance for the reason you describe.  

PV degradation due to shading depends not only on how many cells in a series are shaded, but also whether that row of cells is in parallel with other rows, and how much sunlight those rows are receiving, and the characteristics of the inverter.  The interaction is complex and I am not an expert on PV systems; however my understanding is that several unshaded rows of cells can impress enough voltage across the partially-shaded rows to wipe out most of their output.

To simulate this degradation in detail would require defining the spatial geometry of each module in the array relative to the shading surface, the series/parallel relationships of the modules in the array, diodes that allow shaded cells in series to be bypassed, a description of the inverter characteristics, etc.  I am not aware of any programs that model PV systems in this detail, but would be interested in knowing whether any exist.
 
  ----- Original Message ----- 
  From: Smita Gupta 
  To: BLDG-SIM at GARD.COM 
  Sent: Friday, October 01, 2004 3:49 PM
  Subject: [BLDG-SIM] effect of partial shading on PV performance


  Hi,

  A PV related query again. This time I would like to find out if anyone can suggest an algorithm or tool that accounts for the effect of partial shading on an array. From what I understand, EnergyPlus and eQuest algorithms factor the effect of shading through adjusting the effective irradiation incident on the surface to which the array is attached. Please correct me if I am wrong in understanding this. 

  But the effect of partial shading is quite different depending on the location of the actual cell in series. The shaded cell depending on its location in the series can degrade the performance a lot more than proportional to the % of area shaded. Can anyone confirm if any tool or algorithm takes into account his fact.

  Any help and advise regarding this issue is appreciated.

  Thank you,



  PS- 
  eQuest output for PV performance states that
  "NOTE: PARTIAL SHADING OF A PV ARRAY CAN SEVERELY DEGRADE ITS PERFORMANCE, BUT IS NOT TAKEN INTO ACCOUNT SEE REPORT LS-SH FOR SHADING INFORMATION"

  Smita Gupta
  Energy Analyst, LEED AP

  Buildings & Appliances Office
  California Energy Commission
  1516 Ninth St. MS-25
  Sacramento, CA 95814
  Phone: (916) 657-4034
  Fax: (916) 654-4304


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