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electrical:solar:shading [2024/11/08 15:52]
frater_secessus [so panel designers try to limit damage] 		electrical:solar:shading [2024/11/08 16:30] (current)
frater_secessus [so users try different panel configurations]
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   * since rated wattage is 100w, the cells //must// be 5.55Isc (amps at max power);  100w / 18v = 5.55A.   * since rated wattage is 100w, the cells //must// be 5.55Isc (amps at max power);  100w / 18v = 5.55A.
  
-For reasons discussed below the string of 36 cells will be subdivided into substrings of cells in series. 3 strings of 12 cells is common.  2 strings of 18 cells or 4 strings of 9 cells are also possible, etc.  So we can think of this panel, electrically +For reasons discussed below the string of 36 cells will be subdivided into substrings of cells in series. 2 strings of 18 cells is common((for cost reasons)).  So we can think of this panel
  
-    ############ +    ######### 
-    ############ +    ######### 
-    ############+    ######### 
 +    #########
          
-as+straightened out but electrically identical
  
-    ############ ############ ############   +    ################## ##################
          
  
Line 69: Line 70:
  
 But diodes for each cell would be more expensive in components, assembly, and design.  The compromise most manufacturers make is to have a bypass diode at the substring level.  Our panel is now like this: But diodes for each cell would be more expensive in components, assembly, and design.  The compromise most manufacturers make is to have a bypass diode at the substring level.  Our panel is now like this:
 +             =                 =   
 +    ################## ##################
  
 +...where the equal sign ("=") is a bypass path when needed to prevent damage. 
  
  
-Real-world example: Simplest example:  we have a string of three cells in series.  In full sun all three contribute 0.5v and the total string voltage is 1.5v.  In partial shade one of them drops top 0.2v.  It gets bypassed so this string is now 1.0v.  Without the diode the string would be 1.2v (0.5v + 0.5v + 0.2v) //until the shaded cell failed from overheating//. +Real-world example: we have two substrings, each with 18 cells ins series.  In full sun they will make 18v (2s substrings x 18 cells x 0.5v).   
 + 
 + 
 + 
 + 
 + 
 +In partial shade one of the cells in the 2nd substring are affected.   
 +   
 +             =                 =    
 +    ################## ####0###00######## 
 +   
 +If their were no diodes we would have 9v from the 1st string and 7.5v from the 2nd string for a total of 16.5v. But the shaded cells in the 2nd string would overheat. So it is bypassed and now the panel makes **9v**.   
 +   
 +**Oops**:  9v is not high enough to charge a 12v house bank.  We get nothing.   
 + 
 +**Thought experiment:**  what if we had 3 substrings of 12 cells but the same cells were shaded?  We'd lose one substring and still have 2 6v substrings for 12.0v.  That might be enough to charge a 12v bank that was really low.  If we had 4 substrings of 9 cells and lost one substring we'd have 3 4.5v substrings for 13.5v.  That might be high enough to be actually useful. 
 +   
  
-==== which has observable effects on output ==== 
  
  
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 ==== so users try different panel configurations ==== ==== so users try different panel configurations ====
  
 +=== parallel ===
  
  
 +Let's start using both our panels in parallel
  
 +     #########
 +     #########
 +     #########
 +     #########
 +     
 +     #########
 +     #########
 +     #########
 +     #########
 +     
 +which, unwound and with bypass diodes, would be
  
 +     
 +             =                 =   
 +    ################## ################## 
 +    
 +             =                 =   
 +    ################## ##################     
 +     
  
-**Before we begin:** solar panels are "current sources";  their voltage pops up into the normal range in any kind of meaningful light(>= 20% [[electrical:solar:output|insolation]]) but current will suffer.  +if we have the same partial shading as before
-Partial shading in this context means:+
  
-  * light is falling on the panel 
-  * but not evenly on the panel -- it is different on some cells 
  
-To prevent power from rushing into the shaded string and overheating them, panels have bypass diodes between the strings.  Basically the shaded strings get cut off, electrically speaking, to protect them. In a perfect world each cell would be protect by a lossless, costless diode but that's not possible yet.  :-)+      
 +             =                 =    
 +    ################## ################## 
 +     
 +             =                 =    
 +    ################## ####0###00######## 
 +      
 +      
 +     
  
  
 +To prevent voltage backing up into a panel from a neighboring panel (or battery bank at night) each //panel// typically has a //blocking diode//. So the complete picture is:
  
 +             =                 =   
 +    ################## ################## = 
 +    
 +             =                 =   
 +    ################## ####0###00######## =     
  
 +Note:  in most panels the blocking and bypass diodes are identical parts, just installed in different places, in different orientations, and for different purposes.
  
 +
 +
 +     
 +=== series ===   
 +    
 +             =                 =                   =                  = 
 +    ################## ################## = ################## ####0###00######## =
 +
 +
 +
 +
 +
 +    
 +    
 +    
 +    
 +    
 ===== further reading ===== ===== further reading =====
  
electrical/solar/shading.1731099139.txt.gz · Last modified: 2024/11/08 15:52 by frater_secessus

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