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electrical:solar:shading

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electrical:solar:shading [2024/11/08 15:18]
frater_secessus [how panels work] 		electrical:solar:shading [2024/11/08 16:30] (current)
frater_secessus [so users try different panel configurations]
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 This section will assume an array of 2x generic 100w 18Vmp panels This section will assume an array of 2x generic 100w 18Vmp panels
  
-* the panel is made up of 36 0.5Vmp cells in series for 18v +  * the panel is made up of 36 0.5Vmp cells in series for 18v; 36 x 0.5v = 18v. 
-* since rated wattage is 100w, the cells must be 5.55Isc (amps at max power)+  * 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. 2 strings of 18 cells is common((for cost reasons)).  So we can think of this panel 
 + 
 +    ######### 
 +    ######### 
 +    ######### 
 +    ######### 
 +     
 +straightened out but electrically identical 
 + 
 +    ################## ################## 
 +    
  
 ==== how the work normally ==== ==== how the work normally ====
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 ==== partial shade is bad for panels ==== ==== partial shade is bad for panels ====
  
-* power moves in the direction of higher voltage to lower voltage.   +  * power moves in the direction of higher voltage to lower voltage.   
-* when shaded sufficiently any given cell's voltage will drop off sharply(("collapse")) +  * when shaded sufficiently any given cell's voltage will drop off sharply(("collapse")) 
-* so power from neighboring cells moves //into// the cell instead of out of it +  * so power from neighboring cells moves //into// the cell instead of out of it 
-* the cell converts this power into heat which can damage or destroy the cell, or catch the panel on fire+  * the cell converts this power into heat which can damage or destroy the cell, or catch the panel on fire
  
-When the entire panel is lit or shaded evenly power is reduced but no individual cell is running at a lower voltage than its neighbor.  So overcast days or dense tree canopies are not a problem.  +When the entire panel is lit or shaded evenly the harvest is reduced but **no individual cell is running at a lower voltage than its neighbor**.  So overcast days or dense tree canopies are not a problem.  And when people talk about "shade" problems it usually means "partial shade".
  
  
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 ==== so panel designers try to limit damage ==== ==== so panel designers try to limit damage ====
  
-We can't stop the cell voltage from collapsing in shade so we need a way to remove it, electrically, from the circuit.  +We can't stop the cell voltage from collapsing in shade so we need a way to remove it, electrically, from the circuit.  This is typically done with **diodes**. They are, in effect, electrical valves that allow power to flow in only the intended direction.    
 + 
 +Right now we are concerned with the interatctions of panel internals so we are talking about //bypass diodes//.  They can //bypass// a voltage-collapsed cell or substring and avoids the overheating problem.  
 + 
 +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//.  
 + 
 +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: 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
 +
 +     
 +             =                 =   
 +    ################## ################## 
 +    
 +             =                 =   
 +    ################## ##################     
 +     
 +
 +if we have the same partial shading as before
 +
 +
 +     
 +             =                 =   
 +    ################## ##################
 +    
 +             =                 =   
 +    ################## ####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.
  
-**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.  
-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.  :-)+      
 +=== series ===    
 +     
 +             =                 =                   =                  =  
 +    ################## ################## = ################## ####0###00######## =
  
  
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 +    
 +    
 +    
 +    
 +    
 ===== further reading ===== ===== further reading =====
  
electrical/solar/shading.1731097115.txt.gz · Last modified: 2024/11/08 15:18 by frater_secessus

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