Difference between revisions of "PWM charge controller sizing and selection"

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[[Category:PV source and charge controller sizing and selection]]
 
[[Category:PV source and charge controller sizing and selection]]
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A [[Charge controller#Charge controller types|PWM charge controller]] is rated to operate at a particular [[System voltage parameter|system voltage]] and maximum current. [[PV module|PV modules]] designed to work at the system voltage must be connected in parallel in order to achieve the [[PV source sizing|minimum PV source size]] and the charge controller therefore must be sized to handle this amount of current. If the current rating of a PWM charge controller is exceeded, it can be damaged or destroyed.  
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A [[Charge controller#Charge controller types|PWM charge controller]] is rated to operate at a particular [[DC system voltage]] and maximum current. [[PV module|PV modules]] designed to work at the DC system voltage must be connected in parallel PV source circuits in order to achieve the [[PV source sizing|minimum PV source size]] and the charge controller therefore must be sized to handle this amount of current. If the current rating of a PWM charge controller is exceeded, it can be damaged or destroyed.  
  
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====Step 1: Determine PV module power rating====
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====Step 1: Determine PV module power rating and series configuration====
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The chosen [[System voltage parameter|system voltage]] limits the choices of modules and configurations that are possible:
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The chosen [[DC system voltage]] limits the choices of modules and configurations that are possible with a PWM charge controller. Below is a table of the number of modules that can be connected in series for each PV source circuit depending upon the DC system voltage.
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12 volt system = 1 × 36-cell module per string.
 
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24 volt system = 1 × 72-cell module per string or 2 x 36-cell modules in series per string.
 
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48 volt system = 2 × 72-cell modules in series per string or 4 x 36-cell modules in series per string.
 
  
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36-cell modules are typically below 150W. 72-cell modules are typically 300W-400W.
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{| class="wikitable" border=1
 +
![[DC system voltage]]
 +
!36 cell module
 +
!60 cell module
 +
!72 cell module
 +
|-
 +
|12 V
 +
|1
 +
|—
 +
|—
 +
|-
 +
|24 V
 +
|2
 +
|—
 +
|1
 +
|-
 +
|48 V
 +
|4
 +
|—
 +
|2
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|PV module power rating
 +
! style="text-align:left;"| =
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Number of modules in series
 +
! style="text-align:left;"| =
 +
|}
  
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====Step 2: Calculate minimum number of PV modules====
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====Step 2: Determine proposed module configuration====
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This calculation will give a ''minimum'' number of modules. The final array size should always be larger than this value, thus if the the result of the calculation is a decimal, it should be rounded up. Different modules sizes and configurations can be explored to find the optimal design.
+
This calculation will give a ''minimum'' number of PV modules - the result should always be rounded up. Different modules sizes and configurations can be explored to find the optimal design.
 
{| class="wikitable" border=1 style="width: 80%;"
 
{| class="wikitable" border=1 style="width: 80%;"
 
! style="width: 20%"|Minimum number of PV modules
 
! style="width: 20%"|Minimum number of PV modules
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! style="text-align:left;"| = [[PV source sizing|minimum PV source size]] ÷ PV module power rating (Step 1)
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! style="text-align:left;"| = [[Minimum PV source size|Minimum PV source size]] ÷ PV module power rating (Step 1)
 +
|}
 +
 
 +
The final number of PV modules should always be larger than this value - the result should always be rounded up.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Minimum number of PV source circuits
 +
! style="text-align:left;"| = Minimum number of PV modules ÷ Number of modules in series (Step 1)
 +
|}
 +
 
 +
====Step 3: Verify excess production====
 +
During periods of poor weather or low solar resource, an off-grid PV system is designed to discharge the battery to a certain [[Energy storage sizing and selection#Step 1: Determine depth of discharge parameter|depth of discharge]] which can leave the [[Energy storage|energy storage system]] depleted. It is important that the energy storage system is brought back up to a full [[Energy storage#State of charge (SoC)|state of charge]] in short period of time or the [[Energy storage#Cycle life|cycle life]] of the batteries will be reduced. The PV array therefore must be sized to generate sufficient excess energy, while continuing to meet all of the power needs from the [[Load evaluation|load evaluation]].
 +
 
 +
This step is only necessary for systems that will be used heavily for consecutive days (most systems). If a system is used lightly or infrequently then it will have ample time to recharge during days when it is inactive and this step is not necessary.
 +
 
 +
For a system that is used heavily for consecutive days, it is recommended that the array be sufficiently sized to reach a full state of charge within 7 days or that the system incorporate a generator to ensure adequate charging.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Proposed PV source low insolation production
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! style="text-align:left;"| = PV module power rating (Step 1) × Minimum number of PV modules (Step 2) × [[Minimum PV source size#Step 1: Deteremine PV source loss parameters|Total PV source loss parameter]] × [[Load and solar resource comparison|Design daily insolation]] × [[Minimum PV source size#Step 3: Charge controller efficiency parameter|Charge controller efficiency parameter]] × [[Minimum PV source size#Step 4: Energy storage efficiency parameter|Energy storage efficiency parameter]]
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Daily excess production in Ah
 +
! style="text-align:left;"| = (Proposed PV source low insolation production - [[Load and solar resource comparison|Design daily watt-hours required ]]) ÷ [[DC system voltage]]
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Ah used at full depth of discharge
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! style="text-align:left;"| = [[Energy storage sizing and selection|Final Ah capacity]] × [[Energy storage sizing and selection#Step 1: Determine depth of discharge parameter|Depth of discharge parameter]]
 
|}
 
|}
  
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====Step 3: Determine minimum current rating of charge controller====
 
−
This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The ISC rating of the PV module can be found on its specifications sheet.
 
 
{| class="wikitable" border=1 style="width: 80%;"
 
{| class="wikitable" border=1 style="width: 80%;"
−
! style="width: 20%"|Minimum number of PV modules
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! style="width: 20%"|Time to reach full state of charge
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! style="text-align:left;"| = minimum number of PV modules (from Step 2) × [[PV module|ISC rating]] of chosen module (from Step 2) × [[Irradiance safety parameter|irradiance safety parameter]]
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! style="text-align:left;"| = Ah used at full depth of discharge ÷ Daily excess production in Ah
 +
|}
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 +
====Step 4: Verify charging current====
 +
Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range that depends upon battery type. Flooded lead acid and gel batteries should be charged with current that is between .05-.13 (5-13%) of their C/20 rating.<ref name="trojanpaper"> Trojan Battery Company - User's Guide https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf</ref> AGM batteries can should be charged with a current that is between .05-.2 (5-20%) of their C/20 rating.<ref name="trojanpaper"/> If a system uses many loads during the day, this will limit the available charging current for the energy storage system and should be taken into account by increasing the PV source size.  Most designs should have a charge rate between 5-10% - closer to 10% if the system is used heavily during the day. It is necessary to consult the manual or manufacturer for recommended maximum and minimum charging currents.
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 +
These calculations are performed with the Ah rating of the total energy storage system.
 +
 
 +
It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Available charging current
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! style="text-align:left;"| = [[PV module#Standard test conditions|Maximum power current (Imp)]] × Minimum number of PV source circuits (Step 2)
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|}
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 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Percentage of C/20 rate
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! style="text-align:left;"| = Available charging current ÷ [[Energy storage sizing and selection|Final Ah capacity]]
 +
|}
 +
 
 +
If the number of PV modules does not meet the recommendations outlined above, increasing the PV source in size should be considered.
 +
 
 +
====Step 5: Determine final number of PV modules====
 +
Determine a final number of modules and a PV source configuration that can meet the requirements of Step 1, Step 2, Step 3, and Step 4.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Final number of PV modules in series
 +
! style="text-align:left;"| =
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Final number of PV source circuits
 +
! style="text-align:left;"| =
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|PV source power rating
 +
! style="text-align:left;"| = PV module power rating (Step 1) × Final number of PV modules in series × Final number of PV source circuits
 +
|}
 +
 
 +
====Step 6: Total PV source current====
 +
This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The Isc rating of the PV module can be found on its specifications sheet.
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Total PV source current
 +
! style="text-align:left;"| = Final number of PV source circuits (Step 5) × [[PV module|Isc rating]] of chosen module (Step 1) × [[Irradiance safety parameter|Irradiance safety parameter]]
 +
|}
 +
 
 +
====Step 7: Select a charge controller====
 +
A single charge controller is the simplest and cheapest option, but for larger systems multiple charge controllers often are used in parallel. The final chosen charge controller should:
 +
#Function at the [[DC system voltage]].
 +
#The charge controller(s) should have a total current rating that is larger than the minimum current rating (Step 6). Common charge controller current ratings: 4.5 A, 5 A, 6 A, 10 A, 12 A, 15 A, 20 A, 25 A, 30 A, 35 A, 40 A, 45 A, 50 A, 55 A, 60 A.
 +
#Have a maximum PV source power rating in watts that is lower than the power rating of the PV source. Verify that the maximum PV source power rating is greater than the final PV source power rating. If it is not, the charge controller size needs to be increased.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|PV source and charge controller compatability
 +
! style="text-align:left;"| = Final PV source power rating must be ''less than'' the maximum PV source power rating of the charge controller
 +
|}
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Final charge controller current rating
 +
! style="text-align:left;"| =
 +
|}
 +
 
 +
The result of the following equation should always be rounded up.
 +
 
 +
{| class="wikitable" border=1 style="width: 80%;"
 +
! style="width: 20%"|Number of charge controllers
 +
! style="text-align:left;"| = Total PV source current (Step 6) ÷ Chosen charge controller current rating
 +
|}
  
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====Step 4: Select a charge controller====
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==Notes/references==
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The final chosen charge controller should function at the [[System voltage parameter|system voltage]] and have a current rating that is larger than the minimum current rating calcualted in step 3.
+
<references/>

Latest revision as of 13:06, 19 January 2021

A PWM charge controller is rated to operate at a particular DC system voltage and maximum current. PV modules designed to work at the DC system voltage must be connected in parallel PV source circuits in order to achieve the minimum PV source size and the charge controller therefore must be sized to handle this amount of current. If the current rating of a PWM charge controller is exceeded, it can be damaged or destroyed.

Step 1: Determine PV module power rating and series configuration

The chosen DC system voltage limits the choices of modules and configurations that are possible with a PWM charge controller. Below is a table of the number of modules that can be connected in series for each PV source circuit depending upon the DC system voltage.

DC system voltage 36 cell module 60 cell module 72 cell module
12 V 1 — —
24 V 2 — 1
48 V 4 — 2
PV module power rating =
Number of modules in series =

Step 2: Determine proposed module configuration

This calculation will give a minimum number of PV modules - the result should always be rounded up. Different modules sizes and configurations can be explored to find the optimal design.

Minimum number of PV modules = Minimum PV source size ÷ PV module power rating (Step 1)

The final number of PV modules should always be larger than this value - the result should always be rounded up.

Minimum number of PV source circuits = Minimum number of PV modules ÷ Number of modules in series (Step 1)

Step 3: Verify excess production

During periods of poor weather or low solar resource, an off-grid PV system is designed to discharge the battery to a certain depth of discharge which can leave the energy storage system depleted. It is important that the energy storage system is brought back up to a full state of charge in short period of time or the cycle life of the batteries will be reduced. The PV array therefore must be sized to generate sufficient excess energy, while continuing to meet all of the power needs from the load evaluation.

This step is only necessary for systems that will be used heavily for consecutive days (most systems). If a system is used lightly or infrequently then it will have ample time to recharge during days when it is inactive and this step is not necessary.

For a system that is used heavily for consecutive days, it is recommended that the array be sufficiently sized to reach a full state of charge within 7 days or that the system incorporate a generator to ensure adequate charging.

Proposed PV source low insolation production = PV module power rating (Step 1) × Minimum number of PV modules (Step 2) × Total PV source loss parameter × Design daily insolation × Charge controller efficiency parameter × Energy storage efficiency parameter
Daily excess production in Ah = (Proposed PV source low insolation production - Design daily watt-hours required ) ÷ DC system voltage
Ah used at full depth of discharge = Final Ah capacity × Depth of discharge parameter
Time to reach full state of charge = Ah used at full depth of discharge ÷ Daily excess production in Ah

Step 4: Verify charging current

Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range that depends upon battery type. Flooded lead acid and gel batteries should be charged with current that is between .05-.13 (5-13%) of their C/20 rating.[1] AGM batteries can should be charged with a current that is between .05-.2 (5-20%) of their C/20 rating.[1] If a system uses many loads during the day, this will limit the available charging current for the energy storage system and should be taken into account by increasing the PV source size. Most designs should have a charge rate between 5-10% - closer to 10% if the system is used heavily during the day. It is necessary to consult the manual or manufacturer for recommended maximum and minimum charging currents.

These calculations are performed with the Ah rating of the total energy storage system.

It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.

Available charging current = Maximum power current (Imp) × Minimum number of PV source circuits (Step 2)
Percentage of C/20 rate = Available charging current ÷ Final Ah capacity

If the number of PV modules does not meet the recommendations outlined above, increasing the PV source in size should be considered.

Step 5: Determine final number of PV modules

Determine a final number of modules and a PV source configuration that can meet the requirements of Step 1, Step 2, Step 3, and Step 4.

Final number of PV modules in series =
Final number of PV source circuits =
PV source power rating = PV module power rating (Step 1) × Final number of PV modules in series × Final number of PV source circuits

Step 6: Total PV source current

This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The Isc rating of the PV module can be found on its specifications sheet.

Total PV source current = Final number of PV source circuits (Step 5) × Isc rating of chosen module (Step 1) × Irradiance safety parameter

Step 7: Select a charge controller

A single charge controller is the simplest and cheapest option, but for larger systems multiple charge controllers often are used in parallel. The final chosen charge controller should:

  1. Function at the DC system voltage.
  2. The charge controller(s) should have a total current rating that is larger than the minimum current rating (Step 6). Common charge controller current ratings: 4.5 A, 5 A, 6 A, 10 A, 12 A, 15 A, 20 A, 25 A, 30 A, 35 A, 40 A, 45 A, 50 A, 55 A, 60 A.
  3. Have a maximum PV source power rating in watts that is lower than the power rating of the PV source. Verify that the maximum PV source power rating is greater than the final PV source power rating. If it is not, the charge controller size needs to be increased.
PV source and charge controller compatability = Final PV source power rating must be less than the maximum PV source power rating of the charge controller
Final charge controller current rating =

The result of the following equation should always be rounded up.

Number of charge controllers = Total PV source current (Step 6) ÷ Chosen charge controller current rating

Notes/references

  1. ↑ 1.0 1.1 Trojan Battery Company - User's Guide https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
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