How to design an Off-Grid (hybrid) solar PV system using SMA solar inverters

Information required:
Load =?
Backup time =?
Connection type = single phase or three phase?

 

Design steps:

Step 1

How to decide the battery bank?
Lets say my backup time requirement is 2 hours Then battery capacity (Ah) can be calculated using the following formula
Ah = (Total Load x Backup hours)/(DOD x P.F x SV)
Where

• DOD - Depth of discharge
• P.F - Power factor
• SV - System voltage

So

Ah = (5000 x 2)/(0.6 x 0.8 x 48) = 434.02 Ah approximately equal to 400Ah
Now my battery bank will consist of 8 batteries having 200Ah @ 48V capacity each.

Step 2

How to decide quantity of solar panels?
a) No. of solar panels for running the load directly through solar grid.
Load = 5kW
No. of panels = 5kW x 1.4  --> 7 kW --> 7/235Wp = 30 panels (235 Wp each)
Peak solar hours in worst month of the year = 4 hours (considering the design is for Lahore Pakistan)
so solar PV grid will suport the 5kW load for 4 hours a day. Before and after these four hours, the solar PV grid will give low yield. This low yield will then be compansated by Battery bank and public grid.
b) No. of solar panels for charging the battery bank
400 Ah @ 48V battery bank acts as a 1kW load for solar grid and public grid. (real time observation)
So for a 1kW load we would need 1000 x 1.4 = 1400Wp of panels
No. of panels = 1400/235 = 6 panels
c) Total no. of solar panels
36 + 6 = 42 panels

Step 3

How to choose the inverters?
For choosing the inverters we need to know about following specs of the inverters from its data sheet

• Maximum output power
• Maximum input DC voltage
• Maximum input DC current
• Maximum input Power

Lets say I want to use SMA inverters (Sunny Boy and Sunny Island) then I would choose following inverters
SMC 9000 TL (Solar Inverter)
SI 5048 x 2 (Grid Inverter)

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Circuit Diagram for 5kW Off-Grid Solar PV System
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Circuit Diagram for 5kW Off-Grid Solar PV System

Circuit Diagram for 5kW Off-Grid (Hybrid) Solar PV System

Precautions:

• Use common ground.
• Do not touch DC cables or DC breakers while ON.
• Do not overload the system.
• Use appropriate size of cable.
• In case of any warning on Sunny boy or Sunny island display, contact the installer.

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5kW Off-Grid Solar PV System Design - Hybrid

Project Requirements:

• Total load = 5kW
• Battery backup time = 1 hour
• Primary source = Solar PV
• Secondary source = Grid
• Tertiary source = Battery Bank

System Design:

1. Solar Panels:

Wattage of single panel = 235Wp

No. of solar panels = (Total load x 1.4) / 235

                                = (5000 x 1.4) / 235
                                = 30 (rounded value)
     2.  Inverters:
Company = SMA
Two types of inverters (built in charge controllers) will be used.
a). Sunny Boy     b). Sunny Island

a) Sunny Boy:
Requirement for sunny boy = 7000W*
* 5000W x 1.4 = 7000Wp
Suitable sunny boy (according to the requirement) = SMC 7000 TL

SMC 7000 TL

b). Sunny Island
Requirement for sunny island = 5000W*
* Sunny island runs the load, so its rating depends upon total load.
Suitable sunny island (according to the requirement) = SI 5048

SI 5048

     3. Solar PV array design:
According to the data sheet of SMC 7000 TL, the input DC voltage capacity of SMC 7000 TL is 750V. It has four inputs of solar PV arrays. The input voltage of one array must not be greater than 90 % of 750V. 
Since the open circuit voltage of one solar PV panel is around 34V. Connecting 15 solar panels in series will give us (15*34) 510V DC which is smaller than 675V (90% of 750V).
So we can make two solar PV arrays, having 15 panels each. All the 15 solar panels will be connected in series in one array.

     4. Battery Bank:
Battery rating is defined in terms of Ampere Hours (Ah). We can find out the Ah for battery bank according to the load and backup time.
Ah = (Load x backup time) / (DOD x PF x System Voltage)
Now
   load = 5000
   backup time = 1 hour
   DOD (depth of discharge) = 0.6
   PF (power factor) = 0.8
   System voltage = 48V
So
Ah = (5000 x 1) / (0.6 x 0.8 x 48)
      =  217.01
So now we need 217Ah @ 48V for battery bank.
Rating of single battery = 200Ah @ 12 V
Connecting four batteries of above rating, in series, will give us 200Ah @ 48V.

System Topology:

Circuit Diagram:

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a) HOW TO DESIGN AN OFF-GRID (HYBRID) SOLAR PV SYSTEM USING SMA SOLAR INVERTERS
b) CIRCUIT DIAGRAM FOR 5KW OFF-GRID SOLAR PV SYSTEM

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