Mega Watt Design - IV

In all my previous posts I have laid special emphasis on Site Engineering and how it is accomplished, this post talks of the two methodologies of actual system sizing and the advantages of one over the other that is the Series System and the Parallel System.  So your Solar Mega Watt site is ready after you have accomplished site engineering, how did you size the system and design it :

1.  Series System

When current sources that is the modules themselves are wired in series, the voltages add up
Series circuits were the desired choice earlier

• Because most PV modules deliver power at voltages that range from 25-35 volts (the max power voltage, Vmp, for crystalline silicon modules) to 50 - 100Volts(Vmp for thin films).
• Most Inverters on the other hand require inbound voltages between 240-480 volts.
• Therefore designers must wire up in series so that the voltages add to high enough level for inverter.
• Most crystalline modules are wired in series, 8-12 at a time. Most thin film modules are series wired in groups of 5-6, known as strings.

Interestingly the upward limit of a string size is determined by the open circuit voltage (Voc) of the PV modules.  This value also must fall within the range of the inverter, and normally standard inverters with national governing body approval, upper voltage limit is normally 600 volts.  So for these inverters, the sum of the string's Voc must be under 600 volts.
Finally all strings are wired into a combiner box, which creates a parallel connection, this sums up the current while maintaining the same voltage.

For a 1 MW solar energy series system, the sizing would be as below :

Modules and strings
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• 13,334 modules of 75W each
• At 5 modules per string, the system will contain 2,667 strings
• Each string will require cabling from the PV modules back to the combiner box.  Normally this distance is about 150 feet so we can estimate the wire length based on number of strings: 2667 strings x 2 wires per string x 150 feet per wire run = 8,00,000 feet of wire.

Combiner Boxes
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• The 2667 strings will also require combiner boxes: using 24 pole combiner boxes; the system would require 112 combiner boxes (2667/24 strings per combiner = 111.1 combiner boxes)

Labor
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• Labor associated with terminating the strings in the combiner boxes : 8 person hours to mount and fully install a 24 pole combiner box(including setting and fusing the string terminations).  Therefore there are 896 person hours of electrical labor embedded in the system.
• The cost of these components can show us the cost of the electrical BOS : assume Rs 18/foot for the wire of #10 gauge copper wire, Rs 60,000 for each combiner box and Rs 3900/hour for electrical installation labor.  The total electrical system cost(not counting the PV modules or inverter but everything in between) is Rs 2,59,58,400 or Rs 25.98/watt-peak.

2.   Parallel

When current sources are wired in parallel, the current adds up.

• Generally not an option because the voltage of the PV module is too low for the inverter to handle
• Parallel system design requires a new component to boost the voltage from the levels delivered by the modules(anywhere between 18V to 100V) to the voltages required by the inverter.  Because Voltage Boost units voltage output matches the inverters ideal input voltage, the units can be wired in parallel directly to the inverter.
• With Parallel connection, the current adds, rather than the voltage.  In other words, each cable can be used to its full current carrying capacity, therefore more modules can be connected on a single cable run, which reduces system cost by reducing wiring and combiner box content.

Module and Strings

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• In parallel systems, the number of PV modules on a single cable run is no longer determined by the voltages of the modules and inverter, but by the ampacity of the wire used.
• Each wire has a different characteristic and a #10 gauge copper wire can carry 30 amps of current in standard conditions.
• Each Solar Module delivers 75 watts of power, at assume 68.2 volts (Vmp).  The voltage is then boosted to assume 300V.  At 300V the current contribution of each solar module is only 0.25amps (75W/300V = 0.25A).
• Thus a #10 gauge wire with a 30amp limit can serve upto 120 modules per cable run.

With 13,334 modules and 120 modules per cable run, the Parallel system contains only 112 cable runs (13,334/120 = 111.1).

Wire content is small, there are 112 parallel cable runs, each leading to a combiner box.  Assuming 100 feet as the average distance from the end of the PV to combiner box, this system will only require 22,400 feet of #10 gauge wire

Combiner Box Count

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The combiner box count is far lower in a parallel system.  Only five 24 pole combiner boxes are needed for the 112 cable system.  This also reduces labor requirements.

Labor

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The total cost of a parallel system is thus far lower than the cost for a series system.  With the same global cost assumptions the total cost of the solution is only Rs 919200 or Rs 0.9 /watt-peak(Wp).

As is obvious from the above two calculations, a voltage booster can greatly enhance the economics of a solar energy system in parallel making it more efficient than the existing series stringed system.  

If you have any questions or comments I am always available at info@freshenergy.co or maninder.s.kumar@gmail.com