PVCOMMERCIAL
Table 4: Cost Comparison Summary
Finally, we have labour. As above, we will focus on the labour associated with terminating the strings in the combiner boxes – and here a conservative estimate is eight 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 labour embedded in this system, not counting the hand connections.
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The cost of these components can show us the cost of the electrical balance of system. We will assume $0.30-/foot for the wire, based on #10 gauge copper wire, $1,000 for each combiner box, and $65/hour for electrical installation labour in the USA. Given these assumptions, the total electrical system cost (not counting the PV modules or inverter, but everything in between) is $432,640, or $0.433/watt-peak.
String count in a Parallel PV System We will now look at the same reference system design (1 megawatt, First Solar 75W modules, standard inverter), but assume a parallel system design. First, we need to determine the string count (or “cable run” count) of the system. This is where parallel architecture makes a huge difference.
In parallel systems, the number of PV modules on a single cable run is no longer determined by the voltage of the modules and the voltage of the
In parallel systems, the number of PV modules on a single cable run is no longer determined by the voltage of the modules and the voltage of the inverter but instead by the ampacity of the wire used
inverter, as in the example above, but instead by the ampacity of the wire used. Each wire has different characteristics, and other factors such as temperature come into play, but #10 AWG copper wire can typically carry 30 amps in standard conditions.
Returning to our reference design, each First Solar module delivers 75 watts of power, at 68.2 volts under maximum power. The voltage is then boosted – we will assume to 300V. (Most off-the- shelf inverters, with voltage ranges of 300V to 600V, are most efficient at lower inbound voltages.)
At 300V, the current contribution of each First Solar module is only 0.25 amps. (75W / 300V = 0.25A.) Thus, a #10-gauge wire with a 30-amp limit can handle up to 120 modules per cable run. (Note that we call these “cable runs” rather than “strings” because the voltage does not add as it does in a series system. However, this is essentially a semantic difference.) With 13,334 modules, and 120 modules per cable run, the Parallel system contains only 112 cable runs. (13,334 / 120 = 111.1.)
Note that this represents a 24x improvement in string count over a series system design.
Parallel systems materials and costs With the cable configuration of the Parallel system in place, we can add up the required components.
Starting with wire content, the system no longer has the thousands of strings that require home runs back to the combiner boxes. Instead, 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 AWG wire.
The combiner box count is also far lower in a parallel system. Only five 24-pole combiner boxes
www.solar-pv-management.com Issue V 2010
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