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costs, between the two architectures.

The Current State of Design Recall the basic difference between series and parallel circuits: when current sources (e.g., PV modules) are wired in series, their voltages add; when wired in parallel, the currents add.

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Series circuits are the dominant design choice in most PV systems today. Why? Because most PV modules deliver power at voltages that range from 25-35 volts (the maximum power voltage, Vmp, for most crystalline silicon modules) to 50-100 volts (the Vmp for most thin-film modules). Most inverters, on the other hand, require inbound voltages between 240-480 volts. Thus, designers must wire modules in series so that the voltages add to a high enough level for the inverter. Most crystalline modules are wired in series, eight to 12 at a time. Most thin-film modules are series-wired in groups of five or six. These groups of solar modules, wired in series, are known as “strings.”

Note also that the upward limit of a string size is determined by the open circuit voltage (Voc) of the PV modules. This value must also fall within the range of the inverter – and with any inverter designed to be used in NEC-regulated applications, the upper voltage limit is 600 volts. So for these inverters, the sum of the string’s Voc must be under 600 volts.

2 Note that the PV module voltages will drop as temperatures rise, usually by -0.25 percent to -0.50 percent per degree Celsius. This adds complexity and challenges to string sizing in series designs. For the purposes of this exercise, however, we will ignore that issue.

Finally, all the strings are wired into a combiner box, which creates a parallel connection among them; this sums the current while maintaining the same voltage.

Parallel alternative

As implied in the discussion above, parallel system design is typically 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 from 18 volts to 100 volts) to the voltages required by the

inverter. One such product is the vBoost, sold by eIQ Energy, Inc.2

Because each vBoost unit’s

voltage output matches the inverter’s ideal input voltage, the units can be wired in parallel, directly to the inverter.

With a 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. Some specific examples of these cost savings will be outlined below.

Bill of Materials

The major cost drivers of a solar electrical system are wire (#10 AWG wire is most common), combiner boxes (fused boxes that combine multiple feeds into a single cable), and installation labour. In addition, a key system component for costing is a “string.”

Strings: As mentioned earlier, a string is a self- contained electrical unit comprised of PV modules and their associated cabling. Thus, a string does not represent a single item on the bill of materials (and does not directly incur cost), but is a critical design factor in the overall system layout. String layout determines the quantity and cost of wire, combiner boxes, and installation labour in a system.

Wire: Wire is a factor in several system components. First is the string cabling, which is directly connected to the solar modules, and connects them to the backbone of the system (typically the combiner box). These are typically #12 or #10 AWG copper wires. Thicker cables (#4, #0, or larger) are then used for the “home run” cables, bringing the power from the combiner boxes back to the inverters, completing the circuit.

Combiner boxes: Combiner boxes simply take in multiple pairs of leads, typically between eight and

Image 2: Illustrative schematic of parallel wiring (represents 6.0kW of First Solar modules): Note that the 80 modules are shown on a single parallel cable run

www.solar-pv-management.com Issue V 2010

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