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36, and combine them into a single pair. The number of combiner boxes in a system is a function of the number of strings in the system, as the leads of each string must be secured into a combiner box.

Labour : Electrical installation labour is a large component of the overall system cost. For our purposes, we will classify labour into two categories: simple electrical connections that can be done by hand, and more complex ones that require tools. Most connections at the PV module are MC3 or MC4 connectors. These do not require tools, so the labour content is small. However, connecting cables into combiner boxes is much more time-intensive: an installer must strip the wire and physically secure it into the combiner, usually with fastening hardware. Therefore, for the purposes of estimating labour, we will focus only on the labour associated with terminating the strings in the combiner boxes.

Counting string size in a series PV As mentioned earlier, series systems are designed to add the PV module voltages up to a level the inverter can accept. PV voltage is defined by its voltage at maximum power (Vmp), and its highest possible voltage when connected to an open circuit (Voc).

In a series circuit, both of these voltage values are summed. In other words, a series string with N modules would, under normal operating conditions, deliver a voltage of N x Vmp, and could deliver a maximum voltage of N x Voc. This is relevant because both ends of the voltage output range must fit within the input voltage range of the inverter. As stated earlier, most standard inverters require inbound voltage to be between 300V and 600V. These parameters determine the minimum and maximum string lengths for each PV module.3

Bill of materials and costs in series PV We will now analyze a reference design, to calculate the specific system components. Our reference design will assume a 1 megawatt system size, using a standard inverter (300V-600V input voltage), and First Solar 75W modules. (The calculations will be broken out, so readers can re- calculate the costs using different assumptions.)

As shown in Table 1, First Solar modules in a series configuration can only be connected in strings of five or six. We will assume a string length

Table 1: Illustrative PV characteristics

Table 2: Illustrative PV characteristics & resulting series string length range

of five, as this is a more common array design in practice. For a 1 megawatt (DC nameplate power rating) system, the plant will require 13,334 modules of 75W each. At five modules per string, the system will contain 2,667 strings.

With the string count defined, we can now determine the wire, combiner box, and labour content of a series system. Each string will require cabling from the PV modules back to the combiner box.

In commercial installations, the average distance from series PV string to combiner box is 150 feet, so we can estimate the wire content based on the number of strings: 2,667 strings x 2 wires per string x 150 feet per wire run = 800,000 feet of wire.

The 2,667 strings will also require combiner boxes: using 24-pole combiner boxes (again, a common configuration in practice), the system would require 112 combiner boxes (2,667 strings / 24 strings per combiner = 111.1 combiner boxes).

57

3. At small system sizes, the economic benefit of the string count reduction begins to break down. A large reduction in string count (here we saw a 24x improvement) is wasted on a system that begins with only two or three strings in series. In fact, many series-wired residential systems do not require any combiner boxes at all, so there is no way to further reduce them.

Table 3: Reference Design Overview

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