PVCOMMERCIAL
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
www.solar-pv-management.com Issue V 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80