Wiring Photovoltaics and the Photovoltaic Array
A photovoltaic ("PV") array describes the part of a PV system that converts solar energy into electrical energy. While there are other critically important components of the complete photovoltaic system - most significantly the inverter in all grid-connected systems - the array comprises all of the electrically-connected photovoltaic material.
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A photovoltaic module (or colloquially a "solar panel") is an assembly of wired together PV cells that have been packaged between a rigid backing and a transparent material to provide protection and ease of installation. The material covering the PV cells can be glass, plastic or an apoxy - something lightweight that is strong enough to protect the fragile photovoltaic cells from hail and other environmental hazards while admitting all of the desirable portion of the light spectrum to the photovoltaic material. Very high energy electromagnetic energy such as UV light is actually not desirable because it can overheat the photovoltaic cells, decreasing their efficiency.
Each PV module is rated based on the amount of DC power it is capable of producing under Standard Test Conditions (STC), which are 1,000 W/m2 (or "Peak Sun") at 25 °C module temperature. The modules are then typically wired together into strings, which are then wired together into the complete array. The size of a photovoltaic array can vary dramatically - from two modules used to provide the energy requirements of a recreational vehicle to the 5,200,000 PV modules Agua Caliente Solar Project currently under construction in Yuma County, Arizona.
Typically, photovoltaic systems are wired together so that direct current voltage delivered by the array is high and the current is correspondingly low. This is done because the amount of current dictates how large the wires in the system need to be. To accommodate a lot of current without having large losses to the resistance of the conductor material or worse yet overheating the material and damaging the wiring, the wires need to be very large, which increases the cost of the system. However, this has implications in how the system functions. Because the modules are wired in series, the current in the entire photovoltaic circuit is limited by the photovoltaic module that is producing the least current. Therefore, if one of the PV modules is shaded and producing less or even no current all, the entire string of photovoltaic modules will be similarly affected.
Figures 1 & 2 are the two different wiring configurations of small PV modules that might be used in a classroom setting. Each module is capable of delivering 3 Volts of voltage and 1 Ampere of current.
Figure 1: Series Wiring Configuration
V1total = V11 + V12 + V13 = 3 V + 3 V + 3 V = 9 V
I1total = I11 = I12 + I13 = 1 A
Recalling that Power = Voltage x Current,
P1 = V1total x I1total = 9 V x 1 A = 9 W
Figure 2: Parallel Wiring Configuration
V2total = V21 = V22 = V23 = 3 V
I2total = I21 + I22 = I23 = 1 A + 1 A + 1 A = 3 A
P2 = V2total x I2total = 3 V x 3 A = 9 W
Therefore, regardless of whether the modules are wired in parallel or in series, the amount of power each circuit is capable of producing is the same.
