Solar Panels (Photovoltaic Panels): Overview

Overview of Solar Panels (Photovoltaic Modules)

A photovoltaic (PV) module or solar electric panel is the smallest replaceable unit in a PV array. The module is an integral unit that provides support for a number of PV cells connected electrically and protected from the elements. The electrical output of the module depends on the size and number of cells, their electrical interconnection, and, of course, on the environmental conditions to which the module is exposed. Solar electric panels come in all shapes and sizes, and may be made from different materials. However,the most commonly used module is a "glass-plate-sandwich" that has 36 PV cells connected in series to produce enough voltage to charge a 12 volt battery. The purpose of the structure is to provide a rigid package and protect the inter-cell connections from the environment. Plus (+) and minus (-) connectors are located on the back of the module for interconnection. The modules may have an individual metal frame or be protected by a rubber gasket and intended for installation in a larger mounting system designed to hold several modules.

There are four factors that determine any solar electric panel's output - efficiency of the photovoltaic cells, the load resistance, solar irradiance, and

cell temperature. The solar cell efficiency is set by the manufacturing process - today's commercially available modules are from 3% to 17% efficient at converting the solar energy to electrical energy. The load resistance determines where, on the current and voltage (I-V) curve, the module will operate. The obvious preferred operating point is where maximum power (power is calculated by multiplying the current times the voltage) is generated - called the peak power point. Study the I-V curve shown below. This curve represents the output of any PV generator -from a cell to the largest array.

For a given solar cell area, the current generated is directly proportional to solar irradiance (S) and is almost independent of temperature (T). Thus, as the sun's brightness increases the output voltage and power decreases as temperature increases. The voltage of crystalline cells decreases about 0.5 percent per degree centigrade temperature increase. Therefore, arrays should be mounted in the sunniest place (no shading) and kept as cool as possible by ensuring air can move over and behind the array.