Photovoltaic modules, its types, assembly and performance characteristics

What is a PV Module?

A Photovoltaic (PV) module which is generally termed as a solar panel is an assembly of photovoltaic cells electrically connected to each other and mounted on a laminated frame. The solar cells are primarily made up of silicon material which absorbs the photons emitted by the sun. PV modules are given output ratings in watts (Wp).

Types of PV Modules

There are three major types of solar PV modules.

  1. Mono-crystalline
  2. Poly-crystalline (also known as multi-crystalline)
  3. Thin-film

Each solar PV module type has its own unique features. These PV modules also vary based on how they’re manufactured, their appearance, performance, costs each are best suited for.

  • Mono-crystalline and Poly-crystalline solar PV modules

Both mono-crystalline and poly-crystalline solar PV modules have cells made of silicon wafers. To build a mono-crystalline or poly-crystalline module, wafers are assembled into rows and columns covered with a glass sheet, and framed together. Poly-crystalline cells are square-shaped whereas mono-crystalline cells are square with missing corners.

While both of these types of solar PV modules have cells made from silicon, mono-crystalline and poly-crystalline modules vary in the composition of the silicon itself. Mono-crystalline solar cells are cut from a single, pure crystal of silicon. Alternatively, poly-crystalline solar cells are composed of fragments of silicon crystals that are melted together in a mold before being cut into wafers. Mono-crystalline modules typically have the highest efficiencies and power capacity.

  • Thin-film solar PV modules

Unlike mono-crystalline and poly-crystalline solar PV modules, thin-film modules are made from a variety of materials. The most prevalent type of thin-film solar module is made from cadmium telluride (CdTe).

To make this type of thin-film module, manufacturers place a layer of CdTe between transparent conducting layers that help capture sunlight. This type of thin-film technology also has a glass layer on the top for protection.

Other materials used to manufacture a thin-film PV module are amorphous silicon (a-Si) and Copper Indium Gallium Selenide (CIGS). Thin-film solar modules are thin and often slimmer than other solar PV module types. Thin-film solar modules also tend to have lower efficiencies and power capacities than mono-crystalline or poly-crystalline modules.

How does a PV Module work?

PV module is an assembly of solar cells that work on the principle of photovoltaics. Photovoltaics is the direct conversion of light into electricity that occurs naturally in certain types of material, called semiconductors that exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. Electrons in these materials are freed by solar energy and can be induced to travel through an electrical circuit.

In most cases, the semiconductor material is used to manufacture a solar cell. Solar cells are primarily made up of silicon which absorbs the photons emitted by Sun. When sun ray’s strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of electric current called electricity. A number of solar cells electrically connected to each other and mounted in a frame is called a photovoltaic module.

The working of a solar cell is well explained in the video by SciToons – Brown University, below.

Video copyright and property of SciToons – Brown University, (Youtube)

Components of a Solar PV Module

Module Frame

Glass

Encapsulant

Back Sheet

Junction Box

PV Module Performance Characteristics

As we learned above, PV module is an interconnection of solar cells. One of the most important properties of solar PV systems is the I-V curve characteristic of the PV module. The I-V curve provides important performance information about PV modules such as Open Circuit Voltage (Voc), Short Circuit Current (Isc), Maximum Power Point (MPP), Voltage and Current (Vmp) and (Imp), and module’s efficiency.

The total voltage of a PV module is a scale version of the cell voltage obtained by multiplying the number of cells connected in series, while the total current is a scaled version of the cell current obtained by multiplying the number of strings of cells connected in parallel. We define the Module I-V curve, or the current-voltage curve, as it is illustrated in the figure below. The curve indicates the voltage and current at different operating conditions.

For example, the highest current corresponds to the short-circuit condition (when a PV module’s positive and negative terminals are connected without load, causing very high current to pass), while the highest voltage occurs at open-circuit condition (when a PV module’s positive and negative terminals are not connected to any load, causing no current to pass).

If we observe the current and voltage starting at the open-circuit condition (where voltage is maximum and current is zero), and as we increase the load of the circuit, the current starts increasing and the voltage falls down until it reaches the value of zero at short-circuit condition (where the current is maximum).

The knee of the curve indicates the operating condition in which current and voltage result in maximum power point (MPP). The voltage and current values at MPP are referred to as “Vmp” and “Imp,” respectively.

PV Module Degradation

Although PV Modules are often sold with 25-year limited power output warranty, over years the modules won’t be performing as well as they did on Day 1. Performance declines as solar cells experience degradation due to unavoidable elements like:

  1. Quality of your panels’ materials
  2. Thermal cycling,
  3. Damp heat,
  4. Humidity freeze
  5. UV exposure
  • Thermal cycling can cause solder bond failures and cracks in solar cells.
  • Damp heat has been associated with delamination of encapsulates and corrosion of cells.
  • Humidity freezing can cause junction box adhesion to fail.
  • UV exposure contributes to discolouration and back sheet degradation

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