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Introduction to Solar

What does “photovoltaics” mean? It can best be defined by its two parts: photo, derived from the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta. So, photovoltaics can be simply stated as light-electricity. And that's what photovoltaic (PV) materials and devices do; they convert light energy into electrical energy using the Photoelectric Effect.

These PV materials are then formed into cells. Individual PV cells are electricity-producing devices made of semiconductor materials. PV cells come in many sizes and shapes from smaller than a postage stamp to several inches across. They are often connected together to form PV modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output.

How Solar Works

The driving force behind solar electricity is the photoelectric effect, or more commonly known as the photovoltaic (PV) effect. This is the basic physical process by which a PV cell converts sunlight into electricity. When light shines on a PV cell, it may be reflected, absorbed, or pass right through, but only the absorbed light generates electricity.
The energy of the absorbed light is transferred to electrons in the atoms of the PV cell. The added energy causes the electrons to escape from their normal positions in the atoms of the semiconductor PV material and become part of the electrical flow, or current, in an electrical circuit. This flow of energy follows a process of diffusion between positive and negative layers to eventually become a direct current (DC) of electricity.
Direct current electricity is the unidirectional flow of electric charge. It can be stored and used to power some electronics. However, DC differs from the electricity you would get from a power outlet in your house. Using an inverter, this energy is changed into alternation current (AC). The AC electricity is the final product of the PV system. This then can be used in many different applications, but is most commonly used to offset the use of electricity by the user.

Solar PV Modules

Solar PV modules come in a huge variety of shapes, compositions, and styles. The most popular element used in PV modules is silicon. It is not the only element that modules are composed of but because the element is readily available it is the most popular choice. The three most popular types of silicon modules are amorphous thin film, mono-crystalline, and polly-crystalline.

1. Amorphous

Description:

Named because of their composition at the microscopic scale. Amorphous means "without shape". The silicon in these modules is applied using a technique called chemical vapor deposition. This application technique allows the silicon to be flexible. The cell then can be mounted to either a flexible or rigid frame. Many modules use thick framing and thick glass to ensure structural rigidity. The visual appearance of the amorphous module can vary but generally is one uniform color.
The energy output of this module begins at a rate higher than its nameplate rating and produces at this rate for 90 days. After 90 days it will rapidly decline until it stabilizes. The module will maintain this stable rate for the rest of its life. The efficiency of most thin film modules is lower, roughly 5 to 13%.

Benefits:

Amorphous modules gather more energy in their lifetime compared to crystalline modules. The composition allows the module to respond better to higher temperatures and gather more energy during the summer than a comparable crystalline module. The modules also are more affordable due to the ease of construction.

Drawbacks:

The Amorphous module requires a larger surface area to gather energy. The average increase in required surface area is between 30 and 40%. Also, the amorphous module requires thicker glass for protection and therefore increases the weight. For example, a 55 watt amorphous module is roughly 38” by 38” and weighs 30 lbs and a comparable crystalline module is 31” by 21” weighing only 14 lbs.

2. Mono-Crystaline

Description:

Mono-Crystalline cells are made from one silicon crystal. In single crystal silicon, the crystal lattice of the entire sample is continuous and unbroken with no grain boundaries. As a result these cells can be recognized by an even external coloring. The mono-crystalline cells are rigid. They do not provide any flexibility and are framed with thick aluminum and mounted behind tempered glass.
The energy output of these modules is relatively predictable and has become a standard across the industry. 10 years at 90% and 25 years at 80% of the nameplate rating.

Benefits:

The consistency of the crystal allows these modules to have increased efficiency. The efficiencies of mono-crystalline modules are among the highest reaching up to 18% in commercial use. The modules are very durable and can withstand heavy winds and hail.

Drawbacks:

These modules tend to be more expensive. The added framing and better quality make the demand for these modules higher. Also, the rarity of large silicon crystals creates a shortage of modules.

3. Polly-Crystalline

Description:

polly-crystalline cells are similar to mono-crystalline in that they are made from silicon crystals. However, polly-crystalline cells are composed of man y small crystals. These polly-crystalline cells can be recognized by a visible grain. These cells appear to have a flaking appearance in the metal. Also similarly, these cells are very rigid and strong. Modules composed of these cells appear very similar to the mono-crystalline modules in that they have thick framing and tempered glass.
The energy output of these modules is also predictable and has the same standards as the rest of the industry. 10 years at 90% and 25 years at 80% of the nameplate rating.

Benefits:

polly-crystalline cells can be created for a lower cost. As a result these modules can be purchased for a lower price. The polly-crystalline structure also allows for slightly more flexibility than in the mono-crystalline.

Drawbacks:

These modules are slightly less efficient because there is not a uniform crystal. They cannot perform at the higher efficiencies that the mono-crystalline modules can reach.