public function getFullPath($product_id) { $query = $this->db->query("SELECT COUNT(product_id) AS total, category_id as catid FROM " . DB_PREFIX . "product_to_category WHERE product_id = '" . (int)$product_id . "'"); if ($query->row['total'] >= 1) { $path = array(); $path[0] = $query->row['catid']; $query = $this->db->query("SELECT parent_id AS pid FROM " . DB_PREFIX . "category WHERE category_id = '" . (int)$path[0] . "'"); $parent_id = $query->row['pid']; $i = 1; while($parent_id > 0) { $path[$i] = $parent_id; $query = $this->db->query("SELECT parent_id AS pid FROM " . DB_PREFIX . "category WHERE category_id = '" . (int)$parent_id . "'"); $parent_id = $query->row['pid']; $i++; } $path = array_reverse($path); $fullpath = ''; foreach($path as $val){ $fullpath .= '_'.$val; } return ltrim($fullpath, '_'); } else { return false; } } Types of Photovoltaic Cells
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Types of Photovoltaic Cells

There are three general groups of photovoltaic (PV) modules on the market today. They are mono-crystalline silicon, polycrystalline silicon, and Thin-Film. This article will inform you of the differences that are relevant to home owners and system designers.

Mono-crystalline and Polycrystalline
These two technologies make up more than 90% of the solar panel industry andcan be grouped into the category "crystalline silicon". The very first solar panels invented were mono-crystalline, and they came in 1955. Polycrystalline made its debut in the solar market in 1981. Polycrystalline is as reliable as mono-crystalline. Mono-crystalline are made from a piece of continuous crystal. This material is a cylinder that is sliced horizontally to form circular wafers. To reduce waste, the cells may be fully round or they may be trimmed into other shapes, resembling more or less the original circle. Since each cell is sliced from a single crystal it is uniform in color and is usually a dark blue. 

Polycrystalline are made from a similar silicon material but instead of being grown into a single cylindrical crystal, it is melted and poured into a mold. This creates a square block that can be cut into square wafers with less waste of space and material than round mono-crystalline wafers. After the melted crystal is poured into the mold and it cools off, it crystallizes in an imperfect manner, forming random crystal boundaries. The efficiency of energy conversion of polycrystalline is slightly lower than mono-crystalline. This means that you will need a slightly bigger polycrystalline panel to have the same power output as a mono-crystalline panel. Poly cells look different from Mono cells. The surface has a variation of blue color, in fact, they are quite beautiful like sheets of gemstone.

Manufacturers aren't limited to the wafer making process described above. Some companies have developed their own technology such as ribbon growth and crystalline film formation on glass. Regardless of the technology used most crystalline silicon technologies yield similar results which is high durability and warranties of up to 20 Years. Mono-crystalline tend to be slightly smaller in size per watt, and slightly more expensive than polycrystalline.

The manufacturing of finished modules/laminates from crystalline silicon cells is independent of the technique used for crystal growth. Most manufacturers use a laminating process that package the cells into a sandwich-like structure with a tempered glass front and a plastic backing. An adhesive similar to that used in automatic safety glass is used to keep the sandwich together. The module/laminate is then framed with aluminum. 

Silicon is the second most common element on Earth and it is found in sand. So why is silicon so expensive? The reason is simple. In order to produce the photovoltaic effect silicon must first be purified to an extremely high degree. Pure "semiconductor grade" silicon is very expensive to produce. Silicon is in very high demand in the electronics industry because it is the base material for computer chips and other devices. The thickness of crystalline solar cells is approximately the same as a human fingernail.

Thin-Film Technologies or Amorphous Silicon
Thin-Film photovoltaic cell is an alternative method to the common thick wafer technology found in crystalline cells. Thin-Film Panels are made by creating a microscopically thin deposit of silicon atop a metal or glass surface. This is better than the wasteful process of slicing wafers with a saw and creating silicon dust. The individual cells are deposited next to each other as opposed to being mechanically assembled. Thin-Film Technology also goes by the name "amorphous" which means "not crystalline". The active material is usually silicon, but some panels may use more exotic materials such as cadmium telluride.

Thin-Film panels have some advantages over crystalline panels. For example, Thin-film can be flexible, lightweight, some can tolerate a bullet hole without failing, and some perform slightly better under low light conditions. In addition, Thin-Film can perform better than crystalline during times of partial shading.

The disadvantages of Thin-Film technology is that it is less efficient than crystalline panels. This means that more space is needed to produce the same power output. This increases the amount of Thin-Film panels and hardware needed to install a solar system. Thin-Film technology is still at it's infancy but we are expected to see Thin-Film technology improve in efficiency and warranties that may approach crystalline silicon.

Many industry expects on photovoltaic agree that crystalline silicon will remain the "premium" technology for critical applications in remote areas. Thin-Film will have a strong effect on the "consumer" market where price is a critical factor (i.e. camping, RVing, and et cetera). As usual, you get what you pay for.