Classification of the types of solar plates by its technology
Today’s article is aimed at those who are starting to enter into the world of photovoltaic energy and need information about the types of solar panels that are most commercialized in the current photovoltaic market.
On the basis of what has been said, things have not changed too much in the last 10-15 years, we are in 2017 and there are still 3 technologies that stand out over the rest and that are installed in 90% of current photovoltaic systems. We are talking about monocrystalline, polycrystalline and amorphous solar panels.
# 1 – Monocrystalline pv panels:
Like their given name, the monocrystalline solar panels are composed of monocrystalline cells. They are that type of cell that, a simple view, can be identified by its “black” color and with the corners cut with a chamfer (result of the cut of the cell).
The most common mode of manufacture of monocrystalline silicon cells (sc-Si) consists of starting from a single crystal silicon ingot, obtained by the methods of Czochralski (Cz) or the floating zone (FZ), and cut into wafers which constitute the substrate on which all the remaining process takes place (union “pn”, metallization, etc.).
The above image is an example of this type of cell that we discussed. In addition to this, note that monocrystalline solar panels are the most efficient of all available in the sector.
# 2 – Polycrystalline Solar Modules:
As we discussed with monocrystalline models, polycrystalline solar panels are composed, in this case, of polycrystalline cells. We can differentiate it by its “bluish” color and do not have the chamfer in the corners like the monocrystalline ones.
Polycrystalline silicon (mc-Si) cells also use silicon wafers as a substrate, but unlike monocrystallins, they come from cutting a silicon block that has been slowly allowed to solidify in a crucible and is made up of many small crystals of silicon.
This type of processing is less costly than the previous one, but considerably reduces the efficiency of the cells.
We see in the image the visual difference between a polycrystalline cell and the previous monocrystalline cell from the previous point.
More recent advances, such as diamond wafer cutters, used by manufacturers such as SolarWorld, have led to an improvement in the silicon utilization efficiency (less material is wasted) and to wafer wafer thickness of less than 200 microns, although this thickness is close to its physical limit because we must take into account that the cell must be strong enough not to break in its subsequent manipulation for the manufacture of the panel.
In addition to this, it has been possible to improve the losses by reflection and a better capture of the light inside the cell by means of texturizing techniques and antireflective treaties.
We can see a sample of texturized solar cell in the following picture:
As can be seen in the image, a series of micro-perforations with the aspect of an inverted pyramid have been practiced on the cell, which they do is to help improve the uptake of sunlight from the cell and, consequently, from the solar panel.
As we have already mentioned in previous articles, the conducting grid that forms the front contact of the cell usually consists on a series of “fingers” that are in direct contact with the semiconductor and that are connected between them by means of metallic strips (what is usually called Bus Bar).
The design of these “fingers” and the Bus Bar that cover the cell affects the solar panels efficiency in two ways:
On the one hand, it involves a shading that prevents some of the available radiation from reaching the interior of the cell and, on the other hand, introduces a resistance, due to the metal-semiconductor junction and the resistance of the material used. In this sense, it has evolved from the conventional technique of screen printing to laser buried contact, achieving a 25% improvement in the efficiency of the cell without raising the cost of manufacturing.
Both the mono and polycrystalline cells described have approximately 0.5 volt open circuit (Voc) and about 3 amps short current (the current is directly proportional to the cell area).
Of course, once the cells are obtained, they are assembled and passed a production process to give rise to what we end up seeing in our photovoltaic installation, which are the monocrystalline or polycrystalline solar panels depending on the type of cell.
# 3 – Amorphous silicon panels (thin film):
Although the most common types of panels are the ones described, mono and polycrystalline, we must not forget also amorphous silicon solar panels, or also called “thin film”.
The operation of a thin film solar cell of amorphous silicon is the same as the crystalline but its elaboration is very different. The characteristic aspects of this technology are:
Simple manufacturing process and easy automation.
Need for little active material and reduction of energy and cost.
Easy to realize flexible modules and with optimal quantum efficiency in a wide range of the spectrum.
Amorphous silicon cells have been the first thin-film cells to be marketed. However, due to the drop in prices experienced by crystalline solar panels, they have been losing positions in the market and are currently very small.
A-Si amorphous silicon technology has a considerably lower efficiency than those based on crystalline silicon, mainly due to the poor quality of the silicon used, whose internal structure makes it difficult to collect the photogenerated carriers. However, they are especially suitable for use in cloudy places, in dusty atmospheres, etc.
As can be seen in the picture, amorphous silicon solar panels do not consist on the attachment of individual cells as in the crystalline solar panels, but in a tailor-made sheet in which thin strips are seen separating the cells, created and connected to each other during the elaboration of the module itself, whose framing facilitates the handling and assembly thereof. The range of voltages is also wider than in crystalline silicon, ranging from a few volts to tens of volts and makes them interesting also for solar pumping systems.
Commercially, solar panels are classified by their peak power (Wp), which is the power they can generate under standard conditions of measurement (STC), in other words, the power indicated in the product data sheet. Another of the data that usually look at the nominal voltage, which will indicate if the panel will be suitable for isolated systems of 12V, 24V or for network connection and self-consumption.
These are, in broad strokes, the types of solar panels that we can find at the moment in the photovoltaic solar market. There are more types of panels, and even variations in the technologies described, such as Mono PERC or Panasonic Sanyo HIT, both of higher efficiency than the rest, however, we preferred to leave them for future articles.