PHOTOVOLTAIC SOLAR CELLS, MODULES AND PANELS

Photovoltaic cells are made from silicon, which is one of the most abundant elements on earth. The silicon element is modified by other elements like boron and phosphorus in a process called doping to create a solar cell. There are three types of solar cells made from silicon they are; mono-crystalline, polycrystalline and thin film silicon. Crystalline photovoltaic cells are made from silicon which is first melted and then allowed to crystalize into ingots or casting of pure silicon. Thin slices of silicon called wafers are cut from a single crystal of silicon (mono-crystalline) or from a block of silicon crystal (polycrystalline) to make individual solar cells.

Mono-crystalline cells come from a single crystal ingot of high purity, with typical dimension of 12.5 to 15cm. the ingots have a cylindrical shape, which is cut into thin slices and made round, semi-round or square shapes. These cells are the most electrically efficient, which means they require less surface area than other cell types to produce an equivalent amount of power. They also have a wide range of transparency options. Disadvantages are their higher cost, requirement for ventilation in other to maximize performance, and a distinctive geometric pattern. Mono-crystalline cells are especially suitable for atrium roofs, partial vision glazing in facades, roof top installations in houses and commercial sun shading or roof top retrofits where installation area is limited and maximum electricity generation is desired. Its Commercial modules efficiency, range around 14-19%.

Polycrystalline silicon cells are formed by casting a cuboid form ingot. This ingot is cut into bars and sliced into thin wafers (a thin sheet of semiconductor material) which in turn are used to create the cells. These cells are less efficient than a mono-crystalline cell; however the lower cost per unit area and their distinctive appearance makes them a popular choice for relatively large opaque installations. They have been used extensively in façade spandrel panels and sun shading element on commercial buildings. Polycrystalline silicon differs from mono-crystalline silicon in terms of cost(due to reduction of losses) and efficiency( due to grain boundaries). The difference is small, but still leads to need for larger cell areas (21x21cm) instead of the slight smaller dimension used for equivalent mono-crystalline cells, in order to reach the same efficiency levels. Its Commercial module efficiency, range around 12-15%.

The semiconductor materials used for thin film types of photovoltaic cells are cadmium telluride (CdTe), copper indium diselenide (Cis), amorphous silicon (a.si) and thin film silicon (thin film-si). Usually the mode of thin photovoltaic cells are by spraying or printing a thin semiconductor layer of these photovoltaic cell materials into a glass, metal or plastic foil substrate. The manufacturing is faster and cheaper making thin film photovoltaic cells technology more viable for use economically. Despite this economic advantage, thin film photovoltaic cells suffers from poor cell conversion efficiency, due to their non-single crystal structure, requiring larger cells and area in order to generate equivalent energy compared to the other two methods.

Amorphous silicon is in commercial production, while the other three technologies are slowly reaching the market. One of the advantages of amorphous silicon cells, is that it can be deposited on a variety of low cost rigid and flexible substrate such as polymers, thin metals and plastics as well as tinted glass for building integration as in windows, doors and roofs. The only drawback is its very low efficiency which ranges between 6% to 8%.

It is worth mentioning that a new generation of solar photovoltaic cells technology consisting of the following types of solar cells; Thin film copper zinc tin sulphide (CTZs )solar cells (including materials CZTse and CZTsse), dye sensitized solar cells, quantum dot solar cells, perovskite solar cells and organic solar cells. Out of these cells, only perovskite solar cells appear to be gaining momentum in its march towards commercialization.

Most solar panels are made up of two modules in one frame. Also most common modules are composed of solar cells connected in series, with each cell generating the same voltage (usually -0.5v). These modules are designed to supply electricity at certain designed voltages, such as 12v, 24v or 48v. The current it produces, is directly proportional to the sunlight intensity. Multiple modules or panels are connected together either in parallel or in series to obtain the desired voltage and current. Typically 1m2 solar panel generates 60-120wp in standard condition, depending on the technology. For instance, a typical crystalline silicon module consists of a series circuit of 36 cells encapsulated in glass and plastic package for protection from the environment. Although photovoltaic cells modules are warranted for power output for a period from 10-25 years, but they can still be expected to deliver power for periods of 40 to 50 years.

Photovoltaic solar cells are very useful in powering space vehicles, such as satellites and telescopes and off grid locations on earth. They also are useful in powering objects which would otherwise need expensive and cumbersome fuel sources. Its obvious advantages is that it is clean non-polluting, renewable, noiseless, requires little maintenance, durable, has a long life span and more over no fuel cost or fuel supply problems. Its only disadvantage is at night when there is no sunlight, which would necessitate the use of batteries to extend its performance, expensive initial cost outlay, it’s affected by weather condition and the need of a large area of land to produce equivalent or comparative amount of electricity as other power generation methods.

 

Sources.

• Solar cells from basic to advanced systems.(c)1983; Chenming Hu,Richard M. White-McGraw hill book company.
• Physics of solar cells from basic principles to advanced concepts.(c)2016; Peter Wurfel and Uli Wurfel.-WILEY-VCH,GmbH and co.
• Introduction to solar cell technology.(c)2000. Kiran Ranabhat etal.-iiPP journal.
• Types of solar cells and application.(c)2001. Askari Mohammed Bagher,etal.-American Journal of optics and photonics.
• The physics of solar cells.(c)2003. Jenny Nelson-imperial college press.