Photovoltaic Cell or Solar Cell- The contribution of solar energy to the world total energy supply in the last two decades has grown significantly. Energy from the sun is the most abundant and free of cost that is available energy on earth. The most common solar cells are made up from silicon a semiconductor that is the second most abundant element on earth. Sand which is the second most abundant element will help to utilize this energy. The sand has to be converted in to 99.999% pure crystals to use in the solar panels. To achieve this sand has to go through a complex purification process. The sand and carbon are treated at 2000° C temperature to produce raw silicon. The raw silicon is converted in to gaseous silicon compound form. To obtain highly purified crystalline silicon we mix it with hydrogen. Silicon wafers which are very thin slices which are obtain by reshaped silicon ingots. When we analyse the structure of the silicon atoms that are bonded together the electron in the silicon structure has no freedom of movement as they are bonded.
Assume that phosphorous atoms with five valance electrons are injected in to the silicon. Where one electron will free to move, in this structure when the electron get sufficiently energy it will move freely.
Construction of Photovoltaic cell or Solar Cell:
A photovoltaic cell, often called a solar cell, when the light strike them the electron will gain photon energy and will be free to move the energy in light will be directly converted into electrical potential energy using a physical process called the photovoltaic effect. When the smaller unit called solar cells combine it form Solar panel. In a solar cell crystalline silicon is sandwiched between the conductive layers. Each silicon is connected to the neighbour by four strong bonds which keep the electrons in place so no current can flow. There are two different layers of a silicon solar cell.
However the movement of the electrons will be random and it does not result any current through the load. A driving force is needed to make the electron flow unidirectional. An easy way to produce the driving force is a PN junction. Now we will discuss that how the PN junction in photovoltaic cell produce driving force. Similar to N type of doping if we inject boron with three valance electrons in to the pure silicon there will be one hole for each atom this is called P-type doping. If these two kinds of doped material join together some electrons from the N side will be migrate to the P region and fill the holes available there during this process a positive charge will be created at one side and negative charge at the other side. In this way a depletion region is formed where there are no electrons and holes.
When the electron transfer from the N-side boundary it will becomes slightly positively charged and the P-side becomes negatively charged. The electric field will be definitely formed between these charges. This electric field produce the necessary driving force.
When the light which are the flow of tiny particles called photons shooting out from the sun strikes the PN junction something very interesting happens. Light strikes the N region of the Photo voltaic cell and reaches to the depletion region. This photon energy is sufficient enough to generate electron hole pair in the depletion region. A hole will be created when the electron will be knocked from the bond. The negatively charged electron and location of the positively charged hole are now free to move around. The electric field in the depletion region drives the electron and holes out of the depletion region. Because of the electric field at the PN junction they will go only in one way. The electrons will be drawn to the N-side while the holes will be drawn to the P-side. The top of the cell consist of thin metal finger which will collect the mobile electrons. From there they flow through an external circuit. The concentration of the electrons in the N region and holes in the P region becomes so high that the potential difference will develop between them. As soon as we connect any load between these regions electron will start flowing through the load. When the path will be completed the electrons will recombine with the holes in the P region. In this way photo voltaic cell gives direct current. The top N layer of the solar cell is heavily doped where the P layer is thick and lightly doped. This arrangement will increase the performance of the cell in the solar panel.
Structure of Solar Panel:
Solar photo voltaic cells are made up of six main components these are:
- Anodised aluminium frame
- Highly transparent tempered glass
- Encapsulated material EVA
- Photovoltaic solar cell
- Insulating back sheet
- Junction box
Extruded Aluminium frame:
The extruded aluminium frame play a critical role by both protecting the edge of the laminate section housing the cells and provide solid structure to mount the solar panel in this position.
Highly transparent tempered glass:
Second layer is tough which is 3 to 4 mm thick. This glass protect the solar cell from the weather and impact from hail.
Photovoltaic Solar cell:
The PV cell convert the sunlight directly in to dc electrical energy. The performance of the solar panel dependent upon by the cell type and the characteristics of the silicon which are of two types:
- Mono crystalline silicon
- Polycrystalline silicon
Encapsulation EVA film Layers:
EVA which means Ethylene Vinyl Acetate which is known for its excellent transparency.
Insulating back Sheet:
The back sheet is the rare most layer of the solar panel which acts as a moisture barrier and provides mechanical protection and electrical insulation. The back sheet material is made up of various types of polymers or plastics.
Layers of the Solar Panel:
One of them is layer of cells; the top negative side of this cell is connected to the black side of the negative cell through copper strips and forms a series connection. When we connect these series connected cells parallel to another series we will get the solar panel. A single PV cell produces only around 0.5 volt but we can string them together in modules to get more power. The combination of parallel and series connection of the cells in the solar panel increases the voltage and current values to a usable range. We need twelve photovoltaic cells to charge a cell phone and for to operate the house with the solar panels we will need a large number of photovoltaic cells.
The layer of the EVA sheeting on both sides of the solar panel cells is used to protect them from vibration, shocks, dirt and humidity. There are the two types of appearance on the solar panel. This is because of the difference in the internal crystalline lattice structure. Multi crystals are randomly located in polycrystalline solar panels.
Mono crystalline VS Poly crystalline solar panels:
Mono crystalline solar panels are dark back in colour whereas the poly crystalline is blue in colour. It has single crystal and is made up of pure silicon. The black mono panel with a black aluminium frame. To make the ingot the silicon rocks are melted at 2500° F and then a seed crystal is lowered in to the melted slush and slowly pulled up while rotating. It is almost like making a hand dipped candle but instead of melting wax, we are melting rocks. Because of the round shape there is a lot of material wasted as it is cut in the require shape. That’s why they have rounded corner to minimize the waste. Polycrystalline are made up of multiple cells that are melted into one wafer. Poly cells are made with a newer manufacturing process and it is cheaper than mono crystalline.
Polly crystalline cells are made a different way. They load about 1300 pounds of the silicon rocks in to a 3 by 3 feet quartz mold to create a square shape and then load in to the 2500° F furnace. It takes about 20 hours to melt and about 3 days to cool down. The poly crystalline panel has a blue mottled look like a piece of particle board it looks like it is made up of multiple pieces of silicon pressed together. That is actually caused from when the melted silicon cools and hardens. It crystalize like a frost on a window. Mono crystalline has distinctive shape of the mono cells which are cut off at the corner. The mono panel are more efficient which does not mean that they perform better what it means is that they can produce more power for the same space. Mono crystalline panels also do slightly better in lower light condition so they may produce power for a little bit longer each day. When the angle is not quite as good or the sun is not as strong however in the past large mono installations would suffer from some shading problem that largely been taken care of the by diodes in micro inverters. Poly panels are less heat tolerant which means that the degradation of power they produce is more as temperature rises than mono however because of their bluish colour. They absorb less heat so usually this is not problem. In some places the power generation through Photovoltaic cells are more economical then the other methods of the power generation.. Occasionally, they are used as photo detectors. Due to high cost of manufacturing the mono crystalline panels tends to be a little more expensive than polycrystalline panels. Although the efficiency in manufacturing process are really reducing the cost difference. Mono crystalline panels look different and cost little bit more than polycrystalline. Mono crystalline solar panels tend to be more efficient than polycrystalline solar panels. it has an average of about 17.5 % VS 15.5 % module efficiency. So mono crystalline is 2 % more efficient than polycrystalline. It means that we have slightly more power in same amount of space with mono crystalline than polycrystalline. Let us consider that we have mono crystalline and polycrystalline solar panels which are of same size about 38ʺ × 66ʺ. The mono crystalline output 270 watts and the polycrystalline solar panel output is 260 watts. Mono crystalline panel handle the heat slightly better than polycrystalline. Comparing the temperature coefficient of the two types shows us that mono crystalline short circuit current drops 0.04 % for every degree kelvin over standard test condition of 25 degree Celsius or 77 degree Fahrenheit. So if it is 20° hotter on the roof than in the test suit which is highly likely in the summer the mono crystalline solar panel can loss .06 Amps out of rated 8 Amps. For polycrystalline it loses 0.051% that equals to 0.08 Amps. In extreme desert condition the difference may be big enough to matter but for more residential environment the difference is equal. Finally the mono crystalline panel tend to behave a little better in less than perfect light condition. No solar panel regardless of their type performs well in the shade. If we have slightly shading issues or tend to have hazy skies, mono crystalline panels may perform better. However with the availability of micro inverters and DC optimizers each panel in the solar array, the difference may not be noticeable. Advances in the technology made them practically interchangeable. So the choice of which crystalline technology to use simply comes down to colour performance or space constraints.
Problems in using solar cells:
The solar energy unevenly distributed across the planet. Some areas are sunnier than others and it is also inconsistent. The solar energy is not available at the nigh or when there is cloudy weather. So we will imply such sorts of techniques to get electricity from the sunny spots to cloudy ones, and effective storage of energy. The efficiency of the cell is itself is a challenge too. If the sunlight is reflected instead of absorbed or if dislodged electrons fall back in to a hole before going through the circuit that photon loss energy. When there is full sunny day the solar cell convert only 46 % of the available sunlight to electricity and most commercial system are currently 15 to 20 % efficient. In spite of these limitations it actually would be possible to power the entire world with the solar technology. We need the funding to build the infrastructure and a good deal of space. Estimates ranges from ten to hundred thousands of square miles which seems a lot but the Sahara desert alone is over 3 million square miles in area. Meanwhile solar cells are getting better, cheaper and are competing with electricity from the grid and innovations like floating solar farms may change the landscape entirely. There is a fact that over a billion people do not have access to a reliable electric grid especially in developing countries many of which are sunny. So in places like that solar energy is already much cheaper and safer than available alternatives.