Solar Cell
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.
When research into electricity began and simple batteries were being made and studied, research into solar electricity followed amazingly quickly. As early as 1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce voltage. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.
With Albert Einstein’s explanation in 1905 of the photoelectric effect—metal absorbs energy from light and will retain that energy until too much light hits it—hope soared anew that solar electricity at higher efficiencies would become feasible. Little progress was made, however, until research into diodes and transistors yielded the knowledge necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a silicon solar cell of four percent efficiency in 1954.
Further work brought the cell’s efficiency up to 15 percent. Solar cells were first used in the rural and isolated city of Americus, Georgia as a power source for a telephone relay system, where it was used successfully for many years.
A type of solar cell to fully meet domestic energy needs has not as yet been developed, but solar cells have become successful in providing energy for artificial satellites. Fuel systems and regular batteries were too heavy in a program where every ounce mattered. Solar cells provide more energy per ounce of weight than all other conventional energy sources, and they are cost-effective.
Only a few large scale photovoltaic power systems have been set up. Most efforts lean toward providing solar cell technology to remote places that have no other means of sophisticated power. About 50 megawatts are installed each year, yet solar cells provide only about. 1 percent of all electricity now being produced. Supporters of solar energy claim that the amount of solar radiation reaching the Earth’s surface each year could easily provide all our energy needs several times over, yet solar cells have a long way to go before they fulfill Charles Fritts’s dream of free, fully accessible solar electricity.
Raw Materials
The basic component of a solar cell is pure silicon, which is not pure in its natural state.
To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification.
Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.
The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.
The Manufacturing Process
Purifying the silicon
- 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
- 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure drags the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.
Making single crystal silicon
- 3 Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or boule of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.
Making silicon wafers
- 4 From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer- 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to ‘drag the impurities toward one end of the rod. The impure end can then be removed. Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
Doping
- 6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms burrow into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth. A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.
Placing electrical contacts
- 7 Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated
This illustration shows the makeup of a typical solar cell. The cells are encapsulated in ethylene vinyl acetate and placed in a metal frame that has a mylar backsheet and glass cover.
The anti-reflective coating
- 9 Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide, though others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride.
Encapsulating the cell
- 10 The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.
Quality Control
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect the overall efficiency of the cells. The primary research goal is to find ways to improve the efficiency of each solar cell over a longer lifetime. The Low Cost Solar Array Project (initiated by the United States Department of Energy in the late 1970s) sponsored private research that aimed to lower the cost of solar cells. The silicon itself is tested for purity, crystal orientation, and resistivity. Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects). Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching.
During the entire silicon disk manufacturing process, the temperature, pressure, speed, and quantities of dopants are continuously monitored. Steps are also taken to ensure that impurities in the air and on working surfaces are kept to a minimum.
The completed semiconductors must then undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards. An earlier problem with solar cells was a tendency to stop working when partially shaded. This problem has been alleviated by providing shunt diodes that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.
An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well. The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.
The final test for solar modules is field site testing, in which finished modules are placed where they will actually be used. This provides the researcher with the best data for determining the efficiency of a solar cell under ambient conditions and the solar cell’s effective lifetime, the most important factors of all.
The Future
Considering the present state of relatively expensive, inefficient solar cells, the future can only improve. Some experts predict it will be a billion-dollar industry by the year 2000. This prediction is supported by evidence of more rooftop photovoltaic systems being developed in such countries as Japan, Germany, and Italy. Plans to begin the manufacture of solar cells have been established in Mexico and China. Likewise, Egypt, Botswana, and the Philippines (all three assisted by American companies) are building plants that will manufacture solar cells.
Most current research aims for reducing solar cell cost or increasing efficiency. Innovations in solar cell technology include developing and manufacturing cheaper alternatives to the expensive crystalline silicon cells. These alternatives include solar Windows that mimic photosynthesis, and smaller cells made from tiny, amorphous silicon balls. Already, amorphous silicon and polycrystalline silicon are gaining popularity at the expense of single crystal silicon. Additional innovations including minimizing shade and focusing sunlight through prismatic lenses. This involves layers of different materials (notably, gallium arsenide and silicon) that absorb light at different frequencies, thereby increasing the amount of sunlight effectively used for electricity production.
A few experts foresee the adaptation of hybrid houses; that is, houses that utilize solar water heaters, passive solar heating, and solar cells for reduced energy needs. Another view concerns the space shuttle placing more and more solar arrays into orbit, a solar power satellite that beams power to Earth solar array farms, and even a space colony that will manufacture solar arrays to be used on Earth.
Where To Learn
Books
Bullock, Charles E. and Peter H. Grambs. Solar Electricity: Making the Sun Work for You. Monegon, Ltd., 1981.
Komp, Richard J. Practical Photovoltaics. Aatec Publications, 1984.
Making and Using Electricity from the Sun. Tab Books, 1979.
Periodicals
Crawford, Mark. DOE’s Born-Again Solar Energy Plan, Science. March 23, 1990, pp. 1403-1404.
Waiting for the Sunrise, Economist. May 19, 1990, pp. 95.
Edelson, Edward. Solar Cell Update, Popular Science. June, 1992, p. 95.
Murray, Charles J. Solar Power’s Bright Hope, Design News. March 11, 1991, p. 30.
Photovoltaic Array
If photovoltaic solar panels are made up of individual photovoltaic cells connected together, then the Solar Photovoltaic Array, also known simply as a Solar Array is a system made up of a group of solar panels connected together.
A photovoltaic array is therefore multiple solar panels electrically wired together to form a much larger PV installation (PV system) called an array, and in general the larger the total surface area of the array, the more solar electricity it will produce.
A complete photovoltaic system uses a photovoltaic array as the main source for the generation of the electrical power supply. The amount of solar power produced by a single photovoltaic panel or module is not enough for general use.
Most manufactures produce a standard photovoltaic panel with an output voltage of 12V or 24V. By connecting many single PV panels in series (for a higher voltage requirement) and in parallel (for a higher current requirement) the PV array will produce the desired power output.
A Photovoltaic Solar Array
Photovoltaic cells and panels convert the solar energy into direct-current (DC) electricity. The connection of the solar panels in a single photovoltaic array is same as that of the PV cells in a single panel.
The panels in an array can be electrically connected together in either a series, a parallel, or a mixture of the two, but generally a series connection is chosen to give an increased output voltage. For example, when two solar panels are wired together in series, their voltage is doubled while the current remains the same.
The size of a photovoltaic array can consist of a few individual PV modules or panels connected together in an urban environment and mounted on a rooftop, or may consist of many hundreds of PV panels interconnected together in a field to supply power for a whole town or neighbourhood. The flexibility of the modular photovoltaic array (PV system) allows designers to create solar power systems that can meet a wide variety of electrical needs, no matter how large or small.
It is important to note that photovoltaic panels or modules from different manufacturers should not be mixed together in a single array, even if their power, voltage or current outputs are nominally similar. This is because differences in the solar cell I-V characteristic curves as well as their spectral response are likely to cause additional mismatch losses within the array, thereby reducing its overall efficiency.
The Electrical Characteristics of a Photovoltaic Array
The electrical characteristics of a photovoltaic array are summarised in the relationship between the output current and voltage. The amount and intensity of solar insolation (solar irradiance) controls the amount of output current ( I ), and the operating temperature of the solar cells affects the output voltage ( V ) of the PV array. Photovoltaic panel ( I-V ) curves that summarise the relationship between the current and voltage are given by the manufacturers and are given as:
Solar Array Parameters
- VOC = open-circuit voltage: – This is the maximum voltage that the array provides when the terminals are not connected to any load (an open circuit condition). This value is much higher than Vmax which relates to the operation of the PV array which is fixed by the load. This value depends upon the number of PV panels connected together in series.
- ISC = short-circuit current – The maximum current provided by the PV array when the output connectors are shorted together (a short circuit condition). This value is much higher than Imax which relates to the normal operating circuit current.
- Pmax = maximum power point – This relates to the point where the power supplied by the array that is connected to the load (batteries, inverters) is at its maximum value, where Pmax = Imax x Vmax. The maximum power point of a photovoltaic array is measured in Watts (W) or peak Watts (Wp).
- FF = fill factor – The fill factor is the relationship between the maximum power that the array can actually provide under normal operating conditions and the product of the open-circuit voltage times the short-circuit current, ( Voc x Isc ) This fill factor value gives an idea of the quality of the array and the closer the fill factor is to 1 (unity), the more power the array can provide. Typical values are between 0.7 and 0.8.
- % eff = percent efficiency – The efficiency of a photovoltaic array is the ratio between the maximum electrical power that the array can produce compared to the amount of solar irradiance hitting the array. The efficiency of a typical solar array is normally low at around 10-12%, depending on the type of cells (monocrystalline, polycrystalline, amorphous or thin film) being used.
Photovoltaic I-V characteristics curves provide the information designers need to configure systems that can operate as close as possible to the maximum peak power point. The peak power point is measured as the PV module produces its maximum amount of power when exposed to solar radiation equivalent to 1000 watts per square metre, 1000 W/m 2 or 1kW/m 2. Consider the circuit below.
Photovoltaic Array Connections
This simple photovoltaic array above consists of four photovoltaic modules as shown, producing two parallel branches in which there are two PV panels that are electrically connected together to produce a series circuit. The output voltage from the array will therefore be equal to the series connection of the PV panels, and in our example above, this is calculated as: Vout = 12V 12V = 24 Volts.
The output current will be equal to the sum of the parallel branch currents. If we assume that each PV panel produces 3.75 amperes at full sun, the total current ( IT ) will be equal to: IT = 3.75A 3.75A = 7.5 Amperes. Then the maximum power of the photovoltaic array at full sun can be calculated as: Pout = V x I = 24 x 7.5 = 180W.
The PV array reaches its maximum of 180 watts in full sun because the maximum power output of each PV panel or module is equal to 45 watts (12V x 3.75A). However, due to different levels of solar radiation, temperature effect, electrical losses etc, the real maximum output power is usually a lot less than the calculated 180 watts. Then we can present our photovoltaic array characteristics as being.
Bypass Diodes in Photovoltaic Arrays
Photovoltaic cells and diodes are both semiconductor devices made from a P-type silicon material and a N-type silicon material fused together. Unlike a photovoltaic cell which generates a voltage when exposed to light, PN-junction diodes act like solid state one way electrical valve that only allows electrical current to flow through themselves in one direction only.
The advantage of this is that diodes can be used to block the flow of electric current from other parts of an electrical solar circuit. When used in a photovoltaic solar array, these types of silicon diodes are generally called Blocking Diodes.
In the previous tutorial about photovoltaic panels, we saw that a bypass diode can be used in parallel with either a single or a number of photovoltaic solar cells. The addition of a diode prevents current(s) flowing from a good and well-exposed PV cells, overheating and burning out weak or partially shaded PV cells by providing a current path around the bad cell. Blocking diodes are used differently than bypass diodes.
Bypass diodes are usually connected in “parallel” with a PV cell or panel to shunt the current around it, whereas blocking diodes are connected in “series” with the PV panels to prevent current flowing back into them. Blocking diodes are therefore different then bypass diodes although in most cases the diode is physically the same, but they are installed differently and serve a different purpose. Consider our photovoltaic solar array below.
Diodes in Photovoltaic Arrays
As we said earlier, diodes are devices that allow current to flow in one direction only. The diodes coloured green are the familiar bypass diodes, one in parallel with each PV panel to provide a low resistance path around the panel. However, the two diodes coloured red are referred to as the “blocking diodes”, one in series with each series branch. These blocking diodes ensure that the electrical current only flows OUT of the series array to the external load, controller or batteries.
The reason for this is to prevent the current generated by the other parallel connected PV panels in the same array flowing back through a weaker (shaded) network and also to prevent the fully charged batteries from discharging or draining back through the PV array at night. So when multiple PV panels are connected in parallel, blocking diodes should be used in each parallel connected branch.
Generally speaking, blocking diodes are used in PV arrays when there are two or more parallel branches or there is a possibility that some of the array will become partially shaded during the day as the sun moves across the sky. The size and type of blocking diode used depends upon the type of photovoltaic array. Two types of diodes are available for solar power arrays: the PN-junction silicon diode and the Schottky barrier diode. Both are available with a wide range of current ratings.
The Schottky barrier diode has a much lower forward voltage drop of about 0.4 volts as opposed to the PN diodes 0.7 volt drop for a silicon device. This lower voltage drop allows a savings of one full PV cell in each series branch of the solar array therefore, the array is more efficient since less power is dissipated in the blocking diode. Most manufacturers include blocking diodes within their PV modules simplifying the design.
Build your own Photovoltaic Array
The amount of solar radiation received and the daily energy demand are the two controlling factors in the design of the photovoltaic array and solar power systems. The photovoltaic array must be sized to meet the load demand and account for any system losses while the shading of any part of the solar array will significantly reduce the output of the entire system.
If the solar panels are electrically connected together in series, the current will be the same in each panel and if panels are partially shaded, they cannot produce the same amount of current. Also shaded PV panels will dissipate power and waste as heat rather than generate it and the use of bypass diodes will help prevent such problems by providing an alternative current path.
Blocking diodes are not required in a fully series connected system but should be used to prevent a reverse current flow from the batteries back to the array during the night or when the solar irradiance is low. Other climatic conditions apart from sunlight must be considered in any design.
Since the output voltage of silicon solar cell is a temperature related parameter, the designer must be aware of the prevailing daily temperatures, both extremes (high and low) and seasonal variations. In addition, rain and snowfall must be considered in the design of the mounting structure. Wind loading is especially important in mountain top installations.
In our next tutorial about “Solar Power”, we will look at how we can use semiconductor photovoltaic arrays and solar panels as part of a Stand Alone PV System to generate power for off-grid applications.
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- Standard Test Conditions
- Temperature Coefficient of a PV Cell
- Bypass Diode
- Solar Cell I-V Characteristic
- Photovoltaics Turning Photons into Electrons
- How Many Solar Cells Do I Need
- Photovoltaic Panel
- Photovoltaic Types
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Hi there my name is Matt D’Agati. Solar technology has grown to become probably the most promising and sought-after sourced elements of clean, renewable energy in the past few years. This is due to its numerous benefits, including financial savings, energy efficiency, and also the positive impact this has from the environment. In this specific article, we are going to talk about the advantages of choosing solar energy in homes and businesses, the technology behind it, and just how it could be implemented to increase its benefits. One of many benefits of using solar power in homes may be the financial savings it gives. Solar energy panels can handle generating electricity for your house, reducing or eliminating the need for traditional sourced elements of energy. This will probably bring about significant savings on the monthly energy bill, particularly in areas with a high energy costs. In addition, the expense of solar energy panels and associated equipment has decreased significantly over time, which makes it more affordable for homeowners to invest in this technology. Another good thing about using solar power in homes is the increased value it may provide into the property. Homes that have solar power panels installed are often valued greater than homes that don’t, while they offer an energy-efficient and environmentally friendly replacement for traditional energy sources. This increased value may be a significant benefit for homeowners that are trying to sell their house as time goes by. For businesses, the advantages of using solar power are wide ranging. One of several primary benefits is financial savings, as businesses can significantly reduce their energy costs by adopting solar power. In addition, there are many government incentives and tax credits accessible to companies that adopt solar technology, which makes it much more affordable and cost-effective. Furthermore, companies that adopt solar power can benefit from increased profitability and competitiveness, since they are seen as environmentally conscious and energy-efficient. The technology behind solar technology is not at all hard, yet highly effective. Solar energy panels are made up of photovoltaic (PV) cells, which convert sunlight into electricity. This electricity are able to be kept in batteries or fed straight into the electrical grid, with respect to the specific system design. So that you can maximize the many benefits of solar power, it is critical to design a custom system this is certainly tailored to your unique energy needs and requirements. This may make sure that you have just the right components in position, including the appropriate amount of solar energy panels plus the right types of batteries, to increase your power efficiency and value savings. One of several important aspects in designing a custom solar technology system is knowing the various kinds of solar energy panels and their performance characteristics. There are 2 main kinds of solar power panels – monocrystalline and polycrystalline – each featuring its own benefits and drawbacks. Monocrystalline solar power panels are made of an individual, high-quality crystal, helping to make them more cost-effective and sturdy. However, they are more costly than polycrystalline panels, that are produced from multiple, lower-quality crystals. Along with solar energy panels, a custom solar power system will also include a battery system to keep excess energy, in addition to an inverter to convert the stored energy into usable electricity. It is essential to choose a battery system that is capable of storing the actual quantity of energy you want for the specific energy needs and requirements. This can make certain you have a trusted way to obtain power in the case of power outages or any other disruptions to your time supply. Another advantage of using solar power may be the positive impact this has from the environment. Solar power is on a clean and renewable power source, producing no emissions or pollutants. This makes it an ideal replacement for traditional resources of energy, such as for example fossil fuels, that are a major contributor to polluting of the environment and greenhouse gas emissions. By adopting solar power, homeowners and businesses can really help reduce their carbon footprint and play a role in a cleaner, more sustainable future. In closing, the advantages of using solar energy both in homes and companies are numerous and should not be overstated. From cost benefits, energy savings, and increased property value to environmental impact and technological advancements, solar technology provides a variety of advantages. By knowing the technology behind solar technology and designing a custom system tailored to specific energy needs, you’ll be able to maximize these benefits and then make a positive effect on both personal finances while the environment. Overall, the adoption of solar energy is an intelligent investment for a sustainable and bright future. Should you want to learn about more info on this fact matter take a look at a blog:
Before some time I didn’t have much more knowledge about solar panel but before 2 or 3 yrs I have installed solar panel system in my home’s top roof and that time I examined many companies and much researched on this. So I can say all about the solar PV system. A PV module is an assembly of photo-voltaic cells mounted in a frame work for installation. Photo-voltaic cells use sunlight as source of energy and generate direct current electricity. A collection of PV modules is called a PV Panel, and a system of Panels is an Array. Arrays of a photovoltaic system supply solar electricity to electrical equipment.
can i connect two strings in parallel with different voltage (20 Panels x30V =600V 10 Panels x30V =300V) to a string monitoring Unit ( SMU) ?? if it connected what will be the voltage at common output terminal.
Would you please send a formal quotation for the following item: The Solar Photovoltaic Array system Your quotation should include shipping till Cairo, delivery time and all your terms and conditions.
HI there. Interesting read. I think I understand the logic. I live on a boat that has 4x Shell SM110-24 (rated output 110w; rated current 3.15A; rated voltage 35V). Becuase of the mast and boom these is often a shadow that passes over one or more of the panels – often leaving the other panels fully exposed to direct light. From what I read the shadow on one panel is likely to be affecting the performance of the whole array. It’s almost that I need each individual panel going back to the batteries individually – but thats a lot of wiring. What would be the best set up – series or parallel – and use of blockign diodes to minimise the impact of the shadow on one part of the array? Cheers.
A series or parallel connection for your panels depends on your system requirements. These panels already have Bypass diodes built-in to prevent high currents from partial shading, but yes shading affects the performance and efficiency of the array. Blocking diodes prevent reverse currents from the battery to the panel when the panel is not generating electricity, for example night time. If you are using a charging regulator then blocking diodes are not normally require as it has protection built-in, only if you are charging batteries directly.
In a series connected system, current is common to ALL panels. Therefore the current will be equal to the lowest wattage panel in the series chain as: I = P/V
Dear Sir/Madam, Highly informative and concise artical on the subject. Great service to the electrical engineering community. Keep it up, God bless you with lot of wisdom and knowledge. Best regards.
Photovoltaic Cells (Solar Cells)
Definition: Photovoltaic cells are basically those semiconductor devices that show sensitivity towards light. It has the ability to change radiation energy into equivalent electrical energy. The name of the device itself shows its operation. As the word photo is used for light and voltaic is used for electricity.
Photovoltaic cells are also known as solar cells as it makes use of solar energy in general basis. However, it is not necessary that we only use a natural source of light energy as one can also make use of an artificial light source at the time of operation of photovoltaic cells. It produces a proportional voltage as the output of incident radiation. The working principle of the device is based on the photovoltaic effect.
Now the question arises what is Photovoltaic Effect?
So, a photovoltaic effect is a combination of a physical and chemical process that generates a potential difference when the device is exposed to radiation. This generated voltage corresponds to the intensity of incident radiation.
It is to be noteworthy in the case of solar cells that it does not need an external voltage source. As due to the applied illumination a proportionate voltage is generated at the junction of the device.
Symbol of Photovoltaic Cell
The symbolic representation of a photovoltaic cell is given below:
Construction of Photovoltaic cell
The photovoltaic cell is a semiconductor pn junction device. However, its construction is not the same as a normal junction diode.
It is formed by a combination of p-type semiconductor material with an n-type semiconductor. Usually, silicon and selenium are used as the basic semiconductor material. However, gallium arsenide, cadmium sulphide are also majorly used.
The figure below represents the cross-sectional arrangement of a solar cell:
Here, the above figure clearly represents that the top layer of the device i.e., p region is made very thin in comparison to n region.
The reason why we have constructed the two regions differently is that the region from where the light ray is allowed to incident must be thin. So, that the incident radiation can easily penetrate to the depletion region (pn junction).
A layer of glass is placed at the top surface of the device in order to gather maximum incident radiation. Also, metallic fingers are provided at the top surface of the structure. Here, we have used silver fingers. As they are good conductors and absorbs the released electrons easily in order to provide proper conduction.
Now, the question arises why we have placed the silver fingers at some distance with the other?
The answer is that if we place a layer of metal at the top surface then it does not allow the penetration of radiation to the depletion region. So, it is placed in such a way that light rays can easily reach the junction region.
Also at the bottom, a metallic contact is placed which is generally a nickel plating that acts as negative contact for the whole structure.
Working of Photovoltaic cell
As we have already discussed at the beginning of the article that photovoltaic cells or solar cells are light sensitive devices that produce an electric voltage or current when its surface is illuminated with radiation.
So, now have a look at how potential difference is generated when no external potential is provided but a light ray is incident on it.
The figure below represents a detailed structure showing the working of the solar cells:
When no any light energy is provided to the device then the device does not conduct. Only the intensity of radiation falling at the surface allows a proportional current to flow through the device.
When a light ray is permitted to incident at the top surface of the structure. Then the glass placed gathers the light energy and permit it to reach the p region. As at the time of construction we have discussed that the thin p region is fabricated in order to have easy light penetration towards the junction region.
We know that junction is depletion region composed of neutral atoms. Thus, when a light ray incident at the junction then it produces electron-hole pairs. Due to the presence of depletion region an electric field exists.
Under the influence of the electric field, electrons move to the N side and holes drift towards the P side. In this way, the movement of charge carriers generates an electric current through the device.
However, it is to be noted here that the material is a semiconductor. Hence, it does not permit easy flow of charge carriers. So, to have a sufficient amount of current flow to take place metallic fingers are fabricated at the top surface.
These metallic fingers are nothing but conducting rods that easily absorbs the emitted electrons. This phenomenon gives rise to a potential difference. The generated emf is known as photovoltage. As the voltage is produced due to light.
It is to be noted here that the energy of the incident radiation (or photon energy) must be greater than the energy Band gap between valence and conduction Band. This is so because in order to have proper conduction electron must be free to get excited to the conduction Band.
In this way, an electric potential is generated by a photovoltaic cell without using external bias. It generates voltage nearly from 0.5 to 0.6V.
If we want to increase the overall output through the device, then multiple photocells can be parallelly connected.
Output Characteristic of Photovoltaic cells
The figure below represents the output voltage characteristic of a solar cell:
Here x-axis represents the intensity of incident light and the y-axis shows the output voltage produced. We can see here that with the increase in the intensity of the light there is an increase in the output voltage produced.
Moving further have a look at the output current characteristics of a solar cell with different load resistances:
Here also with the increase in the intensity of light the resultant current through the cell also increases. The output current is low and its value lies in microamperes. The achieved output current relies on the intensity of incident radiation, the size of the cell structure and the conversion efficiency of the device.
Advantages of Photovoltaic Cell
- These devices have a long life span as they are highly durable.
- It only requires excitation through light energy.
Disadvantages of Photovoltaic Cell
- It is expensive.
- The presence of light source is necessary.
- As it generates low output thus for high scale production large solar cells are required.
Applications of Photovoltaic Cell
Solar cells are widely used in space satellites systems. These also find its applications in light meters and solar power charging devices as it efficiently utilizes solar energy.
What is Photovoltaic or Solar Cell? – Construction, Working and Advantages
An electrical device which converts light energy into electrical energy through the photovoltaic effect is known as photovoltaic cell or PV cell or solar cell. A photovoltaic cell is basically a specially designed p-n junction diode.
Construction and Working of Photovoltaic Cell
The construction of a photovoltaic cell is shown in the following figure.
A photovoltaic cell consists of a base metal plate and it is made of either steel or aluminum over which a metallic selenium layer is situated which is light sensitive and acts as the positive terminal.
An electrically conducting layer of cadmium oxide is applied by sputtering over the selenium layer. This cadmium oxide layer is sufficiently thin in order to allow light to reach the selenium and as it is electrically conducting, hence acts as the negative terminal. A strip of metal sprayed on the edge of the top surface which forms the negative contact.
The transparent varnish layer is used to protect the front surface of the photovoltaic cell.
When light falls on the surface of selenium layer through the layer of cadmium oxide, the selenium compound releases the electrons that are sufficient to maintain the flow of current through the external circuit connected between the positive and negative terminals.
Advantages of Using Photovoltaic Cells
The advantages of using photovoltaic cells are listed below −
- Photovoltaic cells do not cause pollution while producing electricity.
- The operating cost of photovoltaic cells is low as source of energy is natural light.
- The maintenance cost of PV cells is also minimum as they need little maintenance.
- Photovoltaic cells have long lifespan. They are highly reliable.
- PV cells are the best renewable energy sources.
Disadvantages of Photovoltaic Cell
Following are some of the disadvantages of using photovoltaic cells −
- The operation of photovoltaic cells depends upon the light energy of the Sun, thus their operation depends upon the weather.
- Storage of electricity produced by the photovoltaic cells is expensive and complicated.
- They require more space for installation.
Applications of Photovoltaic Cell
The applications of photovoltaic cells include the following −
- Remote lighting systems
- Emergency power
- For Satellites power supplies
- In photometric measurements
- As portable power supplies such as solar car, etc.