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LEARNING GOALS:Identify the relationships between PV cells, modules, and arrays. Describe the photovoltaic effect and the fundamental operation of PV devices. • Understand the current-voltage (I-V) characteristics for PV devices and define the key I-V parameters. Understand how the electrical load, solar radiation, and operating temperatures affect the electrical output of a PV device. • Translate the voltage, current, and power output of a PV device from a reference condition to another operating condition. • Determine the electrical output of similar and dissimilar PV devices connected in series and in parallel • Understand the construction and features of PV modules. • Describe the various performance rating conditions for PV modules. PHOTOVOLTAIC CELLSA photovoltaic cell is a semiconductor device that converts solar radiation into direct current electricity. Because the source of radiation is usually the sun, they are often referred to as solar cells. Individual PV cells are the basic building blocks for modules, which are in turn the building blocks for arrays and complete PV systems. Semiconductors PV cells are made from semiconductor materials. A semiconductor is a material that can exhibit properties of both an insulator and a conductor. Semiconductors behave like insulators at very low temperature and their conductivity increases with temperature. At normal temperatures, a semiconductor's electrical conductivity is between that of an insulator and a conductor. Some semiconductors also produce a voltage or exhibit a change in electrical conductivity when exposed to light. Most PV cells use variations of silicon altered by doping to make them suitable semiconductors. Doping is the process of adding small amounts of impurity elements to semiconductors to alter their electrical proper ties. Pure crystalline silicon has four valence (outer) electrons that each bond with the outer electrons of other silicon atoms to form a crystalline structure. When a small amount of boron, which has three valence electrons, is added to silicon crystals, the boron atoms take the places of a few silicon atoms. The crystalline structure where boron bonds to silicon then has an electron void at the location where a fourth electron is absent. This void is also called a hole, since it can be filled by other electrons. This absence of a negative charge is considered a positive charge carrier. A p-type semiconductor is a semiconductor that has electron voids. --1. The basic building blocks for PV systems include cells, modules, and arrays. --2. Semiconductor materials with special electrical properties can be made by adding small amounts of other elements to silicon crystals.: Semiconductors The term photovoltaic Is a combination of the Greek word photos, meaning light, and voltage, which is named after the Italian physicist Alessandro Volta N-TYPE EXTRA ELECTRON MODULE ARRAY BORON ATOM SILICON ATOMS IN CRYSTAL STRUCTURE PHOSPHOROUS ATOM--ELECTRON VOID (HOLE) P-TYPE SILICON ATOMS IN CRYSTAL STRUCTURE The addition of phosphorous, which has five valence electrons, to silicon results in an electron at the location where the phosphorous bonds to the silicon. This electron is only weakly bound to the phosphorous atom and can be easily induced to move through the material. It’s considered a negative charge carrier. An n-type semiconductor is a semiconductor that has free electrons. Photovoltaic Effect The basic physical process by which a PV cell converts light into electricity is known as the photovoltaic effect. The photovoltaic effect is the movement of electrons within a material when it absorbs photons with energy above a certain level. A photon is a unit of electro magnetic radiation. Photons contain various amounts of energy depending on their wave length, with higher energies associated with shorter wavelengths (higher frequency). Pho tons of light transfer their energy to electrons in the material surface. The extra electrons with enough energy to escape from their atoms are conducted as an electric current. Because of the electric result, the photovoltaic effect is also sometimes called the photoelectric effect. A PV cell is a thin, flat wafer consisting of a p-n junction. A p-n junction is the boundary of adjacent layers of p-type and n-type semiconductor materials in contact with one another. When the p-n junction is illuminated, high- energy photons absorbed at the junction impart their energy to extra electrons in the material, moving the electrons to a higher energy state. The electrons gain potential energy and are in a position to do useful work before returning to a lower energy state. PV cells are wafers made of crystalline semiconductors covered with a grid of electrically-conductive metal traces. --3. The photovoltaic effect produces free electrons that must travel through conductors in order to recombine with electron voids, or "holes." - Photovoltaic Effect; LOAD ELECTRON FLOW ELECTRON Many of the photons reaching a PV cell have energies greater than the amount needed to excite the electrons into a conductive state The extra energy imparts heat into the crystalline structure of the cell. When these electrons are excited, they can move around to other atoms, leaving behind voids, or holes. The holes can act in a similar manner to the electrons, appearing to move when a neighboring electron moves to fill a hole, but they are associated with a positive charge. An electrical field produced by the p-n junction prevents the electrons and holes from immediately recombining, which would accomplish no work. The electrons are repelled from the p-type layer toward the top surface of the cell, and the holes are repelled away from the n-type layer toward the bottom surface. This creates a difference in electrical potential (voltage) between the top and bottom surfaces. The free electrons are collected by metal contacts on the top surface of the cell and the holes migrate toward the bottom surface. For the electrons and holes to recombine, the electrons must travel from the top surface to the bottom surface. This is accomplished by connecting the surfaces with conductors and loads. The electrons flow through the loads, doing electrical work, then arrive at the back surface of the cell and recombine with the holes. This process of electrons and holes being separated by photon energy, and doing work before recombining, occurs continuously while PV cells are exposed to light. There is no way to turn off a PV device, other than completely covering the top surface with an opaque material so that no light reaches the cells. Cell Materials Like other semiconductor devices, PV cells must be manufactured in extremely clean environments. So, workers must wear special clothing to keep from contaminating the cells or the machines with dust or hair. PV cells can be produced from a variety of semiconductor materials, though crystalline silicon is by far the most common. The base raw material for silicon cell production is at least 99.99% pure polysilicon, a product refined from quartz and silica sands. Various grades of polysilicon, ranging from semiconductor to metallurgical grades, may be used in PV cell production and affect the quality and efficiency of cells produced. Crystalline silicon (c-Si) cells currently offer the best ratio of performance to cost com pared to competing materials and utilize many of the same raw materials and processes as the semiconductor industry. However, significant research is directed toward developing new PV cell material technologies, as well as improving efficiency and reducing costs of existing technologies. Gallium arsenide (GaAs) cells are more efficient than c-Si cells, but the high cost and toxicity of the GaAs materials have limited their use to space applications. GaAs can be alloyed with indium, phosphorus, and aluminum to create semiconductors that respond to different wavelengths of electromagnetic radiation. This property is utilized to make multi-junction cells, producing highly efficient cells attractive for concentrating PV applications. A multi-junction cell is a cell that maximizes efficiency by using layers of individual cells that each respond to different wavelengths of solar energy. The top layer captures the shortest wavelength radiation, while the longer wavelength components pass through and are absorbed by the lower layers. Thin-film PV devices are module-based approaches to cell design. A thin-film module is a module-like PV device with its entire substrate coated in thin layers of semiconductor material using chemical vapor deposition techniques, and then laser-scribed to delineate individual cells and make electrical connections between cells. Amorphous silicon (a-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe) are among the competing thin-film technologies today. Thin-film modules are less costly to produce and use considerably less raw material than crystalline silicon modules, but most are less efficient than crystalline silicon and may not be as durable in the field. A photo-electrochemical cell is a cell that relies on chemical processes to produce electricity from light, rather than using semiconductors. Photo-electrochemical cells include dye-sensitized cells (Gratzel cells) and polymer (plastic) cells, and are sometimes called organic cells. While the engineering challenges in developing these advanced cells are considerable, some are expected to impact commercial markets within the next decade. Wafer Manufacturing The manufacture of commercial silicon modules involves fabricating silicon wafers, trans forming the wafers into cells, and assembling cells into modules. A wafer is a thin, flat disk or rectangle of base semiconductor material. Wafers are 180 um to 350 um thick and are made from p-type silicon. Crystalline silicon cell wafers are produced in three basic types: monocrystalline, polycrystalline, and ribbon silicon. Each type has advantages and disadvantages in terms of efficiency, manufacturing, and costs. Monocrystalline Silicon. A monocrystalline wafer is a silicon wafer made from a single silicon crystal grown in the form of a cylindrical ingot. Chunks of highly pure polysilicon are melted in a crucible, along with boron. A small seed crystal is dipped into the molten bath and slowly rotated and withdrawn. Over a period of many hours, the seed crystal grows into a large cylindrical crystal up to 40" in length and 8" in diameter. Because the ingot is round, the edges are often cropped to a more rectangular or square shape, which allows cells to be packed more closely in a module. Individual wafers are then cut from the ingot using diamond wire saws. Commercial mono- crystalline cells have efficiencies on the order of 14% to 17%, with some laboratory samples having efficiencies as high as about 25%. Robots are often used in the manufacture of PV cells because they perform repetitive tasks easily and don’t pose a contamination risk. PV Material Efficiencies: Multijunction gallium arsenide (GaAs) Monocrystalline silicon Polycrystalline silicon Copper indium gallium selenide (CIGS) Cadmium telluride (CdTe) Amorphous silicon (a-Si) Dye-sensitized (Gratzel) Polymer (Organic) --4. Various PV materials and technologies produce different efficiencies. === EXAMPLE: Toledo JATC Location: Toledo, OH Type of System: Stand-alone portable unit Peak Array Power: 69 W DC Date of Installation: Jan 2010 Installers: Apprentices and journeymen Purposes: Training While the installation of large rooftop PV systems provides an excel lent opportunity to teach apprentices and journeymen about nearly all aspects of a PV system, typically only a small group of electricians is able to be fully involved in this type of installation process. For subsequent groups, often only demonstrations and walkthroughs of the established and operating PV system are possible, limiting opportunities for hands-on installation training. Several training centers are experimenting with ways to design and implement repeatable PV systems that can be used to provide PV system installation experience to many classes of students. One method of facilitating training and demonstrations is to scale a complete system down to a portable, self-contained unit, such as the system at the Cleveland JATC training center. This unit is configured as a stand-alone PV system and includes a small battery bank. There are several benefits to using small, portable PV systems for training. First, the system is self-contained, which allows it to be easily moved indoors or outdoors as needed. Also, with all the components mounted together and very short conductor runs, the system can be easily disassembled or modified for training purposes. Students can practice selecting appropriate conductors and overcurrent protection devices, making electrical connections, configuring electronic controls, and testing and troubleshooting the system. This type of system can also be augmented by the addition of a small wind turbine generator to add DC power to the inverter input circuit and boost the AC output. The unit's wheels and array mounting allow students to experiment with array tilt and azimuth angles and module shading, observing first-hand how these variables affect power output. The system is configured to operate a variety of loads, such as a battery charge controller, lights, or power tools, or any other AC toad connected to the receptacles. To demonstrate the operation of diversion loads, an attached water heater turns excess power into useful hot water. The only significant drawback to the system is that it cannot provide useful electrical power to the training facility because it’s so small and because, in order to maintain its portability the system is not connected to the on-site electrical system. However, the experience gained from the portable system has encouraged the training center to investigate a future installation of a 5 kW to 6 kW utility-interactive system for the building. In addition to providing an appreciable amount of power to the building, the installation of this system would complement the portable training unit. The portable training unit includes all the components used in a typical stand-alone PV system, but is more versatile. The component configuration allows for easy testing, troubleshooting, and connection of a variety of load types. ==== --5. Monocrystalline silicon wafers are sawn from grown cylindrical ingots. Polycrystalline Silicon. A polycrystalline wafer is a silicon wafer made from a cast silicon ingot that is composed of many silicon crystals. Molten silicon is poured into a crucible to form an ingot, which is slowly and carefully cooled over several hours. During cooling, many silicon crystals form and grow as the molten material solidifies. The cast ingot is then sectioned with wire saws to form square or rectangular wafers. Polycrystalline wafers can sometimes be distinguished from monocrystalline wafers by their square corners and the grain boundaries appearing on the wafer surface. While polycrystalline cells have slightly lower efficiencies (11.5% to 14%) than monocrystalline cells, their lower manufacturing costs and denser packing in modules makes them competitive with monocrystalline modules. --6. Polycrystalline silicon wafers are sawn from cast rectangular ingots. Polycrystalline Ingots Ribbon Silicon. A ribbon wafer is a silicon wafer made by drawing a thin strip from a molten silicon mixture. The melted material is pulled between parallel dies where it cools and solidifies to form a continuous multicrystalline ribbon. The ribbon is then cut at specific intervals to form rectangular-shaped wafers. While cells produced from ribbon silicon wafers have slightly lower efficiencies (11% to 13%) than other silicon cells, this process is less expensive because there is less material waste and it does not require ingot sawing. Cell Fabrication Once a crystalline silicon wafer is produced, it must go through additional processing to become a functional PV cell. First, the wafers are dipped in a sodium hydroxide solution to etch the surface and remove imperfections introduced during the sawing process. The textured surface increases surface area, allows subsequent coatings to adhere better, and minimizes reflected sunlight. --8. Diffusion of phosphorous gas creates a thin n-type semiconductor layer over the entire surface of a p-type wafer. --7. Several steps are involved in turning silicon wafers into PV cells. Cell Fabrication: ETCHED SURFACE EDGE VIEW OF P-TYPE WAFER ETCHING C-SURFACE BECOMES N-TYPE PHOSPHOROUS DIFFUSION-- TRIMMED EDGES ALUMINIZED BACK SURFACE ELECTRICAL CONTACTS EDGE ABRASION -- PATTERN After the wafers are cleaned they are placed on racks and into a diffusion furnace, where phosphorous gas penetrates the outer surfaces of the cell, creating a thin n-type semiconductor layer surrounding the original p-type semiconductor material. The edge of the wafer is then abraded to remove the n-type material. Antireflective coatings are then applied to the top surface of the cell to further reduce reflected sunlight and improve cell efficiency. After the coatings dry, grid patterns are screen printed on the top surface of the cell with silver paste to provide a point for electron collection and the electrical connection to other cells. These grid lines generally include two or more main strips across the cell, with finer lines emanating from the main strips across the cell surface. The configuration of these grid patterns is a critical part of cell design, because they must be of sufficient size and distribution to be able to efficiently collect and conduct current away from the cell, but must be minimized to avoid covering much of the cell surface, which lowers the effective cell surface area exposed to sunlight. Finally, the entire back surface of the cell is coated with a thin layer of metal, typically aluminum, which alloys with the silicon and neutralizes the n type semiconductor layer on the back surface. This results in the bottom surface of the cell being the positive connection, while the top surface is negative. After cells are produced, each is electrically tested under simulated sunlight and sorted according to its current output. This sorting process largely eliminates problems with cur rent mismatch among series-connected cells and allows manufacturers to produce modules that are of the same physical size but have different power ratings. 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