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Cells, Modules, and Arrays

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1 Cells, Modules, and Arrays
Photovoltaic Systems Cells, Modules, and Arrays Photovoltaic Cells • Current–Voltage (I–V) Curves • PV Device Response • Modules and Arrays Arizona Solar Power Society

2 The basic building blocks for PV systems include cells, modules, and arrays.
A 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. See Figure 5-1.

3 Semiconductor materials with special electrical properties can be made by adding small amounts of other elements to silicon crystals. 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 properties. 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. See Figure 5-2.

4 The photovoltaic effect produces free electrons that must travel through conductors in order to recombine with electron voids, or “holes.” 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 electromagnetic radiation. Photons contain various amounts of energy depending on their wavelength, with higher energies associated with shorter wavelengths (higher frequency). Photons 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. See Figure 5-3.

5 Various PV materials and technologies produce different efficiencies.
Crystalline silicon (c-Si) cells currently offer the best ratio of performance to cost compared 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. See Figure 5-4.

6 Monocrystalline silicon wafers are sawn from grown cylindrical ingots.
A monocrystalline wafer is a silicon wafer made from a single silicon crystal grown in the form of a cylindrical ingot. See Figure 5-5. 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 monocrystalline cells have efficiencies on the order of 14% to 17%, with some laboratory samples having efficiencies as high as about 25%.

7 Polycrystalline silicon wafers are sawn from cast rectangular ingots.
A polycrystalline wafer is a silicon wafer made from a cast silicon ingot that is composed of many silicon crystals. See Figure 5-6. 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.

8 Several steps are involved in turning silicon wafers into PV cells.
Once a crystalline silicon wafer is produced, it must go through additional processing to become a functional PV cell. See Figure 5-7. 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.

9 Diffusion of phosphorous gas creates a thin n-type semiconductor layer over the entire surface of a p-type wafer. 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. See Figure The edge of the wafer is then abraded to remove the n-type material.

10 The different materials, processes, and manufacturing steps produce a range of PV cell types.
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. See Figure 5-9.

11 An I-V curve illustrates the electrical output profile of a PV cell, module, or array at a specific operating condition. When voltage is plotted against current for all the operating points, it forms a curve. An I-V curve is the graphic representation of all possible voltage and current operating points for a PV device at a specific operating condition. As voltage increases from zero, the current begins at its maximum and decreases gradually until the knee of the curve is reached. After the knee, small increases in voltage are associated with larger reductions in current, until the current reaches zero and the device is at maximum voltage. See Figure 5-10.

12 Open-circuit voltage is easily measured with test instruments.
The open-circuit voltage (Voc) is the maximum voltage on an I-V curve and is the operating point for a PV device under infinite load or open-circuit condition, and no current output. Since there is no current at the open-circuit voltage, the power output is also zero. The open-circuit voltage is used to determine maximum circuit voltages for modules and arrays. The open-circuit voltage of a PV device can be measured by exposing the device to sunlight and measuring across the output terminals with a voltmeter or a multimeter set to measure DC voltage. See Figure 5-11.

13 Using in-line and clamp-on ammeters are two methods of measuring short-circuit current.
The short-circuit current can be measured by exposing the device to sunlight and measuring current with an ammeter or multimeter. The measuring procedure depends on the actual current and the type of meter. If the short-circuit current is less than the fused current rating of the meter (typically 1 A or 10 A), the test leads can be connected to the output terminals. The meter short-circuits the PV device with a very small resistance and measures the resulting current. If the current is expected to be close to or higher than the meter rating, this in-line method should not be used. Instead, a conductor with a switch is used to short-circuit the output terminals and a clamp-on ammeter is put around the conductor to measure the resulting current. See Figure 5-12.

14 A power versus voltage curve clearly shows the maximum power point.
The operating point at which a PV device produces its maximum power output lies between the open-circuit and short-circuit condition, when the device is electrically loaded at some finite resistance. The maximum power point (Pmp) is the operating point on an I-V curve where the product of current and voltage is at maximum. A variation of the I-V curve plots power against voltage, which clearly shows the maximum power point. See Figure Maximum power is often called peak power and the parameter may be designated by Wp for “peak watts.”

15 Fill factor represents the shape of an I-V curve.
Fill factor (FF) is the ratio of maximum power to the product of the open-circuit voltage and short-circuit current. Fill factor represents the performance quality of a PV device and the shape of the I-V curve. A higher fill factor indicates that the voltage and current at the maximum power point are closer to the open-circuit voltage and short-circuit current, respectively, producing a more rectangular-shaped I-V curve. See Figure 5-14.

16 Efficiency is a measure of how effectively a PV device converts solar power to electrical power.
Efficiency is the ratio of power output to power input. See Figure The efficiency of PV devices compares the solar power input to the electrical power output. Solar irradiance is multiplied by the area of the PV device to determine watts of solar power, which can then be directly compared to watts of electrical power. PV cell efficiencies vary considerably among different PV technologies, and for the same material and technology, efficiencies vary widely between laboratory samples and commercial devices.

17 A PV device can be modeled by a current source in parallel with a diode, with resistance in series and parallel. A PV device can be modeled by a current source in parallel with a diode, with resistance in series and shunt (parallel). See Figure Both series and shunt resistances have a strong effect on the shape of the I-V curve.

18 Increasing series resistance in a PV system flattens the knee in the I-V curve, reducing maximum power, fill factor, and efficiency. Series resistance in PV devices includes the resistance of a cell, its electrical contacts, module interconnections, and system wiring. These resistances are in addition to the resistance of the electrical load. Some amount of series resistance in a PV system is unavoidable because all conductors and connectors have some resistance. However, increasing series resistance over time can indicate problems with electrical connections or cell degradation. Series resistance reduces the voltage over the entire I-V curve. See Figure Increasing series resistance also decreases maximum power, fill factor, and efficiency. If a PV device is operated at constant voltage (such as for battery charging), increasing series resistance results in decreasing operating current.

19 Decreasing shunt resistance reduces fill factor and efficiency and lowers maximum voltage, current, and power, but it does not affect short-circuit current. Shunt (parallel) resistance accounts for leakage currents within a cell, module, or array. Shunt resistance has an effect on an I-V curve opposite to the effect of series resistance. Decreasing shunt resistance reduces fill factor and efficiency, and lowers maximum voltage, current, and power, but does not affect short-circuit current. See Figure Decreasing shunt resistance over time can indicate short-circuits between cell circuits and module frames, or ground faults within an array.

20 Voltage increases rapidly up to about 200 W/m2, and then is almost constant. Current and maximum power increase proportionally with irradiance. Changes in solar irradiance have a small effect on voltage but a significant effect on the current output of PV devices. The current of a PV device increases proportionally with increasing solar irradiance. Consequently, since the voltage remains nearly the same, the power also increases proportionally. See Figure 5-19.

21 Increasing cell temperature decreases voltage, slightly increases current, and results in a net decrease in power. For most types of PV devices, high operating temperatures significantly reduce voltage output. Current increases with temperature, but only slightly, so the net result is a decrease in power and efficiency. See Figure Long-term high temperatures can also lead to premature degradation of cells and module encapsulation. For these reasons, it is desirable to install modules and arrays in a manner that allows them to operate as cool as possible.

22 Modules are constructed from PV cells that are encapsulated by several layers of protective materials. A module is a PV device consisting of a number of individual cells connected electrically, laminated, encapsulated, and packaged into a frame. See Figure The PV cells are laminated within a polymer (plastic) substrate to hold them in place and to protect the electrical connections between cells. The cell laminates are then encapsulated (sealed) between a rigid backing material and a glass cover. Some thin-film laminates use flexible materials such as aluminum or stainless steel substrate and polymer encapsulation instead of a glass cover.

23 An array is a group of PV modules integrated as a single power-generating unit.
An array is a complete PV power-generating unit consisting of a number of individual electrically and mechanically integrated modules with structural supports, trackers, or other components. See Figure 5-22.

24 Several modules may be connected together to form a panel, which is installed as a preassembled unit. The term “panel” is also used in relation to modules and arrays. Sometimes panel is used as an alternate term for a module. More commonly, the term panel refers to an assembly of two or more modules that are mechanically and electrically integrated into a unit for ease of installation in the field. See Figure 5-23.

25 A junction box on the back of a module provides a protected location for electrical connections and bypass diodes. All modules include some means for making intermodule electrical connections, through the use of either pre-wired connectors or a junction box. The junction box may also include bypass diodes and the ability to change the series or parallel configuration of the module cells with certain jumper arrangements. See Figure For example, a module might be changed from 36 series-connected cells to two parallel strings of 18 series-connected cells. This doubles the current and halves the voltage, but the power output remains the same.

26 PV cells or modules are connected in series strings to build voltage.
Individual cells are connected in series by soldering thin metal strips from the top surface (negative terminal) of one cell to the back surface (positive terminal) of the next. Modules are connected in series with other modules by connecting conductors between the negative terminal of one module to the positive terminal of another module. When individual devices are electrically connected in series, the positive connection of the whole circuit is made at the device on one end of the string and the negative connection is made at the device on the opposite end. See Figure 5-25.

27 The overall I-V characteristics of a series string are dependent on the similarity of the current outputs of the individual PV devices. Only PV devices having the same current output should be connected in series. When similar devices are connected in series, the voltage output of the entire string is the sum of the voltages of the individual devices, while the current output for the entire string remains the same as for a single device. Correspondingly, the I-V curve for a string of similar PV devices is the sum of the I-V curves of the individual devices. See Figure 5-26.

28 Strings of PV cells or modules are connected in parallel to build current.
Parallel connections are not generally used for individual PV devices, especially cells, but for series strings of cells and modules. Parallel connections involve connecting the positive terminals of each string together and all the negative terminals together at common terminals or busbars. See Figure 5-27.

29 The overall I-V curve of PV devices in parallel depends on the similarity of the current outputs of the individual devices. When similar devices are connected in parallel, the overall circuit current is the sum of the currents of individual devices or strings. The overall voltage is the same as the average voltage of all the devices connected in parallel. See Figure 5-28.

30 Modules are available in several sizes and shapes, including squares, rectangles, triangles, flexible units, and shingles. PV modules are available in a range of sizes and designed for a variety of applications. See Figure Smaller modules of less than 50 W are typically used individually for low-power battery charging applications, such as navigational aids, accent lighting, motorist-aid call boxes, and small circulation pumps. Smaller modules are often more expensive per unit watt output than larger ones, and are not typically used to build large arrays due to the large number of intermodule connections and mechanical attachments that would be required.

31 Bypass diodes allow current to flow around devices that develop an open-circuit or high-resistance condition. A bypass diode is a diode used to pass current around, rather than through, a group of PV cells. The current is allowed to pass around groups of cells that are shaded or develop an open-circuit or other high resistance condition, preventing an interruption of the continuity of the string. This allows the functional cells or modules in the string to continue delivering power. The consequence, however, is that the string will operate at a lower voltage. See Figure 5-30.

32 A bypass diode limits reverse current through PV devices, preventing excessive power loss and overheating. Without a bypass diode, reverse voltage may decrease until the breakdown voltage is reached. Breakdown voltage is the minimum reverse-bias voltage that results in a rapid increase in current through an electronic device. The high currents can result in potentially damaging levels of power dissipation within the module, and under extreme cases, the resulting high temperatures can melt the module laminate and pose a fire hazard. A bypass diode allows a reverse bias of only 0.7 V, which limits the reverse voltage to a level where only a small amount of power may be dissipated. See Figure 5-31.

33 Module labels must include performance ratings for the module and may include other information used to design a PV system. Standard performance ratings for modules are referred to as nameplate ratings, and they are required by the NEC® to be clearly labeled on every module. At a minimum, each module must be marked with polarity identification, maximum overcurrent device rating, and ratings at specified conditions for key I-V curve parameters, including open-circuit voltage, maximum permissible system voltage, maximum power current, short-circuit current, and maximum power. Additional information may include applicable certifications such as design qualification, fire class rating, ratings at other temperatures, and allowable wire sizes. In addition, all modules must have an installation guide that covers additional requirements for wiring, mounting, and other installation and operation considerations. See Figure 5-32.

34 Various test conditions can be used to evaluate module performance and may produce different results. Due to the dynamic nature of module performance and constantly changing operating conditions, the performance specifications of a module or array have meaning only when the rating conditions are given. These reference conditions are the basis for module performance ratings. Output at other conditions can be determined by translating the data using formulas for temperature and irradiance, the two principal factors affecting PV device performance. See Figure 5-33.

35 Modules are added in series to form strings or panels, which are then combined in parallel to form arrays. The modules or groups of modules are then integrated to form a complete array, using additional series or parallel connections. The result is a complete array that integrates all the modules into a single power-generating unit, with one positive terminal and one negative terminal for connection to other components. See Figure 5-34.

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