solar energy 太阳能第8章模块单元.ppt

Chapter 8 Issues in PV Modules and ArraysMain contestIntroduction How do you get the How do you get the large power output large power output from PV?from PV?A PV module consists A PV module consists of individual solar of individual solar cells electrically cells electrically connected together to connected together to increase their power increase their power output.output.Why are PV cells encapsulated?For mechanical protection;For electrical isolation;And a degree of chemical protection.So it can protect PV cells from the environment and the users is protected from electrical shock.The most important effects in PV modules or arrays are :Losses due to the Losses due to the interconnection of interconnection of mismatched solar cells;mismatched solar cells;The temperature of the The temperature of the module;module;Failure modes of PV Failure modes of PV modules.modules.Module structureA PV module consists A PV module consists of a number of of a number of interconnected solar interconnected solar cells (typically 36 cells (typically 36 connected in series) connected in series) encapsulated into a encapsulated into a single, long-lasting, single, long-lasting, stable unit.stable unit.The purpose of The purpose of encapsulation:encapsulation:A typical bulk silicon PV module used in outdoor remote power applications.Different￿module￿structures￿for￿different￿types￿of￿solar￿cellsModule materialsMost bulk silicon PV Most bulk silicon PV modules consist of a modules consist of a transparent top surface, transparent top surface, an encapsulant, a rear an encapsulant, a rear layer and a frame around layer and a frame around the outer edge.the outer edge.Top surfaceTop surface－－glassglassEncapsulantEncapsulant－－EVA (ethyl-EVA (ethyl-vinyl-acetate)vinyl-acetate)Rear layerRear layer－－TedlarTedlarTypical bulk silicon module materialsAdditional features of an encapsulationThe durability of the encapsulation will The durability of the encapsulation will determine the ultimate operating life of the determine the ultimate operating life of the module, which ideally should be 20 years or so.module, which ideally should be 20 years or so.ØØUV stability;UV stability;ØØTolerance to temperature extremes and thermal Tolerance to temperature extremes and thermal shocks without stressing cells to fracture;shocks without stressing cells to fracture;ØØResistance to abrasion as might occur during dust Resistance to abrasion as might occur during dust storms;storms;ØØSelf-cleaning ability;Self-cleaning ability;ØØAbility to keep cell temperature low to minimize power Ability to keep cell temperature low to minimize power loss, and low cost.loss, and low cost.Requirements of front surface materials(1) (1) a high a high transmissiontransmission in the wavelengths which can be used in the wavelengths which can be used by the solar cells in the PV module.by the solar cells in the PV module.ØØFor For SiSi cells-high transmission of light in wavelength range of 350nm- cells-high transmission of light in wavelength range of 350nm-1200nm.1200nm.(2) (2) thethe reflection reflection from the front surface should be low. from the front surface should be low.(3) Impervious to water, good impact resistance, stable under (3) Impervious to water, good impact resistance, stable under prolonged UV exposure and low thermal resistivity.prolonged UV exposure and low thermal resistivity.(4) Choices for a top surface material: acrylic, polymers, glass (4) Choices for a top surface material: acrylic, polymers, glass –tempered, –tempered, low iron-content glasslow iron-content glass is most commonly used is most commonly used as it is low cost, strong, stable, highly transparent, as it is low cost, strong, stable, highly transparent, impervious to water and gases and has good self-cleaning impervious to water and gases and has good self-cleaning properties.properties.Encapsulant Encapsulant It is used to provide adhesion between the solar cells, It is used to provide adhesion between the solar cells, the top surface and the rear surface of the PV the top surface and the rear surface of the PV module.module.It should be stable at elevated temperatures and It should be stable at elevated temperatures and high UV exposure, should be optically transparent and high UV exposure, should be optically transparent and have a low thermal resistance.have a low thermal resistance.EVA is the most commonly used encapsulant material.EVA is the most commonly used encapsulant material.EVA comes in thin sheets which are inserted between EVA comes in thin sheets which are inserted between the solar cells and the top surface and the rear the solar cells and the top surface and the rear surface.surface.This sandwich is then heated to 150This sandwich is then heated to 150℃℃ to polymerize to polymerize the EVA and bond the module together.the EVA and bond the module together.Rear surfaceLow thermal resistancePrevent the ingress of water or water vaporA thin polymer sheet, typically Tedlar, is used as the rear surface.In bifacial modules both front and the rear must be optically transparent.Tedlar It is a chemically inert, tough, light weight, impervious film It is a chemically inert, tough, light weight, impervious film that remains flexible over a wide temperature range. that remains flexible over a wide temperature range. The unique mechanical, electrical and chemical properties The unique mechanical, electrical and chemical properties make Tedlar an ideal material for many types of surface make Tedlar an ideal material for many types of surface protection. protection. Tedlar also has very good non-stick properties and is widely Tedlar also has very good non-stick properties and is widely used as release film in the manufacture of printed circuit used as release film in the manufacture of printed circuit boards. boards. Tedlar retains its strength and flexibility within the Tedlar retains its strength and flexibility within the temperature range from -70temperature range from -70℃℃ up to +110 up to +110℃℃. . It has an elongation of over 100% and good wear It has an elongation of over 100% and good wear properties. properties. In the sign industry, Tedlar is used as surface protection In the sign industry, Tedlar is used as surface protection because of the weather resistance, UV-resistance and because of the weather resistance, UV-resistance and easy-to-clean properties. easy-to-clean properties. Tedlar film is supplied in thickness 0.013 up to 0.050 mm Tedlar film is supplied in thickness 0.013 up to 0.050 mm and widths up to 1625 mm and widths up to 1625 mm Frame A final structural A final structural component of the component of the module is the edging or module is the edging or framing of the module.framing of the module.A conventional frame is A conventional frame is typically made of typically made of aluminium.aluminium.It should be free of It should be free of projections which could projections which could result in the result in the lodgementlodgement of water, dust or other of water, dust or other matter.matter.Several types of Si PV modules由氟塑料、涤纶复合而成的Tedlar(TPT)背膜组成。

高真空加热层压工艺及配备经防腐处理铝合金边框、水密接线盒，抗紫外老化和抗风强度达2400MpaCells are laminated between high transmissivity, low-iron, 3mm tempered glass and sheet of tedlar-polyeaster-tedlar (TPT) material by two sheets of ethylene Vinyl acetate (EVA). This protects against moisture penetrating into the module. A versatile junction box provides flexibility of connections A heavy-duty anodized aluminium frame provides strength and convenient mounting access. For each 18 cells series strings, one bypass diode is installed.Packing DensityIt refers to the area of the module that is covered It refers to the area of the module that is covered with solar cells compared to that which is blank.with solar cells compared to that which is blank.It affects the output power of the module as well as It affects the output power of the module as well as its operating temperature, and it depends on the its operating temperature, and it depends on the shape of the solar cells used.shape of the solar cells used.The packing density of round and square cellsZero-depth concentratorSparsely packed cells in a module with a white Sparsely packed cells in a module with a white rear surface can provide marginal increases in rear surface can provide marginal increases in output via the “zero depth concentrator” effect – output via the “zero depth concentrator” effect – some of the light striking regions of the module some of the light striking regions of the module between cells and cell contacts is scattered and between cells and cell contacts is scattered and channeled to active regions of the module.channeled to active regions of the module.Module Circuit DesignA bulk silicon PV module consists of multiple individual solar A bulk silicon PV module consists of multiple individual solar cells connected, nearly always in series, to increase the cells connected, nearly always in series, to increase the power and voltage above that from a single solar cell.power and voltage above that from a single solar cell.The voltage of a PV module is usually chosen to be The voltage of a PV module is usually chosen to be compatible with a 12V patible with a 12V battery.An individual silicon solar cell has a voltage of about 0.6V An individual silicon solar cell has a voltage of about 0.6V under 25under 25℃℃ and AM1.5 illumination. and AM1.5 illumination.Module Circuit Design（cont.)Taking into account an expected Taking into account an expected reduction in PV module reduction in PV module voltage due to temperaturevoltage due to temperature and the fact that a  and the fact that a battery may battery may require voltages of 15V or more to chargerequire voltages of 15V or more to charge, most modules , most modules contain 36 solar cells in series. this gives an Vcontain 36 solar cells in series. this gives an Vococ of about  of about 21V under standard test conditions, and an operating 21V under standard test conditions, and an operating voltage at maximum power and operating temperature of voltage at maximum power and operating temperature of about 17 or 18V. about 17 or 18V. The remaining excess voltage is included to account for The remaining excess voltage is included to account for voltage drops caused by other elements of the PV system, voltage drops caused by other elements of the PV system, including operation away from maximum power point and including operation away from maximum power point and reductions in light intensity.reductions in light intensity.In a typical module, 36 cells are connected in series to In a typical module, 36 cells are connected in series to produce a voltage sufficient to charge a 12V battery.produce a voltage sufficient to charge a 12V battery.The voltage from the PV module is determined The voltage from the PV module is determined by the number of solar cells;by the number of solar cells;The current from the module depends primarily The current from the module depends primarily on the size of the solar cells and their efficiency;on the size of the solar cells and their efficiency;The current density from a commercial solar cell The current density from a commercial solar cell is about between 30mA/cmis about between 30mA/cm2 2 to 36mA/cm to 36mA/cm2 2 at at AM1.5 and under optimum tilt condition;AM1.5 and under optimum tilt condition;The current from a module is not affected by The current from a module is not affected by temperature in the same way that the voltage is, temperature in the same way that the voltage is, but instead depends heavily on the tilt angle of but instead depends heavily on the tilt angle of the module.the module.If all the solar cells in a module have identical If all the solar cells in a module have identical electrical characteristics, and they all experience electrical characteristics, and they all experience the same the same insolationinsolation and temperature, then all and temperature, then all the cells will be operating at exactly the same the cells will be operating at exactly the same current and voltage. current and voltage. In this case, the IV curve of the PV module has In this case, the IV curve of the PV module has the same shape as that of the individual cells, the same shape as that of the individual cells, except that the voltage and current are except that the voltage and current are increased.increased.Then the equation for the circuit Where N is the number of cells in series;Where N is the number of cells in series;M is the number of cells in parallel;M is the number of cells in parallel;I IT T is the total current from the circuit; is the total current from the circuit;V VT T is the total voltage from the circuit; is the total voltage from the circuit;I I0 0 is the saturation current from a single solar is the saturation current from a single solar cell;cell;I IL L is the short-circuit current from a single solar is the short-circuit current from a single solar cell;cell;n is the ideality factor of a single solar celln is the ideality factor of a single solar cellAnd And q,kq,k and T are constants. and T are constants.The overall IV curve of a set of identical connected solar cells I-V curve for N cells in series x M cells in parallelMismatch effectsMismatch losses are caused by the Mismatch losses are caused by the interconnection of solar cells or modules which interconnection of solar cells or modules which do not have identical properties or which do not have identical properties or which experience different conditions from one another.experience different conditions from one another.Mismatch losses are a serious problem in PV Mismatch losses are a serious problem in PV modules and arrays under some conditions modules and arrays under some conditions because the output of the entire PV module because the output of the entire PV module under worst case conditions is determined by under worst case conditions is determined by the solar cell with the lowest output.the solar cell with the lowest output.Example:When one solar cell is When one solar cell is shaded while the shaded while the remainder in the module remainder in the module are not, the power being are not, the power being generated by the “good” generated by the “good” solar cells can be solar cells can be dissipated by the lower dissipated by the lower performance cell rather performance cell rather than powering the load, than powering the load, this in turn can lead to this in turn can lead to highly highly localisedlocalised power power dissipation and the dissipation and the resultant local heating resultant local heating may cause irreversible may cause irreversible damage to the module.damage to the module.Shading of one region of a module compared to another is a major cause of mismatch is PV modules.Mismatch in PV modules occurs when the electrical parameters of one solar cell are significantly altered from those of the remaining devices, the impact and power loss due to mismatch depend on:ØØThe operating point of the PV module;The operating point of the PV module;ØØThe circuit configuration;The circuit configuration;ØØThe parameters which are different from the The parameters which are different from the remainder of the solar cells.remainder of the solar cells.The comparison of an ideal and a non-ideal solar cell. For The comparison of an ideal and a non-ideal solar cell. For mismatch, the greatest difference is when the cell is driven mismatch, the greatest difference is when the cell is driven into reverse voltage bias.into reverse voltage bias.Differences in any part of the IV curve between Differences in any part of the IV curve between one solar cell and another may lead to mismatch one solar cell and another may lead to mismatch losses at some operating point. A non-ideal IV losses at some operating point. A non-ideal IV curve and the operating regime of the solar cell curve and the operating regime of the solar cell is shown above, is shown above, large mismatcheslarge mismatches are most are most commonly caused by differences in either the commonly caused by differences in either the short-circuit current or open-circuit voltage.short-circuit current or open-circuit voltage. The impact of the mismatch depends on both The impact of the mismatch depends on both the circuit configuration and on the type of the circuit configuration and on the type of mismatch, and so on.mismatch, and so on.Mismatch for cells connected in seriesAs most PV modules are series-connected, series mismatches are the most common type of mismatch encountered.Of the two simplest types of mismatch considered (mismatch in Isc or in Voc), a mismatch in Isc is more common, as it can easily be caused by shading part of the module. This type of mismatch is also the most severe.For two cells connected in series, the current through the two cells For two cells connected in series, the current through the two cells is the same. The total voltage produced is the sum of the is the same. The total voltage produced is the sum of the individual cell voltages. Since the current must be the same, a individual cell voltages. Since the current must be the same, a mismatch in current means that the total current from the mismatch in current means that the total current from the configuration is equal to the lowest current.configuration is equal to the lowest current.Open circuit voltage mismatch for cells connected in seriesA mismatch in the open-circuit voltage of series-connected cells is a relatively benign form of mismatch.It is shown in the animation. <8-1>We can draw the following conclusion from the above:At short-circuit current, the overall current from At short-circuit current, the overall current from the PV module is unaffected.the PV module is unaffected.At At P Pmaxmax, the overall power is reduced because , the overall power is reduced because the poor cell is generating less powerthe poor cell is generating less powerAs the two cells are connected in series, the As the two cells are connected in series, the current through the two solar cells is the same, current through the two solar cells is the same, and the overall voltage is found by adding the and the overall voltage is found by adding the two voltages at a particular current.two voltages at a particular current.Short-circuit current mismatch for cells connected in seriesA mismatch in the short-circuit of series A mismatch in the short-circuit of series connected solar cells can, depending on the connected solar cells can, depending on the operating point of the module and the degree of operating point of the module and the degree of mismatch, have a drastic impact on the PV mismatch, have a drastic impact on the PV module.module.Please look at the following animation. <8-2>Please look at the following animation. <8-2>Draw something from the above animation:At open-circuit voltage, the impact of a reduced short-circuit At open-circuit voltage, the impact of a reduced short-circuit current is relatively minor. There is a minor change in the open-current is relatively minor. There is a minor change in the open-circuit voltage due to the logarithmic dependence of open-circuit voltage due to the logarithmic dependence of open-circuit voltage on short-circuit current.circuit voltage on short-circuit current.As the current through the two cells must be the same, the As the current through the two cells must be the same, the overall current from the combination cannot exceed that of the overall current from the combination cannot exceed that of the poor cell.poor cell.At low voltages where this condition is likely to occur, the extra At low voltages where this condition is likely to occur, the extra current-generating capability of the good cells is not dissipated current-generating capability of the good cells is not dissipated in each individual cell (as would normally occur at short circuit), in each individual cell (as would normally occur at short circuit), but instead is dissipated in the poor cell.but instead is dissipated in the poor cell.In a series connected configuration with current mismatch, severe power reductions are experienced if the poor cell produces less current than the maximum power current of the good cells and also if the combination is operated at short circuit or low voltages, the high power dissipation in the poor cell can cause irreversible damage to the module.Another relative animation! <8-3>Another relative animation! <8-3>Current mismatch for two cells in series can be Current mismatch for two cells in series can be quite serious and quite common. The Iquite serious and quite common. The Iscsc of the of the combination is limited to the Icombination is limited to the Iscsc of the lowest cell. of the lowest cell.For series connected solar cells with For series connected solar cells with mismatched short circuit currents, the short mismatched short circuit currents, the short circuit current from the poor cell, Isc2, is the circuit current from the poor cell, Isc2, is the maximum current that can flow in the external maximum current that can flow in the external circuit. circuit. Therefore, extra current from the good cell, Therefore, extra current from the good cell, mathematically given by Imathematically given by Isc1sc1-I -Isc2sc2, is forced to , is forced to flow across the good solar cell junction, forward flow across the good solar cell junction, forward biasing it and generating a voltage.biasing it and generating a voltage.An easy method of calculating the combined short-An easy method of calculating the combined short-circuit current of series connected mismatched cells. circuit current of series connected mismatched cells. The current at the point of intersection represents the The current at the point of intersection represents the short-circuit current of the series combination (short-circuit current of the series combination (ieie. . V1+V2=0)V1+V2=0)Hot-spot heatingBack-ground information: there is one low current Back-ground information: there is one low current solar cell in a solar cell in a stringstring of at least several high short- of at least several high short-circuit current solar cell, if the operating current of circuit current solar cell, if the operating current of the overall series string approaches the short-circuit the overall series string approaches the short-circuit current of the “bad” cell, the overall current becomes current of the “bad” cell, the overall current becomes limited by the bad cell. The extra current produced limited by the bad cell. The extra current produced by the good cells then forward biases the good solar by the good cells then forward biases the good solar cells. cells. If the series string is short circuited, then the forward If the series string is short circuited, then the forward bias across all of these cells reverse biases the bias across all of these cells reverse biases the shaded cell. shaded cell. String & block ?Hot-spot heatingHot-spot heating occurs when a large number of occurs when a large number of series connected cells cause a large reverse bias series connected cells cause a large reverse bias across the shaded cell, leading to large dissipation across the shaded cell, leading to large dissipation of power in the poor cell. Essentially the entire of power in the poor cell. Essentially the entire generating capacity of all the good cells is generating capacity of all the good cells is dissipated in the poor cell. The enormous power dissipated in the poor cell. The enormous power dissipation occurring in a small area results in local dissipation occurring in a small area results in local overheating, or “hot-spots”, which in turn leads to overheating, or “hot-spots”, which in turn leads to destructive effects, such as cell or glass cracking, destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell.melting of solder or degradation of the solar cell.Heat dissipated in a shaded cell caused the module to crackHeat dissipated in a shaded cell caused the module to crackBypass diodesThe destructive effects of hot-spot heating may be circumvented through the use of a bypass diode.A bypass diode is connected in parallel, but with opposite polarity, to a solar cell.Play the animation! <8-4>Under normal operation, each solar cell will be forward Under normal operation, each solar cell will be forward biased and therefore the bypass diode will be reverse biased and therefore the bypass diode will be reverse biased and will effectively be an open circuit.biased and will effectively be an open circuit.If a solar cell is reverse biased due to the a mismatch in If a solar cell is reverse biased due to the a mismatch in short-circuit current between several series connected short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the the current from the good solar cells to flow in the external circuit rather than forward biasing each good external circuit rather than forward biasing each good cell.cell.The maximum reverse bias across the poor cell is The maximum reverse bias across the poor cell is reduced to about a single diode drop, thus limiting the reduced to about a single diode drop, thus limiting the current and preventing hot-spot heating. The operation current and preventing hot-spot heating. The operation of a bypass diode and the effect on an IV curve are of a bypass diode and the effect on an IV curve are shown in the last animation.shown in the last animation.The effect of a bypass diode on an IV curve can The effect of a bypass diode on an IV curve can be determined by first finding the IV curve of a be determined by first finding the IV curve of a single solar cell with a bypass diode and then single solar cell with a bypass diode and then combining this curve with other solar cell IV combining this curve with other solar cell IV curves.curves.The bypass diode affects the solar cell only in The bypass diode affects the solar cell only in reverse bias. reverse bias. If the reverse bias is greater than the knee If the reverse bias is greater than the knee voltage of the solar cell, then the diode turns on voltage of the solar cell, then the diode turns on and conducts current. The combined IV curve is and conducts current. The combined IV curve is shown in the figure below.shown in the figure below.IV curve of solar cell with bypass diodeIV curve of solar cell with bypass diodePreventing hot-spot heating with a bypass diode. For clarity, the example uses a total of 10 cells with 9 unshaded and 1 shaded. A typical module contains 36 cells and the effects of current mismatch are even worse without the bypass diode, but are less important with the bypass diode.Play the animation!<8-5>In practice, one bypass diode per solar cell is In practice, one bypass diode per solar cell is generally too expensive and instead bypass diodes generally too expensive and instead bypass diodes are usually placed across groups of solar cells.are usually placed across groups of solar cells.The voltage across the shaded or low current solar The voltage across the shaded or low current solar cell is equal to the forward bias voltage of the other cell is equal to the forward bias voltage of the other series cells which share the same bypass diode series cells which share the same bypass diode plus the voltage of the bypass diode.plus the voltage of the bypass diode.Bypass diodes across groups of solar cells. The Bypass diodes across groups of solar cells. The voltage across the unshaded solar cells depends on voltage across the unshaded solar cells depends on the degree of shading of the poor cell. In the figure the degree of shading of the poor cell. In the figure below, 0.5V is arbitrarily shown.below, 0.5V is arbitrarily shown.The voltage across the unshaded solar cells depends on The voltage across the unshaded solar cells depends on the degree of shading on the low current cell.the degree of shading on the low current cell.For example, if the cell is completely shaded, then the For example, if the cell is completely shaded, then the unshaded solar cells will be forward biased by their short unshaded solar cells will be forward biased by their short circuit current and the voltage will be about 0.6V.circuit current and the voltage will be about 0.6V.If the poor cell is only partially shaded, the some of the If the poor cell is only partially shaded, the some of the current from the good cells can flow through the circuit, current from the good cells can flow through the circuit, and the remainder is used to forward bias each solar cell and the remainder is used to forward bias each solar cell junction, causing a lower forward bias voltage across junction, causing a lower forward bias voltage across each cell. The maximum power dissipation in the shaded each cell. The maximum power dissipation in the shaded cell is approximately equal to the generating capability of cell is approximately equal to the generating capability of all cells in the group.all cells in the group.The maximum group size per diode, without causing The maximum group size per diode, without causing damage, is about damage, is about 15cells/bypass diode15cells/bypass diode, for silicon cells. , for silicon cells. For a normal 36 cell module, therefore, 2 bypass diodes For a normal 36 cell module, therefore, 2 bypass diodes are used to ensure the module will not be vulnerable to are used to ensure the module will not be vulnerable to “ “hot-spothot-spot” damage.” damage.Mismatch for cells connected in parallelIn small modules the cells In small modules the cells are in placed in series so are in placed in series so parallel mismatch is not an parallel mismatch is not an issue.issue.Modules are paralleled in Modules are paralleled in large arrays so the large arrays so the mismatch usually applies mismatch usually applies at a module level rather at a module level rather than at a cell level.than at a cell level.Cells connected in parallel. The voltage across the cellCombination is always the same and the total current from the Combination is the sum of the currents in the individual cells.In the next animation, cell 2 has a lower output In the next animation, cell 2 has a lower output current than cell 1. there is no real problem with current than cell 1. there is no real problem with mismatch as the currents accumulate, while the mismatch as the currents accumulate, while the total current is always higher than the individual total current is always higher than the individual cell currents. cell currents. <8-6><8-6>In the animation of voltage mismatch for two In the animation of voltage mismatch for two cells in parallel. The addition of cell 2 actually cells in parallel. The addition of cell 2 actually reduces Vreduces Vococ below that for good cell by itself. below that for good cell by itself. <8-<8-7>7>An easy method of calculating the combined open circuit An easy method of calculating the combined open circuit voltage (Vvoltage (Vococ) of mismatched cells in parallel. The curve for ) of mismatched cells in parallel. The curve for one of the cells is reflected in the voltage axis so that one of the cells is reflected in the voltage axis so that the intersection pint (where I1+I2=0) is the Vthe intersection pint (where I1+I2=0) is the Vococ of the of the parallel configuration.parallel configuration.Mismatch effects in PV arraysIn a large PV array, individual PV modules are connected in both series and parallel.A series-connected set of solar cells or modules is called a “string”. The combination of series and parallel connections may lead to several problems in PV arrays.Potential mismatch effects in larger PV arrays. Although all Potential mismatch effects in larger PV arrays. Although all modules may be identical and the array does not experience any modules may be identical and the array does not experience any shading, mismatch and hot spot effects may still occur.shading, mismatch and hot spot effects may still occur.Parallel connections Parallel connections in combination with in combination with mismatch effects may mismatch effects may also lead to problems also lead to problems if the bypass diodes if the bypass diodes are not rated to are not rated to handle the current of handle the current of the entire parallel the entire parallel connected array.connected array.Bypass diodes in paralleled modules.Impact of blocking diodes in parallel connected modulesA blocking diode is typically used to prevent the module from loading the battery at night by preventing current from flow from the battery through the PV array. With parallel connected modules, each string to be connected in parallel should have its own blocking diode. This not only reduces the required current carrying capability of the blocking diode, but also prevents current flowing from one parallel string into a lower-current string and therefore help to minimize mismatch losses arising in parallel connected arrays.In additional to the use of bypass In additional to the use of bypass diodes to prevent mismatch diodes to prevent mismatch losses, an additional diode, called losses, an additional diode, called a blocking diode, may be used to a blocking diode, may be used to minimize mismatch losses.minimize mismatch losses.PV module temperatureAn unwanted side-effect of the encapsulation of An unwanted side-effect of the encapsulation of solar cells into a PV module is that the solar cells into a PV module is that the encapsulation alters the heat flow into and out of encapsulation alters the heat flow into and out of the PV module, thereby increasing the operating the PV module, thereby increasing the operating temperature of the PV module.temperature of the PV module.Major impact on the PV module: reducing the Major impact on the PV module: reducing the voltage, lowering the output power; several failure voltage, lowering the output power; several failure or degradation modes of PV modules, as elevated or degradation modes of PV modules, as elevated temperatures increase stresses associated with temperatures increase stresses associated with thermal expansion and also increase degradation thermal expansion and also increase degradation rates by a factor of about two for each 10rates by a factor of about two for each 10℃℃ increase in temperature.increase in temperature.Thermographic image of sixteen cell module with integral bypass Thermographic image of sixteen cell module with integral bypass diode cells under reverse bias conditions. Each color change diode cells under reverse bias conditions. Each color change corresponds to a 4corresponds to a 4℃℃ change in temperature. change in temperature.The operating temperature of a module is determined by The operating temperature of a module is determined by the equilibrium between the heat produced by the PV the equilibrium between the heat produced by the PV module, the heat lost to the environment and the module, the heat lost to the environment and the ambient operating temperature. ambient operating temperature. The heat produced by the module depends on the The heat produced by the module depends on the operating point of the module, the optical properties of operating point of the module, the optical properties of the module and solar cells, and the packing density of the module and solar cells, and the packing density of the solar cells in the PV module.the solar cells in the PV module.The heat lost to the environment can proceed via one of The heat lost to the environment can proceed via one of three mechanisms; conduction, convection and radiation.three mechanisms; conduction, convection and radiation.These loss mechanisms depend on the thermal These loss mechanisms depend on the thermal resistance of the module materials, the emissive resistance of the module materials, the emissive properties of the PV module, and the ambient conditions properties of the PV module, and the ambient conditions (particularly wind speed) in which the module is (particularly wind speed) in which the module is mounted. mounted. Heat generation in PV modulesA PV module exposed to sunlight A PV module exposed to sunlight generates heat as well as electricity. For a generates heat as well as electricity. For a typical commercial PV module operating typical commercial PV module operating at its maximum power point, only 10-15% at its maximum power point, only 10-15% of the incident sunlight is converted into of the incident sunlight is converted into electricity, with much of the remainder electricity, with much of the remainder being converted into heat.being converted into heat.The factors which affect the heating of the The factors which affect the heating of the module are:module are:ØØThe reflection from the top surface of the The reflection from the top surface of the module;module;ØØThe electrical operating point of the module;The electrical operating point of the module;ØØAbsorption of sunlight by the PV module in Absorption of sunlight by the PV module in regions which are not covered by solar cells;regions which are not covered by solar cells;ØØAbsorption of low energy (infrared) light in the Absorption of low energy (infrared) light in the module or solar cells;module or solar cells;ØØThe packing density of the solar cells.The packing density of the solar cells.Front surface reflectionlLight reflected from the front surface of the module does not contribute to the electrical power generated. Such light is considered an electrical loss mechanism which needs to be minimized. Neither does reflected light contribute to heating of the PV module. The maximum temperature rise of the module is therefore calculated as the incident power multiplied by the reflection. lFor typical PV modules with a glass top surface, the reflected light contains about 4% of the incident energy.Operating point and efficiency of the moduleThe operating point and efficiency of the solar cell determine the fraction of the light absorbed by the solar cell that is converted into electricity.If the solar cell is operating at short-circuit current or at open-circuit voltage, then it is generating no electricity and hence all the power absorbed by the solar cell is converted into heat.Absorption of light by the PV moduleThe amount of light absorbed by the parts of the module other than the solar cells will also contribute to the heating of the module.How much light is absorbed and how much is reflected is determined by the color and material of the rear backing layer of the module.Absorption of infra-red lightLight which has an energy below that of the Light which has an energy below that of the band gap of the solar cells cannot contribute to band gap of the solar cells cannot contribute to electrical power, but if it is absorbed by the solar electrical power, but if it is absorbed by the solar cells or by the module, this light will contribute to cells or by the module, this light will contribute to heating . The aluminium at the rear of the solar heating . The aluminium at the rear of the solar cell tends to absorb this infrared light. In solar cell tends to absorb this infrared light. In solar cells which do not have full aluminium coverage cells which do not have full aluminium coverage at the rear of the solar cell, the infrared may at the rear of the solar cell, the infrared may pass through the solar cell and exit from the pass through the solar cell and exit from the module.module.Packing factor of the solar cellsSolar cells are specifically designed to be efficient absorbers of solar radiation. The cells will generate significant amounts of heat, usually higher than the module encapsulation and rear backing layer. Therefore, a higher packing factor of solar cells increases the generated heat per unit area.Heat loss in PV modulesThe operating temperature The operating temperature of a PV module is an of a PV module is an equilibrium between the equilibrium between the heat generated by the PV heat generated by the PV module and the heat loss module and the heat loss to the surrounding to the surrounding environment. There are environment. There are three main mechanisms of three main mechanisms of heat loss: conduction, heat loss: conduction, convection and radiation.convection and radiation.The module temperature is determined by the equilibrium between heat generated in the PV module by the sun and the conduction, convection and radiative heat loss from the module.Heat conduction Conductive heat losses are Conductive heat losses are due to thermal gradients due to thermal gradients between the PV module and between the PV module and other materials (including the other materials (including the surrounding air) with which the surrounding air) with which the PV module is in contact. The PV module is in contact. The ability of the PV module to ability of the PV module to transfer heat to its transfer heat to its surroundings is characterized surroundings is characterized by the thermal resistance and by the thermal resistance and configuration of the materials configuration of the materials used to encapsulate the solar used to encapsulate the solar cells.cells.A is the area of the surface conducting heat;l is the length of the material through which heat must travel;K is the thermal conductivity in units of Wm-1℃Where Pheat is the heat (power) generated by the PV module;Φ is the thermal resistance of the emitting surface in ℃W-1;ΔT is the temperature difference between the two materials in ℃.To find the thermal resistance of a more To find the thermal resistance of a more complicated structure, the individual thermal complicated structure, the individual thermal resistances may be added in series or in parallel.resistances may be added in series or in parallel.Since both the front and the rear surface Since both the front and the rear surface conduct hear from the module to the ambient, conduct hear from the module to the ambient, these two mechanisms operate in parallel with these two mechanisms operate in parallel with one another and the thermal resistance of the one another and the thermal resistance of the front and rear accumulate in parallel. front and rear accumulate in parallel. Alternatively, in a module, the thermal resistance Alternatively, in a module, the thermal resistance of the encapsulation and that of the front glass of the encapsulation and that of the front glass would add in series. would add in series. Convection Convection heat transfer arises from the transport of heat Convection heat transfer arises from the transport of heat away from a surface as the result of one material moving away from a surface as the result of one material moving across the surface of another.across the surface of another.In PV modules, convective heat transfer is due to wind In PV modules, convective heat transfer is due to wind blowing across the surface of the module. blowing across the surface of the module. The heat which is transferred by this process is given by the equation:Where A is the area of contact between the two materials;H is the convection heat transfer co-efficient in units of Wm-2℃-1, (experimental parameter) ΔT is the temperature difference between the two materials in ℃.Radiation Any object will emit radiation based on its temperature Any object will emit radiation based on its temperature including the PV module as discussed in the Blackbody including the PV module as discussed in the Blackbody Radiation.Radiation.The net heat or power lost from the module due to radiation is the difference between the heat emitted from the surroundings to the module and the heat emitted from the PV module to the surroundings, or in mathematical format: Where Tsc is the temperature of the solar cell;Tamb is the temperature of the ambient surrounding the solar cell;ε is the emissivity of the surfaceσ is the Stafan-Boltzmann constant.Nominal operating cell temperatureA PV module will be typically rated at 25℃ under A PV module will be typically rated at 25℃ under 1KW/m1KW/m2 2.  . However, when operating in the field, they typically However, when operating in the field, they typically operate at higher temperatures and at somewhat lower operate at higher temperatures and at somewhat lower insolationinsolation conditions. conditions.The nominal operating cell temperature (NOCT) is defined as the temperature reached by open circuit cells in a module under the conditions as listed below: Irradiance on cell surface=800W/m2;Air temperature=20℃;Wind velocity=1m/s;Mounting=open back side.An approximate expression for the cell temperature is T Tcellcell( (℃)=T℃)=T℃)=T℃)=Tambientambientambientambient(℃)+0.3(℃)+0.3(℃)+0.3(℃)+0.3××××intensity (in mW/cmintensity (in mW/cmintensity (in mW/cmintensity (in mW/cm2 2 2 2) (℃)) (℃)) (℃)) (℃)Thermal expansion and thermal stressesUse of stress relief loops to accommodate expansion Use of stress relief loops to accommodate expansion between cells with increases in temperature.between cells with increases in temperature.The spacing between cells tries to increase an amount given by:Where аG,аC , are the expansion coefficients of the glass and the cell respectively;D is the cell width;C is the cell centre to centre distance.Typically, interconnections between cells are looped to minimize cyclic stress. Double interconnects are used to protect against the probability of fatigue failure caused by such stress.In addition to interconnect stresses, all module interfaces are subject to temperature-related cyclic stress which may eventually lead to delamination.Electrical insulationThe encapsulation system has to be able to withstand voltage differences at least as large as the system voltages. Metal frames must also be earthed, as internal and terminal potentials can be well above the earth potential. Any leakage currents to earth must be low to prevent interference with earth leakage safety devices.Mechanical protectionSolar modules must have adequate strength and rigidity to allow normal handling before and during installation.If glass is used for the top surface, it must be tempered, since the central areas of the modules hotter than areas near the frame. This places tension at the edges, and can cause cracking. In an array, the modules must be able to accommodate some degree of twisting in the mounting structure, as well as to withstand wind induced vibrations and the loads imposed by high winds, snow and ice.Possible module twisting on a distorted mounting frame.Degradation and failure modes for bulk silicon PV modulesA PV module’s operating A PV module’s operating life is largely determined life is largely determined by the stability and by the stability and resistance to corrosion of resistance to corrosion of the materials from which the materials from which it is constructed.it is constructed.There are several failure There are several failure modes and degradation modes and degradation mechanisms which may mechanisms which may reduce the power output reduce the power output or cause the module to or cause the module to fail, and these fail, and these mechanisms are related mechanisms are related to water ingress or to water ingress or temperature stress.temperature stress.Example of PV module degradation or failure. shown here is the degradation of the ARC of a solar cell caused by water vapor ingress.Reversible reductions in output powerSoiling of the top surface of the array Soiling of the top surface of the array may cause either mismatch losses or may cause either mismatch losses or a more uniform reduction in power a more uniform reduction in power output.output.(1) Shading by a tree;(2) Soiling the front surface;et al.These reductions in power are all reversible provided that the original cause is rectified.Solar cell degradation;Solar cell degradation;Short-circuited cells;Short-circuited cells;Open-circuited cells;Open-circuited cells;Interconnect open-circuits;Interconnect open-circuits;Module open-circuits;Module open-circuits;Module short-circuits;Module short-circuits;Degradation and failure of PV modulesDegradation mechanisms may involve either a gradual Degradation mechanisms may involve either a gradual reduction in the output power of a PV module over reduction in the output power of a PV module over time or an overall reduction in power due to failure of time or an overall reduction in power due to failure of an individual solar cell in the module.an individual solar cell in the module.module glass module glass breakage;breakage;Module Module delamination;delamination;Hot-spot failures;Hot-spot failures;By-pass diode By-pass diode failure;failure;Encapsulant failure;Encapsulant failure;Cell failure through interconnect shortingCracked cell indicating how “interconnect”. Cracked cell indicating how “interconnect”. BusbarsBusbars can can help prevent open-circuit failure.help prevent open-circuit failure.A typical solar cell consists of a glass or plastic cover or other encapsulant, an antireflective layer, a front contact to allow electrons to enter a circuit, a back contact to allow them to complete the circuit, and the semiconductor layers where the electrons begin and complete their journey. PV BoSBalance of SystemBalance of SystemRepresents all components and Represents all components and costs other than the PV modules. costs other than the PV modules. It includes design costs, land, site It includes design costs, land, site preparation, system installation, preparation, system installation, support structures, power support structures, power conditioning, operation and conditioning, operation and maintenance costs, indirect maintenance costs, indirect storage, and related costs. Big storage, and related costs. Big items are:items are:n nInverter (if needed)Inverter (if needed)n nInstallationInstallationn nMounting hardwareMounting hardwaren nWiring (conduit)Wiring (conduit)n nBatteries (if needed)Batteries (if needed)What is solar system? Major photovoltaic system componentsUS National Association House Building demonstration in US National Association House Building demonstration in Bowie, Maryland USA withBowie, Maryland USA with PV standing seam metal roofPV standing seam metal roofExample of BIPVInstalling solar cells on urban roof tops will allow Installing solar cells on urban roof tops will allow customers to sell electricity to their local distributor customers to sell electricity to their local distributor during the day and buy electricity back at night. during the day and buy electricity back at night. Example of utility generationOptical air mass and AM0, AM1 and AM1.5Optical air mass and AM0, AM1 and AM1.5S-W effect (S-W effect (Staebler-Wronski Effect)Staebler-Wronski Effect)Photovoltaic effectPhotovoltaic effectHot-spot effect, how is the hot-spot caused and Hot-spot effect, how is the hot-spot caused and how to avoid the hot spot in the PV modulehow to avoid the hot spot in the PV moduleDrift current density and diffusion current density of Drift current density and diffusion current density of the silicon solar cellthe silicon solar cellV Vococ and and I Iscsc and FF and efficiency and and FF and efficiency and R Rs s and and R RsHsH and Pand Pmm of bulk silicon solar cell of bulk silicon solar cell I-V curve of PV cell, and explain the FF and Pm, I-V curve of PV cell, and explain the FF and Pm, how to improve the efficiency of the PV cell.how to improve the efficiency of the PV cell.How is the pn junction of the solar cell formed? How is the pn junction of the solar cell formed? Simply explain the phosphorous (P) dopant Simply explain the phosphorous (P) dopant diffusion mechanisms in silicon with the liquid diffusion mechanisms in silicon with the liquid dopant source (POCldopant source (POCl3 3). ).Equivalent circuit of a solar cell with a load.Equivalent circuit of a solar cell with a load.。