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Some thin-film modules are flexible and portable and are often used to charge batteries in small handheld devices such as cell phones. The current-voltage (I-V) characteristic is the basic electrical output profile of a PV device. The I-V characteristic represents all possible current-voltage operating points (and power output) for a given PV device (cell, module, or array) at a specified condition of incident solar radiation and cell temperature. 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. A PV device can operate anywhere along its I-V curve, depending on the electrical load. At any specific voltage on an I-V curve there is an associated current, and this operating point is known as an I-V pair. Since the product of cur rent and voltage is power, each I-V pair also rep resents a specific power output, which changes from point to point along the I-V curve. Certain points on an I-V curve are used to rate module performance and are the basis for the electrical design of arrays. The basic I-V curve parameters include the open- circuit voltage (17 short-circuit current maximum power voltage (V), maximum power current ('mp)' and maximum power (P_mp) It’s important to note that the I-V curve changes with cell temperature and irradiance, and that I-V parameters have meaning only when these conditions are specified. PV Cells --9. The different materials, processes, and manufacturing steps produce a range of PV cell types. --10. An I-V curve illustrates the electrical output profile of a PV cell, module, or array at a specific operating condition. I-V Curve MAXIMUM POWER POINT SHORT-CIRCUIT CURRENT - MAXIMUM POWER CURRENT OPEN-CIRCUIT VOLTAGE MAXIMUM POWER VOLTAGE_\ Open-Ckt Voltage Measurement -VOLTMETER OR MULTIMETER SET TO MEASURE DC VOLTAGE --11. Open-circuit voltage is easily measured with test instruments. Open-Circuit Voltage The open-circuit voltage (V) 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 a current at the open-circuit voltage, the power output is also zero. The open-circuit voltage is used to determine maxi mum circuit voltages for modules and arrays. The open-circuit voltage of a PV device can he measured by exposing the device to sunlight and measuring across the output terminals with a voltmeter or a multimeter set to measure DC voltage. 1. The open-circuit voltage of a PV device is determined by the semiconductor material properties and temperature. Increasing temperature reduces the open-circuit voltage for crystalline silicon. However, the open-circuit voltage is independent of cell area. A large cell can be sectioned into a number of smaller cells, and each will have the same open-circuit voltage. For individual crystalline silicon cells, the open-circuit voltage is typically 0.5 V to 0.6 V at 25°C (77°F). Some thin-film cells have an open-circuit voltage of 1.0 V or higher. Consequently, individual cells are connected in series to build higher, usable voltage levels in modules. Short-Circuit Current The short-circuit current (I) is the maximum current on an I-V curve and is the operating point for a PV device under no load or short- circuit condition, and no voltage output. Since voltage is zero at the short-circuit current, the power output is also zero. The short-circuit current of a PV device is used to determine maximum circuit design currents for modules and arrays, and is significantly affected by varying solar irradiance. Special equipment can be used to electronically load PV devices while measuring voltage and current in order to automatically generate I-V curves. 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 1A or 10A), 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. Special test instruments and software can automatically generate I-V curves, calculate key parameters, and produce reports. --12. Using in-line and clamp-on ammeters are two methods of measuring short-circuit current. Short-Circuit Current Measurement SHORT-CIRCUIT CURRENT Only PV devices can be safely short-circuited. Never short-circuit other electrical sources. DEVICE CLAMP-ON AMMETER SHORT-CIRCUIT CURRENT CONDUCTO CLAMP-ON METER METHOD PV DEVICE-AMMETER OR MULTIMETER SET TO MEASURE CURRENT IN-LINE METER METHOD Short-Circuiting----Short-circuiting most current sources, such as an AC wall outlet or a battery, is extremely dangerous and can cause immediate damage to components if there is no overcurrent protection. Since there is essentially no resistance, the current flow is very high and can cause a potentially fatal electric shock and extremely high temperatures that may cause electrical burns or fire. Furthermore, short-circuiting a battery can cause it to explode, scattering acid and other toxic materials onto nearby people or components. PV devices, however, can be safely short-circuited because they are inherently current- limited. In fact, some types of battery charge controllers short-circuit PV arrays as a means of controlling the charging current into battery systems. A short-circuited PV device will flow current only up to certain point, because the electrons making up the current are not free to flow unless they are released by photons. The number of photons striking the PV device is finite, and only some of those photons transfer enough energy to free an electron. Therefore, the current flow cannot exceed the supply of free electrons. Only greater irradiance can increase the short-circuit current, and that is feasible only up to a point. The current of a PV device is directly proportional to surface area and solar irradiance. In other words, for a given device, doubling the surface area exposed to solar radiation will double the current output. Likewise, doubling solar irradiance on the device surface will double current. 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 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. Maximum power is often called peak power and the parameter may be designated by W for "peak watts". The I-V pair at the maximum power point is composed of the maximum power voltage and the maximum power current. The maximum power voltage (V_mp) is the operating voltage on an I-V curve where the power output is at maximum. The maximum power current (I_mp) is the operating current on an I-V curve where the power output is at maximum. Maximum power is calculated using the following formula: P =V x I_mp p where mp = maximum power (in W) V_mp = maximum power voltage (in V) I_mp = maximum power current (in A) For example, what is the maximum power of a PV module with a maximum power volt age of 23.5 V and a maximum power current of 7.1 A? P =V x I_mp P =23.5x7.1 mp P =166.8W mp --13. A power versus voltage curve clearly shows the maximum power point. I-V Curve with Power VOLTAGE (V) The maximum power point is located on the knee of the I-V curve and is the highest efficiency operating point for a PV device for the given conditions of solar irradiance and cell temperature. Due to the shape of the curve, maximum power voltage is typically about 70% to 80% of the value of the open-circuit voltage, while maximum power current is typically about 90% of the value of the short- circuit current. Maximum power voltage and current can be measured only while the PV device is connected to a load that operates the device at maximum power. Alternatively, the maximum power voltage and current can be determined from I-V curve data by multiplying each I-V pair and determining which pair results in the maximum power. Since horizontal and vertical lines drawn from any current and voltage point to its corresponding axes form a rectangle, and the area of the rectangle equals power, the rectangle with the largest area graphically represents the maximum power point. The maximum power point is also used to rate the performance of PV devices under specific conditions of solar irradiance and cell temperature. Fill Factor. 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. Fill factor is expressed as a percentage and is calculated with the following formula: where FF = fill factor mp = maximum power (in W) = open-circuit voltage (in V) = short-circuit current (in A) --14. Fill factor represents the shape of an I-V curve. Fill Factor HIGHER; FILL FACTOR FILL FACTOR I EQUALS 1 MAXIMUM POWER POINTS LOWER FILL FACTOR For example, what is the fill factor of a PV cell with a maximum power of 3.0W, an open- circuit voltage of 0.6 V and a short-circuit current of 7.0 A? FF = 0.714 or 71.4% Most commercial crystalline silicon PV cells have fill factors exceeding 70%, while the fill factor for many thin-film material is somewhat less. For a higher fill factor cell, the current decreases much less with increasing voltage up to the maximum power point, and decreases much more with increasing voltage beyond maximum power. A decrease in fill factor over time indicates problems with PV devices, including degradation of the cells or, more commonly, increased resistance of the wiring or connections in the system. Efficiency. Efficiency is the ratio of power output to power input. 5. 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. Efficiency is expressed as a percentage and is calculated with the following formula: Where... TI = efficiency P = maximum power (in W) E = solar irradiance (in W/m^2) A = area (in m) === PV DEVICE ,- ELECTRICAL / POWER / OUTPUT I SOLAR POWER INPUT_\ MORE SOLAR POWER IS J CONVERTED TO ELECTRICITY HIGH EFFICIENCY PV DEVICE_ SOLAR POWER INPUT_\ LESS SOLAR POWER IS CONVERTED TO ELECTRICITY LOW EFFICIENCY --15. Efficiency is a measure of how effectively a PV device converts solar power to electrical power. === For example, what i the efficiency of a PV module with a surface area of 1.2m and a maximum power output of 160W when ex posed to 1000W/mi solar irradiance? Cells with higher efficiencies require less surface area to produce each watt of power, which saves some costs for raw materials, mounting structures, and other equipment. However, higher efficiency modules are generally no less expensive than less efficient ones, because the price for modules is generally based on the maximum power rating and not on the size. For modules, efficiencies are often based on the entire module laminate area including the frame, and spacing between individual cells in the module. For individual cells, there is none of this extra area to affect the efficiency. This is one reason why module efficiencies are lower than their associated best cell efficiencies. Operating Point The operating point on an I-V curve is determined by the electrical load of the system. For example, if a battery is connected to a PV module, the battery voltage sets the operating voltage of the module. It also establishes the operating current that flows between the device and battery. If an incandescent lamp or DC motor is connected to a PV device, the effective resistance of the lamp filament or motor determines the operating point. Short-circuit current is associated with zero load resistance and open-circuit voltage is associated with infinite load resistance. Every point in between the two states has a specific load resistance that increases from left to right along the I-V curve. PV cells operate most efficiently at their maximum power points. However, the maxi mum power point is constantly changing due to changes in solar irradiance and cell temperature. Consequently, some systems use maximum power point tracking (MPPT) to dynamically match the electrical loads to PV output in order to maximize the performance. This function is included in most interactive inverters and some battery charge controllers. The electrical load resistance required to operate a PV device at any point can be calculated using Ohm's law. For the maximum power point, the formula is: R_mp = where R_mp = resistance at maximum power point (in W) V_mp = maximum power voltage (in V) I_mp = maximum power current (in A) For example, a module has maximum power voltage of 15 V and maximum power current of 3 A. What resistance is required to operate the module at the maximum power point, and what is its maximum power? R mp - mp R mp – R_mp P =V x I_mp P =15x3 mp P =45W mp System Resistance A PV device can be modeled by a current source in parallel with a diode, with resistance in series and shunt (parallel). Both series and shunt resistances have a strong effect on the shape of the I-V curve. 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. 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. Schematic Symbols PV CELL SYMBOL A PV device can be modeled by a current source in parallel with a diode, with resistance in series and parallel. Series Resistance F z w; 0 EQUIVALENT CIRCUIT V VOLTAGE (V). Increasing series resistance in a PV system flattens the knee in the I-V curve, reducing maximum power, fill factor, and efficiency. Shunt (parallel) resistance accounts for leakage currents within a cell, module, or as ray. 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. Decreasing shunt resistance over time can indicate short circuits between cell circuits and module frames, or ground faults within an array. I z w; 0 Parallel Resistance I _V VOLTAGE(V) --18. Decreasing shunt resistance reduces fill factor and efficiency and lowers maximum voltage, current, and power, but it does not affect short-circuit current. A variable resistive load, such as a rheostat or adjustable resistor, can be used to load a PV device over nearly its entire I-V curve When combined with meters measuring voltage and current, this method can be used to generate the I V curves of small PV devices or individual modules, which can then be used to identify the key I V curve parameters. An ideal PV device would have no series resistance and infinite shunt resistance, producing a rectangular I-V curve with a fill factor of 100%. In reality, however, both have a finite value and only series resistance can be practically managed. This is why it’s important to minimize PV system series resistance as much as possible, especially where long wiring distances are involved. Next: PV Cells, Modules, and Arrays--Device Response |