I-V characteristics curves for solar panels
The I-V characteristics curve usually defines the PV cell performance. A combined performance of all the solar cell in a module defines a I-V characteristics of a PV module and similarly it also defines I-V characteristics of a string. The following figure shows the I-V characteristics curve, P-V characteristics curve and datasheet of a PV module: –
The vertical axis denotes the current (I) while the horizontal axis denotes the voltage (V). Following terms are defined to understand the I-V characteristics curve: –
Short circuit current (ISC) – It is the maximum current that can flow through a cell when the voltage across the solar cell is zero (0) (i.e., when the solar cell is short circuited). It is denoted by ISC.
Open circuit voltage (VOC) – It is the maximum voltage a solar cell can produce when no load is connected to it (i.e., measures with a multimeter across the open ends of the wires). It is denoted by VOC.
In a typical 545 Wp module, the short circuit current (ISC) is 18.47 Amps and Open circuit voltage (VOC) is 37.7 V, both measured at standard test condition (STC). The power output from the solar module is the product of current and voltage at a particular instant on the I-V characteristics curve. The highest power output is realised at a particular point on the I-V characteristics curve which is known as Maximum Power Point Tracking (MPPT) and the respective current and voltage are known as Current at maximum power (IMPP) voltage at maximum power (VMPP). Typically, the I-V characteristics curve is drawn at one sun radiation (1000 W/m2) however, variation in solar radiation value predominantly changes the current output from the solar panel and subsequently the power output. The output voltage from solar panel is highly dependent on the operating temperature of the solar cells. With increase in temperature the open circuit voltage (VOC) of cells reduces which is defined as temperature co-efficient of solar panel. The typical temperature co-efficient of a solar panel of 545 Wp mono perc are follows: –
|Temperature coefficients of PMAX||-0.34 %/℃|
|Temperature coefficients of VOC||-0.25 %/℃|
|Temperature coefficients of ISC||0.04 %/℃|
Consider the below info-graphic for better understanding of effects of irradiance and temperature on I-V curves: –