The Total electrical power output (wattage) of a photovoltaic module is equal to its operating voltage multiplied by its operating current. Photovoltaic modules may produce current over a wide range of voltages. This is unlike voltage sources such as batteries, which produce current at a relatively constant voltage.
The output characteristics of any given module are characterized by a performance curve, called an I-V curve, that shows the relationship between current and voltage output. Voltage (V) is plotted along the horizontal axis. Current (I) is plotted along the vertical axis. Most I – V curves are given for the standard test conditions (STC) of 1000 watts per square meter irradiance (often referred to as one peak sun hour) and 25 o C (77oF) cell temperature and 1.5 ATM spectral conditions. It should be noted that STC represent the optimal conditions as a consistent means for measuring – rarely are these conditions recreated in outside environments. The I – V curve contains three significant points:
- Maximum power Point (both Vmp and Imp)
- Open Circuit Voltage (Voc)
- Short Circuit Current ( Isc)
Maximum Power Point (Vmp & Imp)
This point
The current at the maximum power point can be determined by extending a horizontal line from the curve to the left to read a value on the vertical current scale ( Imp).
The wattage at the maximum power poin.t is determined by multiplying the voltage at maximum power by current at maximum power. This power is represented by the rectangle under the curve.
The power output decreases as the voltage drops. Current and power output of most modules drops off as the voltage increases beyond the maximum power point
Open Circuit Voltage (Voc)
This point, labeled Voc, is the maximum potential voltage achieved when no current is being drawn from the module. Since no current is flowing, the module experiences maximum electrical pressure. The power output at Voc is zero watts.
Open Circuit Voltage can be measured in the field in several common circumstances. When buying a module, it is recommended to test the voltage to see if it matches the manufacture’s specifications. When testing voltage with a digital multimeter from the positive to the negative terminal, an open circuit is created by the meter which allows Voc to be measured. It is also common to see a module operating at voc early in the morning and late in the evening. Intensity of sunlight and cell temperature, as they affect voltage and current, will be discussed later this chapter.
Short Circuit Current (Isc)
This point, labeled Isc, is the maximum current output that can be reached by the module under the conditions of a circuit with no resistance or a short circuit.
When first purchasing a module, it is recommended to test the short circuit current to see if it matches the specification sheet. The short circuit current can be measured only when making a direct short across the positive and negative terminals of a module. Creating a direct short across more than one module at a time (or a module with voltage greater than 24 V nominal) is not recommended an can be extremely dangerous. All Isc measurements should be taken when the module is not connected to other component in the system.
Note: when testing modules with ‘quick connects’ it is recommended to use test leads
To avoid leaving carbon deposits (which cause high resistance) on the
Module’s leads.
Before testing amperage with a digital multimeter, check to ensure the module’s Isc does not exceed the meter’s DC amperage rating and always use the appropriate personal protective equipment. Intensity of sunlight will affect the amperage reading and is discussed in the next section.
Specification Label
All of the values found on the I-V curve are used to create a specification label for each module. All module are rated under standard test conditions, thereby allowing their values to be compared. The specification label can be found on the back side of the module or through the manufacturer.
Incoming search terms:
- high temperature effects on characteristic of solar panels or modules (1)
- pv modules irradiation effect on open voltage circuit (1)
- pv panel irradiance on open circuit voltage (1)
- solar panel performance curve over 24 hours (1)
- the curve between power and hour for the solar cell (1)
- what is the relation between voltage squre metre solar panel (1)
- what wattage can one sqaure meter of a solar panel produce (1)