Solar Panel PV Module Performance

  Date: 02/17/2015

FAQS Summary:

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.

Solar Panel PV Module Performance

FAQS Content:


Solar Panel PV Module Performance

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. The chart shows a typical I-V

curve. Voltage (V) is plotted along the horizontal axis. The current is

plotted along the vertical axis. Most I-V curves are given for the standard

test conditions (STC) of 1,000 watts per square meter irradiance (often

referred to as one peak sun) and 25 °C (07 °F) cell temperature. 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 IV curve contains three significant points:

• Maximum Power Point (both Vmp and Imp)

• Open Circuit Voltage (Voc)

• Short Circuit Current (Isc)

1. Maximum Power Point (Vmp & Imp)

This point, labeled Vmp and Imp, is the operating point at which the

maximumoutput will be produced by themodule at operating conditions

indicated for that curve. In other words, the Vmp and Imp of the module

can bemeasuredwhen the system is under load at 25 °C cell temperature

and 1,000 watts per square meter. The voltage at the maximum power

point can be determined by extending a vertical line from the curve

downward to read a value on the horizontal voltage scale. The example

in the chart above displays a voltage of approximately 17 volts at the

maximum power (Vmp). 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 example above

displays a current of approximately 2.5 amps at the maximum power.

The wattage at the maximum power point is determined by multiplying

the voltage at maximum power by the current at maximum power. In

the chart, the maximum wattage at STC would be approximately 43

watts. 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.

2. 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

example at left displays an open circuit voltage of approximately 21

volts. 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 manufacturers specifications. When testing voltage with

a digital multi-meter 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.

3. 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. The example to the left displays

a current of approximately 2.65 amps. The power output at Isc is

zero watts. 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

and can be extremely dangerous. All Isc measurements should be

taken when the module is not connected to other components in

the system. Note: When testing modules with ‘quickconnects’ 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 multi-meter, check to ensure the module’s

Isc does not exceed the meter’s DC amperage rating and always use

the appropriate personal protective equipment.

4. Specification Label

All of the values found on the I-V curve to the left are used to

create a specification label for each module. All modules 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.

Solar Panel PV Module Performance

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