Investigating the Characteristics of Photovoltaic (Solar) Panels - Theory

The IV Characteristic Curve and the Three Points of Interest

A photovoltaic (solar) panel will produce differing amounts electrical power depending on its set-up and circuit to which it connects. The typical current-voltage output of a solar panel is shown below with three points of interest discussed.

Graph

This maximum current is called the short circuit current, abbreviated I(sc). When the module is shorted (zero load resistance) the voltage in the circuit is zero.
Isc occurs when the panel voltage is zero.

There is a point on the "knee" of the curve where the maximum power output is located. This point is called the Maximum Power Point. Different resistances drawn on different amounts of power from the panel under constant sunlight conditions. The optimal resistance draws the greatest power. The optimal resistance is the ratio of voltage to current determined from the maximum power point. At this point, the load resistance is equal to the solar panel internal resistance.

Conversely, the maximum voltage is produced when there is a break in the circuit. This is called the open circuit voltage, abbreviated Voc. Under this condition the resistance is infinitely high and there is no current, since the circuit is incomplete.
Voc occurs when the panel current is zero.

Example:

From the graph above, Isc = 2.65 A and Voc = 21.3 V

At the maximum power point, Imp = 2.5 A and Vmp = 17.3 V => Pmp = 2.5 x 17.3 = 43.25 W

The optimal resistance for operation = Vmp/ Imp = 17.3/2.5 = 6.9 ohms

Current Output and Brightness

Standard sunlight conditions on a clear day are assumed to be 1000 watts of solar energy per square meter (1000 W/m2). This is sometimes called "one sun," or a "peak sun." Less than one sun will reduce the current output of the module by a proportional amount. For example, if only one-half sun (500 W/m2) is available, the amount of output current is roughly cut in half. Thus, the brightness affects the current output only.

Graph

Voltage Output and Temperature

The greater the temperature, the smaller the open-circuit voltage. Higher module temperatures will reduce the voltage by 0.04 to 0.1 volts for every one Celsius degree rise in temperature.

This is why modules should not be installed flush against a surface. Air should be allowed to circulate behind the back of each module so its temperature does not rise, reducing its output.

Graph

Thus, the photovoltaic (solar) panel provides the greatest power output when:

  1. the external load (resistance) equals the value of the internal resistance of the panel at the maximum power point
  2. the sun is at its brightest i.e. no clouds are present and the panel's face is at right angles to the sun's rays
  3. the panel's surrounds are as cool as possible to avoid sunlight energy losses as heat

Source: http://www.polarpowerinc.com/info/operation20/operation23.htm

Maximum Power Point Tracker

A maximum power point tracker (or MPPT) is a high efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic (PV) cell, most commonly for a solar panel or array, and converts the power to a voltage or current level which is more suitable to whatever load the system is designed to drive.

Photovoltaic (PV) cells have a single operating point where the values of the current (I) and Voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I as specified by Ohm's Law. A PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). Maximum power point trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell.

Battery-less grid-tied PV inverters utilize MPPTs to extract the maximum power from a PV array, convert this to alternating current (AC) and sell excess energy back to the operators of the power grid.

MPPT charge controllers are desirable for off-grid power systems to make the best use of all the energy generated by the panels. MPPT charge controllers are quickly becoming more affordable and are more common in use now than ever before.

The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged.

Source: http://en.wikipedia.org/wiki/Maximum_power_point_tracker

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