Understanding the Voltage – Current (I-V) Curve of a Solar Cell

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The behavior of an illuminated solar cell can be characterized by an I-V curve. Interconnecting several solar cells in series or in parallel merely to form Solar Panels increases the overall voltage and/or current but does not change the shape of the I-V curve. The I-V curve contains three significant points: Maximum Power Point, MPP (representing both Vmpp and Impp), the Open Circuit Voltage (Voc), and the Short Circuit Current (Isc).

The I-V curve is dependent on the module temperature and the irradiance. An increasing irradiance leads to an increased current and slightly increased voltage, as illustrated below:
As shown above, an increasing temperature has a detrimental effect on the voltage as the voltage tends to reduce. Note that Most I-V curves are given for the standard test conditions (STC) of 1000 watts per square meter sunlight (often referred to as one peak sun) and 25 degrees C (77 degrees F) cell temperature.

The operating point of a PV module is the defined as the particular voltage and current, at which the PV module operates at any given point in time. For a given irradiance and temperature, the operating point corresponds to a unique (I, V) pair which lies onto the I-V curve. The power output at this operating point is given by:

P = I * V

The operating point (I, V) corresponds to a point on the power-voltage (P-V) curve, For generating the highest power output at a given irradiance and temperature, the operating point should such correspond to the maximum of the (P-V) curve, which is called the maximum power point (MPP) defined by (Impp* Vmpp). If a PV module (or array) is directly connected to an electrical load, the operating point is dictated by that load. 

For getting the maximal power out of the module, it thus is imperative to force the module to operate at the maximum power point. The simplest way of forcing the module to operate at the MPP, is either to force the voltage of the PV module to be that at the MPP (called Vmpp) or to regulate the current to be that of the MPP (called Impp).

However, the MPP is dependent on the ambient conditions. If the irradiance or temperature change, the I-V and the P-V characteristics will change as well and hence the position of the MPP will shift. Therefore, changes in the I-V curve must be tracked continuously such that the operating point can be adjusted to be at the MPP after changes of the ambient conditions.

This process is called Maximum Power Point Tracking or MPPT. The devices that perform this process are called MPP trackers and are integral part of the charge controllers in Solar PV installations.

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