Maximum Power Point Tracking is a family of control algorithms that aims at optimizing the use of a power source that possesses a fluctuating power profile.
Indeed, some power sources, like solar panels, present power characteristics that strongly depend on the operating conditions. For instance, the cloud coverage significantly impacts the capability of a panel to deliver electricity. As such, maximizing the extracted power requires identifying – and tracking – the operating point that provides the highest power level as a function of the operating conditions.
Therefore, Maximum Power Point Tracking (MPPT) is often applied in renewable energy systems – e.g. photovoltaic plants or wind turbines – as their power delivery capability varies significantly and in an unpredictable manner. Other special operating points may be interesting to track, such as the maximum efficiency point tracking (MEPT), or other optimum, e.g. related to operating costs.
Principles of operation
For practically all real power sources, the power that can be extracted varies with the operating point. While electrical sources are related to the voltage/current pair, the same principle also applies to force/speed, flux/surface, etc.
In all cases, the inevitable internal resistance (or equivalent quantity) limits the maximum possible output power. Non-linear or more complex characteristics also exist, but with the same result: the maximum power point is not located at the [max. voltage · max. current] point (or equivalent quantity). Therefore, the operating point that delivers the maximum power must be constantly tracked by searching for the best voltage · current combination.
Photovoltaic solar panel example
For instance, photovoltaic panels (PV panels) possess a well-known output characteristic, featuring an internal resistance that quickly decreases close to the open-circuit voltage (assuming a current source model).
This results in a bump-shaped power-voltage characteristic, whose top is typically located between 60-80% of the open-circuit voltage. This point is however not fixed but varies with the output current, which depends itself on the temperature and irradiance, i.e. the operating conditions of the PV cells themselves.
Typical IV-PV curves of a solar panel