Maximum power point tracking
[5] The technique is most commonly used with photovoltaic (PV) solar systems but can also be used with wind turbines, optical power transmission and thermophotovoltaics.
As these conditions vary, the load characteristic (impedance) that gives the highest power transfer changes.
The system is optimized when the load characteristic changes to keep power transfer at highest efficiency.
This optimal load characteristic is called the maximum power point (MPP).
Circuits can be designed to present optimal loads to the photovoltaic cells and then convert the voltage, current, or frequency to suit other devices or systems.
[7][8] MPPT samples cell output and applies the proper resistance (load) to obtain maximum power.
[9] MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors.
[10] Photovoltaic cells have a complex relationship between their operating environment and the power they produce.
The nonlinear I-V curve characteristic of a given cell in specific temperature and insolation conditions can be functionally characterized by a fill factor (FF).
Fill factor is defined as the ratio of the maximum power from the cell to the product of open circuit voltage Voc and short-circuit current Isc.
Tabulated data is often used to estimate the maximum power that a cell can provide with an optimal load under given conditions: For most purposes, FF, Voc, and Isc are enough information to give a useful approximate view of the cell's electrical behavior under typical conditions.
[12] However, at a photovoltaic cell's MPP region, its curve has an approximately inverse exponential relationship between current and voltage.
A load with resistance R=V/I equal to the reciprocal of this value draws the maximum power from the device.
This is a dynamic quantity that changes depending on the level of illumination, as well as other factors such as temperature and cell condition.
If a full power-voltage (P-V) curve is available, then the maximum power point can be obtained using a bisection method.
When directly connecting a load to cell, the operating point of the panel is rarely at peak power.
The I-V curve of the panel can be considerably affected by atmospheric conditions such as irradiance and temperature.
MPPT algorithms frequently sample panel voltages and currents, then adjust the duty ratio accordingly.
Modern implementations often utilize more sophisticated computers for analytics and load forecasting.
[15] Perturb and observe method may result in top-level efficiency, provided that a proper predictive and adaptive hill climbing strategy is adopted.
[24] This is usually a value that has been predetermined to be the MPP, either empirically or based on modelling, for expected operating conditions.
may be chosen to give optimal performance relative to other factors as well as the MPP, but the central idea is that
This algorithm calculates the following equation: where: Both P&O and incremental conductance are examples of "hill climbing" methods that can find the local maximum of the power curve for the array's operating condition, and so provide a true MPP.
[15] However, this method can produce oscillations and can perform erratically under rapidly changing atmospheric conditions.
[25] Although simple and low-cost to implement, the interruptions reduce array efficiency and do not ensure finding the actual MPP.
In such systems the same current, dictated by the inverter, flows through all modules in the string (series).
Because different modules have different I-V curves and different MPPs (due to manufacturing tolerance, partial shading,[27] etc.)
[28] Instead, MPPTs can be deployed for individual modules, allowing each to operate at peak efficiency despite uneven shading, soiling or electrical mismatch.
(An alternative approach commonly used in spacecraft is to divert surplus PV power into a resistive load, allowing the panel to operate continuously at its peak power point in order to keep the panel as cool as possible.
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