Perturb and observe

In this method the controller adjusts the voltage from the array by a small amount and measures power; if the power increases, further adjustments in that direction are tried until power no longer increases. This is called perturb and observe (P&O) and is most common, although this method can cause power output to oscillate.^[14][15] It is also referred to as a hill climbing method, because it depends on the rise of the curve of power against voltage below the maximum power point, and the fall above that point.^[16] Perturb and observe is the most commonly used method due to its ease of implementation.^[14] Perturb and observe method may result in top-level efficiency, provided that a proper predictive and adaptive hill climbing strategy is adopted.^[17][18]

Incremental conductance

In this method, the controller measures incremental current and voltage changes to predict the effect of a voltage change. This method requires more computation in the controller, but can track changing conditions more rapidly than P&O. Power output does not oscillate.^[19] It utilizes the incremental conductance (${\displaystyle dI/dV}$) of the photovoltaic array to compute the sign of the change in power with respect to voltage (${\displaystyle dP/dV}$).^[20] The incremental conductance method computes MPP by comparison of the incremental conductance (${\displaystyle I_{\Delta }/V_{\Delta }}$) to the array conductance (${\displaystyle I/V}$). When these two are the same (${\displaystyle I/V=I_{\Delta }/V_{\Delta }}$), the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the process is repeated.

The incremental conductance method is based on the observation that at MPP, ${\displaystyle dP/dV=0}$, and that ${\displaystyle P=IV}$. The current from the array can be expressed as a function of the voltage:

${\displaystyle P=I(V)V}$.

Therefore, ${\displaystyle dP/dV=VdI/dV+I(V)}$. Setting this equal to zero yields: ${\displaystyle dI/dV=-I(V)/V}$. Therefore, MPP is achieved when the incremental conductance is equal to the negative of the instantaneous conductance. The power-voltage curve characteristic shows that: when the voltage is smaller than MPP, ${\displaystyle dP/dV>0}$, so ${\displaystyle dI/dV>-I/V}$; when the voltage is bigger than MPP, ${\displaystyle dP/dV<0}$ or ${\displaystyle dI/dV<-I/V}$. Thus, a tracker can know where it is on the power-voltage curve by calculating the relation of the change of current/voltage and the current voltage themselves.

Current sweep

The current sweep method uses a sweep waveform for the array current such that the I-V characteristic of the PV array is obtained and updated at fixed time intervals. MPP voltage can then be computed from the characteristic curve at the same intervals.^[21][22]

Constant voltage

Constant voltage methods include one in which the output voltage is regulated to a constant value under all conditions and one in which the output voltage is regulated based on a constant ratio to the measured open circuit voltage (${\displaystyle V_{OC}}$). The latter technique may also be labeled the "open voltage" method.^[23] If the output voltage is held constant, there is no attempt to track MPP, so it is not strictly a MPPT technique, though it does function in cases when MPP tracking tends to fail, and thus it is sometimes used supplementally. In the open voltage method, power delivery is momentarily interrupted and the open-circuit voltage with zero current is measured. The controller then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage ${\displaystyle V_{OC}}$.^[24] This is usually a value that has been predetermined to be the MPP, either empirically or based on modelling, for expected operating conditions.^[19][20] The array's operating point is thus kept near MPP by regulating the array voltage and matching it to the fixed reference voltage ${\displaystyle V_{ref}=kV_{OC}}$. The value of ${\displaystyle V_{ref}}$ may be chosen to give optimal performance relative to other factors as well as the MPP, but the central idea is that ${\displaystyle V_{ref}}$ is determined as a ratio to ${\displaystyle V_{OC}}$. One of the inherent approximations in the method is that the ratio of MPP voltage to ${\displaystyle V_{OC}}$ is only approximately constant, so it leaves room for further possible optimization.

Temperature method

This method estimates the MPP voltage (${\displaystyle V_{mpp}}$) by measuring the temperature of the solar module and comparing it against a reference.^[25] Since changes in irradiation levels have a negligible effect on the MPP voltage, its influences may be ignored - the voltage is assumed to vary linearly with temperature.

This algorithm calculates the following equation:

${\displaystyle V_{mpp}(T)=V_{mpp}(T_{ref})+u_{V_{mpp}}(T-T_{ref})}$

Where:

${\displaystyle V_{mpp}}$ is the voltage at the maximum power point for a given temperature;

${\displaystyle T_{ref}}$ is a reference temperature;

${\displaystyle T}$ is the measured temperature;

${\displaystyle u_{V_{mpp}}}$ is the temperature coefficient of ${\displaystyle V_{mpp}}$ (available in the datasheet).

Comparison of methods

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.^[6][16][19]

P&O produces power output oscillations around the maximum power point even under steady state irradiance.

Incremental conductance can determine the maximum power point without oscillating.^[14] It can perform MPPT under rapidly varying irradiation conditions with higher accuracy than P&O.^[14] However, this method can produce oscillations and can perform erratically under rapidly changing atmospheric conditions. The sampling frequency is decreased due to the higher complexity of the algorithm compared to P&O.^[20]

In the constant voltage ratio (or "open voltage") method, energy may be lost during the time the current is set to zero.^[20] The approximation of 76% as the ${\displaystyle V_{MPP}/V_{OC}}$ ratio is not necessarily accurate.^[20] Although simple and low-cost to implement, the interruptions reduce array efficiency and do not ensure finding the actual MPP. However, efficiencies of some systems may reach above 95%.^[24]