•
•
19.1.1
temperature change, the I-V and the P-V characteristics will change as well and hence the
position of the MPP may shift. Therefore, changes in the I-V curve have to 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. We can distinguish between two categories
of MPPT:
Indirect MPPT, where the position of the MPP is estimated via a hard-coded
algorithm.
Direct MPPT, where the actual I-V data is used to determine the position of the
MPP.
All the MPPT algorithms that we discuss in this section are based on finding and
tuning the voltage until V MPP is found. Other algorithms, which are not discussed in this
section, work with the power instead and aim to find I MPP .
Indirect MPPT
First, we discuss indirect MPP tracking, where simple assumptions are made for
estimating the MPP based on a few measurements.
Fixed voltage method
For example, in the fixed voltage method (also called constant voltage method), the
operating voltage of the solar module is adjusted only on a seasonal basis. This model is
based on the assumption that for the same level of irradiance, higher MPP voltages are
expected during winter than during summer. This method is not very accurate; it works
best at locations with minimal irradiance fluctuations between different days.
Fractional open circuit voltage method
One of the most common indirect MPPT techniques is the fractional open circuit voltage
method. This method exploits the fact that – in a very good approximation – the V MPP is
given by
where k is a constant. For crystalline silicon, k usually takes values between 0.7 and 0.8. In
general, k of course is dependent on the type of solar cells.
As changes in the open circuit voltage can be easily tracked, changes in the V MPP can
be easily estimated just by multiplying by k. This method can thus be implemented easily.
•
19.1.1
temperature change, the I-V and the P-V characteristics will change as well and hence the
position of the MPP may shift. Therefore, changes in the I-V curve have to 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. We can distinguish between two categories
of MPPT:
Indirect MPPT, where the position of the MPP is estimated via a hard-coded
algorithm.
Direct MPPT, where the actual I-V data is used to determine the position of the
MPP.
All the MPPT algorithms that we discuss in this section are based on finding and
tuning the voltage until V MPP is found. Other algorithms, which are not discussed in this
section, work with the power instead and aim to find I MPP .
Indirect MPPT
First, we discuss indirect MPP tracking, where simple assumptions are made for
estimating the MPP based on a few measurements.
Fixed voltage method
For example, in the fixed voltage method (also called constant voltage method), the
operating voltage of the solar module is adjusted only on a seasonal basis. This model is
based on the assumption that for the same level of irradiance, higher MPP voltages are
expected during winter than during summer. This method is not very accurate; it works
best at locations with minimal irradiance fluctuations between different days.
Fractional open circuit voltage method
One of the most common indirect MPPT techniques is the fractional open circuit voltage
method. This method exploits the fact that – in a very good approximation – the V MPP is
given by
where k is a constant. For crystalline silicon, k usually takes values between 0.7 and 0.8. In
general, k of course is dependent on the type of solar cells.
As changes in the open circuit voltage can be easily tracked, changes in the V MPP can
be easily estimated just by multiplying by k. This method can thus be implemented easily.