What is Maximum Power Point Tracking and How Does it
Work?
| What is MPPT? |
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Maximum Power Point
Tracking, frequently referred to as
MPPT, is an electronic system
that operates the Photovoltaic (PV) modules in
a manner that allows the modules to
produce all the power they are capable
of.
MPPT is not a
mechanical tracking system that “physically
moves”the modules to make them point
more directly at the sun.
MPPT is a fully
electronic system that varies the electrical
operating point of the modules so that
the modules are able to deliver maximum
available power. Additional power harvested
from the modules is then made available as
increased battery charge current. MPPT can be
used in conjunction with a mechanical tracking
system, but the two systems are completely
different.
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| To understand how MPPT
works, let’s first consider the operation of a
conventional PWM or Relay Type (non-MPPT)
charge controller. When a conventional controller
is charging a discharged battery, it simply
connects the modules directly to the battery. This
forces the modules to operate at battery voltage,
typically not the ideal operating voltage at which
the modules are able to produce their maximum
available power. The PV Module
Power/Voltage/Current Graph (at right)
shows the traditional Current/Voltage curve for a
typical 75W module at standard test conditions of
25C cell temperature and 1000W/m2 of insolation.
This graph also shows PV module power delivered vs
module voltage. For the example shown, the
conventional controller simply connects the module
to the battery and forces the module to operate at
12V. By forcing the 75W module to operate at 12V
the conventional controller artificially limits
power production to 53W. |
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| Rather than simply
connecting the module to the battery, an
MPPT charge controller calculates the voltage at
which the module is able to produce the most power
and connects to the panel at that voltage.
In this example the maximum power voltage of the
module (Vmp) is 17V. The MPPT system then operates
the modules at 17V to extract the full 75W,
regardless of present battery voltage. A high
efficiency DC-to-DC power converter converts the
17V module voltage at the controller input to
battery voltage at the output. If the whole system
was 100% efficient, battery charge current in this
example would be VMODULE VBATTERY x IMODULE,
or 17V 12V x 4.45A = 6.30A. A charge current
increase of 1.85A or 42% would be achieved by
harvesting module power that would have been left
behind by a conventional controller and turning it
into useable charge current. But, nothing is 100%
efficient and actual charge current increase will
be some what lower as some power is lost in wiring,
fuses, circuit breakers, and in the MPPT
controller. |
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Actual charge current
increase varies with operating conditions. As
shown above,the greater the difference between
PV module maximum power voltage Vmp and battery
voltage, the greater the charge current
increase will be. Cooler PV module cell
temperatures tend to produce higher Vmp and
therefore greater charge current increase.This
is because Vmp and available power increase as
module cell temperature decreases as shown in
the PV Module Temperature Performance
Graph (at right). Modules with a
25C Vmp rating higher than 17V will also tend
to produce more charge current increase because
the difference between actual Vmp and battery
voltage will be greater. A highly discharged
battery will also increase charge current since
battery voltage is lower,and output to the
battery during MPPT could be thought of as
being “constant power”.
What most people see in cool
comfortable temperatures with typical battery
conditions is a charge current increase of
between 10 – 25%. Cooler temperatures and
highly discharged batteries can produce
increases in excess of 30%. Customers in cold
climates have reported charge current increases
in excess of 40%. What this means is that
current increase tends to be greatest when it
is needed most(during the winter); in cooler
conditions when days are short, sun is low on
the horizon, and batteries may be more highly
discharged. In conditions where extra power is
not available (highly charged battery and hot
PV modules) most MPPT charge controllers will
perform as a conventional PWM type
controller.
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Another way to think of an MPPT charge controller is to
think of it like the transmission of a car. If the
transmission of your car is in the wrong gear the wheels do not
receive the maximum power available at the best efficiency
possible. This is because the engine is either operating at a
higher or lower rpm than its ideal. The transmission is there
to transfer the power to the wheels in a way that allows the
engine to operate at the most efficient speed whether driving
on the highway at 110 km per hour or a bumpy country road at 20
km per hour. Without the transmission you would be very limited
to what speed you could drive your car.
For the analogy let's make the solar module the car
engine. The solar module, like the car engine, likes
to operate at a certain voltage (or speed) to make the most
power or operate at the best efficiency. This is called the
Vmp (maximum power point voltage) or the Vpp
(the peak power voltage). The Vmp of a solar
module changes with changes in temperature and sunlight
intensity. The brighter the sun the higher the
Vmp. The colder the module, the higher the
Vmp.
The battery voltage is similar to the speed of the
car's wheels. It varies according to battery type,
battery temperature, battery state of charge and any loads that
are applied to the battery. Large loads will temporarily
decrease the battery voltage. For a 12 volt nominal system, the
battery voltage could vary between 11.0 volts and 16.0
volts.
To charge a battery, we need to apply a voltage that
is higher than the battery voltage. If the solar
module has a Vmp lower than the battery
voltage the current will not flow. This is similar to your
engine operating slower than the wheels of the car. That is why
solar modules must have a Vmp significantly
higher than the battery voltage. If it was not, the module
would not charge the battery when the module is warm or the
battery is almost full. To make sure the module will charge on
a hot day, manufacturers make the Vmp of the
voltage somewhere around 17-18 volts. On a hot day this may go
down to 15 volts, and on a cold day, as high as 19 volts .
If the Vmp is higher than the voltage of a
battery (without an MPPT charge controller), the module voltage
is dropped down to the battery voltage which is generally less
than ideal. PWM and relay type charge controllers connect the
solar module directly to the battery giving you no additional
power. MPPT charge controllers connect to the solar
module at the Vmp (maximum power point voltage) and then reduce
the output voltage to get all the power available into the
battery. Most MPPT charge controllers have to adjust
every few seconds to minutes as the sun intensity, module
temperature and battery voltage changes.
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