19-12-2012, 05:45 PM
Analysis of Photovoltaic Charging System Based on MPPT
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Abstract
With the daily worsening of global energy crisis and
environmental pollution, characteristics of the
non-pollution, non-noise and non-raw material except
the sun of photovoltaic generation makes it a promising
green energy. At present, the maximum output power of
photovoltaic battery changes with the external changing
environment, and it plays the match role in internal
resistance and external load impedance of photovoltaic
battery. A photovoltaic charger based on MPPT is
designed in this paper, and the design of special
photovoltaic charging inverter, problems of charging
batteries and other issues are discussed. Asymmetric
fuzzy control is applied to the maximum power point
tracking control of photovoltaic charger, which can
guarantee the maximum power point tracks
photovoltaic battery quickly when the external
environment changes dramatically, and has high control
precision and stability. Finally, it is proved that the
efficiency of the photovoltaic charger is relatively high.
Introduction
At present, the conversion efficiency of
monocrystalline silicon cell, with the most efficient
photoelectric conversion efficiency, is about 20%.
Because of improper control of photoelectric conversion,
the utilization of solar energy and spectrum is only 50%
to 70%; in addition, the particles, dust and so on in the
air may land at photovoltaic battery board, which will
reduce the amount of light irradiation and conversion
efficiency. On the other hand, the outside environment
plays a great role in the output of photovoltaic battery:
declining intensity of sunlight results in short-circuit
photovoltaic battery and decreasing of current and
output power; while rising temperature results in
declining of open circuit voltage and output power of
photovoltaic battery.
Design of photovoltaic charging system
A photovoltaic charging system with MPPT function
is composed of solar battery, battery management
module, CPU dominating module. When charging the
battery, solar battery generates electricity which is
stored in battery by the sun radiation, surplus electric
power is used to supply for the monitoring unit; when
taking use of electricity, battery releases the stored
energy supply for the system to ensure the system runs
without power-down [3 ,4 ].
CPU dominating module maintains the maximum
power output of the array and manages the battery
optimally. The dominating module regulates the DC /
DC module to ensure the maximum power output of the
solar battery array. The dominating module which is
responsible for the group's macro-charging cycle
management sends orders through the I 2C interface to
battery management module, including setting up the
charge and discharge mode of battery management
module, calibrating the battery power and so on, as well
as gets status information of the battery from the battery
management module, including the battery voltage, the
charge current, electricity left over and so on.
Management strategy of battery energy
Design of battery management module
Photovoltaic charging system designs a management
module specifically for the battery; with the command
of CPU dominating module, the management module
can take charge of charging management, supplying
management, calibrating the battery power and the
corresponding protection. With the charging and
discharging characteristic of lithium-ion battery, battery
management module charges in phased, when the
battery voltage is low, it will charge with a rate of
current less than 0.1C; then open the switch to high-rate
charge switch will be opened to charge with large
current less than 1C; finally, it will enter the floating
charging stage. Power management is mainly
responsible for monitoring the battery voltage and
collecting surplus electricity statistics; when the power
fell to 15% and the voltage is lower than a certain
threshold, CPU dominating module will cut off charging,
and the selection of the threshold voltage considers the
impact of environmental temperature.
Asymmetric fuzzy control principle
The membership function and control rule of
traditional fuzzy control are symmetrical; otherwise, the
characteristics of photovoltaic battery on either side of
the maximum power point are asymmetry, and there is a
significant difference in control effect on either side of
the maximum power point when taking use of
asymmetric fuzzy control; although it can track changes
of the external environment quickly, there is still a
power fluctuation nearby the maximum output power
point. For maximum power point tracking control
methods and the issue of the characteristics of
photovoltaic cells, we apply the asymmetric fuzzy
control in controlling the maximum power point
tracking when designing the photovoltaic charger taking
into account the problem of controlling the maximum
power point tracking and the characteristic of
photovoltaic battery.
Conclusion
We design a photovoltaic charging system in this
paper, the result of testing shows that the system in
accordance with the design of operation has well
stability, reliability and efficiency. Study shows that
fixed-voltage track can be taken used to supply with the
chopper to the main circuit of the inverter in order to
pursuit the maximum photovoltaic conversion efficiency.
At the same time, the asymmetric fuzzy PID control is
applied to MPPT of photovoltaic battery in accordance
with its characteristic. The method can guarantees that
the photovoltaic battery tracks the maximum power
point quickly when the external environment changes
dramatically, which basically eliminates the oscillation
phenomenon that traditional method tracks maximum
power point and is with high control precision and
stability.