Abstract
Background
The purpose of this project is to build a clean self-sustained photovoltaic
energy harvesting system. The system will accept input from multiple,
high-efficiency solar panels and store the energy in Lithium-Ion battery
cells. The system will be used to drive a resistive load from either the
battery or directly from the solar panels, depending on the state of the
system. The system will accept input power from the solar panels
regardless of the amount of light incident on each panel.
Typically, large arrays of solar panels are less efficient with increasing size.
This is because any reduction in the power produced by a single panel will
cause that panel to sink current from the system, which reduces efficiency and
can damage the photovoltaic substrate. By breaking up the solar array into
separate branches, this project aims to eliminate this wasteful current backflow
and increase total system efficiency from panel to load.
Harvesting Solar Energy using MPPT
Storing Energy with Li-Ion Batteries
The I-V curve of a solar panel is
widely variant, based on the
intensity and angle of the
incident light. In order to extract
the maximum power from a
panel, a Maximum Power Point
Tracking (MPPT) algorithm
must be employed. This
algorithm maintains the correct
voltage-current relationship at
the output of the solar panel in
order to provide maximum
power output.
In order to safely charge LithiumIon batteries, a special, nonlinear
charging curve must be followed.
The battery will only accept a
high-current charge input when
its output voltage is above 10%
nominal, requiring a tricklecharge for voltages less than
10%. At 90% nominal output
voltage, the charge current must
taper off. This charging profile is
implemented with the use of
specialty Li-Ion charging IC’s.
System Implementation
Solar Panel
MPPT Controller
Buck-Boost
Converter
Solar Panel
Battery
Charger
Power-switching
output circuitry
To Load
Li-Ion
Battery
MPPT Controller
Multiple branches of solar panels and MPPT controllers are merged at the input of a buck-boost converter, with low-dropout diodes to prevent damaging
current backflow. The buck-boost chip circuitry conditions the power for the battery charger, which charges the battery using a Li-Ion charging algorithm.
Power-switching circuitry at the output allows the load to be driven by either the battery or the buck-boost output directly, depending on the battery’s charge.
Microcontroller for
monitoring and
data acquisition
Input from
panel/MPPT
branches
Output to load
Buck/boost
converter
Power-switching
output circuitry
Battery Charger
Battery connection
PCB Implementation
System Charging Efficiency
3.5
Efficiency Analysis
3.2785
Power (Watts)
3
2.35125
2.5
2.1465
2
1.5
MPPT Output
Buck-Boost
Output
1
Battery
Charging
Power
0.5
0
Relative Charging Efficiency
100
100%
80
60
40
20
0
71.7%
65.4%
Total
Photovoltaic
Power
Buck-boost
Efficiency
Final System
Total System
Efficiency
In order to measure the system’s
charging efficiency, the current and
voltage at each node is measured
at a given time interval during the
battery’s full-current charge state.
These measurements are
performed by current-sense
amplifiers and voltage dividers
connected to ADC pins on the
microcontroller. Multiplying the
currents and voltages together
allows the total power efficiency to
be directly observed across each
stage of the system.