P12407: Multiple-input Photovoltaic Energy Harvesting

SystemAbstractThe 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.

BackgroundTypically, 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 BatteriesThe 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 Lithium-Ion 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 trickle-charge 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

Li-Ion Battery

Power-switching output circuitry

Solar Panel MPPT Controller

Battery Charger

To Load

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.

Input from panel/MPPT branches

Buck/boost converter

Battery ChargerPower-switching output circuitry

PCB Implementation

Microcontroller for monitoring and data acquisition

Output to load

Battery connection

Final System

Efficiency Analysis

0

0.5

1

1.5

2

2.5

3

3.5 3.2785

2.351252.1465

System Charging Efficiency

Pow

er (W

atts

)

MPPT Output

Buck-BoostOutput Battery

ChargingPower

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.

0102030405060708090

100100%

71.7%65.4%

Relative Charging Efficiency

TotalPhotovoltaic

PowerBuck-boost Efficiency Total System

Efficiency