Transcript Solar Power Array Management for the Solar Racing Team

Solar Power Array Management
for the Solar Racing Team
Mark Calotes
Ginah Colón
Alemneh Haile
Nidhi Joshi
Michael Lu
November 1, 2010
GT Solar Jackets
School of Electrical and Computer Engineering
Georgia Institute of Technology
Project Overview
• Design a system to charge car batteries
with the energy received from solar cells
• To be used for the Solar Jackets’ solar car
in the World Solar Challenge
• Estimated cost: $222
Design Goals
• Maximum power point tracking
• Switching power supply
• Microcontroller programming
• Board/PC communication
Design Overview
Solar Array
Material: CZ silicon (monocrystalline)
Predicted Efficiency: 17.8%
10 cells in series
Testing outdoors at 2:30 p.m.
on a sunny day
Voc = 5.4 V
Isc = 5.45 A
Individual Solar Cell I-V Curve
Battery
HR9-12 Sealed Lead Acid Battery
12 V (6 cells in series)
Power Switching Circuit Layout
Preliminary Test Waveforms
Input: 6V
Output: 12 V
Control of Circuit
Output voltage controlled
by changing duty cycle of
PWM that is used to
switch MOSFET
Frequency of
MOSFET Switching
PWM = 1 kHz
Duty Cycle of
MOSFET Switching
PWM = 21%
Circuit Efficiency
Efficiency = 85%
Causes for low efficiency:
Solutions:
• MOSFET IRF 520: Ron = 0.27 Ω
• Use MOSFET STW75NF20 with Ron =
0.034 Ω
• Regular Diodes
• Diode with lower voltage drop
(SBR10U45SP5 for solar cells)
• Inductor Series Resistance = 0.186 Ω
Power MOSFETs and Gate Driver
Need for lower rise
time in MOSFET
switching
• Losses occur during
MOSFET switching
• Rise time limits
maximum switching
frequency
Gate Driver:
• Higher switching voltage
→ lower rise time
• Lower input resistance
Initial Testing MOSFET IRF 520:
Rise Time = 73.3 μs
Inductors
• Inductor value chosen on the basis of
maximum switching frequency
• Lower value of L → smaller components
Scaling of System for Final Solar
Car
• Final system will use a battery at 96 V
• Main Issue: Final solar array characteristics unknown
• Component Characteristics
Component
Key Characteristics
MOSFET STW75NF20
200 V, 75 A
Diode SBR10U45SP5
Vbr = 45 V, Max. I = 10 A
Current Sensor AC715
Range = 0 to 20 A
• Components that need to be changed for compatibility
Inductor: Select depending on final array characteristic
• Modify range of frequency and duty cycle for circuit control
PIC18F4321 Microcontroller
• 44-Pin MCU
• Important Pins:
– 30, 31: Internal
Oscillators
– 11, 36: PWM Module
– 1, 44: PC I/O Port
• Accurate Clock Speed:
– DC – 40 MHz
• Low Supply Voltage:
– 2.2 V – 5.5 V
• Max. Current Draw:
– 1.750 mA
• C compiler available
• Unreserved pins for
power usage
designated as I/O Pins
•
Qwik-and-Low Board (MCU On
Board)
Advantages:
– LCD display information linked to
I/O ports already
– Built-in RS-232 serial port
– GT-based compiler and
debugging software
• Disadvantages:
– Limited user of I/O pins due to
default assignment
– Unused board space; needs to
be customized for S.P.A.M.
design
• Current S.P.A.M. Progress:
– Have initialized testing and
editing template programs to
become accustomed with the
MCU
– Specific I/O pins for algorithm
implementation not yet specified
Perturb and Observe (P & O)
Algorithm
• Implementation Goals:
– Take current voltage (V) and current (I),
then compare with most previous value
– Incrementally increase duty cycle for
greater voltage; decrease duty cycle for
less voltage
– Repeat searching continuously
• Algorithm Strengths:
– Popular implementation; open-source
code available
– Effective in dynamically changing
environments
• Algorithm Disadvantages:
– Inevitable oscillation around the
maximum power point (MPP)
– May mistake a local maximum as the
absolute maximum
Coding Progress and Problems
• Progress
– Attained capability of using the Qwik-and-Low Board
and PIC microcontroller to program prototypes
– Besides algorithm, no C coded program available
from group yet
• Problems/Setbacks
– Unsure about how to proceed with programming MCU
on a custom PCB board
– Physical implementation significantly delayed due to
time spent understanding MCU behavior and
characteristics
Printed Circuit Board
• Small, compact PCB
– Switching power supply
– Microcontroller
– Serial communication
– Voltage- & current-measuring circuits
• Two modules per board
– Maximize microcontroller I/O resources
– Anticipate multiple solar sub-arrays
Future Work Schedule
• Finalize circuit components & design by
11/19/2010
– Minimize inductor & maximize efficiency
• Design printed circuit board by 11/24/2010
• Program microcontroller by 12/1/2010
– Code MPPT
• Test that the circuit adjusts to changing
light intensity by 12/8/2010
Circuit Components
MOSFET STW75NF20
200 V, 75 A, Ron = 0.034 Ω
Rise Time = 33 ns when Vgs = 10 V
Diode SBR10U45SP5: Bypass Diode for Solar Panels
Forward Voltage Drop = 0.42 V, Reverse Breakdown Voltage = 45 V, Max. Current = 10 A
UC2714 MOSFET GATE DRIVER
High Current Power FET Driver,
1.0 A Source / 2 A Sink
Sources: Datasheets
Current Sensor
ACS715
Current Range = 1 to 20 A
R = 1.2 mΩ
Output Sensitivity = 133 to 185 mV/A