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Solar Power Source for
Sensors
Group 10
Steven Portscheller
Pavlina Akritas &
Gunjan Tejani
ECE 445 Senior Design
April 28, 2006
Introduction
Solar panel and battery system provide
independent power source for a wireless
sensor node
Utilizes peak power tracking to extract the
maximum power from the solar panel under
variable conditions (sunlight, load)
Provides a regulated 3.3 Volts independent of
a power grid
Objectives
Provide approximately 30-50 mW at 3.3 V
Small size
Implement maximum peak power tracking on
the solar panel
Monitor battery charge status
Overall Design
Initial Idea
Constant Voltage Fraction
Voltage at which peak power occurs is a constant
fraction of open circuit voltage under given light
conditions
Not necessary to measure current
Model based design (non adaptable)
Dependant on accuracy of preproduction testing
Challenges
Hard to search for PV’s
that are small.
Power Film 3.6 V, 50 mA
Flexible Solar Panel
MPT3.6-75 by Sundance
Solar with size 2.9" X 3“.
Testing
Solar panel tests to determine peak power voltage
constant k
Determining k
2.5
y = 0.74*x - 0.094
2
1.5
Solar Panels 1
Vmax (V)
V 1
Rv ar
2 0.5
0
data
linear fit
-0.5
0 0.5 1 1.5 2 2.5 3 3.5
Voc (V)
Testing cont.
BUT, the current is too low, therefore the resulting
power was low.
-3 -3
x 10 V versus P with a Lamp x 10 V versus I with a Lamp
3 1.8
2.8
1.6
2.6
1.4
2.4
1.2
2.2
Current (A)
Power (W)
2 1
1.8
0.8
1.6
0.6
1.4
0.4
1.2
1 0.2
1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2
Voltage (V) Voltage (V)
Challenges Overcame
Final PV used, Edmund Scientific item
#3039811 with specifications of 0.45 V, 800 mA
and size of 33/4" x 29/16" x 1/4“.
Given the low VOC, used 6 PV’s in series.
I SC is never reached so model would work.
Temperature Test Results
When cooled VOCincreases (from 2.870 V to 3.035 V)
When heated VOCdecreases (from 2.837 V to 2.685 V)
Possible reasons include the solar insolation, different
ambient temperatures, electrons and holes
Another possible reason could be the internal cell diode
Id I0 e qVd / kT
1
Typical Test Results
Temperature results:
• When cooled
V increases (from 2.870 V to 3.035 V)
OC
• When heated
V decreases (from 2.837 V to 2.685 V)
OC
• V versus I, V variation throughout a day
OC
V versus I at Voc = 3.301V P versus V at Voc = 3.301V
0.7 1.6
0.6 1.4
0.5 1.2
Voltage (V)
0.4 1
0.3 Power (W)
0.8
0.2
0.6
0.1
0.4
0
0.5 1 1.5 2 2.5 3 3.5 0.2
Current (A) 0.5 1 1.5 2 2.5 3 3.5
Voltage (V)
Voc throughout a day
Voc throughout a day
3.8
3.6
3.4
3.2
Voc (V)
3
2.8
2.6
2.4
2.2
6 8 10 12 14 16 18 20
Time
Changing ideas
Maximum power point tracking (MPPT) based on
measuring voltage and current and calculating
power
Operating points kept at knee of the PV I-V curves.
Adaptable.
Power Tracking Hardware
Dc/dc converter with large (1 F) capacitor
Current sense amplifier and sense resistor
PIC to measure voltage and current and
enable/disable converter accordingly
Dc/dc converter
Acts as voltage control for solar panel
Dc/dc converter is enabled to discharge
capacitor and lower operating voltage of solar
panel
Converter is disabled to allow capacitor to
charge and raise operating voltage of solar
panel
Boosts solar panel voltage (0 to 3.3 V) to a
voltage sufficient to charge the battery
Dc/dc converter
MAX1675 chosen for small size and high
efficiency (up to 94 % claimed)
Cooper Bussmann PowerStor® carbon
aerogel supercapacitor chosen for high
energy density and low equivalent series
resistance (ESR)
Testing
Testing indicated an efficiency of approx.
85%
MAX1675
0.9
0.85
0.8
Efficiency
0.75
R= 100
0.7 R= 90
R= 68
R= 48
0.65
1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4
Input Voltage (V)
Microcontroller
MSP430 used in target circuit
PIC16F876A was chosen for availability and
support provided in the class (examples, etc.)
Software written in C so algorithms could be
adapted
Current Sense Amplifier
MAX4173 chosen for small size, low power
consumption
Testing showed 20 V/V amp with 0.2 ohm
sense resistor allowed measurable current
range of 0.004 to 0.75 amps (approx.)
Care must be taken in choosing resistor/amp
combination to prevent saturation of amplifier
Testing
MAX4173 w/ Rsense = 0.2 ohms
3
2.5
2
Vout (V)
1.5
1
0.5
0
0 0.5 1 1.5 2 2.5 3
Current (A)
Algorithm
Takes voltage and current readings each
cycle and multiplies for power
Compares current power to previous two
power readings
If three progressively lower power readings
have been sensed, compares three voltage
readings to determine if converter needs to
be enabled or disabled
Algorithm
readADC_1(); //voltage
readADC_2(); //current sense amplifier output
tempVOLT = VOLT_2;
VOLT_2 = VOLT_1;
VOLT_3 = tempVOLT;
VOLT_1 = ADC_1; //most recent voltage reading
tempPOWER = POWER_2;
POWER_2 = POWER_1;
POWER_3 = tempPOWER;
mult(); //POWER_1 is updated
Algorithm
if(POWER_2 > POWER_1)
{
if(POWER_3 > POWER_2 // P3 > P2 > P1
{
if(VOLT_1 >= VOLT_2)
{
if(VOLT_2 >= VOLT_3) // V3 < V2 < V1 V rising
{
output_high(PIN_C0); //enable converter
}
}
else // V2 > V1
{
if(VOLT_3 > VOLT_2) //V3 > V2 > V1 V dropping
{
output_low(PIN_C0); //disable converter
}
}
}
Circuit
Testing
Tested indoors with a lamp
Determine voltage of maximum power for the
solar panel under controlled conditions
Monitor voltage on solar panel with tracking
circuit implemented
Tracker would maintain an average voltage
within approx. 10% of peak power voltage
Testing Current Signal
V*I
Voltage
Rechargeable Battery
Uses 3-cell 3.6V NiMH rechargeable battery
with 700-mAH capacity
Features
Light weight
No memory-effect – trickle charging
Environmentally friendly – no toxic chemicals
Constant charging and discharging rates
70% efficiency
Discharge Characteristic
Battery Voltage Discharge Vs. Time
Discharge across various 1.4
load conditions 1.2
Smaller the resistance 1
Voltage (V)
faster the discharge 0.8 V1 w / 2.3Ohms
V2 w / 5 ohms
0.6
At a maximum applicable V3 w / 3.4 ohms
0.4
load of 50 mW plus the
0.2
circuit consumption, the
0
battery can supply up to
0
7
0
0
0
15
25
45
60
75
0
7
10
10
12
13
15
21 hours without Time (mins)
charging up
Bq2012 - Gas Gauge IC
Maintain accurate record of available battery
charge
Monitor voltage across a sense resistor to
determine charge or discharge activity
The bq2012 also estimates self-discharge,
monitors the battery for low-battery voltage
thresholds, and compensates for temperature
and charge/discharge rates
Bq2012 Features
Three ways to communicate with the gas gauge IC
1. EMPTY output
2. LED display
3. DQ serial I/O
communication function
Serial Communication
Transmit bits 03hex to read NACH register
and receive bits for currently stored charge
capacity in the battery
Using two available pins on the controller
Shut-off
- 20% of the charge capacity remains
Serial Communication
Switching Regulator
Buck-boost dc/dc converter
TPS61131PW
operation
Takes the input from the
positive battery terminal,
which is connected to the
dc/dc converter MAX1675
Higher efficiency than the
linear regulator
Fixed 3.3 V output at a
maximum of 1300 mA
output current capacity
Load and Input Variation
• Output voltage (Ch 1) sweep • Output voltage (Ch 1) for variation
across resistance 200 ohms and in input voltage from 2.0 V to 4.5V
1000 ohms at a constant input (Ch 2)
voltage of 3.6V
Regulator Efficiency
Efficiency versus Input Voltage for the Switching Regulator TPS61131PW
86
56 ohms
47 ohms
39 ohms
85.5
Efficiency (%)
85
84.5
84
83.5
3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4
Input Voltage (V)
• Efficiency increases as the input voltage increases as well as the load decreases
• The average efficiency is 85%
PCB
Overall Design
Conclusions
Need to maintain voltage greater than
nominal battery voltage in order to prevent
back current and associated losses
Probably necessary to design a dc/dc
converter instead of using off the shelf design
Circuit still provides necessary power, but
peak power tracking did not appear to offer
significant gains
Experience Gained
Solar panel characteristics
MPPT algorithms
PIC programming
Battery monitoring and charge tracking
Serial communication
Low voltage power systems
PCB design and fabrication
Cost Analysis
Component Price ($) Description
MAX1675 5.06 dc/dc converter
MAX4173 1.88 current sense amplifier
PIC16F876A 4.71 controller
Sumida CR54-220 1.17 inductor
Cooper Bussmann PB-5R0H474 5.13 capacitor
MPF102 0.37 fet
HD01DICT-ND 0.7 diode
13FR200E 1.55 sense resistor
Edmund Scientifics #3039811(x6) 53.7 solar panels
Bq2012 3.81 gas gauge IC to monitor battery
TPS61131PW 4.61 switching Regulator
DR74 2.19 inductor
SNN5542 Battery Pack 15.96 3-cell 3.6 V NiMH battery pack
TOTAL 100.84 Single Unit Cost
Credits
Dwayne Hagerman
Prof. Scott Carney
Joel Jordan
Jonathan Kimball
Sriram Narayanan
Machine and Part Shop Staff
