Solar Power Source for Sensors

1. Solar Power Source for Sensors Group 10 Steven Portscheller Pavlina Akritas & Gunjan Tejani ECE 445 Senior Design April 28, 2006

2. 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

3. 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

4. Overall Design

5. 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

6. 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“.

7. Testing • Solar panel tests to determine peak power voltage constant k

8. Testing cont. • BUT, the current is too low, therefore the resulting power was low.

9. 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 , used 6 PV’s in series. • is never reached so model would work.

10. Temperature Test Results • When cooled increases (from 2.870 V to 3.035 V) • When heated decreases (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

11. Typical Test Results • Temperature results: • When cooled increases (from 2.870 V to 3.035 V) • When heated decreases (from 2.837 V to 2.685 V) • V versus I, variation throughout a day

12. Voc throughout a day

13. 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.

14. 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

15. 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

16. 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)

17. Testing • Testing indicated an efficiency of approx. 85%

18. 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

19. 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

20. Testing

21. 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

22. 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

23. 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 } } }

24. Circuit

25. 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

26. Testing Current Signal V*I Voltage

27. 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

28. Discharge Characteristic • Discharge across various load conditions • Smaller the resistance faster the discharge • At a maximum applicable load of 50 mW plus the circuit consumption, the battery can supply up to 21 hours without charging up

29. 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

30. Bq2012 Features • Three ways to communicate with the gas gauge IC • EMPTY output • LED display • DQ serial I/O communication function

31. 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

32. Serial Communication

33. Switching Regulator • Buck-boost dc/dc converter 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 TPS61131PW

34. Load and Input Variation • Output voltage (Ch 1) sweep across resistance 200 ohms and 1000 ohms at a constant input voltage of 3.6V • Output voltage (Ch 1) for variation in input voltage from 2.0 V to 4.5V (Ch 2)

35. Regulator Efficiency • Efficiency increases as the input voltage increases as well as the load decreases • The average efficiency is 85%

36. PCB

37. Overall Design

38. 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

39. Experience Gained • Solar panel characteristics • MPPT algorithms • PIC programming • Battery monitoring and charge tracking • Serial communication • Low voltage power systems • PCB design and fabrication

40. Cost Analysis

41. Credits • Dwayne Hagerman • Prof. Scott Carney • Joel Jordan • Jonathan Kimball • Sriram Narayanan • Machine and Part Shop Staff