Solar Power for Indoor Sensor Systems. Presented By: Dan Stieler, PhD President, PowerFilm, Inc. June 27, 2018

Transcription

1 Solar Power for Indoor Sensor Systems Presented By: Dan Stieler, PhD President, PowerFilm, Inc. June 27, 2018

2 What Is Energy Harvesting? Collecting and using ambient energy Light (solar), Motion (piezoelectric), Electrodynamic (regenerative braking), Heat (thermoelectric), and Electromagnetic (radio frequency) Is It Really Free Energy?? Energy harvesting external to system light, electromagnetic, heat Energy harvesting internal to system motion, heat, electrodynamic

3 Why Add Energy Harvesting? Extend system life Lower cost of ownership Maintenance safety Class 1 Hazardous environments Low installation cost Installation location flexibility Improve customer satisfaction Enables large number of devices IoT: Number of Connected Devices Worldwide

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5 Design for Low Power Sleep power consumption IC sleep mode I 2 R losses High impedance Operating voltage Low Storage element Transmit/receive peak power Self discharge Wireless protocol Time to connect Beacon or 2-way communication Limit bandwidth Use transmission latency Increase connection interval Turn off unused I/O

6 Perception, Measurement, and Collection LIGHT

7 Lux measurement Common Low cost Spectral response Based on human eye CIE 2005 Eye perceived intensity Capable of 1,000,000,000:1 Only 1,000:1 dynamic contrast Lux meter spectral response 2

8 Lighting Scenarios Activity Illumination (lux) Public areas with dark surroundings Simple orientation for short visits Working areas where visual tasks are only occasionally performed Warehouses, Homes, Theaters, Archives 150 Easy Office Work, Classes 250 Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories 500 Supermarkets, Mechanical Workshops, Office Landscapes 750 Environment Illumination (Lux) Direct Sunlight 107,527 Overcast 1,000-10,000 Twilight 10 Full Moon 0.1 Overcast Night Activity Illumination (lux) Normal Drawing Work, Detailed Mechanical Workshops, Operation Theaters 1,000 Detailed Drawing Work, Very Detailed Mechanical Works Performance of visual tasks of low contrast and very small size for prolonged periods of time Performance of very prolonged and exacting visual tasks Performance of very special visual tasks of extremely low contrast and small size

9 Light levels will vary significantly based on position of panel within room Light intensity is not linear with distance from light source

10 Changes in light level with height are much smaller and change more slowly next to a wall Maximum intensity when mounted on wall will depend on luminaries' beam angle

11 Available Power Indoors Light intensity Light source type (LED, Fluorescent, Incandescent) Room position (walls, floor, ceiling) Luminaire beam angle Illumination time/cycle

12 What s commercially available? How does it work? SOLAR

13 a-si c-si GaAs CIGS Weight Light Moderate Light to Moderate Light to Moderate Area Moderate Low Very Low Moderate Indoor Performance High Low to Moderate Low to High Very Low Durability Excellent Poor Good Moderate Flexibility/Conformability Excellent Poor Poor to Good Good Thickness Thin Thick Thin to Thick Thin to Thick Cost Moderate Low High Moderate Shade Tolerance Excellent Poor Poor Poor to Moderate Manufacturing Moderate Moderate to Complex Complex Moderate

14 Solar efficiencies quoted under light spectrum of AM1.5 at 1000W/m^2 and 25 C 4 Spectrum of energy efficient light sources limited to visual range

15 Quantum Efficiency Parameter that describes a solar cell s absorption efficiency at each wavelength Cell matching will limit indoor performance of multi-junction cells 5

16 Solar cells are diodes Collected power depends on resistance across cell Current changes proportional to light intensity Voltage depends on a cell s shunt resistance Solar Cell IV Curve

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22 Rated efficiency has no correlation with indoor performance Quantum efficiency and leakage do Light source spectrum has a major impact a-si collects the most power in energy efficient indoor lighting Rated Current Density (ma/cm 2 ) at 1000Lux Technology Efficiency % Fluorescent LED Incandescent CIGS 8.5% Si 18.0% a-si 6.0% Triple Junction GaAs 30.0%

23 ENERGY HARVESTING (EH) ELECTRONICS FOR SOLAR

24 Controllable input impedance to maximize power collection Fixed set point Ratio power point Maximum power point

25 Other Features Offered by EH ICs Integrated battery charging Switcher shutdown for reduced EMI during wireless transmission UV/OV protection Very low power collection ( 80mV, 1 A) Power Good Output Signals Integrated comparator Primary battery backup 2s super cap balancer Dual EH inputs Regulated outputs High efficiency (>90%) Table of commercially available EH ICs is available in Appendix A

26 STORAGE ELEMENT CONSIDERATIONS 6

27 Storage Element Its more than Coulombs In and Coulombs Out Capacity of batteries significantly decrease with increased current draw. Specified capacities usually for low steady current ML mAh at 0.03mA ML mA, 0.1s pulses with 25uA, 2s standby 7 7 Pulsed battery discharges allow battery to relax and recover maintaining capacity Capacitors are impacted less by high current discharges

28 Battery lifetime varies depending on depth of discharge 9 8 Capacity varies from vendor to vendor

29 Storage Element Tips Understanding peak and continuous current is critical For high peak currents on small storage elements capacity can be increased by adding a parallel tank capacitor Vet batteries in actual scenarios with brand of battery you plan to use Plan for a low depth of discharge to increase battery lifetime Consider temperature of operating environment self discharge is temperature dependent Additional information on batteries can be found in Appendix B

30 EXAMPLE SOLAR POWERED BLUETOOTH SENSOR

31 Measures Light intensity using Lux sensor TI OPT3001 Battery voltage using ADC TI CC2650 Temperature using internal microcontroller sensor TI CC2650 Energy Harvesting PowerFilm LL a-si solar panel Texas Instruments BQ25570 EH IC 1800 F electrolytic capacitor Radio Texas Instruments CC2650 wireless MCU running a Bluetooth Low Energy stack

32 Estimating Battery Requirements Measure peak current Measure system average power draw Specify typical illumination per day Specify safety factor for max time with no light Specify battery chemistry 1mA/V, 11mA Peak 2.5ms duration

33 Lab Devices makes an inexpensive ($300) µa current probe for an oscilloscope Active differential probe 1Ω series resistance 200kHz to 20MHz bandwidth Requires oscilloscope 10

34 Estimate or measure power consumption For this example a 1800uF capacitor was used as storage element to estimate power consumption Seconds Voltage (mv) Average Voltage (V) Average Current (ma) Average Power (mw) Daily Power Consumption (mwh) Connection Interval: ms, Sensor Read Interval: 100ms Connection Interval: 765ms, Sensor Read Interval: 10s Using 1800uF capacitor as storage element

35 Choose safety factor (10 days) Calculate capacity required Daily Power Consumption (mwh) Safety Factor (Days) Capacity Required (mwh)

36 Choose battery chemistry (Li-ion self discharge ~0.25% per day) Choose battery capacity (40maH/3.7V Li-ion DTP301120, Capacity rated at 0.2C discharge, ~148Wh)

37 Calculate % battery capacity used in worst case scenario based on the safety factor. Ensure when accessing that capacity the voltage does not drop below system operating voltage Verify that % capacity used will give adequate battery cycle life 50%, 10% to maintain 80% initial capacity) Daily Power Consumption (mwh) Safety Factor (Days) Capacity Required (mwh) % Capacity Used % % % % Output regulator efficiency included in Daily Power Battery Capacity Including Self Discharge is 144mWh

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39 Estimating Solar Requirements Energy harvester efficiency (90%, high solar voltage improves EH IC efficiency) Desired days to recharge from worst case given by safety factor (5 days) Illumination (12 hours/day) Illumination Intensity PowerFilm a-si Solar Module LL , 200Lux, 1000Lux Daily Power Consumption (mwh) Capacity Required (mwh) Days to Fully Recharge Illumination Time (hours/day) Solar Generation Capability (mw) Charger efficiency is 90%

40 Demo Video

41 Conclusions Solar enables self sustaining sensors indoors and out Rated solar efficiency has no correlation with indoor or low light intensity performance a-si excels at producing power indoors EH ICs optimize the amount of power collected Specifying a storage element takes careful consideration and testing The devil is in the details!

42 Questions Do you have systems that you d like to optimize and power via energy harvesting? PowerFilm can help! We would love to hear about your application and help make your vision a reality. Visit us at Sensor Expo Booth 1535 or contact us at or kjohnson@powerfilmsolar.com.

43 References 1. IoT: Number of Connected Devices Worldwide Statista, 2. LEDs and Photosynthesis. Berry Gardener, 9 Feb. 2016, standard%20spectrum%20(AM1-5g).aspx Arthur, Battery Types Used in Solar Lighting. LEDWatcher.com. 1/15/ /20/ Zhang, Yin, "Performance Characteristics of Lithium Coin Cells for Use in Wireless Sensing Systems" (2012). All Theses and Dissertations. Paper M. Jensen. Coin cells and peak current draw. White Paper SWRA Particle. Whitepaper, "Power Management for IoT Devices". (2017). papers/power-management-for-iot-devices?sent=true&submissionguid=5f28f939-95a3-439b-ab68- c1677f6d80da

44 Appendix A

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47 Appendix B Battery Chemistry Cell Voltage Self-Discharge Rate (Room Temperature) Energy Density Charge Efficiency (W*hr/Kg) (%) Cycle Durability Flat Discharge Curve NiCd 26,35,36, % per day Y 5-20% first day, NiMH 27,35,36, % per day Y NiMH - Low Discharge 28,29,30,35,36,37, % per day Y <25% discharge = Alkaline % per day < 50% discharge = N Lithium Ion 32,35,36, % per day N Lithium Iron Phosphate %/day Y Lead Acid % per day N Super Capacitors 39, % per day 5 >99 >1,000,000 N