Harvestingaware Power Management for Sensor Networks

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Harvesting-aware Power Management forSensor
Networks
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Energy Harvesting in Sensor Networks

• Energy neutrality a holy grail for sensor
  networks used in long-term monitoring
  applications
• Minimize logistical and access costs associated
  with replacement of batteries
• Wireless sensor nodes with energy harvesting
  capabilities

Trio/Prometheus(Solar, Berkeley)
Piezoelectric Windmill(Wind, UT Arlington)
Commercial Platforms(Solar/Mechanical, EnOcean)
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This Talk

• Platform design considerations
• Experience in designing and deploying
  HelioMote,a solar-powered wireless sensor node
  platform
• Power management techniques
• Harvesting-aware energy management of sensor
  nodes and sensor networks

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HelioMote A Solar Energy Harvesting Wireless
Sensing Platform
Solar Cells
Overcharge Protection
NiMH Batteries
Monitor
Undercharge Protection
DC Step-Up Converter
5
Design Challenges
What energy modality? Single or multiple?
Harvesting Circuit
Energy Storage
Energy Transducer
Energy Harvesting Storage Manager
CPU Radio Sensors Actuators
Sensor Node
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Environmental Energy Sources
Highest power density
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Design Challenges
How to maximize energy extraction?
Harvesting Circuit
Energy Storage
Energy Transducer
Energy Harvesting Storage Manager
CPU Radio Sensors Actuators
Sensor Node
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Harvesting Circuit Design

• Solar panels behave very differently from
  batteries
• Voltage-limited current source with Maximal Power
  Point (MPP)
• Commercial MPP ICs too power hungry
• digitally controlled switching regulators that
  isolate the load and present desired impedance to
  the panel
• HelioMote opts for low-overhead near-MPP
  operation by careful choice of panel and
  secondary battery
• Clamps panel to a battery forcing operation at a
  battery-dictated voltage

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Design Challenges
Is energy buffer needed? Capacitor or
battery? What battery chemistry?
Harvesting Circuit
Energy Storage
Energy Transducer
Energy Harvesting Storage Manager
CPU Radio Sensors Actuators
Sensor Node
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Energy Storage Technologies
Rechargeable Battery
Ultracapacitor
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Choice is a Function of Duty Cycle
(b) Switched better at low-to-moderate duty
cycles with near-neutralambient energy
availability
Energy Consumer (Application)
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Design Challenges
Harvesting Circuit
Energy Storage
Energy Transducer
Energy Harvesting Storage Manager
How to route energy? Analog or digital or s/w?
CPU Radio Sensors Actuators
Sensor Node
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Energy Storage Management
Independent Load
Micropower Reference
AC
Switch
AC
Micropower Reference
Digital
Analog

• Active all the time
• Comparators 3-5uA
• References 1-2uA

• Sleep energy is wasted
• CPU (sleep) 5-50uA
• Input protection 5-20uAActive energy is
  huge
• ADC 200-400uA, CPU 10 mA

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Summary of HelioMote Design Choices

• Battery 2 AA NiMH (2400 mAH)
• Management Autonomous, Analog
• Solar Panel Autonomous, optimal power point
  operation
• 225 mW effective at peak sun
• Data Collection High-accuracy charge
  accumulation, temperature, run-time,and voltage
• Power Characteristics
• Voltage 2.91V regulated
• Consumption 20 mA (active), 0.09 mA (sleep)
• Efficiency 80 (active), 50 (sleep)
• Roundtrip battery efficiency 66
• Self-dischagre 1 per day

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HelioMote in Real-life Deployments
Battery Voltage vs. Time
Current accumulator vs. Time
Snapshot from a 3-month deployment in LA

• Many academic and industrial users across several
  countries
• Open-source hardware and software, as well as
  commercial ruggedized version

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How long will HelioMote last?

• NASA surface meteorology and solar energy data
  forLos Angeles (34? N, 118? W) for December
• Average daily insolation (horizontal) 2.60 kWH /
  m2
• Worst case NO-SUN days over 14 day period is 4.99
  days
• Solar panel provides 585mWH (2106J) per day
• Panel directly powers Heliomote for 2 hours a day
• Energy is partially drawn from battery the rest
  of the time
• Two scenarios analyzed
• Node receives unobstructed sunlight throughout
  day
• Node is in shade for 50 of the time
• Perpetual operation feasible?

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Results of Analysis HelioMote inLA Winter
Lifetime min (time to first outage, battery
degradation to 80)

• Even with obstructions, sustained operation at 7
  duty cycle is feasible (18 without obstructions)
• Experimental numbers show sustained operation at
  60 duty cycle in LA summer and 20 during LA
  winter
• Energy supply is 3X higher in Summer (7.25 kWH/m2)

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Realistic Notion of Perpetuity

• Component failures and degradation
• Battery 5-20 years
• Ultracapacitor 2-20 years
• Solar panel 2 10 years
• Thin-film 2-10 years
• Crystalline 20 years
• http//www.boatus.com/boattech/SolarPanels.htm
• Environmental issues
• Dust and debris accumulates on surface and blocks
  light (forcing premature servicing, so just
  change the battery)
• Seasonal changes affect light availability at a
  given point
• Vegetation growth over time
• So, realistically, lifetime beyond 10-20 yearsis
  wishful!

• Debris and Vegetation greatly reduce solar panel
  efficiency

• Solar panel shows sign of rust after 2 months of
  deployment

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Design Challenges
Harvesting Circuit
Energy Storage
Energy Transducer
Energy Harvesting Storage Manager
CPU Radio Sensors Actuators
How to schedule nodeoperations?
Sensor Node
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Management of Energy Harvesting

• Variation in harvesting opportunities
• E.g. harvested energy is a function of node
  location,time-of-day, aging, duration of energy
  storage etc.
• How to extract maximum performance?
• How to achieve energy neutral operation?

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Isnt Residual Battery EnergyAwareness Enough?
Node A
Eb
Path 1
Destination
Source
Es per day, all before 12N
Node B
Path 2
Es per day, all after 12N
Eb

• Scenario
• Routing costs Er per hour
• One hour of routing before 12N, and one hour
  after 12N
• Roundtrip battery efficiency ?

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Residual Battery at 12N
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Residual Battery at End-of-day
Harvesting-aware Routing
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Harvesting-aware Power Management

• Goal is not power minimization but energy
  neutrality
• Indefinitely long lifetime, limited only by h/w
  longevity
• Subject to performance constraints and
  optimization
• Unknown spatiotemporal profile of harvested
  energy
• At a node adapt temporal profile of workload
• In a n/w adapt spatial profile of workload
  (across nodes)

Learn AmbientEnergy Characteristics
Resource Scheduling
Predict Future Energy Opportunity
Learn Consumption Statistics
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Understanding Energy NeutralityA Harvesting
Theory

• Condition for energy neutrality with a battery
  with roundtrip efficiency ? and leakage ?leak is
• Modeling bursty energy source Ps(t) and consumer
  Pc(t)
• Sufficient conditions for energy neutrality

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At a NodeHarvesting-aware Duty Cycling

• Duty cycling between active and low-power states
  for power scaling
• Approach
• System utility function as a function of D
• Time slots ?T with duty cycle calculated for a
  window of Nw slots
• ?TxNw a natural energy neutral period such as
  1 day
• At start of window predict harvested energy level
  for next ?TxNw slots using history and external
  weather predictions
• Calculate D for Nw slots for max U subject to
  energy neutrality
• Revise duty cycle allocations based on actual
  observed Ps(t)

Application Utility vs. Duty Cycle
Stored vs. Direct Solar Energy Usage
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Practical Dynamic Duty Cycle Adaptation
100
Optimal
90
Adaptive

• Optimal
• Oracle, LP solution
• Naive
• Constant over a day based on predicted total
  energy
• Dynamic
• Adaptive control based on error and duty cycle
  limits

Simple
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Solar Energy Utilization()
70
60
50
40
0.4
0.5
0.6
0.7
0.8
0.9
1
Battery roundtrip efficiency (?)
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Across a NetworkHarvesting-aware Routing
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Harvesting-aware Routing Performance
morning
Afternoon
Battery Aware
Harvesting Aware
Simulation using light traces from James Reserve
Energy snapshots
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Summary

• Energy harvesting emerging as a viable technology
  for sensor network deployments
• Experience with first generation of platforms
  though significant platform issues remain
• Efficiency, aging biofouling, multimodal
  harvesting
• Challenges in providing performance and lifetime
  assurance under highly-variable ambient energy
  availability
• Harvesting theory for fundamental insights
• Practical node and network level methods
• For more info, visit http//nesl.ee.ucla.edu/proje
  cts/heliomote
• Acknowledgements
• Collaborators Jonathan Friedman, Sadaf Zahedi
• Research support CENS, DARPA, NSF, ONR

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Backup Slides
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Impact on Solar Panel Efficiency
Capacitor Voltage
Radio Operation Threshold
Normalized Wasted Energy
Capacitor induced voltage clamping lasting for 18
minutes leads to 36 waste of solar panel energy
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The Bottom Line
Environmental Energy Availability (J/Day)
Application Duty Cycle

• 90,720,000 delta energy points analyzed