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Design and Analysis of Micro-Solar Power Systems for Wireless Sensor Networks. Jaein Jeong with Xiaofan Jiang and David Culler Computer Science, UC Berkeley INSS08, June 19 th , 2008. Great Duck Island [SMP+04] . Golden Gate Bridge [Kim07]. Typical Wireless Sensornet Application.

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design and analysis of micro solar power systems for wireless sensor networks

Design and Analysis of Micro-Solar Power Systems for Wireless Sensor Networks

Jaein Jeongwith Xiaofan Jiang and David CullerComputer Science, UC Berkeley

INSS08, June 19th, 2008

typical wireless sensornet application

Great Duck Island [SMP+04]

Golden Gate Bridge [Kim07]

Typical Wireless Sensornet Application
  • Typical sensornet application runs on battery.

Limited Lifetime with

Battery-Powered Node!

previous works on micro solar power systems

MPP Tracking

SimpleDesign

Multi-Level Storage

[Everlast, 2006]

[Prometheus, 2005]

[Heliomote, 2005]

[Ambimax, 2006]

[Trio, 2006]

[Fleck, 2006]

Previous Works on Micro-Solar Power Systems
  • Solar-energy harvesting can be used as alternative to battery.
  • Several systems exist with a unique set of requirements.
  • But, they represent only particular points in the design space.Little analysis on performance in entire range of situations.
contributions

Heliomote [Raghunathan et al 05]

Trio [Dutta et al 06]

Contributions
  • Present a model for micro-solar power systems andDevelop a taxonomy of micro-solar design space.
  • Empirical analysis of two well-studied designs.
  • A design guideline for micro-solar systems.
organization
Organization
  • System Architecture for Micro-Solar System
  • Design Considerations for Four Components.
    • External Environment
    • Solar Collector
    • Energy Storage
    • Load
  • Concrete Examples: Trio and Heliomote
  • Conclusion
system architecture
System Architecture

External Environment

Sun

Storage Monitoring (optional)

Esolar_in

Energy Storage

Load

Solar Collector

EL1

ELn

Esol =

Econs

Solar Panel

Estorage_in

Level-1 storage

Level-n storage

Mote

Regulating Circuit

Charging Controllerand Switch

Software Charging Control (optional)

architecture external environment

N

Θ

Vs

Solar Panel

Architecture –External Environment
  • Astronomical Model
    • Estimate solar radiation using angle Θ.
    • Solar panel output is given as Psol = cos Θ * Effpanel * A
  • Statistical Model
    • Refines the astronomical model by using weather variation statistics.
  • Effect of Obstructions
architecture solar collector
Architecture –Solar Collector
  • Converts solar energy to electricity.
  • Solar panel I-V curve describes possible operating point.
  • I-V curve moves depending on solar radiation.
  • Operating point dictated by output impedance.
architecture energy storage
Architecture –Energy Storage
  • Buffers energy and delivers in a predictable fashion.
  • Considerations:
    • System Requirements: Lifetime, capacity, current draw, size and weight.
    • Trade-offs between efficient energy transfer and charging logic.
  • Storage Elements
    • NiMH, Li+ for high energy density and supercap for long lifetime.
  • Configurations of energy storage :
    • Single element or multiple-level of storage elements
architecture load
Architecture –Load
  • Mote is end consumer of energy in micro-solar system.
  • We abstract its behavior as load.
    • Radio, sensing and computation are main causes.
    • Duty-cycling is used to save energy consumption.
    • When the duty-cycle rate is R, average load is given as :

Iestimate = R * Iactive + (1 – R) * Isleep

comparative study trio and heliomote
Comparative Study - Trio and Heliomote

Trio Block Diagram

Sun

Storage Monitoring using uC ADC(CapV, BattV, Status)

Esolar_in

Energy Storage

Load

RU6730Solar Cell

Esol =

Switch

Ecap

Ebat

Estorage_in

Econs

Telosrev.BMote

Zener (SMAZ5V6)

and Schottky (LLSD103A)Diodes

Supercap(L1)

Li+(L2)

DC/DC

Solar Collector

Software Charging Control(Charging Switch, Thresholds)

Heliomote Block Diagram

Sun

Esolar_in

Energy Storage

Load

SolarWorld4-4.0-100Solar Cell

HW Battery Monitor

Econs

Esol =

Mica2Mote

2x AA NiMH

DC/DC

Ebat

Estorage_in

Diode

HW Charge Controllerand Switch

Solar Collector

comparative study 1 solar collector operation
Comparative Study(1) Solar-Collector Operation
  • Evaluate solar-collector matching by comparing Eop with Empp
    • Eop : daily solar radiation from the solar collector.
    • Empp : daily solar radiation that can be achieved with MPP.
  • Experiment (a) measures operating point (Iop, Vop)
  • Experiment (b) measures I-V curve at that moment.
comparative study 1 solar collector operation1
Comparative Study(1) Solar-Collector Operation
  • Difference between Eop and EmaxP :
    • Trio: 4.8% of MPP, Heliomote: 22.0% of MPP
    • For Trio, SW charging allows setting Vop close to MPP after the measurement.
    • For Heliomote, Vop is set by battery voltage and protection circuit.This makes it hard to change Vop once the system is designed.

Trio

Heliomote

comparative study 1 solar collector operation2
Comparative Study(1) Solar-Collector Operation
  • Useful range of the solar panel in a particular system is very narrow.
  • Power tracking circuits or algorithms are only meaningful within this small range.
comparative study 2 energy flow and energy efficiency
Comparative Study(2) Energy Flow and Energy Efficiency
  • System efficiency for daily operation
    • Effsys = (Ebat + Ecap + Econs) / Esol
  • Daily cycle of a system:
    • Charge, Discharge, Saturation
  • Efficiency at different daily phase
    • Effbat−dis = Econs / Ebat−dis
    • Effcap−dis = Econs / Ecap−dis
    • Effchg = (Ebat−chg + Ecap−chg + Econs) / Esol

Discharge(supercap)

Discharge(battery)

Discharge(battery)

Charge

comparative study 2 energy flow and energy efficiency1
Comparative Study(2) Energy Flow and Energy Efficiency
  • System Energy Efficiency
    • Trio node : 19.5% to 33.4%
    • Heliomote : 6.9% to 14.6%
  • What makes this difference?
comparative study 2 energy flow and energy efficiency2
Comparative Study(2) Energy Flow and Energy Efficiency
  • Charging-discharging efficiency of Heliomote is as good as that of Trio, but its system efficiency is much smaller.
  • Much of solar energy is wasted during saturation phase.
  • Efficiency of Heliomote would be 31.9% to 41.9% without saturation.
comparative study 2 energy flow and energy efficiency3
Comparative Study(2) Energy Flow and Energy Efficiency
  • With Trio, supercap discharge period exists.
    • System runs on the supercap not on battery.
    • Effective battery lifetime increases by Tcap-dis / (Tbat-dis + Tcap-dis)
conclusion
Conclusion
  • Presented a system model for micro-solar power system.
  • Analyzed two well-studied platforms, Trio and Heliomote.
  • Insights from the analysis:
    • Solar-collector:
      • Useful range of solar-panel voltage is narrow.
      • Can closely match operating point to MPPby setting operating point to this range without using MPPT.
    • Energy storage:
      • Multi-level storage improves system energy efficiency and lifetime.