Design and Analysis of Micro-Solar Power Systems for Wireless by lee92256

VIEWS: 8 PAGES: 19

									    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 19th, 2008
Typical Wireless Sensornet Application

• Typical sensornet application runs on battery.

 Great Duck Island [SMP+04]          Golden Gate Bridge [Kim07]




                        Limited Lifetime with
                       Battery-Powered Node!


                    GDI                                  GGB
Sampling Rate       8.33 x 10-4 Hz   Sampling Rate       1 KHz
TX Rate             0.03 B/s         TX Rate             441 B/s
Power Consumption   1.6 mW           Power Consumption   358.2 mW – 672.3 mW
Battery Capacity    860mAh           Battery Capacity    4 x 18000mAh at 6V
Lifetime            63 days          Lifetime            35 days               2
                   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.
             Simple             Multi-Level           MPP
             Design              Storage            Tracking




      [Heliomote, 2005]   [Prometheus, 2005]   [Everlast, 2006]




        [Fleck, 2006]         [Trio, 2006]     [Ambimax, 2006]      3
                       Contributions

• Present a model for micro-solar power systems and
  Develop a taxonomy of micro-solar design space.
• Empirical analysis of two well-studied designs.
• A design guideline for micro-solar systems.




  Heliomote [Raghunathan et al 05]   Trio [Dutta et al 06]

                                                             4
                       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



                                               5
                      System Architecture


                                  External Environment
            Sun                                                  Storage Monitoring
                                                                          (optional)
Esolar_in


   Solar                          Energy Storage                                Load
   Collector        Esol =          EL1                         ELn
                    Estorage_in                                         Econs
      Solar Panel                   Level-1              Level-n
                                    storage              storage
      Regulating                          Charging Controller                      Mote
        Circuit                              and Switch


                                                   Software Charging Control (optional)



                                                                                          6
                                                                                               External Environment
                                                                         Sun                                                  Storage Monitoring
                                                                                                                                       (optional)
                                                             Esolar_in



       Architecture –                                           Solar
                                                                Collector        Esol =
                                                                                 Estorage_in
                                                                                               Energy Storage
                                                                                                 EL1                         ELn
                                                                                                                                     Econs
                                                                                                                                             Load




   External Environment
                                                                    Solar Cell                   Level-1              Level-n
                                                                                                 storage              storage
                                                                   Regulating                          Charging Controller                      Mote
                                                                     Circuit                              and Switch


                                                                                                                Software Charging Control (optional)




• Astronomical Model                                      • Effect of Obstructions
   – Estimate solar radiation using angle Θ.
   – Solar panel output is                  N
     given as                           Θ
                                   Vs
     Psol = cos Θ * Effpanel * A
                                            Solar Panel

• Statistical Model
   – Refines the astronomical model
     by using weather variation statistics.




                                                                                                                                                       7
                                                               External Environment
                                         Sun                                                  Storage Monitoring
                                                                                                       (optional)
                             Esolar_in



        Architecture –          Solar
                                Collector        Esol =
                                                 Estorage_in
                                                               Energy Storage
                                                                 EL1                         ELn
                                                                                                     Econs
                                                                                                             Load




        Solar Collector
                                    Solar Cell                   Level-1              Level-n
                                                                 storage              storage
                                   Regulating                          Charging Controller                      Mote
                                     Circuit                              and Switch


                                                                                Software Charging Control (optional)




• 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.


                                                                                                                       8
                                                                                External Environment
                                                          Sun                                                  Storage Monitoring
                                                                                                                        (optional)
                                              Esolar_in



      Architecture –                             Solar
                                                 Collector        Esol =
                                                                  Estorage_in
                                                                                Energy Storage
                                                                                  EL1                         ELn
                                                                                                                      Econs
                                                                                                                              Load




     Energy Storage
                                                     Solar Cell                   Level-1              Level-n
                                                                                  storage              storage
                                                    Regulating                          Charging Controller                      Mote
                                                      Circuit                              and Switch


                                                                                                 Software Charging Control (optional)




• 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




                                                                                                                                        9
                                                                                        External Environment
                                                                  Sun                                                  Storage Monitoring
                                                                                                                                (optional)
                                                      Esolar_in



     Architecture –                                      Solar
                                                         Collector        Esol =
                                                                          Estorage_in
                                                                                        Energy Storage
                                                                                          EL1                         ELn
                                                                                                                              Econs
                                                                                                                                      Load




         Load
                                                             Solar Cell                   Level-1              Level-n
                                                                                          storage              storage
                                                            Regulating                          Charging Controller                      Mote
                                                              Circuit                              and Switch


                                                                                                         Software Charging Control (optional)




• 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




                                                                                                                                                10
Comparative Study - Trio and Heliomote
                                          Trio Block Diagram
                Sun
                                                                Storage Monitoring using uC ADC
    Esolar_in                                                         (CapV, BattV, Status)


                                          Energy Storage                                     Load
          RU6730
          Solar Cell
                            Esol =        Ecap         Switch            Ebat
       Zener (SMAZ5V6)
                            Estorage_in                                            Econs          Telos
             and                          Supercap                    Li+                         rev.B
      Schottky (LLSD103A)                   (L1)                     (L2)                         Mote
            Diodes
                                                       DC/DC
          Solar Collector

                                                                    Software Charging Control
                                                                  (Charging Switch, Thresholds)


                                     Heliomote Block Diagram
                Sun
    Esolar_in

                                            Energy Storage                                   Load
         SolarWorld
                                                  HW Battery Monitor
         4-4.0-100
         Solar Cell                                                                Econs
                            Esol =               2x AA NiMH     DC/DC                             Mica2
                                          Ebat
                            Estorage_in                                                           Mote
            Diode                                HW Charge Controller
          Solar Collector                            and Switch

                                                                                                          11
                   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.




                                                                    12
                    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




                                                                                13
                 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.




                                                                           14
                 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
                                                     Discharge                Discharge
                                                                 Charge Discharge
   – Effchg = (Ebat−chg + Ecap−chg + Econs) / Esol   (battery)                (battery)
                                                                        (supercap)




                                                                                    15
                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?




                                               16
               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.                                                  17
             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)




                                                                                18
                             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 MPP
         by setting operating point to this range without using MPPT.
   – Energy storage:
       • Multi-level storage improves system energy efficiency and lifetime.




                                                                               19

								
To top