7/9/2007 AIIT Summer Course - D# 1 Wireless Embedded Systems and Networking Foundations of IP-based Ubiquitous Sensor Networks Micro-Power Systems David E. Culler University of California, Berkeley Arch Rock Corp. July 13, 2007

7/9/2007AIIT Summer Course - D#2 Micro-Power System Architecture Evaluation Metrics –Eff solar = P on / P maxP –Eff system = (E L1 + … + E Ln + E cons ) / E sol

7/9/2007AIIT Summer Course - D#3 An Example Solar energy scavenging system for Telos –Super capacitors buffer energy –Lithium rechargeable battery as a backup –Uses MCU to manage charge cycles to extend system lifetime –Manage limited recharges –Simple, carefully developed design Redesigned for TRIO deployment –Boosting and current limiting Developed reactive power management software architecture Demonstrated in REALITY Duty CycleLight RequiredSystem Lifetime 1%5 hrs / 1 mo43 years 10%5 hrs / 4 days4 years 100%10 hrs / 1 day1 year Prometheus Design estimates Perpetual Environmentally Powered Sensor NetworksPerpetual Environmentally Powered Sensor Networks, Jiang, Polastre, Culler, IPSN/SPOTS, 2005

7/9/2007AIIT Summer Course - D#4 Facts E = P * T

7/9/2007AIIT Summer Course - D#5 Energy Storage

7/9/2007AIIT Summer Course - D#6 Energy and Power Density

7/9/2007AIIT Summer Course - D#7 Battery Chemistry

7/9/2007AIIT Summer Course - D#8 Energy Stroage Requirements: –Lifetime, Capacity, Current draw, Size/Weight Types of storage: –NiMH: capacity and cost –Li+: energy density and capacity –Supercap: lifetime Storage configuration: –Combination of battery and supercap provides good lifetime as well as capacity. Charging mechanisms: –HW vs. SW, Complexity vs. Efficiency

7/9/2007AIIT Summer Course - D#9 The Load

7/9/2007AIIT Summer Course - D#10 Load (Sensor Node): Estimating Node Consumption Energy consumption with radio comm: –I est = R*I awake + (1-R) * I sleep

7/9/2007AIIT Summer Course - D#11 The Ambient Source Solar Vibration Movement Flow Heat transfer

7/9/2007AIIT Summer Course - D#12 External Environment: Estimating Solar Radiation Statistical Model Mathematical Model

7/9/2007AIIT Summer Course - D#13 Solar Collector: Solar-cell Characteristics Solar-cell I-V curve Regulator

7/9/2007AIIT Summer Course - D#14 Charging to Energy Storage Element Supercap for primary, lithium-ion for secondary. –Reduces battery charging frequency. Software-controlled battery charging. –Unlike other batteries, Li+ battery should be charged only when there is sufficient charge in the supercap. –Pros: Simple hardware: micro-controller, DC-DC converter, analog switch. –Cons: Requires correct software for charging control. Energy Storage Controller Energy Storage Element Solar Cell Sunlight Super- capacitor Power Selection SW Regulating Circuit Li-ion Battery DC- Converter Wireless Sensor Node (Micro- controller & Radio) Solar Cell Circuit Solar Energy Harvesting Unit VCC Set Charge Set Power Charging Characteristic Time Voltage (V) Current (A) Cell Voltage (V) Charge Current (A)

7/9/2007AIIT Summer Course - D#15 Consideration of other types of storage element (1) Trio [DHJ+06] (2) Heliomote [RKH+05] (3) Everlast [SSC05] Storage One Li+ battery with one 22F cap Two AA NiMH batteriesOne 100F capacitor Capacity E bat = 2625mWhE bat = 4320mWhE cap = 86.8mWh B day 14.5 days at 10% 6.7 days at 25% 23.9 days at 10% 11.0 days at 25% 0.48 days at 10% 0.22 days at 25% Charging control Software, pulse charging Hardware, trickle charging overcast days? YES NO Battery is needed during overcast days. –Supercap-only method doesn’t have sufficient capacity. Comparison of charging efficiency is not available yet.

7/9/2007AIIT Summer Course - D#16 Comparative Study: Solar-Collector Operation Compare P on with P maxP a.solar-cell operating point b.maximum possible value Trio – P on – P maxP = 4.83mW (5.3%) Heliomote – P on – P maxP = mW (-23.2%)

7/9/2007AIIT Summer Course - D#17 Comparative Study: Energy flow and efficiency Compare mote consumption (E cons ) and stored energy (E bat and E cap ) with solar energy income (E sol ). Trio: up to 33.4%, Heliomote: up to 14.6%

7/9/2007AIIT Summer Course - D#18 Solar-Collector Operation: Trio

7/9/2007AIIT Summer Course - D#19 Solar-Collector Operation: Heliomote

7/9/2007AIIT Summer Course - D#20 Energy flow and efficiency (Heliomote) - Energy loss due to regulator Solar energy income: 08:00 to 17:00. Clipped after 12:00. Two-third loss in daily energy income.

7/9/2007AIIT Summer Course - D#21 Related Work on Solar Powered Sensor Network Trio [DHJ+06] –Real deployment of large sensor nodes. –Multi-hop routing. –Operate only for several hours with full radio cycle. Other Previous Works –RF transmit beacon [ROC+03], Prometheus [JPC05] Heliomote [RKH+05], ZebraNet [ZSLM04] RF TX beacon Prometheus Heliomote ZebraNetTrio Trio [DHJ+06] RF TX beacon [ROC+03] Prometheus [JPC05] Heliomote [RKH+05] ZebraNet [ZSLM04] Multi-hopYesNo Sustainable Operation No No (No battery) Yes Duty-cycling On-off duty-cycle GPS assisted time-sync Deployment~ 500Lab bench ~ 10

7/9/2007AIIT Summer Course - D#22 Energy Management Architecture