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eShare: A Capacitor-Driven Energy Storage and Sharing Network for

Long-Term Operation(Sensys 2010)Ting Zhu, Yu Gu, Tian He, Zhi-Li ZhangDepartment of Computer Science and Engineering, University of Minnesota, Twin Cities

Presenter: Junction Date: 2010.10.28

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Outline•Motivation•System Overview•Evaluation•Conclusion & Contribution

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Outline•Motivation•System Overview•Evaluation•Conclusion & Contribution

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Motivation• Energy sharing locally consumed

▫ Allow energy to efficiently and quantitatively flow back and forth among multiple energy storage systems

• Application:▫ Greenhouse Application (ClimateMinder’s GrowFlex

Technology)

▫ Wearable Computing Application (UbiComp 2008)

Battery/solar-powered(backup Bettery 6-

8months)

Environmental conditions:

Soil moistureLeafwetness

Ambient temperatureIrrigation/vents control

Harvesting power from 6 body

locations

Locations ?Wrist: 115 ±106

mWArm: 1.01 ±0.46

mW wired

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Batteries v.s. Capacitors• Requirements of energy sharing

▫ Fast▫ Highly efficient▫ Quantitatively controllable

• Limitation of batteries▫ Low charge efficiency (6%)▫ Limited charge current▫ Inaccurate remaining energy prediction

• Capacitors▫ High charge efficiency (90%)▫ Have more than 1 million recharge cycles ( > 10 years)▫ Can be charged very quickly

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Ultra-Capacitors

• Leakage▫ Physical size and remaining energy ↑, The leakage power ↑

3000F capacitor: first 48hrs29% of total energy leaked

away

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

•Evaluation•Conclusion & Contribution

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System Overview

1.Remaining energy inside ultra-capacitors2. Samples the harvesting power

1. calculate the energy

leakage rate2. Forward leakage

info, remaining/harvest

pw

Leakage model & energy supply/demand => control discharge/charge state

Decide the most efficient routes for energy distribution

Control energy exchange between neighboring nodes

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

•Evaluation•Conclusion & Contribution

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Hardware Layer• Single v.s. capacitor array

▫ Slow boot-up time▫ High remaining energy▫ Inflexibility in fine-grained control (A/D converter)

• Requirements▫ Generality▫ Simplicity▫ Stability

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

•Evaluation•Conclusion & Contribution

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Control Layer• Charging & discharging

▫ Minimize leakage -> improve efficiency• Energy Leakage Model

▫

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Charging• Basic Alternative Charging Control

• Adaptive Charging Control▫ Based on the charge current

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Discharging• Serial connected capacitors

▫ different voltage combination -> different remaining energy levels

• The less energy remain, the more energy share

▫ Adaptively discharged: higher leakage power first▫ Until voltage value reaches the calculated min voltage▫ Excluded from discharging

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

Energy Access Protocol Energy Network Protocol

•Evaluation•Conclusion & Contribution

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

Energy Access Protocol Energy Network Protocol

•Evaluation•Conclusion & Contribution

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Energy Access Protocol• Directly connect through power cord

▫ Not through DC/DC converter▫ Consumes large amount of power

• Protocol▫ Receiver-initiated▫ Both receiver and sender can terminate

transmission

monitor monitor

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Outline•Motivation•System Overview

▫Hardware Layer▫Control Layer▫Energy Sharing Layer

Energy Access Protocol Energy Network Protocol

•Evaluation•Conclusion & Contribution

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• Finding the minimum energy loss path▫ Transfer Efficiency (eij)▫ Energy Sharing Efficiency (ESEij)

• Energy optimal sharing among devices

Energy Network Protocol

For node a:E = 100JESEac = 0.9, ESEad = 0.81, ESEab = 0.72

c -> a 80J => 80 * 0.9 = 72, E = 100 – 72 = 28Jd -> a ? =>28/0.81 = 34.6J E = 28 – 28 = 0

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Outline•Motivation•System Overview•Evaluation

▫Evaluation of Efficient Control▫Evaluation of Energy Sharing

•Conclusion & Contribution

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Outline•Motivation•System Overview•Evaluation

▫Evaluation of Efficient Control▫Evaluation of Energy Sharing

•Conclusion & Contribution

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Evaluation of Effective Control• Baseline & metrics

▫ No Efficient Control (NEC)▫ Remaining energy & Voltage

• Implementation▫ MICAz node (TinyOS & NesC)▫ (a) indoor

56 hours

2 Ultra-Capacitors

100F & 400F

NEC / EC

48.7J

Charging control selects the lowest leakage power to store energy -> low

energy leaked away

48.7J = MICAz 1% duty cycle more than 16hrs

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Evaluation of Effective Control• Implementation

▫ (b) Mobile Phone Discharging

▫ (c) Outdoor Energy Harvesting

EC: 19 hrs (17.3% service time of the

NEC)

872.8J (14.4% more)

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Outline•Motivation•System Overview•Evaluation

▫Evaluation of Efficient Control▫Evaluation of Energy Sharing

•Conclusion & Contribution

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Evaluation of Energy Sharing• Evaluation of Energy Access Protocol

▫ One-to-One Many-to-One2.5V 1.6V

1.2V

0.4V

Energy sharing: 1 ~ 3.1(s)

2.37V2.35V1.71V

0.64V

113J => MICAz 1% duty cycle

38hrs

Energy sharing: 1 ~ 2.3(s)

2.378V

2.35V

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Evaluation of Energy Sharing• Evaluation of Energy Network Protocol

▫ oil pipeline monitoring▫ climate monitoring and control in greenhouses

NES (No Energy Sharing) LES (Local Energy Sharing): with direct connected neighbors

(baseline) GES (Global Energy Sharing) Network Lifetime Wasted Energy

Energy leaked away inside the capacitor array Energy consumption of the energy sharing control and

communication Energy loss when energy flows from on device to the other

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Experiments46m

21m

•2 days (48hrs)•Collected energy pattern -> for simulation input•Randomly generated working pattern• Mean duty cycle = 5%

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Performance Analysis• Simulation Results

LES Control: 0.406J

GES Control: 0.7836J

A/D converterNegative >

Positive

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Outline•Motivation•System Overview•Evaluation•Conclusion & Contribution

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Conclusion & Contribution• First Ultra-capacitor based energy router for sharing

energy among embedded sensor devices• By energy sharing the network lifetime is extended

▫ Efficient Control (Charge & Discharge) Using an array of capacitors to minimize leakage based on leakage

model▫ Energy Sharing (Supply & Demand)

Collaboration between data networks and energy networks for efficient energy management

Energy access protocol -> share energy among neighboring devices Energy network protocol -> optimally distribute energy among

network Quantitatively control the amount of energy transferred

• No experiments with real system deployment

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