1. eShare: A Capacitor-Driven Energy Storage and Sharing Network for Long-Term Operation(Sensys 2010) Ting Zhu, Yu Gu, Tian He, Zhi-Li Zhang Department of Computer Science and Engineering, University of Minnesota, Twin Cities Presenter: Junction Date: 2010.10.28 2010/10/28 1

2. Outline Motivation System Overview Evaluation Conclusion & Contribution 2010/10/28 2

3. Outline Motivation System Overview Evaluation Conclusion & Contribution 2010/10/28 3

4. 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 moisture Leafwetness Ambient temperature Irrigation/vents control Harvesting power from 6 body locations Locations ? Wrist: 115 ±106 mW Arm: 1.01 ±0.46 mW wired 2010/10/28 4

5. 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 2010/10/28 5

6. Ultra-Capacitors Leakage ▫Physical size and remaining energy ↑, The leakage power ↑ 3000F capacitor: first 48hrs 29% of total energy leaked away 2010/10/28 6

7. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer Evaluation Conclusion & Contribution 2010/10/28 7

8. System Overview 1.Remaining energy inside ultra-capacitors 2. Samples the harvesting power 1. calculate the energy leakage rate 2. 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 2010/10/28 8

9. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer Evaluation Conclusion & Contribution 2010/10/28 9

10. 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 2010/10/28 10

11. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer Evaluation Conclusion & Contribution 2010/10/28 11

12. Control Layer Charging & discharging ▫Minimize leakage -> improve efficiency Energy Leakage Model ▫ 2010/10/28 12

13. Charging Basic Alternative Charging Control Adaptive Charging Control ▫Based on the charge current 2010/10/28 13

14. 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 2010/10/28 14

15. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer  Energy Access Protocol  Energy Network Protocol Evaluation Conclusion & Contribution 2010/10/28 15

16. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer  Energy Access Protocol  Energy Network Protocol Evaluation Conclusion & Contribution 2010/10/28 16

17. 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 monitormonitor 2010/10/28 17

18. Outline Motivation System Overview ▫Hardware Layer ▫Control Layer ▫Energy Sharing Layer  Energy Access Protocol  Energy Network Protocol Evaluation Conclusion & Contribution 2010/10/28 18

19. Finding the minimum energy loss path ▫Transfer Efficiency (e ij ) ▫Energy Sharing Efficiency (ESE ij ) Energy optimal sharing among devices Energy Network Protocol For node a: E = 100J ESEac = 0.9, ESEad = 0.81, ESEab = 0.72 c -> a 80J => 80 * 0.9 = 72, E = 100 – 72 = 28J d -> a ? => 28/0.81 = 34.6J E = 28 – 28 = 0 2010/10/28 19

20. Outline Motivation System Overview Evaluation ▫Evaluation of Efficient Control ▫Evaluation of Energy Sharing Conclusion & Contribution 2010/10/28 20

21. Outline Motivation System Overview Evaluation ▫Evaluation of Efficient Control ▫Evaluation of Energy Sharing Conclusion & Contribution 2010/10/28 21

22. 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 2010/10/28 22

23. 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) 2010/10/28 23

24. Outline Motivation System Overview Evaluation ▫Evaluation of Efficient Control ▫Evaluation of Energy Sharing Conclusion & Contribution 2010/10/28 24

25. Evaluation of Energy Sharing Evaluation of Energy Access Protocol ▫One-to-One Many-to-One 2.5V1.6V 1.2V 0.4V Energy sharing: 1 ~ 3.1(s) 2.37V 2.35V 1.71V 0.64V 113J => MICAz 1% duty cycle 38hrs Energy sharing: 1 ~ 2.3(s) 2.378V 2.35V 2010/10/28 25

26. 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 2010/10/28 26

27. Experiments 46m 21m 2 days (48hrs) Collected energy pattern -> for simulation input Randomly generated working pattern Mean duty cycle = 5% 2010/10/28 27

28. Performance Analysis Simulation Results LES Control: 0.406J GES Control: 0.7836J A/D converter Negative > Positive 2010/10/28 28

29. Outline Motivation System Overview Evaluation Conclusion & Contribution 2010/10/28 29

30. 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 2010/10/28 30