Energy Neutral Systems

1. Energy Neutral Systems CSE 591 – Green Computing

2. Randomness in Available Energy • Intermittent energy supply • Depends on the environment • Solar cells produce energy only in the day time • Cloudy weather hampers energy production • Statistical variance in available power • The scavenging sources put a cap on the instantaneous power and not on energy • Principal difference from the batteries, which put a cap on the energy.

3. Shift in Design Consideration • Energy efficiency alone is not enough • Optimized power profile is more important • System design under an energy constraint is no longer valid • The energy constraint itself varies over time, given the randomness in the energy source • More relevant design objective – • Perpetual energy neutrality – • Maximize the performance of a system without depleting the battery ever

4. System Model Battery • Scavenging source generates energy • The computing system uses the energy from the scavenging source to perform computational tasks Scavenging Source Scavenging Source • Scavenging source generates energy • The energy is used to charge a battery • The computing system uses the energy from the battery to perform computational tasks Computing System Computing System

5. Scavenging Source Metrics • Design considerations – • Conversion Efficiency – Percentage of energy available from the source that can be used as electrical energy for powering systems • Cost constraints • Form factor constraints • Technology constraints • E.g. Solar cells have a conversion efficiency of 5% • Reliability – Failure to supply power, uncontrolled power supply • E.g. scavenging from ambulation has low reliability

6. I-V characteristics Battery Scavenging Sources • Voltage sources • Limit on the current drawn (Irating) • Current depends on the load across the battery • Current sources • Limit on the voltage supplied • The operating point, both voltage and current depends on the load Highest Instantaneous power Voltage (V) Current (A) Voltage (V) Aim – Operate in the Maximal Power Point (MPP) Harvesting Aware Power Management for Sensor Networks by AmanKansal, Jason Hsu, Mani B Srivastava, and Vijay Raghunathan

7. Techniques for using harvested energy • Adjust the load on the scavenging source. • Use a current switch in the circuit • Essentially a voltage regulator or a transformer. • Allow the scavenging source to operate in the MPP • The voltage or current is then scaled according to the load using the switch • Inefficiencies – • The switching circuit itself draws power • Leakage current of the transformer will cause wastage of power. • Storage of scavenged energy • Use fuel cells or ultracapacitors • Inefficiencies – • Round trip efficiency – Percentage of energy lost in storing in the capacitor or battery and then transferring for use. • Main cause is the leakage of energy • Depth of Discharge (DoD) – The residual battery capacity just before it is recharged • The lifetime of a battery increases exponentially with increasing depth of discharge • System level workload scheduling • Schedule workload to match the available power profile from the sources.

8. Harvesting theory • Energy Neutrality – A system is energy neutral, for a given time t, if the energy stored in the battery after the system operation for time t is greater than or equal to the previous. • Mathematically, if is the battery energy at time t = 0 and is the battery energy after time t. • If then the system is energy neutral • If then the system is perpetually energy neutral

9. Modeling of harvesting system • is the power obtained from a scavenging source • is the efficiency of the storage (fraction of usable electrical energy ) • is the leakage power • is the power consumed by the computing unit • For a given time t, if , then the storage unit stores energy • Else the storage unit gets depleted of the energy • From the stored energy only a fraction can be used to power computing units

10. Energy Neutrality Criteria Amount of energy stored from scavenging sources Amount of energy depleted from storage units due to lack of scavenged energy Amount of energy depleted from storage units due to leakage Previously stored energy Harvesting Aware Power Management for Sensor Networks by AmanKansal, Jason Hsu, Mani B Srivastava, and Vijay Raghunathan

11. Scavenging Source • Definition of scavenging source • (ρ1,σ1,σ2) tuple defines a scavenging source • ρ1is the constant power supply model • σ1 is the upper limit • σ2 is the lower limit • If Ps(t) is the power obtained from the scavenging souce at any time then we have, Average case – σ1 and σ2 are zero. Performance Aware Tasking for Environmentally Powered Sensor Networks by AmanKansal, Dunny Potter and Mani B Srivastava

12. Power source consumer • Definition of variable power source consumer • Consumer (ρ2 ,σ3 ) has a power consumption which satisfies the following equation for any given time T

13. Theorem – Energy Neutral Operation • The sufficient conditions for a system to be energy neutral are – • Here Bmax is the storage capacity while B0 is the initially stored energy

14. System Performance Characterization • Utility – A function of duty cycling • Example - speed tracking sensors

15. Performance maximization objective • Maximize average utility • Constraints – • Energy consumption model • Energy neutrality constraint • Performance constraint

16. Prediction of available power • The optimization assumes that the available power is known before hand • However, scavenged power depends on several environmental factors along with diurnal variations • Exponentially weighted predictive function where is a historically weighted function for the energy available from solar source and x(i) is the current amount of available energy

17. Typical BSN Workload • Ayushman [2] health monitoring application is considered as the workload • Ayushman has three phases of operation – • Sensing Phase – Sensing of physiological values (Plethysmogram signals) from the sensors and storing it in the local memory • Transmission Phase – Send the stored data to the base station in a single burst • Security Phase – Perform network wide key agreement for secure inter-sensor communication using Physiological value based Key Agreement Scheme (PKA) [3]. • The Security phase occurs once in a day • The Sensing phase and Transmission phase alternate forming a sleep cycle • (the processor can sleep during sensing phase while it can be active during the transmission phase) Ayushman Workload Frequency Throttling during security phase Sensing Phase Enables Sleep Scheduling Transmission Phase Sleep Cycle Sensor CPU Utilization Security Phase Time

18. Sustainability Analysis of Atom • Duty Cycling of Atom operation during Ayushman execution • Sleep mode (C6) during Sensing Phase • Power consumption = Psleep for time ts • Active mode during data transmission phase • Power consumption = Pactive + radio power Pradio for data transmission time ttx • Active mode during PKA execution • Power Consumption = Pactive + PKA execution power PPKA for time tPKA • PKA involves transmission of security related information (vault) between two sensors • The Atom processor must be in active state with the radio on. Power Consumption = Pactive + Pradio for time tvault • Total energy consumption for n BSN nodes • x is the number of sleep cycles required in a day Total Energy Sensing Energy Transmission Energy PKA Computation Energy (Pair wise PKA) PKA communication Energy (Pair wise PKA) Total Sleep Cycle Time Pair wise PKA Execution Time

19. Sustainability Analysis Results • Four energy scavenging sources were considered • Body Heat, Ambulation, Respiration and Sun Light • Three energy efficient techniques used • processor level sleep scheduling and communication (radio sleep) scheduling (P-M), • no processor level sleep scheduling but with communication scheduling (NP-M), and • no processor level sleep scheduling or communication scheduling (NP-NM). • Here we consider the average case power consumption of the scavenging sources. • Variance in the power availability can be captured using the theory discussed earlier.

20. Sustainability Results

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