Design, Modeling, and Capacity Planning for Micro-Solar Power Sensor Networks Jay Taneja, Jaein Jeong, and David Culler IPSN/SPOTS 2008 – 4/23/2008

Applications and Science Zebras in the savanna People tracking Bird Nest Monitoring However, publications report that scientific results have been mostly underwhelming. Why?

What Does It Mean To Be Systematic? Need to develop models for planning of every subsystem Must formalize application-driven constraints Real science demands real engineering If one thing fails – everything can fail

HydroWatch: Water in Motion

The Challenges Capacity Planning for Microsolar Mechanical Design - Weatherproof with Correctly Exposed Sensors Incorporating off-the-shelf and custom-built pieces

Storage Charge-Discharge All Ideal Components8 Hours of Sun Per Day Half Hour of Exposure Per Day Components of a Solar Circuit Regulator Efficiencies E in : E out 66% 2% 60%50%

Application Load Application Load Starting point for capacity planning Most time is spent sleeping (~20 uA) with short active periods (~20 mA) ColorDeviceAverage Current Sensors9 uA (550 uA at 1.67% DC) Radio0.206 mA (20.6 mA at 1% DC) MCU9.6 uA (2.4 mA at 0.4% DC) Quiescent15 uA Total0.24 mA Total Current

Energy Storage Energy Storage TypeLead AcidNiCadNiMHLi-ionSupercap Operating Voltage Range V V V V V* Volume Energy Density 67 Wh/L102 Wh/L282 Wh/L389 Wh/L5.73 Wh/L Charge/Discharge Efficiency 70-92%70-90%66%99.9%97-98% Charging MethodTrickleTrickle/Pulse Pulse Est. Lifetime (79.2 mWh/day) 98.5 days33.3 days (2)75.8 days (2)35.4 days3.8 days * estimated

Solar Panel Solar Panel Cells in series and parallel Important parameters IV and PV Curves Physical Dimensions MPP: 3.11 Volts

Regulators Regulators Regulators are glue matching primary components 50-70% efficiency for typical sensornet load range Input regulator Regulates voltage from solar panel to battery Can be obviated by matching panel directly to storage Output Regulator Regulates mote voltage Provides stability for sensor readings Model estimates that load requires 28 minutes of sunlight

Mechanical Considerations Enclosure design is often application-driven Sensor Exposure WaterproofingEase-of-Deployment Internal Mechanicals Temp / RH Sensor TSR, PAR Sensors

Network Architecture Used Arch Rock Primer Pack for multi-hop network stack, database for stored readings, and web-based network health diagnosis

The Urban Neighborhood 20 Nodes for 5 Days Mounted on house, around trees, and on roof Meant to emulate forest floor conditions Important for systematic approach -- provided validation of model

Urban Neighborhood Energy Harvested Every node received enough sunlight

Three Nodes, Three Solar Inputs

The Forest Watershed 19 Nodes for over a Month Mounted on 4-ft stakes throughout the transsect

Forest Watershed Site

Forest Watershed Energy Harvested Watershed Most nodes struggle to harvest sunlight

Three Nodes at the Watershed

Reflected Light Though only minimally, a cloudy day helps a sun-starved node harvest solar energy. Sunny Overcast Sunny

Results and Conclusions Open issues not captured in model Battery sizing for days or for seasons? Solar spotting effect Enclosures with holes? Systematic approach resulted in 97% collection of an unprecedented spatiotemporal data set New view of watershed microclimates

Questions?

Outline Microsolar Model Sizing the Pieces Network and Node Architecture Deployment Results

Daily Solar Model Discharge: No incoming solar energy, battery discharging Transition: Some incoming solar energy, battery still discharging Recharge: Incoming solar energy for charging battery Saturation: Incoming solar energy shunted, battery full

Application Load Most time is spent sleeping (~20 uA) with short active periods (~20 mA) Variation in hardware results in RMS currents from 0.25 mA-0.6 mA. Empirically measured 0.53 mA. DeviceAverage Current Sensors9 uA (550 uA for 5s in 5m) Radio0.206 mA (20.6 mA at 1% DC) MCU9.6 uA (2.4 mA at 0.4% DC) Quiescent15 uA Total mA

Node Voltages

HydroWatch Node

A Tale of Two Deployments Urban Neighborhood Deployment 20 Nodes for 5 Days Mounted on house, around trees, and on roof to emulate conditions on forest floor Provided validation of model Forest Watershed Deployment 19 Nodes for over 1 Month Mounted throughout forest on 4-ft stakes Rainy season

Network Diagram

Conclusions and Future Work Good Engineering Needed to Undertake Good Science Microsolar has subtle differences from mesosolar Enclosure design requires careful engineering Experimenting with flexible panels and application duty cycling

Solar Voltages Solar energy is available