Develer Workshop: A workshop focused on the principles and benefits of applying the Energy Harvesting techniques on Wireless Sensor Networks. The contents come from my Better Embedded 2013 talk.

1. DEVELOPMENT OF A WIRELESS SENSOR
NETWORK POWERED BY ENERGY
HARVESTING TECHNIQUES
Daniele Costarella
Develer – Campi Bisenzio, FI, Italy – November 6th, 2013

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Energy Harvesting Workshop
Outline
• Energy Harvesting Basics
• What are the benefits? Where is it useful? Important aspects.
• Piezoelectric, Thermoelectric and Solar Sources
• Selecting the Right Transducers, piezogenerator models,
capabilities, limitations
• Converting Harvested Energy into a Regulated Output
• Rectification, start-up, efficiency, and over-voltage concerns
• Integrated solution in a WSN
• Challenges Design of a EH-WSN node, prototyping
• Data analysis
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Common EH Systems
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Energy Harvesting Basics
• Energy Harvesting is the process by which energy readily available
from the environment is captured and converted into usable electrical
energy
• This term frequently refers to small autonomous devices, or micro
energy harvesting
• Ideal for substituting for batteries that are impractical, costly, or
dangerous to replace.

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Common EH Sources
Energy Source
Performance
(Power Density)
Notes
Solar:
• Outdoor, direct sunlight
• Outdoor, cloudy
• Indoor
15 mW / cm2
0.15 mW /cm2
10 uW / cm2
Power per unit with a
Conversion efficiency of 15%
Mechanical
• Machinery
100-1000 uW /cm3
•
Human body
110 uW / cm3
Ex. 800 uW / cm3 @ 2mm e 2.5
kHz
Ex. 4 uW / cm3 @ 5 mm and 1
Hz
•
•
Acoustic noise
Airflow
1 uW / cm2 @ 100 dB
750 uW / cm2 @ 5 m/s
Thermic
• Temperature gradients
•
EM radiation
1-1000 uW / cm3
It depends on the specific
conditions with respect to the
Betz limit
Depends on the average
temperature.
Distance: 5 m from a 1W source
@ 2.4 GHz (free space)

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Design challenges in conventional WSN
• Sensor node has limited energy supply
• Hard to replace/recharge nodes’ batteries once deployed, due to
• Number of nodes in network is high
• Deployed in large area and difficult locations like hostile environments,
forests, inside walls, etc
• Nodes are ad hoc deployed and distributed
• No human intervention to interrupt nodes’ operations
• WSN performances highly dependent on energy supply
• Higher performances demand more energy supply
• Bottleneck of Conventional WSN is ENERGY

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Energy Harvesting in Wireless Sensor
Networks
• Wireless Sensor nodes are designed to operate in a very
low duty cycle
• The sensor node is put to the sleep mode most of the time and it is
activated to perform sensing and communication when needed
• Moderate power consumption in active mode, and very
low power consumption while in sleep (or idle) mode
• Advantages:
• Recharge batteries or similar in sensor nodes using EH
• Prolong WSN operational lifetime or even infinite life span
• Growing interest from academia, military and industry
• Reduces installation and operating costs
• System reliability enhancement

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Wireless Sensor Node
Main subsystems
Power unit
Piezoelectric
generator
Sensing
subsystem
Sensors
Solar source
TEG
ADC
Computing
subsystem
Communication
subsystem
MCU
• Memory
• SPI
• UART
Radio

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Wireless Sensor Node
Power consumption distribution for a wireless sensor node
25%
Computing Subsystem
Sensing Subsystem
60%
15%
Communication Subsystem

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Energy sources
• Vibrating piezos generate an A/C output
• Electrical output depends on frequency and acceleration
• Open circuit voltages may be quite high at high g-levels
• Output impedances also quite high
• TEGs are simply thermoelectric modules that convert a
temperature differential across across the device, and
resulting heat flow through it, into a voltage
• Based on Seebeck effect
• Output voltage range: 10 mV/K to 50 mV/K
• A solar cell converts the energy of light directly into
electricity by the photovoltaic effect
• The output power of the cell is proportional to the
brightness of the light landing on the cell, the total area
and the efficiency

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Energy Harvesting Workshop
Energy Storage
Option 1: Traditional Rechargeable Batteries
• Inefficient charging (lots of energy converted to heat)
• Limited numbed of charging cycles
Option 2: Capacitors
• Efficient charging
• Limited capacity
Option 3: Super Capacitors
• Small size
• High efficiency
• Very high capacity ( from 1 up to 5000F or so)
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Supply management: LTC3588
• The LTC3588 is a high efficiency
integrated hysteretic buck DC/DC
converter
• Collects energy from the piezoelectric
transducer and delivers regulated
outputs up to 100mA
• Integrated low-loss full-wave bridge
rectifier
• Requires 950nA of quiescent current
(in regulation) and 450nA in UVLO
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Supply management: LTC3588
A simple circuit simulation
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Supply management: LTC3588
A simple circuit simulation with a 47uF output capacitor
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Supply management: LTC3588
We could increase the output capacitance to 2200uF
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Supply management: LTC3588
And if we choose an even larger capacity? Ex. 1F
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Anatomy of the WSN node
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Battery Output vs. EH Module Output

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Energy Available vs. Time
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Demoboard Project
• Design of a multisource Energy
Harvesting Wireless Sensor Node
• Development of a demoboard with
Energy Harvesting capabilities,
including RF communication and
Temperature sensor
• Additional supercap for longer
backup operation
• Very customizable to the end users’
needs
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Power supply circuit
Piezo
Solar
TEG
Primary Charge
Supercap
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Prototyping
On board:
• 40-Pin Flash Microcontroller
with nanoWatt XLP Technology
• Low Power 2.4GHz GFSK
Transceiver Module
• Low Power Linear Active
Thermistor
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Signal analysis
Fig. A: Duty cycle
Fig. B: TX pulse length (Zoom View)
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Code Diagram
Fig. A: Init
Fig. B: Main Loop
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Payload structure
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Data analysis
• Web interface
• Real time graphics
• History
• Views
• Temperature
• Supercapacitor Voltage
• Input Voltage
• Charging
• Backup status
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Data analysis: examples
Fig. A: Temperature
Fig. B: Input Voltage (VIN)
Fig. C: Supercap charging
Fig. D: Supercap discharge
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28. DEMO

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Board specifications
Feature
Description
Sources:
Solar / TEG / Piezoelectric
Input voltage ranges:
Solar: 5 ÷ 18 VDC
TEG: 20 ÷ 500 mVDC
Piezoelectric: max 18 VAC
Temperature Sensor:
0 ÷ 50 °C
Resolution:
0.4 °C
Wireless communication:
2400-2483.5 MHz ISM (GFSK)
Transmission rate:
1 and 2 Mbps support
Current/Power IDLE mode:
9 uA / 30 uW
Current/Power TX mode:
18.9 mA / 62 mW
Maximum TX distance:
100 m
Backup operation:
> 24 h
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References
Energy Harvesting Technologies
Springer
By Shashank Priya and Daniel J. Inman
Covers a very wide range of interesting topics
My Master Thesis
Università degli Studi di Napoli “Federico II”
By Daniele Costarella
Available online: http://danielecostarella.com

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Thank you
@dcostarella
http://it.linkedin.com/in/danielecostarella
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