RF Energy Harvesting and Battery-

Free Wireless Sensors

Pierre Mars, VP Applications Engineering, CAP-XX

Charlie Greene, Head of Technology Platforms, Powercast

Darnell nanoPower Forum, May 2009

Overview

• About CAP-XX

• About Powercast

• Application – Wireless Sensors

• System Overview

• System Performance

• Next Steps

• Summary

2

About CAP-XX

• World leader in thin, flat, small supercapacitors suitable for portable electronic devices

• Research-based, market-driven electronic components manufacturer. Founded in Australia in 1997. Listed on AIM in London, April 2006

• Turn-key power design solutions

• Production in Sydney & Malaysia

• Significant sales to big brand customers in Europe, Asia and North America

• CAP-XX supercapacitor technology licensed to Murata in 2008

• Distributors throughout USA, Europe and Asia

3

What is a supercapacitor?

4

Aluminum foil

Carbon coating:2000m2 / gm surface area

Separator

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+ve -ve

Separation distance:Solid-liquid interface (nm)

C α A / d

A , d = C

Physical charge storage,

Not electrochemical Ions in Solvent

Basic Electrical Model

A supercapacitor buffers the load from the source. Source provides low

average power, supercapacitor provides peak power to the load.

C: 210mF – 1.8F

ESR: 25m – 85m,

2.75V/cell, -30 to +85C

ESR: 16m – 50m, 2.25V/cell

2.25V/cell; -30 to +70C

– No dielectric, working voltage

determined by electrolyte

– 2 cells in series for 5.5V

operation

About Powercast

• Driving innovation in the transmission, reception, and conversion of RF Energy

– Wireless Power Systems

– Renewable Power

• Applications

– Wireless sensors, RTLS, Long-range RFID

– Device recharging

– Lighting

– Defense

• Privately held, Founded 2003

5

Powerharvester™

Module

2009 Product

Showcase

Wireless Power

• Dedicated source transmits common radio waves– Ambient sources also available: Mobile/cellular, TV, Radio, etc.

• Proprietary receiver– captures the RF energy with an antenna

– efficiently converts the RF energy to the appropriate DC voltage

• Energy transfer is controllable and predictable by design

6

Wireless Sensors

• Building automation

• Energy management

• Process monitoring

• Condition monitoring

• Location tracking

• Reduced wiring

• Sealed devices

• Reduced maintenance

• Controllable power

• Difficult locations

7

Applications Benefits of Wireless Power

Issues with Primary Batteries

in Wireless Sensor Networks

• Intentional constraints to save power

– design, operation, application

• Reliability

• Temperature performance

• Battery replacement cost

• Limitations of scale

• Majority of energy is consumed in sleep mode

10s 100s 1000s

Man

ag

em

en

t E

ffo

rt

Size of Sensor Network

Battery-Free

Battery-Powered

Battery

Replacement

Sensor Inactive

Capacitor

Voltage

Power

Broadcast

Sensor Power

Consumption

Sensor Active

Battery-Free Concept

Send power as needed - 1) continuously, 2) scheduled, or 3) on-demand

9

VMIN

VMAX

GND

System Overview

10

Powerharvester™ Module

Front Back

TI eZ430-RF2500TSimpliciTI protocol

CAP-XX GZ115

Sleeve Dipole Antenna

Simple “2 wire” hardware integration for any RF module

High Efficiency

Low Power

Small Form Factor

Integrated, 915 MHz

Powerharvester™ ModuleP2100 – 915MHz, Charge & Fire

• High Conversion Efficiency

• Internal Charge Management

• High Sensitivity

• Configurable Output Voltage

• 50mA Output Current

• Capacitor Overvoltage Protection

• Internally Matched to 50 ohms

• Low Quiescent Current (<1A)

• Simple Integration

• Small Footprint

11

Features

P2100 Charge Current

12

1

10

100

1000

10000

-15 -10 -5 0 5 10 15

Ou

tpu

t C

urren

t (u

A)

Module Input Power (dBm)

P2100 Capacitor Charge Current at 1.1V

Advantages of Low

Supercapacitor Voltage

• The P2100 charges the superapacitor to only ~1.2V

• Supercapacitor cell voltage is limited by the voltage stability of the electrolyte, there is no dielectric. Cell voltage of supercapacitors with organic electrolytes < 2.3V to 2.7V depending on the electrolyte.

• A low supply voltage allows the use of a single cell supercapacitor. This avoids the need for balancing between cells

– No balancing circuitry

– No current drawn by balancing circuit

• Reduced rate of supercapacitor ageing at lower voltage

• Reduced leakage current at lower voltage

13

GZ115 Leakage Current @ 23 deg C, 2.3V

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90 100

Time (Hours)

Le

ak

ag

e C

urr

en

t (m

icro

Am

ps

)Supercapacitor Leakage Current

14

Diffusion current: Ions

migrate deeper into the

pores of the carbon

electrode.

It takes many hours for

diffusion current to decay and

leakage current to settle to its

equilibrium value.

Leakage current behaviour means

a minimum charge current is

required to charge a

supercapacitor. Leakage current

increases exponentially with temp

& decreases exponentially with

voltage.

CAP-XX GZ115

Charge Rates for Low Charge Currents

0.00

0.50

1.00

1.50

2.00

0 5 10 15 20

Time (Hours)

Vo

lta

ge

10uA Cap 1

10uA Cap 2

10uA Cap 3

10uA Cap 4

15uA Cap 5

20uA Cap 6

20uA Cap 7

20uA Cap 8

30uA Cap 9

30uA Cap 10

Supercapacitors Need a Minimum Charge Current

15

High diffusion current in the early phase of

supercapacitor charging means a minimum charging

current is required. Need at least 20A to charge the

supercapacitor. The larger the supercapacitor C, the

greater the minimum current required.

Supercapacitor Ageing: C Loss

16

GW214@ 3.6V, 23C, Ambient RH

y = 1.136E-01e-1.416E-05x

R2 = 9.889E-01

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Time (hrs)

Ca

pa

cit

an

ce

(F

)

C Loss rate = 1.4%/1000hrs

GW214 ESR @ 3.6V, 23C, Ambient RH

y = 0.0027x + 71.706

R2 = 0.9168

0

20

40

60

80

100

120

140

160

180

200

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Time (hrs)

ES

R (

mO

hm

s)

ESR rise rate of

2.7mOhms/1000hrs, or 3.4%

of initial ESR/1000hrs

Supercapacitor Ageing: ESR Rise

17

Supercapacitor Ageing

• Allow for ageing when sizing the supercapacitor

• Ageing depends on operating temperature and voltage

• GZ115 initial values 0.15F, 60m. At 1.2V, 23C:

– C loss after 10years 30% GZ115 C = 0.1F

– ESR increase after 10yrs 40% GZ115 ESR = 83m

• At 1.2V, 50C:

– C loss after 10years 70% GZ115 C = 0.047F

– ESR increase after 10yrs 40% GZ115 ESR = 230m

– This still operates the wireless transmitter. Supercapacitor will discharge from 1.16V to 1.04V.

• As a “rule of thumb”, double ESR and reduce C by 1/3 when sizing the supercapacitor.

18

Sizing the supercapacitor

19

If the supercapacitor is supplying a constant power load, such as a DC:DC

converter, where supercapacitor current increases as supercapacitor voltage

decreases, to maintain V x I constant, then supercapacitor ESR may become

significant, and you should solve:

ESR

C

VSUPERCAP

VLOAD

ILOAD

PLOAD

ESR

PESRVVI

PIVESRI

IESRIVP

IVP

ESRIVV

SUPERCAPSUPERCAP

LOAD

LOADLOADSUPERCAPLOAD

LOADLOADSUPERCAPLOAD

LOADLOADLOAD

LOADSUPERCAPLOAD

2

4

0

)(

2

2

If load current is very small, then ILOAD•ESR << VSUPERCAP and can use an energy

balance to size the supercapacitor. Otherwise, use a spread sheet to solve the above

and simulate V & I over time, or use SPICE.

20

Energy StorageChoosing the Supercap Value

0

1

2

3

4

0.5

0

0.5

5

0.6

0

0.6

5

0.7

0

0.7

5

0.8

0

0.8

5

0.9

0

0.9

5

1.0

0

1.0

5

1.1

0

1.1

5

1.2

0

1.2

5

1.3

0

1.3

5

1.4

0

1.4

5

1.5

0

Ou

tpu

t V

olt

ag

e (

V)

Capacitor Voltage (V)

Voltage Window (Hysteresis)

DC-DC conversion

efficiency

Energy Available

Capacitor Value

= required load energyGZ115 cap size = 0.16F (measured)

Stored energy = 22.7 mJ

VMIN VMAX

C = 7.02E/e e 0.82

C = 8.57E

Energy Harvesting Performance

21

1

10

100

1000

10000

5 10 15 20 25 30 40 45 50

Ch

arg

e T

ime (

sec)

Charge Time vs. Distance

Sleeve Dipole (G=1.5)

Air Dipole (G=4.1)

Yagi (G=6.1)

Distance (ft)

Measured @10ft (152 sec)

Measured @ 15 ft(103 sec)

Measured @20ft (145sec)

3W EIRP Patch Antenna Transmitter

TI eZ430-RF2500T

22

Start-up and Data

3.70 mJ @ 3.3V

Note: V1.5 Software

Initial start-up Data

Energy Storage• Load energy = 3.7mJ

• From slide 20, C > 8.57 x 0.0037 > 31.7mF

• GZ115 = 0.15F >> 31.7mF, allows for ageing and more

• From the previous slide, max load current 25mA. Therefore max supercapacitor current = 25mA x 3.3/1.15/0.82 90mA.

• Voltage drop due to ESR (aged supercapacitor) = 120m x 90mA 11mV <<1.15V, so the energy calculation for min C is a good approximation.

23

Solving the energy balance for min V = sqrt(Vinit2 –2xE/(eC)) = 1.12V.

This is verified by solving the quadratic eqn for constant power using C = 0.1F, ESR = 120m, Load power = 3.3V x 25mA/0.82, load duration = 50ms. With starting voltage = 1.16V, the supercapacitordischarges to 1.10V

System Summary

Energy Calculations

• Stored energy = 22.7 mJ

• Usable energy = 18.6 mJ (current design)

• Initial start-up and data transmission = 3.7 mJ

Optimizing Performance

• Narrow the voltage window (hysteresis) of the Powerharvester module

• Adjust the capacitor size

• Improve DC-DC conversion efficiency

• Modify software start-up sequence

24

Next Steps

• Enable user configurable voltage hysteresis for additional flexibility

• Optimize software startup sequence to minimize the “energy overhead”

• Improve harvester input sensitivity to extend range

• Release system as a reference design

25

Summary

• Battery-free = maintenance-free

• Voltage window and cap sizing is important for operation cycle

• Low voltage charging increases life cycle performance of supercap

26

27

www.cap-xx.com

www.powercastco.com