Ultra Low Power RF Energy Harvesting System using a Super Capacitor

as an Energy Reservoir for an IoT Node

Florian Grante, Ghalid Abib, Muriel Muller and Nel Samama

Laboratoire Samovar, Institut Polytechnique de Paris, T

ecom SudParis, D

epartement Electronique et Physique (EPh), 19,

rue Marguerite Perey, 91120, Palaiseau, France

Keywords:

RF Electromagnetic Energy Harvesting, WiFi, Autonomous Sensor, Schottky Diode, RF/DC Rectiﬁer, Boost,

Energy Budget Analysis, Green IoT, DC/DC Boost, Super Capacitor.

Abstract:

In this paper, we propose a 2.45 GHz RF energy harvesting system to power a battery free Internet of Things

(IoT) sensor node. The system operates from an RF signal with a power as low as -20 dBm and includes an

RF to DC converter, a storage super capacitor and a voltage DC/DC up-conversion. The system components

are sized to provide the required electric energy to operate the connected object. The tested connected object

includes an ARM microcontroller, an ultra low-power sensor (temperature, pressure and humidity) and a

Bluetooth Low Energy interface for a wireless transmission toward a terminal such as a smartphone or tablet.

It requires 200 µJ per transmission cycle, which must be provided by the proposed harvesting system. This

system is tested in an anechoic chamber and using a constant power RF signal to characterize the energy

recovery time needed for measuring and transmitting the sensor data. Promising results show that the system

is capable of sending data after 5 hours of RF energy harvesting from a source distant with one meter.

1 INTRODUCTION

Climate change issues are pushing us to innovate to

solve the question of our energy sources for the fu-

ture. How can we reduce our carbon footprint? How

can we produce electricity that is healthier for the en-

vironment? We also see steps to reduce our consump-

tion with the least possible impact on our lifestyles.

We then provide an optimization work of the energy

consumption of our industries. To achieve this, the

key element of the solution is the ever increasing feed-

back of information on all our systems.

This is where the Internet of Things (IoT) comes

into play. Today, the IoT represents billions of con-

nected objects and the number is still growing expo-

nentially. It has multiple uses such as Industry 4.0

with predictive maintenance, the development of con-

nected cities with smart grids to optimize the distri-

bution of electricity in the network or more simply,

the management of heating at home which represents

more than two thirds of energy consumption in Eu-

rope (Commission, 2020).

However, the multiplication of these connected

objects has the side effect of increasing the environ-

mental impact of digital technology. Indeed, these

billions of connected objects are generally powered

by batteries that need to be renewed every year on

average. While the carbon footprint of an AAA bat-

tery is 65 gCO2e (ADEME, 2019), the production of

batteries for the IoT sector could generate 0.01 % of

Greenhouse Gas (GHG) in 2025 and 0.02 % by 2030

(IEA, 2021).

A new ﬁled of research is developing to ﬁnd alter-

native energy sources for the IoT called energy har-

vesting.

The principle is to recover energy from our envi-

ronment, natural or not. We then have connected ob-

jects powered by solar energy with the development

of organic photovoltaic panels without rare-earth el-

ements (Wu et al., 2017); but also with mechani-

cal energy as proposed by the company ZF Electron-

ics (ZFE, 2022) allowing the installation of switch

connected lamps without batteries. We also ﬁnd

other sources of energy in research such as thermo-

electric energy (Correa-Betanzo et al., 2019) work-

ing with Seebeck effect, vibratory energy (Balgu-

vhar and Bhalla, 2018) and the one that interests

us in this work, electromagnetic energy (Franciscatto

et al., 2013) and more speciﬁcally here the WiFi radio

waves.

The objective is to develop a system capable of

recovering and converting the surrounding RadioFre-

Grante, F., Abib, G., Muller, M. and Samama, N.

Ultra Low Power RF Energy Harvesting System using a Super Capacitor as an Energy Reservoir for an IoT Node.

DOI: 10.5220/0011143400003286

In Proceedings of the 19th International Conference on Wireless Networks and Mobile Systems (WINSYS 2022), pages 15-21

ISBN: 978-989-758-592-0; ISSN: 2184-948X

quency (RF) WiFi signals into a direct current (DC)

capable of powering a sensor in a connected object.

The principle of radio energy harvesting has been de-

veloped in research for nearly twenty years now and

has seen several evolutions, especially on the type

of signals used. Indeed, we can see research papers

focused on TV signals (Parks et al., 2013) (Lacerna

et al., 2022) in the early 2010s, whose frequencies are

around a few hundred MHz to papers more on the In-

dustrial, Scientiﬁc and Medical (ISM) band at 2.45

GHz (Berg

es et al., 2015) and GSM at 900 MHz and

1800 MHz (Ho et al., 2016) with the massive develop-

ment of WiFi, 3G, 4G systems in the world in recent

years.

The RF energy harvesting discipline is also split

into two: remote power on the one hand with Ra-

dioFrequency Identiﬁcation (RFID) and the Near

Field Communication (NFC) standards for short dis-

tances or over longer distances that can have uses in

particular in the medical (Jin et al., 2019) or appli-

cation like the one proposed by Archos at CES2022.

A system with a power station supplying energy to

bateryless sensors operating with the principle of

beam forming to optimize power transmission over

radio waves (Archos, 2021). On the other hand, we

have power supply by opportunistic harvesting of am-

bient signals and this is what interests us here. Can we

power a connected object by harvesting the surround-

ing WiFi signals in a building?

We can ﬁnd several works dealing about this prin-

ciple (Kim et al., 2014) (Tran et al., 2017), especially

in the 2.45 GHz ISM band that interests us. They con-

cern the development of the RF/DC converter which

is more or less complex with its theoretical characteri-

zation. In this work, we wish to go further by propos-

ing a study of the complete ecosystem in which the

RF energy harvester is supposed to operate properly

in order to conclude about the feasibility of such a

system. We present in section 2 the development of

a harvester for 2.45 GHz WiFi signals which will be

integrated into our system presenter in section 3. Fi-

nally, the section 4 presents the measurement results

under laboratory conditions.

2 RECTIFIER

To convert a RF WiFi signal into DC voltage, it is

necessary to develop a Rectenna circuit following the

schematic represented in FIG.1, meaning the combi-

nation of an antenna and a rectiﬁer circuit which will

convert the RF signal into DC voltage.

The received RF signal is converted into DC

thanks to a non-linear device based on a Schottky

Figure 1: A simple rectenna schematic.

diode. A matching network must be inserted between

the antenna and the diode for a maximal power trans-

fer. The diode is loaded by a low pass ﬁlter using a

capacitor and a resistor corresponding the equivalent

impedance of the connected object.

For the antenna part, we decided to use a classical

WiFi whip antenna as we can ﬁnd it on routers, and

we focus on the development of the rectiﬁer. Some

works on speciﬁc antenna can be found like (Srinivasu

et al., 2020), (Shen et al., 2018) or (Trinh et al., 2019).

We wish to work with ambient WiFi signals and

our different measurement campaigns of the available

power at a distance of 1 meter from the WiFi access

point led us to realize a system that must operate at an

incident power as low as -20 dBm.

For this reason, we restrict ourselves to a single-

wave rectiﬁer composed of a single Schottky diode in

order to minimize the losses. We use the Skyworks

SMS7630 Schottky diode that is widely used in the

state of the art on rectennas for RF energy harvest-

ing (Trinh et al., 2019) (Kim et al., 2015). The cir-

cuit is optimized thanks to simulations performed on

Keysight Advanced Design System (ADS) software

and represented in FIG.2. The matching network is

synthesized thanks to microstrip transmission lines.

We used a 10 MΩ load resistance, which can be as-

similated to an inﬁnite resistance in DC, because we

will use a capacitor to store the energy and whose

impedance tends to inﬁnite in DC. The recovered out-

put DC voltage is about 178 mV for an input RF

power of -20 dBm at 2.45 GHz. Then, the optimized

rectiﬁer circuit is realized (FIG.2) and characterized.

A measurement of |S

|, the input reﬂection coef-

ﬁcient of the rectiﬁer circuit, is shown in FIG.3 The

measured value below -8 dB at 2.45 GHz ensures

that our impedance matching network optimizes the

power transfer from the antenna to the diode.

As the goal is to recover a DC voltage at the recti-

ﬁer’s output, we can characterize the performance of

the circuit by measuring this DC output voltage as a

function of the signal frequency, which can be seen in

FIG.3. We can note a maximum voltage of 159 mV

obtained for an incident signal of 2390 MHz at -20

WINSYS 2022 - 19th International Conference on Wireless Networks and Mobile Systems

dBm, which is close to the 178 mV obtained through

simulation. According to FIG.3, we can expect an

output voltage between 98 mV and 155 mV over the

ISM band (2.4 GHz - 2.5 GHz).

(a) Rectiﬁer schematic (Keysight ADS)

(b) PCB made in our laboratory

Figure 2: Schematic and PCB of the rectiﬁer.

(a) |S

| measurement with an Anritsu MS46122B USB

VNA

(b) DC output voltage measurement Vs. input signal fre-

quency at -20 dBm

Figure 3: Rectiﬁer characterization.

3 VOLTAGE BOOST

A DC voltage of 155 mV obtained in the ISM band is

insufﬁcient to power a connected object whose supply

voltage is usually deﬁned between 1.8 V and 3.3 V.

We must therefore use a voltage DC/DC boost to up-

convert the recovered low voltage.

The only voltage boost we know of that can op-

erate over such a low voltage range in the industry is

the LTC3108 from Analog Devices. Still, this voltage

boost is designed for a thermoelectric generator such

as a Peltier module, which has, in particular, a very

low impedance of few Ohms. If we connect this volt-

age boost directly to the rectiﬁer output, it will lead to

an insufﬁcient rectiﬁer output voltage because of this

low impedance.

We have therefore imagined an intermediate sys-

tem between the rectiﬁer and the voltage DC/DC

boost: a kind of energy reservoir that come and

go composed of a super capacitor surrounded by

switches. The complete system is shown in FIG.4.

The super capacitor reservoir is then sometimes

connected to the rectiﬁer to recover the DC energy.

This capacitor being then the only load on the rectiﬁer

and thus allow the best possible voltage by its inﬁnite

DC impedance. And sometimes, it is connected to the

voltage DC/DC boost to discharge and thus, convert

the accumulated energy to a voltage level adapted to

our connected object. This boosted DC voltage will

be stored in the capacitor placed at the boost output.

This raises the question of the technology for the

switches. It is indeed essential to choose switches that

have the lowest possible leakage current and that can

toggle with a low voltage and current. As our key

value here is the voltage, we can eliminate the bipo-

lar transistors, which is controlled by a current whose

value cannot be controlled. MOSFETs on the other

hand can be considered because they are controlled in

voltage by the gate.

Another solution is the electromechanical relays.

Indeed, the switching is mechanical, so a real open

circuit is present and can be considered as no leakage

switches. The challenge is to ﬁnd electromechanical

relays or MOSFETs without leakage that we can con-

trol with a voltage as low as 100 mV (Liu et al., 2014).

Voltage supervisors under 1 V exist and are used

in applications close to ours like the UB20M (UB2,

Figure 4: Schematic of our proposed circuit.

Ultra Low Power RF Energy Harvesting System using a Super Capacitor as an Energy Reservoir for an IoT Node

2017) which is able to detect a rectiﬁer output volt-

age as low as 0.6 V. Even if it is still too high for our

system which must operate around 0.1 V, research pa-

pers seem promising on the subject, especially on the

side of electromechanical relays (Qian et al., 2017).

Even if they present constraints that do not allow to

use them as they are presented in the study, they al-

low to realize a switch activating at 100 mV with a

hysteresis up to 20 mV.

4 MEASUREMENTS AND TESTS

Now that we are able to rectify the RF WiFi signal

into a 3.3 V DC voltage, we would like to test the

whole system to see if it is able to power a connected

object to transmit data.

To ensure if our proposed design works, we will

use classical relays, which will be controlled and

controller, here an Arduino. The electric consumption

of these relays is not considered because they are not

supposed to be present in the ﬁnal circuit. The com-

plete schematic of our system is represented FIG.5.

The circuit after the voltage DC/DC boost is a ca-

pacitor that stores the energy and a voltage supervisor

that is supposed to enable the Low DropOut (LDO)

regulator when the voltage is above the threshold of

2.63 V.

The connected object is made up of an ON-

Semi RSL10-SIP microcontroller based on an ARM

Cortex-M3 core and a Bluetooth Low Energy (BLE)

5.2 interface for a wireless transmission. It measures

temperature, pressure and humidity using an ultra-

low-power BME680 sensor from Bosch Sensortec. It

advertises a 25 bytes data frame following the pattern

on TAB.1 where UUID refers to a custom Universally

Unique IDentiﬁer to identify which node is advertins-

ing the data. The energy need of this connected object

is around 200 µJ per transmission cycle. The details

of its energy consumption have been the subject of

work and a previous publication (Grante et al., 2020).

For a proper study and characterization and in or-

der to have control over the power of the RF sig-

nal source, a controlled environment is preferred.

Thus, we carried out our measurements in an anechoic

chamber (FIG.6) using a dedicated RF signal genera-

Figure 5: Schematic of our complete testing system.

Table 1: Sensor node data frame using BLE 5.2.

Byte Field Description

0 Frame Type

FLAGS for

the receiver

1-16 UUID[0]-[15]

Custom

128-bit

Service

UUID

17-18 TEMP[0]-[1]

Beacon

temperature

19-20 HUM[0]-[1]

Beacon

humidity

21-23 PRESSURE[0]-[2]

Beacon

atmospheric

pressure

24 VERSION

Service frame

version

Figure 6: Photo of the test setup inside the anechoic cham-

ber at Telecom Paris.

tor for transmission.

The output power of the RF generator is adjusted

so as to obtain an Effective Isotropic Radiated Power

(EIRP) of 20 dBm, which is the maximum emission

power allowed by the European legislation (ETSI,

2019) on this frequency range.

A few tests have shown that a 100 µF capacitor

loaded at 3.3 V ensure to give the 200 µJ necessary to

power the connected object to measure and transmit

its data. We were also able to determine that a su-

per capacitor of 2 F at 100 mV upstream of the volt-

age boost can achieve these conditions at the voltage

boost output in terms of voltage and amout of energy.

We followed the following protocol:

1. Connect a transmitting whip antenna (Tx) to an

RF signal generator and set the frequency to 2.45

GHz. The power is set to the maximum of what a

device in Europe can transmit, i.e., 20 dBm EIRP

WINSYS 2022 - 19th International Conference on Wireless Networks and Mobile Systems

Table 2: Time needed to reach 100 mV across the super

capacitor.

Distance

(cm)

Time

(seconds)

Time

(HH:mm:ss)

25 786 00:13:06

50 7810 02:10:10

100 18716 05:11:56

2. The receiving whip antenna (Rx) is placed at 25

cm distant from the transmitter and is connected

to our rectiﬁer system

3. Relays are setup to connect the super capacitor to

the rectiﬁer to store energy

4. Monitoring the voltage using a voltmeter at the

terminals of the super capacitor as a function of

time until reaching 100 mV

5. When 100 mV is reached, the relays are switched

to isolate the rectiﬁer and connect the super ca-

pacitor to the voltage DC/DC boost

6. The 100 µF capacitor is charged, its voltage rises

7. When 3.3 V are reached, the voltage supervisor

triggers the LDO regulator and allow discharging

the 100 µF capacitor in the connected object to

perform the measurement and transmit the data

through a BLE. The data are received and dis-

played by a smartphone application (FIG.7).

8. This procedure is repeated for a Tx/Rx distance of

50 cm and 100 cm.

We measured the evolution of the super capacitor

voltage as a function of time for the three distances.

We present the curves in the FIG.8 and the duration

to reach 100 mV in TAB.2. At 25 cm, a transmis-

sion is obtained after 786 seconds (13 minutes and 6

seconds). The curve has a linear appearance, which

can be explained by the proximity between the Tx

and Rx antennas. Indeed, the power received must

be greater than -20 dBm and thus makes it possible

to obtain a DC voltage at the rectiﬁer’s output greater

Figure 7: The connected object that send data to the tablet

application.

Figure 8: Evolution of super capacitor voltage Vs.time us-

ing a 2450 MHz, 20 dBm EIRP emitter signal generator at

25, 50 and 100 cm.

than 100 mV and therefore, to reach this threshold

more quickly.

At 50 cm, we get 7810 seconds (2 hours, 10

minutes and 10 seconds). At constant efﬁciency,

since we have doubled the distance, the well known

Friis’ formula for free-space path loss (dependence

on the square of the distance) allows us to assume

that it should take 4 times longer to complete a

transmission. In practice, we observe that it takes

10 times longer. This can be explained by the

non-linearity of the diode which implies a decrease

in efﬁciency when the received powers are lower as

we have shown in the characterization of a rectiﬁer

in (Grante et al., 2020). We can then approximate

that we have an average efﬁciency that is 10/4 = 2.5

times lower than if our system was linear.

At 100 cm, a transmission takes place after 18716

seconds (5 hours, 11 minutes and 56 seconds). Un-

like the measurement at 25 cm, here we see a curve

on FIG.8 that looks like the theoretical exponential

charge curve of a capacitor which leads us to think

that we have a rectiﬁer output voltage close to the 100

mV threshold. We can then consider that it will be

difﬁcult to go beyond a Tx/Rx distance of 100 cm be-

cause we would not have the 100 mV necessary at the

output of the rectiﬁer.

5 CONCLUSION

We have proposed a rectiﬁer capable of converting

WiFi signals into a DC voltage in the order of 150

mV for a received power of -20 dBm. We have used

the simplest possible rectiﬁer using a single Schottky

diode because the state of the art on the subject leads

us to believe that it is not necessary to use more com-

plex conﬁgurations for incident powers as low as -20

dBm as indicated on TAB.3.

To allow a more relevant comparison with some

of the works in the table, a measurement of the output

Ultra Low Power RF Energy Harvesting System using a Super Capacitor as an Energy Reservoir for an IoT Node

Table 3: State of the art rectiﬁer output voltage comparison.

Architecture

Number

Diodes

Load

(kΩ)

(dBm)

Frequency

(MHz)

out

(mV)

Ref.

Full rectiﬁer 2 8.2 -20 890 114 (Ho et al., 2016)

Full rectiﬁer 2 5 -20 2450 <200 (Selim et al., 2020)

Full rectiﬁer 4 NA NA 2450 50 (Chen et al., 2017)

Single ended rectiﬁer 1 Inf -20 2450 150 This work

Single ended rectiﬁer 1 Inf -8 2450 542 This work

Dickson 4 NA -8 2450 500 (Fan et al., 2018)

Full rectiﬁer 2 5 -8 2450 <500 (Selim et al., 2020)

voltage (V

out

) of our rectiﬁer for an incident power

) of -8 dBm has been performed.

This table shows some voltage values (V

out

) ob-

tained with different rectiﬁer conﬁgurations. We

can see that our single-ended rectiﬁer obtains simi-

lar performance in terms of voltage as more complex

schematics. This is due to the use of a super capaci-

tor as a load that allows us to consider an inﬁnite load

and therefore to obtain a better voltage.

We could see that our system allowed us to trans-

mit data from a connected object, which is battery

free and powered thanks to an RF energy harvesting

at 2.45 GHz. The association of our rectiﬁer with a

voltage DC/DC boost and storage capacitors is able

to supply a voltage of 3.3 V with enough energy to

allow the sensor data transmission. Whether at 25,

50 or even 100 cm Tx/Rx distance, we can expect at

least, 4 transmissions per day.

We can then imagine application cases such as

temperature monitoring in ofﬁces in order to optimize

the heating of the building and thus hope to save fossil

fuels.

However, currently, this has only been possible

with an external system that manages the switches al-

lowing passing to one side or the other of the system

without losing energy. In-depth study work is nec-

essary on these switches to have a fully autonomous

device.

It will also be necessary to test this system not

with a constant power RF signal generator but with

a WiFi signal to determine a minimum network activ-

ity to allow transmission in a reasonable time.

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Ultra Low Power RF Energy Harvesting System using a Super Capacitor as an Energy Reservoir for an IoT Node