Ambient RF Energy Harvesting

Abstract

In this paper, we present a study of ambient RF energy harvesting techniques. The measurement of the ambient RF power density is presented. The average of the density in broadband (1GHz-3.5GHz) is in the order of -12dBm/m² (63 Two systems have been studied to recover the RF energy. The first is a broadband system without matching circuit. The second is a narrow band system (1.8-1.9GHz) with a matching circuit. The rectifier circuit RF / DC and the choice of the load to optimize the DC power recovered are presented. The preliminary results indicate that the recovered energy is not sufficient to directly power devices but could be stored in a super-capacity or micro-batteries.

Full text

1
European Association for the Development of Renewable Energies,
Environment and Power Quality (EA4EPQ)
International Conference on Renewable Energies and Power Quality
(ICREPQ’10)
Granada (Spain), 23th to 25th March, 2010
Ambient RF Energy Harvesting
D. BOUCHOUICHA
1
, F. DUPONT
1
, M. LATRACH
2
, L.VENTURA
3
1
STMicroelectronics, 16 Rue Pierre et Marie Curie 37071 Tours France
dhaou.bouchouicha@st.com , francois.dupont@st.com
laurent.ventura@univ-tours.fr
2
Groupe RF& Hyperfréquence- école supérieure d’électronique de l’ouest(ESEO)
4 Rue Merlet de la Boulaye, BP 30926, 49009 Angers, France
mohamed.latrach@eseo.fr
3
Laboratoire de microélectronique de puissance-Université de Tours.
16 rue Pierre et Marie Curie 37071 Tours France.
Abstract:
In this paper, we present a study of ambient RF energy harvesting
techniques. The measurement of the ambient RF power density is
presented. The average of the density in broadband (1GHz-
3.5GHz) is in the order of -12dBm/m² (63µW/m²). Two systems
have been studied to recover the RF energy. The first is a
broadband system without matching circuit. The second is a
narrow band system (1.8-1.9GHz) with a matching circuit. The
rectifier circuit RF / DC and the choice of the load to optimize
the DC power recovered are presented.
The preliminary results indicate that the recovered energy is not
sufficient to directly power devices but could be stored in a
super-capacity or micro-batteries.
Keywords -
harvesting energy, RF energy, wireless
sensor, rectenna.
1. Introduction:
In recent years the use of wireless devices is growing in
many applications like mobile phones or sensor networks.
This increase in wireless applications has generated an
increasing use of batteries. Many research teams are
working on the autonomy of the batteries by reducing the
consumption of the devices. Others teams have chosen to
recycle ambient energy like in MEMS [1]. The charging of
multiple applications is easy because the user can do it
easily, like for mobile phones. But for other applications,
like wireless sensor nodes located in difficult access
environments, the charging of the batteries remains a
major problem. This problem increase when the number of
devices is large and are distributed in a wide area or
located in inaccessible places. The uses of the Wireless
Power Transmission (WPT) allow the overcoming of these
problems.
The rectification of microwave signals to DC power has
been proposed and researched in the context of
high-power
beaming since the 1950s [2]. It has been proposed for
helicopter powering [3], solar power satellite (SPS) [4],
the SHARP System [5], and recently for RFID system.
The principle of this kind of power transfer is presented in
the Figure 1.
Fig.1. Conceptual view of the WPT system.
In this paper we focus on ambient RF energy. We propose
to use the energy from commercial RF broadcasting stations
like GSM, TV, WIFI or Radar to supply energy for wireless
sensor nodes or other applications. This powering method
can be especially interesting for sensor nodes located in
remote places, where other energy sources like solar or wind
energies are not feasible.
The DC power depends on the available RF power and
conversion efficiency RF/DC.
RFDCRFdc PP .
/
η
=
The choice of antenna and frequency band is very important
to optimize the DC power harvested.
In the section 2 of this paper we present the measurements
of the ambient RF power density. The evolution of this
density is studied as a function of the
frequency and time.
The section 3 discusses the design of the broadband
rectenna without a matching circuit. The section 4
presents a
study of the rectenna with a matching circuit.
2. Measurements of density RF power:
Multiple sources of different frequencies are radiating
power in all directions in a rich scattering environment (Fig.
2).
Rectifier RF/DC Devices
DC
Emitter
RF power
Reception
RF power

2
freq (100.0MHz to 3.500GHz)
S(1,1)
S11
Fig.2. examples of the different radiating sources
We have measured the RF power density in the different
points in the urban environments. The variation of this
density in dBm/m² depends on the frequency and time in
the 680MHz-3.5GHz band and is presented in the Fig.3.
The power density variation is found to be between -
60dBm/m² and -14.5dBm/m² (1nW/m² and 35.5µW/m²)
and is constant over time. The maximum of this power
density has been measured in the 1.8GHz-1.9GHz band.
The summation of the power density of all the measured
signals (Fig.4) provides a greater power density around-
12dBm/m². The RF energy harvesting system principle is
presented in the Fig.5
Fig.3 Measured RF power density versus time (680MHz-
3500MHz)
Fig.4. the total RF power density measured versus time
Fig.5. Principle of RF energy harvesting system
3. Broadband system:
The broadband system consists of two parts; the rectifier
without matching circuit and the broadband antenna. The
goal of the system is to maximize the DC power harvested
and is designed to recover all signals available. For this
issue we must use an omni-directional broadband antenna.
3.1. Rectifier
The RF/DC converter is a voltage doubler and has been
designed and simulated by using the Advanced Design
System (ADS) software, which uses the harmonic-balance
method. This circuit is optimized and achieved by using a
commercial zero biased Schottky diode HSMS2850 (Fig.6)
PORT1
C1
di_hp_HSMS2850_20000301
D2
C2 R1
di_hp_HSMS2850_20000301
D1
Fig.6: Schema of the rectifier
Fig.7 Show the impedance versus frequency. It is equivalent
to a parallel RC circuit. This type of impedance can be
adapted for a wide frequency range, but the loss in the
matching circuit will be very important [6]. To avoid these
losses we connect the antenna directly to the rectifier
without a matching circuit.
Fig.7: The impedance of rectifier without matching circuit as a
function of the frequency
Matching
Rectifier
Storage
Devices
Waves

3
The fabrication of the microwave rectifier is done by using
FR4 as a substrate (relative permittivity 4.4, tangent losses
0.02, thickness 0.8mm). Fig.8 shows the variations of the
output DC power versus the input RF power at 1.5GHz.
The simulation and measurement results are in good
agreement for RF power more than -32dBm.
Fig.8: Simulated and measured DC power as a function of RF
power (1.5GHz)
The impedance of the antenna affects the DC power
recovered. In Fig.9, the DC power as a function of
resistance of antenna for a wide band frequency is
presented. For resistances lower than 50Ω the DC power is
very low. The optimum resistance value to increase the
DC power over the entire frequency range (1GHz-3GHz)
is about 100Ω.
Fig.9: Simulated result of the DC power as a function of the
antenna resistance. The input RF power is Pin=-40dBm.
3.2. Spiral Antenna
As seen in the previous section, the major problem in broad-band
rectenna design is linked to the matching circuit. For maximal
power transfer, the antenna impedance must be matched to the
optimal diode impedance for all frequencies. Our approach is to
present a constant impedance of 100Ω to the diode by using a
frequency-independent antenna.
An equiangular spiral with dimensions shown in Fig. 10 was
chosen for the following reasons:
1) Uni-planar with convenient feed point for diode connection.
2) Possible dual polarization;
3) Broadband antenna
4) Omni-directional radiation pattern
Fig.10 Spiral antenna
The spiral antenna was simulated with HFSS tools [7].As
shown in Fig 11, measured and simulated return losses are
in good agreement. In all band (1GHz- 3GHz) the return
loss is lower than -10dB.
Fig. 12 shows the radiated antenna energy in the space. The
shape of radiation varies according to the frequency. A
quasi Omni directional radiation is obtained for low
frequencies around 1GHz. In the all frequency band (1GHz-
3GHz) the gain is more than 2.5dBi (Fig.13). The gain can
reach 7dBi for a frequency of 3 GHz.
Fig.11. Antenna return loss
1GHz 1.5GHz
2GHz 3GHz
Fig.12: Radiation pattern of spiral antenna.
9cm
8cm
1.2mm
Measured
Simulated
-50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
Pin en dBm
Pout en dBm
simulation
mesure
Simulated
Measured

4
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.81.0 3.0
-20
-10
-30
0
freq, GHz
dB(S(1,1))
Fig.13 Maximum gain
The Maximum RF power that the spiral antenna can
capture in the order of -42dBm (63nW).
In Fig.14, the DC power as a function of load RL for a
wide band frequency is presented. The optimum load to
maximize the DC power is 18KΩ. With this load the DC
power is estimated between 5pW and 10pW (-83dBm -
80dBm). The rectenna is presented in Fig.15. The
measured power in environment is around -79dBm
(12.5pW). It is in the range of estimated DC power.
Fig.14: Output power as function of RL load for Pin=-42dBm
(63nW)
Fig.15 Rectenna prototype
The Dc power harvested with broadband system is very
low. The level of RF energy and the mismatching of the
antenna to rectifier are the causes of this low level DC
power. To increase the DC power harvested and the
efficiency of conversion RF/DC we can use the special RF
source to feed the devices (WPT). The use of antenna
arrays can increase the RF power and the DC power but
for attended the significant DC level the size of the array
become very large.
4. Narrow band system:
As seen in the section I, the maximum of the power density
in the urban environment has been measured in the 1.8GHz-
1.9GHz band. This power density is -14.5dBm/m² and is
constant over time. It is approximately equal to half of the
total power density (Fig. 16)
Fig.16: Total measured RF power density as time (1.8GHz-
1.9GHz)
In this section we present only the results of simulations.
For the narrowband the matching circuit is essay to achieve.
It is presented in Fig.17. The return loss of the rectifier is
presented in Fig.18.
Fig.17: Schema of the rectifier with matching circuit
Fig.18. Rectifier return loss
vout
PORT1
L2
L3L1
C
C3
C1
di_h p_HSMS 2850 _200 0030 1
D2
di_ hp_HSMS 285 0_200 0030 1
D1
C2 R1
Matching circuit Rectifier

5
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.81.0 3.0
0.2
0.4
0.6
0.0
0.8
Freq(GHz)
Effeciency (%)
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.81.0 3.0
100
200
300
0
400
Freq(GHz)
DC Power(pW)
The estimate DC power scavenged with the narrow band
system is presented in Fig.19. The input RF power
recovered with antenna is estimated around -42dBm
(63nW). The efficiency of rectifier is estimated in order of
0.6% (Fig.20). The DC power can be attended the 400pW.
Fig. 19: estimated of DC power scavenged with a narrowband
system
Fig. 20: Estimated efficiency as function frequencies
The use of the matching circuit has increased the DC
power. But the efficiency of conversion RF/DC is very
low (0.6%). This is due to characteristics of the diodes at
low power and the voltage junction (V
j
=0.35V for
HSMS2850). To increase the DC power harvested and the
efficiency of
conversion RF/DC we should be use the
diodes more sensitive than HSMS2850 with a junction
voltage near 0V and use an antenna arrays to increase the
input RF power.
5. Conclusion:
In this paper we have presented a study of feasibility to
harvesting the ambient RF energy. The measurement of
the RF power density available in urban environment
shows the RF power is very low and is distributed in a
large wide band frequency. To scavenge a maximum of
DC power we have presented a wideband system when
able to deliver a DC power around 12.5pW. A narrowband
system is also presented. The first study for this system
show the attended DC power can be about a 400pW.
For the two systems the scavenged DC power is very low to
ensure autonomous operation of devices. But this energy
harvesting can be store in micro-battery or super capacity.
To increase the DC power scavenged we can increase the
RF input power by using for example an antenna arrays.
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