Design of a compact RF energy harvester for wireless sensor networks T. Ajmal, D. Jazani and B.Allen Centre for Wireless Research, University of Bedfordshire, Park Square, Luton, United Kingdom, LU1 3JU
[email protected]
Keywords: Energy Harvesting, wireless sensor networks, loop antenna, medium waves
propagation of these lower frequencies is well known to be better than higher frequencies in terms of range and building penetration.
Abstract We present a method for powering electronic devices from ambient Radio Frequency (RF) signals. Our proposed design of RF energy harvester is made of a compact ferrite rod antenna together with the electronics necessary for converting the received signal into a form that can directly replace batteries. We show that our device can, in principal, power a wireless sensor node from ambient medium wave transmissions so long as the node is within 120 km of a 150kW transmitter.
A block diagram of a generic energy harvester is shown in figure 1. In the case of an RF energy harvester, the transducer is an antenna; the signal conditioning circuit may consist of an AC-DC convertor and a voltage multiplier; and the user device or a storage device such as a rechargeable battery or capacitor.
Signal Conditioning
Transducer
Device/WSN
1 Introduction Figure1: Block diagram of proposed Energy Harvester
Wireless Sensors are usually used in hazardous and/or inaccessible environments making battery replacement undesirable, difficult or impossible. Energy harvesting solutions for powering Wireless Sensor Nodes (WSNs) hence have a significant advantage as they provide a sustainable solution to their power needs. Examples of ambient energy sources include: solar, heat differential and kinetic. It is immediately apparent that technologies which address these issues are highly desirable from environmental, social and economic perspectives. An alternative is to harness the power in RF signals, such as radio and television broadcasts, which is the focus of this paper. Harnessing energy from radio waves is not new and the crystal radio receiver is a well-known example [1]. More recently research has targeted TV transmissions [2] but this requires a cumbersome antenna and penetration of these signals into buildings is low. The scheme reported in [3] requires a custom radio transmission to power the devices. Reference [4] reports of the possibility of charging a mobile phone from ambient mobile network transmissions and there are several related devices reported in the literature.
The rest of this paper is structured starting with the Principle of Operation in sections 2. Section 3 gives a mathematical calculation of energy/voltage received by a loop antenna from a MW transmitter. Section 4 describes the antenna parameters that can be optimised to maximize power. Section 5 illustrates using CAD models, retro-fitting antenna in a battery socket. Section 6 gives the circuit simulation results and paper concludes with Section 7.
2 Principle of Operation The transducer of our RF energy harvester is a loop antenna chosen because it provides a compact solution at low frequencies such as medium waves, which we target for operating our device. It consists of n turns of wire wound around a ferrite rod of diameter D and length ℓ which has a relative permeability of μr. If E is the incident Electric field strength then the induced voltage can be given by [5]:
Vout QV Q
and
E
Q
where In this paper, we examine the possibility of harnessing energy from medium wave broadcast transmissions since the transmitted power can be as much as 400kW and the
2
L
D 2 4
2fL Rs
n 2 A r o
n
(1)
Q is the unloaded coil quality factor and L is the coil inductance. Here, f is the carrier frequency, λ is the wavelength, Rs is the antenna coil series resistance, A is the cross-sectional area of the coil, μo is absolute permeability. The RF energy harvester is located at a distance d from the MW transmitter. E can be calculated using the ground wave propagation model described in [5]. For an antenna with a figure-of-merit, FM, and transmitted power, Pt electric field at a distance d can be given as
Pt
E FM
d
The RF energy harvesting device is constructed of 200 turns of silk covered 0.07 mm diameter litz wire wound around an insulator tube with a diameter of 30 mm and length 120 mm. The insulator is placed around a ferrite rod of relative permeability of 300. The device is orientated to maximize the available voltage across the terminals of the windings With the device aligned to maximise the resulting induced voltage, this yields an alternating voltage reducing from 1V to 20μV across the open circuit terminals of the windings as the distance increases to 200 km, as shown in figure 3.
(2)
A
Here A is the attenuation factor and depends on distance from transmitter (d), carrier frequency (f) and the earths‟ conductivity (σ) and relative permittivity of the ground path (εr). This is given as:
A
2 0.3 p
2 p 0.6 p 2
[sin b ]
1 p exp 5 p / 8 (3) 2
where the auxiliary parameters, p and bo are defined as
p
0.582df 2 cos b
( 1) f b tan 1 r 18 Equations (1)-(3) present us with an analytical background for determining the RF power harvesting performance. 3. RF Energy from MW transmitters For a medium wave transmitter that emits 150kW of RF power at 1MHz, E is computed using the model described above for a transmitter antenna figure-of-merit of FM=420 and height h=300m, both typical of medium wave broadcast transmitters [5] The earths‟ propagation parameters are chosen to be for an average ground propagation with εr = 3 and σ =1 mS/m [6]. The RF field strength versus distance is shown in figure 2 to vary from 5V/m close to the transmitter to 0.1mV/m at a range of 200km.
Figure2: Incident Electric field strength versus distance from transmitter
Figure 3: Open-circuit voltage across the antenna terminals versus distance from the transmitter With a 1kΩ load resistance, the dissipated power versus distance is shown in figure 4 to reduce from 1mW to 1nW. This can be converted into a direct voltage using a voltage rectifier multiplier [7] which achieves an efficiency of around 50%, depending on the number of stages, input voltage and matching arrangements. This requires consideration when determining the power that is delivered to the user equipment. The output of the rectifier is connected to the device to be powered or charged. For a low-power WSN requiring 5μW [8] can be powered when placed within 120km of a 150kW transmitter.
Figure 4: Power at the antenna output versus distance from the transmitter for a 1kΩ load
4 Optimising the antenna design A number of design parameters can be varied and impact the available voltage and power. These parameters include core material permeability, core dimensions and number of windings. 4.1 Core dimensions: Diameter, D and length, l The curves for different core dimensions are plotted in Figure5. The coil diameter used for the reference design was 30mm and a length of 120mm. By reducing the diameter to 10mm yields the curves labelled „D:10, L:120‟, which indicate the expected reduction in voltage (and hence power) by a factor of 100. Increasing the diameter to 50mm and reducing the length to 20mm, which yields a ferrite disc rather than a rod shape, produces the performance curves labelled „D:50,L:20‟, which show the expected increase in voltage and power by a factor of 80 to levels above 1mV and 1nW respectively. Increasing the diameter further to 100mm yields the curves labelled „D:100, L:20‟ and voltage and power levels in excess of 20mV and 1μW respectively. This gives a voltage in excess of 1V for distances of less than 20km from the transmitter.
retrofit in a standard battery holder. Alternatively, for larger cores (e.g., D=100mm, L=20mm), a dummy battery can be connected to the coil and inserted into a standard battery holder. Received power across a 1KΩ resistor for different core dimensions is shown in Figure 6 below. A low-power WSN requiring 5-10μW [8] can be easily powered when placed within a short distance from the transmitter.
Figure 6: Plots showing the effect of core dimension on the load power 4.2 Number of windings, n The available voltage and power can also be varied by changing the number of windings on the coil. These are plotted for two values of n, i.e. 100 and 200, shown in figure 7 below for the same size core dimensions as used in the previous design example. Curves labelled „n=100‟ show the expected reduction in voltage and power by a factor of 10 when the number of windings are reduced to 100. (a)
(b) Figure 5: Plots showing the effect of core dimensions on the output voltage The diameter may be increased still further to yield further increases in available voltage and power. These larger diameter coils are larger than standard battery dimensions in common use Figure 5(b) shows the output voltage for standard battery sizes, i.e., AA, C or D. This would allow
Figure 7: Plots showing the effect of number of windings on the output voltage. 4.3 Core Permeability. The impact of the ferrite rod is demonstrated by comparing the „ferrite core‟ curve with those labelled „air core‟ (μr=1) in figure 8. Here, the coil has the ferrite rod removed and therefore has an air core with relative permeability of μ r=1.
This has reduced the available voltage and power by a factor of 300 to very low levels.
form-factor of a D-cell battery, which may be fitted directly into a conventional battery holder as a battery replacement.
Figure 8: Plots showing the effect of core on the output voltage. 4.4 Transmitter Power.
Figure 10 : Decomposition of example aparatus based on a ferrite rod antenna and with a standard D-cell battery
The effect of transmitted power is shown in Figure 9.The curves labelled „1kW‟ show the available voltage and power when the transmit power is reduced to 1kW (from 150kW). The impact of this is a factor of approximately 12.
Figure 9: Plots showing the effect of transmitted power on the output voltage.
Figure 11: Decomposition of dummy battery with switchable poles and power circuit
We can optimise the design of the harvester by changing either of the parameters mentioned above to achieve required voltage and power levels. The parameters chosen for optimisation largely depend on the final requirement scenario for the application of energy harvester, and include volume, weight, cost and location of the device.
The example in figure 11 is a dummy battery, containing the power handling circuit, it has built-in polarity reversing switch and only requires connection through a ribbon connector to an antenna or series of antennas, regardless of the antenna size. So long as there is sufficient energy available from the ambient RF signals, this device would not ordinarily require further intervention.
5 Design of Energy Harvester
6. Circuit Simulation Results
The energy harvester can be retro fitted into a standard battery case. The connection is made via the anode and cathode that are arranged to fit into an existing battery holder of the user device. With the exception of the end caps, the remaining components are encapsulated in a non-conducting material which may also be bio-degradable. An example material is thermally treated pressed wood pulp, which is a bi-product from the paper industry. The example in figure 9 has the
The AC voltage needs to be rectified and increased to a suitable level before this can be used for powering a device. Fig 11 shows such a circuit that includes a rectifier and a multiplier circuit using schottky diode, IN5817. Also shown are the circuit simulation results for the resulting in sufficient voltage. This circuit gives an average power efficiency of 50% [7].
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Figure 12: Circuit diagram for bridge rectifier and voltage doubler AC/DC convertor
DC Output Voltage
Figure 13: Simulated waveforms for the bridge rectifier
7. Conclusion To get sufficient power for a small electronic device, requiring for example 5μW [8] , the device should be within 100km of a high power transmitter or 20km from a low power Medium Wave transmitter. There may be more than one transmitter contributing to the received power and the effect is cumulative The RF energy harvester has hence great potential in powering small devices essentially replacing batteries and eliminating the need for replacement or un/expected cessation. This enables placement of WSN nodes in inaccessible locations and for long duration deployments. This type of energy harvester would also be much less sensitive to performance degradation at low temperatures, unlike conventional batteries. The power requirement of a wireless sensor node largely depends on its application and configuration, but this can be reduced by using power management algorithms and energy efficient routing protocols [9]-[12]. Harvested energy is expected to power the operations of the entire wireless sensor node, hence integrating the harvester in the design of a wireless sensor would further optimise the antenna design for the sensor node.
