Abstract This paper proposes a novel harvester for sensors powering by spurious electromagnetic emissions from compact fluorescent lamps. The proposed device is based on a magnetic coupling and consists of a resonant loop and an RF-to-DC rectifier. Experimental results are reported and discussed. It is shown that up to 0.61 mW can be generated from a 30 W compact fluorescent lamp, thus demonstrating that the proposed harvester is an optimum candidate for powering low-power sensors.
1. Introduction
Electromagnetic energy harvesting represents a promising solution for energizing low-power sensors and devices [1-16]. The research in this area has focused on the design of efficient rectennas (rectifying antennas) to harvest the energy associated with wireless communication systems by a far-field coupling. A rectenna consists of an antenna used to receive a free propagating electromagnetic wave and an Radio Frequency (RF)-to-Direct Current (DC) rectifier [17 - 20]. However, as a consequence of the very low power density introduced into the environment by wireless communication systems, it is difficult to harvest a DC power of interest for practical applications.
In this paper, we focus on the possibility of energizing sensors for home monitoring applications by spurious emissions of devices that can be found in a common household environment. In particular, we propose to exploit a near-field magnetic coupling to harvest spurious emissions of Compact Fluorescent Lamps (CFLs). In fact, experimental studies have demonstrated that CFLs emit a relatively strong electromagnetic field in the frequency range from a few tens to some hundreds of kHz [21-24]. More specifically, the device here presented consists of a resonant loop and a rectifier. Results obtained by coupling the proposed harvester with several CFLs differing by power consumption are presented and discussed.
2. Architecture of the Proposed Harvester and Experimental Results Photographs of the proposed harvester are given in Figs. 1a-1c; it consists of a resonator, an RF-to-DC bridge rectifier and a DC-pass lumped capacitor in shunt configuration with the load. The resonator is a spiral loop loaded by a lumped capacitor. The corresponding equivalent circuit is illustrated in Figs. 1d-1e; the resonator coupled with spurious emissions of a CFL is represented by an Alternate Current (AC) power generator.
Taking into account measurements reported in [25] which demonstrate that spurious emissions of CFL have a peak at about 50 kHz, the value of the lumped capacitor used to tune the resonator was accorded to resonate at the frequency of 50 kHz. In order to evaluate the power received from the resonator when it is coupled with a CFL (i.e., PAC), an LC matching network was optimized so as to match the impedance of the resonator to that of the HP E4411B spectrum analyzer (i.e., 50 Ω ). From these measurements, a peak at around 41 kHz of the coupling between the harvester and the analyzed CFLs was noticed. As a consequence, a new value of the lumped capacitor was set to tune the resonant frequency to 41 kHz: the new value was 1.89 µF and it was implemented by using two shunt capacitors of 1.5 µF and 390 nF.
Measurements of the spectrum of the signal received by the resonator when coupled with different types of lamps were performed. The CFLs used are shown in Figs. 2a-2b and they differ by the power consumptions which are: 5 W, 15 W, 23 W and 30 W. Fig. 2b shows the experimental setup adopted for measurements; experimental data obtained this way are given in Fig. 2c. A peak of 1.44 dBm at 41 kHz was measured for the 30 W lamp, while the total received power (channel power) calculated by integrating the spectrum over the bandwidth [20, 220] kHz is of about 4.1 dBm.
Results given in Fig. 2c were used to optimize the RF-to-DC rectifier. In more detail, the spectrum of the signal received in the case of coupling between the resonator and the 30 W lamp was used to optimize the LC matching so to satisfy at 41kHz the conjugate matching condition between the EM harvester (represented by the AC generator) and the rectifier. With reference to the use of the proposed harvester to power high-impedance sensors for home monitoring
applications, optimization were performed with a load of 1 MΩ. Finally, measurements of the DC power generated by coupling the prototype illustrated in Fig. 1 with a 30 W CFL
were performed. Corresponding results are illustrated in Fig. 3: for a load of 1 MΩ a DC power of about 0.61 mW was obtained.
(a) (b) Front view (c) Back view
(d) (e)
Fig. 1. (a)-(b)-(c) Photographs of the device proposed in this paper for power generation by spurious emissions of CFLs. (d) Schematic of the resonant loop when coupled with a CFL. (e) Schematic of the proposed device.
5. Conclusion
A novel device for the harvesting of spurious emissions of Compact Fluorescent Lamps (CFLs) has been presented. The proposed device is based on a near-field coupling and consists of a resonant loop and a rectifier. Measurements of the RF power harvested by CFL with different power consumptions are reported and discussed. From experimental tests, it is demonstrated that a DC power of 0.6 mW can be generated by coupling the proposed harvester with a 30 W CFL.
(a)
(b) (c)
Fig. 2: (a) From left, proposed EM harvester coupled with a: white Imperia (23 W), white Elettro GT (15 W), white Elettro GT (5 W). (b) Experimental setup used to estimate the RF spectrum received by the proposed harvester when coupled with the CFLs investigated in this paper (the photograph shows the case of a 30 W CFL). (c) Spectrum measured with the HP E4411B spectrum analyzer.
Fig. 3: Measured DC output power. Data obtained for different values of the load by coupling the proposed harvester with a 30 W CFL.
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