A proposal to improve the feasibility of widespread RF ambient energy harvesting Olivia McCombe s3423574

ABSTRACT

Ambient radio frequency (RF) is an ever-present free power source in urban areas, however its low energy levels currently prevent its widespread uptake. Following efforts to concurrently harvest RF energy from an expanded frequency range and to improve conversion efficiency, this is a proposal to firstly – investigate the current potential for ambient RF energy to power small-device batteries, secondly – clarify the effect of rectenna band number on true conversion efficiency, and finally – determine whether there is a diode type which could improve the conversion efficiency of dual-resonant circuits at very low energy levels.

LITERATURE REVIEW As a byproduct of the growth in wireless communications in the last decade, there has been an increase in the existing ambient radio frequency (RF) energy, which has prompted some groups of researchers to improve its conversion to usable power [1]. RF energy is collected by an antenna and converted to direct current (DC) power using a rectifier circuit – which as a system is known as a ‘rectenna’. The main drawback of RF energy is that it typically exists at very low levels [2]. Therefore, research in the last five years has focussed on increasing rectenna power output by expanding the frequency range from which rectennae concurrently harvest and improving their conversion efficiency. The significance of studies in this area is ultimately determined by whether they test rectenna prototypes at realistically low and variable RF energy levels: some studies have gone so far as to test with energy levels from urban surveys that they have conducted [1]-[3], while others with a progressive focus on dual-band rectennae are undermined by their failure to analyse output over a broad energy input range [4][5]. In terms of operating range, single and dual-band rectennae have been operated at as low as -29 to -30 dBm [2][3][6], with the study by Pinuela et al. claiming that the dual-band type lowered the operating range by 4 dBm and was able to operate at half the measured energy levels surveyed from around Greater London. Kim et al. confirmed that that power level could operate a battery-free sensor [2], but in order to define the feasibility of RF energy harvesting’s uptake beyond Wireless Power Transmission (WPT) (battery-free) technologies, it would be useful to determine how long it would take to charge a low power device’s battery at those input power levels. Another area lacking certainty is multiband rectenna efficiency: there were two contrasting positions on it from the studies surveyed: in comparison to a single-band rectenna, Pinuela et al. [3] and Liu et al. [7] asserted that a multiband rectenna was less efficient (with the latter noting that as the number of bands increased, the conversion efficiency ‘breakdown’ occurred at a lower input power), while Kim et al. [2], Kuhn et al. [8], and Shariati et al. [1] stated that it was more efficient (with Kuhn et al. claiming the highest conversion efficiency in the literature of 84% for a four-band rectenna at 5.8 dBm [8]). This difference may be at least partially due to a general lack of specification about the type of efficiency mentioned: the former studies could have calculated efficiency by dividing output power by the summed input power from all bands (true efficiency), while the latter divided by input power from one band (effective efficiency), yielding higher results. Given the potential of multiband rectennae, it is necessary to clarify the relationship between rectenna band number, conversion efficiency, and breakdown efficiency. Alternatively, strategies to improve rectifier circuit efficiency have included implementing: a voltage doubler [9], duty cycles tailored to capacitor-wireless sensor network (WSN) node relationships [10] or lower power input [2], a complementary metal-oxide-semiconductor (CMOS) rectifying charge pump [11], a pseudomorphic high-electron mobility transistor (pHEMT) to overcome the multiband breakdown efficiency issue [7], and eliminating the bandpass filter by impedance-matching the circuit to the antenna frequency [4][12] – which has recently been extended to dual resonances [1]. This final technique is very interesting, with the potential for

combining resonant circuits for other frequency bands to improve conversion efficiency, if appropriate diodes are chosen, particularly ones sensitive to low power inputs (-20 dBm) [1].

RESEARCH QUESTIONS

1. How long would it take to charge a typical lithium-ion smartphone battery with a rectenna receiving the input energy levels recorded in the surveys previously considered [1]-[3] at a range of conversion efficiencies from 20-50%? In other words, what is the value of this energy level in ampere-hours (Ah)?

2. What is the exact relationship between rectenna band number, true conversion efficiency, and breakdown efficiency?

3. As part of a dual resonant circuit strategy, is there a diode type which would be more sensitive to power inputs below -20 dBm, thus improving conversion efficiency from that level?

RATIONALE Ambient energy sources are worth pursuing because they are free and they negate a need for additional power generation, which is already a burden on our environment and the atmosphere [1]. Ambient energy is particularly useful with the emergence of WPT technologies and WSNs, in which case small devices such as sensors can operate independently in locations where traditional power sources would be impractical [2]. Among ambient energy sources, RF is competitive for being ever-present in urban areas and for the smallness of the rectenna and its potential to be embedded in other structures [3]. CONTEXT IN CURRENT BODY OF KNOWLEDGE Research question 1 would be approached by taking the averages and the modes from the upper and lower halves of the energy levels from the surveys mentioned previously [1]-[3], reducing them by 50-80% (representing the average conversion efficiencies recorded from the literature), at intervals of 5%, delivering them at a constant rate to a typical lithium-ion smartphone battery, and measuring the time it took for the battery to charge from 0-100%. This would give some idea of how long it would take for existing rectennae to charge a small-device battery with real input energy levels from cities. Research question 2 would be approached by building basic otherwise-identical prototypes of single, dual, three and four-band rectennas as represented by the relevant literature [1]-[3][7][8], inputting energy levels from -60 dBm to +20dBm, and measuring true conversion efficiency. These measurements would then be compared across the range of energy levels for different rectenna bands to clarify whether band number affects conversion efficiency and the trajectory of conversion efficiency across the input energy level range. Research question 3 would be approached firstly with a literature review of the diode types most likely to increase sensitivity to energy input levels below -20 dBm. Subsequently, a dual resonant rectenna would be built following the methodology of Shariati et al. [1] with the option of switching out the diodes. Each selected diode type would be tested in the rectenna with an energy input range of -20 dBm to -60 dBm, in order to confirm whether any of them increased the conversion efficiency of this rectenna type.                 OUTCOMES The outcomes of this research would define the current value of ambient RF energy, by determining whether real low input energy levels are sufficient to recharge a small battery-powered device within a reasonable amount of time. The outcomes could also clarify the potential for rectenna output to be improved, by determining whether there is an optimal implementation of multi-band rectennae in relation to true efficiency and breakdown efficiency, and furthermore whether the conversion of very low energy input levels could be converted more efficiently with particular diode types in dual-resonant rectennae.

References [1] N. Shariati et al., “Multi-service highly sensitive rectifier for enhanced RF energy scavenging”, Scientific Reports, vol. 5, May 2015. [2] S. Kim et al., “Ambient RF energy-harvesting technologies for self-sustainable standalone wireless platforms”, Proceedings of the IEEE, November 2014, vol. 102, no.11, pp. 1649-1666. [3] M. Pinuela et al., “Ambient RF energy harvesting in urban and semi-urban environments”, IEEE Transactions on Microwave Theory and Techniques, vol. 61, no. 7, July 2013, pp. 2715-2726. [4] P. Kim, G. Chaudhary, & Y. Jeong, “A dual-band energy harvesting using frequency limited dual-band impedance matching”, PIER, vol. 141, 2013, pp. 443–461. [5] B. Li, “An antenna co-design dual band RF energy harvester”, IEEE Trans. Circuits Syst. I, Reg. Papers. vol. 60, 2013, pp. 3256–3266. [6] Collado, A. & Georgiadis, A., “Conformal hybrid solar and electromagnetic (EM) energy harvesting rectenna”, IEEE Trans. Circuits Syst. I, Reg. Papers. vol. 60, 2013, pp. 2225–2234. [7] Z. Liu, et al., “Enhanced dual-band ambient RF energy harvesting with ultra-wide power range”, IEEE Microwave and Wireless Components Letters, vol. 25, no. 9, September 2015. [8] V. Kuhn et al., “A multi-band stacked RF energy harvester with RF-DC efficiency up to 84%”, in IEEE Transactions on Microwave Theory and Techniques, vol. 63, no. 5, May 2015. [9] N. M. Din, C. K. Chakrabarty, A. Bin Ismail, K. K. A. Devi, & W. Y. Chen, “Design of RF energy harvesting system for energizing low power devices”, PIER, vol. 132, 2012, pp. 49–69. [10] R. Shigeta et al., “Ambient RF energy harvesting sensor device with capacitor-leakage-aware duty cycle control”, IEEE Sensors Journal, vol.13, no.8, August 2013, pp. 2973 – 2983. [11] W. Wang et al., “A high-efficiency full-wave CMOS rectifying charge pump for RF energy harvesting applications”, Microelectronics Journal, August 2015. [12] H. Sun, Y. X. Guo, M. He, & Z. Zhong, “Design of a high-efficiency 2.45-GHz rectenna for low-input-power energy harvesting”, IEEE Antennas Wireless Propag. Lett. Vol. 11, 2012, pp. 929–932.