RF harvesting circuits have been demonstrated for more than 50 years, but only a few have been able to harvest energy from freely available ambient (i.e., non-dedicated) RF sources. In this paper, our objectives were to realize harvester operation at typical ambient RF power levels found within urban and semi-urban environments. To explore the potential for ambient RF energy harvesting, a city-wide RF spectral survey was undertaken from outside all of the 270 London Underground stations at street level. Using the results from this survey, four harvesters (comprising antenna, impedance-matching network, rectifier, maximum power point tracking interface, and storage element) were designed to cover four frequency bands from the largest RF contributors (DTV, GSM900, GSM1800, and 3G) within the ultrahigh frequency (0.3-3 GHz) part of the frequency spectrum. Prototypes were designed and fabricated for each band. The overall end-to-end efficiency of the prototypes using realistic input RF power sources is measured; with our first GSM900 prototype giving an efficiency of 40%. Approximately half of the London Underground stations were found to be suitable locations for harvesting ambient RF energy using our four prototypes. Furthermore, multiband array architectures were designed and fabricated to provide a broader freedom of operation. Finally, an output dc power density comparison was made between all the ambient RF energy harvesters, as well as alternative energy harvesting technologies, and for the first time, it is shown that ambient RF harvesting can be competitive with the other technologies.

Ambient RF Energy Harvesting in Urban and Semi-Urban Environments

More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC–DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.

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Micropower energy harvesting

More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC-DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.

This paper presents an overview of principles and requirements for powering wireless sensors by radio-frequency (RF) energy harvesting or transport. The feasibility of harvesting is discussed, leading to the conclusion that RF energy transport is preferred for powering small sized sensors. These sensors are foreseen in future Smart Buildings. Transmitting in the ISM frequency bands, respecting the transmit power limits ensures that the International Commission on Non-Ionizing Radiation Protection (ICNIRP) exposure limits are not exceeded. With the transmit side limitations being explored, the propagation channel is next discussed, leading to the observation that a better than free-space attenuation may be achieved in indoors line-of-sight environments. Then, the components of the rectifying antenna (rectenna) are being discussed: rectifier, dc-dc boost converter, and antenna. The power efficiencies of all these rectenna subcomponents are being analyzed and finally some examples are shown. To make RF energy transport a feasible powering technology for low-power sensors, a number of precautions need to be taken. The propagation channel characteristics need to be taken into account by creating an appropriate transmit antenna radiation pattern. All subcomponents of the rectenna need to be impedance matched, and the power transfer efficiencies of the rectifier and the boost converter need to be optimized.

RF Energy Harvesting and Transport for Wireless Sensor Network Applications: Principles and Requirements

Wireless sensor nodes (WSNs) are employed today in many different application areas, ranging from health and lifestyle to automotive, smart building, predictive maintenance (e.g., of machines and infrastructure), and active RFID tags. Currently these devices have limited lifetimes, however, since they require significant operating power. The typical power requirements of some current portable devices, including a body sensor network, are shown in Figure 1.

Energy Harvesting for Autonomous Wireless Sensor Networks

To assess the feasibility of ambient RF energy scavenging, a survey of expected power density levels distant from GSM-900 and GSM-1800 base stations has been conducted and power density measurements have been performed in a WLAN environment. It appears that for distances ranging from 25 m to 100 m from a GSM base station, power density levels ranging from 0.1 mW/m2 to 3.0 mW/m2 may be expected. First measurements in a WLAN environment indicate even lower power density values, making GSM and WLAN unlikely to produce enough ambient RF energy for wirelessly powering miniature sensors. A single GSM telephone however has proven to deliver enough energy for wirelessly powering small applications on moderate distances.

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Ambient RF Energy Scavenging: GSM and WLAN Power Density Measurements

Wireless batteries or rectennas- rectifying antennas - are conceived for converting wireless RF power into DC power. Although power conversion efficiencies exceeding 80% have been reported for high (20 dBm) rectenna input power levels, wireless batteries will be most beneficial at large distances from sources that will radiate at power levels limited by national and international regulations. Therefore, the challenge is in maximizing the power conversion efficiency of wireless batteries for low input power levels, say 0 dBm and below. By directly conjugate matching a rectifying circuit to a microstrip patch antenna, the need for a matching network between the two no longer exists. Thus the efficiency of the wireless battery will improve. Moreover, this matching technique automatically suppresses the reradiation of harmonics by the microstrip patch antenna since the harmonics will be mismatched. Thus, the impedance matching and filtering network encountered in traditional wireless battery designs has become obsolete. With the aid of analytical models developed for antenna and rectifier, single-layer, internally matched and filtered PCB rectennas have been designed for low input power levels. An efficiency of 52% for OdBm input power has been realized at 2.45 GHz for a wireless battery realized on FR4, showing an improvement - next to the size and complexity reduction - of more than 10% over a traditional rectenna design. A series connection of these wireless batteries is shown to be able to power a standard household wall clock.

Efficient, Compact, Wireless Battery Design

An accuracy improvement procedure for microstrip patch antenna design is presented. The classical cavity model of a patch antenna is discussed with emphasis on the fringe fields and the compensation for these fringe fields in the form of effective patch dimensions. The effective patch dimensions are improved using a single full-wave simulation, drastically increasing accuracy at the cost of only a small increase in computation time. Using the input impedance of the patch antenna determined via a full-wave simulator, effective patch dimensions are adjusted for each single mode, matching the cavity model results to the full-wave results. It is found that adjustment of non-resonant modes cause a shift in amplitude of the input impedance and a adjustment of a resonant mode results in a shift in frequency. The error in predicted impedance decreases from 75% to 5%. An example is given where an exotic input impedance is required, to match the antenna directly to a nonlinear system. This shows that this method results in fast, efficient, reliable and optimal microstrip patch antenna design.

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J.A.C. Theeuwes

Papers

Ambient RF Energy Scavenging: GSM and WLAN Power Density Measurements

Efficient, Compact, Wireless Battery Design

Accuracy Improvement of Cavity Model Effective Patch Dimensions using a Single Full-Wave Iteration