Traditional power supply cords have become less important because they prevent large-scale utilization and mobility. In addition, the use of batteries as a substitute for power cords is not an optimal solution because batteries have a short lifetime, thereby increasing the cost, weight, and ecological footprint of the hardware implementation. Their recharging or replacement is impractical and incurs operational costs. Recent progress has allowed electromagnetic wave energy to be transferred from power sources (i.e., transmitters) to destinations (i.e., receivers) wirelessly, the so-called wireless power transfer (WPT) technique. New developments in WPT technique motivate new avenues of research in different applications. Recently, WPT has been used in mobile phones, electric vehicles, medical implants, wireless sensor network, unmanned aerial vehicles, and so on. This review highlights up-to-date studies that are specific to near-field WPT, which include the classification, comparison, and potential applications of these techniques in the real world. In addition, limitations and challenges of these techniques are highlighted at the end of the article.

Opportunities and challenges for near-field wireless power transfer: A review

This paper introduces a novel compact ultralightweight multiband RF energy harvester fabricated on a paper substrate. The proposed rectenna is designed to operate in all recently released LTE bands (range 0.79–0.96 GHz; 1.71–2.17 GHz; and 2.5–2.69 GHz). High compactness and ease of integration between antenna and rectifier are achieved by using a topology of nested annular slots. The proposed rectifier features an RF-to-dc conversion efficiency in the range of 5%–16% for an available input power of −20 dBm in all bands of interest, which increases up to 11%–30% at −15 dBm. The rectenna has been finally tested both in laboratory and in realistic scenarios featuring a superior performance to other state-of-the-art RF harvesters on flexible substrates.

A Novel Ultra-Lightweight Multiband Rectenna on Paper for RF Energy Harvesting in the Next Generation LTE Bands

A dual-port triple-band L-probe microstrip patch rectenna design for ambient RF energy harvesting using the GSM-900, GSM-1800, and UMTS-2100 bands is described. The compact dual-port L-probe patch antenna is implemented by stacking two single-port patch antennas back to back. Each port can independently harvest RF signal from a half-space with gain greater than 7 dBi, and together with both ports in a dc combining configuration, the antenna can acquire RF energy from nearly all directions. We also provide a design for a high-efficiency triple-band rectifier operating at GSM-900, GSM-1800, and UMTS-2100, which is replicated on each port and concatenated together to allow dc combining and near doubling of the output dc voltage. Measurement results show that our prototyped dual-port triple-band rectenna can receive RF power from nearly all directions with an efficiency of greater than 40% and an output voltage of more than 600 mV when the power density is greater than 500 $\mu \mathrm{W}/\mathrm{m}^{2}$ .

A Dual-Port Triple-Band L-Probe Microstrip Patch Rectenna for Ambient RF Energy Harvesting

Efficiency in RF energy harvesting systems: A comprehensive review

Magnetically coupled resonance wireless power transfer systems (MCR WPT) have been developed in recent years. There are several key benefits of such systems, including dispensing with power cords, being able to charge multiple devices simultaneously, and having a wide power range. Hence, WPT systems have been used to supply the power for many applications, such as electric vehicles (EVs), implantable medical devices (IMDs), consumer electronics, etc. The literature has reported numerous topologies, many structures with misalignment effects, and various standards related to WPT systems; they are usually confusing and difficult to follow. To provide a clearer picture, this paper aims to provide comprehensive classifications for the recent contributions to the current state of MCR WPT. This paper sets a benchmark in order to provide a deep comparison between different WPT systems according to different criteria: (1) compensation topologies; (2) resonator structures with misalignment effects; and, (3) electromagnetic field (EMF) diagnostics and electromagnetic field interference (EMI), including the WPT-related standards and EMI and EMF reduction methods. Finally, WPT systems are arranged according to the application type. In addition, a WPT case study is proposed, an algorithm design is given, and experiments are conducted to validate the results obtained by simulations.

Magnetically Coupled Resonance WPT: Review of Compensation Topologies, Resonator Structures with Misalignment, and EMI Diagnostics

This letter presents a novel dual-band rectifier with extended power range (EPR) and an optimal incident RF power strategy in the settings where the available RF energy fluctuates considerably. It maintains high power conversion efficiency (PCE) in an ultra-wide input power range by adopting a pHEMT in the proposed topology. Simultaneous RF power incident mode is proposed and preferred to the traditional independent mode for multi-band harvesting. Measured results show that more than 30% PCE is obtained with input power ranging from $-15~{\rm dBm}$ to 20 dBm and peak PCE of 60% is maintained from 5 to 15 dBm. Positive power gain is achieved from $-20~{\rm dBm}$ to more than 10 dBm. Investigation about the effect of RF power incident ratio on dual-band harvesting's performance is presented and it provides a good reference for future multi-band harvesting system design.

Enhanced Dual-Band Ambient RF Energy Harvesting With Ultra-Wide Power Range

This paper presents a systematic design of a high-efficiency magnetic resonant wireless power transfer (MR-WPT) system for biomedical implants based on the unconventional multisine (MS) transmission waveform. The MS waveform featuring a high peak to average power ratio can boost the system efficiency with an elaborate design. Optimizations on flexible coil, system model, and rectifier design are all considered to strengthen its efficiency improvement as well as eliminate its drawbacks. With all the proposed optimizations implemented, the MS waveform has been applied for the first time in practical MR-WPT system for in vivo power delivery. A 6.78-MHz MR-WPT system with 18-mm implant depth is realized for wirelessly powering a spinal cord stimulator. It achieves an overall system efficiency (both coils and rectifier included) of 50.7% with a three-tone 230-kHz tone spacing MS waveform postimplantation in the rodent model. Low specific absorption rate and the tissue temperature rising from electromagnetic fields in the body are also verified to guarantee a safe and practical wireless power link for biomedical implants.

In Vivo High-Efficiency Wireless Power Transfer With Multisine Excitation

This paper proposes a triple-band rectifying circuit for ambient RF energy harvesting, which can be served as a power supply for the central unit in Wireless Body Sensor Networks (WBSN). It fully utilizes the RF energy in ambient and converts it into DC power with high efficiency. It covers the public telecommunication bands of GSM-900, GSM-1800 and UTMS-2100, which are measured to hold the major RF energy in ambient. Simultaneous multiband energy collection helps in achieving high conversion efficiency, which is around twice that of the conventional separate single-band inputs. Measurement results show that 31.2% RF-to-DC conversion efficiency and 159mV output DC voltage are gained over a resistive load of 3.3 KΩ with an input RF power of −20dBm.

High-efficiency triple-band ambient RF energy harvesting for wireless body sensor network

A rapid design approach to achieve the optimal efficiency in a magnetic resonant wireless power transfer (MR-WPT) system is presented. A concise design guideline is provided to determine the optimal efficiency condition and related coupling coefficients. Experimental validations are demonstrated by both simulation and measurement results of a 6.78 MHz MR-WPT system prototype with the maximum efficiency of 76.1% at a 50 mm distance between transmitter and receiver.

Rapid design approach of optimal efficiency magnetic resonant wireless power transfer system

This letter presents a reconfigurable diode topology which obtains the independently tuned ultra-low threshold voltage and large breakdown voltage simultaneously. Measured I-V curves show that the proposed topology obtains both ultra-low threshold voltage (250 mV) and tunable breakdown voltage up to −51 V. By applying the topology in rectifier design, high efficiency of above 50% can be maintained within a wide power range from 0.1 W to above 10 W in an inductive wireless power transfer (WPT) test environment. It provides a practical diode topology design for rectifiers to maintain high efficiency for large power variation scenarios such as coil misalignment.

Zhongtao Liu

Papers

Enhanced Dual-Band Ambient RF Energy Harvesting With Ultra-Wide Power Range

In Vivo High-Efficiency Wireless Power Transfer With Multisine Excitation

High-efficiency triple-band ambient RF energy harvesting for wireless body sensor network

Rapid design approach of optimal efficiency magnetic resonant wireless power transfer system

A Reconfigurable Diode Topology for Wireless Power Transfer With a Wide Power Range