Ana Collado

Ambient RF Energy-Harvesting Technologies for Self-Sustainable Standalone Wireless Sensor Platforms

In this paper, various ambient energy-harvesting technologies (solar, thermal, wireless, and piezoelectric) are reviewed in detail and their applicability in the development of self-sustaining wireless platforms is discussed. Specifically, far-field low-power-density energy-harvesting technology is thoroughly investigated and a benchmarking prototype of an embedded microcontroller-enabled sensor platform has been successfully powered by an ambient ultrahigh-frequency (UHF) digital TV signal (512-566 MHz) where a broadcasting antenna is 6.3 km away from the proposed wireless energy-harvesting device. A high-efficiency dual-band ambient energy harvester at 915 MHz and 2.45 GHz and an energy harvester for on-body application at 460 MHz are also presented to verify the capabilities of ambient UHF/RF energy harvesting as an enabling technology for Internet of Things and smart skins applications.

Rectenna design and optimization using reciprocity theory and harmonic balance analysis for electromagnetic (EM) energy harvesting

A rectenna design methodology combining electromagnetic (EM) simulation and harmonic balance (HB) analysis is presented. It consists of applying reciprocity theory to calculate the Thevenin equivalent circuit of the receiving antenna and optimizing the rectifying circuit parameters using HB analysis. The method is demonstrated by designing a 2.45-GHz rectenna based on a square aperture-coupled patch antenna with dual linear polarization. A compact implementation is achieved by etching a cross-shaped slot on the patch surface leading to a 32.5% patch side reduction. Voltage-doubling circuits convert the received RF power from each port to dc permitting the rectenna to receive arbitrarily polarized signals. The circuit is optimized for low input power densities and a simulated maximum efficiency of 38.2% was obtained for 1.5 nWcm-2 input RF power density at 2.43 GHz.

Conformal Hybrid Solar and Electromagnetic (EM) Energy Harvesting Rectenna

This paper presents the design of a cost effective, hybrid energy harvesting circuit combining a solar cell and a rectenna capable to harvest ambient electromagnetic energy. Electromagnetic analysis is used to model and optimize the designed circuits in order to allow the antenna and solar cell to share the same area leading to a compact structure. Nonlinear harmonic balance optimization is used to maximize the RF-to-DC conversion efficiency of the rectenna circuitry in the presence of the solar cell and both wideband and multiband topologies are presented. Furthermore, a low cost and flexible polyethylene terephthalate PET substrate and a flexible amorphous silicon solar cell are chosen, providing for both a low cost and conformal structure. A prototype able to generate a maximum DC power of 56 mW when the solar cell is illuminated with 100 mW/cm2 solar irradiance, and a dual band rectenna demonstrating an efficiency of 15% around 850 MHz and 1850 MHz when illuminated by a microwave signal of available power is presented.

Wireless Power Transmission: R&D Activities Within Europe

Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery-free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections. WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe.

Design of a 2.45 GHz rectenna for electromagnetic (EM) energy scavenging

A compact dual polarized rectenna operating at 2.45 GHz is presented. It consists of a square aperture coupled patch antenna with a cross shaped slot etched on its surface that permits a patch side reduction of 32.5%. The patch size is 3.4 cm by 3.4 cm. The antenna is dual linearly polarized with each orthogonal polarization received by an appropriately placed coupling slot. The received signal from each slot output is rectified by a voltage doubling circuit and the doubler DC output signals are combined allowing the rectenna receive signals of arbitrary polarization. The circuit is optimized for low input power densities using harmonic balance. Simulated rectifier maximum RF-to-DC conversion efficiency values of 15.7% and 42.1% were obtained for input available power levels of -20 dBm and -10 dBm respectively at 2.45 GHz. The measured results are in agreement with the simulation.

No Battery Required: Perpetual RFID-Enabled Wireless Sensors for Cognitive Intelligence Applications

Over the last decade, radio frequency identification (RFID) systems have been increasingly used for identification and object tracking due to their low-power, low-cost wireless features. In addition, the explosive demand for ubiquitous rugged low-power, compact wireless sensors for Internet-of-Things, ambient intelligence, and biomonitoring/ quality-of-life application has sparked a plethora of research efforts to integrate sensors with an RFID-enabled platform. The rapid evolution of large-area electronics printing technologies (e.g., ink-jet printing and gravure printing) has enhanced the development of low-cost RFID-enabled sensors as well as accelerated their large-scale deployment. This article presents a brief overview of the recent progress in the area of RFID-based sensor systems and especially the state-of-the-art RFID-enabled wireless sensor tags realized through the use of ink-jet printing technology.

Optimum behavior: Wireless power transmission system design through behavioral models and efficient synthesis techniques

In this article, the different challenges and open issues regarding WPT systems have been presented considering both the transmitting and receiving ends. The challenges in the receiving side include the accurate modeling of the rectifying elements, the adequate selection of the rectifier topology, and the joint optimization of antenna and rectifier circuit. The challenges in the transmitting side are mainly in the improvement of the dc-to-RF conversion efficiency and in the selection of the optimum transmitted signal waveform. The final goal of most of these challenges is to take advantage of the nonlinear nature of the rectifying elements in order to maximize the RF-to-dc conversion efficiency in the receiving end of WPT systems.

Improving wireless power transmission efficiency using chaotic waveforms

The use of chaotic signals as an optimal source for wireless power transmission as well as electromagnetic energy harvesting is proposed. The improved performance of rectifier circuits, in terms of higher RF to DC conversion efficiency, when using chaotic signals in comparison to one-tone signals is demonstrated. A 433MHz chaotic generator and a rectifier circuit are designed and implemented in order to demonstrate the improved efficiency of the system when using chaotic waveforms.

Boosting the Efficiency: Unconventional Waveform Design for Efficient Wireless Power Transfer

Traditionally, wireless power is delivered through single-carrier, continuous-wave (CW) signals. Most research efforts to enhance the efficiency of wireless power transfer systems have been confined to the circuit-level design. However, in recent years, attention has been paid to the waveform design for wireless power transmission. It has been found that signals featuring a high peak-to-average power ratio (PAPR) can provide efficiency improvement when compared with CW signals. A number of approaches have been proposed, such as multisines/multicarrier orthogonal frequency division multiplex (OFDM) signals, chaotic signals, harmonicsignals, ultrawideband (UWB) signals, intermittent CW (ICW) signals, or white-noise signals. This article reviews these techniques with a focus on multisines/multicarrier signals, harmonic signals, and chaotic signals. A theoretical explanation for efficiency improvement is provided and accompanied by experimental results. Circuit design considerations are presented for the receiver side, and efficient transmission architectures are also described with an emphasis on spatial power combining.

DTV band micropower RF energy-harvesting circuit architecture and performance analysis

A design architecture for a micropower RF energy-harvesting in the Digital TV (DTV) band is presented. The rectifying antenna (rectenna), given a single tone excitation at 550 MHz, has a measured conversion efficiency of 0.4% for −40 dBm input and 18.2% for −20 dBm input, respectively. A DC-DC boost converter circuit is designed and fabricated to be used along with this rectenna. The DC-DC boost converter circuit is able to operate with voltages as low as 400mV DC while the measured DC-DC conversion efficiency is at least 11.3%.