Joint research project

Radio Frequency Energy Harvesting for Wireless Rechargeable Sensor Networks.

Project leaders
Giuseppe Virone, Joseph Costantine
LIBANO - CNRS-L - National Council for Scientific Research of Lebanon
CNR- CNRS (Libano) 2017-2018
Engineering, ICT and technologies for energy and transportation
Thematic area
Engineering, ICT and technologies for energy and transportation
Status of the project

Research proposal

Wireless Sensor Networks (WSN) have recently found a wide variety of applications. One of their key features is the ability to perform inexpensive measurements on the environment. Some relevant examples of WSN applications include smart energy management and structural monitoring of both commercial and industrial buildings as well as smart grid and sub-station monitoring. As far as the food and agricultural security area is concerned, WSN are adopted for effective environmental monitoring, irrigation, greenhouse, livestock, cold chain control and traceability/security.
The increase of sensor boards' performance, such as computational power, CPU speed and memory, leads to higher energy consumption. Energy is usually provided by chemical batteries whose replacement is often not cost effective and in some cases not practical, not feasible, or risky, depending on the application. Instead of using batteries, some sensor networks harvest energy from external renewable energy sources such as solar radiation, vibrations, wind, or temperature variations.
However, such option is only valid when sensors are exposed to external energy sources. Indeed, in many applications (e.g. concrete-embedded sensors for structural health monitoring) sensors cannot be directly connected to external sources, therefore other solutions must be adopted.
In order to overcome these limitations the new concept of Wireless Rechargeable Sensor Networks (WRSN) has recently been introduced. In these networks, sensors exploit the energy carried by radio-frequency (RF) signals impinging on their antennas and convert them to direct current (DC) power by employing RF rectifying systems embedded within the sensors boards. In practice such RF signals are those used for data transfer among network nodes and typically emitted by nodes not suffering from power limitations. In general, the energy carried by RF electromagnetic signals is enough for powering small sensor devices which remain idle most of the time and wake up with very long duty cycles. By harvesting such energy the need of replacing sensor batteries can thus be avoided.
A critical parameter in the RF to DC power conversion is the conversion efficiency whose optimization constitutes a major challenge. By increasing this efficiency the performance of the whole network is maximized. RF harvesting efficiency is improved if an efficient antenna system as well as a rectifier circuit are designed so as to match with each other. The antenna system is responsible for collecting and delivering the maximum available power to a diode in the rectifier circuit.
In this framework we propose to investigate the energy conversion efficiency in WRSN and in particular on the design of the rectifying system. Although there are multiple techniques to design a rectifying system, the problem resides in designing the most efficient one that allows higher conversion efficiency, despite the variability of the RF signal power level. Moreover, the antenna systems must be compact, lightweight, circularly polarized, with sufficient gain to be integrated on various platforms.
Such challenges will be addressed by a team composed of very well-known experts in the fields of antennas, RF circuits design and wireless sensor networks. The design of the antenna and rectifying system will be executed at the American University of Beirut while the implementation and the measurement of the designed prototypes will be performed at CNR in Italy. The research effort will include four major tasks which will be executed sequentially, as described below:

Task 1: Determine the antenna topology
The first task in this research is focused on determining the antenna type and topology that can be implemented as the front end of the complete system. The antenna must be conformal, robust and miniature for sensor integration. Planar antennas can constitute a natural and popular choice for this type of application.

Task 2 Design and develop a rectifying circuit
Based on the antenna designed in Task 1, we will design and develop a new rectifying circuit that caters to the needs of the received signal. A typical rectifier circuit consists of several elements that include a non-linear component which is usually a Schottky barrier RF diode. An input Low Pass Filter (LPF), an output DC pass filter and a resistive load are part of the rectifier circuit. The diode element converts the input RF power at the fundamental frequency (f0) to DC and higher order harmonic components (2f0,3f0,etc...). The higher order harmonics generated by the diode can lead to low RF-DC conversion efficiency. In addition, these higher order harmonics can be re-radiated through the antenna, if the latter resonates at those frequencies, leading to Electromagnetic Interference (EMI). Given these hazardous impacts, generated harmonics must be suppressed, which is typically done through the input LPF and output DC pass filter. The rectifier circuit is then built and tested as a stand-alone circuit.

Task 3: Test the rectenna system in real scenarios
The successful antenna and rectifier circuit designs from Tasks 1 and 2 will then be assembled together in a full system. The system will be tested with input radio waves in order to evaluate the rectification performance. The whole system will also be tested in real scenarios and platforms. The final prototype will be tested on sensors that are integrated in food storage areas or other venues to verify its performance.

Task 4: Fine tune the design and integrate with additional energy sources
The system is checked for any difference with predicted data. If difference exists, fine tuning of the design is achieved and repetition of prototyping and measurements is executed until a complete match is verified. The system is also investigated for intgration with additional renewable power sources such as solar panels.

Research goals

This proposed research effort is focused on the possibility of powering rechargeable wireless sensor networks using ambient RF sources. Such objective meets well with the CNRS-CNR objectives of new techniques for renewable energy, aiding in food security monitoring and assisting in the management of natural resources. It proposes an alternative recharging method for embedded sensors and insures their continuous functioning by providing and achieving a regulatory powering ability. This research effort will result in a prototype that is composed of a compact planar antenna in conjunction with a rectifying circuit that composes a "rectenna" system. The rectenna integrated in a sensor node for harvesting transmitted RF signals and converting them into DC. The resulting DC power is used to charge the sensor.
To that extent the objectives of this research effort are summarized below:
I. Design planar antenna systems to harvest signals from RF transmitters
II. Design rectifying circuits that can operate at frequencies determined in objective 1
III. Optimize the operation of the rectifying circuit for various distances and directions of the transmitted signal
IV. Build the antenna and optimized rectifying circuit
V. Integrate the complete system composed of antenna and rectifying circuit on a sensor network platform in real scenario
VI. Test the full system
VII. Investigate possible integration with supplementary power sources
VIII. Fine tune the final model and propose recommenda

Last update: 27/11/2021