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Researchers at the National University of Singapore (NUS) have made a significant breakthrough in green energy technology with the development of a new battery-free system that harnesses ambient radiofrequency (RF) signals to power electronic devices and sensors. This innovative approach could revolutionise the way we power small electronic devices, eliminating the need for traditional batteries.
Wireless technologies such as Wi-Fi, Bluetooth, and 5G rely on RF signals to transmit data. Leveraging these ubiquitous signals, the NUS research team has created a prototype energy harvesting module capable of converting ambient RF signals into direct current (DC) voltage. This advancement holds promise for powering small electronic devices without the dependency on batteries.
RF energy harvesting is a critical technology for reducing battery reliance, extending device lifespans, and minimising environmental impact. It also enhances the practicality of wireless sensor networks and Internet of Things (IoT) devices, especially in remote locations where regular battery replacement is challenging.
Traditional RF energy harvesting technologies face significant hurdles. Ambient RF signals are often weak, typically less than -20 dBm, making it difficult for existing rectifier technologies to operate efficiently. Improving antenna efficiency and impedance matching can boost performance but at the cost of increased on-chip size, complicating integration and miniaturisation.
To overcome these challenges, NUS researchers, in collaboration with scientists from Tohoku University (TU) in Japan and the University of Messina (UNIME) in Italy, have developed a cutting-edge rectifier technology using nanoscale spin-rectifiers (SRs). This technology effectively converts ambient RF signals into DC voltage, even at power levels below -20 dBm.
The team optimised SR devices to operate across two configurations: a single SR-based rectenna functioning between -62 dBm and -20 dBm, and an array of ten SRs in series achieving 7.8% efficiency with a zero-bias sensitivity of approximately 34,500 mV/mW. By integrating the SR array into an energy harvesting module, they successfully powered a commercial temperature sensor at -27 dBm.
Professor Yang Hyunsoo from NUS, who led the project, highlighted the importance of this breakthrough: “Harvesting ambient RF signals is key to advancing energy-efficient devices, but current modules struggle with low power due to limitations in rectifier technology.”
He explained that traditional Schottky diodes, limited by thermodynamics, focus on antenna efficiency, whereas nanoscale spin-rectifiers offer a compact and efficient RF-to-DC conversion. The new SR technology significantly improves sensitivity and efficiency at much lower power levels compared to previous rectifiers.
The research team optimised several parameters of the SR, including perpendicular anisotropy, device geometry, and dipolar fields, to enhance performance. They also incorporated compact co-planar waveguides to couple RF power into the SRs, resulting in a high-efficiency, compact on-chip design. Their findings indicate that SR technology is potentially the most compact and efficient rectifier technology available.
Dr. Raghav Sharma, the Lead Author of the study, commented on the implications of the research. “Despite extensive global research, fundamental constraints in rectifier technology for low ambient RF power have remained unresolved. Our spin-rectifier technology surpasses current Schottky diode efficiency and sensitivity, setting a new benchmark for RF rectifier technologies and paving the way for advanced ambient RF energy harvesters and sensors.”
Looking ahead, the NUS team is exploring further integration of on-chip antennas to enhance the efficiency and compactness of SR technologies. They are also developing series-parallel connections to fine-tune impedance in large SR arrays, which could significantly increase the harvested RF power and potentially generate rectified voltages of several volts, reducing the need for DC-to-DC converters.
The researchers aim to collaborate with industry and academic partners to advance self-sustained smart systems based on SR rectifiers. This collaboration could lead to the development of compact on-chip technologies for wireless charging and signal detection systems.
