RF Wireless Energy Transfer driving Sustainable IoT
The Internet of Things (IoT) revolution calls for innovative technologies to meet long-term operational demands and optimize resource utilization while ensuring performance and connectivity guarantees. Millions of IoT devices monitor our well-being, measure environmental pollution, track assets, and assist in managing natural resources. However, as IoT applications expand, maintaining the uninterrupted operation of a massive number of battery-powered devices becomes increasingly challenging. In fact, due to a small form factor, most IoT devices can only carry a tiny battery whose effective lifetime does not match that of the device’s electronics. As a result, battery replacements may outpace the growth of connected devices, driving up business operational costs and environmental pollution if the electronic waste is not disposed of correctly. Moreover, the maintenance of IoT devices deployed in remote areas, embedded in civil infrastructure, or integrated into medical implants represents a risky and possibly costly operation.
Energy-saving and energy-repletion mechanisms are necessary for enabling the perpetual autonomy of future sustainable IoT networks. Energy-saving mechanisms aim to minimize the energy needed for completing tasks such as sensing, processing, and communication without significantly degrading the system performance. In contrast, energy repletion comprises energy harvesting (EH) techniques for recharging the batteries by exploiting ambient or dedicated energy sources, also known as power beacons (PBs). We focus on the latter here, which refers to wireless energy transfer (WET) and can be achieved through various dedicated sources such as light, electrostatic, induction, magnetic, acoustic, piezoelectric, thermal, or radio-frequency (RF), each with its strengths and limitations. The choice of a specific WET technology for recharging IoT devices depends on multiple factors, such as required range, energy density, hardware complexity, cost, regulatory constraints, and operational environment, to ensure a viable and efficient energy supply. Nevertheless, RF-WET may be particularly appealing for charging ultra-low-power IoT devices. This is because it allows a radio transmitter to broadcast energy over relatively long distances and to charge multiple devices simultaneously, even in non-line-of-sight conditions. Moreover, RF-EH circuit form factors and manufacturing costs allow seamless integration of this technology in existing devices and enable dual EH from both dedicated and ambient energy sources.
Figure 1: Block diagram of the architecture of an RF-WET system and main energy consumption/loss sources.
Unfortunately, the end-to-end power transfer efficiency (PTE) of RF-WET technology is inherently low due to the many energy consumption/loss sources, as illustrated in Figure 1, and there are safety-related apprehensions. These have motivated mostly customised low-power IoT charging applications relying on energy beamforming and waveform optimisation, distributed and massive antenna systems, smart reflect arrays and metasurfaces, and flexible PBs (i.e., moving PBs and PBs with rotary antennas). Note that next-generation base stations may be designed and configured to support RF-WET with predefined performance guarantees in addition to legacy and enhanced data transmission services. However, this might be only possible in highly dense network deployments since system efficiency decreases exponentially with the charging distance. In other scenarios, the deployment of separate PBs is needed. Although this entails higher upfront costs, overall network costs can be reduced compared with traditional IoT setups, especially as the number of IoT devices increases and considering that inaccurate hardware power profiling, battery imperfections, and/or operating conditions can significantly and unexpectedly shorten battery lifespans.
Still, reducing the network’s operational costs and carbon footprint is crucial and may be enabled by self-sustained PBs, which can autonomously harvest, manage, and exchange energy. This agrees with the vision of sustainable RF-WET, illustrated in Figure 2:
Sustainable RF-WET synergises economic prosperity, social equity, and environmental health to cope with current and future sustainable IoT quality-of-service charging goals, including minimum wastage of resources.
Figure 2: Towards sustainable RF-WET.
PBs have a larger form factor and better hardware/connectivity capabilities than IoT devices, which allows them to incorporate a more efficient and higher-power EH circuitry that could possibly harvest from multiple ambient sources simultaneously, hence reducing the uncertainty of the total harvested energy. Such green PBs (gPBs) contribute to sustainable RF-WET in economic and environmental dimensions since the autonomous generation of electricity reduces the overall costs and the carbon footprint associated with grid-based energy distribution. Meanwhile, conveniently deploying multiple gPBs is crucial to eliminate blind spots in the network, distribute the energy according to the application requirements, and contribute to social equity by promoting ubiquitous wireless charging and enabling new use cases.
Sustainable RF-WET still faces numerous challenges that must be urgently addressed. The following are open questions underscoring the evolving landscape of RF-WET and inviting further exploration and innovation in the pursuit of a more sustainable and resilient future:
- How can gPBs be deployed efficiently, considering devices’ energy demands, ambient energy availability, and the availability (or lack thereof) of channel state information?
- What strategies and protocols can optimise charging during periods of scarce or unavailable ambient energy?
- How to assess the carbon footprint of RF-WET across its entire life cycle?
- When are RF-WET-based solutions preferred to contemporary and other competing technologies?
- What is the most suitable RF spectrum for operation?
- What technologies and protocols can help maximise the end-to-end performance given the complex interdependencies of nonlinear components within the RF-WET chain?
- What are the implications of security breaches on system performance, and how can cyber threats be mitigated?
- Lastly, what defines a truly sustainable and high-performing RF-WET system, and what metrics should guide its design?
Further Reading
O. López et al., “Energy-Sustainable IoT Connectivity: Vision, Technological Enablers, Challenges, and Future Directions,” in IEEE Open Journal of the Communications Society, vol. 4, pp. 2609-2666, 2023.
O. Rosabal, et al., “Sustainable RF Wireless Energy Transfer for Massive IoT: Enablers and Challenges,” in IEEE Access, vol. 11, pp. 133979-133992, 2023.
O. López and H. Alves, Wireless RF Energy Transfer in the Massive IoT Era: Towards Sustainable Zero-Energy Networks, John Wiley & Sons 2021.
O. López, et al., “High-power and safe RF wireless charging: Cautious deployment and operation,” IEEE Wireless Communications, vol. 31, no. 6, pp. 118-125, Dec. 2024.
O. López, et al., “Massive Wireless Energy Transfer: Enabling Sustainable IoT Toward 6G Era,” IEEE Internet of Things Journal, vol. 8, no. 11, pp. 8816-8835, 2021.
A. Azarbahram, O. López and M. Latva-Aho, “Waveform Optimization and Beam Focusing for Near-Field Wireless Power Transfer With Dynamic Metasurface Antennas and Non-Linear Energy Harvesters,” in IEEE Transactions on Wireless Communications, vol. 24, no. 2, pp. 1031-1045, Feb. 2025.
O. López, et al., “Massive MIMO With Radio Stripes for Indoor Wireless Energy Transfer,” in IEEE Transactions on Wireless Communications, vol. 21, no. 9, pp. 7088-7104, Sept. 2022.
A. Azarbahram, et al., “Deep Reinforcement Learning for Multi-User RF Charging with Non-linear Energy Harvesters,” in IEEE Globecom, Aug. 2024.
O. Rosabal, et al., “Average Local EMF Exposure and Power Consumption of a RIS-assisted WET System,” ISWCS, Rio de Janeiro, Brazil, 2024, pp. 1-6.
A. Azarbahram, et al., “Energy Beamforming for RF Wireless Power Transfer with Dynamic Metasurface Antennas,” in IEEE Wireless Communications Letters, vol. 13, no. 3, pp. 781-785, Mar. 2024.
O. López, et al., “CSI-Free Rotary Antenna Beamforming for Massive RF Wireless Energy Transfer,” in IEEE Internet of Things Journal, vol. 9, no. 10, pp. 7375-7387, May. 2022.
O. Rosabal, O. López, and H. Alves, “Energy-Efficient Analog Beamforming for RF-WET With Charging Time Constraint,” in IEEE Transactions on Vehicular Technology, vol. 73, no. 8, pp. 12160-12165, Aug. 2024.
About the authors
Associate Professor
Onel L. A. López
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PhD Researcher