Technical and Ecological Limits of 2.45-GHz Wireless Power Transfer for Battery-Less Sensors

Wireless power transfer (WPT) is a possible alternative to batteries for supplying smart sensors. It is often described as 'greener' because it avoids the use of batteries. However, this claim usually lacks the proper assessment of the environmental impacts of WPT and is particularly questionable given the poor efficiency of wirelessly transmitting power over a few meters of distance. In this article, we design and thoroughly optimize a complete state-of-the-art WPT system and compare it to an equivalent battery-powered solution to assess whether it effectively represents an eco-friendly alternative. The proposed WPT system is a 2.45 GHz simultaneous wireless information and power transfer (SWIPT) system with beamforming and a high peak-to-average power ratio waveform for supplying a battery-less passive infrared sensor for room occupancy tracking. The final prototype of the smart sensor consumes 4μW} and can operate in steady state at 5m from the remote power head with an overall power transfer efficiency of 17μW/W and can cold-start at 3.5m. We carry out a life-cycle assessment (LCA) of the environmental impacts through four indicators, i.e., primary energy demand, global warming potential, terrestrial ecotoxicity, and freshwater consumption. Results show that for a 10-year lifetime, the WPT alternative has at least 5.5-10.3 × higher environmental impacts than its battery-powered equivalent. This demonstrates the importance of LCA during the design of IoT smart sensors and shows that the use of meter-range 2.45-GHz SWIPT should be limited to applications where conventional battery-powered systems cannot be used.