We describe a radio frequency (RF) energy harvester and power management circuit that trickle charges a battery from incident power levels as low as -20dBm. We designed the harvester for the 2.4 GHz RF band to leverage the ubiquity of energy that is produced by Wi-Fi, Bluetooth, and other devices. This paper reports on the design and current status of the harvester and compares our performance to other published results. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. This paper describes a novel rectenna design, boost converter, and battery charger for RF energy harvesting specifically tuned to this low-power regime. At -20dBm RF input power, the harvesting system (rectenna, boost converter, and battery charger) sources 5.8µJ into a rechargeable battery after 1 hour.

Abstract-We present a new system for measuring ambient RF energy in the 2.4GHz ISM band. This apparatus is intended to establish the feasibility of harvesting ambient RF energy to power emerging ultra-low-power sensors and microcontrollers. We simultaneously acquire RF measurements from a spatial and polarization diversity antenna system, with both a spectrum analyzer (frequency-selective but slow), and a log amp (wideband but fast), explain key tradeoffs in the measurement configuration, and present a post-processing algorithm which provides a reliable characterization of the RF energy available in the 2.4GHz ISM band. Preliminary results suggest enough energy is available to support a low duty cycle wireless sensor node system. An average RF power of 11nW is observed 10m away from a typical Wi-Fi access point in an office environment, suggesting the possibility of low duty cycle, wirelessly powered sensing and communication using a Bluetooth Low Energy (BLE) or another ultra low power uplink. I.INTRODUCTION While some battery-powered sensing nodes promise multiyear lifetimes, the inevitable labor costs to eventually replace batteries are often prohibitively expensive, even when amortized over long lifetimes. Energy harvesting is a promising way to supplant batteries, the most common sources of energy harvesting being photovoltaic, thermal, and vibrational energy sources, or a hybrid of these sources [1,2]. Human motion energy harvesting has also been shown to be realistic [3,4], which can provide energy for mobile or wearable sensor networks. Lately, the feasibility of harvesting ambient RF energy, particularly from the crowded 2.4GHz ISM band, has been the subject of speculation in the literature. Most reported results claim that there is insufficient 2.4GHz RF energy available to power sensors despite the increasing number of devices using this band (many of which are not WiFi devices) combined with the decreasing power requirements of such sensors [5,6]. Recent research has thus focused on digital television or other non-2.4GHz sources of ambient RF energy [7].

An RF energy harvesting and storage system is described that trickle charges a battery from incident power levels as low as -25 dBm referred to the feedpoint of an 8 dBi patch antenna. The circuit is optimized for the indoor ambient power range typically observed in the 2.4 GHz ISM band so that we can harvest the energy provided by nearby Wi-Fi, Bluetooth and other devices. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. We present several improvements to our prior work that significantly improve its performance, including a novel wideband multi-element antenna array, an improved boost converter, and a redesigned battery charger. At -25dBm RF input power, the new harvesting system sources 150μJ into a rechargeable battery after 1 hour. We believe that this work represents the lowest reported startup power yet achieved in battery-storage RF energy harvesting systems.

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