Tragedy of the Coulombs: Federating Energy Storage for Tiny, Intermittently-Powered Sensors

Untethered sensing devices have, for decades, powered all system components (processors, sensors, actuators, etc) from a single shared energy store (battery or capacitor). When designing batteryless sensors that are powered by harvested energy, this traditional approach results in devices that charge slowly and that are more error prone, inflexible, and inefficient than they could be. This paper presents a novel federated approach to energy storage, called UFoP, that partitions and prioritizes harvested energy automatically into multiple isolated smaller energy stores (capacitors). UFoP simplifies task scheduling, enables efficient use of components with differing voltage requirements, and produces devices that charge more quickly under identical harvesting conditions than a traditional centralized approach. We have implemented a UFoP reference design and conducted extensive experimental evaluation, including a short deployment. Our experimental results using an MSP430-based prototype show that UFoP provides as much as 10% more computational availability, and as much as four times more radio availability than the centralized approach. For all applications and energy environments evaluated, UFoP harvested 0.7-10.2% more energy than the centralized equivalent; meaning UFoP adds zero energy overhead.