Energy-adaptive Buffering for Efficient, Responsive, and Persistent Batteryless Systems

Batteryless energy harvesting systems enable a wide array of new sensing, computation, and communication platforms untethered by power delivery or battery maintenance demands. Energy harvesters charge a buffer capacitor from an unreliable environmental source until enough energy is stored to guarantee a burst of operation despite changes in power input. Current platforms use a fixed-size buffer chosen at design time to meet constraints on charge time or application longevity, but static energy buffers are a poor fit for the highly volatile power sources found in real-world deployments: fixed buffers waste energy both as heat when they reach capacity during a power surplus and as leakage when they fail to charge the system during a power deficit. To maximize batteryless system performance in the face of highly dynamic input power, we propose REACT: a responsive buffering circuit which varies total capacitance according to net input power. REACT uses a variable capacitor bank to expand capacitance to capture incoming energy during a power surplus and reconfigures internal capacitors to reclaim additional energy from each capacitor as power input falls. Compared to fixed-capacity systems, REACT captures more energy, maximizes usable energy, and efficiently decouples system voltage from stored charge – enabling low-power and high-performance designs previously limited by ambient power. Our evaluation on real-world platforms shows that REACT eliminates the tradeoff between responsiveness, efficiency, and longevity, increasing the energy available for useful work by an average 25.6 event responsiveness by an average 7.7x without sacrificing capacity, and enabling programmer directed longevity guarantees.