  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% over static buffers optimized for reactivity and capacity, improving
event responsiveness by an average 7.7x without sacrificing capacity, and
enabling programmer directed longevity guarantees.
