Supercapacitor Energy Buffering for Self-Sustainable, Continuous Sensing Systems

Continuous, high-data-rate visual sensing and processing in the field is important for intelligent transportation, environmental monitoring, and area / context awareness. Rechargeable batteries in self-sustainable systems suffer from adverse environmental impact and fast aging. Advancements in supercapacitor energy density and low-power processors have reached an inflection point, where a data-intensive visual sensing system can rely solely on supercapacitors for energy buffering. This paper demonstrates the first working prototype of such a system, consisting of eight 3,000-Farad supercapacitors, a custom-designed 70-mW controller / harvester board, and a Nexus 7 tablet computer. We leverage the voltage-to-stored-energy relationship in capacitors to enable precise energy buffer modeling (no more than 3% error in time-to-depletion prediction). We find the supercapacitor self-discharge (or leakage) to be a minor issue in practice where the operating power significantly exceeds the leakage power. However, accurate energy budgeting must account for the variation of effective capacitance—particularly lower capacitance at lower voltages nearing energy depletion. Precise energy budgeting allows model-driven system adaptation (such as dynamic CPU configurations on low-power chipsets) and consequently realizes high, stable operational quality-of-service. Our working prototype has been successfully deployed at a campus building rooftop where it analyzes nearby traffic patterns continuously. This work was supported in part by the National Science Foundation grants CNS-1217372, CNS-1239423, and CCF-1255729. Figure 1: Picture of our deployed system on a seven-story building rooftop (center picture) that includes a camera, a block of solar panels, and a system box. The system box (left-side picture) contains a Nexus 7 tablet computer (without its internal rechargeable battery) sustained by eight Maxwell 3000 Farad supercapacitors (wrapped in black tapes), and a custom-built controller board (at the top of the box). The energy buffering capacity of the supercapacitor block is about 1.4× that in the original Nexus 7 battery.