Analysis of Supercapacitor Energy Loss for Power Management in Environmentally Powered Wireless Sensor Nodes

This paper analyzes supercapacitor energy loss and investigates its impact on power management in environmentally powered wireless sensor nodes that use supercapacitor-based energy storage systems. While supercapacitor self-discharge is usually taken into account for system design and power management in wireless sensor nodes, supercapacitor charge redistribution and the associated energy loss have not been considered. In an environmentally powered wireless sensor node, supercapacitor charge redistribution may have more significant impact on power management than self-discharge. This paper analyzes supercapacitor energy loss during charge redistribution based on the variable leakage resistance (VLR) model. It is concluded that supercapacitor energy loss is dependent on charge redistribution time and supercapacitor initial state, which is defined as the initial voltages across the capacitors in the VLR model. The impact of supercapacitor energy loss on power management is studied by considering the task scheduling problem. A set of simulation cases that covers various supercapacitor initial state and energy harvesting capability is designed to investigate the impact of supercapacitor energy loss on task scheduling policy. It is demonstrated that greedy or lazy scheduling policy is preferred depending on supercapacitor initial state and energy harvesting capability in terms of two performance evaluation metrics.