Battery and ultra-capacitor hybrid energy storage system and power management scheme for solar-powered Wireless Sensor Nodes

This paper presents a Wireless Sensor Node (WSN) architecture with solar power generation and a hybrid energy storage scheme. The WSN is composed of three key modules: Energy Harvesting, Energy Storage, and the Control/Processing unit. The harvesting module consists of a miniature 179 mW solar array and MPPT hardware. A rechargeable 350 mAh Lithium-Ion battery and an ultra-capacitor are used as the energy storage elements. The low-ESR ultra-capacitor efficiently supplies the load power, which can reach as high as 295 mW peak, while the battery provides high-density storage. These elements are interfaced through a digitally controlled bi-directional dc-dc converter, which efficiently regulates the power-flow in the WSN. Multiple sensors and circuitry are implemented to measure positional and environmental data, as well as receiving and transmitting data via RF communication. A long-term test of the WSN is conducted to demonstrate the effective system functionality.