Wensi Wang

Design considerations of sub-mW indoor light energy harvesting for wireless sensor systems

For most wireless sensor networks, one common and major bottleneck is the limited battery lifetime. The frequent maintenance efforts associated with battery replacement significantly increase the system operational and logistics cost. Unnoticed power failures on nodes will degrade the system reliability and may lead to system failure. In building management applications, to solve this problem, small energy sources such as indoor light energy are promising to provide long-term power to these distributed wireless sensor nodes. This article provides comprehensive design considerations for an indoor light energy harvesting system for building management applications. Photovoltaic cells characteristics, energy storage units, power management circuit design, and power consumption pattern of the target mote are presented. Maximum power point tracking circuits are proposed which significantly increase the power obtained from the solar cells. The novel fast charge circuit reduces the charging time. A prototype was then successfully built and tested in various indoor light conditions to discover the practical issues of the design. The evaluation results show that the proposed prototype increases the power harvested from the PV cells by 30% and also accelerates the charging rate by 34% in a typical indoor lighting condition. By entirely eliminating the rechargeable battery as energy storage, the proposed system would expect an operational lifetime 10--20 years instead of the current less than 6 months battery lifetime.

Autonomous wireless sensor network based Building Energy and Environment Monitoring system design

Wireless sensor network technology emerged in recent years with numerous potential applications. The building environment and energy monitoring (BEEM) is among the most important ones. The design of such smart wireless sensing system is presented in this paper. The proposed system consists of low power Tyndall wireless sensor node hardware with light energy harvesting featured power supply and energy management system for long-term deployment. Energy consumption, light level, temperature and humidity parameters are measured and transmitted via a 2.4GHz Zigbee wireless network. Evaluations of the system are conducted in a local office building with a total of 62 nodes operating with varying functions. The evaluation results of the system including the measured energy and environmental data are presented. The evaluation results show that this design is worlds first known indoor light energy harvesting powered BEEM system.

Energy harvesting embedded wireless sensor system for building environment applications

For many wireless sensor networks applications, indoor light energy is the only ambient energy source commonly available. Many advantages and constraints co-exist in this technology. However, relatively few indoor light powered harvesters have been presented and much research remains to be carried out on a variety of related design considerations and trade-offs. This work presents a solution using the Tyndall mote and an indoor light powered wireless sensor node. It analyses design considerations on several issues such as indoor light characteristics, solar panel component choice, maximum power point tracking, energy storage elements and the trade-offs and choices between them.

Thermoelectric Energy Harvesting for Building Energy Management Wireless Sensor Networks

A thermoelectric energy harvester powered wireless sensor networks (WSNs) module designed for building energy management (BEM) applications is built and tested in this work. An analytic thermoelectric generator (TEG) electrical model is built and verified based on parameters given in manufacturer data sheets of Bismuth Telluride TEGs. A charge pump/switching regulator two-stage ultra-low voltage step-up DC/DC converter design is presented in this work to boost the <0.5 V output voltage of TEG to usable voltage level for WSN (3.3 V). The design concept, device simulation, circuits schematic, and the measurement results are presented in detail. The prototype device test results show 25% end-to-end conversion efficiency in a wide range of input temperatures/voltages. Further tests demonstrate that the proposed thermoelectric generator design can effectively power WSN module which operates with a 1.7% duty cycle (5.8 seconds measurement time interval) when the prototype is placed on a typical wall-mount heater (60°C surface temperature). The thermoelectric energy harvesting powered WSN demonstrates duty cycles significantly higher than the required duty cycle for BEM WSN applications.

Design of miniaturized wireless sensor mote and actuator for building monitoring and control

In this paper, a wireless sensor network mote hardware design and implementation are introduced for building deployment application. The core of the mote design is based on the 8 bit AVR microcontroller, Atmega1281 and 2.4 GHz wireless communication chip, CC2420. The module PCB fabrication is using the stackable technology providing powerful configuration capability. Three main layers of size 25 mm2 are structured to form the mote; these are RF, sensor and power layers. The sensors were selected carefully to meet both the building monitoring and design requirements. Beside the sensing capability, actuation and interfacing to external meters/sensors are provided to perform different management control and data recording tasks. Experiments show that the developed mote works effectively in giving stable data acquisition and owns good communication and power performance.

Power management for sub-mW energy harvester with adaptive hybrid energy storage

Energy harvesting with its ubiquitous availability is intensively investigated to extend the wireless sensor nodes lifetime. While the harvested power is often less than 1 mW, the commercial-off-the-shelf power management circuits designed for conventional battery applications operate in much higher power. A designated power management module for sub-mW energy harvester is proposed in this article to increase the energy conversion efficiency and extend the energy storage lifetime for small input power. The proposed module consists of discrete component–based pulse-width modulation (PWM) controlled synchronous boost converter structured impedance matching circuit and supercapacitor/micro-battery hybrid energy storage. The synchronous boost converter-based power management circuit achieves 79% conversion efficiency at the input power of 0.5 mW. The power loss during the energy conversion is significantly reduced and thus allows wireless sensor nodes to be powered from a minimum of 120 Lux fluorescent light with a photovoltaic cell smaller than a credit card. The supercapacitor/micro-battery energy storage provides lifetime of more than 73 h when no external energy is available. The experimental results with 25 energy harvester–powered wireless sensor nodes demonstrate that the practical ultra-low–power design makes sub-1-mW energy harvester power management applicable.

Patterning Submicrometer Thick Inorganic Nanoparticle Films by Solution Process and Application for Light Trapping in Solar Cells

We present a low-cost fabrication process to deposit patterned inorganic nanoparticle films with submicrometer thickness and in turn to build higher dimensional structures through sequential multilayer deposition. Oxide nanoparticle films including semiconductors, dielectrics, and conductors have been patterned by moulding or imprinting from their solvent-suspension/paste using polydimethylsiloxane stamps. The easily controlled film thickness and good duplication fidelity with high resolution allows one to fabricate various layered structures, such as double layer and multilayer structures with minimized residual materials between them to finally define quasi-3D structures. Our experiment shows that colloidal suspension of materials can readily be patterned by stamping techniques with similar quality as compared to well-developed thermal or UV imprinting using solvent-free molecule-based materials. The usability of the fabricated structure is further demonstrated by integration of a 2-D grating on dye sensitized solar cell for improved power conversion efficiency.

A rotating machine acoustic emission monitoring system powered by multi-source energy harvester

This working paper introduces a multi-source energy harvesting powered acoustic emissions (AE) monitoring system for the applications of rotating machine fault detections. This wireless sensor prototype records the AE signal at approximately 200KHz frequency. It performs fast Fourier transform (FFT) and compares the results with known fault patterns. An energy harvesting solution utilizes vibrational, thermal and light energy as the power supply for the AE wireless sensor node. The multiple-source energy harvester proposed in this work generates 1.56mW from 25--48mg vibration energy and 3.37mW from thermoelectric energy when deployed on the 62°C metal surface of air compressor. This AE wireless sensor prototype combines with the batteryless energy harvesting power supply in order to provide a self-powered health monitoring solution for wide range of rotating machinery.

Bulk Material Based Thermoelectric Energy Harvesting for Wireless Sensor Applications

The trend towards smart building and modern manufacturing demands ubiquitous sensing in the foreseeable future. Self-powered Wireless sensor networks (WSNs) are essential for such applications. This paper describes bulk material based thermoelectric generator (TEG) design and implementation for WSN. A 20cm2 Bi0.5Sb1.5Te3 based TEG was created with optimized configuration and generates 2.7mW in typical condition. A novel load matching method is used to maximize the power output. The implemented power management module delivers 651μW to WSN in 50 °C. With average power consumption of Tyndall WSN measured at 72μW, feasibility of utilizing bulk material TEG to power WSN is demonstrated.

Electrical/optical dual-function redox potential transistor

We demonstrate a new type of transistors, the electrical/optical “dual-function redox-potential transistors”, which is solution processable and environmentally stable. This device consists of vertically staked electrodes that act as gate, emitter and collector. It can perform as a normal transistor, whilst one electrode which is sensitised by dye enables to generate photocurrent when illuminated. Solution processable oxide-nanoparticles were used to form various functional layers, which allow an electrolyte to penetrate through and, consequently, the current between emitter and collector can be controlled by the gate potential modulated distribution of ions. The result here shows that the device performs with high ON-current under low driving voltage (<1 V), while the transistor performance can readily be controlled by photo-illumination. Such device with combined optical and electrical functionalities allows single device to perform the tasks that are usually done by a circuit/system with multiple optical and electrical components, and it is promising for various applications.