A survey of multi-source energy harvesting systems <s> I. INTRODUCTION <s> Environmental energy is an attractive power source for low power wireless sensor networks. We present Prometheus, a system that intelligently manages energy transfer for perpetual operation without human intervention or servicing. Combining positive attributes of different energy storage elements and leveraging the intelligence of the microprocessor, we introduce an efficient multi-stage energy transfer system that reduces the common limitations of single energy storage systems to achieve near perpetual operation. We present our design choices, tradeoffs, circuit evaluations, performance analysis, and models. We discuss the relationships between system components and identify optimal hardware choices to meet an application's needs. Finally we present our implementation of a real system that uses solar energy to power Berkeley's Telos Mote. Our analysis predicts the system will operate for 43 years under 1% load, 4 years under 10% load, and 1 year under 100% load. Our implementation uses a two stage storage system consisting of supercapacitors (primary buffer) and a lithium rechargeable battery (secondary buffer). The mote has full knowledge of power levels and intelligently manages energy transfer to maximize lifetime. <s> BIB001 </s> A survey of multi-source energy harvesting systems <s> I. INTRODUCTION <s> AmbiMax is an energy harvesting circuit and a supercapacitor based energy storage system for wireless sensor nodes (WSN). Previous WSNs attempt to harvest energy from various sources, and some also use supercapacitors instead of batteries to address the battery aging problem. However, they either waste much available energy due to impedance mismatch, or they require active digital control that incurs overhead, or they work with only one specific type of source. AmbiMax addresses these problems by first performing maximum power point tracking (MPPT) autonomously, and then charges supercapacitors at maximum efficiency. Furthermore, AmbiMax is modular and enables composition of multiple energy harvesting sources including solar, wind, thermal, and vibration, each with a different optimal size. Experimental results on a real WSN platform, Eco, show that AmbiMax successfully manages multiple power sources simultaneously and autonomously at several times the efficiency of the current state-of-the-art for WSNs <s> BIB002 </s> A survey of multi-source energy harvesting systems <s> I. INTRODUCTION <s> This paper describes a systematic approach to building micro-solar power subsystems for wireless sensor network nodes. Our approach composes models of the basic pieces - solar panels, regulators, energy storage elements, and application loads - to appropriately select and size the components. We demonstrate our approach in the context of a microclimate monitoring project through the design of the node, micro-solar subsystem, and network, which is deployed in a challenging, deep forest setting. We evaluate our deployment by analyzing the effects of the range of solar profiles experienced across the network. <s> BIB003 </s> A survey of multi-source energy harvesting systems <s> I. INTRODUCTION <s> This paper presents the design, implementation and characterization of an energy--efficient smart power unit for a wireless sensor network with a versatile nano-Watt wake up radio receiver. A novel Smart Power Unit has been developed featuring multi-source energy harvesting, multi-storage adaptive recharging, electrochemical fuel cell integration, radio wake-up capability and embedded intelligence. An ultra low power on board microcontroller performs maximum power point tracking (MPPT) and optimized charging of supercapacitor or Li-Ion battery at the maximum efficiency. The power unit can communicate with the supplied node via serial interface (I2C or SPI) to provide status of resources or dynamically adapt its operational parameters. The architecture is very flexible: it can host different types of harvesters (solar, wind, vibration, etc.). Also, it can be configured and controlled by using the wake-up radio to enable the design of very efficient power management techniques on the power unit or on the supplied node. Experimental results on the developed prototype demonstrate ultra-low power consumption of the power unit using the wake-up radio. In addition, the power transfer efficiency of the multi-harvester and fuel cell matches the state-of-the-art for Wireless Sensor Networks. <s> BIB004
Energy harvesting has become a commercial reality, with many deployed embedded devices being powered from naturally-occuring or unwanted energy in their environment. Electrical energy can be harvested from light, wind, vibration, or temperature difference and used to power embedded devices. The increasing popularity of these systems has been driven by a convergence between the increasing power output from harvesters and the decreasing energy demand of electronic devices. While batteries have conventionally been used to power such devices, they have a finite capacity and must be replaced or recharged when depleted. For this reason, energy harvesting is an attractive power source as it potentially offers a perpetual source of energy, provided there is sufficient and appropriate energy in the deployment environment. A drawback of many existing systems is that they only support one energy harvester type and may only be used where that type of energy source is present; changing the energy harvester requires changes to the system's hardware and software. The design trade-offs for single-source energy harvesting systems have been extensively considered BIB003 , BIB001 . To increase the availability of energy, and address the drawbacks of existing single-harvester systems, a number of reported works have proposed the simultaneous use of several energy harvesters BIB002 - BIB004 . By using a small wind turbine  and a solar cell, for example, more energy can potentially be generated (and for a longer period per day) than if a single harvester is used. Furthermore, the size of the energy buffer 978-3-9815370-0-0/DATE13/ c ⃝2013 EDAA (e.g. a supercapacitor or rechargeable battery) can potentially be reduced as there may be a shorter period where energy is not generated. While the use of multiple energy harvesters is a simple concept, their selection and integration into a complete system involves choices which impact on factors including efficiency and ease of deployment. This paper aids the effective design of multi-source energy harvesters by surveying the design of existing systems. It explores the many interrelated trade-offs and design choices that are essential for effective design of efficient and easilydeployable multi-source energy harvesting systems. A taxonomy for these systems is introduced with the aid of two example architectures developed by the authors of this paper (Sec. II), and subsequently used to classify the design of existing systems (Sec. III). The paper concludes (Sec. IV) with a discussion on the open research challenges and likely direction of future developments.