A survey of multi-source energy harvesting systems <s> II. SYSTEM DESIGN TAXONOMY <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> II. SYSTEM DESIGN TAXONOMY <s> Wireless sensors are normally powered by nonrechargeable batteries, but these must be replaced when depleted. Recent developments in energy harvesting technology allow sensors to be powered by environmental energy where it is present, but the wide range of situations where sensors are deployed means that it is desirable for the energy components of a sensor node (i.e. batteries, supercapacitors, and power generation devices) to be selected and configured at the time of node deployment. Previous energy harvesting-powered systems have been designed for specific energy hardware and been difficult to adapt for different resources. Energy-awareness is useful for state-of-the-art network algorithms, but present systems do not provide a standardized or straightforward way for nodes to monitor and manage their energy hardware. ::: ::: The developments reported in this paper deliver a reconfigurable energy subsystem for wireless autonomous sensors. The new system permits energy modules to be selected and fitted to the sensor node in-situ, in a plug-and-play manner, without the need for reprogramming or the modification of hardware. The node can monitor and intelligently manage its energy resources and assess its overall energy status by analyzing its level of stored energy and rate of power generation. These activities are facilitated by a proposed common hardware interface (which allows multiple energy modules to be connected) and an electronic datasheet structure for the energy modules. The system has been verified through the development and testing of a prototype wireless sensor node which operates from a mix of energy sources. <s> BIB002 </s> A survey of multi-source energy harvesting systems <s> II. SYSTEM DESIGN TAXONOMY <s> Supercapacitors are often used in energy harvesting wireless sensor nodes (EH-WSNs) to store harvested energy. Until now, research into the use of supercapacitors in EH-WSNs has considered them to be ideal or oversimplified, with non-ideal behavior attributed to substantial leakage currents. In this brief, we show that observations previously attributed to leakage are predominantly due to redistribution of charge inside the supercapacitor. We confirm this hypothesis through the development of a circuit-based model, which accurately represents non-ideal behavior. The model correlates well with practical validations representing the operation of an EH-WSN and allows behavior to be simulated over long periods. <s> BIB003 </s> A survey of multi-source energy harvesting systems <s> II. SYSTEM DESIGN TAXONOMY <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 </s> A survey of multi-source energy harvesting systems <s> II. SYSTEM DESIGN TAXONOMY <s> Sensor nodes in wireless sensor networks (WSNs) are often powered by energy harvesting devices used to extend the lifetime of the nodes and energy storage elements are fundamental to collect the exceeding power incoming from the ambient. The stored energy is then consumed to supply the sensor node when the ambient energy is scarce or insufficient to satisfy the current requests from the load. To date, the choice of the storage element is made between rechargeable batteries and electric double-layer capacitors (EDLC or supercapacitors), and many works in literature exploit hybrid storage architectures. In this paper we characterize an innovative technology available on the market, namely the lithium-ion capacitor (LIC). We show that this cutting edge technology combines the high cell voltage, the energy density and the low self-discharge of a lithium battery with the power density, the high capacity and long cycle life of a supercapacitor. <s> BIB005
Illustrative examples of two multi-source energy harvesting systems, developed by the authors of this paper, are shown in figures 1 (the 'Smart Power Unit' BIB004 , hereafter referred to as 'System A') and 2 (the 'Plug-and-Play Architecture' BIB002 , 'System B'). In these examples, energy harvesters and storage devices are connected via a power unit to an embedded device (wireless sensor). System A is intended for outdoor operation, harvesting energy from wind and light and also using a fuel cell as energy backup; its power budget is of the order of a few milliwatts. Conversely, System B is intended for indoor use, harvesting energy opportunistically from a selection of modules as appropriate to the available energy in the deployment environment; its power budget is <1mW. This section considers options for system architectures, of particular interest when noting the differences between the two example systems. It looks at methods for connecting devices, and options for intelligent management and power processing. 1) Power Conditioning Functionality: Most energy harvesting devices produce power intermittently, so it is necessary to buffer the energy they produce. This allows the bursty loads of embedded devices such as wireless sensor nodes to be accommodated. As a minimum, an input power conditioning circuit is required to go between the harvester and the storage device -to prevent the backflow of energy to the harvester, and in many cases to rectify and regulate its output. The amount of power harvested depends on the environment, the harvester, and its match with the power conditioning circuit. There is a trade-off between the efficiency and the complexity/quiescent power consumption of the power conditioning circuit. System A uses a maximum power point tracking (MPPT) arrangement that works to ensure that the energy harvesters operate at their optimal point. In addition System A uses a hydrogen fuel cell which has a high energy density comparated with traditional battery and which starts to work when the stored energy coming from the environmental sources is running out. Conversely, System B has devolved this functionality to the individual modules, but the demonstration modules produced operate at a fixed point which offers a compromise between efficiency and quiescent current draw. Most devices also have an output conditioning circuit between the storage device and the load, to supply a suitable voltage to the embedded device. In the case of System A, a Buck-Boost converter is used. System B uses a low quiescent current linear regulator, which again is a compromise between its conversion efficiency and quiescent current draw. 2) Exchangeable Hardware: In contrast with early singlesource systems like Prometheus BIB001 , which are designed for fixed energy devices, some reported systems provide the facility to connect a range of different energy devices. However, there are several levels of functionality for this: • Swappable energy harvesters. The most basic systems allow energy harvesters to be exchanged, but options are limited by the input power conditioning. • Swappable energy harvesters and storage. More complex systems allow the harvesters and energy storage devices to be exchanged, with similar constraints. In particular different storage technologies offer different characteristics well known in literature BIB003 , BIB005 . • Completely flexible. The most flexible system architecture permits the harvesters and energy storage devices to be exchanged, but each harvester/storage device has to have its own interface circuitry. Providing the facility to change the energy device for one of a different type means that additional power conditioning circuitry is likely to be required to interface between the storage device and the rest of the system. In other words, functionality and flexibility must be traded off against system complexity. Some systems have the energy devices soldered directly to their circuit board. For System A, which is designed to be used outdoors, the natural choice is to use wind and light energy harvesters, so these are assumed by the system design. Conversely, for System B which is designed for industrial monitoring indoors, a range of energy harvesting techniques are applicable and would only be decided by the properties of the deployment location. For this reason, System A assumes certain energy hardware (although the device size is changeable within certain bounds), while System B can accommodate any device provided that it has an appropriate interface circuit. 3) Energy Monitoring/Control Capability: Intelligent features allow the system to monitor its energy status so that it can respond by, for example, adjusting its duty cycle to conserve energy when resources are limited, or selecting auxiliary storage such as the fuel cell. At their most basic, energy-aware systems may provide an analog line to allow the microcontroller to monitor the store voltage; complex systems allow the amount of energy stored and the power being generated to be monitored (and perhaps controlled). For systems with a fixed input power conditioning architecture, they may only allow the microcontroller to observe the input current and stored energy level. More complex devices which allow the energy storage device to be exchanged may offer the facility for this monitoring to take account of hardware changes. The means of communication with devices may be analog or digital. They may also be twoway, allowing the microcontroller to impose changes on the power conditioning circuitry (e.g. to adjust its supply voltage or to move energy between storage devices). Here, the main trade-off is between the complexity and loss of efficiency by adding the extra functionality, and the advantages gained by the improved energy-awareness of the system. System A is a particularly capable example, as it has an embedded microcontroller on the SPU which communicates via an I 2 C bus, allowing the energy status to be monitored and controlled. The provision of the facility to exchange the energy hardware normally means that energy awareness will be sacrificed. System B is a notable exception, as it has an electronic datasheet on each energy module which may be individually interrogated to determine their properties. They communicate via a digital interface to the embedded system. 