Abstract
Sustainable operation of battery powered wireless embedded systems (such as sensor nodes) is a key challenge, and considerable research effort has been devoted to energy optimization of such systems. Environmental energy harvesting, in particular solar based, has emerged as a viable technique to supplement battery supplies. However, designing an efficient solar harvesting system to realize the potential benefits of energy harvesting requires an in-depth understanding of several factors. For example, solar energy supply is highly time varying and may not always be sufficient to power the embedded system. Harvesting components, such as solar panels, and energy storage elements, such as batteries or ultracapacitors, have different voltage-current characteristics, which must be matched to each other as well as the energy requirements of the system to maximize harvesting efficiency. Further, battery nonidealities, such as self-discharge and round trip efficiency, directly affect energy usage and storage decisions. The ability of the system to modulate its power consumption by selectively deactivating its sub-components also impacts the overall power management architecture. This paper describes key issues and tradeoffs which arise in the design of solar energy harvesting, wireless embedded systems and presents the design, implementation, and performance evaluation of Heliomote, our prototype that addresses several of these issues. Experimental results demonstrate that Heliomote, which behaves as a plug-in to the Berkeley/Crossbow motes and autonomously manages energy harvesting and storage, enables near-perpetual, harvesting aware operation of the sensor node.
I. INTRODUCTION
The application space for wireless sensor networks is dominated by the longevity constraint, since the cost of physically deploying the sensor nodes often outweighs the cost of the nodes themselves. Energy is the limiting factor in achieving extreme (months to years) systemwide lifetime. Fortunately, a promising technique to forestall this network energy crisis is emerging – environmental energy harvesting. Exploiting energy sources ubiquitous to the operating space of the sensor nodes raises the possibility of infinite lifetime. Achieving this (through harvesting aware design) represents a new frontier in the natural progression of energy optimization techniques, which started from low power design [1], evolved into power aware design [2], and recently, battery aware design [3]. Table I shows the power generation potential of several energy harvesting modalities [4]. While a wide variety of harvesting modalities are now feasible, solar energy harvesting through photo-voltaic conversion provides the highest power density, which makes it the modality of choice to power an embedded system that consumes several mW using a reasonably small harvesting module. However, the design of a solar energy harvesting module involves complex tradeoffs due to the interaction of several factors such as the characteristics of the solar cells, chemistry and capacity of the batteries used (if any), power supply requirements and power management features of the embedded system, application behavior, etc. It is, therefore, essential to thoroughly understand and judiciously exploit these factors in order to maximize the energy efficiency of a solar harvesting module. TABLE I POWER DENSITIES OF HARVESTING TECHNOLOGIES Harvesting technology Power density Solar cells (outdoors at noon) 15mW/cm2 Piezoelectric (shoe inserts) 330μW/cm3 Vibration (small microwave oven) 116μW/cm3 Thermoelectric (10oC gradient) 40μW/cm3 Acoustic noise (100dB) 960nW/cm3 A. Paper contributions This paper makes the following contributions. First, we identify the various considerations and tradeoffs that are involved in the design of a solar energy harvesting module, and describe their impact on harvesting efficiency. We illustrate how these considerations differ from conventional battery based systems. Second, we discuss the desired features of such a solar harvesting module, and the services it should provide to the rest of the system to enable harvesting aware power management. We also illustrate how such harvesting aware operation can further improve system lifetime compared to state-of-the-art battery aware power management. Finally, we present the design, implementation, and performance evaluation of Heliomote, our plug-and-play solar energy harvesting module for the Berkeley/Crossbow motes. Heliomote autonomously performs energy harvesting, storage, and power routing, and enables harvesting aware operation by providing instantaneous solar and battery-state information through a simple one wire interface. Our experimental results demonstrate the feasibility of self-sustained near-perpetual operation of outdoor sensor networks using solar energy harvesting. II. RELATED WORK Energy efficient design techniques have been studied for sensor networks [5] at all levels from hardware design [6] to protocols for medium access control [7], routing [8], data gathering [9], topology management [10], [11], etc. Tools and techniques for energy and battery life estimation have also been proposed [12]. Environmental energy harvesting has been considered for improving the sustainable lifetimes of wearable computers [13], [14], sensor networks [15]–[17], etc. Numerous harvesting modalities have been successfully demonstrated including solar, vibrational, biochemical, and motion based [18]–[21], and several others are currently being developed [22]. While harvesting technology provides the ability to extract energy from the environment, it must be efficiently integrated into an embedded system to translate that harvested energy into increased application performance and system lifetime. A solar harvesting augmented high-end embedded system was described in [23], in which a switch matrix was used to power individual


Download full report
http://googleurl?sa=t&source=web&cd=1&ve....1.89.4757%26rep%3Drep1%26type%3Dpdf&ei=xyNTTpCREMHnrAeH4JjADg&usg=AFQjCNHLqTtvV17d8Q8eSWH_HC1-pHR47g