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Unformatted text preview: 1536-1268/05/$20.00 2005 IEEEPublished by the IEEE CS and IEEE ComSocPERVASIVEcomputing69DISTRIBUTED ENERGYHARVESTING FOR ENERGY-NEUTRAL SENSOR NETWORKSAman Kansal and Mani B.Srivastava,University of California,Los AngelesEmbedded deployments, such as sen-sor-actuator networks, constitute a largeclass of pervasive computing devices.Unlike cell phones or laptops, which userscan periodically recharge, pervasive de-vices must operate on their initial batter-ies. The highest reported energy densitiesfor current battery technologies rangearound 3.78 kJ/cm3, which implies thatfor a low-power device operating at anaverage consumption of 1 mW to have a10-year lifespan, it needs a large 100 cm3battery. Thus, energy supply is a majorbottle neck for system lifetime, and har-vesting energy from the deployment envi-rons can help alleviate this. UCLAs energyharvesting project ( is making a two-pronged effort to address the various chal-lenges in building practical energy har-vesting sensor networks.First, weve started developing a theoryfor energy-neutral systems. These systemsstrive to meet application performancerequirements using only environmentallyavailable energy and can thus sustainthemselves infinitely (until the hardwareor application becomes outdated). In con-trast to battery-operated systems, powermanagement in energy harvesting systemsdiffers fundamentally in that its the avail-able power thats limited and not the totalenergy. Also, power availability varies intime and might be different at differentnodes in the network. While most appli-cations result in energy consumption atmultiple nodes, some flexibility in deter-mining which specific nodes are used isusually possible. For instance, when a sen-sor network records an event in the envi-ronment, theres a choice in determiningthe exact route used for routing the datato the user. Clearly, the achievable systemlifetime depends on how the relay nodesmake these choices with respect to theenvironmental energys spatio-temporalprofile. Notably, the network as a wholemust make the task allocation decisionsin a distributed manner because no singlenode has complete knowledge of theentire networks energy opportunity. Wedeveloped analytical models for the har-vesting and consuming entities andderived theorems that characterize theirachievable performance.1We also demon-strated distributed protocols to scheduletasks in accordance with the environ-mental harvesting opportunity availableat different network nodes.2The second part of the effort involvesdeveloping working prototypes, whichhas also revealed several interestingissues in the harvesting systems hard-ware and software design. Our first pro-totype consists of a network of solarenergy harvesting sensor nodes calledheliomotes. Each heliomote (shown inFigure 1) consists of a solar energy har-Energy Harvesting ProjectsEditor: Anthony D. Joseph UC Berkeley adja@cs.berkeley.eduWorks in Progress...
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