Reliable Energy Sources As a Foundation for Reliable Intermittent Systems
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Reliable Energy Sources as a Foundation for Reliable Intermittent Systems Dhananjay Jagtap Pat Pannuto [email protected] [email protected] University of California, San Diego University of California, San Diego ABSTRACT be accessible to change batteries [11]. This is the case when de- This paper defines architectural and operational principles for sim- ploying sensor networks for structural or condition monitoring ple and reliable energy harvesting devices that can be used in service in industrial environments [38] or for environmental monitoring of high-level applications. For many maintenance and monitoring applications where there is no built infrastructure. This is part of tasks, we propose that it is more valuable to have reliable, trusted what makes energy harvesting designs so exciting: they present affirmations that nothing has changed than it is to know theexact the potential for set-it-and-forget it systems that can go anywhere moment that something has failed. This presents an opportunity for and last forever [16]. a new class of highly reliable, but not necessarily timely, intermit- What often makes energy harvesting designs challenging, how- tent devices. These are devices that are capable of sending messages ever, is that energy sources are unreliable, unpredictable, or un- at reasonable, fixed intervals (e.g. once an hour or once a day). They controllable. Early designs simply ran whenever they could for as cannot activate more often, but they also promise not to activate long as they could [10, 31, 35]. However, this wholly uncontrolled less often. To establish this reliability, we look to opportunities for intermittency made it difficult for device and system designers energy scavenging that are often ignored as their instantaneous to build applications. This has led to wide array of solutions to power delivery capability is very limited. However, unlike higher resolve device-level intermittency, which include hybrid and hier- power scavenging opportunities, many of these sources are not archical power systems [19, 28], physically and logically federated intermittent. Such sources may provide less than a microwatt, but energy storage banks [1, 8, 15], and energy-aware checkpointing their trickle of energy will be continuously, reliably available for and energy-atomic task models [17, 26]. While these techniques months to decades. This enables the creation of devices that can be can help address and mask intermittency for applications when guaranteed to activate at predictable intervals, which allows for the energy is available, to build systems of devices whose aggregate construction of non-intermittent systems atop intermittent devices. operation is reliable, we need to go back to the energy source. Unreliably intermittent energy sources are the root cause of un- CCS CONCEPTS predictability for energy harvesting devices. Not all energy sources are intermittent, however. Indeed, we observe that the world is • Computer systems organization → Embedded and cyber- full of what we term ambient batteries. These are ambient energy physical systems; • Hardware → Batteries; • Human-centered sources that are able to provide similar reliability to that which computing → Ubiquitous and mobile computing. is afforded to non-harvesting systems by traditional batteries. We ACM Reference Format: define ambient batteries as easily harvestable sources that have Dhananjay Jagtap and Pat Pannuto. 2020. Reliable Energy Sources as a Foun- deterministic and stable current and voltage characteristics over dation for Reliable Intermittent Systems. In The 8th International Workshop very long time periods (i.e. months to decades). on Energy Harvesting and Energy-Neutral Sensing Systems (ENSsys ’20), No- Such reliable energy would eliminate many of the historical vember 16–19, 2020, Virtual Event, Japan. ACM, New York, NY, USA,7 pages. challenges of energy harvesting systems, so why have they not https://doi.org/10.1145/3417308.3430276 been used from the beginning? It is not per lack of availability, such stable sources are shockingly prevalent (e.g. every outdoor tree, 1 INTRODUCTION as discussed in Section 4.2.1). Notably absent from our definition With the advent of IoT, an ever-growing panoply of sensors are of ambient batteries, however, is any minimum power delivery being conceived, constructed, and deployed to grow the reach of capability. In practice, the sources able to provide such longitudinal, computational infrastructure into the physical world. Generally reliable power may only be able to provide a microwatt or less, with these devices are powered using batteries or from the mains power only tens of millivolts of potential. It is only the recent confluence supply. However, we want to be able to embed intelligent sensing of very low power electronics and highly efficient harvesting ICs where there may not be mains power or where devices may not that have made such energy budgets viable for real-world systems. At first glance, devices with an average power budget of less than one microwatt may seem very limited. As we will show in Section 4, Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed however, when integrated over time, even a very small trickle of for profit or commercial advantage and that copies bear this notice and the full citation energy can power useful devices and applications. In particular, we on the first page. Copyrights for third-party components of this work must be honored. focus on wide-area, longitudinal monitoring. These are challenging For all other uses, contact the owner/author(s). ENSsys ’20, November 16–19, 2020, Virtual Event, Japan applications as they require large-scale deployment and must last © 2020 Copyright held by the owner/author(s). for the lifetime of physical infrastructure. Consider the fine-grained ACM ISBN 978-1-4503-8129-1/20/11. measure of soil moisture, where topography, weather, and watering https://doi.org/10.1145/3417308.3430276 1 ENSsys ’20, November 16–19, 2020, Virtual Event, Japan Dhananjay Jagtap and Pat Pannuto Table 1: Mapping Prior Applications to the General Energy Harvesting Architecture. This table looks at recent and older energy harvesting systems, and shows how the major, conceptual architecture has remained steady. One of the most interesting trends is the evolution of the energy buffer and charge controller from boutique, carefully hand-tuned circuits towards plug-’n-play, off-the-shelf components. The modularization of energy capture and management has allowed complexity to shift to the energy consumers: they have grown from transmitting fixed, 12-bit identifiers to emulating full game consoles. Application Energy Source Energy Buffer Charge Controller Energy Consumer Pushbutton (2001) [31] Transient mechanical pulse (pushed button) 4.4 µF MAX666 (LDO only) Event-reporting radio WISP (2006) [35] Radio waves (targeted RFID) Capacitors (unspecified; < 1 mF) Custom circuit (Minimalist) wireless sensor node WATTR (2010) [6] Variation in water pressure 3200 µF capacitor Custom circuit Event-reporting DoubleDip (2012) [28] Temperature difference between pipe & air ML2020 (45 mAh battery) LTC 3109 Sensing ¸ event-reporting Monjolo (2013) [10] Magnetic field generated by wire supplying AC load 500 µF tantalum capacitor LTC 3588 Energy metering for AC loads Battery-Free Solar energy and button presses 3.3 mF super capacitor BQ25570 Gaming console Gameboy (2020) [9] influence hour-to-hour changes but erosion and climate change harvesting devices where each subsystem is simple and reliable, influence year-over-year effects. An application like this does not which in turn allows for the creation of highly deployable devices. require tight latency; it is not important to know the exact second We start with a short summary of the major components: it started raining. But it does require reliability; it must be able to report if an area has not been watered (e.g. broken infrastructure). Energy Source. This provides current and potential. It can stem And it cannot require maintenance; it is costly to repeatedly send from a variety of physical phenomenon such as electromagnetic personnel to deploy and maintain sensors over large physical ar- radiation, heat, pressure, or electrostatic charge buildup. The source eas. The value from these applications is positive affirmation that refers to both the phenomenon which generates the energy and, if nothing has changed – and nothing needs to be fixed. needed, the conversion element or transducer that converts to elec- We propose that the key to enabling real-world deployments for trical energy that is accessible to traditional electronics frontends. such applications is reliable intermittency. This is a primitive that addresses the intermittency of systems of intermittent devices. Prior Energy Buffer. This accumulates electric charge when it is avail- applications-focused deployments built atop intermittent devices able from the source and stores it. It delivers this energy to the could not distinguish between an absence of events and broken Energy Consumer when it is time to execute an event. intermittent devices [5]. To build robust, reliable applications it is equally important for devices to report when phenomena under Charge Controller. This provides and maintains the desired study have not occurred as it is to report when they do. voltage and current levels. It is responsible for energy into and