technologies Article Energy-Harvesting Powered Variable Storage Topology for Battery-Free Wireless Sensors † Firdaous El Mahboubi *, Marise Bafleur, Vincent Boitier * and Jean-Marie Dilhac LAAS-CNRS, Université de Toulouse, CNRS, INSA, UPS, 31042 Toulouse, France; marise.bafl[email protected] (M.B.); [email protected] (J.-M.D.) * Correspondence: [email protected] or fi[email protected] (F.E.M.); [email protected] (V.B.) † El Mahboubi, F.; Bafleur, M.; Boitier, V.; Dilhac, J.-M. Energy-Harvesting Powered Variable Storage Topology for Battery-Free Wireless Sensors. In Proceedings of the IEEE International Conference on Modern Circuits and Systems Technologies (MOCAST2018) on Electronics and Communications, Thessaloniki, Greece, 7–9 May 2018; doi:10.1109/MOCAST.2018.8376624. Received: 29 October 2018; Accepted: 13 November 2018; Published: 16 November 2018 Abstract: The energy autonomy of wireless sensors is one of the main roadblocks to their wide deployment. The purpose of this study is to propose simple adaptive storage architecture, which combined with energy harvesting, could replace a battery. The main concept is based on using several ultracapacitors (at least two) that are reconfigured in a series or in parallel according to its state of charge/discharge, either to speed up the startup of the powered system or to provide energy autonomy. The proposed structure is based on two ultra-capacitors, one of small capacitance value and one of big value. Powered by an energy-harvesting source, the devised control circuitry allows cold start up with empty ultra-capacitors, pre-regulated output voltage, and energy usage efficiency close to 94.7%. Keywords: energy harvesting; autonomy; variable energy storage; ultra-capacitors; wireless sensor 1. Introduction The energy autonomy of wireless sensors is one of the main roadblocks to their wide deployment in the different following areas: structural health monitoring (SHM) in severe environment such as aeronautics [1] or in building or infrastructure such as bridges with very long lifetime (>25 years) [2]. A way to strengthen the energy autonomy of these systems is to use energy harvesting from the surrounding environment coupled to a storage unit. The device used for storage is either a battery or an ultra-capacitor. In some cases, ultra-capacitors are more interesting than batteries: in conditions of extreme temperatures (in specific locations of an aircraft); in terms of safety (batteries may cause fire or explode); the peak power that can deliver ultra-capacitors is significantly larger than that of batteries (high power density); high charge/discharge efficiency and lifetime much greater (up to 500,000 cycles) than that of a battery (3000 to 4000 cycles). Even if the new technologies of low-power integrated circuits have made it possible to extend their lifetime, the replacement of hundreds or even thousands of batteries is not economically viable. Moreover, getting rid of primary batteries to avoid costly maintenance would be a must. However, the storage in ultra-capacitor has some drawbacks and requires a compromise to satisfy two important objectives: a sufficient voltage during the initial charge must be rapidly reached (small capacitance) to get and maintain the powered system operational as quickly as possible and a large amount of energy should be stored (big capacitance) to increase its energy autonomy. A self-adaptive storage architecture consisting of four ultra-capacitors (UCs) was already proposed to address these constraints [3]. This structure is based on reconfiguring the storage elements from all Technologies 2018, 6, 106; doi:10.3390/technologies6040106 www.mdpi.com/journal/technologies Technologies 2018, 6, 106 2 of 13 Technologies 2018, 6, x 2 of 13 in-series to all in-parallel and reversely according to the availability of harvested energy and to the loadenergy consumption. and to the Thisload autonomous consumption. adaptive This autonomous storage strategy adaptive allows astorage fast start-up strategy and allows an increased a fast energystart-up usage and an (~10%) increased compared energy to usage a single (~10%) big capacitor. compared However, to a single it does big capacitor. not provide However, a pre-regulated it does voltagenot provide and inducesa pre-regulated abrupt voltagevoltage changesand induces upon ab capacitorrupt voltage switching changes that upon could capacitor induce unwantedswitching perturbations.that could induce We unwanted propose a perturbations. variable storage We topology propose constituteda variable storage of only topology two ultra-capacitors, constituted of a smallonly two value ultra-capacitors, one (Csmall) and a a small large valuevalue oneone (C (Cbigsmall), which) and area large appropriately value one switched (Cbig), towhich provide are fastappropriately start-up of switched the system to toprovide be powered, fast start-up large energyof the system storage, to output be powered, voltage large pre-regulation, energy storage, and autonomyoutput voltage of the pre-regulation, system. and autonomy of the system. 2. Basic Operation Principle This adaptive storage system, aimed at supplyingsupplying a wireless sensor node poweredpowered byby ambientambient energy harvesting, is not new. new. Se Severalveral solutions solutions were were already already propos proposeded in in the the literature: literature: one one needs needs a avery very large large number number of of switches switches causing causing heavy heavy lo lossessses [4], [4], another another requires requires a a large number of ultra-capacitors [[5],5], andand a a third third one one uses uses a a complex comple architecturex architecture with with multi-stacked multi-stacked dc-dc dc-dc converters converters [6]. In[6]. this In paper,this paper, we propose we propose an alternative an alternativ and verye simpleand very self-adaptive simple self-adaptive energy storage energy architecture. storage Wearchitecture. analyze and We experimentally analyze and experimental compare twoly differentcompare architectures.two different architectures. 2.1. Basic Principle of Adaptive Storage 2.1. Basic Principle of Adaptive Storage The proposed adaptive storage system, schematically described in Figure1, is self-powered The proposed adaptive storage system, schematically described in Figure 1, is self-powered by by an energy-harvesting source and is composed of at least two ultra-capacitors (UCs). It is meant an energy-harvesting source and is composed of at least two ultra-capacitors (UCs). It is meant to to replace a battery. The basic idea is to adapt the size of the storage so that the system can start replace a battery. The basic idea is to adapt the size of the storage so that the system can start rapidly rapidly (small capacitance by implementing the different capacitor elements in series) but also provide (small capacitance by implementing the different capacitor elements in series) but also provide a a good autonomy (large capacitance by implementing the different capacitor elements in parallel). good autonomy (large capacitance by implementing the different capacitor elements in parallel). These configuration changes require a control circuitry that should be able to operate at low voltage, These configuration changes require a control circuitry that should be able to operate at low voltage, since it is supplied by the energy stored in the ultra-capacitors. The main challenges of this adaptive since it is supplied by the energy stored in the ultra-capacitors. The main challenges of this adaptive storage system are twofold: storage system are twofold: • First, it should allow a cold startup operation that corresponds to the case when ultra-capacitors • First, it should allow a cold startup operation that corresponds to the case when ultra-capacitors are empty. are empty. • • Second, itsits architecturearchitecture shouldshould bebe optimizedoptimized toto minimizeminimize losses.losses. Figure 1.1. BlockBlock diagram diagram of of the the proposed proposed self-powered self-powered adaptive adaptive storage storage architecture architecture (dashed-line (dashed-line block). 2.2. Referenceblock). Self-Adaptive Switched Architecture Storage with 4 Ultra-Capacitors 2.2. ReferenceThe reference Self-Adaptive architecture Switched already Arch proposeditecture Storage in a previous with 4 Ultra-Capacitors work [7] uses a matrix of four identical ultra-capacitors (Ci = C for I = 1, 2, 3, 4), interconnected by nine switches. As shown in Figure2, three The reference architecture already proposed in a previous work [7] uses a matrix of four Schottky diodes allow a default serial structure at start-up. identical ultra-capacitors (Ci = C for I = 1, 2, 3, 4), interconnected by nine switches. As shown in Figure 2, three Schottky diodes allow a default serial structure at start-up. Technologies 2018, 6, 106 3 of 13 Technologies 2018, 6, x 3 of 13 DuringDuring thethe chargecharge phase,phase, thethe changechange ofof configurationconfiguration dependsdepends onon thethe statestate ofof chargecharge ofof thethe ultra-capacitorultra-capacitor connectedconnected toto groundground (V(VC4C4).). This allows for three different configurations:configurations: • • “All“All in-series”in-series” allowsallows fastfast startupstartup (C(Ceqeq = C/4) • • “Series-parallel”“Series-parallel” (C(Ceqeq = C) • eq • “All“All in-parallel”in-parallel” in in order order to maximizeto maximize the amount the amount of stored of energystored (Cenergyeq = 4 C)(C without = 4 C) increasing without