Contributions to the design of energy harvesting systems for autonomous sensors in low power marine applications

Tesi en modalitat de compendi de publicacionsOceanographic sensor platforms provide biological and meteorological data to help understand changes in marine environment and help to preserve it. Lagrangian drifters are autonomous passive floating platforms used in climate research to obtain surface marine data. They are low-cost, versatile, easy-to-deploy and can cover large extensions of the ocean when deployed in group. These deployments can last for years, so one of the main design challenges is the autonomy of the drifter. Several energy harvesting (EH) sources are being explored to reduce costs in battery replacement maintenance efforts such as solar panels. Drifters must avoid the impact of the wind because this may compromise proper surface current tracking and therefore, should ideally be mostly submerged. This interferes with the feasibility of solar harvesting, so other EH sources are being explored such as the oscillatory movement of the drifter caused by ocean waves. Wave energy converters (WEC) are the devices that turn this movement into energy. The motion of the drifter can principally be described by 3 oscillatory degrees of freedom (DoF); surge, heave and pitch. The heave motion includes the buoyancy’s response of the drifter, which can be explained by a mass-spring-damping model. By including the wave’s hydrodynamic load in this model, it is converted into a nonlinear system whose frequency response includes the wave’s frequency and the natural frequencies from the linear system. A smart option to maximize the captured energy is to design the inner WEC with a natural frequency similar to that of the drifter's movement. In this thesis, a 4 DoF model is obtained. This model includes the heave, the surge and the pitch motion of the drifter in addition to the inner pendulum motion relative to the buoy. Simultaneously, different pendulum-type WECs for small-size oceanic drifters are proposed. One of these converters consists of an articulated double-pendulum arm with a proof mass that generates energy through its relative motion with the buoy. Different experimental tests are carried out, with a prototype below 10 cm in diameter and 300 g of total mass, proving the capability of harvesting hundreds of microwatts in standard sea conditions EH sources require an additional power management unit (PMU) to convert their variable output into a constant and clean source to be able to feed the sensor electronics. PMUs should also ensure that the maximum available energy is harvested with a maximum power point tracking (MPPT) algorithm. Some sources, such as WECs, require fast MPPT as its output can show relatively rapid variations. However, increasing the sampling rate may reduce the harvested energy. In this thesis, this trade-off is analyzed using the resistor-based fractional open circuit voltage-MPPT technique, which is appropriate for low-power EH sources. Several experiments carried out in marine environments demonstrate the need for increasing the sampling rate. For this purpose, the use of a commercial PMU IC with additional low-power circuitry is proposed. Three novel circuits with a sampling period of 60 ms are manufactured and experimentally evaluated with a small-scale and low-power WEC. Results show that these configurations improve the harvested energy by 26% in comparison to slow sampling rate configurations. Finally, an EH-powered oceanographic monitoring system with a custom wave measuring algorithm is designed. By using the energy collected by a small-size WEC, this system is capable of transmitting up to 22 messages per day containing data on its location and measured wave parameters.Les plataformes d’observació oceanogràfiques integren sensors que proporcionen dades físiques i biogeoquímiques de l’oceà que ajuden a entendre canvis en l’entorn marí. Un exemple d’aquestes plataformes són les boies de deriva (drifters), que són dispositius autònoms i passius utilitzats en l’àmbit de la recerca climàtica per obtenir dades in-situ de la superfície marina. Aquests instruments són de baix cost, versàtils, fàcils de desplegar i poden cobrir grans superfícies quan s’utilitzen en grup. L’autonomia és un dels principals desafiaments en el disseny de drifters. Per tal d’evitar els costos en la substitució de bateries, s’estudien diferents fonts de captació d’energia com per exemple la solar. Els drifters utilitzats per l’estudi dels corrents marins superficials han d’evitar l’impacte directe del vent ja que afecta al correcte seguiment de les corrents i, per tant, cal que estiguin majoritàriament submergides. Això compromet la viabilitat de l’energia solar, fet que requereix l’estudi d’altres fonts de captació com el propi moviment de la boia causat per les onades. Els convertidors d’energia de les onades (WEC, wave energy converters) compleixen aquesta funció. El moviment dels drifters pot explicar-se bàsicament a través de 3 graus de llibertat oscil·latoris: la translació vertical i la horitzontal i el balanceig. La translació vertical inclou la flotabilitat del dispositiu, que es pot descriure mitjançant el model massamolla- amortidor. Incloure la càrrega hidrodinàmica de l’onada en aquest model el converteix en un sistema no lineal amb una resposta freqüencial que inclou la de l’onada i les naturals del sistema lineal. Una opció per maximitzar l’energia captada és dissenyar el WEC amb una freqüència natural similar a la del moviment de la boia. En aquesta tesis es proposa un model de 4 graus de llibertat per a l’estudi del moviment del drifter. Aquest inclou els 3 graus de llibertat de la boia i el moviment del pèndul relatiu a ella. En paral·lel, es proposen diferents WEC del tipus pendular per drifters de reduïdes dimensions. Un d’aquests WEC consisteix en un doble braç articulat amb massa flotant que genera energia a través del seu moviment relatiu al drifter. S’han dut a terme diferents proves experimentals amb un prototip inferior a 10 cm de diàmetre i 300 g de massa, les quals demostren la seva capacitat de captar centenars de microwatts en condicions marines estàndard. Utilitzar fonts de captació d’energia requereix incloure una unitat gestora de potència (PMU, power management unit) per tal de convertir la seva sortida variable en una font constant i neta que alimenti l’electrònica dels sensors. Les PMU també tenen la funció d’assegurar que es recull la màxima energia mitjançant un algoritme de seguiment del punt de màxima potència. Els WEC requereixen un seguiment d’aquest punt ràpid perquè la seva sortida consta de variacions relativament ràpides. Tanmateix, augmentar la freqüència de mostreig pot reduir l’energia captada. En aquesta tesi, s'analitza a fons aquesta relació utilitzant la tècnica de seguiment de la tensió en circuit obert fraccionada basada en resistències, que és molt adequada per a fonts de baixa potència. Diversos experiments realitzats en el medi marí mostren la necessitat d'augmentar la freqüència de mostreig, així que es proposa l'ús de PMU comercials amb una electrònica addicional de baix consum. S’han fabricat tres circuits diferents amb un període de mostreig de 60 ms i s’han avaluat experimentalment en un WEC de reduïdes dimensions. Els resultats mostren que aquestes configuracions milloren l'energia recollida en un 26% en comparació a PMU amb mostreig més lent. Finalment, s’ha dissenyat un sistema autònom de monitorització marina que inclou un algoritme de mesura d'ones propi. Aquest sistema és capaç de transmetre fins a 22 missatges al diaPostprint (published version

Data comparison between three acoustic doppler current profilers deployed in OBSEA platform in North-Western Mediterranean

Three different Acoustic Doppler Current Profilers (ADCP) have been deployed in OBSEA platform, a 20 meters depth underwater observatory cabled with a 4 km mixt cable to Vilanova i la Geltru’s coast. Two months of continuous data have been collected in order to confirm their proper operation and long term North current characteristic from the area.Peer Reviewe

Convertidor puente H-NPC con devanados abiertos como inversor central en aplicaciones fotovoltaicas trifásicas de gran escala

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Power gain from energy harvesting sources at high MPPT sampling rates

Energy harvesting (EH) sources require the tracking of their maximum power point (MPP) to ensure that maximum energy is captured. This tracking process, performed by an MPP tracker (MPPT), is performed by periodically measuring the EH transducer’s output at a given sampling rate. The harvested power as a function of the sampling parameters has been analyzed in a few works, but the power gain achieved with respect to the case of a much slower sampling rate than the EH source’s frequency has not been assessed so far. In this work, simple expressions are obtained that predict this gain assuming a Thévenin equivalent for the EH transducer. It is shown that the power gain depends on the relationship between the square of AC to DC open circuit voltage of the EH transducer. On the other hand, it is proven that harvested power increases, using a suitable constant signal for the MPP voltage instead of tracking the MPP at a low sampling rate. Experimental results confirmed the theoretical predictions. First, a function generator with a series resistor of 1 k¿ was used, emulating a generic Thévenin equivalent EH. Three waveform types were used (sinus, square, and triangular) with a DC voltage of 2.5 V and AC rms voltage of 0.83 V. A commercial MPPT with a fixed sampling rate of 3 Hz was used and the frequency of the waveforms was changed from 50 mHz to 50 Hz, thus effectively emulating different sampling rates. Experimental power gains of 11.1%, 20.7%, and 7.43% were, respectively, achieved for the sinus, square, and triangular waves, mainly agreeing with the theoretical predicted ones. Then, experimental tests were carried out with a wave energy converter (WEC) embedded into a drifter and attached to a linear shaker, with a sinus excitation frequency of 2 Hz and peak-to-peak amplitude of 0.4 g, in order to emulate the drifter’s movement under a sea environment. The WEC provided a sinus-like waveform. In this case, another commercial MPPT with a sampling period of 16 s was used for generating a slow sampling rate, whereas a custom MPPT with a sampling rate of 60 Hz was used for generating a high sampling rate. A power gain around 20% was achieved in this case, also agreeing with the predicted gain.This work was supported by the European Innovation Council under the EU Horizon 287 Europe program ‐ Grant agreement No 101071179, project SUSTAIN (Smart Building Sensitive to 288 Daily Sentiment)”. “The second author was supported by the European Union – NextGenerationEU 289 and the Ministerio de Universidades – Plan de Recuperación, Transformación y Resiliencia under a 290 Margarita Salas post‐doctoral research fellowship (ref. 2022UPC‐MSC‐94068).Peer ReviewedPostprint (author's final draft

Data comparison between three acoustic doppler current profilers deployed in OBSEA platform in north-western Mediterranean

Three different Acoustic Doppler Current Profilers (ADCP) have been deployed in OBSEA platform, a 20 meters depth underwater observatory cabled with a 4 km mixt cable to Vilanova i la Geltru’s coast. Two months of continuous data have been collected in order to confirm their proper operation and long term North current characteristic from the areaPeer ReviewedPostprint (published version

Dynamic analysis of a pendulum-type wave energy converter for oceanic drifters by means of a 4 DoF model

Drifters are Lagrangian instrumentation widely used in oceanography and climate research. They are designed to obtain data from oceans by passively following the water currents. They provide information about the ocean surface such as currents or water temperature. One of the main challenges faced at drifter’s design is their autonomy [1]. The battery exchange is not possible because of the excessively high cost, both from the economic and the environmental point of view. Therefore, some studies tried to deal with this issue by embedding a wave energy converter (WEC) on the drifter: the waves motion is used to generate power through an inner mechanism, so no battery exchange is needed.Peer Reviewe

A new nonlinear compliant mechanism for harvesting energy from ocean waves

Traditonal linear oscillators, such as cantilevers or pendulums, are only sensitive to specific resonant frequencies. They have then very narrow frequency bandwidths when harvesting energy from ocean waves. In order to enhance the dynamic performance of the wave energy converters (WECs), used to expand the autonomy of Lagrangian Drifters, a statically balanced compliant mechanism (SBCM) is investingated. It is based on finite element analysis (FEA) simulaitons. The design of the SBCM is introduced and its static force-displacement curve is obtained in FEA. The dynamic response of the SBCM to harmonic base excitiaons at low frequencies and low accelerations is investigated based on time-domian FEA simulations. The close agreement between simulations, numerial and analytical results verifies that the SBCM is sensitive to ultra-low frequencies with weak accelerations in a wide frequency range. The applicability of the SBCM in WECs is demonstrated by adding PVDF films in the FEA model. In the time-domian simulation, the SBCM-based WEC is excited by the drifter motion pattern obtained from Orcaflex and corresponding to two typical ocean waves (i.e. synthesized Airy and Jonswap models). Relative displacement between the base and mass and the electric outputs are obtained. According to this work, the SBCM provides a structural solution for WECs with enhanced energy harvesting performance.Postprint (published version

Evaluation of two MPPT techniques in low-power pendulum-type wave energy converters

Lagrangian Drifters are autonomous foating passive devices that provide oceanographic surface data. They are low-cost, low-power and easy-deployable marine instrumentation used in climate research. One of the main challenges of drifters is energy autonomy. Wave Energy Converters (WEC) have proved their viability in high-power electric generation plants to work as Kinetic Energy Harvesters (KEH) [1] and now they are also showing up at smaller power rate applications in order to power devices such as ocean buoys and drifters [2]. In this latter case, a wise and efcient energy management is crucial to maximize the lifespan of the device. To achieve this goal, a Power Management Unit (PMU) is used, which can also include a Maximum Power Point Tracker (MPPT) to extract maximum energy from the KEH device.Peer Reviewe

Underwater acoustic slant range measurements related to weather and sea state

Underwater range measurements are key factor in underwater acoustic positioning, used in Long Base-Line (LBL) or Ultra Short Base-Line (USBL) computing techniques. These measurements are commonly carried out through acoustic communications between modems and their accuracy can be affected by different factors, such as sea state, weather conditions, and obstacles in the line of sight propagation. This is especially important in shallow waters areas, where others phenomena such as multi-path have to be considered. Therefore, range accuracy and the associated position estimation errors are an important area of research. Here, we addressed the relation between range measurements variability and sea state (i.e. currents or waves height) as proxy of real-world conditions, affecting acoustic positioning performances. For that purpose, a long-term deployment have been carried out in the underwater cabled observatory OBSEA, which provide different measurements of the sea and weather state.Peer ReviewedPostprint (published version