Accurate supercapacitor modeling for energy-harvesting wireless sensor nodes

Supercapacitors are often used in energy-harvesting wireless sensor nodes (EH-WSNs) to store harvested energy. Until now, research into the use of supercapacitors in EH-WSNs has considered them to be ideal or over-simplified, with non-ideal behavior attributed to substantial leakage currents. In this brief, we show that observations previously attributed to leakage are predominantly due to redistribution of charge inside the supercapacitor. We confirm this hypothesis through the development of a circuit-based model which accurately represents non-ideal behavior. The model correlates well with practical validations representing the operation of an EH-WSN, and allows behavior to be simulated over long periods

Accurate Parameters Identification of a Supercapacitor Three-Branch Model

Supercapacitors are becoming increasingly important storage system components. To effectively control their terminal voltage, even in real time, numerous circuit models capable of faithfully simulating their behavior in energy systems and various applications are being explored. The three-branch supercapacitor model appears to be a good compromise between simplicity and accuracy. Typically, this model lacks accuracy in dynamic cycling and long stand-by periods. In this study, a new model identification method based on the state equations of the circuit is described and tested on a 400 F supercapacitor, and the obtained results are validated by measurements. Such an approach, suitably optimized, provides good agreement with the measurements, with discrepancies below 50 mV even in repeated cycles. In the static identification, after 90 minutes of self-discharge, the discrepancy was approximately 5 mV. The study also discusses the sensitivity of the model output to the circuit parameters, which is useful for choosing the appropriate timespan for parameter optimization and introduces variable leakage resistance and a method for its determination. Through this parameter, good agreement with the measurements is observed during the long self-discharging phases. A discrepancy of less than 50 mV between the measured and computed results is observed after one week. The union of the circuit state equations based model and the nonlinear leakage resistance determination allows the three-branch circuit model to achieve a high accuracy both in real-time simulation and in the presence of long stand-by phases

Analysis of nickel-copper metallic foam supercapacitor for electric vehicles with hybrid battery-supercapacitor energy storage system

Trabalho final de mestrado para obtenção do grau de mestre em Engenharia Eletrotécnica - Ramo de EnergiaFor centuries, the indicators of CO2 had never been so higher than past half century indicators, accounting for an increase rate of 1,5 ppm/year. This is the direct reflect of global warming, a problem that has motivated researchers on developing adequate alternatives for a variety of human needs, which is the case of transitioning from fossil fuels to renewable electrical powering sources, such as Electric Vehicles trending. This dissertation concerns one of the most delicate aspect about electric vehicles (EVs) design, the Energy Storage System, which regarding autonomy, is the key to overcome the current transportation electric energy sources limitations. Among the numerous known devices, Batteries and Supercapacitors are the main technologies in which electric vehicles energy storage relies on. However, batteries because of their chemical nature can be environmentally harmful when considering disposal aspects, concern which has motivated the energetic development and improvement of greener technologies, such as supercapacitos. This dissertation presents a proposition for the development procedure of a positive electrode nickel-copper hybrid supercapacitor theoretical cell, which was simulated, and the resulting data analyzed and discussed. Therefore, the hybridization of both technologies was subjected to computational simulations in order to test the hypothesis that a direct parallel connection between both technologies could interact collaborating to extend battery’s lifespan, hence, reducing its environmental impact. The simulations have shown that such interaction results in reduced current demand, causing the state of charge wave period to increase, thus, extending the lifespan of batteries.Durante séculos, os indicadores de CO2 nunca estiveram tão elevado quanto no último meio século, o que representa uma aumento de 1,5 ppm/ano. Este dado é reflexo direto dos efeitos do aquecimento global, problema o qual tem motivado pesquisadores no intuito de desenvolver novas e mais adequadas alternativas para nossas mais diversas necessidades, à exemplo disto a transição do consumo de combustíveis fósseis para fontes renováveis de energia elétrica, impactando diretamente nos veículos elétricos. Este trabalho refere-se à um dos aspectos maior peso no projeto de veículos elétricos (VEs), o Sistema de Armazenamento de Energia, o qual em termos de autonomia é crucial para superar as atuais limitações das fontes de energia elétrica para apicações móveis. Entre os inúmeros dispositivos conhecidos, baterias e supercapacitores são as principais tecnologias nas quais o armazenamento de energia de veículos elétricos é hoje dependente, sendo implementado por diversos fabricantes em uma grande variedade de categorias veículares. Entretanto, devido à composição das baterias eletroquímicas, estes dispositivos acabam por se tornarem ambientalmente prejudiciais consoante ao processo de descarte e reciclagem de seus componentes uma vez que compostos químicos possuem processos normalizados e controlados para o descarte. Sendo assim, esta dentre outras questões tem motivado pesquisadores a desenvolverem novos materiais com uma viés ecologicamnte correta assim como o esforço no desenvolvimento de outras tecnologias de armazenamento de enrgias, atuais ou novas, com base nos mesmos princípios já citados. Dentre as tecnologias pesquisadas encontram-se os supercondensadores, os quais devido à diferenciação para com as baterias consoante à tecnologia de armazenamento, não deixam resíduos químicos como consequência de seu descarte, sendo assim considerados dispositivos ecologicamente corretos. Com o intuito de paralelamente estudar e desenvolver um procedimento para a modelização de células de supercapacitores a partir de materiais ensaiados laboratorialmentes com o intuito da utilização como eletrodos de tais dispositivos, as espumas metálicas de níquel-cobre [Eugénio13] foram escolhidas para tal processo, as quais apresentam elevada área de superfície específica para as reações eletroquímicas ocorrerem, fator o qual é conhecidamente motivo de influência nos valores de capacitância obtidos. Devido a proposta original do material estudado ser relacionada ao uso como eletrodo positivo, o processo desenvolvido deu origem a um supercondensador do tipo híbrido, o qual contou com um eletrodo negativo de carbono ativado complementar ao de níquel-cobre. Tal dispositivo mostrou-se capaz de apresentar capacitâncias superiores (454F) quando comparados ao dispositivo base de projeto, Maxwell 2.3V 300F. Entretanto, devido ao potencial eletroquímico inferior do Ni-Cu, o dispositivo híbrido apresentou uma tensão nominal reduzida de 2V quando em comparação ao dispositivo base. Para a implementação dos dados gerados em um bloco padrão do software Matlab Simulink, foi seguido o procedimento descrito por [Zubieta00] sendo que devido a falta de informações, complementarmente foi desenvolvido um procedimento de otimização não linear através do software Microsoft Excel via plugin Solver para a obtanção de tais dados. Esta etapa mostrou-se efetiva uma vez que os dados resultantes mostraram-se correspondentes ao esperado com base em [Zubieta00]. A partir disso, o perfil de corrente imposto ao sistema, conforme apresentado em [Omar09], foi validado através de uma segunda simulação computacional, também via Matlab Simulink, na qual a mesma configuração de elementos de armazenamento de energia, baterias e supercapacitores, foi implementada e submetidos ao perfil de carga mencionado. Esta simulação resultou em dados os quais validaram o apresentado em [Omar09] através da comparação dos perfis de correntes resultantes do banco de baterias e módulo de supercondensadores. Como a vida útil das baterias é determinada pelo consumo de cíclos face ao número máximo determinado em ficha técnica, os quais são computados pelo conjunto dos eventos de carga de descarga (ciclo) do dispositivo, uma terceira simulação foi desenvolvida, esta com o intuito de controlar a imposição de corrente e geração de ciclos completos e uniformes. Tal simulação demonstrou uma redução no na corrente demandada e número de ciclos completados pela bateria em módulo híbrido quando comparada ao mesmo procedimento em modo solo, sendo que esta redução é caracterizada por períodos mais longos entre picos, sendo assim uma extensão teórica da vida útil do dispositivo de aproximadamente 36 minutos (2%). Com o intuito de utilizar a empilhadeira elétrica Carer R45NCF [Omar09] um modelo tridimensional foi desenvolvido através do software AutoCAD 2014 com o intuito de identificar dois parâmeteros necessários, área frontal do veículo e localização do centro de gravidade. A partir dos dados obtidos uma simulação do veículo elétrico (empilhadeira) foi desenvolvida, apresentando como sistema de armazenamento de energia as mesmas configurações já citadas. Nesta simulação o perfil de carga foi convertido de corrente para velocidade angular através das equações das dinamicas de movimento para veículos. Para além disso, neste modelo foram aplicados ambos os supercapacitores, Maxwell 2.3V 300F e supercapacitor híbrido de Ni-Cu/AC. Os resultados obtidos para o dispositivo da Maxwell foram concordantes aos obtidos nas simulações anteriores, sendo que também foi observada a redução nas correntes de descarga das baterias submetidas aos sistemas solo e híbrido. Já para o supercondensador híbrido de Ni-Cu/AC, notou-se um lijeiro aumento na contribuição fornecida para com a bateria, resultado esperado face a diferença de capacitância, consequentemente também de carga, quando comparado ao supercapacitor Maxwell 2.3V 300F. O dispositivo contribuiu para a redução da demanda do sistema para com a bateria fornecendo maior corrente nos momentos de picos de descargas quando comparado ao dispositivo base.N/

Online Parameter Estimation for Supercapacitor State-of-Energy and State-of-Health Determination in Vehicular Applications

WOS:000536291000079Online accurate estimation of supercapacitor state-of-health (SoH) and state-of-energy (SoE) is essential to achieve efficient energy management and real-time condition monitoring in electric vehicle (EV) applications. In this article, for the first time, unscented Kalman filter (UKF) is used for online parameter and state estimation of the supercapacitor. In the proposed method, a nonlinear state-space model of the supercapacitor is developed, which takes the capacitance variation and self-discharge effects into account. The observability of the considered model is analytically confirmed using a graphical approach. The SoH and SoE are then estimated based on the supercapacitor online identified model with the designed UKF. The proposed method provides better estimation accuracy over Kalman filter (KF) and extended KF algorithms since the linearization errors during the filtering process are avoided. The effectiveness of the proposed approach is demonstrated through several experiments on a laboratory testbed. An overall estimation error below 0.5% is achieved with the proposed method. In addition, hardware-in-the-loop experiments are conducted and real-time feasibility of the proposed method is guaranteed

Supercapacitor Electro-Mathematical And Machine Learning Modelling For Low Power Applications

Low power electronic systems, whenever feasible, use supercapacitors to store energy instead of batteries due to their fast charging capability, low maintenance and low environmental footprint. To decide if supercapacitors are feasible requires characterising their behaviour and performance for the load profiles and conditions of the target. Traditional supercapacitor models are electromechanical, require complex equations and knowledge of the physics and chemical processes involved. Models based on equivalent circuits and mathematical equations are less complex and could provide enough accuracy. The present work uses the latter techniques to characterize supercapacitors. The data required to parametrize the mathematical model is obtained through tests that provide the capacitors charge and discharge profiles under different conditions. The parameters identified are life cycle, voltage, time, temperature, moisture, Equivalent Series Resistance (ESR) and leakage resistance. The accuracy of this electro-mathematical model is improved with a remodelling based on artificial neuronal networks. The experimental data and the results obtained with both models are compared to verify and weigh their accuracy. Results show that the models presented determine the behaviour of supercapacitors with similar accuracy and less complexity than electromechanical ones, thus, helping scaling low power systems for given conditions

The nonlinearities in the galvanostatic charging curves of supercapacitors provide insights into charging mechanisms

Supercapacitors are often charged using constant currents. The capacitance can be determined from the slope of the voltage-time curve if the measured voltage over the supercapacitor increases linearly with time. However, the resulting voltage-time curve is often nonlinear, which may lead one to interpret the capacitance as being either time or voltage dependent. In the current work, systematic experimental studies of the nonlinearity of galvanostatic charging curves as a function of applied current and temperature are undertaken for commercial supercapacitors in the range 1–1000 F. A consistent theory is developed to explain the available data. It is demonstrated that the nonlinearity in the voltage-time curve can be attributed to a constant capacitance in parallel with a resistance, the latter which is inversely proportional to the applied current. The influence of faradaic charge transfer reactions or surface charge reorganization on this parallel resistance is analyzed. The proposed theory is also used to analyze galvanostatic charging data available in the research literature, and the different types of nonlinearities observed provide new insight into the mechanisms occurring during charging of various types of supercapacitors.publishedVersio

Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors