Modeling of the Electrochemical Reactions at the Electrode-Electrolyte Interface of Nickel/Metal Hydride Batteries by an Equivalent Electrical Circuit

Based on the experimental impedance spectra, the electrochemical reactions that are deposed at the electrode-electrolyte interface can be modeled by equivalent electrical circuits. Each element used in the circuit must have a physical correspondence in the electrochemical system. In this work, a model has been proposed to a NiMH battery electrode to describe, in detail, the electrochemical process at the interface of this electrode. The theoretical impedance of a proposed circuit is a function of several variables. These adjusted variables to reach a good agreement between the theoretical spectra and the experimental spectra in the studied frequency. The Z-simplex software allows refining the experimental results. These results show a good superposition between the experimental spectra and the theoretical spectra corresponding to the proposed electric circuit. This leads to the conclusion that the proposed circuit describes the phenomena that take place at the interface of the hydride electrode

Dynamic Evolution of Bitterness Units in Beer Worts: Modeling and Concerns

Beer bitterness is reported in International Bitterness Units (IBU), which is the concentration (in ppm) of iso-alpha-acids in the product. Such acids result from the isomerization of alpha-acids originally found in hops which is carried out by boiling the beer wort. This concentration can be measured by HPLC, which led, in the past, to some empirical predictions for the effects of boil time and wort density on the dynamic evolution of IBU in barley worts. Since HPLC is an expensive and time onerous procedure, the organizations devoted to the standardization of procedures in the brewing industry established protocols that indirectly evaluate the IBU by spectrophotometry. This work investigated the dynamic evolution of IBU values as a function of the wort density when evaluated by this standard procedure, which involves the extraction with iso-octane and the UV absorbance measurement in a spectrophotometer. Three barley worts were prepared with dry malt extract (DME) having specific gravities (SG) of 1.030, 1.040 and 1.050. These worts and a fourth sample constituted of distilled water (SG = 1.000) had their pH buffered in 5.2 and were boiled under atmospheric pressure. During the boiling step, Czech Saaz hop pellets were added according to a time schedule (1, 3, 5, 10, 25, 40 and 60 minutes). All reactions were conducted in small (500 ml) aluminum reactors with triplicates for each isomerization time.  Finally, the IBU of the samples were measured and used to fit non-linear empirical models for the IBU evolution in the wort, based on a maximum likelihood statistical criterion along with deterministic optimization methods. Results indicated that IBU values, evaluated by the standard procedure, rise to their final value within a few minutes, instead of one hour, which is traditionally assumed

Algoritmo para estimação dos parâmetros equivalentes de modelos elétricos de baterias de iões de lítio

As fontes renováveis são cada vez mais utilizadas na produção de energia elétrica em todo o mundo. À medida que a população global aumenta e os respetivos preços das tecnologias de produção diminuem, prevê-se um aumento exponencial da sua utilização. Face às características intrínsecas dos recursos naturais, as soluções de armazenamento da energia elétrica são cada vez mais importantes e determinantes no balanço energético. O recente crescimento exponencial da eletrificação e mobilidade elétrica tem sido um fator determinante no aumento da produção de sistemas de armazenamento de energia, principalmente de baterias de lítio. De entre as várias soluções disponíveis, as baterias de lítio apresentam-se, atualmente, como a tecnologia mais madura para suportar as soluções de armazenamento atuais. O presente trabalho tem como objetivo o estudo da impedância das baterias de lítio através da técnica de Espectroscopia de Impedância Eletroquímica (EIS). Para modelar a bateria, desenvolveu-se um modelo equivalente elétrico baseado nesta técnica. Os dados obtidos são analisados através de gráficos de Nyquist. Para reduzir o erro encontrado entre os resultados estimados e os obtidos experimentalmente, foi desenvolvido um algoritmo de otimização capaz de extrair os parâmetros elétricos do modelo. Desta forma, os resultados estimados são equivalentes aos obtidos experimentalmente. Esta modelação matemática foi feita para diferentes condições de funcionamento, i.e., diferentes estados de carga da bateria e diferentes valores de temperatura. Pretendeu-se avaliar o desenvolvimento do estado de saúde da bateria, pelo que se provocou um envelhecimento cíclico à bateria e procedeu-se a uma análise posterior. No final são estudados os gráficos de Nyquist, os parâmetros estimados e a progressão da degradação do estado da bateria é analisada.Renewable sources are increasingly used in the production of electricity worldwide. As the overall population increases and the respective prices of production technologies decreases, an exponential increase in its use is expected. Given the intrinsic characteristics of natural resources, electric energy storage solutions are becoming more important and determining on the energy balance. The recent exponential growth of electrification and electrical mobility has been a determining factor in developing the production of energy storage systems, especially of lithium batteries. Among the various solutions available, lithium batteries are currently present as the most mature technology to support current storage solutions. The present work aims to study the impedance of lithium batteries by using the technique of Electrochemical Impedance Spectroscopy (EIS). To create a model of the battery, an electrical equivalent model based on this technique was developed. The data obtained from this technique is analyzed through Nyquist charts. To reduce the error found between the estimated results and those obtained experimentally, an optimization algorithm capable of extracting the model parameters from the model was developed. In this way, the estimated results are equivalent to those obtained experimentally. This mathematical modeling was made for different operating conditions, i.e., different state of charge and temperature values. For the evaluation of the state of the health parameter, a cyclical aging process was imposed, which allowed a posterior data analysis. In the end the Nyquist graphics and the estimated parameters are studied, and the progression of the battery state degradation is analyzed