Pyroelectric conversion: harvesting energy from temperature fluctuations

Peer Reviewe

Effects of cycling on lithium-ion battery hysteresis and overvoltage

Currently, lithium-ion batteries are widely used as energy storage systems for mobile applications. However, a better understanding of their nature is still required to improve battery management systems (BMS). Overvoltages and open-circuit voltage (OCV) hysteresis provide valuable information regarding battery performance, but estimations of these parameters are generally inaccurate, leading to errors in BMS. Studies on hysteresis are commonly avoided because the hysteresis depends on the state of charge and degradation level and requires time-consuming measurements. We have investigated hysteresis and overvoltages in Li(NiMnCo)O2/graphite and LiFePO4/graphite commercial cells. Here we report a direct relationship between an increase in OCV hysteresis and an increase in charge overvoltage when the cells are degraded by cycling. We fnd that the hysteresis is related to difusion and increases with the formation of pure phases, being primarily related to the graphite electrode. These fndings indicate that the graphite electrode is a determining factor for cell efciency.Peer ReviewedPostprint (published version

Entropy characterisation of overstressed capacitors for lifetime prediction

We propose a method to monitor the ageing and damage of capacitors based on their irreversible entropy generation rate. We overstressed several electrolytic capacitors in the range of 33 µF–100 µF and monitored their entropy generation rate View the MathML source(t ). We found a strong relationship between capacitor degradation and View the MathML source(t ). Therefore, we proposed a threshold for View the MathML source(t ) as an indicator of capacitor time-to-failure. This magnitude is related to both capacitor parameters and to a damage indicator such as entropy. Our method goes beyond the typical statistical laws for lifetime prediction provided by manufacturers. We validated the model as a function of capacitance, geometry, and rated voltage. Moreover, we identified different failure modes, such as heating, electrolyte dry-up and gasification from the dependence of View the MathML source(T) with temperature, T. Our method was implemented in cheap electrolytic capacitors but can be easily applied to any type of capacitor, supercapacitor, battery, or fuel cell.Peer ReviewedPostprint (author's final draft

Battery Impedance Spectroscopy obtained from Electrochemical Model

We investigate impedance spectra obtained from an electrochemical model using pulsed signals. We characterize the dependence of NiMH battery parameters such as electrodes porosity and conductivity on impedance. We are able to relate impedance to simulated battery aging. Impedance as a function of state of charge is also investigated. Further characterization on accuracy and resolution and results on temperature dependence will be submitted.Postprint (published version

Impedance characterization of an LCO-NMC/graphite cell: ohmic conduction, SEI transport and charge-transfer phenomenon

Currently, Li-ion cells are the preferred candidates as energy sources for existing portable applications and for those being developed. Thus, a proper characterization of Li-ion cells is required to optimize their use and their manufacturing process. In this study, the transport phenomena and electrochemical processes taking place in LiCoO2-Li(NiMnCo)O2/graphite (LCO-NMC/graphite) cells are identified from half-cell measurements by means of impedance spectroscopy. The results are calculated from current densities, instead of absolute values, for the future comparison of this data with other cells. In particular, impedance spectra are fitted to simple electrical models composed of an inductive part, serial resistance, and various RQ networks—the parallel combination of a resistor and a constant phase element—depending on the cell. Thus, the evolution of resistances, capacitances, and the characteristic frequencies of the various effects are tracked with the state-of-charge (SoC) at two aging levels. Concretely, two effects are identified at the impedance spectrum; one is clearly caused by the charge transfer at the positive electrode, whereas the other one is presumably caused by the transport of lithium ions across the solid electrolyte interphase (SEI) layer. Moreover, as the cells age, the characteristic frequency of the charge transfer is drastically reduced by a factor of around 70%.Postprint (published version

Battery aging impedance spectroscopy and incremental capacity analysis

We analize electrochemical LiFePO4 cells with impedance spectroscopy and incremental capacity analysis in order to establish a correlation with capacity fade. We found that polarization and diffusion impeances increased with aging, but at different rates depending on the aging stage. This aging stage dependence was also found in ICA analyzes, where the lithium intercalation was investigated. A correlation with capacity decrease measured with Coulomb counting was established.Postprint (author's final draft

State of charge dependency of the overvoltage generated in commercial Li-ion cells

The overvoltage that is produced in the cells under operation limits the capacity and power they can deliver. A detailed study about the mechanisms that contribute to that overvoltage—and thus to their lifetime—is required for optimizing the use of batteries as well as their manufacturing process. We investigate galvanostatic discharge at low and moderate rates in an LCO-NMC/graphite cell in order to quantify the ohmic voltage drop and activation and concentration polarizations. For doing so, we compare half-cells to full cell overvoltages. We find that the ohmic drop and concentration polarization dominate at high rates and low rates, respectively. Moreover, we track the evolution of concentration polarization with State-of-Charge (SoC) and we observe that there exists a relationship between diffusion and phase transformations. Specifically, we validate experimentally that initial stages of a phase formation are not dominated by diffusion. Phase transitions are commonly evaluated by incremental capacity analysis. However, we determine that it is more appropriate to obtain that information from the full cells by the overvoltage analysis. Furthermore, we suggest that the working SoC range can be optimized from the overvoltage analysis by avoiding the particular SoCs at which the most detrimental phase transitions take place.Peer ReviewedPostprint (published version

Desarrollo de un sistema de recuperación de amoníaco de purines de cerdo mediante evaporación al vacío

Peer ReviewedPostprint (published version

MATLAB® modeling of a microgrid: towards a vision based on entropy balance

A microgrid is defined as the link of consumer loads and decentralized energy assets with the ability to control the entire system as a whole. The modeling of an electromagnetic microgrid is discussed in this work to analyze energy effectiveness and entropy balance. The method computes the associated losses and simulates the electrical behavior of the components. The model is built and tested by using the MATLAB Simulink application. The system is made up of several blocks consisting of solar panels, batteries, consumer loads, and the inverter needed to connect them all. Their energy efficiency is assessed using the findings from each of these components. The system's entropy generation is also calculated to study how it relates to performance.Peer ReviewedPostprint (published version

Determination of a System’s Entropy Using Pyroelectric Sensors

We propose a system for measuring entropy variations, S, in thermal systems using pyroelectric sensors. These sensors convert time-dependent temperature variations into electrical current. Consequently, heat and temperature variations are obtained, and sensor entropy is inferred. Various polyvinylidene fluoride and lead zirconate titanate sensors have been tested. Two types of measurements are performed. One in the volume, to measure entropy variations in a heat source, and the other on the surface, to measure entropy flux delivered by the heat source. Thermodynamic models and heat transfer dynamic simulations agree with the experimental results and relate the sensor entropy to the heat source entropy. These results show that pyroelectric sensors can enable entropy monitoring of thermal processes to improve system performance.Postprint (author's final draft