End Of Discharge Process Research Articles

Simulation of single and two-phase refrigerant compression in rotary compressors

Liquid slugging, which often happens during startup and defrosting processes of the air conditioning, is a great threat to the reliability of rotary compressors, because of its high pressure in the compression chamber. In this paper, a coupled mathematical model that can predict the compression process of refrigerants in three different states in the compressor cylinder is proposed. The refrigerant phase change, heat exchange between refrigerant and wall, leakage through leak passage and refrigerant kinetic energy change rate are considered in the model. The coupled solution method and its Fortran calculation program are carried out. This mathematical model is experimentally verified by using R290 as refrigerant. The results show that the simulation results of pressure variation in the compressor cylinder during the refrigerant two-phase compression and discharge processes are consistent with the experimental values. The liquid slugging happens with the maximum pressure of 2400 kPa in the cylinder under low suction gas mass fraction. The R290 is more prone to condense during in-cylinder compression than that of R22 and R410A under low wall temperature condition. The larger the volume change rate of the compression chamber, the higher compressor rotation speed, and the smaller the flow area at the end of discharge process will exacerbate the liquid compression phenomenon.

A novel designed visualized Li-ion battery for in-situ measuring the variation of internal temperature

Since thermal safety is a significant topic in Li-ion batteries (LIBs), monitoring the internal temperature can offer valuable information to clarify the mechanism of heat accumulation. However, it is a challenge to measure the internal temperature of LIBs due to the harsh electrochemical environment and unexpected impacts on the battery performance. For addressing such issue, a novel internal temperature measurement technique with a minimized influence on the electrochemical performance is developed for in situ measuring the internal temperature of the LIB. In the specific design, one end of the LIB is sealed with a BaF2 optical glass, through which the infrared camera is allowed for real-time monitoring the internal temperature during charging and discharging processes. In addition to a stable electrochemical performance in such the visualized LIB, the observation results indicate that the temperatures are different during the charging and discharging processes. A maximum temperature difference 2.0°C between internal and surface of the visualized LIB is observed at the end of discharge process under 2C rate condition. It indicates that internal temperature is more important to analyze the effects of temperature on the thermal safety of the high capacity LIBs. The proposed internal temperature measurement technique could be widely used for analyzing the effects of temperature on the thermal safety of LIBs in further work.

A Semi-Empirical Aging Model for Lithium Iron Phosphate Electrode

First, galvanostatic performance of a pristine lithium iron phosphate (LFP) electrode is studied by introducing a variable resistance single particle model (SPM) that is verified by experimental data from a Li/LFP coin cell. The empirical variable resistance is coupled with SPM to account for the poor ionic and electrical conductivity features of LFP. This variable resistance represents two features of LFP active material: 1. The low ionic conductivity of LFP active material results in increasing the diffusion overpotential especially at the end of discharge where the larger particles participate in the intercalation/deintercalation of ions. 2. The resistive-reactant feature of this material increases the ohmic resistance where poorly coated particles (intraparticle resistance) and poorly connected particles to the matrix (interparticle resistance) play important roles at the end of discharge process. Based on an inverse method, a Parameter Estimation (PE) process is conducted to provide the most influential electrochemical parameters of the LFP positive electrode. These parameters are the solid diffusion coefficient (Ds,p), the intercalation/deintercalation reaction-rate constant (Kp), the total electroactive area of particles (Sp), and the unknown coefficients in the cell resistance equation. In this regard, a least square function and the Genetic Algorithm (GA) are employed as the objective function and the optimizer of the inverse method, respectively. After finding all unknown parameters for a pristine Li/LFP half-cell, the most important parameters, which change by aging, are detected from the analysis of experimental data. These data are extracted from a high-power Li/LFP coin cell built at the Laboratoire d'électrochimie interfaciale et appliquée (LÉIA) of Université de Sherbrooke. The experimental data consist of galvanostatic 1C charge/discharge curves and electrode impedance spectroscopy (EIS) of the cathode versus Li foil as the counter and reference electrode. Based on EIS, we conclude that the charge transfer and the electrolyte resistances both increase with cycling. In fact, damage made in the conductive coating around active material particles results in both decreasing reaction sites and losing active materials. Consequently, charge transfer resistance increases. A variable total electroactive area, representing the reduction in the reaction sites and the increase of the charge transfer resistance, is considered as the main parameter changed by aging. The increase of the electrolyte resistances, on the other hand, is addressed by the variable resistance, which also introduced to consider diffusion overpotential and ohmic resistance at the end of discharge process. Comparisons between the experimental results and the model predictions show that the variable resistance SPM is able to predict the performance of LFP positive electrode. Decreasing total electroactive surface area and increasing resistivity of LFP active material are found to be the most important parameters to simulate aging phenomena in this active electrode material.

Analysis of Polarization in Realistic Li Ion Battery Electrode Microstructure Using Numerical Simulation

In order to explore the local effects of electrode microstructural inhomogeneity, a half cell Li ion battery was built with reconstructed realistic LiCoO2 cathode electrode which was divided equally into eight subdomains. A previously developed C++ code was used to simulate the discharge and charge processes using finite volume method. The geometric properties of each subdomain were characterized by porosity, tortuosity and specific interfacial area. The average variables, such as Li ion concentration and reaction current density, and polarization due to different sub-processes, such as activation of electrochemical reactions (PAER) and charge transport of species were calculated in each subdomain. The inhomogeneous geometric properties result in non-uniform polarization distribution. The polarization due to ionic transport depends on the position, tortuosity and porosity. The porosity, specific interfacial area, state of discharge/charge and position play an important role in the distribution of PAER. The PAER difference between subdomains tends to enlarge at the end of discharge process but to reduce at the end of charge process although the main electrochemical reaction always takes place in the subdomains with smaller porosity. In addition, the PAER is much larger than the polarization due to ionic transport. By comparing with the results from pseudo 2D model, we found that microstructural inhomogeneity in the 3D model has significant impact on global polarization and polarization distribution in the electrode. The presented method provides a possibility to study local effect of inhomogeneous geometric properties.

Charge-Discharge Characteristics of an Adsorbed Natural Gas Storage System under Ambient Conditions

The performance of an adsorbed natural gas (ANG) storage system with natural convection heat transfer between the ANG bed and the ambient air is studied. Results are obtained for the bed without and with external fins on ambient air side. A one dimensional transient conduction model with suitable kinetic equation is formulated to simulate the performance of the bed filled with a homogeneous mixture of activated carbon and graphite. The model duly considers non-ideal behaviour of natural gas, variable specific heat of the adsorbed phase and heat of adsorption. Results are obtained for the case of constant pressure charging and constant flow discharging. The performance of the ANG bed is evaluated in terms of delivery capacity and discharge time. Results are obtained at an ambient temperature of 308 K and 35 bar for a charging time of 3.34 min. It is found that under this condition, the bed temperature increases by 70 and 45K and the storage capacity reduces by 75 and 60% without and with external fins, respectively. During discharge also, due to insufficient heat supply the bed temperature drops to very a low value thereby increasing the amount of adsorbate retained at the end of discharge process. This study clearly shows the need for improving the heat transfer rate from or to the ANG bed for higher delivery capacity.

Analysis of Polysulfide Dissolved in Electrolyte in Discharge-Charge Process of Li-S Battery

In this paper, the qualitative and semi-quantitative analysis of polysulfide species dissolved in electrolyte during the discharge-charge process and cycling of Li-S batteries was reported. ICP-OES (Inductively coupled plasma- Optical emission spectrometer) measurement was used to estimate the total sulfur content dissolved in electrolyte. Lithium polysulfide with different order were separated and confirmed by LC-MS (Liquid chromatography coupled with mass spectrometry). Li2S4 and Li2S6 were proved to be the most stable form of lithium polysulfide species. At the end of discharge process, total sulfur content in the form of Li2S4 and Li2S6 remained in electrolyte was about 20% of the active material in initial cathode. At the end of charge process, 45% total sulfur content was preserved in electrolyte mainly in the form of Li2S6. The partial transformation of active material from liquid phase to solid phase resulted in relatively low practical specific capacity than the theoretical. In cycles, active material transferred between liquid and solid phase kept a balance, and the content of total sulfur, Li2S4 and Li2S6 were changed slightly at the end. Consequently, polysulfide dissolved in electrolyte just took limited responsibility for the capacity fading with cycles of Li-S batteries.