Enhanced Equivalent Electrical Circuit Model of Lithium-based Batteries Accounting for Charge Redistribution, State-of-Health and Temperature Effects

Accurate models capable to predict the dynamic behavior and the State-of-Charge (SoC) of Battery Energy Storage Systems (BESSs) is a key aspect for the definition of model-based controls in electric vehicles and in power grid applications of these energy storage systems. In this context, the paper presents an enhanced electrical BESS model capable to accurately represent the effects of charge redistribution in Lithium-based cells. In fact, this phenomenon is the main source of nonlinearity in the behavior of such devices. The improvement of the proposed model is achieved by a virtual DC current generator accounting for the internal charge transfer. The behavior of this virtual generator is inferred from experimental results describing the effects of the charge redistribution during different sub-phases, namely charge, discharge and rest phases. The proposed model, along with its parameter assessment, has been experimentally validated for: i) two types of Li-ion chemistry, ii) aged cells; iii) different cell operating temperatures ranging from -20°C up to 55°C, iv) a complex battery cell pack of 25 kWh rated energy.

Toward Ultrafast Charging of Electric Vehicles

Massimiliano Capezzali, Stefano Grioni, Hardi Hõimoja, Hans Björn Püttgen, Alfred Rufer, Michail Vasiladiotis

The paper is devoted to the problems arising from the ultrafast (≤ 10 min) charging of an electric vehicle (EV). An ultrafast charging station (UFCS) must provide high power output with minimal influence on the electricity transmission system, which can only be achieved by the application of energy storage acting as an additional buffer between the vehicle and the grid. Besides storage, interfaces between a fast charging station and the outside environment (vehicle, utility grid) must be designed to fulfil a set of requirements. The main challenge is to be found within the specification of parameters for the design of future energy supply systems, providing for fast charging of the vehicle batteries while avoiding solicitations of the local distribution system which exceed its instantaneous power capabilities. The possible impact of an UFCS on the power distribution system is analysed with the stochastic approach, based on the utilisation of such a station. The general aspects of highly variable load profile clearly include the use of energy storage means that must be specified regarding both the energy storage and the instant power capabilities. Different technologies are analysed in terms of performances and costs. In conclusion, one of the important problems facing electric vehicles is the possibility of short-time charging, both as seen from the EV battery itself as well as from the local supply system. In this context, large load variations as seen from the local power system, at multiple levels, must be carefully assessed with special attention to feasible load changes at the coupling points. By addressing these technical issues as well as the financial constraints, the research aims at providing viable solution for broad ultrafast charging systems integration into the power distribution infrastructure.

2012

Photocatalytic and Electrocatalytic Reduction of Carbon Dioxide in Pressurized Systems

Patrick Voyame

The depletion of carbon-based fossil fuels and the rise in atmospheric carbon dioxide concentration will force an inevitable change in the future global energy landscape. CO2 reduction presents the advantages of decreasing its atmospheric concentration and storing energy in chemical form in CO2 reduction products. With a predicted conversion to renewable energy such as solar or wind energy, energy storage will become a key process in the near future for buffering the fluctuating energy production. The objective of the present work was to study the efficiency towards CO2 reduction of different molecular catalysts. In particular, conducting experiments in high-pressure and supercritical conditions and observing the effect of CO2 pressure or concentration on the efficiency and selectivity of the reaction. As supercritical CO2 (scCO2) has poor solubilisation capabilities, experiments were conducted in biphasic systems or with addition of an organic co-solvent. To drive the reduction reaction of CO2, a catalyst is needed to overcome the kinetic limitations of the reaction, but an energy input is also necessary. Three different forms for this energy input were used in this work. In a first time a sacrificial product, decamethylferrocene (DMFc), was used to transfer electrons to CO2 in biphasic water/scCO2 systems. Complete oxidation of the DMFc was observed in presence of anion capable of transporting protons from water to the DMFc present in the supercritical phase. A photosensitization cycle was used to supply a water soluble catalyst, NI(II)Cyclam, in electron at the required potential to drive the reduction of CO2 into carbon monoxide in water/scCO2 system. The creation of the interface in the system appeared highly favourable to the efficiency of the catalyst. A second catalyst, a ruthenium polypyridyl carbonyl complex, was used for the photocatalytic reduction of CO2. Pressure had an important impact on the production of one of the two reduction product, carbon monoxide, while the production of formate was unaffected by CO2 pressure. As limitations in productivity in photocatalytic experiments were coming principally from the photosensitizer cycle or the sacrificial electron donor, photosensitizer was replaced by an electrode to provide the catalyst in electrons. Voltammetry in CO2-expanded liquids was described and determination of important parameters such as catalyst concentration and diffusion coefficient as a function of pressure was performed. Electrocatalytic reduction of CO2 by ruthenium and rhenium polypyridyl carbonyl complexes was studied in CO2-expanded liquids. The catalytic mechanisms were observed to be highly influenced by CO2 concentration.

EPFL2016

Non-Parametric Estimation of Surface Temperature of Li-ion Cells using Thermal Impulse Response

Mario Paolone, Dimitri Torregrossa, Ying Xiao

This paper focuses on the prediction of temperature profiles on the surface of lithium-ion cells using a non-parametric method. In particular, this paper proposes the adoption of the impulse response technique to compute and interpret cell surface temperatures through generic discharge conditions. The method requires data obtained from dedicated experiments performed on the targeted cell. In order to develop the proposed method, these experiments aim at obtaining cell surface overshoot in temperature profiles consequent to current discharge impulses that are used as reference inputs. A set of dedicated experimental validations is included in the paper to verify the accuracy of the proposed method for different state-of-health of the cell and environmental conditions. The results show an error in the temperature prediction of less than 1.8% in the entire predicted temperature subject to generic current discharge profiles as well as a negligible error on the predicted peak temperature. It is important to underline that the proposed technique does not require any knowledge of the internal structure of the targeted cell or any properties of material.