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Electric vehicle (EV) adoption continues to rise, yet EV sales still represent a small portion of vehicle sales in most countries. An expansion of the DC fast charging (DCFC) network is likely to accelerate this revolution towards sustainable transportation, giving drivers more flexible options for charging on longer trips. However, DCFC presents a large load on the grid which can lead to costly grid reinforcements and high monthly operating costs – adding energy storage to the DCFC station can help mitigate these challenges. This thesis first performs a comprehensive review of DCFC stations with energy storage, including motivation, architectures, power electronic converters, and detailed simulation analysis for various charging scenarios. The review is closely tied to current state-of-the-art technologies and covers both academic research contributions and real energy storage projects in operation around the world. It is identified that the battery energy storage systems (BESSs) with active front end converter provides high efficiency with reasonable power density in a DCFC station. It is also realized that the isolated DC/DC converter interfacing BESS and EV determines the overall efficiency of a DCFC station with a low grid connection. Secondly, this thesis analyzes the impact of active front end based DCFC stations connected to a grid distorted with background voltage harmonics. In active front end based DCFC stations, background voltage harmonics produce current not only at the frequencies of the distorted voltage, but also at other coupled frequencies. Various mitigation techniques, such as increasing inner control loop gain, grid voltage feedforward, and selective harmonic compensation, have been adopted in industry to reduce the emissions originating from distorted background voltage. However, although these techniques are effective in suppressing the current at the harmonic orders present in the background voltage, they deteriorate the emission at coupled frequencies. This thesis provides the theoretical explanation of this phenomenon, which is verified by simulation of a two-level active front end in PSCAD/EMTDC. This thesis also discusses the proper treatment of current emission due to background voltage harmonics. Thirdly, the thesis identifies the semi dual active bridge (semi-DAB) converter as an ideal candidate as the interfacing isolated DC/DC converter between the BESS and the BEV. A novel control strategy is proposed for the semi-DAB converter to achieve wide voltage gain while increasing the efficiency at operational points with high input voltage and low output voltage, which is a commonly occurring scenario when the BESS is fully charged, and the EV battery is at low charge. Furthermore, this thesis also provides an algorithm to determine the required phase-shift in real time for any operating point, eliminating the need to devise the control trajectory offline. A 550 V, 10 kW experimental prototype is built and tested to validate the proposed control strategy. With a 25 A constant charging current, the prototype shows the proposed control strategy can improve efficiency by up to 3.5% compared to the well-known dual phase shift control at operating points with high input voltage (450 – 550 V) and low output voltage (150 – 275 V), with a peak efficiency of 97.6%. Finally, this thesis proposes a novel variable turns-ratio semi-DAB converter to improve its overall efficiency even further when the input voltage is high and the output voltage is low. Furthermore, a control law is also proposed to determine the turns-ratio, i.e., the operational structure of the converter, which reduces the converter peak and rms current. The 550 V, 10 kW prototype is modified to accommodate the variable turns-ratio high frequency transformer to test the proposed converter and control. The proposed converter with control can further improve the efficiency at many operating points compared to single turns-ratio semi-DAB with DPS control. The peak efficiency achieved is 98.5%. / Thesis / Doctor of Philosophy (PhD)

This study investigates the economic benefits of installing a lithium-ion battery storage (lithium iron phosphate, LFP and lithium titanate, LTO) at an electric bus fast charging station.  It is conducted on a potential electric bus system in the Swedish city Västerås, and based on the existing bus schedules and routes as well as the local distribution system. The size of the energy storage as well as the maximum power outtake from the grid is optimized in order to minimize the total annual cost of the connection. The assessment of the distribution system shows that implementing an electric bus system based on opportunity charging in Västerås does not cause over-capacity in the 10 kV grid during normal feeding mode. However, grid reinforcements might become necessary to guarantee potential backup feeding modes. Batteries are not a cost effective option to decrease grid owner investments in new transformers. However, battery energy storage have the possibility to decrease the annual cost of connecting a fast charging station to the low-voltage grid. The main advantage of the storage system is to decrease the fees to the grid owner. Of the studied batteries, LTO is the most cost effective solution because of its larger possible depth-of-discharge for a given cycle life. The most important characteristics, that determine if a fast charging station could benefit economically from an energy storage, is the bus frequency. The longer the time in between buses and the higher the power demand, the more advantageous is the energy storage.

Un modèle décrivant les phénomènes physiques internes de batteries lithium-ion est développé pour une détection précise de leur état, avec application au domaine de l'industrie automobile. Pour pouvoir utiliser le modèle à des fins de contrôle de charge rapide, un observateur de vieillissement est tout d'abord conçu et intégré au modèle de batterie. Dans un second temps, une stratégie de contrôle de charge rapide robuste est conçue. Elle est basée sur un contrôleur Crone capable de gérer les grandes incertitudes paramétriques du modèle de batterie tout en atteignant l'objectif de charge rapide. Enfin, quelques simplifications du modèle de batterie, de la technique d'optimisation et de la définition des profils de charge rapide sont proposées et évaluées afin de rendre l'ensemble de la stratégie de recharge rapide applicable à un système embarqué de gestion de batterie. / A physics-based battery model is developed for an accurate state-detection of batteries in the automotive industry. In order to use the model for the purpose of fast charging control an aging observer is designed and integrated to the battery model. In a subsequent step a robust fast charging control is introduced to design a controller able to deal with large parametric uncertainties of the battery model while achieving the fast charging target. Finally some simplifications in the battery model structure, in the optimization technique and in the definition of fast charging profiles are proposed and evaluated to make the whole model applicable for an onboard battery management system.

L'objectif de cette thèse est l’étude de la charge rapide de batteries lithium-ion basée sur la méthode de la compensation chute ohmique. Cette méthode permet théoriquement de réduire le temps total de charge des batteries. Dans cette thèse, cette méthode a été mise en œuvre sur trois types différents de cellules de format 18650 : C/ FP, C/NMC et LTO/LFP. Cette méthode montre de bons résultats pour les batteries C/LFP et LTO/LFP avec une réduction du temps de charge total d'environ 70% par rapport à la méthode classique. Néanmoins, cette méthode présente des inconvénients comme notamment l’élévation de la température interne de la batterie pendant la charge rapide. De plus, cette méthode implique un courant élevé et conduit à des potentiels élevés qui peuvent engendrer également des dégradations. En particulier, nous avons démontré que la batterie C / LFP subissait des dégradations internes notamment une déformation mécanique de l’enroulement et une dégradation de la composition d’électrolyte. / The aim of this thesis is to study fast-charging of lithium-ion, battery using the ohmic-drop compensation method. The latter method theoretically will reduce the total charging of the batteries considered. In this thesis, the ODC method was implemented on three different types of 18650 battery cells. These batteries are C/LFP, C/NMC and LTO/LFP. This method show a good result for C/LFP and LTO/LFP batteries with a reduction of total charging time of about 70% in comparison with the classical method. Nevertheless, there are some issues regarding this method; the temperature elevation of the battery is high during fast-charging. Indeed, almost all fast-charging procedure experiences the same problem concerning that matter. Moreover, with ODC fast-charging method, high current rate and high voltage will worsen the situation. These complications of the ODC fast-charging method are key points for both performance and durability of the batteries. Particularly, we have demonstrated that C/LFP battery underwent internal degradation as a mechanical deformation of the active materials and degradation of electrolyte.

This diploma thesis refers to Terminal project 1 and Terminal project 2 from former years of my studies. This work engages in design and construction of high-current charger for the electric vehicle Peugeot 106. Firstly, there is given an account of electric vehicle. There is made reference to its parameters and the way of its battery charging part. Secondly, the work deals with design concept of high-current charger and with its theoretical lay-out. Respectively, this work substantially touches on issues of chosen power, control, regulative and protective circuits of high-current charger. Thirdly, practical assembly procedure of high-current charger is detailed. Dimensioning of particular elements is described in this work. Photos and schemes of given apparatus are shown. Last but not least, results of measurement on assembled high-current charger are stated in this work.

Potential users of plug-in electric vehicles often ask for public charging facilities before buying vehicles. Furthermore, the speed of public charging is often expected to be similar to conventional refueling. For this reason, research on and political interest in public charging focus more and more on fast charging options with higher power rates, yet estimates for future needs are rare. This paper tries to fill this gap by analyzing current charging behavior from a large charging data set from Sweden and Norway and take the findings to calibrate a queuing model for future fast charging infrastructure needs. We find that the ratio of battery electric vehicles to public fast charging points can be similar to other alternative fuels in the future (close to one fast charging point per 1000 vehicles for high power rates of 150 kW). In addition, the surplus on the electricity prices for payoff is only 0.05–0.15 €/kWh per charging point. However, charging infrastructure needs highly depend on battery sizes and power rates that are both likely to increase in the future.

For a successful market take-up of plug-in electric vehicles, fast-charging stations along the highway network play a significant role. This paper provides results from a first study on estimating the minimum number of fast-charging stations along the European highway network of selected countries (i.e., France, Germany, the Benelux countries, Switzerland, Austria, Denmark, the Czech Republic, and Poland) and gives an estimate on their future profitability. The combination of a comprehensive dataset of passenger car trips in Europe and an efficient arc-cover-path-cover flow-refueling location model allows generating results for such a comprehensive transnational highway network for the first time. Besides the minimum number of required fast-charging stations which results from the applied flow-refueling location model (FRLM), an estimation of their profitability as well as some country-specific results are also identified. According to these results the operation of fast-charging stations along the highway will be attractive in 2030 because the number of customers per day and their willingness to pay for a charge is high compared to inner-city charging stations. Their location-specific workloads as well as revenues differ significantly and a careful selection of locations is decisive for their economic operation.

Plug-In electric vehicles (PEV) are in an early market phase in almost all markets. Still, the lack of public charging infrastructure is a barrier to PEV adoption. The assessment of future charging infrastructure needs is often based on key figures, mainly the ratio of PEV to public charging points. However, countries differ regarding their framework conditions, e.g. the availability of home charging, and the question of how much public charging infrastructure is needed cannot be answered equally for all countries. Yet, studies analyzing the framework conditions for the medium- to long-term demand for charging infrastructure are rare. Here, we review the existing literature and summarize the evidence for the importance of framework conditions on charging infrastructure needs. Furthermore, we illustrate the literature evidence by comparing the framework conditions for charging infrastructure in different countries based on a comprehensive dataset of framework parameters. We find public charging infrastructure as alternative to home charging is only needed in some densely populated areas. However, framework conditions vary largely among countries. Accordingly, findings from literature for specific countries can only be transferred to other countries to a limited extent.

abstract: Lithium titanium oxide (LTO), is a crystalline (spinel) anode material that has recently been considered as an alternative to carbon anodes in conventional lithium-ion batteries (LIB), mainly due to the inherent safety and durability of this material. In this paper commercial LTO anode 18650 cells with lithium cobalt oxide (LCO) cathodes have been cycled to simulate EV operating condition (temperature and drive profiles) in Arizona. The capacity fade of battery packs (pack #1 and pack#2), each consisting 6 such cells in parallel was studied. While capacity fades faster at the higher temperature (40°C), fading is significantly reduced at the lower temperature limit (0°C). Non-invasive techniques such as Electrochemical Impedance Spectroscopy (EIS) show a steady increase in the high-frequency resistance along with capacity fade indicating Loss of Active Material (LAM) and formation of co-intercalation products like Solid Electrolyte Interface (SEI). A two-stage capacity fade can be observed as previously reported and can be proved by differential voltage curves. The first stage is gradual and marks the slow degradation of the anode while the second stage is marked by a drastic capacity fade and can be attributed to the fading cathode. After an effective capacity fading of ~20%, the battery packs were disassembled, sorted and repackaged into smaller packs of 3 cells each for second-life testing. No major changes were seen in the crystal structure of LTO, establishing its electrochemical stability. However, the poor built of the 18650-cell appears to have resulted in failures like gradual electrolytic decomposition causing prominent swelling and failure in a few cells and LAM from the cathode along with cation dissolution. This result is important to understand how LTO batteries fail to better utilize the batteries for specific secondary-life applications. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2019