Bandwidth Based Methodology for Designing a Hybrid Energy Storage System for a Series Hybrid Electric Vehicle with Limited All Electric Mode

The cost and fuel economy of hybrid electrical vehicles (HEVs) are significantly dependent on the power-train energy storage system (ESS). A series HEV with a minimal all-electric mode (AEM) permits minimizing the size and cost of the ESS. This manuscript, pursuing the minimal size tactic, introduces a bandwidth based methodology for designing an efficient ESS. First, for a mid-size reference vehicle, a parametric study is carried out over various minimal-size ESSs, both hybrid (HESS) and non-hybrid (ESS), for finding the highest fuel economy. The results show that a specific type of high power battery with 4.5 kWh capacity can be selected as the winning candidate to study for further minimization. In a second study, following the twin goals of maximizing Fuel Economy (FE) and improving consumer acceptance, a sports car class Series-HEV (SHEV) was considered as a potential application which requires even more ESS minimization. The challenge with this vehicle is to reduce the ESS size compared to 4.5 kWh, because the available space allocation is only one fourth of the allowed battery size in the mid-size study by volume. Therefore, an advanced bandwidth-based controller is developed that allows a hybridized Subaru BRZ model to be realized with a light ESS. The result allows a SHEV to be realized with 1.13 kWh ESS capacity. In a third study, the objective is to find optimum SHEV designs with minimal AEM assumption which cover the design space between the fuel economies in the mid-size car study and the sports car study. Maximizing FE while minimizing ESS cost is more aligned with customer acceptance in the current state of market. The techniques applied to manage the power flow between energy sources of the power-train significantly affect the results of this optimization. A Pareto Frontier, including ESS cost and FE, for a SHEV with limited AEM, is introduced using an advanced bandwidth-based control strategy teamed up with duty ratio control. This controller allows the series hybrid’s advantage of tightly managing engine efficiency to be extended to lighter ESS, as compared to the size of the ESS in available products in the market

Voltage equalisation techniques for high capacitance device modules

Phd ThesisTraditionally, the electrochemical battery has been the prime medium by which electrical energy is stored for future use. Increasingly, the demands of modern systems such as electric vehicles, renewable energy, distributed generation, smart grid and others has stretched the development of new chemistries, materials and assembly techniques for electrochemical batteries. Additionally, some load profiles in these applications demand extremely high dynamic behaviour which is either undeliverable by conventional electrochemical batteries or is undesirably damaging to these technologies. As such, a family of electrochemical storage, known generally as supercapacitors or ultracapacitors, have been developed and implemented for such applications. In recent years advancements in electrochemical technology has led to hybridisation of high capacitance devices. Lithium-ion capacitors that are used in this work are, with their higher cell voltage and modern packaging, expected to be among the next emerging families of state-of-the-art electrical energy storage devices. The relatively low cell voltage of high capacitance cells requires them to be connected in series to attain a system level voltage. During charging and discharging, manufacturing tolerances between the cells results in voltage mismatch across the stack. Mismatched voltages are an inefficient use of the energy storage medium and can lead to dangerous failures in the cells. Several techniques exist to limit the variance in cell voltages of supercapacitors across a series connected stack. These range from simple systems which discharge the cells at higher voltages through resistors to more complex active converter systems which equalise the cell voltages through charge redistribution via a power electronic converter. Whilst the simpler schemes are effective they are very inefficient and as such are not suitable for use in many applications. A number of active converter voltage equalisation schemes have been proposed in literature, however, each of these equalisation schemes exhibit flaws which either makes them less desirable or less effective for a broad range of applications. Therefore, a new equalisation converter topology is proposed which is designed for greater equalisation effectiveness, modularity and size. The proposed equalisation converter differs from previously published equalisation schemes by allowing energy transfer between any pair of cells without the cumbersome multi-winding transformers employed in existing equalisation converters. The new equalisation scheme uses a bi-directional arrangement of MOSFET switches for galvanostatic isolation allowing the converter to be multiplexed to the stack. This arrangement allows the total size of the equalisation scheme to be reduced whilst maintaining performance.EPSRC