Clay S. Hearn

Utilization of Optimal Control Law to Size Grid-Level Flywheel Energy Storage

This paper presents a method for sizing grid-level flywheel energy storage systems using optimal control. This method allows the loss dynamics of the flywheel system to be incorporated into the sizing procedure, and allows data-driven trade studies to be performed which trade peak grid power requirements and flywheel storage capacity. A demonstration of the sizing methodology will be illustrated through a case study based on home consumption and solar generation data collected from the largest smart grid in Austin, Texas, USA.

Low Cost Flywheel Energy Storage for a Fuel Cell Powered Transit Bus

This paper presents work that was performed to design a compact flywheel energy storage solution for a fuel cell powered transit bus with a focus on commercialization requirements. For hybrid vehicle applications, flywheels offer much higher power densities than conventional batteries. The presented design attempts to maximize the use of lower-cost technologies. The rotor relies primarily on steel for the flywheel structure, and emphasis is placed on size reduction for vehicle packaging advantages Simulations of bus configurations on measured routes was performed using PSAT to correctly size the flywheel energy storage system.

Electrically dewatering microalgae

Microalgae are being developed as a source of fuels and/or chemicals. A processing challenge is dewatering the algae. Electrical approaches to dewatering include exploiting electrophoresis or electroflocculation. The reported experiments show that electrophoresis does occur but is complicated by the effects of the fluid motion. It appears that the coupling of the algal cell and the fluid can be sufficiently strong such that fluid motion effects can influence or dominate behavior. Electroflocculation appears to be a robust process. It does, however, inherently leave electrically induced trace metal flocculants in the dewatered algae.

Modeling and evaluation of a plug-in hybrid fuel cell shuttle bus

The Center for Electromechanics at The University of Texas at Austin acquired a plug-in hybrid fuel cell bus for demonstration and model development under a program funded through the USDOT-FTA. The purpose of this program was to evaluate the performance and use of the bus while developing a model that could predict overall performance and energy consumption on daily driving routes. A model of the fuel cell bus was developed using PSAT (Powertrain Analysis Toolkit). The model development involved verifying component characteristics and a parametric study of drivetrain efficiencies to relate predicted to measured vehicle energy consumption data from on-road testing. The PSAT model was able to predict net energy consumption to within 5% over varying route profiles and vehicle conditions. Further investigations with advanced energy storage were performed to evaluate the benefits of ultracapacitor assisted batteries by using the correlated PSAT model. Ultracapacitors act as an additional load leveling device in the hybrid vehicle for peak propulsion and braking vehicle loads, thereby reducing stress on the batteries. The model simulation results show that ultracapacitors can increase overall vehicle economy by 2 to 4% and deliver a net increase in battery efficiency of 3 to 4%.

Splits of windage losses in integrated transient rotor and stator thermal analysis of a high-speed alternator during multiple discharges

For a high speed electrical alternator, the rotor outer banding and stator inner liner are typically made of high strength graphite epoxy composites due to their high strength and stiffness. Machine structural integrity at high rotating speeds degrades significantly as the composite resins lose their strength at high temperatures. The magnitude of the frictional windage losses generated in the air gaps and the splits of the windage losses between the rotor and stator become crucial to the machine design since these windage losses greatly influence the rotor outer and stator inner surface temperatures. Splits of windage losses generated by an enclosed high speed composite rotor in low air pressure environments were investigated by The University of Texas at Austin Center for Electromechanics and described in a companion paper. The windage splits are dictated by the air temperature gradients at the rotor outer and stator inner surfaces. Unique heating, cooling, and component material properties of a typical highspeed alternator during repetitive-discharge events make its transient air-gap windage splits very much different from those of the test setup. This paper describes transient windage splits in integrated rotor and stator thermal analyses of a high-speed alternator designed for multiple discharges. The transient windage splits in the air-gap airflow were obtained through multiple iterations on windage losses, air-gap air temperatures, and rotor and stator surface temperatures.

Reduced-Order Dynamic Model of Permanent Magnet and HTSC Interaction in an Axisymmetric Frame

This paper presents a reduced order model for a permanent magnet (PM) and high-temperature superconductor (HTSC) in an axisymmetric frame. This model is formulated as a bond-graph to aid integration into system models for applications such as lift bearings, where the nonlinear force-displacement interactions are important for stability analysis and control design. The reduced-order model is based on the mechanical and electromagnetic interaction between a PM and bulk HTSC. Performance of the proposed reduced-order model is compared to finite element method (FEM) analysis and experimental tests to confirm the static and transient performance.

Ultra-stiff, low mass, electromagnetic gun design

High performance in an electromagnetic (EM) gun implies high velocity with minimal transition from a solid to plasma armature. Factors that affect gun performance include armature integrity, bore straightness, and bore stiffness. Experiences firing solid armature at the Center for Electromechanics at The University of Texas at Austin since 1987 have shown that the lack of one or more of these three ingredients will result in less than desirable performance. This work presents a simple, ultra-stiff and low mass EM gun design that provides five to six times the rail-to-rail structural stiffness than a conventional bolted, composite sidewall-type EM gun construction. This translates into minimal bore deflection which lessens the amount the armature must distort to maintain a low voltage contact with the rails. The EM gun design incorporates a passive preloading mechanism that maintains a compressive stress state in the bore components without the use of hydraulics. Bore preload is provided that exceeds the maximum rail-to-rail EM loads and is reacted by a composite structure that also provides longitudinal barrel stiffness. Manufacturing techniques are presented that would allow the design to be built on a small or large scale using standard manufacturing tolerances and demonstrated assembly processes. Material selection and impact on the ability to actively cool a railgun is also presented.

3-D Transient Modeling of Bulk High-Temperature Superconducting Material in Passive Magnetic Bearing Applications

Bulk high-temperature superconductors are being considered for use in several engineering applications, including passive magnetic bearings. These bearings, apart from being passive, i.e., inherently stable, also offer the promise of lower bearing losses; thus, they are being considered for use with flywheels for energy storage in applications related to frequency regulation and for correcting forecasting errors associated with renewable energy sources. The effort presented in this paper was undertaken to characterize the performance of these bearings such as longitudinal and transverse stiffness and loss characteristics. To this end, a finite-element method (FEM) using the T- Ω potentials was used for the formulation. The results of the FEM were verified with experiments. These experiments are described. This FEM tool was also used to guide the development of a reduced-order model, which could run faster and, therefore, could be used in larger system simulations. Some discussions about the run time on a desktop PC are also presented.

Lumped-Parameter Model to Describe Dynamic Translational Interaction for High-Temperature Superconducting Bearings

This paper discusses a lumped parameter modeling methodology to describe the dynamic vertical and translational force interaction between a bulk superconductor and levitated permanent magnet (PM). The model is formulated to be easily incorporated into larger rigid body system models to aid design of superconducting bearings for flywheel energy storage applications. The proposed modeling technique will significantly reduce the computational expense in order to shorten the design cycle process. The validity of the proposed lumped parameter model is demonstrated by comparing results from finite-element method analysis and measurements of the force displacement interaction between a PM and a bulk high-temperature superconductor.

The Effects of Membrane Properties and Structural Parameters on the Non-Minimum Phase Behavior of the PEM Fuel Cell Humidification System

The water vapor transfer across a Nafion® membrane exhibits an undesired non-minimum phase behavior. This paper will show that even in the disturbance-to-output loop, the non-minimum phase zero adversely affects the feedback controller design because of the coupling effect between the disturbance-to-output and the input-to-output loops. The non-minimum phase zero location is influenced by the channel plate structure and the membrane material property. The structural parameters examined in this research include channel plate dimensions and heat transfer coefficients. The membrane properties studied include membrane vapor transfer properties described in the Arrhenius’ equation. A governing equation to link the non-minimum phase zero and the parameters is developed in this paper. This equation shows that the non-minimum phase zero arises from the competing heat and mass transfer dynamics, and is determined by the structural parameters and membrane properties. A sensitivity study is presented and shows that structural and material property optimization can be used with the control system design to mitigate the non-minimum phase behavior in the PEM fuel cell humidification system.