A. Rowe

Mathematical modeling of proton exchange membrane fuel cells

A one-dimensional non-isothermal model of a proton exchange membrane (PEM) fuel cell has been developed to investigate the effect of various design and operating conditions on the cell performance, thermal response and water management, and to understand the underlying mechanism. The model includes variable membrane hydration, ternary gas mixtures for both reactant streams, phase change of water in the electrodes with unsaturated reactant gas streams, and the energy equation for the temperature distribution across the cell. It is found that temperature distribution within the PEM fuel cell is affected by water phase change in the electrodes, especially for unsaturated reactant streams. Larger peak temperatures occur within the cell at lower cell operating temperatures and for partially humidifed reactants as a result of increased membrane resistance arising from reduced membrane hydration. The non-uniform temperature rise can be significant for fuel cell stacks. Operation on reformed fuels results in a decrease in cell performance largely due to reduced membrane hydration, which is also responsible for reduced performance at high current densities for high cell operating pressures. Model predictions compare well with known experimental results.

A techno-economic analysis of using mobile distributed pyrolysis facilities to deliver a forest residue resource.

Distributed mobile conversion facilities using either fast pyrolysis or torrefaction processes can be used to convert forest residues to more energy dense substances (bio-oil, bio-slurry or torrefied wood) that can be transported as feedstock for bio-fuel facilities. Results show that the levelised delivered cost of a forest residue resource using mobile facility networks can be lower than using conventional woodchip delivery methods under appropriate conditions. Torrefied wood is the lowest cost pathway of delivering a forest residue resource when using mobile facilities. Cost savings occur against woodchip delivery for annual forest residue harvests above 2.5 million m(3) or when transport distances greater than 300 km are required. Important parameters that influence levelised delivered costs are transport distances (forest residue spatial density), haul cost factors, and initial moisture content of forest residues. Relocating mobile facilities can be optimised for lowest cost delivery as transport distances of raw biomass are reduced.

Magnetic heat pumps: An overview of design principles and challenges

Active magnetic regeneration is one of the most promising alternative technologies for the development of heat pumps and cooling systems for applications around room temperature. In the open literature, numerous works can be found in which much effort has been put on the development of magnetocaloric materials, magnetic circuits and prototypes. In this article, the authors discuss some of the main challenges encountered in the literature and how design choices impact cooling power and work requirements from a system engineering perspective. First, based on a generic schematic representation of a magnetocaloric heat pump, or refrigerator, various problems and challenges found in the current state of the art are pointed out and discussed. Second, different design principles for magnetic heat pumps are examined. As a means to improving performance, an extended design/optimization methodology is proposed based on entropy generation minimization with performance criteria. Finally, some initial optimization results are presented and discussed.

Numerical Analysis of an Active Magnetic Regenerator (AMR) Refrigeration Cycle

An alternative cycle proposed for refrigeration and gas liquefaction is active magnetic regenerator (AMR) refrigeration. This technology relies on solid materials exhibiting the magnetocaloric effect (MCE), a nearly reversible temperature change induced by a magnetic field change. This work focuses on numerical simulations of the AMR refrigeration cycle. A transient one‐dimensional finite element model developed in FEMLAB™ incorporates energy equations for the refrigerant and the heat transfer fluid. The results of the model are validated by comparison to room temperature experiments with Gd. Predictions are then made for the performance of DyAl2 AMR beds near 70 K. Numerical results for simulations significantly above the Curie temperature are found to be dependent upon the initial conditions.

An Overview of Operating Experience Using the AMR Test Apparatus

Active Magnetic Regenerative (AMR) Refrigeration is currently being investigated by the authors in the hopes of creating compact and efficient devices for the liquefaction of cryofuels. In the past, progress has been hindered by a lack of understanding as to how one creates an effective AMR. Measurements of the thermodynamic properties of potential refrigerants were found to be insufficient and dynamic tests of materials undergoing an AMR cycle were needed. To address this need, an AMR Test Apparatus was developed. An overview of the operating experience and some key experimental results produced by the AMRTA are reported. Areas of ongoing study will also be discussed.

Material screening metrics and optimal performance of an active magnetic regenerator

A variety of metrics to rank the magnetocaloric materials can be found in the literature, but a quantitative assessment showing their efficacy has not been reported. A numerical model of an active magnetic regenerator cycle is used to assess the predictive ability of a set of material metrics. The performance of eight cases of known magnetocaloric material (including first order MnFeP1-xAsx and second order materials Gd, GdDy, Tb), and 15 cases of hypothetical materials are considered. Using a fixed regenerator matrix geometry, magnetic field, and flow waveforms, the maximum exergetic cooling power of each material is identified. Several material screening metrics such as relative cooling power (RCP) are tested and a linear correlation is found between maximum RCP and the maximum exergetic cooling power. The sensitivity of performance to variations in the hot side and cold side temperatures from the conditions giving maximum exergetic power are determined.

Modeling and simulation of a hybrid electric propulsion system of a green ship

In this work, the hybrid electric propulsion system of a marine vehicle was modeled in MATLAB Simulink and SimPowerSystems. Models of each of the propulsion components were developed and incorporated into a complete system propulsion model. A rule-based supervisory mode controller was constructed which specifies the combination of onboard power sources to be used throughout the mission cycle. The hybrid electric propulsion and control model was simulated on a dSPACE hardware-in-the-loop platform. For each simulation, the energy storage system state of charge, HEV mode, propeller motor drive speed set point, and hotel load were specified. This study forms the foundation for further research in ship hybrid electric propulsion system design and power management.

Cryogenic Active Magnetic Regenerator Test Apparatus

An AMR Test Apparatus (AMRTA) used in experiments near room‐temperature required a number of modifications to allow for testing at cryogenic temperatures and with a 5 T magnetic field. The impacts of parasitic heat leaks, frictional heat generation, and eddy current heating in the AMRTA are analyzed. A low temperature gas circulation (LTGC) system to control the operating temperature was developed. The LTGC consists of a GM cryocooler coupled to a compressor and helium circuit which circulates fluid through a set of heat exchangers and flexible transfer lines connected to the AMRTA. Design features are discussed as is some initial test data.

Demagnetizing Effects in Active Magnetic Regenerators

A process for cleaning a deposit from an interior surface of a processing chamber includes generating a plasma from a cleaning gas including SO2F2 and contacting the interior surface with the plasma for a time sufficient to convert the deposit into a volatile product, thereby cleaning the deposit from the interior surface, in which the process is conducted in the absence of SF6. The deposits, which may be removed by the process of the invention, include silicone, silicone oxide, silicone nitride, tungsten, copper and aluminum.

Multiple points of equilibrium for active magnetic regenerators using first order magnetocaloric material

First order transition material (FOM) usually exhibits magnetocaloric effects in a narrow temperature range which complicates their use in an active magnetic regenerator (AMR) refrigerator. In addition, the magnetocaloric effect in first order materials can vary with field and temperature history of the material. This study examines the behavior of a MnFe(P,Si) FOM sample in an AMR cycle using a numerical model and experimental measurements. For certain operating conditions, multiple points of equilibrium (MPE) exist for a fixed hot rejection temperature. Stable and unstable points of equilibriums (PEs) are identified and the impacts of heat loads, operating conditions, and configuration losses on the number of PEs are discussed. It is shown that the existence of multiple PEs can affect the performance of an AMR significantly for certain operating conditions. In addition, the points where MPEs exist appear to be linked to the device itself, not just the material, suggesting the need to layer a regenerator...