Søren Knudsen Kær

Modelling the motion of cylindrical particles in a nonuniform flow

The models currently used in computational fluid dynamics codes to predict solid fuel combustion rely on a spherical shape assumption. Cylinders and disks represent a much better geometrical approximation to the shape of bio-fuels such as straws and woods chips. A sphere gives an extreme in terms of the volume-to-surface-area ratio, which impacts both motion and reaction of a particle. For a nonspherical particle, an additional lift force becomes important, and generally hydrodynamic forces introduce a torque on the particle as the centre of pressure does not coincide with the centre of mass. Therefore, rotation of a nonspherical particle needs to be considered. This paper derives a model for tracking nonspherical particles in a nonuniform flow field, which is validated by a preliminary experimental study: the calculated results agree well with measurements in both translation and rotation aspects. The model allows to take into account shape details of nonspherical particles so that both the motion and the chemical reaction of particles can be modelled more reasonably. The ultimate goal of such a study is to simulate flow and combustion in biomass-fired furnaces using nonspherical particle tracking model instead of traditional sphere assumption, and thus improve the design of biomass-fired boilers.

Co-firing straw with coal in a swirl-stabilized dual-feed burner: modelling and experimental validation.

This paper presents a comprehensive computational fluid dynamics (CFD) modelling study of co-firing wheat straw with coal in a 150kW swirl-stabilized dual-feed burner flow reactor, in which the pulverized straw particles (mean diameter of 451microm) and coal particles (mean diameter of 110.4microm) are independently fed into the burner through two concentric injection tubes, i.e., the centre and annular tubes, respectively. Multiple simulations are performed, using three meshes, two global reaction mechanisms for homogeneous combustion, two turbulent combustion models, and two models for fuel particle conversion. It is found that for pulverized biomass particles of a few hundred microns in diameter the intra-particle heat and mass transfer is a secondary issue at most in their conversion, and the global four-step mechanism of Jones and Lindstedt may be better used in modelling volatiles combustion. The baseline CFD models show a good agreement with the measured maps of main species in the reactor. The straw particles, less affected by the swirling secondary air jet due to the large fuel/air jet momentum and large particle response time, travels in a nearly straight line and penetrate through the oxygen-lean core zone; whilst the coal particles are significantly affected by secondary air jet and swirled into the oxygen-rich outer radius with increased residence time (in average, 8.1s for coal particles vs. 5.2s for straw particles in the 3m high reactor). Therefore, a remarkable difference in the overall burnout of the two fuels is predicted: about 93% for coal char vs. 73% for straw char. As the conclusion, a reliable modelling methodology for pulverized biomass/coal co-firing and some useful co-firing design considerations are suggested.

Evaluation of Fuel-Cell Range Extender Impact on Hybrid Electrical Vehicle Performance

The use of electric vehicles (EVs) is advantageous because of zero emission, but their market penetration is limited by one disadvantage, i.e., energy storage. Battery EVs (BEVs) have a limited range, and their batteries take a long time to charge, compared with the time it takes to refuel the tank of a vehicle with an internal combustion engine (ICE). Fuel cells (FCs) can be added to an EV as an additional energy source. These are faster to refill and will therefore facilitate the transition from vehicles running on fossil fuel to electricity. Different EV setups with FC strategies are presented and compared. The results of the setups are presented by range, efficiency, and price. These show the negative effect on the range when purpose-designed setups are driven above the design requirement as the range drops considerably. The simulations also showed the necessity of good FC control when driving in start/stop city cycles. Simulations with the New European Driving Cycle (NEDC) showed that efficiency fell by at least 15% for the FC hybrid EV (FCHEV) when compared with BEVs.

A Computational Analysis of Multiphase Flow Through PEMFC Cathode Porous Media Using the Multifluid Approach

A three-dimensional multiphase model that describes the liquid water flux through the porous media and into the gas flow channel of a proton exchange membrane fuel cell (PEMFC) cathode is presented. The model is based on the multifluid approach, thus solving one complete set of transport equations for each phase. It is used to investigate the effect of different material parameters on the predicted liquid water saturation in various layers, i.e., catalyst layer (CL), microporous layer (MPL), and gas diffusion layer (GDL). Each layer can be characterized by its porosity, permeability, and contact angle, while the Leverett function was used to describe the capillary pressure vs saturation. The irreducible saturation can be specified for each layer independently. An expression for the channel/GDL interface condition was developed, which accounts for the number of droplets per unit area at the GDL interface. Describing the different porous media by Leverett equations leads to a jump condition in saturation across each interface. In accordance with previous studies it is found that the MPL remains at a low saturation level, while the CL might become flooded depending on the fraction of hydrophilic pores. Under-the-land compression leads to an increased level of flooding compared to the channel section.

Lifetime Estimation of the Nanophosphate

There are currently many different lithium ion (Li-ion) chemistries available on the market, and several new players are in the research and development process; however, none of them is superior to the other chemistries in all aspects. Relatively low price, long cycle and calendar lifetime, and intrinsic safety of the nanophosphate LiFePO4/C Li-ion chemistry make it possible to consider this chemistry for electric vehicle (EV) applications. This paper investigates the lifetime of the nanophosphate LiFePO4/C battery chemistry when it is used for full electrical vehicles. The investigation is performed considering a semiempirical calendar and cycle lifetime model, which was developed based on extended accelerated lifetime tests. Both capacity and power capability degradations during calendar and cycle life aging are considered and quantified. Finally, the developed battery cell lifetime model is used to study the capacity and power capability degradation behavior of the tested nanophosphate LiFePO4/C battery for two EV operational scenarios.

\hbox{LiFePO}_{4}\hbox{/C}

Degradation tests of two phosphoric acid (PA) doped polybenzimidazole (PBI) membrane based high temperature polymer electrolyte membrane (HT-PEM) fuel cells were reported in this paper to investigate the effects of start/stop and the presence of methanol in the fuel to the performance degradation. Continuous tests with H2 and simulated reformate which was composed of H2, water steam and methanol as the fuel were performed on both single cells. 12-h-startup/12-h-shutdown dynamic tests were performed on the first single cell with pure dry H2 as the fuel and on the second single cell with simulated reformate as the fuel. Along with the tests electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy (EIS) were employed to study the degradation mechanisms of the fuel cells. Both single cells showed an increase in the performance in the H2 continuous tests, because of a decrease in the oxygen reduction reaction (ORR) kinetic resistance probably due to the redistribution of PA between the membrane and electrodes. EIS measurement of first fuel cell during the start/stop test showed that the mass transfer resistance and ohmic resistance increased which can be attributed to the corrosion of carbon support in the catalyst layer and degradation of the PBI membrane. During the continuous test with simulated reformate as the fuel the ORR kinetic resistance and mass transfer resistance of both single cells increased. The performance of the second single cell experienced a slight decrease during the start/stop test with simulated reformate as the fuel. [DOI: 10.1115/1.4029081]

Battery Chemistry Used in Fully Electric Vehicles

Methanol-fueled, high-temperature polymer electrolyte membrane fuel cell (HTPEMFC) power systems are promising as the next generation of vehicle engines, efficient and environmentally friendly. Currently, their performance still needs to be improved, and they still rely on a large Li-ion battery for system startup. In this article, to handle these two issues, the potential of thermoelectric (TE) devices applied in a HTPEMFC power system has been preliminarily evaluated. First, right after the fuel cell stack or the methanol reformer, thermoelectric generators (TEGs) are embedded inside a gas–liquid heat exchanger to form a heat recovery subsystem jointly for electricity production. It is calculated that the recovered power can increase the system efficiency and mitigate the dependence on Li-ion battery during system startup. To improve the TEG subsystem performance, a finite-difference model is then employed and two main parameters are identified. Second, TE coolers are integrated into the methanol steam reformer to regulate heat fluxes herein and improve the system dynamic performance. Similar modification is also done on the evaporator to improve its dynamic performance as well as to reduce the heat loss during system startup. The results demonstrate that the TE-assisted heat flux regulation and heat-loss reduction can also effectively help solve the abovementioned two issues. The preliminary analysis in this article shows that a TE device application inside HTPEMFC power systems is of great value and worthy of further study.

Performance Degradation Tests of Phosphoric Acid Doped Polybenzimidazole Membrane Based High Temperature Polymer Electrolyte Membrane Fuel Cells

A three-dimensional, multicomponent, two-fluid model developed in the commercial CFD package CFX 13 (ANSYS Inc.) is used to investigate the effect of porous media compression on water transport in a proton exchange membrane fuel cell (PEMFC). The PEMFC model only consist of the cathode channel, gas diffusion layer, microporous layer, and catalyst layer, excluding the membrane and anode. In the porous media liquid water transport is described by the capillary pressure gradient, momentum loss via the Darcy-Forchheimer equation, and mass transfer between phases by a nonequilibrium phase change model. Furthermore, the presence of irreducible liquid water is taken into account. In order to account for compression, porous media morphology variations are specified based on the gas diffusion layer (GDL) through-plane strain and intrusion which are stated as a function of compression. These morphology variations affect gas and liquid water transport, and hence liquid water distribution and the risk of blocking active sites. Hence, water transport is studied under GDL compression in order to investigate the qualitative effects. Two simulation cases are compared; one with and one without compression.

Potential Usage of Thermoelectric Devicesin a High-Temperature Polymer ElectrolyteMembrane (PEM) Fuel Cell System: Two Case Studies

The paper investigates the feasibility of employing a battery thermal management system (BTMS) in different applications based on a techno economic analysis considering the battery lifetime and application profile, i.e. current requirement. The preliminary objective is to set the decision criteria of employing a BTMS and if the outcome of the decision is positive, to determine the type of the employed BTMS. However, employing a BTMS needs to meet a number of application requirements and different BTMS associates a different amount of capital cost to ensure the battery performance over its lifetime. Hence, the objective of this paper is to develop and detail the method of the feasibility for commissioning BTMS called “The decision tool framework” (DTF) and to investigate its sensitivity to major factors (e.g. lifetime and application requirement) which are well-known to influence the battery pack thermal performance, battery pack performance and ultimately the performance as well as utility of the desired application. This DTF is designed to provide a common framework of a BTMS manufacturer and designer to evaluate the options of different BTMS applicable for different applications and operating conditions. The results provide insight into the feasibility and the required specification and configuration of a BTMS.

The Effect of Inhomogeneous Compression on Water Transport in the Cathode of a Proton Exchange Membrane Fuel Cell

Solid oxide fuel cell micro-cogeneration systems have the potential to reduce domestic energy consumption by providing both heat and power onsite without transmission losses. The high-grade heat produced during the operation of the power causes high thermal transients during the startup/shutdown phases and degrades the fuel cells. To counteract the degradation, the system should not be stressed with rapid load variation during the operation. The analysis will consider an average profile for heat and power demand of a family house. Finally data analysis and power system limitations will be used to develop a viable strategy of operation.