Athanasios G. Konstandopoulos

SOLAR HYDROGEN PRODUCTION BY A TWO-STEP CYCLE BASED ON MIXED IRON OXIDES

A very promising method for the conversion and storage of solar energy into a fuel is the dissociation of water to oxygen and hydrogen, carried out via a two-step process using metal oxide redox systems such as mixed iron oxides, coated upon multi-channeled honeycomb ceramic supports capable of absorbing solar irradiation, in a configuration similar to that encountered in automobile exhaust catalytic converters. With this configuration, the whole process can be carried out in a single solar energy converter, the process temperature can be significantly lowered compared to other thermo-chemical cycles and the re-combination of oxygen and hydrogen is prevented by fixing the oxygen in the metal oxide. For the realization of the integrated concept, research work proceeded in three parallel directions: synthesis of active redox systems, manufacture of ceramic honeycomb supports and manufacture, testing and optimization of operating conditions of a thermochemical solar receiver-reactor. The receiver-reactor has been developed and installed in the solar furnace in Cologne, Germany. It was proven that solar hydrogen production is feasible by this process demonstrating that multi cycling of the process was possible in principle.

Reciprocating flow regeneration of soot filters

Particulate filter systems will most likely be required for compliance of diesel-powered vehicles and equipment to future emission legislation. The major issue with such systems is that their reliable and cost-effective regeneration (through oxidation of the collected soot particles) is not currently possible under all engine operating conditions without additional external thermal energy. In the present work we exploit the autothermal properties of the reverse flow reactor and study its application for the improvement of the regeneration process of diesel particulate filters. The reciprocating flow regeneration of soot filters is investigated with the aid of a mathematical model for the regeneration process, after the model has been assessed against conventional experimental regeneration data. The numerical results confirm the capability of the new technique to provide a clean filter where conventional regeneration fails. In this way the operating limits of current regeneration techniques (thermal or catalyst-assisted) can be extended substantially, and the stage is set for the construction of an industrial prototype.

Solar hydrogen: fuel of the near future

Renewable hydrogen produced using solar energy to split water is the energy fuel of the future. Accelerated innovation in both major domains of solar energy (photovoltaics and concentrated solar power) has resulted in the rapid fall of the solar electricity price, opening the route to a number of practical applications using solar H2. Referring to several examples as well as to new technologies, this article provides insight into a crucial technology for our common future.

Flow Resistance Descriptors for Diesel Particulate Filters: Definitions, Measurements and Testing

In the present work a practice for conducting experiments and analyzing the fundamental descriptors of Diesel Particulate Filter (DPF) flow resistance behavior is described. The recommended practice addresses each of the following areas: definition of flow resistance descriptors, filter geometrical characteristics, experimental setup and data analysis methods. The sources of errors in each area are identified and a sensitivity analysis is performed. Finally, the recommended practice is applied to several filter materials and configurations tested in the laboratory to determine objectively a set of flow resistance descriptors (namely the filter wall permeability, inlet/outlet loss coefficient) and their error bars. It is expected that application of the recommended practice for the determination of the flow resistance descriptors of DPFs will lead to better quality control procedures and meaningful comparisons of experimental data collected at different laboratories. As a useful outcome of the suggested practice a permeability-pore structure correlation extending over 5 orders of magnitude for cordierite and SiC porous media is obtained and the effect of catalyst coating on the filter flow resistance is analyzed.

Evolution of aggregate size and fractal dimension during Brownian coagulation

Fractal aggregate coagulation is described within a general framework of multivariate population dynamics. The effect of aggregate morphology on the coagulation rate, is taken into account explicitly, introducing in addition to aggregate particle size, the aggregate fractal dimension, as a second independent variable. A simple constitutive law is derived for determining the fractal dimension of an aggregate, resulting from a coagulation event between aggregates with different fractal dimensions. An efficient Monte Carlo method was implemented to solve the resulting bivariate Brownian coagulation equation, in the limits of continuum and free molecular flow regimes. The results indicate that as the population mean fractal dimension goes from its initial value towards its asymptotic value, the distribution of fractal dimension remains narrow for both flow regimes. The evolution of the mean aggregate size in the continuum regime is found to be nearly independent of aggregate morphology. In the free molecular regime however, the effects of aggregate morphology, as embodied in its fractal dimension, become more important. In this case the evolution of the aggregate size distribution cannot be described by the traditional approach, that employs a constant fractal dimension.

Multi-channel simulation of regeneration in honeycomb monolithic diesel particulate filters

In recent years advanced computational tools of diesel particulate &lter (DPF) regeneration have been developed to assist in the systematic and cost-e5ective optimization of next generation particulate trap systems. In the present study, we employ a previously validated, state-of-the-art multichannel DPF simulator to study the regeneration process over the entire spatial domain of the &lter. Particular attention is placed on identifying the e5ect of inlet cones and boundary conditions, &lter can insulation and the dynamics of “hot spots” induced by localized external energy deposition. Lateral heat losses through the insulation and the periphery of the &lter can, as captured by the magnitude of the Nusselt number,Nu, are detrimental to the e5ectiveness of the regeneration process. A &lter can Nu number less than 10 and preferably less than 5 is a good design target for high regeneration e=ciency. For the case studied, insulation of the inlet cones can lead to a gain of 30% in regeneration e=ciency by eliminating radial temperature gradients at the inlet &lter face. The multichannel simulator provides an instructive illustration of the well-appreciated e5ects of localized hot spot on &lter regeneration: hot spots play a more signi&cant role (spread over) when located near the entrance of the &lter. ? 2003 Elsevier Ltd. All rights reserved.

Deposit growth dynamics: particle sticking and scattering phenomena

Abstract A common phenomenon in deposition processes is the impaction of a particle on a pre-deposited particle in a deposit (e.g., as in thin film growth via aerosol routes or in gas-side fouling of heat exchange equipment). The fate of an incident particle (i.e., whether it sticks on the deposit or it escapes) affects the net deposition rate, as well as the resulting deposit microstructure, phenomena that we have been studying, using discrete particle computer simulations. Here, we summarize the sticking behavior of impacting particles in terms of appropriate macroscopic “boundary conditions” that can be used in continuum level simulations of the dynamics of deposit growth. In addition, we study the properties of rebounding/scattered particles from a “rough” particulate deposit surface, in terms of rebounding linear and angular velocities and scattering angle distributions. Interestingly enough, the rebounding velocity distributions exhibit a multimodal character which becomes less pronounced with the degree of “rigidity” of the deposit, reflecting the influence of local microstructural details (coordination number distribution) on the scattering process. Scattering angle distributions are unimodal, resembling a “distorted” sinusoidal. The remaining challenge is to develop, with further parametric studies, appropriate “boundary conditions” for the scattering quantities of interest as well, and open the door to continuum level simulations of macroscopic systems in realistic geometries.

A Study Describing the Performance of Diesel Particulate Filters During Loading and Regeneration - A Lumped Parameter Model for Control Applications

A computational lumped parameter model (MTU-Filter- Lumped) was developed to describe the performance of diesel particulate filters (DPFs) during loading and regeneration processes. The model was formulated combining three major sub-models: a filtration model, a pressure drop model, and a mass and an energy balance equation for the total filter volume. The first two submodels have been widely validated in the literature, while the third sub-model is introduced and combined with the first two sub-models in the present study. The three sub-models combined can give a full description of diesel particulate filter behavior during loading and regeneration processes, which was the objective of the present work. The total combined lumped parameter model was calibrated using experimental data from the literature covering a range of experimental conditions, including different catalytic regeneration means and engine- operating conditions. The model predictions showed very good agreement with the experimental data in terms of pressure drop across the filter, mass retained in the filter, and filter temperature. A diesel particulate filter system was selected to illustrate the control application of the lumped model equations. This system involves a diesel particulate filter for the collection and oxidation of the engine out particulate matter emissions, and the injection of hydrocarbons upstream of an oxidation catalytic converter (OCC) in order to raise the exhaust gas temperature and in turn achieve filter regeneration. Two model-based control strategies were developed aiming to minimize the fuel penalty of the regeneration process described above.

New Exposure System To Evaluate the Toxicity of (Scooter) Exhaust Emissions in Lung Cells in Vitro

A constantly growing number of scooters produce an increasing amount of potentially harmful emissions. Due to their engine technology, two-stroke scooters emit huge amounts of adverse substances, which can induce adverse pulmonary and cardiovascular health effects. The aim of this study was to develop a system to expose a characterized triple cell coculture model of the human epithelial airway barrier, to freshly produced and characterized total scooter exhaust emissions. In exposure chambers, cell cultures were exposed for 1 and 2 h to 1:100 diluted exhaust emissions and in the reference chamber to filtered ambient air, both controlled at 5% CO(2), 85% relative humidity, and 37 degrees C. The postexposure time was 0-24 h. Cytotoxicity, used to validate the exposure system, was significantly increased in exposed cell cultures after 8 h postexposure time. (Pro-) inflammatory chemo- and cytokine concentrations in the medium of exposed cells were significantly higher at the 12 h postexposure time point. It was shown that the described exposure system (with 2 h exposure duration, 8 and 24 h postexposure time, dilution of 1:100, flow of 2 L/min as optimal exposure conditions) can be used to evaluate the toxic potential of total exhaust emissions.

The Diesel Exhaust Aftertreatment (DEXA) Cluster: A Systematic Approach to Diesel Particulate Emission Control in Europe

The DEXA Cluster consisted of three closely interlinked projects. In 2003 the DEXA Cluster concluded by demonstrating the successful development of critical technologies for Diesel exhaust particulate after-treatment, without adverse effects on NO x emissions and maintaining the fuel economy advantages of the Diesel engine well beyond the EURO IV (2000) emission standards horizon. In the present paper the most important results of the DEXA Cluster projects in the demonstration of advanced particulate control technologies, the development of a simulation toolkit for the design of diesel exhaust after-treatment systems and the development of novel particulate characterization methodologies, are presented. The motivation for the DEXA Cluster research was to increase the market competitiveness of diesel engine powertrains for passenger cars worldwide, and to accelerate the adoption of particulate control technology.