Output Densities Research Articles

A 1D+1D Model of Direct Ethanol Fuel Cells Based on an Optimized Kinetic Mechanism for Ethanol Electro-Oxidation Involving Free and Adsorbed Intermediate Species

A 1D+1D model for liquid-feed direct ethanol fuel cells with detailed ethanol electro-oxidation kinetics is presented. One-dimensional convective transport along the flow channels is coupled to a one-dimensional description for species transport across the MEA accounting for the effect of the electrochemical reactions and the mixed potential due to ethanol and acetaldehyde crossover. The model, validated against previous experimental data, provides the variation of the concentrations of the free species and the output and parasitic current densities along the flow channels. The downstream evolution of ethanol and acetaldehyde affects the anode reaction rates and the ethanol and acetaldehyde crossover fluxes, which in turn impacts on the cathode reaction rates. Polarization curves obtained at different positions along the flow channels show the effect of ethanol consumption and acetaldehyde upstream production and downstream consumption. Power density, parasitic current, fuel utilization and product selectivity curves are presented and discussed for different ethanol feed concentrations and flow rates.

An efficient coupling system using a thermophotovoltaic cell to harvest the waste heat from a reforming solid oxide fuel cell

The coupling system proposed here is composed of a reforming solid oxide fuel cell (R-SOFC) and a thermophotovoltaic cell (TPVC), where the synthesis gases (CH4 and CO) replacing H2 is used as fuel. The performance characteristics of the coupling system and R-SOFC are evaluated and compared. The optimal regions of the coupling system are determined and the parametric selection criteria are provided. The maximum power output densities of the R-SOFC-TPVC and SOFC-TPVC systems operated at 973–1273 K are calculated and compared with those of other SOFC-based coupling systems. It is found that the SOFC-TPVC or R-SOFC-TPVC system has some obvious advantages.

Porous medium based burner for efficient and clean combustion of ammonia–hydrogen–air systems

Due to its high hydrogen density and extensive experience base, ammonia (NH3) has been gaining special attention as a potential green energy carrier. This study focuses on premixed ammonia–hydrogen–air flames under standard temperature and pressure conditions using an inert silicon-carbide (SiC) porous block as a practical and effective medium for flame stabilization. Combustion experiments conducted using a lab scale burner resulted in stable combustion and high combustion efficiencies at very high ammonia concentration levels over a wide range of equivalence ratios. Noticeable power output densities have also been achieved. Preliminary results of NOx emission measurements indicate NOx concentrations as low as 35 ppm under rich conditions. The remarkable capability of this specific burner to operate efficiently and cleanly at high ammonia concentration levels, which can easily be achieved by partial cracking of NH3, is believed to be a key accomplishment in the development of ammonia fired power generation systems.

Using electrochemical cycles to efficiently exploit the waste heat from a proton exchange membrane fuel cell

With the help of the current models of a proton exchange membrane fuel cell (PEMFC) and a thermally regenerative electrochemical cycle (TREC), a novel model of the hybrid system composed of a PEMFC and N TRECs is established. The TRECs in the system can effectively exploit the waste heat released form the PEMFC. The expressions for the power output and efficiency of the hybrid system are analytically derived. The effects of some key parameters including the electric current density and electrode area of the PEMFC and the electric current and regenerative efficiency of TRECs on the performance of the system are discussed in detail. It is found that when the regenerative efficiency of TRECs is in the range of 0.2–0.8, the maximum power output density of the hybrid system is about 1.11–1.20 times of that of the PEMFC. The maximum power output densities and corresponding efficiencies of the hybrid system at the differently operating temperatures of the PEMFC are calculated and compared with those of other PEMFC-based hybrid systems, and consequently, it is revealed that the proposed model can more efficiently utilize the waste heat produced in a PEMFC than other PEMFC-based hybrid systems.

High performance solid-oxide fuel cell: Opening windows to low temperature application

A key hindrance of operating solid oxide fuel cells (SOFCs) at low temperature is the relatively high cell resistance resulting in low power output density. In this work, we report an SOFC design based on an anode-supported cell (ASC) with thin film Yttria stabilized zirconia electrolyte (YSZ), capable of high power output densities of 1050 mW cm−2 using H2 as fuel, at an operating temperature of 873 K. Such high cell performances have been realized by applying three optimization steps: (1) using La0.6Sr0.4CoO3−δ-perovskite (LSC) as high performing cathode material at low temperature; (2) integration of an optimized Ce0.8Gd0.2O1.9 (CGO) inter-diffusion barrier layer and (3) optimization of the microstructure of the anode substrate by means of increasing the substrate porosity.

Establishing daily quality control (QC) in screen-film mammography using leeds tor (max) phantom at the breast imaging unit of USTH-Benavides Cancer Institute

Daily QC tests performed on screen film mammography (SFM) equipment are essential to ensure that both SFM unit and film processor are working in a consistent manner. The Breast Imaging Unit of USTH-Benavides Cancer Institute has been conducting QC following the test protocols in the IAEA Human Health Series No.2 manual. However, the availability of Leeds breast phantom (CRP E13039) in the facility made the task easier. Instead of carrying out separate tests on AEC constancy and light sensitometry, only one exposure of the phantom is done to accomplish the two tests. It was observed that measurements made on mAs output and optical densities (ODs) using the Leeds TOR (MAX) phantom are comparable with that obtained from the usual conduct of tests, taking into account the attenuation characteristic of the phantom. Image quality parameters such as low contrast and high contrast details were also evaluated from the phantom image. The authors recognize the usefulness of the phantom in determining technical factors that will help improve detection of smallest pathological details on breast images. The phantom is also convenient for daily QC monitoring and economical since less number of films is expended.

Evaluation of Heat Output Densities of Lithium Chloride-Modified Magnesium Hydroxide for Thermochemical Energy Storage

Lithium chloride-modified magnesium hydroxide is a candidate material for thermochemical energy storage. In this work, the effects of lithium chloride mixing ratio, hydration temperature, and water vapor pressure on the hydration behavior of the material are investigated. Heat output densities for all experimental conditions are evaluated. The heat output density per unit weight of lithium chloride-modified magnesium hydroxide, at a molar mixing ratio of 0.10 mol of lithium chloride per mol of magnesium hydroxide, is 1.40 × 103 kJ kg–1 at a hydration temperature of 110 °C and a water vapor pressure of 57.8 kPa. This value is higher than the heat output density of authentic magnesium hydroxide.

Utilization of Raw Glycerol for Pyocyanin Production from Pseudomonas Aeruginosa in Half-Microbial Fuel Cells: Evaluation of Two Electrochemical Approaches

Power generation via glycerol in Pseudomonas aeruginosa fuel cells was evaluated under improved culture conditions using two different electrochemical systems. One used Pt-black carbon felt air cathodes and proton exchange membranes. The other used ammonia pre-treated carbon felt electrodes immersed in ferricyanide as cathode and saline bridges as a cation exchange system. In all cases, ammonium pre-treated carbon felts were used as anodes. Experiments were conducted during 120 h at 37°C. Biochemical parameters such as microbial growth, substrate consumption and pyocyanin production were evaluated. Electrochemical studies of chronoamperometry, power output generation and coulombic efficiency (CE) were also performed. Initial concentrations of 25 g L−1 of pure glycerol were used. Small differences between systems, in terms of PCN production, glycerol consumption or microbial growth were observed. Also, good results in terms of current densities (Idmax) of 42 ± 2.1 μA cm−2, CE of 48 ± 2.4% and power output densities (Pdmax) of 350 ± 17.5 mW m−2, were achieved. The power output densities were at least 4 times higher than other previously reported MFCs based also on PCN production from pure glycerol or glucose. In contrast, when both systems were supplemented with raw glycerol obtained as by-product from biodiesel industry, lower values for Idmax, CE and Pdmax ranging 23 ± 1.15 μA cm−2, 36 ± 1.18% and 240 ± 12.0 mW m−2, were respectively achieved, depending on the electrochemical system used.

Piezoelectric vibrational energy harvesting

Vibrational energy harvesting spans both new technology and traditional technology space. The latter is represented well by wind up watches and the former by self-powered autonomous sensor networks, which is the subject of intense academic and industrial research and development. In this study, the authors introduce the concepts of vibrational energy harvesting with an emphasis on piezoelectric technology. However, much can be learned from more traditional transduction technologies such as electromagnetic so they also discuss some of these systems. The transfer of vibrational energy from its mechanical domain to a useful electrical domain is complex and in this section they explore ways in which the electrical charge (electrical energy state) can be efficiently rectified from its AC character to a viable DC voltage for storage or immediate loading. Finally, they examine some of the very new technologies which permit harvesting of vibrational energy on the very small scale using micro-electromechanical systems based technologies. This length scale is important and better matches the energy density of small scale vibrational sources and power output densities of piezoelectric materials in particular.

Determining and Approaching Achievable Rates of Binary Intersymbol Interference Channels Using Multistage Decoding

By examining the achievable rates of a multistage decoding system on stationary ergodic channels, we derive lower bounds on the mutual information rate corresponding to independent and uniformly distributed (i.u.d.) inputs, also referred to as the i.u.d. information rate. For binary intersymbol interference (ISI) channels, we show that these bounds become tight as the number of decoding stages increases. Our analysis, which focuses on the marginal conditional output densities at each stage of decoding, provides an information rate corresponding to each stage. These rates underlie the design of multilevel coding schemes, based upon low-density parity-check (LDPC) codes and message passing, that in combination with multistage decoding approach the i.u.d. information rate for binary ISI channels. We give example constructions for channel models that have been commonly used in magnetic recording. These examples demonstrate that the technique is very effective even for a small number of decoding stages

Discriminative speaker adaptation using articulatory features

This paper presents an automatic speech recognition system using acoustic models based on both sub-phonetic units and broad, phonological features such as voiced and round as output densities in a hidden Markov model framework. The aim of this work is to improve speech recognition performance particularly on conversational speech by using units other than phones as a basis for discrimination between words. We explore the idea that phones are more of a short-hand notation for a bundle of phonological features, which can also be used directly to distinguish competing word hypotheses. Acoustic models for different features are integrated with phone models using a multi-stream approach and log-linear interpolation. This paper presents a new lattice based discriminative training algorithm using the maximum mutual information criterion to train stream weights. This algorithm allows us to automatically learn stream weights from training or adaptation data and can also be applied to other tasks. Decoding experiments conducted in comparison to a non-feature baseline system on the large vocabulary English Spontaneous Scheduling Task show reductions in word error rate of about 20% for discriminative model adaptation based on articulatory features, slightly outperforming other adaptation algorithms.

Temporal changes in sebum excretion and propionibacterial colonization in preadolescent children with and without acne

It is generally accepted that the onset of sebum secretion occurs before puberty in boys and girls as a result of increasing androgen output during the adrenarche. Propionibacteria are part of the commensal skin flora and, in adults, are found in highest numbers in sebum-rich areas of skin such as the face and upper trunk. Previous studies investigating the association between sebum output and propionibacterial population densities have been cross-sectional and have been carried out mainly in adults. The purpose of this study was to examine the association between the onset of sebum secretion and expansion of the propionibacterial flora in a population of early adolescent children aged between 5.5 and 12 years, and to evaluate the temporal relation between the two factors longitudinally. In addition, the study aimed to evaluate the change with age in sebaceous gland activity and propionibacterial colonization on the skin and in the nares between children who developed acne and those who did not. Biannual examinations of volunteers included age, pubertal (Tanner) stage, weight and height, lesion counting on the face, propionibacterial colonization on the skin surface and in the nares and sebum secretion. A longitudinal analysis based on all observations of each subject throughout the study was applied to examine the change of sebaceous gland activity and propionibacterial colonization with age and pubertal stage. A generalized estimating equation was used with a 0.05 level of significance. The commencement of sebum production was asynchronous, with only a small number of follicles initially starting to secrete sebum onto the skin surface. The number of secreting follicles and the area of sebum increased with age and pubertal stage (P < 0.0001, P < 0.05, respectively). Numbers of propionibacteria on the skin tended to increase after the age of 9 years, but not significantly so. In contrast, numbers of propionibacteria in the nares increased significantly with age (P < 0.0001) but not with pubertal maturation. Children who developed acne had higher sebum output and propionibacterial densities with increasing age than children who did not develop acne. This effect was significant for the increase of total sebum area with age in pubertal children (P = 0.0023), the increase in number of secreting follicles with age (P = 0.020) in prepubertal children, and the increase in propionibacteria densities in the nares with age (P = 0.0005) in pubertal children. Sebaceous gland activity and propionibacterial numbers on the skin surface remained unchanged with increasing age in children who did not develop acne. Propionibacterial population densities in the nares increased with age regardless of the development of acne. Onset of sebum secretion and consequently expansion of the propionibacterial skin flora occur earlier in children who develop acne than in children of the same age and pubertal status who do not develop acne. These observations suggest that postponing the onset of sebum production or the expansion of the propionibacterial skin flora until after puberty may represent ways of preventing the disease or minimizing its severity. Determinants of propionibacterial colonization on the skin and in the nares may be different.

Light-reflection random-target method for measurement of the modulation transfer function of a digital video-camera

This article reports the suggestion, realization and verification of the newly developed measuring means of the noiseless and locally shift-invariant modulation transfer function (MTF) of a digital video camera in a usual incoherent visible region of optical intensity, especially of its combined imaging, detection, sampling and digitizing steps which are influenced by the additive and spatially discrete photodetector, aliasing and quantization noises. Such means relates to the still camera automatic working regime and static two-dimensional spatially continuous light-reflection random target of white-noise property. The introduced theoretical reason for such a random-target method is also performed under exploitation of the proposed simulation model of the linear optical intensity response and possibility to express the resultant MTF by a normalized and smoothed rate of the ascertainable output and input power spectral densities. The random-target and resultant image-data were obtained and processed by means of a processing and evaluational PC with computation programs developed on the basis of MATLAB 6.5. The present examples of results and other obtained results of the performed measurements demonstrate the sufficient repeatability and acceptability of the described method for comparative evaluations of the performance of digital video cameras under various conditions.

Comparison of low angle radar clutter models

Three low angle clutter models, the compound-K, the noncentral gamma-gamma (NCGG), both of which model speckle, and the Weibull distributions are compared. The derivations of the first two densities are surprisingly similar, differing on only one critical point. The compound-K assumes that the Gaussian random variables that are the input to the square law detector have zero means, while the NCGG assumes that the Gaussian random variables have nonzero means. The compound-K must then assume that it is the output variance that is fluctuating to produce speckle, while the NCGG assumes that it is the noncentrality parameter, resulting from the addition of the nonzero means that is fluctuating. The difference in the resulting models is significant. The Weibull density has long been used to model clutter successfully, and it too will be considered for comparison, but its use is not based upon a physically motivated development. Rather, its use is based on comparisons of clutter output densities being well matched by the Weibull density with appropriate parameters.

Use of near-IR to monitor the influence of external heating on dental composite photopolymerization

Objectives: This study was conducted to determine the effect of modest external heating on the photopolymerization kinetics and conversion of commercial dental composite restorative materials. Methods: A transmission-mode, real-time near-infrared spectroscopic technique was used to monitor the photopolymerization process in the composite materials at various temperatures between 23 and 70 °C. Several light curing units, differing in spectral output and power densities were compared at the different cure temperatures. Several significantly different commercial composites were compared for their response. Results: Regardless of the curing light or composite material used, photopolymerization at a moderate curing temperature of 54.5 °C resulted in significantly higher immediate and final conversion values compared with room temperature photocuring. Contrary to the room temperature cured materials, at the elevated cure temperature the extent of post-cure was minor and different curing lights produced very uniform conversion values within a given material. The time required to reach a given level of conversion, established as full conversion with the room temperature cure, was reduced typically by 80–90% using the elevated curing conditions. Complementary kinetic studies confirmed the effect of cure temperature on increasing the polymerization rate in dental composites as significant. Significance: Increasing the temperature of composite resin within potentially biologically compatible limits can significantly influences resin polymerization. These increased rates and conversion could lead to improved properties of composite restorative materials.

Some Recent Developments in Econometric Inference

Recent results in information theory, see Soofi (1996; 2001) for a review, include derivations of optimal information processing rules, including Bayes' theorem, for learning from data based on minimizing a criterion functional, namely output information minus input information as shown in Zellner (1988; 1991; 1997; 2002). Herein, solution post data densities for parameters are obtained and studied for cases in which the input information is that in (1) a likelihood function and a prior density; (2) only a likelihood function; and (3) neither a prior nor a likelihood function but only input information in the form of post data moments of parameters, as in the Bayesian method of moments approach. Then it is shown how optimal output densities can be employed to obtain predictive densities and optimal, finite sample structural coefficient estimates using three alternative loss functions. Such optimal estimates are compared with usual estimates, e.g., maximum likelihood, two‐stage least squares, ordinary least squares, etc. Some Monte Carlo experimental results in the literature are discussed and implications for the future are provided.

Hidden Gauss-Markov models for signal classification

Continuous-state hidden Markov models (CS-HMMs) are developed as a tool for signal classification. Analogs of the Baum (1972), Viterbi (1962), and Baum-Welch algorithms are formulated for this class of models. The CS-HMM algorithms are then specialized to hidden Gauss-Markov models (HGMMs) with linear Gaussian state-transition and output densities. A new Gaussian refactorization lemma is used to show that the Baum and Viterbi algorithms for HGMMs are implemented by two different formulations of the fixed-interval Kalman smoother. The measurement likelihoods obtained from the forward pass of the HGMM Baum algorithm and from the Kalman-filter innovation sequence are shown to be equal. A direct link between the Baum-Welch training algorithm and an existing expectation-maximization (EM) algorithm for Gaussian models is demonstrated. A new expression for the cross covariance between time-adjacent states in HGMMs is derived from the off-diagonal block of the conditional joint covariance matrix. A parameter invariance structure is noted for the HGMM likelihood function. CS-HMMs and HGMMs are extended to incorporate mixture densities for the a priori density of the initial state. Application of HGMMs to signal classification is demonstrated with a three-class test simulation.

Transient electroluminescence in a para-hexaphenyl based multilayer device

An organic blue light emitting device (OLED) is investigated in terms of transient electroluminescence (EL) and current characteristic. The driving voltage pulse mode is varied with respect to amplitude and pulse duration covering the range of 1ms to 250 ms. The OLED performance is studied with respect to response times, EL output and current densities. It is further demonstrated that reverse biasing the device prior to the driving voltage pulse increases the external quantum yield by up to 25% by virtue of depleting space charge carriers and charge filled trap sites.

Preferable disequilibrium in a nonlinearcobweb economy

This study demonstrates the possibility that chaotic fluctuations may be preferable to acompetitive equilibrium. For this purpose, we use a linear cobweb model augmented by thelower and upper bounds for variations of output and construct densities of ergodically chaoticfluctuations. We demonstrate with the aid of numerical examples that the long‐run averageprofit and utility obtained in perpetual disequilibrium can be greater than the profit andutility at equilibrium.

The semi-relaxed algorithm for estimating parameters of hidden Markov models

For hidden Markov model (HMM) based speech recognition where the basic speech unit is smaller than the recognizer's output unit, the standard full Baum-Welch re-estimation procedure for the HMM training is very costly in computation. This is because it requires evaluation of the HMM output densities and of the forward/backward probabilities in the entire region of the state-frame trellis. In this paper, we present an algorithm which exploits the fact that the entries of the trellis are essentially zero except near the block diagonal and hence achieves significant computational saving. The algorithm is evaluated in experiments with a large vocabulary word recognizer based on mixture-density HMM representation of phonemes. The HMM parameters trained with the new algorithm are essentially identical to those trained with the full Baum-Welch algorithm in that the resulting HMMs have nearly the same likelihood values on the same set of training data. Identical word recognition accuracies are yielded using the HMMs trained with the two algorithms. However, the new algorithm is shown to be about an order of magnitude faster than the full Baum-Welch algorithm.