Unified mathematical modelling of steady-state and dynamic voltage-current characteristics for PEM fuel cells

In this study, a unified mathematical modelling technique for computing the steady-state and dynamic voltage-current (V-I) characteristics of PEM fuel cell stacks is developed. The proposed modelling method is based on the least squares technique and a set of electrochemical equations representing the PEM fuel cells. Three PEM fuel cell systems are considered for validating the proposed model. Furthermore, the authors investigated load current optimization by using the proposed method, in order to maximize the power output. Hence, this study provides a valuable approach for optimization of operating points of fuel cells and design of power conditioning units, simulators, and system controllers.     

Modelling a continuous devulcanization in an extruder

An engineering kinetic model is applied to EPDM devulcanization in an extrusion process. Parameters that were included in the model are crosslink density, devulcanizing agent concentration, time, shear rate, and temperature. The extruder is considered as a series of plug flow and stirred tank reactors. The residence time is calculated using solid flow or liquid flow model, depending on the degree of decrosslinking. The model can accurately predict the decrosslinking degree inside the temperature boundary defined by the applied experimental conditions. Outside this boundary, the effect of chemical degradation on the measured conversion becomes more significant.     

Nature of palm oil in centrifuge sludge waste water

In attempting to improve oil recovery, palm oil retained in the centrifuge sludge waste has been categorized and quantified. It is composed of (i) 30% free oil droplets; (ii) 56% oil released after cellulase treatment followed by detergent washing, and (iii) 14% residual oil which remains diffused and bound to the cell debris. The fatty acid composition indicates that all three oils are palm oil. The separated oil is highly contaminated.     

On the solution of maximization problems of optimal design by geometric programming

Summary A geometric programming method, recently developed for the constrained maximization of posynomials, is presented and illustrated by an application to the optimal design of a torsion bar spring.Research supported by the National Science Foundation, Project GP 13546.     

Development of a circuit for functional electrical stimulation

This paper examines the various design of a multiple-purpose portable functional electrical stimulator which is used in surface stimulation of paralyzed muscle of patients with stroke and results in limb activation. The functionality, circuit performance and reliability of the circuits will be examined. Analysis, design, and experimental results are presented.     

Key Recovery Attacks on Iterated Even–Mansour Encryption Schemes

Iterated Even–Mansour (EM) encryption schemes (also named “key-alternating ciphers”) were extensively studied in recent years as an abstraction of commonly used block ciphers. A large amount of previous works on iterated EM concentrated on security in an information-theoretic model. A central question studied in these papers is: What is the minimal number of rounds for which the resulting cipher is indistinguishable from an ideal cipher? In this paper, we study a similar question in the computational model: What is the minimal number of rounds, assuring that no attack can recover the secret key faster than trivial attacks (such as exhaustive search)? We study this question for the two natural key scheduling variants that were considered in most previous papers: the identical subkeys variant and the independent subkeys variant. In the identical subkeys variant, we improve the best known attack by an additional round and show that \(r=3\) rounds are insufficient for assuring security, by devising a key recovery attack whose running time is about \(n/\log (n)\) times faster than exhaustive search for an \(n\)-bit key. In the independent subkeys variant, we also extend the known results by one round and show that for \(r=2\), there exists a key recovery attack whose running time is faster than the benchmark meet-in-the-middle attack. Despite their generic nature, we show that the attacks can be applied to improve the best known attacks on several concrete ciphers, including the full \({\hbox {AES}^{2}}\) (proposed at Eurocrypt 2012) and reduced-round LED-128 (proposed at CHES 2012).     

Self‐Crystallized Multifunctional 2D Perovskite for Efficient and Stable Perovskite Solar Cells

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.     

Effect of humic acid on pyrene removal from water by polycation-clay mineral composites and activated carbon

Pyrene removal by polycation-montmorillonite (MMT) composites and granulated activated carbon (GAC) in the presence of humic acid (HA) was examined. Pyrene, HA, and sorbent interactions were characterized by FTIR, fluorescence and zeta measurements, adsorption, and column filtration experiments. Pyrene binding coefficients to the macromolecules were in the order of PVPcoS (poly-4-vinylpiridine-co-styrene) > HA > PDADMAC (poly diallyl-dimethyl-ammonium-chloride), correlating to pyrene-macromolecules compatibility. Electrostatic interactions explained the high adsorption of HA to both composites (～100%), whereas HA adsorption by GAC was low. Pyrene removal by the composites, unlike GAC, was enhanced in the presence of HA; removal by PDADMAC-MMT increased from ～50 (k(d) = 2.2 × 10(3) kg/L) to ～70% (k(d) = 2.4 × 10(3) kg/L) in the presence of HA. This improvement was attributed to the adsorption of pyrene-HA complexes. PVPcoS-MMT was most efficient in removing pyrene (k(d) = 1.1 × 10(4) kg/L, >95% removal) which was explained in terms of specific π donor-π acceptor interactions. Pyrene uptake by column filters of GAC reached ～50% and decreased to ～30% in the presence of HA. Pyrene removal by the PVPcoS-MMT filter was significantly higher (100-85% removal), exhibiting only a small decrease in the presence of HA. The utilization of HA as an enhancing agent in pollutant removal is novel and of major importance in water treatment.     

Rotor heating as an indicator of system voltage instability

Voltage stability hinges on the continuing control of the total power system's supply and demand of reactive power within the overall transmission, subtransmission and distribution networks which connects the power stations to the consumers. This is complicated by the elements interspersed within this extensive network that both supply and consume reactive power. Extensive collapses have followed disturbances more severe than considered at the planning stages, but there is a puzzling delay of minutes between the disturbance and collapse. By simulating the physical process, the critical factor has been found to be the action of rotor overcurrent protection in curtailing rotating unit reactive power output. The puzzling delay has been solved by the mathematical modelling of the generator rotor's thermal response which shows that above rated rotor current would take minutes to reach the rotor's temperature limit. The paper describes the heat run tests from which the parameters for mathematical rotor thermal models for 500 MW and 660 MW generators have been obtained. The algorithm that has been devised is both simple and reliable and allows computer evaluation, in a fraction of a second, of the time for rotor temperature limit to be reached. When integrated into SCADA, this information would give an early warning and so provide a valuable tool for the control system voltage stability