Sustainable ammonia production through process synthesis and global optimization

Current ammonia production technologies have a significant carbon footprint. In this study, we present a process synthesis and global optimization framework to discover the efficient utilization of renewable resources in ammonia production. Competing technologies are incorporated in a process superstructure where biomass, wind, and solar routes are compared with the natural gas-based reference case. A deterministic global optimization-based branch-and-bound algorithm is used to solve the resulting large-scale nonconvex mixed-integer nonlinear programming problem (MINLP). Case studies for Texas, California, and Iowa are conducted to examine the effects of different feedstock prices and availabilities. Results indicate that profitability of ammonia production is highly sensitive to feedstock and electricity prices, as well as greenhouse gas (GHG) restrictions. Under strict 75% GHG reductions, biomass to ammonia route is found to be competitive with natural gas route, whereas wind and solar to ammonia routes require further improvement to compete with those two routes. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16498 2019     

Modeling and CW operation of a quasi-three-level 946 nm Nd: YAG laser

A model is developed for an end-pumped quasi-three-level laser with population in the lower laser level at equilibrium such as for transitions to the manifolds⁴I9/2in Nd³⁺,⁴I15/2in Er³⁺,⁵I8in Ho³⁺and³H6in Tm³⁺. It is shown that the effect of residual lower laser level population on laser operation can be treated as a saturable loss. Room temperature operation on the⁴F3/2-⁴I9/2transition in Nd:YAG under CW dye laser pumping has been demonstrated with a threshold as low as 11.5 mW incident power and a slope efficiency of 7 percent with 0.3 percent output coupling. Performance is limited by the low output coupling and diffraction loss.     

The optimal diamond wheels for grinding diamond tools

Grinding is the most suitable process for manufacturing good quality diamond tools. In this paper, diamond wheels have been studied. From the grinding of polycrystalline diamond (PCD) insets, the effects of certain factors such as the bonding material, the grit size and structure of a diamond wheel have been investigated. It is concluded that vitrified bond diamond wheels are the most suitable for grinding PCDs and the recommended grit size is mesh number 1000, which can get a good surface quality within an appropriate time. The wheel structure is another important factor. Rougher wheels (mesh #800, #1000) with the softer grade scale P yield a higher material removal rate (MRR) than scale Q. However, a finer wheel (mesh #1200) needs a tougher structure to promote its grinding ability and to have a higher MRR.     

The modeling and implementation of tri-rotor flying robot

The objective of this study is to develop a tri-rotor flying robot, which adopts the Y-shaped three-rotor structure. In order to balance the yaw torque produced by the three rotors, the RC servomotor and linkage is installed on the tail axis in order to improve the angle of the rolling axis of the tail motor. Moreover, through the torque generated by the horizontal component of the lift from the inclined motor on the tail axis, it balances the yaw torque of the three rotors. The dynamic equations of the tri-rotor flying robot are determined in this paper. The relationship between motor thrust, angular acceleration, and voltage input was also studied in this research. In order to study the effect of control parameters on the flight stability completely, this study develops a universal stability experimental platform to help tuning the control parameters safely. Based on this, the tri-rotor flying robot can rapidly change flying gesture and avoid oscillation. Finally, we conducted some indoor and outdoor flight tests. From the experimental results, the tri-rotor flying robot can fly and hover stably in the sky.     

Efficient and short certificateless signatures secure against realistic adversaries

The notion of certificateless cryptography is aimed to eliminate the use of certificates in traditional public key cryptography and also to solve the key-escrow problem in identity-based cryptography. Many kinds of security models have been designed for certificateless cryptography and many new schemes have been introduced based on the correspondence of the security models. In generally speaking, a stronger security model can ensure a certificateless cryptosystem with a higher security level, but a realistic model can lead to a more efficient scheme. In this paper, we focus on the efficiency of a certificateless signature (CLS) scheme and introduce an efficient CLS scheme with short signature size. On one hand, the security of the scheme is based on a realistic model. In this model, an adversary is not allowed to get any valid signature under false public keys. On the other hand, our scheme is as efficient as BLS short signature scheme in both communication and computation and, therefore, turns out to be more efficient than other CLS schemes proposed so far. We provide a rigorous security proof of our scheme in the random oracle model. The security of our scheme is based on the k-CAA hard problem and a new discovered hard problem, namely the modified k-CAA problem. Our scheme can be applied to systems where signatures are typed in by human or systems with low-bandwidth channels and/or low-computation power.     

神经网络在暂态稳定评估中应用的研究述评

对神经网络在暂态稳定评估问题中应用的研究文献进行调研和综述,对稳定评估的目标、输入特征的选择、所采用的神经模型等进行了述评,对目前存在的问题进行了讨论。建议今后应在特征抽取和选择、训练样本集的压缩、在线训练机制以及类间边界区识别等方面开展深入的研究工作。     

Deriving a transient stability index by neural networks for power-system security assessment

This paper proposes an approach for establishing a transient stability classifier and derives a continuous transient stability index, using a three-layer feed-forward artificial neural network (ANN), for on-line security assessment in large power systems. With the derived stability index, a novel classification scheme creating an ‘indeterminate’ class, is introduced to minimize misclassifications and to improve the reliability of the classification results. Several post-fault abstract attributes about the system generators’ acceleration rates and kinetic energies provide the basis for the stability classification. In order to derive the transient stability index, a semi-supervised backpropagation (BP) learning algorithm, making use of a specially defined error function, is developed. The proposed approach can not only distinguish whether a power system is stable or unstable, on the basis of the specific post-fault attributes, but can also provide a relative stability quantifier. Furthermore, as the number of the selected abstract attributes is independent of the system size, the methodology of the proposed approach can realistically be applied to large power systems. The 10-unit 39-bus New England power system is employed to demonstrate the proposed approach. The numerical results show that the ANN-based classifier can assess the transient stability reasonably well.     

A hierarchically structured and constraint-based data model for intuitive and precise solid modeling in a virtual reality environment

This article proposes a hierarchically structured and constraint-based data model for intuitive and precise solid modeling in a virtual reality (VR) environment. The data model integrates a high level constraint-based model for intuitive and precise manipulation, a middle level solid model for complete and precise representation and a low-level polygon mesh model for real-time interactions and visualization in a VR environment. The solid model is based on a hybrid B-rep/CSG data structure. Constraints are embedded in the solid model and are organized at hierarchical levels as feature constraints among internal feature elements, part constraints among internal features and assembly constraints between individual parts. In addition to providing a complete and precise model representation and the support for real-time visualization, the proposed data model permits intuitive and precise interaction through constraint-based manipulations for solid modeling in a VR environment. This is a critical issue for product design in a VR environment due to the limited resolutions of today's VR input and output devices.     

Modeling a solar-powered double bed novel composite adsorbent (silica activated carbon/CaCl2)-water adsorption chiller

During the past few decades, the growing demand for air conditioning has caused a significant increase in demand for primary energy resources. Adsorption cooling system is one of the technologies which could be powered by renewable energy. This study aims to improve the performance of a solar-powered adsorption chiller by applying a novel composite adsorbent, a mixture of activated carbon, silica gel and calcium chloride. Modeling is established to investigate the cooling performance of a composite adsorbent based adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: (1) single glazed cover; (2) double glazed cover and (3) transparent insulation material (TIM) cover. The simulation results show that the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller depend hugely on the solar collector temperature. It is found that a double glazed cover shows the best cooling performance and 30 m² is the most optimized solar collector area. Two to three hours of pre-heating time is required to initiate the desorption process of the adsorber in a day of operation. This newly developed silica activated carbon/CaCl₂ composite material as adsorbent used in the adsorption chiller could achieve a high mean COP_sc of 0.48. Its satisfactory performance suggests that this novel composite material has a potential to be used in the adsorption chiller system even if it is powered by unstable solar energy.     

A note on the existence conditions for fault detection and isolation observers

This paper proves that the necessary and sufficient conditions for the existence of FDI observers given by Hou and Muller (Theorem 3) are incorrect. A necessary correction to their theorem is presented and proved. A simple and useful condition is also proposed for the existence of the FDI observers with a rigorous proof. Examples illustrate the effectiveness of the proposed existence condition.