Nominal Operating Conditions Research Articles

Off-design performance analysis of a turbocharged solid oxide fuel cell system

The aim of this study is to analyse the off-design performance of an innovative turbocharged solid oxide fuel cell system, fed by biogas and designed to generate 30 kW during nominal operating conditions. The layout of such a plant combines the high efficiencies of the solid oxide fuel cell with a reduced-cost option for fuel cell pressurization: fuel cells are usually pressurized by a micro gas turbine, but the alternative use of a turbocharger, a mass-produced component widely used in the automotive field, could reduce significantly the capital cost of the system. This kind of layout has been rarely investigated in literature, and a detailed performance analysis of a turbocharged SOFC system would be a valuable source of information for both academia and industry. To perform this analysis, a steady-state model has been created using a modular tool developed in Matlab®-Simulink® that includes off-design models of the system components. The model has been used to compare different control strategies. The most suitable control strategy, based on wastegate and cold bypass valves, was adopted, obtaining compliance with the operative constraints and high system efficiency. Afterward, the system steady-state operation was simulated for various electric power loads and ambient temperatures. The performance analysis focused on the effect of these two variables on the system behaviour and has provided insight on the influence on the most significant system outputs, including global efficiency, temperatures and pressures. A system efficiency increase was observed at part load, with values growing from 50.8% to 57.3%. Higher values of ambient temperature resulted in a more significant pressurization of the fuel cell, affecting positively the system efficiency: from 50.5% at 0 °C to 51.0% at 30 °C. Great attention was paid to the system constraints, to verify that the plant could operate properly in all the considered conditions. The proposed control strategy was tested and proved to be effective at achieving this objective.

High-Density 3-D Stackable Crossbar 2D2R nvTCAM With Low-Power Intelligent Search for Fast Packet Forwarding in 5G Applications

By virtue of hardware parallelism, ternary content addressable memory (TCAM) is attractive for low-latency search for packet forwarding in routers for network communications in the upcoming fifth-generation (5G) era. However, the demand for high-density TCAM encounters remarkable costs in silicon area and power consumption. In this work, a 16-kb nonvolatile ternary content addressable memory (nvTCAM) test chip based on resistive memory (ReRAM) is demonstrated in 28-nm process with two techniques to deal with the aforementioned issues. First, the crossbar array with 2-diode-2-ReRAM (2D2R) nvTCAM cell is proposed to realize >3× improvement in storage density. The back-end-of-line integration of both diode and ReRAM resistor also allows for further 3-D stacking to realize larger storage density. Second, the machine learning concept is exploited to realize intelligent search operation. K -means clustering is employed to allocate the entry storage and then the search of destination IP address can be targeted to a specific bank for low power. The evaluation shows >70% reduction in search energy with 2% overhead in silicon area for bank count of four. The test chip also achieves a match delay of 2 ns under nominal operating conditions.

A method of using fuzzy hypergraphs to evaluate structural and technological survivability of attitude control systems for unmanned spacecraft

Successful completion of a mission by an unmanned spacecraft (USC), both under nominal operational conditions both under examined contingencies and unexamined off-nominal situations is possible through designing survivability into the USC onboard system (OS). An analysis of current methods for evaluating USC OS survivability during their configuration management and reconfiguration under conditions of examined in-flight contingencies widely used in the design and development of the said USC has shown that these methods are not acceptable for evaluating the USC OS survivability in case of unexamined off-nominal situations in flight. This calls for development of conceptually novel methodological and procedural framework for evaluating structural survivability of USC OS configurations that take into account the level of participation of functional elements (FE) and OS subsystems in the USC control operations under various scenarios of the mission plan implementation. The paper proposes an original approach to evaluating the structural and technological survivability of the USC OS based on a fuzzy hypergraph formal representation of the operations to control the USC attitude, where the edges of the hypergraph connect the FE and OS subsystems that support the implementation of this or that specific control process. The paper also shows how one could use for the quantitative evaluation of the structural and technological survivability of a specific USC OS configuration the results of differentiation of a fuzzy hypergraph that could be visualized as a fuzzy hypergraph of technological independence of OS FE. Such an approach makes it possible to analyze the effects of FE on OS, identify the most critical elements, which have the lowest technological independence under mission plan implementation conditions, which could be used for providing a rationale for the required level of structural and functional redundancy of USC elements and subsystems introduced during various phases in its life cycle. Keywords: unmanned spacecraft, onboard systems survivability, fuzzy hypergraph derivative.

Detailed Simulation of the Nominal Flow and Temperature Conditions in a Pre-Konvoi PWR Using Coupled CFD and Neutron Kinetics

The aim of the numerical study was the detection of possible vortices in the upper part of the core of a Pre-Konvoi Pressurized Water Reactor (PWR) which could lead to temperature cycling. In addition, the practical application of this Computational Fluid Dynamic (CFD) simulation exists in the full 3D analysis of the coolant flow behavior in the reactor pressure vessel of a nuclear PWR. It also helps to improve the design of future reactor types. Therefore, a CFD simulation of the flow conditions was carried out based on a complex 3D model. The geometry of the model includes the entire Reactor Pressure Vessel (RPV) plus all relevant internals. The core is modelled using the porous body approach, the different pressure losses along and transverse to the main flow direction were considered. The spacer-grid levels were taken into account to the extent that in these areas no cross-flow is possible. The calculation was carried out for nominal operating conditions, i.e., for full load operation. Furthermore, a prototypical End of Cycle (EOC) power distribution was assumed. For this, a power distribution was applied as obtained from a stationary full-core calculation with the 3D neutron kinetics code DYN3D. In order to be able to adequately reproduce flow vortexes, the calculation was performed transiently with suitable Detached Eddy Simulations (DES) turbulence models. The calculation showed fluctuating transverse flow in the upper part of the core, starting at the 8th spacer grid but also revealed that no large dominant vortices exists in this region. It seems that the core acts as a rectifier attenuating large-scale vortices. The analyses included several spacer grid levels in the core and showed that in some areas of the core cross-section an upward increasingly directed transversal flow to the outlet nozzle occurs. In other areas of the core cross-section, on the other hand, there is nearly any cross-flow. However, the following limitations of the model apply: In the model all fuel elements are treated identical and cross flows due to different axial pressure losses for different FA types cannot be displayed. The complex structure of the FAs (eg. flow vanes in spacer grids) could also influence the formation of large-scale vortices. Also, the possible influence of two-phase flows was not considered.

Optimal design of flexible heat-integrated crude oil distillation units using surrogate models

The design of distillation columns often considers a given fixed feedstock and nominal operating conditions. Here we present an optimization-based approach for the optimal design of these units considering a flexible operation under a range of potential feedstocks. Our method combines an artificial neural network with a support vector machine to model the crude oil distillation unit. The artificial neural network model predicts the performance of the distillation unit for a given crude oil feedstock whilst the support vector machine classifier filters out infeasible design alternatives from the solution space (i.e., designs that are unlikely to converge when simulated using a rigorous model). The inputs to the artificial neural network include the column structural variables and operating conditions, whilst the outputs are process variables linked to the column performance. The artificial neural network models and support vector machines constructed for different crude oil feedstocks are integrated into a two-stage optimization framework in order to optimize the column structural variables and operating conditions, where the minimum utility demand is estimated using the pinch analysis. An effective solution strategy that combines stochastic and deterministic optimization algorithms is applied to search for economically viable and flexible design alternatives that can operate over a given range of crude oil feedstocks while satisfying the product quality specifications. The capabilities of the proposed approach are illustrated using an industrially-relevant case study, where we clearly show that the proposed approach can identify design alternatives capable of handling various feedstocks effectively.

Exploiting Residue Curve Maps to Assess Thermodynamic Feasibility Boundaries under Uncertain Operating Conditions

The very first step of almost any separation process design procedure is the thermodynamic feasibility analysis. In the case of distillation, residue curve maps (RCMs) represent an essential tool to assess whether the separation is feasible or not. However, the analysis is generally carried out by referring to nominal operating conditions and product purities as specification. This means that, when process parameters are likely to undergo fluctuations, the prediction of the system response is not that obvious. An ABE/W (acetone−butanol−ethanol/water) mixture was then selected as a case study since it allows us to discuss several non-ideal thermodynamic behaviors and because of the renewed interest in biorefinery and sustainable processes during recent years. Residue curve mapping was then exploited to determine the thermodynamic feasibility range for multicomponent distillation processes as well as for distillation trains and process-intensified solutions taking into account both product purity and product recovery specifications. The final product of this study is a thorough procedure to determine the flexibility boundaries of feed and product compositions as well as an immediate and intuitive graphical representation from a binary standard distillation column to a complex multicomponent dividing wall column application

Detecting loss-of-coolant accidents without accident-specific data

This paper develops an automated fault detection tool to detect very small LOCAs in pressurized water reactors that would be difficult for operators to detect manually. One of the primary challenges with previous automated fault detection methods, which are data-driven, is that they require data from LOCAs; however, it may be difficult to capture real operational data from LOCA scenarios. This work uses a physics-inspired approach that equates the physical effects of a LOCA to changes in known variables. This approach enables the detection of very small LOCAs using data-driven approaches that use nominal operating data without the need for LOCA data. The approach combines data-driven modeling with control-theoretic estimation techniques to detect LOCAs and estimate their magnitudes in real-time. First, simulated process data for a variety of nominal operating conditions is collected using a generic pressurized water reactor simulator. Then, that data is used to train an artificial neural network regression model that captures the nonlinear plant dynamics. Finally, the regression model is used in a particle filter to detect the onset and estimate the magnitude of the leak. These methods are successfully verified using LOCA simulations that would be hard to manually distinguish from normal operating transients.

Robust optimization of an organic Rankine cycle for geothermal application

A robust design optimization (RDO) methodology for Organic Rankine Cycles (ORC) is presented, allowing to ensure an improved, stable performance over a large range of operating conditions. In contrast with classical ORC design methods, whereby all modeling hypotheses and operating conditions are considered as perfectly known, i.e. deterministic, the RDO approach allows to account for the manifold sources of uncertainty affecting the system. For geothermal ORC, the latter are related on one hand with the ill-known properties of the geothermal source and, on the other, with intrinsically random parameters, such as the condensation temperature. The proposed RDO approach selects values of the design parameters that maximize the expected (average) performance while minimizing its variance under uncertain nominal operating conditions. The optimal design delivered by the proposed strategy outperforms the one derived from the standard deterministic approach: specifically, the expected power output is increased by 1.5%, while its standard deviation is reduced by 8.5% and the surface of the heat exchangers by 34%. • Geothermal Organic Rankine cycle performance is subject to many uncertainties. • A robust optimization strategy is devised for cycle design under uncertainty. • The design maximizes average performance while minimizing the variance. • Nested Kriging surrogates are used to efficiently speed-up the design loop. • Robust optimum has +1.5% power output and smaller variance wrt standard design.

МОДЕЛИРОВАНИЕ ОДНОФАЗНОГО ДВУХУРОВНЕВОГО АВТОНОМНОГО ИНВЕРТОРА НАПРЯЖЕНИЯ С ШИРОТНО-ИМПУЛЬСНОЙ МОДУЛЯЦИЕЙ

The work is devoted to modeling the circuit of a single-phase two-level the autonomous inverter of the voltage with pulse-width modulation in the MATLAB environment. Pulse-width modulation generation unit developed. Static characteristics of the converter are obtained. With the help of a set of measuring instruments, the efficiency of the сonverter is measured under nominal operating conditions.

Development of an organic Rankine cycle-based micro combined heat and power system for residential applications

Micro combined heat and power systems can achieve higher overall energy efficiencies than conventional home heating appliances. In the present paper, a prototype compact organic Rankine cycle-based micro combined heat and power system is built and tested for use in residential houses in order to increase energy efficiency and security. The heat generated by the energy system is used for both space heating and domestic hot water. The organic Rankine cycle’s working fluid is n-Pentane. At nominal operating conditions, the system generates 1.0 kWe of electricity of which 0.65 kWe can be used in the house or exported to the grid. Its heat output ranges from 7.0 to 14.0 kWth. A thermodynamic model for the organic Rankine cycle is developed to determine its thermodynamic performance under varying operating conditions. The model is then used to calculate the power output and energy conversion efficiency. The advantages of the micro combined heat and power system include a high overall energy efficiency, automated operation, low maintenance, low noise levels, and compactness. The value provided to the end-user by this integrated system is found in both its reliability and independence from the electrical grid.

Testing technique for jackhammers at the «СОРП» test bench

A methodology for measuring the parameters of jackhammers at the «СОРП» test bench is presented, which is distinguished by metrological impeccability and correctness. All parameters are measured in the same nominal operating conditions, which was not provided by the previously used methods. Keywords jackhammer, bench, tests, manual machine, object, processing, energy characteristics, impact energy, dynamic characteristics. vvanaev@mail.ru

Design, manufacture and test of a micro-turbine renewable energy combustor

The ever-increasing demand on highly efficient decentralized power generation with low CO2 emission has made microturbines for power generation in micro gas turbine (MGT) systems popular when running on biofuels as a renewable source of energy. This document presents a state-of-the-art design, and optimization (in terms of design, performance and emission control) of a micro-turbine renewable energy combustor that fits into the existing Bladon 12kWe recuperated microturbine plenum while running on a range of biofuels as it can successfully provide the required power of the MGT. Governing equations for in-depth analysis of the combustor consist of manufacturer empirical data to simulate system-level operation with respect to replacement of the fossil with biofuels. The Model developed and validated at the company’s ISO conditions confirms the output power of the new combustor fits the conventional system with slight eco-energy improvements. The modeling of the combustor in a complete microturbine assembly system is performed, then was utilized to further analysis of the microturbine with the designed combustor. The results gave on average 46.7% electrical efficiency, 83.2% system efficiency, 12 kWe electrical power, and 90% recuperator effectiveness at nominal operating conditions of microturbine (MT). Sensitivity analyses evaluate changes in performance with respect to fuel phase (e.g., liquid or gaseous) and design variables (e.g., orientation, shape, and dimensions of combustor), leading to energy optimization of the unit. Findings demonstrate that the combustor in microturbine can meet the target performance specifications of a company conventional diesel microturbine with significant savings. An objective function including both combustor and recuperator technical energy data is defined for finding the best ratio of fuel and air and their flow rates to find the most effective operating points for the operation of MT. Annual time series simulations completed for Coventry, West Midlands, United Kingdom indicate a new combustor can reduce operational costs of diesel fuel combustor by 8%, 2%, 36%, and 25% when supplying bioethanol, DME, biogas, and NG, respectively. Annual operating time of the renewable microturbine combustor at rated capacity included an 11% reduction in exergy loss with biogas fuel relative to diesel fuel.

An Enhanced <bold>Σ</bold> -Y Spray Atomization Model Accounting for Diffusion due to Drift-Flux Velocities

<div class="section abstract"><div class="htmlview paragraph">Spray modeling techniques have evolved from the classic DDM (Discrete Drops Method) approach, where the continuous liquid jet is discretized into “drops” or “parcels” till advanced spray models often based on Eulerian approaches. The former technique, although computationally efficient, is essentially inadequate in highly dense jets, as in the near nozzle region of compression ignition engines, while the latter could lead to extreme levels of computational effort when resolved interface capturing methods, such as VoF (Volume of Fluids) and LS (Level-Set) types, are used. However, in a typical engineering calculation, the mesh resolution is considerably coarser than in these high fidelity computations. If one presumes that these interfacial details are far smaller than the mesh size, smoothing features over at least one cell ultimately results in a diffuse-interface treatment in a Eulerian framework. Therefore, it is explained that currently the greatest interest is focused on the development of diffuse interface computational models which use a variable called surface density of the interface (Σ), solved using an averaged or filtered transport equation, to model the atomization process. However, in this type of approach a single-fluid formulation is used solving a single momentum equation but neglecting some features of the relative velocity between the liquid and the gas phases. This practice is not an issue for diesel sprays under nominal operating conditions of current engines, i.e., under high injection pressure and chamber density. Nevertheless, in certain combustion strategies, due to lower ambient densities (highly premixed combustion) or the usage of substitute fuels, the slip between phases becomes more important and must be fully considered. In this work, we propose a new developed Σ-Y Spray Atomization Model that accounts for diffusion due to Drift-Flux velocities, providing a predictive behavior under all engine conditions. In addition, the proposed formulation creates a mechanism by which the interfacial dynamics can impact the transport of the liquid mass fraction, therefore, making the interfacial density an active scalar, fully coupled with the rest of the flow, overcoming the limitations of previous formulations.</div></div>

Simulation of cryogenic high pressure regulator dynamics

ABSTRACT The most critical component in an aerospace tank pressurisation system is a pneumatic pressure regulator with helium gas as the medium. The current paper focused on dynamic analysis of high pressure, spring-loaded, cryogenic medium pressure regulator. In order to evaluate its dynamic behaviour, a nonlinear model has been developed based on the fundamental equations by considering various forces – pressure forces, spring forces, bellow forces and Coulomb friction forces. The simulation is carried out for nominal operating conditions of the pressure regulator using commercial available AMESIM simulation tool. The effect of damping orifice size and downstream ullage volume are studied and obtained results are presented in the paper. Also experimental tests have been carried out for the same parameters to validate the simulation model using a proto model developed in-house with emphasis on machining of various precise parts of the high pressure regulator. These parts are manufactured in-house using high precision surface grinding, jig boring, electro discharge machining process, etc. The machined parts are realised to achieve precise sliding diametrical clearances between 32 microns and 50 microns, eccentricity within 20 microns and average surface roughness of 0.4 microns. The dimensional parameters of the machined parts are evaluated using the co-ordinate measuring machine (CMM), Talyrond and Talysurf equipments. The simulation results and experimental test results are found to be in good agreement. The developed model serves as a tool for optimisation of the regulator performance parameters.

Automotive Subzero Cold-Start Quasi-Adiabatic Proton Exchange Membrane Fuel Cell Fixture: Design and Validation.

Subzero automotive cold-starts of proton exchange membrane fuel cell (PEMFC) stacks require accelerated thermal rises to achieve nominal operating conditions and close-to-instantaneous usable output power. Advances in the material, structure and operational dependence on the balance between the maximum power output and the electrochemical conversion of hydrogen and oxygen into water requires validation with subzero cold-starts. Herein are presented the design and validation of a quasi-adiabatic PEMFC to enable single-cell evaluation, which would provide a more cost-effective option than stack-level testing. At –20 °C, the operational dependence of the preconditioned water content (3.2 verse 6.2) for a galvanic cold-start (<600 mA cm−2) was counter to that of a laboratory-scale isothermal water fill test (10 mA cm−2). The higher water content resulted in a faster startup to appreciable power output within 0.39 min versus 0.65 min. The water storage capacity, as determined from the isothermal water fill test, was greater, for the lower initial water content of 3.2, than 6.2, 17.4 ± 0.3 mg versus 12.8 ± 0.4 mg, respectively. Potentiostatic cold-starts produced usable power in 0.09 min. The versatility and reproducibility of the single cell quasi-adiabatic fixture avail it to future universal cold-start stack relevant analyzes involving operational parameters and advanced materials, including: applied load, preconditioning, interchanging flow field structures, diffusion media, and catalyst coated membranes.

Fixed Low-Order Wide-Area Damping Controller Considering Time Delays and Power System Operation Uncertainties

This paper presents a procedure for the design of a Wide-Area Damping Controller (WADC) towards improving the low-frequency oscillation modes of the power system considering uncertainties in both nominal power system operation condition and time delay on the WADC communication channels. A set of possible operating conditions is linearized around their equilibrium points and used for the construction of the uncertainty model of the system. The lower and upper limits of the time delay are also included in the set. The WADC operation requires remote signals provided by Phasor Measurement Units and highly susceptible to failures or cyber-attacks that may compromise the controller performance. The proposed procedure evaluates in each stage if the resulting controller presents robustness to communication channel permanent failure. Modal analysis and time-domain nonlinear simulations were conducted in the IEEE 68-bus power system.

Nanoscale electro-thermal interactions in AlGaN/GaN high electron mobility transistors

Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V) resulted in an ∼44% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5 V) condition. The difference arises primarily due to reduction in the heat generating volume when operating under partially pinched-off (i.e., high VDS) conditions. Self-heating amplifies with this reduction as heating now takes place primarily over length scales less than the mean free path of the phonons tasked with energy dissipation. Being less efficient, the subcontinuum transport restricts thermal transport away from the device hot-spot causing a net increase in channel temperature. Taken together, even purely thermally driven device mean-time-to-failure is not, therefore, based on power dissipation alone as both bias dependence and subcontinuum thermal transport influence device lifetime.

A cyber‐secure control‐detector architecture for nonlinear processes

AbstractThis work presents a detector‐integrated two‐tier control architecture capable of identifying the presence of various types of cyber‐attacks, and ensuring closed‐loop system stability upon detection of the cyber‐attacks. Working with a general class of nonlinear systems, an upper‐tier Lyapunov‐based Model Predictive Controller (LMPC), using networked sensor measurements to improve closed‐loop performance, is coupled with lower‐tier cyber‐secure explicit feedback controllers to drive a nonlinear multivariable process to its steady state. Although the networked sensor measurements may be vulnerable to cyber‐attacks, the two‐tier control architecture ensures that the process will stay immune to destabilizing malicious cyber‐attacks. Data‐based attack detectors are developed using sensor measurements via machine‐learning methods, namely artificial neural networks (ANN), under nominal and noisy operating conditions, and applied online to a simulated reactor‐reactor‐separator process. Simulation results demonstrate the effectiveness of these detection algorithms in detecting and distinguishing between multiple classes of intelligent cyber‐attacks. Upon successful detection of cyber‐attacks, the two‐tier control architecture allows convenient reconfiguration of the control system to stabilize the process to its operating steady state.

Optimisation of the Autothermal NH3 Production Process for Power-to-Ammonia

The power-to-ammonia process requires flexible operation due to intermittent renewable energy supply. In this work, we analyse three-bed autothermal reactor systems for design and off-design performance for power-to-ammonia application. The five reactor systems differ in terms of inter-stage cooling methods, i.e., direct cooling by quenching (2Q), combination of indirect and direct cooling (HQ and QH) and indirect cooling (2H) with variations. At optimum nominal operation conditions, the inter-stage indirect cooling (2H) reactor systems result in the highest NH3 production. For off-design performance analysis, NH3 production is minimised or maximised by varying one of the following process variables at a time: inert gas, feed flow rate or H2-to-N2 ratio. For each variation, the effect on H2 intake, recycle stream load and recycle-to-feed ratio is also analysed. Among the three process variables, the H2-to-N2 ratio provided ca. 70% lower NH3 production and 70% lower H2 intake than at nominal operation for all five reactor systems. Operation of autothermal reactor systems at significantly lower H2 intake makes them reliable for power-to-ammonia application; as during energy outage period, shutdown can be delayed.

A phenomenological base semi-physical thermodynamic model for the cylinder and exhaust manifold of a natural gas 2-megawatt four-stroke internal combustion engine

This paper presents the application of a systematic methodology to obtain a semi-physical model of phenomenological base for a 2 MW internal combustion engine to generate electric power operating with natural gas, as a function of the average thermodynamic value normally measured in industrial applications. Specifically, the application of the methodology is focused on the cylinders, exhaust manifold, and turbocharger turbine sections. The proposed model was validated with actual operating data, obtaining an error rate not exceeding 5%, which allow a thermal characterization of the Jenbacher JMS 612 GS-N based on the model. A parametric analysis is conducted; considering the volumetric efficiency, the output electric power, the effective efficiency, the exhaust gas temperature, the turbine mass flow, the specific fuel consumption under the nominal operation conditions, which is 1.16 bar in the gas pressure, 65 °C in the cooling water temperature, 35 °C in the average ambient temperature, and 1500 rpm. The results of this model can be used to evaluate the thermodynamic performance parameters of waste heat recovery systems. On the other hand, new control strategies and the implementation of state observers for the detection and diagnosis of failures can be developed based on the proposed model.