Steady-state Performance Of System Research Articles

Biogas Production from Co-digestion of Pennisetum Pururem cv. Pakchong 1 Grass and Layer Chicken Manure Using Completely Stirred Tank

In this research, the production of biogas from co-digestion of Pennisetum purpureum cv. Pakchong1 grass and layer chicken manure using completely stirred tank was investigated. The experiment was defined to examine effect of the change in carbon to nitrogen (C/N) ratios and the organic loading rates (OLRs) on biogas production and system steady-state performance. Primary analyses suggested that an approximate content of grass and manure was 50: 50% by weight to achieve C/N ratio of 20 and 70: 30% by weight for C/N ratio of 30 respectively. The experimental reactor was set to operate at a fixed total solid content of 4% with two cases of C/N ratio at four different OLRs of 1.1, 1.4, 1.7 and 2.2kg VS/(m3.d). Each condition was operated for 1.5 time of reactor retention time. The result suggested that maximum steady-state methane yield of 0.27 ± 0.01 L CH4/kg VSadded can be achieved at C/N ratio of 20 with OLR of 1.1kg VS/(m3.d). Moreover, the results also suggested that methane yield decreased for an increase in OLR. Nonetheless, the work presented herein can provide an insight information for design and operation optimization according to economical investment analysis.

Steady State Analysis of 5kW SOFC System with Novel Configuration

For stand-alone solid oxide fuel cell (SOFC) systems, delivering demanded power, maintaining system operating constraints, and achieving high system efficiency are the primary design objectives. We consider a novel SOFC design that includes an air bypass valve to achieve greater controllability. Through model-based analysis, the steady-state system performance is investigated. And the optimal operating points (OOPs) for different loads are explored to achieve the high system efficiency. The effects of bypass valve are quantified. The analysis demonstrates that the design is feasible and the bypass valve can play a substantial role on improving the overall system performance and operationability.

Thermal management oriented steady state analysis and optimization of a kW scale solid oxide fuel cell stand-alone system for maximum system efficiency

This research attempts to ensure system safety while to maximize system efficiency by addressing steady state analysis and optimization for solid oxide fuel cell (SOFC) systems. Firstly, a thermal management oriented kW scale SOFC stand-alone system (primarily comprising a planar SOFC stack, a burner, and two heat exchangers) is developed, in which a special consideration for stack spatial temperature management is conducted by introducing an air bypass manifold around heat exchangers. The dynamic model of the system is performed using transient energy, species, and mass conservation equations. Secondly, based on the system model, the effects of operating parameters including fuel utilization (FU), air excess ratio (AE), bypass ratio (BR), and stack voltage (SV) on the system steady-state performances (e.g. system efficiency, stack inlet, stack outlet, and burner temperatures) are revealed. Particularly, an optimal relationship between the system efficiency and the operating parameters is proposed; the maximum system efficiency can certainly be obtained at the inlet outlet temperature critical point of the BR-AE or FU-AE planes for all SV operating points. Finally, according to the optimal relationship, a traverse optimization process is designed, and the maximum system efficiency and safe operating parameters at any efficient SV operating point are calculated. The results provide an optimal reference trajectory for control design, where the system is safe and efficiency optimization. Moreover, the results reveal two important system characteristics: (1) the burner operates within safe temperature zone as long as the temperature of the upstream stack is well controlled; (2) the control design for the system is a nonlinear optimal control with switching structure, which is a challenging control issue.

Steady-state performance optimization for nonlinear control systems of Lur’e type

In this paper, we develop a methodology for the steady-state performance optimization, in terms of the sensitivity to disturbances, for Lur’e type nonlinear control systems. For linear systems, steady-state performance is well defined and related to frequency-domain characteristics. The definition and analysis of steady-state performance of nonlinear systems are, however, far from trivial. For a practically relevant class of nonlinear systems and disturbances, this paper provides a computationally efficient method for the computation of the steady-state responses and, therewith, for the efficient performance assessment of the nonlinear system. Based on these analysis tools, a strategy for performance optimization is proposed, which can be employed for the optimized tuning of system and controller parameters. The results are illustrated by application to a variable gain controlled short-stroke wafer stage of a wafer scanner.

Performance of a combined solar heating system for residential applications in Greece

Combined solar heating is an emerging technology that offers the potential for realising more efficient use of solar thermal energy in order to cover space heating and domestic hot-water loads. This paper is focused on small-scale applications of single-family residential dwellings in Greece. A system-level model was developed and validated. The model was exercised to evaluate strategies for optimal design and operation of solar combisystems (SCSs), using a range of performance criteria as objective functions for optimisation. The effect of SCS design and operating variables on steady-state system performance has been investigated. System design and operation optimisation has been performed through the integration and synthesis of system-level modelling with system optimal design, optimisation, and annual simulation studies to provide a comprehensive assessment.

Low-Frequency Learning and Fast Adaptation in Model Reference Adaptive Control

While adaptive control has been used in numerous applications to achieve system performance without excessive reliance on dynamical system models, the necessity of high-gain learning rates to achieve fast adaptation can be a serious limitation of adaptive controllers. This is due to the fact that fast adaptation using high-gain learning rates can cause high-frequency oscillations in the control response resulting in system instability. In this note, we present a new adaptive control architecture for nonlinear uncertain dynamical systems to address the problem of achieving fast adaptation using high-gain learning rates. The proposed framework involves a new and novel controller architecture involving a modification term in the update law. Specifically, this modification term filters out the high-frequency content contained in the update law while preserving asymptotic stability of the system error dynamics. This key feature of our framework allows for robust, fast adaptation in the face of high-gain learning rates. Furthermore, we show that transient and steady-state system performance is guaranteed with the proposed architecture. Two illustrative numerical examples are provided to demonstrate the efficacy of the proposed approach.

WAITING TIME ANALYSIS OF MULTI-CLASS QUEUES WITH IMPATIENT CUSTOMERS

In this paper, we study three delay systems where different classes of impatient customers arrive according to independent Poisson processes. In the first system, a single server receives two classes of customers with general service time requirements, and follows a non-preemptive priority policy in serving them. Both classes of customers abandon the system when their exponentially distributed patience limits expire. The second system comprises parallel and identical servers providing the same type of service for both classes of impatient customers under the non-preemptive priority policy. We assume exponential service times and consider two cases depending on the time-to-abandon distribution being exponentially distributed or deterministic. In either case, we permit different reneging rates or patience limits for each class. Finally, we consider the first-come-first-served policy in single- and multi-server settings. In all models, we obtain the Laplace transform of the virtual waiting time for each class by exploiting the level-crossing method. This enables us to compute the steady-state system performance measures.

Performance analysis of a membrane liquid desiccant air-conditioning system

A new membrane liquid desiccant air-conditioning (LDAC) system is proposed and investigated in this paper. Liquid-to-air membrane energy exchangers (LAMEEs) are used as a dehumidifier and a regenerator in the proposed membrane LDAC system, which can eliminate the desiccant droplets carryover problem occurring in most direct-contact LDAC systems. A parametric study on steady-state performance of the membrane LDAC system is performed using the TRNSYS energy simulation platform. The impacts of various climatic conditions and key system parameters on the system performance are evaluated. Results show that the proposed membrane LDAC system is capable of achieving recommended supply air conditions for productive, comfort and healthy environments if the key system parameters are effectively controlled. The system coefficient of performance (COP) at the design condition is 0.68, and the sensible heat ratio (SHR) for the dehumidifier lies in the range between 0.3 and 0.5 under different climatic, operating and design conditions. The proposed membrane LDAC system is able to effectively remove latent load in applications that require efficient humidity control.

Stochastic decomposition in production inventory with service time

We study an (s,S) production inventory system where the processing of inventory requires a positive random amount of time. As a consequence a queue of demands is formed. Demand process is assumed to be Poisson, duration of each service and time required to add an item to the inventory when the production is on, are independent, non-identically distributed exponential random variables. We assume that no customer joins the queue when the inventory level is zero. This assumption leads to an explicit product form solution for the steady state probability vector, using a simple approach. This is despite the fact that there is a strong correlation between the lead-time (the time required to add an item into the inventory) and the number of customers waiting in the system. The technique is: combine the steady state vector of the classical M/M/1 queue and the steady state vector of a production inventory system where the service is instantaneous and no backlogs are allowed. Using a similar technique, the expected length of a production cycle is also obtained explicitly. The optimal values of S and the production switching on level s have been studied for a cost function involving the steady state system performance measures. Since we have obtained explicit expressions for the performance measures, analytic expressions have been derived for calculating the optimal values of S and s.The technique developed here could be applied to a few other problems in inventory. To substantiate this claim we analyze in detail a variant (which is discussed in Schwarz et al. (2006)) of the above problem. For that model, we assume that in a production run, production occurs only once in a cycle and the amount produced (in bulk) is sufficient to take the inventory level back to S. A brief discussion on the application of our method to inventory system with lead-time for replenishment has also been provided.

Slotted ALOHA performance for FU-FB in frequency selective fading environment

Slotted ALOHA is a simple and straightforward random multiple access technique, which has been used extensively in data and cellular networks as the protocol for random access. The complexity of state space-based analysis methods for finite user finite buffer systems increases exponentially with buffer size and number of users. The presence of multipath frequency selective fading channel further adds to the complexity, making the analysis practically intractable. This paper uses an approximate analysis technique called tagged user analysis TUA to analyze the performance parameters of slotted ALOHA over multipath and frequency selective fading channels for finite user finite buffer systems. In TUA, the steady state system performance is evaluated from the analysis of a single user. Moreover, the state flow graph of TUA has just four states, thus reducing the complexity of the analysis. Simulation results confirm the validity of the TUA analysis. Copyright © 2013 John Wiley & Sons, Ltd.

Effects of Imperfect Secondary Path Modeling on Adaptive Active Noise Control Systems

Implementation of adaptive active noise control (ANC) systems requires an estimate model of the secondary path to be uploaded onto digital control hardware. In practice, this model is not necessarily perfect; however, to avoid mathematical difficulties, theoretical analysis of these systems is usually conducted for a perfect secondary path model. This paper conducts a stochastic analysis on performance of Filtered-x LMS (FxLMS)-based ANC systems when the actual secondary path and its model are not identical. This analysis results in a number of mathematical expressions, describing effects of a general secondary path model on stability, steady-state performance and convergence speed of FxLMS-based ANC systems. As a surprising result, it is found that intentional misadjustment of secondary path models can enhance performance of ANC systems in practice. Theoretical results are found to be in a good agreement with the results obtained from numerical analysis. Also, experimental results confirm the validity and accuracy of the theoretical results.

Stochastic decomposition in production inventory with service time (abstract only)

We study an (s, S) inventory system with positive service time (for an overview of the work reported so far in inventory with positive service time one may refer to Krishnamoorthy, Lakshmi and Manikandan: A survey on inventory models with positive service time, OPSEARCH, DOI 10.1007/s12597-010-0032-z). This leads to a queue of demands being formed. The process of demand arrival constitutes a Poisson process. The duration of each service is exponentially distributed. Our model is a supply chain where items are added to the inventory through a production process. This starts each time the inventory level goes down to s and continues to be on until inventory level reaches S with the time required to add one unit of the item into the inventory when the production is on, are independent, identically distributed exponential random variables. Further all distributions involved in this paper are assumed to be mutually independent. We assume that no customer joins the queue when the inventory level is 0. This assumption leads us to an explicit product form solution for the steady state probability vector, using a simple approach. This is despite the fact that there is a strong correlation between lead time (the time required to add an item into the inventory) and the number of customers joining the queue during the lead time (except when the inventory level is zero during which time no customer joins the queue). The technique is to combine the steady state probability vector of the classical M/M/1 queue and that of the production inventory system where each service requires negligible time and no backlogs are allowed. Using a similar technique, the expected length of a production cycle is also obtained explicitly. The optimality of the highest inventory level S and the production switching on level s has been studied using a cost function constructed using the steady state system performance measures. Since we have obtained explicit expressions for these measures, analytic expressions have been derived for the optimal values of S and s. To show that our method can be applied to other similar problems, we analyze in detail a variant of the above problem (discussed in Schwarz M, Sauer. C, Daduna, H., Kulik, R and Szekli, R: M/M/1 Queueing systems with inventory, Queueing Systems, 54,55-78, 2006). For that model, we assume that in a production run, production occurs only once in a cycle and the amount produced is sufficient to take the inventory level back to S. A brief discussion on the application of our method to inventory system with lead time for replenishment has also been provided.

Dynamic Stability Analysis of a Tidal Power Generation System Connected to an Onshore Distribution System

This paper presents the analyzed results of both dynamic simulations and steady-state performance of a tidal power generation system (TPGS) containing a permanent-magnet synchronous generator (PMSG) driven by a tidal turbine through a gearbox. The stator windings of the tidal PMSG are connected to an onshore distribution system through an ac-to-dc converter, a dc-to-ac inverter, a step-up transformer, and a transmission cable. A d-q axis equivalent-circuit model is employed to establish complete dynamic equations of the studied system under three phase balanced loading conditions. A frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear-model simulations are both carried out to systematically determine the dynamic stability of the studied system under different operating conditions. It can be concluded from the simulation results that the studied TPGS subject to different disturbance conditions can maintain stable operation.

Thermal response of basement wall systems with low-emissivity material and furred airspace

In basement wall systems, airspaces can contribute in obtaining a higher thermal resistance, if a low-emissivity material such as reflective foil is installed within a furred-airspace. In this study, numerical simulations were conducted using the hygrothermal model ‘hygIRC-C’ that was developed at the National Research Council of Canada’s Institute for Research in Construction to investigate the steady-state and transient thermal performance of basement wall systems. This model solves simultaneously the energy equation in the various material layers, surface-to-surface radiation equation in the furred-airspace assembly, Navier–Stokes equation for the airspace, and Darcy and Brinkman equations for the porous material layers. The wall systems used in the simulations incorporate a low-emissivity material (foil with emissivity = 0.04) bonded to a moulded/expanded polystyrene foam that is installed in a furred-airspace assembly. The furring is installed horizontally and covered with a gypsum board. The structural element of the wall (external layer) is a poured-in-place concrete. Walls with and without furred-airspace assembly were considered in this study. Also, consideration was given to investigate the effect of the above-grade and below-grade portions of the wall on the thermal performance when these walls are subjected to two different Canadian climates. Results showed that at steady-state condition, the effective thermal resistance ( R-value) of the wall with a furred-airspace assembly depends on the soil, outdoor, and indoor temperatures. Additionally, these wall configurations resulted in an energy savings of ~17% compared to walls without furred-airspace assembly when these walls are subjected to two different climate conditions.

Numerical study on the steady state and transient performance of a multi-type heat pump system

Numerical methods are provided to analyze dynamic characteristics as well as steady-state performance of a multi-type heat pump system. Lumped parameter method was applied to simulate the compressor and expansion device. Additionally, fully distributed method was used for analyzing the condenser and evaporators. The transient terms in the governing equations for heat exchangers were solved adopting the wave equation solutions. The simulation results predict the steady-state performance of a heat pump system with the deviation of ±10%. The transient simulation showed satisfactory responses of temperatures and pressures for various compressor speeds or expansion valve openings. From the results, it was shown that the operating conditions of secondary fluid of one indoor unit had minor influence on the performance of the other unit, while opening of expansion valve affected the transient responses of both evaporators significantly.

Modelling of Optimal Unified Power Flow Controller (OUPFC) for optimal steady-state performance of power systems

In this paper a hybrid configuration of a FACTS controller called Optimal Unified Power Flow Controller (OUPFC) which is composed of a mechanical phase shifting transformer augmented with a small scale Unified Power Flow Controller (UPFC) is introduced. The steady-state model of OUPFC is developed as a power injection model. This model is used to develop an Optimal Power Flow (OPF) algorithm including OUPFC to find the optimum number, location, and settings of OUPFCs to minimize the total fuel cost and power losses. Simulation results are presented for the IEEE 14-, 30-, and 118-bus systems. The optimization method is numerically solved using Matlab and General Algebraic Modelling System (GAMS) software environments. The results demonstrate the effectiveness of the proposed approach to solve the optimal location and settings of OUPFCs incorporated in OPF problem and improve the power system operation. Furthermore, the ability of OUPFC to optimize the objective functions is compared to that of PST and UPFC.

Experimental study on the improvement of CO 2 air conditioning system performance using an ejector

Several preceding researches have evidenced that the transcritical air conditioning system using CO 2 as a refrigerant has an inherent inefficiency resulting in degraded steady-state system performance of a CO 2 air conditioning system compared with that of a conventional air conditioning system. As a practical improvement, two-phase ejector was considered in place of expansion device in this study. The two-phase ejector for CO 2 air conditioning system was designed and developed considering the non-equilibrium state for evaluating the sonic velocity and the critical mass flux. The experiments of performance with respect to variation of ejector geometry such as the motive nozzle throat diameter, mixing section diameter and the distance between motive nozzle and diffuser were carried out. There exist optimum design parameters in each test. Experiments showed that the coefficient of performance of the system using an ejector was about 15% higher than that of the conventional system.

Stability Analysis of a Wave-Energy Conversion System Containing a Grid-Connected Induction Generator Driven by a Wells Turbine

This paper presents the dynamic-stability analyzed results of both dynamic simulations and steady-state performance of a wave-energy power generation system containing a grid-connected induction generator (IG) driven by a Wells turbine. The stator windings of the IG are connected directly to a power grid through a step-up transformer and a transmission line. A d-q axis equivalent-circuit model is employed to establish the IG, the transmission line, and the grid to derive the complete dynamic equations of the studied system under three-phase balanced loading conditions. A frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear-model simulations are both carried out to systematically determine the dynamic stability of the studied system under various operating conditions. It can be concluded from the simulation results that the studied wave-energy power generation system subject to different disturbance conditions can maintain stable operation.

Fast response low harmonic distortion control scheme for voltage source inverters

A fast response low harmonic distortion control scheme is proposed that combines the useful features of the sliding mode control and the repetitive control to achieve excellent transient and steady-state system performances in voltage source inverters. The principle of s domain equivalent control is adopted to integrate the two methodologies and to facilitate the design and analysis of the proposed scheme. An optional specific low-pass filter can be introduced into the system to improve the transient response of the regulation. Experimental results show that fast dynamic responses are achieved through the sliding mode control, whereas low harmonic distortions are achieved through the repetitive control.

Study on Fuzzy-Neural Network for Inverted Plasma Arc Cutting Power Supply

A variable interval fuzzy quantification algorithm with self-adjustable factor in full domain is proposed in this paper. It focuses on digital inverted plasma arc cutting power and studies strong nonlinearity and uncertainty of power. The neural network is also introduced to decouple cutting parameters variables in the multi-parameters coupling cutting process. This algorithm avoids complex nonlinear system modeling and realizes real-time and effective online control of cutting process by combining advantages of fuzzy control and neural network control. Furthermore, the optimized fuzzy control improves steady-state precision and dynamic performance of system simultaneously. The experimental result shows that this control improves precision, ripples, finish and other comprehensive index of work piece cut, and plasma arc cutting power supply based on fuzzy-neural network has excellent control performance.