Nominal Input Research Articles

Techno-Economic Optimization of Medium Temperature Solar-Driven Subcritical Organic Rankine Cycle

The present work focuses on the techno-economic assessment and multi-objective genetic algorithm optimization of small-scale (40 kWth input), solar Organic Rankine Cycle (ORC) systems driven by medium-to-high temperature (up to 210 °C) parabolic dish (PDC) and trough (PTC) collectors. The ORCs are designed to maximize their nominal thermal efficiency for several natural hydrocarbon working fluids. The optimization variables are the solar field area and storage tank capacity, with the goal of minimizing the levelized cost of produced electricity (LCoE) and maximizing the annual solar conversion efficiency. The lowest LCOE (0.34 €/kWh) was obtained in Athens for a high solar field area and low storage tank capacity. Meanwhile, the maximum annual solar conversion efficiencies (10.5–11%) were obtained in northern cities (e.g., Brussels) at lower solar field locations. While PTCs and PDCs result in similar efficiencies, the use of PTCs is more cost-effective. Among the working fluids, Cyclopentane and Cyclohexane exhibited the best performance, owing to their high critical temperatures. Notably, the systems could be more profitable at higher system sizes, as indicated by the 6% LCoE decrease of the solar ORC in Athens when the nominal heat input was increased to 80 kWth.

Analysis of a Resonant Converter with Wide Input Voltage

A DC-DC converter with a 16:1 (Vin,max = 16Vin,min) wide input voltage operation is presented for auxiliary power supplies on solar power conversion circuits or railway vehicles. The solar cell output voltage is associated with the solar intensity (day or night) and geographical location. Thus, the wide input voltage capability of DC converters is required for photovoltaic power conversion. For low power supplies on railway vehicles, the nominal input voltages are 24 V~110 V for the electric door system, motor drive, solid state lighting systems and braking systems. The presented converter uses buck/boost and resonant circuits to achieve the wide input voltage range operation from 18 V to 288 V. If Vin stays on a low input voltage range (18 V~72 V), the buck/boost circuit is operated at a voltage boost characteristic. On the other hand, the buck/boost circuit is operated at a voltage buck characteristic when the input voltage climbs to a high voltage range (72 V~288 V). Thus, the buck/boost circuit can output a constant voltage. Then, the resonant circuit in the second stage is worked at a constant input voltage case so that the frequency variation range is reduced. Finally, to investigate the performance and effectiveness of the studied circuit, experiments with a 500 W prototype were conducted to investigate the performance of the studied circuit.

Wide Range Series Resonant DC-DC Converter with a Reduced Component Count and Capacitor Voltage Stress for Distributed Generation

This paper proposes a galvanically isolated dc-dc converter that can regulate the input voltage in a wide range. It is based on the series resonance dc-dc converter (SRC) topology and a novel boost rectifier. The proposed topology has a smaller number of semiconductors than its SRC-based existing topologies employing an ac-switch in the boost rectifier. The proposed dc-dc converter comprises only two diodes and one switch at the output side, while the existing solutions use two switches and two diodes to step up the voltage. The proposed converter boosts the input voltage within a single boosting interval in the positive half-cycle of the switching period. In addition, the resonant current in the negative half-cycle is sinusoidal, which could enhance the converter efficiency. The resonant capacitor voltage is clamped at the level of the output voltage. Therefore, the voltage stress of the capacitor could significantly reduce at various input voltage and power levels. This makes it perfect for distributed generation applications such as photovoltaics with wide variations of input voltage and power. The converter operates at the fixed switching frequency close to the resonance frequency to obtain the maximum efficiency at the nominal input voltage. The zero-voltage switching (ZVS) feature is achieved in the primary semiconductors, while the diodes in the output-side rectifier turn off at nearly zero current switching. The mathematical model and design guidelines of the proposed converter are discussed in the paper. The experimental results confirmed the theoretical analysis based on a 300 W prototype. The maximum efficiency of the converter was 96.8% at the nominal input voltage, and the converter has achieved a wider input voltage regulation range than that with the boosting cell comprising an ac-switch.

Non‐linear adaptive tracking control for quadrotor aerial robots under uncertain dynamics

This paper presents the design, analysis, and implementation of an adaptive backstepping controller for underactuated quadrotors to track time-varying trajectories with parameter uncertainties. Quadrotor systems are subject to complicated non-linearity and coupling dynamics, so the ignorance of parameter uncertainties may cause performance degradation and even instability. With the concept of nominal input, the uncertain mass and inertia are decoupled from the lifting force and moment torque. By utilizing the backstepping technique, the design procedure with adaptive laws for dynamic parameters is proposed to ensure stability and convergence of tracking errors to the origin asymptotically. The proposed control scheme is extended to control a quadrotor with velocity motor input while the motor coefficients and geometric parameters are handled by the adaptive laws. A trajectory generation for the proposed adaptive tracking controller is addressed subsequently. The proposed controller requires only the position and orientation of the quadrotor with the twice-differentiable trajectories, while previous work demanded acceleration information. Simulation and experimental results are illustrated to show the efficacy of tracking performance for object transportation.

Calculating the Effect of Ribs on the Focus Quality of a Therapeutic Spherical Random Phased Array.

The overlaying rib cage is a major hindrance in treating liver tumors with high intensity focused ultrasound (HIFU). The problems caused are overheating of the ribs due to its high ultrasonic absorption capability and degradation of the ultrasound intensity distribution in the target plane. In this work, a correction method based on binarized apodization and geometric ray tracing approach was employed to avoid heating the ribs. A detailed calculation of the intensity distribution in the focus plane was undertaken to quantify and avoid the effect on HIFU beam generated by a 1-MHz 256-element random phased array after the ultrasonic beam passes through the rib cage. Focusing through the ribs was simulated for 18 different idealized ribs-array configurations and 10 anatomically correct ribs-array configurations, to show the effect of width of the ribs, intercostal spacing and the relative position of ribs and array on the quality of focus, and to identify the positions that are more effective for HIFU applications in the presence of ribs. Acoustic simulations showed that for a single focus without beam steering and for the same total acoustic power, the peak intensity at the target varies from a minimum of 211 W/cm2 to a maximum of 293 W/cm2 for a nominal acoustic input power of 15 W, whereas the side lobe level varies from 0.07 Ipeak to 0.28 Ipeak and the separation between the main lobe and side lobes varies from 2.5 mm to 6.3 mm, depending on the relative positioning of the array and ribs and the beam alignment. An increase in the side lobe level was observed by increasing the distance between the array and the ribs. The parameters of focus splitting and the deterioration of focus quality caused by the ultrasonic propagation through the ribs were quantified in various possible different clinical scenarios. In addition to idealized rib topology, anatomical realistic ribs were used to determine the focus quality of the HIFU beam when the beam is steered both in axial and transverse directions and when the transducer is positioned at different depths from the rib cage.

Distributed Model Predictive Control for Linear Constrained Systems Based on Time-Varying Terminal Sets

A novel distributed model predictive control (DMPC) strategy with time-varying terminal set for linear constrained systems is presented in this paper. To decrease the load of computation of DMPC while ensuring the global optimization, the nominal system is introduced by treating the influence of neighboring subsystems as a bounded disturbance. Then, under the distributed control structure, a distributed predictive control optimization problem containing the nominal state and input can be designed for each subsystem. Furthermore, different from most DMPC approaches, a novel approach to design a terminal constraint set that can be updated in every update time based on the predicted state of the system is proposed. Additionally, the analysis of feasibility and the stability of the proposed DMPC algorithm are described under kinds of the system constraints. Finally, experimental simulation is shown to prove validity by the control scheme in this paper.

Robust distributed model predictive platooning control for heterogeneous autonomous surface vehicles

This paper proposes to apply a robust distributed model predictive platooning control approach for a group of heterogeneous autonomous surface vehicles (ASVs) with the input constraint and bounded external disturbances. The control input for each ASV is composed of two parts: the optimal nominal control input and the ancillary control input. The optimal nominal control input is generated by solving a distributed MPC (DMPC) problem based on the state information of itself and its neighbors. The offline ancillary control law aims to ensure that the actual system state trajectory evolves in a hyper-tube centered along the optimal nominal state trajectory. A coupled inter-vehicle safety constraint is designed for the DMPC optimization problem to guarantee the inter-ASV collision avoidance. Theoretical results on ensuring the feasibility of the proposed robust DMPC algorithm are provided and the closed-loop systems are proved to be input-to-state stable. Numerical simulations are performed to verify the theoretical results.

Methane yield prediction of ultrasonic pretreated sewage sludge by means of an artificial neural network

In this study, anaerobic digestion of sewage sludge with different ultrasonic pretreatment (USp) conditions was investigated. USp were applied at a constant ultrasonic density of 0.5 W/mL with sonication times of 0–240 min. While the methane yield of the untreated (control) reactor was 170.1 ± 4.7 mL/g volatile solids (VS), the highest methane yield was 266.1 ± 7.5 mL/g VS in the reactor where sonication was applied for 120 min. Actual specific energy input (SEA)/nominal specific energy input (SEN) ratios after USps were measured for pretreatment yield. These values varied between 77.0 and 14.87% according to different sonication times. Sonication times of more than 30 min did not significantly increase the methane yield due to possible reflocculation. After the USps, various cumulative methane yields were predicted via the modified Gompertz Model, modified Logistic Model, and Artificial Neural Network (ANN) model (Scenario 1); of these the ANN model made predictions that were closest to the experimental data. In the other part of the study, ANN was trained by experimental pretreatment conditions and methane yields were successfully predicted by taking different USp parameters (different sonication times, SEA values and incremental soluble chemical oxygen demand % values) as input variables (Scenarios 2, 3 and 4). Regression analyses showed values close to 1, indicating that the prediction of the ANN model correlated linearly with experimental data.

The effect of process parameters and plate thickness on in-plane inherent deformations in T-joint fillet weld

Purpose The purpose of this paper is to quantitatively investigate the effect of process parameters (including welding current, voltage and speed) and plate thickness on in-plane inherent deformations in typical fillet welded joint; meanwhile, the plastic strains remaining in the weld zone are also analyzed under different influencing factors. Design/methodology/approach To achieve the purpose of this study, a thermal-elastic-plastic finite element (TEP FE) model is developed to analyze the thermal-mechanical behavior of the T-welded joint during the welding process. Experimental measurements have verified the validity of the established TEP FE model. Using the effective model, a series of numerical experiments are performed to obtain the inherent deformations under the conditions of different influencing factors, and then the calculation results are discussed based on the relevant data obtained. Findings Through numerical simulation analysis, it is found that the longitudinal and transverse inherent deformations decrease with the increase of welding speed and plate thickness, whereas as the nominal heat input increases, the inherent deformations increase significantly. The longitudinal shrinkage presents a quasi-linear and nonlinear distribution in the middle and end of the weld, respectively. The plastic strains in the cross section of the T-joint also vary greatly because of the process parameters and plate thickness, but the maximum value always appears near the location of the welding toe, which means that this point faces a relatively large risk of fatigue cracking. The inherent deformations are closely related to the plastic strains remaining in the weld zone and are also affected by many influencing factors such as process parameters and plate thickness. Research limitations/implications In this study, relatively few influencing factors such as welding current, voltage, speed and plate thickness are considered to analyze the inherent deformations in the T-welded joint. Also, these influencing factors are all within a certain range of parameters, which shows that only limited applicability can be provided. In addition, only in-plane inherent deformations are considered in this study, without considering the other two out-of-plane components of inherent deformations. Originality/value This study can help to expand the understanding of the relationship between the inherent deformations and its influencing factors for a specific form of the welded joint, and can also provide basic data to supplement the inherent deformation database, thereby facilitating further researches on welding deformations for stiffened-panel structures in shipbuilding or steel bridges.

Printed and Flexible Microheaters Based on Carbon Nanotubes.

This work demonstrates a cost-effective manufacturing method of flexible and fully printed microheaters, using carbon nanotubes (CNTs) as the heating element. Two different structures with different number of CNT layers have been characterized in detail. The benchmarking has been carried out in terms of maximum operating temperature, as well as nominal resistance and input power for different applied voltages. Their performances have been compared with previous reports for similar devices, fabricated with other technologies. The results have shown that the heaters presented can achieve high temperatures in a small area at lower voltages and lower input power. In particular, the fully printed heaters fabricated on a flexible substrate covering an area of 3.2 mm2 and operating at 9.5 V exhibit a maximum temperature point above 70 °C with a power consumption below 200 mW. Therefore, we have demonstrated that this technology paves the way for a cost-effective large-scale fabrication of flexible microheaters aimed to be integrated in flexible sensors.

Scalable time-constrained planning of multi-robot systems

This paper presents a scalable procedure for time-constrained planning of a class of uncertain nonlinear multi-robot systems. In particular, we consider N robotic agents operating in a workspace which contains regions of interest (RoI), in which atomic propositions for each robot are assigned. The main goal is to design decentralized and robust control laws so that each robot meets an individual high-level specification given as a metric interval temporal logic (MITL), while using only local information based on a limited sensing radius. Furthermore, the robots need to fulfill certain desired transient constraints such as collision avoidance between them. The controllers, which guarantee the transition between regions, consist of two terms: a nominal control input, which is computed online and is the solution of a decentralized finite-horizon optimal control problem (DFHOCP); and an additive state feedback law which is computed offline and guarantees that the real trajectories of the system will belong to a hyper-tube centered along the nominal trajectory. The controllers serve as actions for the individual weighted transition system (WTS) of each robot, and the time duration required for the transition between regions is modeled by a weight. The DFHOCP is solved at every sampling time by each robot and then necessary information is exchanged between neighboring robots. The proposed approach is scalable since it does not require a product computation among the WTS of the robots. The proposed framework is experimentally tested and the results show that the proposed framework is promising for solving real-life robotic as well as industrial applications.

Ultrasound-assisted hydration of wheat grains at different temperatures and power applied: Effect on acoustic field, water absorption and germination

Wheat has been increasingly employed as malt in the food industry. As the malting process demands long processing time, ultrasound can be a clean intensifying alternative. This work studied the ultrasound-assisted hydration of wheat grains at different nominal input power (0−1500 W) and temperatures (15−25 °C). The actual acoustic field within the reactor was characterized. The effects of the studied variables on hydration kinetics were assessed separately by the Weibull and Peleg Models. Thermodynamic parameters were evaluated. Grains germination was also addressed under the studied conditions. Increasing temperature as well as higher acoustic density caused an improvement in the hydration kinetics, reaching 72 % when both variables were combined. Hydration was an endothermic and non-spontaneous process. In the range of actual acoustic density (0.061–0.102 W mL−1) and hydration temperature, the maximum germination was kept at ∼5 days with 86 % germinated grains.

The influence of energy recovery on the overall efficiency of acoustic sources at low frequencies

This paper defines a criterion called the overall efficiency (OE) for evaluating the efficiency of acoustic sources. In practice, the conventional efficiency (CE), is not capable of giving a comprehensive overview of the power consumption, especially in the connected amplifier drivers. The reason for this is that the CE is defined based on the nominal input electrical power. The nominal input electrical power is defined as the real part of the power that is delivered by the electrical amplifier to the actuation unit of an acoustic source. Therefore, power loss in the amplifier unit is not included in the CE definition. With the aid of the OE criterion, the effect of power loss in the connected amplifier units is taken into account. In particular, the application of the OE is crucial for acoustic sources that operate in a low frequency range. This is due to the reactive nature of power in both actuators and connected amplifiers. In the current paper, various combinations of amplifiers and actuators are studied. In particular, voice coil actuators and piezoelectric stack elements including both Lead Zirconate Titanate (PZT) ceramics and single crystal Lead Zirconate Niobate-Lead Titanate (PZN-PT) piezoelectric materials are investigated. In addition, the effect of a connected power driver is investigated. A class AB and a class D amplifier are studied respectively as analogue and switching amplifiers. Unlike a class AB amplifier, a class D amplifier is capable of energy recovery. A perforated flat acoustic source is examined in this paper as a practical example to verify the OE criterion. The numerical simulation on a thin acoustic source shows that for a single actuator, thanks to energy recovery, the OE is higher in a class D amplifier than that in a class AB amplifier. This study reveals that if a class D amplifier is the driver, using piezoelectric actuators results in a higher OE compared to using a voice coil actuator. Measurements on the sample acoustic source verify the numerical results presented in the current study.

Active fault-diagnosis method using adaptive allocator and fault-tolerant adaptive control system design

A new approach to the active fault diagnosis (AFD) for input redundant plants is presented in this paper. A test signal which helps the diagnosis is injected to the plant in addition to nominal control input in the AFD typically. One feature of the proposed AFD is adaptive allocation of the test signal. The adaptive allocator which distributes the test signal and injects it to the redundant actuators is introduced in this paper. A fault-diagnosis (FD) system that estimates fault location and its magnitude from adaptive parameter of the adaptive allocator is constructed with a simple neural network (NN) model. Furthermore, a A fault-tolerant adaptive control system which includes the adaptive test signal allocator is designed based on the model reference adaptive control (MRAC) technique. The adaptive laws of the adaptive allocator and controller are derived by using a suitable Lyapunov function. By performing experiments using a two-input redundant plant, we show the effectiveness and applicability of the proposed AFD and MRAC system.

Sizing of dc-link capacitor for a grid connected solar photovoltaic inverter

Objective: To determine the optimum size of a dc-link capacitor for a grid connected photovoltaic inverter. Methods: Dc-link capacitors are considered as one of the sensitive parts of the grid connected photovoltaic systems and needs effort to design a reliable and optimal size capacitor as its reliability is concerned with the overall system reliability. The double line frequency power flows between the input and outside of a Φ grid connected PV system which produces voltage ripples at the capacitor and dc link. This voltage ripple increases temperature of passive components and dc source which affects the MPP operation of the photovoltaic modules and the system life. Therefore, it is essential to limit the voltage ripples at the input side of the system. The easiest way to limit the double frequency ripple voltage is to connect a capacitor in parallel to the PV module and the inverter which buffers the double line frequency power and supply a constant power to the inverter. This study proposed a general method for sizing a dc-link capacitor for a Φ grid connected voltage source inverter. It is seen that the capacitance is inversely proportional to the nominal dc and ripple voltage. Thus an increase in the nominal system voltage decreases the size of the capacitor and at the same time increases the voltage ripple. Therefore to limit voltage ripple within permissible limits and to ensure better system performance the dc-link capacitor must be appropriately sized. The simulations based on 3kW grid connected PV system are carried out in DIgSILENT Power Factory software. Findings: A capacitor of 410µF is needed to be connected in parallel with a 3kVA inverter having an nominal input voltage of 370V and maintaining a voltage ripple under 8.5%. Novelty: After determining optimized dc-link capacitor size we will limit the voltage ripple under permissible limits and hence improves the system efficiency and life of the grid connected PV system. Keywords: Voltage source inverter; voltage ripple; Dc-link capacitor sizing; distributed generation; grid connected PV system

A Computationally Efficient Robust Model Predictive Control Framework for Uncertain Nonlinear Systems

In this article, we present a nonlinear robust model predictive control (MPC) framework for general (state and input dependent) disturbances. This approach uses an online constructed tube in order to tighten the nominal (state and input) constraints. To facilitate an efficient online implementation, the shape of the tube is based on an offline computed incremental Lyapunov function with a corresponding (nonlinear) incrementally stabilizing feedback. Crucially, the online optimization only implicitly includes these nonlinear functions in terms of scalar bounds, which enables an efficient implementation. Furthermore, to account for an efficient evaluation of the worst case disturbance, a simple function is constructed offline that upper bounds the possible disturbance realizations in a neighborhood of a given point of the open-loop trajectory. The resulting MPC scheme ensures robust constraint satisfaction and practical asymptotic stability with a moderate increase in the online computational demand compared to a nominal MPC. We demonstrate the applicability of the proposed framework in comparison to state-of-the-art robust MPC approaches with a nonlinear benchmark example.

Enhancing competitiveness of Zimbabwe’s cotton production under contract farming

This study was designed to assess the competitiveness of cotton production by smallholder farmers under contract farming in Zimbabwe and propose strategies for enhancing sector performance. Data were collected from secondary sources, key informant interviews and focus group discussions (FGDs). A multiple criteria analysis (MCA) was used to identify key constraints in cotton production. Cotton budgets were developed and a policy analysis matrix (PAM) was derived to assess the competitiveness of cotton farming. Key constraints identified were related to policy and institutional failure. Cotton production was found to be not competitive with an overall disincentive of USD-132.30/ha of cotton produced. The nominal protection coefficient output (NPCO) of 0.75 and nominal protection coefficient input (NPCI) of 1.5 indicate taxation in the product and factor markets respectively. The effective protection coefficient (EPC) of 0.5 indicates that the operating environment does not encourage production. To sustain cotton farming under contract farming, there is a need to strengthen private and public institutions to reduce behavioural uncertainties. Alternatively, financing of cotton production might need remodelling to incentivise banks to finance farmers directly through group lending and government guaranteed funding. Human capital development through farmer training programmes is recommended to enhance farm productivity.

Investigation on the performance evaluation of gas-fired combi-boilers with factor analysis and cluster analysis

This study presents a performance evaluation method of gas-fired heating and hot water combi-boilers. Seven factors related to the operation performance are selected. Twenty combi-boilers of different brands and heat input are selected randomly and tested according to GB25034-2010. The factor analysis and cluster analysis method are selected to evaluate the performance of combi-boilers. Analysis results show that, the evaluation model F = 0.652F1 + 0.190F2 + 0.158F3 is achieved, and the combi-boilers can be ranked with the evaluation model. The gas-fired combi-boilers selected can be copolymerized into 3 classes. The thermal efficiency at rated input in heating mode, thermal efficiency at 50% of the nominal input in hot water mode, concentration of O2 and CO2 of dry combustion products have the greatest impact on the operation performance of combi-boilers. This study provides a method for quantitative evaluation of the operation performance of combi-boilers and a clear direction for product optimization.

An error-based active disturbance rejection control with memory structure

The paper studies the control problem for nonlinear uncertain systems with the situation that only the current reference signal is available. By constructing a memory structure to save the previous reference signals, a novel error-based active disturbance rejection control with an approximation for the second-order derivative of reference signal is proposed. The transient performance of the proposed method is rigorously studied, which implies the high consistence of the closed-loop system. More importantly, to attain the satisfactory tracking performance, the necessary condition for nominal control input gain is quantitatively investigated. Furthermore, the superiority of the proposed method is illuminated by contrastively evaluating the sizes of the total disturbance and its derivative. The proposed method can alleviate the burden of the estimation and compensation for total disturbance. Finally, the experiment for a manipulator platform shows the effectiveness of the proposed method.

Techno-economic assessment of hybrid CSP-biogas power plants

This study aims to investigate the performance and economic benefits arising from the integration of concentrating solar power (CSP) plants with anaerobic digestion processes. To demonstrate the capabilities of hybrid CSP-biogas plants, the CSP section of the Ottana solar facility (Italy) is considered as a case study. A simulation model for the performance analysis of the hybrid system is developed, and the effects of variation in the volume of the anaerobic digester and the biogas storage capacity on the main performance indexes are evaluated. Furthermore, two different operating strategies for energy storage management are compared and the possible requirements for energy curtailment are analysed. Finally, a preliminary economic analysis is carried out. The results demonstrate the benefits and improvements in plant capacity factor and efficiency arising from proper sizing of the biogas section. Conversely, oversizing of the biogas section results in significant curtailments in biogas and/or solar field energy production, due to the limited storage capacity. Consequently, the optimal configuration, even from an economic point of view, is achieved by a biogas section of a size that is able to supply part (in the range of 10%–65%) of the nominal thermal power input required by the power block.