Direct Current Voltage Research Articles

A Communication‐Free Master–Slave Control of Cascaded‐Type DC Microgrids With Nondispatchable Generations

ABSTRACTThis paper addresses the challenge of integrating dispatchable and nondispatchable distributed generators (DGs) in cascaded‐type direct current (DC) microgrids and proposes a communication‐free master–slave control strategy. In the proposed method, all DGs are treated as voltage sources. A master dispatchable DG is primarily responsible for maintaining a constant DC voltage at the point of common coupling (PCC). The rest slave nondispatchable DGs adjust their output power based on a front maximum power point tracking (MPPT) controller. This communication‐free approach enhances system reliability and ease of implementation. Moreover, it ensures load voltage stability, providing a high‐quality power supply for users. Additionally, it enables nondispatchable DGs to achieve MPPT‐based output power and enables power curtailment operation. Subsequently, a small‐signal stability analysis confirms the efficacy of the proposed strategy. Finally, simulation and experiment results further validate its efficacy.

A Lyapunov Theory-Based SEIG–STATCOM Voltage Regulation Control Strategy

To improve the voltage regulation of asynchronous generators during load switching, a Lyapunov-based control strategy has been proposed to stabilize the generator’s voltage by connecting a static synchronous compensator. By constructing a Lyapunov function from the mathematical model, the error tracking problem is transformed into a global asymptotic stability problem of the Lyapunov function at the equilibrium point. The outer loop linearizes the direct current (DC) voltage control process, while the inner loop replaces integral terms with differential terms. The proposed Lyapunov method achieves linearized voltage control with a quadratic outer loop structure and the inner loop differential structure exhibits a shorter transient process, outperforming traditional methods. Simulation and experimental tests were then used, where the latter was a down-scale laboratory prototype experiment. Compared to traditional (voltage-oriented control) VOC, the outer loop (Lyapunov-function-based control) LBC reduces the DC voltage transient processes by approximately 9.4 milliseconds, while the inner loop LBC reduces both alternating current (AC) and DC voltage transient processes by approximately 2.6 ms and 8.7 ms, respectively.

Strain detection based on magnetic domain wall motion in amorphous FeSiBNb thin film

The strain dependence of the pulse voltage induced in a pickup coil originating from the domain wall motion in an amorphous FeSiBNb thin film was investigated. No significant change in the peak pulse voltage was observed when tensile strain was applied to the thin film, whereas the peak pulse voltage changed steeply when compressive strain was applied. In addition, strain susceptibility could be controlled via appropriate annealing. Evaluation of the strain gauge comprising the amorphous FeSiBNb thin film, a pickup coil, an electrical circuit (which converts the pulse voltage into direct current voltage), and a Helmholtz coil revealed that strain gauge has a notably high gauge factor of approximately 37,500.

Memristive behaviour of Al/rGO-CdS/FTO device at different temperatures: A MATLAB-integrated study

In the present work, a comprehensive study is carried out to investigate the memristive behaviour of reduced graphene oxide (rGO) conjugated cadmium sulphide quantum dot (CdS QD) nanocomposites (rGO-CdS), offering insights into their dynamic response under varying thermal conditions. The study integrates experimental analysis with MATLAB simulation to provide a detailed understanding of the complex interplay between different operating temperature (300K, 350K, 400K, and 450K) and memristive behavior in rGO-CdS nanocomposites. Different structural and chemical characterizations were carried out which confirms the formation of rGO-CdS nanocomposites. A sandwich structured device was fabricated with the synthesized rGO-CdS nanocomposites using Aluminum (Al) as top and Fluorine doped tin oxide (FTO) as bottom electrode. The influence of operating temperature on hysteresis behaviour of the fabricated Al/rGO-CdS/FTO device was investigated using Keithley 2450 source meter by sweeping a direct current (dc) voltage (−5 V → 5 V → −5 V). Notably, we observe a positive temperature coefficient in the device current, with maximum and minimum recorded current of |1.29mA| and |0.66mA| at 450K and 300K respectively. The current-voltage (I-V) behavior observed in the device reveals that in the low resistance state (LRS), conduction is dominated by bulk-limited mechanisms. However, in the high resistance state (HRS), conduction involves contributions from both Schottky barriers and the Pool-Frenkel effect. A MATLAB based linear drift model was used to simulate the device responses at different temperatures using the experimental data. The study provides first comprehensive analysis of temperature dependent hysteresis behaviour of Al/rGO-CdS/FTO device, integrating MATLAB simulation to glean valuable insights into its operation and possible applications as memristive material across different temperature regimes.

Study on photovoltaic primary frequency control strategy at different time scales

AbstractDuring the participation of photovoltaics in grid frequency regulation, different frequency regulation tasks are required at different time scales. The grid demands that photovoltaics (PVs) improve steady‐state frequency when facing short‐term load fluctuations, while also enhancing frequency response to long‐term environmental and load changes. Therefore, this study takes different time scales as the starting point. First, a two‐stage PV grid‐connected inverter generation system model is established, and an overall control strategy is proposed. Next, for short‐term time scales, a virtual inertia strategy based on direct current (DC) voltage droop control is proposed to utilize the energy storage effect of DC capacitors to suppress frequency fluctuations. For long‐term time scales, a strategy for controlling the variable reactive power reserve capacity is proposed to address the inadequacy of frequency regulation caused by traditional fixed de‐rating rate strategies and redundant standby power. Finally, the effectiveness of the proposed strategies is verified through model validation in MATLAB/Simulink. Simulation results demonstrate the effectiveness of the strategies at different time scales, aiding in improving grid frequency response.

True Random Number Generator Based on Chaotic Oscillation of a Tunable Double-Well MEMS Resonator.

Chaotic systems have aroused interest across various scientific disciplines such as physics, biology, chemistry, and meteorology. The deterministic but unpredictable nature of a chaotic system is an ideal feature for random number generation. Microelectromechanical systems (MEMS) are a promising technology that effectively harnesses chaos, offering advantages such as a compact footprint, scalability, and low power consumption. This paper presents a true random number generator (TRNG) based on a double-well MEMS resonator integrated with an actuator and position sensor. The potential energy landscape of the proposed MEMS resonator is actively tunable with a direct current voltage. Experimental demonstrations of tunable bistability and chaotic resonance are reported in this paper. A chaotic time sequence is generated through piezoresistive sensing of the position of the MEMS resonator once it is driven into the chaotic regime. Subsequently, the randomness of the bit sequence, achieved by applying the exclusive or function to a digital chaotic sequence and its delayed differential is confirmed to meet the National Institute of Standards and Technology specifications. Moreover, the throughput and energy efficiency of the proposed MEMS-based TRNG can be adjusted from 50 kb s-1 and 0.44 pJ per bit at a low energy barrier to 167 kb s-1 and 6.74 pJ per bit at a high energy barrier by changing the MEMS device's potential well. The tunability of the proposed double-well MEMS resonator not only offers continuous adjustments in the energy efficiency of TNRG but also unveils vast and diverse research opportunities in analog computing, encryption, and secure communications.

An enhanced maximum power point tracking and voltage control for proton exchange membrane fuel cell using predictive model control techniques

Proton Exchange Membrane Fuel Cells serve as sustainable devices for converting renewable energy sources into electricity, offering advantages such as rapid startup, high power density, and operation at low temperatures. They are highly efficient and environmentally friendly energy sources, yet their non-linear output characteristics under variable conditions present challenges in maximizing power output. Furthermore, they generates direct current, while many electrical devices rely on alternating current. This research addresses the necessity of enhancing the performance and efficiency through the development of an advanced maximum power point tracking technique and voltage control strategy. A modified finite-control-set model predictive control technique is proposed for both maximum power point tracking and voltage control. Specifically, a finite-control-set model predictive control technique approach is employed to modulate the switching signal for both the DC-DC boost converter and the DC-AC inverter. The DC-DC boost converter step up the fuel cell output voltage to the desired level, ensuring it reaches the maximum power point, and a DC-AC inverter to convert the direct current voltage to a pure sinusoidal alternating waveform. The investigation demonstrates the effectiveness of the proposed method in achieving its objectives. The proposed maximum power point tracking technique accuracy achieved the maximum power with a rate of 97 % with excellent respond time within 7 ms. For alternating current power, only less than 1.5 % of total harmonic distortion is recorded. The study evaluates the control scheme under robust operating conditions, demonstrating its effectiveness in optimizing PEMFC output and providing high-quality AC voltage.

DC vs AC Electrokinetics-Driven Nanoplasmonic Raman Monitoring of Charged Analyte Molecules in Ionic Solutions.

Electrokinetic surface-enhanced Raman spectroscopy (EK-SERS) is an emerging high-order analytical technique that combines the plasmonic sensitivity of SERS with the electrode interfacial molecular control of electrokinetics. However, previous EK-SERS works primarily focused on non-Faradaic direct current (DC) operation, limiting the understanding of the underlying mechanisms. Additionally, developing reliable EK-SERS devices with electrically connected plasmonic hotspots remains challenging for achieving high sensitivity and reproducibility in EK-SERS measurements. In this study, we investigated the use of two-tier nanolaminate nano-optoelectrode arrays (NL-NOEAs) for DC and alternating current (AC) EK-SERS measurements of charged analyte molecules in ionic solutions. The NL-NOEAs consist of Au/Ag/Au multilayered plasmonic nanostructures on conductive nanocomposite nanopillar arrays (NC-NPAs). We demonstrate that the NL-NOEAs exhibit high SERS enhancement factors (EFs) of ∼106 and can be used to modulate the concentration and orientation of Rhodamine 6G (R6G) molecules at the electrode surface by applying DC and AC voltages. We also performed numerical simulations to investigate the ion and R6G dynamics near the electrode surface under DC and AC voltage modulation. Our results show that AC EK-SERS can provide additional insights into the dynamics of molecular transport and adsorption processes compared to DC EK-SERS. This study demonstrates the potential of NL-NOEAs for developing high-performance EK-SERS sensors for a wide range of applications.

Memristive gas sensor (gasistor) based on Ag/ordered TiO2 nanorods/FTO sandwich structure for evaluation of ethanol concentration in mixed ambient

Herein, we designed a memoristor-type gas sensor (gasistor) with Ag/ordered TiO2 nanorods/FTO sandwich structure for recognizing ethanol concentration in ethanol-methanol mixtures. The gas adsorption characteristics of different TiO2 substrates were first investigated by first-principle calculations. The results showed that the TiO2 surface in the low resistance state exhibited better immunity to other interfering gas molecules. However, similar adsorption energies and charge transfers calculated for adsorption of methanol and ethanol mean that the interference of methanol to the response of ethanol cannot be completely ignored. In next experiments, the Ag/TiO2/FTO gasistor with well-aligned TiO2 nanorods as the resistive layer was constructed successfully and demonstrated the stable resistive switching function and excellent retention characteristic. Further device tests confirm the ability of the gasistor in the low resistance state to discriminate between ethanol and methanol, as well as better anti-interference for other organic vapors. Also, an ultra-short recovery time (∼1.8 s) was observed by applying a direct current scanning voltage to the gasistor. Finally, to improve the recognition rate of ethanol in ethanol-methanol mixed atmosphere, a neural network algorithm based on back-propagation was incorporated and good prediction accuracy was obtained at ethanol concentrations ≥ 5 ppm.

Hierarchical power reserve control of string-inverter-based photovoltaic power plant for primary frequency control

Retaining a certain power reserve is the precondition for a photovoltaic power plant (PVPP) to provide primary frequency control. Usually, a string-inverter-based PVPP may consist of hundreds of string inverters, and each string inverter has several direct current (DC) input channels. Thus, how to allocate the power reserve among so many controllable units is a challenging problem. In this paper, a hierarchical power reserve control method for a string-inverter-based PVPP to provide primary frequency control is introduced. In the inverter layer, the power reserve of each DC input channel is determined to balance the DC voltage variation of each channel during primary frequency control. In the plant layer, the power reserve is allocated to each PV cluster based on the predicted power generated by long short-term memory (LSTM) network, and the PV clusters are formed based on the historical generated power of each string inverter using K-means method. Furthermore, the string inverters within the same PV cluster evenly share the power reserve. To validate the proposed method, case studies in both the inverter layer and plant layer are carried out; the results verify the effectiveness of the proposed method.

Electrolytic biodiesel production from spent coffee grounds: Optimization through response surface methodology and artificial neural network

BackgroundThe disposal of waste spent coffee grounds (SCG) presents a pressing environmental concern, necessitating effective pre-treatment strategies to mitigate potential damage. Transesterification emerges as a viable solution for converting SCG lipids into biodiesel, offering both environmental and economic benefits. MethodsIn this study, the utilization of SCG as a renewable feedstock for biodiesel production through an innovative electrolysis process has been explored, aiming to address the dual challenges of waste management and sustainable energy production. To obtain maximum conversion of SCG oil to biodiesel, a comprehensive analysis of the fatty acid profile using Gas Chromatography-Mass Spectrometry (GC–MS), was conducted allowing for precise characterization of lipid content. Additionally, Fourier Transform Infrared (FTIR) spectroscopy was employed to categorize functional groups and Nuclear Magnetic Resonance (1H NMR) spectroscopy was utilized to analyze the molecular structure of the SCG oil. Optimization of process parameters namely, catalyst concentration, electrolysis time, and direct current (DC) voltage was performed using Response Surface Methodology (RSM) and Artificial Neural Network (ANN) techniques. The ANN model, with its ability to capture complex nonlinear relationships, was particularly effective in identifying the optimal combination of parameters, thereby maximizing biodiesel yield. Significant findingsThe extracted SCG oil was characterized using FTIR, GC–MS and 1H NMR analysis. The GC- MS analysis of bio-oil has reported 44.6 % Linoleic acid, 31.6 % Palmitic acid. The extracted oil had got significant amount of key saturated and unsaturated fatty acid making it suitable for transesterification process. Through ANN, the optimal combination of parameters for electrolytic transesterification i.e.,0.75 wt% catalyst loading, 2 h electrolysis time, and 40 V DC voltage, yielded the highest biodiesel production (98.32 wt.%). Comparative analysis indicated superior performance of the ANN model (R2 = 0.9931, MAE = 0.123) over RSM (R2 = 0.9636, MAE = 1.546). The artificial neural network (ANN) provided a more accurate forecast of the process yield; however, the RSM model effectively predicted the interactions and significance of the pyrolysis factors. The artificial neural network (ANN) provided a more accurate forecast of the process yield; however, the RSM model effectively predicted the interactions and significance of the pyrolysis factors. Biodiesel characterization via FTIR and 1H NMR analysis showed physiochemical properties within standard limits for SCG biodiesel.

Ultra‐Stable Phonon Laser Through Closed‐Loop Feedback Control for Optomechanical Sensing

AbstractThe resonant amplification of the optical and mechanical components in cavity optomechanical systems enhances the performance of sensing applications, which rely on precise frequency control and narrowed mechanical linewidth. This study proposes a direct closed‐loop feedback technique for a narrow linewidth phonon laser based on optomechanical self‐oscillation. By continuously pumping a mechanical resonator with a laser, a phonon laser is achieved with an oscillation frequency of 17.58 MHz, exhibiting a narrow linewidth of just 0.44 mHz and an effective mechanical Q of 4 × 1010, while preserving the linear frequency response. The oscillator phase is fed back to the pump laser, yielding a mechanical frequency stability of 7.2 × 10−11 and providing a real‐time output of direct current voltage corresponding to the oscillator frequency changes. The system's real‐time sensing capability is tested using fiber probes and polystyrene particles, demonstrating a performance approach to the theoretical resolution limit. This work opens new avenues for real‐time precision measurement technology and can be extended to different optomechanical systems.

Reversibly photochromic wood constructed by electric field-assisted self-assembled chitosan and tungsten trioxide coatings

ABSTRACT The photochromic properties are highly desirable for advanced wood applications, but they have not been extensively studied. To incorporate photochromic capabilities into wood materials, composite coatings were developed on the wood surface using an electric field-assisted layer-by-layer (LBL) self-assembly technique. The composite coatings consist of cationic chitosan and anionic tungsten trioxide (WO3) nanoparticles. By applying a direct current voltage during the deposition process, a well-organized distribution of WO3 nanoparticles was achieved, resulting in a uniform coverage of the wood substrate. This assembly process facilitated the creation of structured photochromic coatings with minimal nanoparticle fillers, a straightforward and rapid procedure, and potential for large-scale production. Lab colorimetric results demonstrated that the wooden substrate changed color from its natural state to blue when exposed to UV light. UV-vis spectroscopy further confirmed the efficient absorption of WO3 and photochromic properties of the modified wood.

The experimental and DFT approaches on electronic, thermal and conductivity properties of non-linear optical bearing fused aromatic chalcones towards prospective OLEDs

New derivatives of push-pull chalcone was successfully synthesized to demonstrate as NLO active on the impact of organic light emitting diode (OLED) materials. A total of three fused-aromatic chalcones (AA, AB, AC) with different substituents have been characterized through TGA, Z-scan analysis, 1H & 13C NMR, IR and UV–Vis. All the targeted compounds are nonlinear refraction (NLR) active with the range of χ(3) (∼ × 10–6 esu). AC reveals the highest value of first order hyperpolarizability (βtot) at 528.08 × 10–30 esu upon the substitution of strong electron acceptor. Theoretical investigation was performed by employing the theory of B3LYP/6–31 G (d,p) level via density functional theory (DFT) to optimize the geometries, molecular electrostatic potential (MEP) and frontier molecular orbitals (FMO). The predicted electronic properties of the molecule are supported by the experimental results, confirming the theoretical conclusions. The examination of its potential as emissive layer in OLED application was constructed on ITO substrate for electroluminescence behaviour evaluation under various direct current (DC) voltages supply. The preliminary outcomes reveal that these simple chalcone derivatives are capable of being utilised in any optoelectronic application.

Experimental Validation of a Modular All-Electric Power Take-Off Topology for Wave Energy Converter Enabling Marine Renewable Energy Interconnection

Power electronic converters are an enabling technology for the emerging marine energy applications, such as using ocean waves to produce electricity. This paper outlines the power take-off system and its key components used in a wave energy converter offering modularity and scalability to generate power efficiently. The proposed power take-off system was implemented based on a modular multilevel converter and could be deployed to convert any alternating current electrical energy to a different alternating current for interconnection to grid or non-grid applications. Examples of widespread deployment are supplying electricity to coastal communities or producing clean drinking water. The analysis using both the simulation tests and laboratory experiments verified the design objectives and basic functionality of the developed power take-off system. An acceptable response using a field programmable gate array-based controlled laboratory testbench was achieved, complying with guidelines specified in the prevalent industry standards. Seamless operation during steady-state and transients for the studied wave energy converter was achieved as supported by the obtained results. The key findings of this work were experimentally examined under different load conditions, direct current bus voltage fluctuations, and generator speed–torque regulation. The ability of the power take-off system to generate high-power quality of the waveforms, e.g., against adhering to the IEEE 519-2022 standard for total harmonic distortion limits, is also confirmed.

Design of a Ka‐band low‐cost microstrip line phase shifter and a sum‐difference beam antenna based on liquid crystal technology

AbstractThis letter presents a Ka‐band low‐cost microstrip line phase shifter based on liquid crystal (LC) technology. Furthermore, a sum‐difference beamforming antenna array based on the LC phase shifter is designed. The phase shifter is composed of two coaxial connectors, a metal base, an intermediate dielectric layer, a gradient metal sheet and a top glass layer. The simulated results show that a nearly 360° phase shift coverage can be achieved with the effective permittivity of LC varying from 2.5 to 3.5. The sum‐difference beam antenna array is composed of a circulator, a sum‐difference network, a one‐to‐eight power‐division network, eight liquid crystal phase shifters, and a serious fed antenna array. The simulation results show that the antenna array can achieve sum‐difference beam scanning range of ±30°. A prototype of LC phase shifter is fabricated and tested. The results show that the designed LC phase shifter can achieve a 360° phase coverage by applying a direct current (DC) voltage ranging from 0 to 12 V. Compared to traditional phase shifters, the proposed design offers a simple structure at a lower cost. When applied in the design of a sum‐difference beam antenna array, it can reduce the producing cost and enable integrated design.

Noncontact Rotational Speed Measurement with Near-Field Microwave of Open-Ended Waveguide

Rotational speed measurement is important for many applications. Here, a noncontact rotational speed test method based on the detection of the periodically perturbed near-field microwave of an open-ended waveguide is proposed. Both simulations and experiments were conducted to verify the near-field microwave rotational speed sensor. The constructed rotation speed sensing system was composed of a standard open-ended WR-42 waveguide (in our measurements, a waveguide-to-coaxial adapter was used to represent an open-ended waveguide) working at ~18 GHz, a radio frequency (RF) circulator, a signal generator, a, RF detector and an oscilloscope. A rotating fan to be measured was placed close to the waveguide’s mouth and, thus, the waveguide’s reflection coefficient was periodically modulated by the rotating fan blades. Then, the RF detector converted this varying reflection coefficient into a direct current (DC) voltage, namely, a periodical waveform. Finally, the rotational speed of the fan could be extracted from this waveform. Measurements using both the proposed near-field microwave method and conventional optical transmission/reflection methods were conducted for verification. The effect of the rotating fan’s location relative to the waveguide’s mouth was also studied. The results show the following: 1. The proposed method works well with a rotational speed of up to ~5000 RPM (rounds per minute), and an accuracy of 1.7% can be achieved. 2. Metallic or non-metallic fan blades are all suitable for this method. Compared with the existing radar method, the proposed method may be advantageous for rotation detection in a constrained space.

Electrically reflectance-tunable circular polarization reflector based on thin polymer-stabilized cholesteric liquid crystals

This paper presents a reflectance-tunable circular polarization reflector that utilizes thin polymer-stabilized cholesteric nematic liquid crystals (PSCLCs) with negative anisotropy. The switching mechanism involves direct-current (DC) field-induced movement in the polymer network, which generates the mechanical force that causes a pitch-gradient helical structure and thus a distorted Bragg reflection. The pitch-gradient helical structure of the 5 μm-thick cells only allows for a limited number of pitch lengths, resulting in a decrease in reflectance at the designed Bragg reflectance and a reflectance tunable range determined by applying DC voltages. The PSCLCs reflecting red, green, and blue colors were measured for their reflectance tuning ability and circular polarization selectivity. A tunable polarization volume grating was fabricated using a PSCLC cell, and the ability of the grating to diffract light intensity was demonstrated.

Periodic spiking pulse formation in broadband nonlinear optoelectronic oscillator

An approach to generating periodic spiking pulses in a broadband opto-electronic oscillator (OEO) without external injection is proposed and demonstrated. Through biasing the electro-optic Mach-Zehnder modulator (MZM) in the OEO cavity at its nonlinear working point, spiking pulses are excited by the self-generated chaos under the combined action of the loop filter response time and the nonlinear excitation effect. An interaction force is introduced between pulses. As a result, the pulse number in a single roundtrip duration and the temporal distribution of the generated spiking pulses can be tuned by adjusting the loop gain and the direct-current (DC) bias voltage of the MZM. Both numerical simulation and experiment are carried out to demonstrate the spiking pulse generation mechanism, where ultra-short spiking pulse trains with even and uneven temporal distribution are generated.

Double barrier structure of Mo/AlN/Mo/AlN/Mo multilayer and its resonant tunneling effect

We prepare a symmetrical double barrier structure of Mo/AlN/Mo/AlN/Mo by using the reactive magnetron sputtering on a sapphire substrate and examine its resonant tunneling effect. Characterizations of multilayers demonstrate that the AlN (0002) aligns well along the Mo (110). Direct current-voltage (DC I–V) test finds significant negative differential resistance at room temperature around 0.7 and 0.9 V carrier energy and with peak-to-valley current ratios of about 1.1 and 1.2. The simulation using conventional transfer matrix method suggests good consistence between the designed structure, the quantized energy level and the switching voltage. The large signal DC model also fits the experimental I–V properties. Our work provides a new type of resonant tunneling device in room temperature.