Dc Flow Research Articles

Performance improvement of a Stirling-type two-stage pulse tube cooler by using stepped displacer plus bypasses phase shifter

Stirling-type pulse tube cooler (SPTC) is the most prospective thermoacoustic refrigerator due to its long lifetime, high reliability and low vibration, and the phase shifter significantly affects the energy conversion efficiency of SPTC. A novel phase shifter of the stepped displacer plus bypasses (SD-B) is proposed in this paper. Thermoacoustic impedance characteristics of the stepped displacer (SD) and the stepped displacer plus bypasses (SD-B) are analyzed. Based on a two-stage SPTC, the effect of additional combined DC flows in the system is first investigated. Results indicate that the cooling capacity could be improved through introducing additional DC flows in the positive direction from the hot end of regenerator to pulse tube. While for SD, negative DC flows are induced due to the clearance seal gaps between the displacer piston and cylinder shell. Thus, SD-B is explored to improve energy conversion efficiency through naturally introducing DC flows by bypasses. The influence of the opening diameter of two bypasses is investigated, and comparisons between the SD and SD-B are performed. Results indicate that with SD-B, the cooling capacity at 20 K achieves a maximum of 2.04 W with a relative Carnot efficiency of 13.0 %, 56.9 % higher than that of SD.

Efficiency enhancement in a heat-driven single-unit thermoacoustic refrigeration system

Heat-driven thermoacoustic cooling technology provides promising alternatives for eco-friendly refrigeration systems due to its high reliability and minimal environmental impact, meanwhile facing limited efficiency compared to current prevailing cooling technologies. The present work introduces an innovative bypass configuration in heat-driven thermoacoustic refrigeration systems aimed at enhancing the energy coupling between the engine and refrigerator, thereby enhancing system efficiency, particularly when the sources temperature is high. The acoustic power diversion mechanism of the bypass tube is firstly explored in an ideal thermoacoustic core unit through theoretical analysis. This core unit is subsequently studied in Sage platform and integrated into an actual single-stage looped traveling-wave thermoacoustic refrigeration system, respectively, and the effects of the bypass tube dimensions and operating conditions on the system are investigated. Furthermore, a comparative analysis is conducted between the proposed bypass tube-based system and the conventional bypass tube-free system, considering aspects such as system performance, key parameter distribution, and exergy loss analysis. The results demonstrate a significant efficiency enhancement, with a 68.8% increase in Coefficient of Performance (COP) and a 6.2 percentage point boost in relative Carnot efficiency compared to the system without a bypass tube. Additionally, the impact of the bypass dimensions and the DC flow introduced by the bypass tube on the system performance is further discussed at a numerical level. These findings offer valuable insights and serve as a reference for the future development of more efficient thermoacoustic refrigeration systems.

Study on transferring the refrigeration power in the temperature-distributed regenerative refrigeration cycle

The temperature-distributed regenerative refrigeration cycle utilizes the real-gas properties to generate the temperature-distributed refrigeration power and improves the cooling performance and liquefaction rate of cryogenic gases. The temperature-distributed refrigeration power is distributed in the volume of the regenerator. It is challenging to transfer this distributed refrigeration power, and the regenerative heat loss is usually inevitable. The transfer process of the temperature-distributed refrigeration power is studied in this paper. A comprehensive investigation into associated influencing factors is presented in this paper. Furthermore, a parameter of the temperature-distributed transfer coefficient (TTC) is proposed to evaluate the transfer process. This study extends into a broader range of the discrete transfer method from liquid-helium temperatures to ambient temperatures and further analyzes the rules from scattered points to multiple points. The internal DC flow method is adopted to eliminate the radial thermal resistance and improve the TTC, recognizing the constraints of the traditional distributed transfer method (coiling heat exchange tubes outside the regenerator). The TTC of the distributed method with the DC flow is improved around 1–2 times by optimizing the mass flux of multi-strand DC flow and temperature range. It is also improved 1.5–2 times by combining the DC flow and discrete method. Additionally, the TTC is improved about 1–1.5 times with the DC flow when using the refrigeration power for gas liquefaction.

Current density profiles in a compact dipole plasma

This article presents current density profiles due to Lorentz and hydrodynamic forces in the presence of spatially varying plasma parameters, electrostatic field (E0→), and microwave electric field (E1→̃) obtained from experiments in a plasma confined by a dipole magnet driven at the steady state. The electric field E0→ (or E1→̃) and the pressure tensor P0¯ (or P1¯) were determined to obtain the total current density J0→ (or J1→̃) at various spatial locations employing the electrical conductivity tensor S¯DC (or S¯AC) as obtained in the previous work [Nanda et al., Phys. Plasmas 29, 062105 (2022)]. The results show that the DC density due to hydrodynamic force dominates over those due to the Lorentz force, and the converse is observed in the case of AC density. Furthermore, the DC flow due to the Lorentz force is regulated by bounce motion (along r̂ and θ̂) and grad-curvature drift (along ϕ̂), whereas E→×B→ drift controls the AC density along the three directions, where r̂, θ̂, and ϕ̂ represent unit vectors in spherical polar co-ordinates. The dominance of DC density due to Lorentz and hydrodynamic forces along r̂ and θ̂ directs the particles along the azimuthal direction by J→×B→ force. This prevents the loss of particles along the radial and polar directions, thus helping in overall plasma confinement. The work reveals interesting features of current density profiles, guided by bounce motion, magnetic drifts, and anisotropic pressure tensor, which would be beneficial for understanding current flow in laboratory and space dipole plasmas.

An Electrical Power System Reconfiguration Model Based on Optimal Transmission Switching under Scenarios of Intentional Attacks

Currently, operating electrical power systems (EPS) is a complex task that relies on the experience of the operators or the strength of algorithms developed for autonomous operation. The continuous operation of EPS is vulnerable to intentional cybernetic and physical attacks. With the most significant extension and distribution in the EPS, the transmission lines are most exposed to potential attacks. Before this, the entire behavior of the EPS changes, and, on occasions, a blackout can even be generated. The present investigation focused on developing a methodology for reconfiguring the power system against intentional attacks, considering the topology change through optimal switching of transmission lines (OTS) based on optimal DC flows and quantifying the contingency index, which allows for the identification of the weaknesses of the EPS. The methodology was applied to the IEEE 30−bus system, and contingencies were randomly generated, as is typical with intentional attacks. The study successfully identified the reconfiguration strategy of EPS based on OTS-DC, mitigating potential problems such as line loadability and voltage angle deviation in the nodes.

가드의 설치 위치가 PCB의 트래킹 현상에 대한 저항성능 향상에 미치는 영향

The purpose of this study was to identify the characteristics of the tracking phenomenon in the PCB where DC flows, and to increase the resistance to the tracking phenomenon by using a guard. To this end, electronic boards (hereinafter, basic-test sample) printed with needle to needle electrodes at 4 ㎜ intervals was manufactured. In addition, a +guard test sample, a –guard test sample, and a double guards test sample were made by attaching a guard on the basic-test sample, and tracking experiments were performed 25 times for each sample, and the results were compared. The number of droplets required for tracking failure was 1.73 times more for the +guard test sample, 2.64 times more for the –guard test sample, and 5.39 times more for the double guard test sample than the basic-test sample. In particular, as a result of verifying the resistance to tracking at a significance level of 99%, it was found that the resistance to tracking was improved for all three types of samples. Based on the above results, we confirmed the possibility of using a guard to reduce the fire caused by the tracking phenomenon in the PCB.

Automated Two-stage Conversion of Hourly Optimal DC Flow Solution to AC Power Flow

Automated Two-stage Conversion of Hourly Optimal DC Flow Solution to AC Power Flow

Guidelines for DC preparation and flow cytometry analysis of mouse lymphohematopoietic tissues.

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy, and functional characterization of mouse and human DC from lymphoid organs, and various non-lymphoid tissues. Within this chapter, detailed protocols are presented that allow for the generation of single-cell suspensions from mouse lymphohematopoietic tissues including spleen, peripheral lymph nodes, and thymus, with a focus on the subsequent analysis of DC by flow cytometry. However, prepared single-cell suspensions can be subjected to other applications including sorting and cellular enrichment procedures, RNA sequencing, Western blotting, and many more. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all co-authors, making it an essential resource for basic and clinical DC immunologists.

Transformation of conventional transformers for enhanced DC mitigation in AC power networks with advanced grid support

The flow of quasi-DC or DC in AC networks may trigger half-cycle saturation of electrical grid transformers that could lead to their internal heating or grid collapse. Apart from the requirement of DC mitigation measures, the conversion of conventional power grid into an intricate network demands the need for power equipment with dynamic control capability. The transformers provide the most strategic point in the grid for the introduction of DC protection and grid-support features. This paper proposes a fractionally rated power-electronics module coupled to neutral and ground terminals of conventional transformers that delivers efficient power network protection against quasi-DC or DC flow with different advanced grid-support features injected on the transformer primary. The proposed concept is validated on an experimental hardware prototype employing power hardware-in-the-loop (P-HIL) configuration of Typhoon HIL-402 and compared with the simulation results in this paper. Also, a transformer and associated module protection approach employing a hybrid bypass switch has been suggested and experimentally validated in this work. The experimental results validate the capability of the module to counter DC injection, perform harmonics mitigation, voltage control or unbalance compensation, impedance matching and power flow control at the same injection point.

Safety management of Internet of Things engineering construction based on GPU parallel computing

This article introduces the calculation method for continuous safety analysis of GPU speed. Combine the configuration of CPU-GPU software with hardware and software configuration, logically allocate computing resources, and use the DC flow system to diagnose whether there are errors in the entire network. Combining the characteristics of network connectivity after a failure, a true N-1 error analysis based on the same calculation strategy of data collection is proposed, and comparisons are made. Various levels of granularity are designed and calculations are performed. This method can create power lines for each group, and ensure fast, real-time safety analysis, and propose an improved method to analyze Web applications. In the multi-level parallel computing model, the key steps of multi-GPU integration are described. The virtual container technology detects applications that intensively interact between multiple GPU panels in managing security analysis and sensitivity analysis applications, as well as application design for security application analysis and sensitivity analysis. This paper selects several typical construction project cases through the research on the construction safety management of construction companies based on the Internet of Things, which mainly involve five issues of on-site safety inspection, staff safety education, safety conflict management and safety awareness, and the application of BIM technology. Highlights the company’s problems in the safety management of engineering construction; it also consults and integrates relevant construction team types and actual management conditions, and combines viewpoints and actual conditions to provide reasonable and contradictory suggestions from five perspectives, namely: completion Safe construction project system, using advanced technology to improve the efficiency of construction safety management.

Utilizing Parallel Superconducting Element as a Novel Approach of Flux-Coupled Type SFCL to Limit DC Current in the System

To lessen the amount of energy lost during transmission, electricity is increasingly being sent using high-voltage lines. Transmission loss in a DC system is lower than in an AC system over long distances. The DC system can improve the efficiency of long-distance transmission by connecting power grids with different requirements. The DC method is becoming popular since it helps to keep the grid stable. Managing and blocking DC flow is crucial to system functionality. In this study, we explore the operation of a flux-coupled type superconducting fault current limiter (SFCL) in a DC system, where the two windings are connected in parallel to limit the fault current flow. A flux-coupled type SFCL is built by connecting two coils in parallel and a superconducting element (SE) in series with the secondary coil. The functions of an SFCL of the flux-coupled kind are equivalent in both direct and alternating current systems. Because of the opposing magnetic fluxes produced by the two coils, the voltage generated by the parallel connection of the coils is always zero. Inadequate SE leads to an increase in resistance, inhibiting the cancellation of opposing magnetic fluxes and hence a loss in power. Connecting the two coils in series allows voltage to be generated while the fault current is limited. To further validate the performance of SFCL with varying resistance and inductance, the system is additionally tested on the IEEE 39 bus system. The MATLAB/SIMULINK software suite is used to run the test system.

The effect of the aftercooler on the regenerator temperature non-uniformity in a high-capacity pulse tube cryocooler

High-power Stirling-type pulse tube cryocoolers (SPTC) are expected to be an ideal candidate for cooling high temperature superconductivity (HTS) for its advantages of compact structure, low maintenance and long service life. However, the regenerator temperature non-uniformity is still a key problem needs to be solved since it significantly deteriorates the cooling performance. As a main heat exchanger, the insufficient and asymmetric heat exchange in the aftercooler may be one of the factors affecting the temperature inhomogeneity inside the regenerator. However, it has been seldom investigated. In this study, the relationship between the performance of the aftercooler and the temperature non-uniformity of the regenerator was investigated based on a high-power SPTC working at ∼ 80 K. With the help of Sage simulation, it is found that the inlet temperature of the regenerator has a large effect on its inside temperature distribution. Even an inapparent temperature inhomogeneity at the inlet of the regenerator will induce a large temperature difference in the middle of the regenerator, due to an obvious DC flow generated in the regenerator. By changing the water inlet and outlet directions of the aftercooler, the temperature non-uniformity of the regenerator will consequently change, which means the uneven heat transfer of aftercooler can be one of the main inducements to the temperature non-uniformity inside the regenerator. Further, methods of using copper wire mesh filling at the hot end of the regenerator, and optimizing heat exchange tubes inside the aftercooler were studied to reduce the temperature non-uniformity.

On the Importance of using an AC or DC Network Model in the Multi-Period Secure Stochastic Optimal Power Flow for Settling a Multidimensional Day-Ahead Market

As the penetration of renewable energy sources increases, their variability and uncertainty have pushed the development of secure stochastic approaches to solving the secure day-ahead and intra-day multi-period optimal power flow. Some of these approaches, in addition to settling the energy market, also procure other products that generators can offer such as spinning reserves and ramping capability, the latter of which becomes even more important under high penetration of renewable sources. The resulting large scale nonlinear minimization problem makes using the more simple DC flow model of the network appealing, at least in the day-ahead stage. This work focuses on the impact of using an AC vs. a DC model of the network in the results of these multi-dimensional markets, using the Colombian system as a test case. The study implies that although the overall energy allocations may not change much from one model to another, other products may exhibit very different allocations under different network models.

Electric polarization and nonlinear optical effects in noncentrosymmetric magnets

We study electric polarization and nonlinear optical effects in spin systems with broken inversion symmetry. We apply strong coupling expansion to the underlying electronic Hamiltonians, and systematically derive expressions for electric polarization in spin systems that are represented in terms of spin operators. The magnon representation of the obtained electric polarization operator allows us to compute linear and nonlinear optical responses by the standard diagrammatic method. We apply our formalism to Heisenberg model with alternating coupling constants and $J_1$-$J_2$ model with inversion symmetry breaking. We demonstrate that these inversion broken spin systems support dc current flow upon magnon excitations which arises from the shift current mechanism.

Flow Rectification in Loopy Network Models of Bird Lungs.

We demonstrate flow rectification, valveless pumping, or alternating to direct current (AC-to-DC) conversion in macroscale fluidic networks with loops. Inspired by the unique anatomy of bird lungs and the phenomenon of directed airflow throughout the respiration cycle, we hypothesize, test, and validate that multiloop networks exhibit persistent circulation or DC flows when subject to oscillatory or AC forcing at high Reynolds numbers. Experiments reveal that disproportionately stronger circulation is generated for higher frequencies and amplitudes of the imposed oscillations, and this nonlinear response is corroborated by numerical simulations. Visualizations show that flow separation and vortex shedding at network junctions serve the valving function of directing current with appropriate timing in the oscillation cycle. These findings suggest strategies for controlling inertial flows through network topology and junction connectivity.

Congestion management via increasing integration of electric and thermal energy infrastructures

Congestion caused in the electrical network due to renewable generation can be effectively managed by integrating electric and thermal infrastructures, the latter being represented by large scale District Heating (DH) networks, often fed by large combined heat and power (CHP) plants. The CHP plants could further improve the profit margin of district heating multi-utilities by selling electricity in the power market by adjusting the ratio between generated heat and power. The latter is possible only for certain CHP plants, which allow decoupling the two commodities generation, namely the ones provided by two independent variables (degrees-of-freedom) or by integrating them with thermal energy storage and Power-to-Heat (P2H) units. CHP units can, therefore, help in the congestion management of the electricity network. A detailed mixed-integer linear programming (MILP) optimization model is introduced for solving the network-constrained unit commitment of integrated electric and thermal infrastructures. The developed model contains a detailed characterization of the useful effects of CHP units, i.e., heat and power, as a function of one and two independent variables. A lossless DC flow approximation models the electricity transmission network. The district heating model includes the use of gas boilers, electric boilers, and thermal energy storage. The conducted studies on IEEE 24 bus system highlight the importance of a comprehensive analysis of multi-energy systems to harness the flexibility derived from the joint operation of electric and heat sectors and managing congestion in the electrical network.

A Novel DC Bias Suppression Method Considering the Cooperation of Multiple Devices

The dc bias phenomenon caused by the excessive current of the neutral point will pose significant threats to the transformers. The conventional suppression method based on capacitive dc blocking device (BD) can prevent dc bias current from penetrating the transformer winding and is the most widely used. However, due to the high investment cost of the BD, it is not appropriate to install the BD for all transformers around the grounding electrode. Thus the BD may redirect the dc flow from one transformer with a BD to the peripheral transformers without BDs, causing the neutral current to exceed undesirably. To address the problems above, a new type of current balancing device (CBD) is introduced, which can lead to flexible suppression of neutral current. Based on coordination between the BD and CBD, a global optimization configuration method is proposed. Unlike the existing suppression strategy based on dc bias suppression devices, the proposed method optimizes the configuration of suppression devices for each transformer from a global perspective in the planning stage, which is an effective supplement to the current suppression method. MATLAB-based simulation results verify the effectiveness and superiority of the proposed method by numerous comparative case studies.

Auroral ionospheric plasma flow extraction using subsonic retarding potential analyzers.

Thermal ion retarding potential analyzers (RPAs) are used to measure in situ auroral ionospheric plasma parameters. This article analyzes data from a low-resource RPA in order to quantify the capability of the sensor. The RPA collects a sigmoidal current-voltage (I-V) curve, which depends on a non-linear combination of Maxwellian plasma parameters, so a forward-modeling procedure is used to match the best choice plasma parameters for each I-V curve. First, the procedure is used, given constraining information about the flow moment, to find scalar plasma parameters-ion temperature, ion density, and spacecraft sheath potential-for a single I-V curve interpreted in the context of a Maxwellian plasma distribution. Second, two azimuthally separated I-V curves from a single sensor on the spinning spacecraft are matched, given constraining information on density and sheath potential, to determine the bulk plasma flow components. These flows are compared to a high-fidelity, high-resource flow diagnostic. In both cases, the procedure's sensitivity to variations in constraining diagnostics is tested to ensure that the matching procedure is robust. Finally, a standalone analysis is shown, providing plasma scalar and flow parameters using known payload velocity and International Reference Ionosphere density as input information. The results show that the sensor can determine scalar plasma measurements as designed, as well as determine plasma DC flows to within hundreds of m/s error compared to a high-fidelity metric, thus showing their capability to replace higher-resource methods for determining DC plasma flows when coarse-resolution measurements at in situ spatial scales are suitable.

A Multi-Function Integrated Circuit Breaker for DC Grid Applications

The protection and current flow regulation of high-voltage direct-current (HVDC) grids requires the deployment of additional semiconductor-based equipment including dc circuit breakers (DCCBs) and current flow controllers (CFCs). However, the inclusion of multiple devices could significantly increase the total cost of an HVDC system. To potentially reduce costs, this paper presents an innovative multi-function integrated DCCB (MF-ICB). The proposed device exhibits a reduced number of semiconductor switches and can fully block dc faults at different locations while regulating dc currents. The configuration of the integrated solution and its operating principle are assessed, with its performance being examined in PSCAD/EMTDC using a three-terminal HVDC grid. Simulation results demonstrate the capability and effectiveness of the MF-ICB to regulate grid current and isolate dc faults.

Effects of DC flow on a cryogen-free Vuilleumier type pulse tube cryocooler

Abstract DC flow may exist in a pulse tube cryocooler if a closed loop is formed by double inlet, etc. It has been found that, given the right amount and the right direction, DC flow may be favorable to the cooling performance of a PTC working at liquid hydrogen temperature and liquid helium temperature with Stirling type or G-M type PTC. In case of Vuilleumier type PTC, which is driven by a thermal compressor, none has been studied. In this paper, the effects of DC flow on a Vuilleumier type PTC working around 10 K have been studied through numerical analysis. In the simulation, the software SAGE is utilized to study the DC flow with a double-inlet module capable of adjusting the DC flow rate. The dependence of performance for thermal compressor on DC flow was firstly investigated. Then the influence of DC flow on the cooling performance was studied, including the cold end enthalpy, cooling capacity, expansion efficiency for pulse tube, etc. Main factors affecting the pulse tube expansion efficiency was distinguished. Finally, the operating parameters effects on cooling performance with varying DC flow rate were carried out. It is found that certain amount of DC flow is beneficial to the system performance with the optimum ratio between DC flow rate and AC flow rate amplitude lying within the approximate range from 0.05% to 0.08%.