Design and experimental investigation of the high efficiency 1.5 W/4.2 K pneumatic-drive G-M cryocooler

Narrow rectangular channel has been widely used at evaporator of cryogenic engineering and electronics cooling. At present paper, saturated flow boiling heat transfer at vertical narrow rectangular channel with gaps of 1.0, 1.5 and 2.5 was experimentally investigated, the heat transfer characteristics at various operating conditions were tested and discussed, and the correlation of saturated boiling heat transfer coefficient was regressed used Least Square Method according to experimental results. It shows that heat transfer coefficient decreases at wider gaps and higher inlet pressure, and it increases at higher flow rate, heat flux and increased vapor quality. Compared with traditional correlation for ordinary channel, the discrepancy is significant. Considering the size effects, a new correlation was proposed to estimate the heat transfer coefficient. The error of the regressed correlation is within ±12%.

SOFC can utilize a variety of carbon-containing fuels stably and efficiently. Many studies have proved that anode recirculation can effectively improve the system fuel utilization, enhancing the system's electrical efficiency and lowering operating costs. More precisely, anode recirculation takes high power output, high efficiency, and high stability into account at the system level, while these three indexes are contradictory at the SOFC stack level. In this study, multiple SOFC system process models were built with MATLAB, which involved four anode recirculation modes, namely, "conventional low recirculating rate" mode, "conventional high recirculating rate" mode, "recirculation with water gas shift reaction reactor" mode and "recirculation with adiabatic reformer" mode. These modes were compared under different operating conditions. The key parameters that affect the system performance include fuel flow rate, recirculation rate, current per cell, and reformer temperature. Finally, the measured data of industrial-size fuel cells were used to make verification. It is found that the "recirculation with adiabatic reformer" mode can achieve high electrical efficiency while maintaining SOFC stacks working at low fuel utilization conditions. In other words, high power output, high electrical efficiency, and high stability are achieved at the same time. Moreover, it also has considerable feasibility in terms of system thermal balance. This study can provide powerful guidance for SOFC system design and construction.

Superconductor technology has recently attracted increasing attention in power-generation- and electrical-propulsion-related domains, as it provides a solution to the limited power density seen by the core component, electrical machines. Superconducting machines, characterized by both high power density and high efficiency, can effectively reduce the size and mass compared to conventional machine designs. This opens the way to large-scale purely electrical applications, e.g., all-electrical aircrafts. The alternating current (AC) loss of superconductors caused by time-varying transport currents or magnetic fields (or both) has impaired the efficiency and reliability of superconducting machines, bringing severe challenges to the cryogenic systems, too. Although much research has been conducted in terms of the qualitative and quantitative analysis of AC loss and its reduction methods, AC loss remains a crucial problem for the design of highly efficient superconducting machines, especially for those operating at high speeds for future aviation. Given that a critical review on the research advancement regarding the AC loss of superconductors has not been reported during the last dozen years, especially combined with electrical machines, this paper aims to clarify its research status and provide a useful reference for researchers working on superconducting machines. The adopted superconducting materials, analytical formulae, modelling methods, measurement approaches, as well as reduction techniques for AC loss of low-temperature superconductors (LTSs) and high-temperature superconductors (HTSs) in both low- and high-frequency fields have been systematically analyzed and summarized. Based on the authors’ previous research on the AC loss characteristics of HTS coated conductors (CCs), stacks, and coils at high frequencies, the challenges for the existing AC loss quantification methods have been elucidated, and multiple suggestions with respect to the AC loss reduction in superconducting machines have been put forward. This article systematically reviews the qualitative and quantitative analysis methods of AC loss as well as its reduction techniques in superconductors applied to electrical machines for the first time. It is believed to help deepen the understanding of AC loss and deliver a helpful guideline for the future development of superconducting machines and applied superconductivity.

A novel process to separate submicron particles from gases — a cascade of packed columns

An active magnetic regenerative refrigerator (AMRR) is expected to be useful for hydrogen liquefaction due to its inherent high thermodynamic efficiency. Because the temperature of the cold end of the refrigerator has to be approximately liquid temperature, a large temperature span of the active magnetic regenerator (AMR) is indispensable when the heat sink temperature is liquid nitrogen temperature or higher. Since magnetic refrigerants are only effective in the vicinity of their own transition temperatures, which limit the temperature span of the AMR, an innovative structure is needed to increase the temperature span. The AMR must be a layered structure and the thermophysical matching of magnetic field and flow convection effects is very important. In order to design an AMR for liquefaction of hydrogen, the implementation of multi-layered AMR with different magnetic refrigerants is explored with multi-staging. In this paper, the performance of the multi-layered AMR using four rare-earth compounds (GdNi2, Gd0.1Dy0.9Ni2, Dy0.85Er0.15Al2, Dy0.5Er0.5Al2) is investigated. The experimental apparatus includes two-stage active magnetic regenerator containing two different magnetic refrigerants each. A liquid nitrogen reservoir connected to the warm end of the AMR maintains the temperature of the warm end around 77 K. High-pressure helium gas is employed as a heat transfer fluid in the AMR and the maximum magnetic field of 4 T is supplied by the low temperature superconducting (LTS) magnet. The temperature span with the variation of parameters such as phase difference between magnetic field and mass flow rate of magnetic refrigerants in AMR is investigated. The maximum temperature span in the experiment is recorded as 50 K and several performance issues have been discussed in this paper.

With the rapid development in portability of the consumer electronics, high conversion efficiency and high power density are going to be more and more important challenges of the power supply adapter design. Conventional centre tapped rectifier configuration with the capacitive output filter is widely used in the front-end Dc/Dc converters because of its inherent advantages in high efficiency and small occupied area. However, the intrinsic resonance between the leakage inductance of the power transformer and the output junction capacitance of the rectifier still exists in practical and the consideration voltage spike increase the voltage stress across the rectifier. This paper presents the detail design considerations for using the voltage stress clamping rectifier configuration in the front-end converter in order to obtain relative high conversion efficiency and high power density. A 300Watts prototype is built up to verify all the theoretical analysis.

Recently, energy network systems with energy storage such as batteries have been focused on using for electric vehicles and power grid systems. Therefore, the Dual Active Bridge (DAB) dc-dc converter has been focused on because it has some unique advantages, such as bi-directional power transmission and high efficiency by Zero-Voltage-Switching (ZVS). However, hard switching and surge current are critical issues at light load. For the improvement of this problem, "masked-switching control (MSC)" was reported previously. However, another hard switching and surge current has confirmed at medium load by MSC. In this paper, the adaptive control method is proposed that can reduce surge current and operate ZVS at medium load. By experimental results, 1.4% of maximum power efficiency improvement at the medium load is confirmed. From the results, it can be seen that the proposed control method can achieve ZVS and realization of high power efficiency with wide load range.

Cryogenic power electronics can significantly improve the converter power density and efficiency. For the transportation applications, cryogenically cooled superconducting machine and cable are adopted to achieve high efficiency performance. If the power electronics circuit can also be designed with cryogenic environment, the overall system performance can be further improved with reduced cost and weight. In this article, the dynamic cryogenic performances of a commercial 650 V gallium nitride (GaN) high electron mobility transistors (HEMT) device are presented. The cryogenic power electronics structure is discussed first. Then, comparisons of room temperature and cryogenic temperature operations are made to illustrate the dynamic switching differences. With the aid of cryogenic cooling, the excessive heat generated by the device can be extracted and junction temperature is lower than room temperature to enable pushing more current flows through the device. The overcurrent test demonstrates that the device is saturated at 250 A under cryogenic temperature, which is much greater than the room temperature value (160 A). Operating mechanisms and explanations are given for the overcurrent test results.

Titanium nitride thin films are used as an electrode material in superconducting (SC) applications and in oxide electronics. By controlling the defect density in the TiN thin film, the electrical properties of the film can achieve low resistivities and a high critical temperature (Tc) close to bulk values. Generally, low defect densities are achieved by stoichiometric growth and a low grain boundary density. Due to the low lattice mismatch of 0.7%, the best performing TiN layers are grown epitaxially on MgO substrates. Here, we report for the first time a Tc of 4.9 K for ultrathin (23 nm), highly textured (111), and stoichiometric TiN films grown on 8.75% lattice mismatch c-cut Al2O3 (sapphire) substrates. We demonstrate that with the increasing nitrogen deficiency, the (111) lattice constant increases, which is accompanied by a decrease in Tc. For highly N deficient TiN thin films, no superconductivity could be observed. In addition, a dissociation of grain boundaries (GBs) by the emission of stacking faults could be observed, indicating a combination of two sources for electron scattering defects in the system: (a) volume defects created by nitrogen deficiency and (b) defects created by the presence of GBs. For all samples, the average grain boundary distance is kept constant by a miscut of the c-cut sapphire substrate, which allows us to distinguish the effect of nitrogen deficiency and grain boundary density. These properties and surface roughness govern the electrical performance of the films and influence the compatibility as an electrode material in the respective application. This study aims to provide detailed and scale-bridging insights into the structural and microstructural response to nitrogen deficiency in the c-Al2O3/TiN system, as it is a promising candidate for applications in state-of-the-art systems such as oxide electronic thin film stacks or SC applications.

<div class="section abstract"><div class="htmlview paragraph">High efficiency, fuel flexibility, and seamless integration with electrified systems are fundamental prerequisites for the next generation of internal combustion engines. In this context, the free-piston linear generator (FPLG) evolves the traditional internal combustion engine concept (ICE) by replacing the crankshaft mechanism with a linear generator, directly converting piston motion into electricity. The FPLG offers several advantages, including higher efficiency in converting mechanical energy to electricity, the ability to operate with a variable compression ratio, and reduced heat losses during the expansion stroke. Among the various tested architectures, the two-stroke, opposed-piston FPLG appears to be the most promising. However, detailed numerical and experimental investigations are necessary to fully understand how performance and efficiency are influenced by the intricate interplay of processes governing electricity generation. In particular, the significant differences between conventional crankshaft-based engines and FPLG kinematics have a profound impact on gas exchange and combustion processes. This study presents a numerical analysis of the key parameters affecting the performance and efficiency of spark-ignition opposed-piston FPLGs. Simulations were conducted using a modified 1D code, which accounts for the effects of electrical load and gas spring pressure on piston motion. Given the unconventional geometry featuring uni-flow scavenging and a side-mounted spark plug, preliminary CFD simulations were performed to develop realistic intake and exhaust system schematics and to establish an appropriate heat release rate profile. Methane was chosen as the fuel for two main reasons: it can be produced from biogenic sources and is applicable to both mobility and power generation. Additionally, its high octane number makes it particularly suitable for FPLG operation at high compression ratios. A single-cylinder unit (~250 cm<sup>3</sup>) was simulated as an initial step toward developing a small-scale prototype. Simulations examined the effects of gas spring pressure, charging pressure, electrical load, and spark timing. The results indicate that efficiency is maximized by applying the highest possible load under given operating conditions and introducing backpressure on the exhaust side to improve trapping efficiency.</div></div>

The extended continuous class-F mode RF power amplifier (PA) is presented for the first time. The introduction and experimental validation of this novel PA mode demonstrates a new design space over a wide band of frequencies. This paper will show that high output power and drain efficiency, equivalent to the class-F mode, can be maintained by varying the reactive components of fundamental and second harmonic impedances in accordance with the new formulation of the voltage waveform. Additionally it will be shown that, by varying both phase and magnitude of the fundamental and second harmonic impedances, a yet wider design space can be achieved, where the efficiency is maintained at a level greater than a certain target value. For the validation of this new theory, an experimental investigation was carried out on GaAs pseudomorphic HEMT devices and demonstrates that high output power and drain efficiency between 75%-83% can be achieved over a wide range of fundamental and second harmonic loads.

Biomimetic low carbonization efficient solar-driven thermochemical energy storage reactor design inspired by the diatoms’ superior photosynthesis capacity

An experimental investigation into the effect of fins on heat transfer in circulating fluidized beds

A hydro-mechanical transmission (HMT) transmits power both hydraulically and mechanically, allowing higher efficiencies than hydrostatic transmissions while maintaining continuously variable transmission ratios. Ordinary HMTs consist of a hydraulic pump/motor pair in parallel with a mechanical transmission, but their large sizes and high costs are major disadvantages. The Hondamatic inline HMT (iHMT) significantly reduces the size, cost, and complexity of an HMT through an inline design that eliminates gears. However, low power efficiency is the principal barrier to its more widespread use. A dominant loss is throttling due to fluid compressibility and non-optimal valve timing, especially in the hydraulic motor. In this paper a method of controlling the motor displacement is proposed, using a rotatable motor valve cam and check valves in conjunction with the Hondamatic’s existing adjustable swashplate. The proposed modification implements near ideal pre-compression and decompression and is applicable for other swashplate type hydraulic motors. For the iHMT, power efficiency improvements of 4.8% to 14.4% at various operating conditions can be achieved.

We propose the concept of superconducting straintronic devices based on the proximity effect at superconductor-ferromagnet interfaces and the ability of ferromagnets with a strong magnetoelastic coupling to change magnetization orientations under strain. Since strains can be generated electrically with the aid of a piezoelectric transducer attached to a superconductor-ferromagnet nanostructure, the power consumption of the resulting hybrid straintronic device could be greatly reduced in comparison with a superconducting spin valve operated by external magnetic field. To demonstrate the feasibility of our proposal, we theoretically describe the strain-mediated magnetoelectric control of the critical temperature Tc in asymmetric ferromagnet-superconductor-ferromagnet trilayers integrated onto a ferroelectric substrate. To that end, Tc is first calculated as a function of the misorientation angle Δφ between the in-plane magnetizations of the two ferromagnetic layers. Numerical calculations performed for CoFe/Nb/NiCu trilayers show that, at optimal nanoscale thicknesses of superconducting and ferromagnetic films, the variation of Δφ may change Tc significantly (ΔTc ∼ 1 K). Then electric-field-induced magnetization rotations are determined for CoFe/Nb/NiCu trilayers coupled to the Pb(Zn1/3Nb2/3)O3-6%PbTiO3 single crystal. It is found that, owing to the opposite sign of the magnetoelastic coupling in CoFe and NiCu alloys, the misorientation angle Δφ can be changed by up to 90° without the introduction of an antiferromagnetic pinning layer. As a result, the electrical switching of the Nb interlayer between the superconducting state and a finite-resistance state appears to be possible in a narrow temperature range. Hence the described multiferroic hybrid can be employed as an electrically controlled resistive switch useful for memory and logic applications in cryogenic electronics.