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This article proposes a methodology and model to optimize the torque of the torsional electromagnetic actuators. Specifically, the analytical expression of the torque is derived, and then optimization of coil’s configuration and the size of the permanent magnet are investigated based on this, which is of great significance to improve the torque and reduce driving current. Additionally, three times of device’s torque performance is achieved when testing the fabricated samples with different sizes of permanent magnets. The experimental results, finite element simulation, and theoretical analysis are in good agreement, illustrating the reliability, practicality, and simplicity of the proposed methodology in a wide range of magnetic actuators.

A wearable flexible sensor fabricated on an ultrathin substrate is proposed for sleep respiratory monitoring in nasal cavity. The sensitive unit is composed of aurum multiturn microcoil fabricated on the surface of a polyimide catheter by the rotated lithography system, and the signal measurement is based on the principle of thermal resistance. The signal simulation and performance tests of the respiration sensor have been conducted, which demonstrate that the sensor has great performance in anti-interference, response speed, sensitivity, durability, and stability in long-time usage. The practical human tests in various simulated respiratory disease situations, mainly including apnea and hypopnea, display clear differences between normal and abnormal breathing signals measured by the sensor. The respiratory signals measured by this sensor can be utilized to analyze the various respiratory disease conditions. The presented wearable ultrathin flexible respiratory sensor has shown great potential for preliminary diagnosis of sleep respiratory diseases and sleep health monitoring.

A self-powered micro-electromechanical systems (MEMS) inertial switch has been proposed for potential wake-up application without additional power consumption. In comparison with the previous inertial switches powered by constant voltage sources, the proposed device embedded with the charged electret could output a pulse signal without an external power supply. The self-powered inertial switch has two working states: It could harvest vibration energy under excitation of small acceleration when the applied vibration acceleration was smaller than its threshold; It would output a triggering signal once the applied vibration acceleration arrived up to its threshold. The dynamic responses of the switch in the two working states were analyzed. The drop hammer system and the vibrator system were used in conjunction to test the feasibility of the fabricated self-powered inertial switch. The experimental results showed that under excitation of 0.3 g at 137 Hz, the device could work as a vibration energy harvester (VEH), charging the capacitor of the self-charging power unit to 0.8 V in one minute. When a shock with an amplitude of 40 g was applied to the device, a pulse signal was generated. In addition, electrical measurements during energy harvesting indicated durability and adaptability of the device. [2021-0080]

Electrochemical exfoliation of graphite stands out as a promising alternative to the existing methods for scalable graphene fabrication. However, factors governing the electrochemical process and the underlying mechanism are complex and how to effectively control the exfoliation process is far from completely clear despite many attempts in previous works. Herein, for the first time, capillary infiltration, anodic oxidation and their dependence on temperature were found to be critical in determining the electrolyte infiltration and the anodic oxidation process. On this basis, we achieved tuning of sheet dimensions (both thickness and lateral size) and surface chemistry of graphene by facilely controlling the temperature (5-95 °C). Four kinds of graphene materials featuring small size, porosity, water dispersibility and large size can be selectively fabricated in the same electrolyte system at different temperatures. Especially, low-temperature exfoliation results in high yields (99.5%) of small-sized graphene, which is a new breakthrough for electrochemical methods. The finding and associated mechanism of temperature's influence on both capillary infiltration and anodic oxidation not only deepen our understanding of the electrochemical exfoliation, but also make electrochemistry a versatile technique for graphene fabrication.

The working principle of the triboelectric nanogenerator (TENG), contact electrification and electrostatic induction, has been used to harvest raindrop energy in recent years. However, the existing research is mainly concentrated on solid-liquid electrification, and adopts traditional electrostatic induction (TEI) for output. As a result, the efficiency of droplet electricity generators (DEGs) is severely constrained. Therefore, previous studies deem that the DEG output is limited by interfacial effects. This study reveals that this view is inappropriate and, in reality, the output strategy is the key bottleneck restricting the DEG performance. Here, a switch effect based on an electric-double-layer capacitor (EDLC) is introduced, and an equivalent circuit model is established to understand its working mechanism. Without pre-charging, a single droplet can generate high voltage over 100 V and the output is directly improved by two-orders of magnitude compared with TEI, which is precisely utilizing the interfacial effect. This work provides insightful perspective and lays solid foundation for DEG applications in large scale.

This article presents a flexible implantable device fabricated on a polyimide (PI) catheter for targeted treatment of cardiovascular diseases. The copper (Cu) microcoil was fabricated on a PI catheter surface using the rotated UV lithography system and surrounding electroplating technology. The simulation on the magnetic fields generated from the Cu microcoil with 0.2 A current and the driving force analysis on magnetic nanoparticles (MNPs) were conducted. The results demonstrated the precise control of MNPs motions within the magnetic fields generated from the Cu microcoil fabricated on the PI surface. The maximum temperature that increased from the magnetized Cu microcoil was 2 °C at room temperature, suggesting no induced hyperthermia. Moreover, the electrical performance of Cu microcoil was excellently stable in a variety of environments, such as wet, dry, and mechanical fatigue conditions. In vivo tests displayed great biocompatibility of the fabricated device within the carotid artery of a rat model. The directional assembly of MNPs can be achieved by a Cu microcoil fabricated on the surface of a PI catheter. The fabricated device shows notable advantages in promoting the targeted therapeutic strategy of cardiovascular diseases.

To pursue an environmentally friendly and efficient heating solution, a novel hybrid PV/T-air dual source heat pump based on a three-fluid heat exchanger was proposed. In the heating season, the system can absorb energy from solar and air simultaneously; in the non-heating season, the PV modules can be cooled actively or passively by heat pipe. The system was designed to provide heating and electricity for a rural single-family house in Beijing, and a mathematical model was developed. The results indicated that: (1) compared to a conventional air-source heat pump, the energy consumption of this system throughout the heating season was reduced by 13.1%; (2) for electricity generation, compared to a conventional PV panel, the electrical energy output increased by 14.7%. Additionally, the proposed integrated system has the advantages of a compact structure, standardized production, easy maintenance, and no risk of freezing.

Variable refrigerant flow systems (VRFS) are gradually gaining popularity in commercial and residential buildings owing to their satisfactory part-load performance, flexible control, and ease of installation and maintenance. However, the characteristic complex system structure of VRFS makes it difficult to understand their performance. This study presents a comprehensive review of VRFS concerning their design and optimisation, control strategies, modelling and simulation, and fault detection and diagnosis (FDD). This review begins with the developments in design and optimisation of VRFS in terms of alternative refrigerants, crucial components, and innovative systems. Subsequently, the control strategies of VRFS including energy efficiency, improved humidity, and defrosting operation are reviewed. Meanwhile, the research objectives on modelling and simulation of VRFS could be divided into three aspects: investigation of the inherent energy performance, prediction of energy consumption in buildings, and comparisons of energy saving potential. In addition, the advances in FDD for VRFS, based on the rapid development of data mining technologies, are presented. The main conclusions drawn from this detailed review provide a comprehensive overview of VRFS and inspire new insights and feasible solutions for relevant researchers, product developers, practitioners, and policymakers in this field.

We propose an electrochemical route that can prepare few-layered water-dispersible graphene (W-Gr) ≤ 6 atomic layers in thickness and 1–50 μm in lateral size with a remarkably high yield (99%). Simultaneously, high dispersibility (2.5 mg mL−1) and excellent stability (over 6 months) of the W-Gr solution were achieved. All these are attributed to the rational design of electrolyte (NaOH + PTA) and choosing of HOPG as the anode, which ensures enhanced anode oxidation and exfoliation of the large size graphite precursor. This large size W-Gr was used to obtain surface enhanced Raman scattering (SERS) for Rhodamine 6G (R6G) (10−8 M).

To cater to the application in various micro/nano devices, this paper reports a compatible and efficient approach combining electrodeposition and dealloying to fabricate bi-continuous nanoporous copper films. In the electrodeposition step, effect of concentration ratio of different ions, pH of the solution and cathode current density on the elementary composition and microscopic morphology of the deposited alloy are systematically investigated, obtaining an optimum condition with good stability and process compatibility. A uniform copper-zinc alloy with its zinc content improved to ~67 at.% is prepared under this condition. A uniform nanoporous copper with an average pore size of 150 nm is fabricated by ultrasonic assisted dealloying whose efficiency is significantly improved compared to free dealloying. This method is promising to be used in the mass production of nanoporous copper films, benefiting researches on various practical applications in micro/nano devices.

Many of graphene’s industrial applications such as electromagnetic interference (EMI) shielding demand efficient fabrication of high-quality graphene in large scale. Existing electrochemical exfoliation is a trade-off but never an ideal solution in this regard. Herein, we propose a delocalized electrochemical exfoliation (DEE) strategy to revolutionize the way graphite is electrochemically exfoliated. By transmitting the electric potentials with electron transfer reactions, the electrochemical exfoliation is firstly delocalized from electrode/electrolyte interfaces to the whole electrolyte system, thus making deep yet non-destructive exfoliation possible for every dispersed graphite particle. The as-prepared DEE-graphene possesses an ultralow defect density (~1.3 × 1010 cm−2) and significantly high carbon-to-oxygen ratio (~28). Remarkably, record high yields (greater than 98%, 1–10 layers) and production rates (~72.7 g h−1) are achieved in up-scaled DEE in a reproducible manner. More importantly, processing this high-quality graphene into membranes with optimal orientation brings a superior EMI shielding performance (1.9 × 105 dB cm2 g−1) outperforming the best membranes fabricated from metals and many other 2-D materials including reduced graphene oxide and MXenes. The highly efficient DEE with a fundamentally different mechanism and the effective orientation angle modulation strategy for EMI shielding would inspire research and applications of graphene and other two-dimensional materials.

Two-phase natural loops (TPNLs) have been widely used in HVACs for energy conservation. Under harsh conditions, a mechanical pump is usually added in the loop (TPPLs), which is expected to enhance the performance. Unfortunately, the extent and reasons for the enhancement is unclear owing to a lack of comparative studies between TPNLs and TPPLs. In this study, an experimental setup that includes both TPNL and TPPL was established, which were measured comparatively for five typical cases. The results indicate that: (1) When a TPNL performs poorly (under-charged refrigerant, over-charged refrigerant, or insufficient height difference), a pump can enhance the performance remarkably (up to 400%), approaching the “ideal cycle” of TPNL, and the main contribution to the enhancement is eliminating the overheating and sub-cooling, rather than increasing heat convection. (2) For a well-designed TPNL, the pump cannot further enhance the performance, and the pump is unnecessary. (3) When the flow resistance is considerable, the pump even degrades the performance owing to a considerable pressure drop. Finally, (4) the “ideal cycle” of TPNL is the boundary of the enhancement, and the application principle of the pump is to approach the “ideal cycle” at as low a speed as possible.

For energy conservation, two-phase thermosyphon loops (TPTLs) have been widely used in air conditioning systems. Unfortunately, the evaluation index and design criterion of the TPTL is currently lacking, and causes for poor performance are inconclusive. In this study, an “ideal cycle” (optimal state) of the TPTL was proposed, and an approaching degree e was defined to evaluate the extent of approaching the “ideal cycle”. Furthermore, a visual experimental platform was developed, and real cycles under different conditions were measured and evaluated. The results indicate that: (1) the performance of the “ideal cycle” is the upper limit of the performances of all real cycles, and the approaching degree provides an effective evaluation index and design criterion; (2) causes for low approaching degree have been identified and illustrated visually, from the perspective of refrigerant distribution, including under-charged refrigerant, over-charged refrigerant, insufficient height difference, and considerable flow resistance; and (3) the “ideal cycle” can be approached, only when the filling ratio (100%) and the height difference (1.2 m) is optimal, and the flow resistance is negligible simultaneously.

An integrated cooling system comprising passive cooling-thermosyphon loop and active cooling-vapor compression was proposed for the cooling of electronics, especially avionics, under a wide range of heat loads and ambient temperatures. A prototype of the integrated system with the weight of 2.8 kg was developed by microchannel heat exchangers and a micro rotary compressor. The dynamic characteristics of the thermosyphon loop mode and vapor compression mode, and the switching mode were investigated experimentally, with second as time scale. Under the premise that the average wall temperature does not exceed 50 °C, the thermosyphon loop mode can handle a heat load of 600 W with COP of 35.7, and the vapor compression mode can handle a heat load of about 1000 W with COP of 3.1, at the ambient temperature of 20 °C. The developed integrated cooling system can effectively handle the sharp change in heat load (two heat loads of 400 W and 800 W like square wave) and ambient temperature (from 10 °C to 32 °C) by switching working modes. It automatically switched to TL mode at low heat load and ambient temperature for energy conservation, and switched to VC mode at high heat load and ambient temperature for safety.

Triboelectric nanogenerator (TENG) is a kind of capacitive conduction to convert mechanical triggering into electricity, which is the realized applications of Maxwell's equations in the field of nano energy and self-powered systems. As a conductor to participate electrostatic induction, the induction electrode has received little cognition. According to the unified theory of electromagnetics, the investigation of magnetic medium in TENG should not be ignored. Here we fabricate flexible single-electrode triboelectric nanogenerator (S-TENG) by the Micro Electromechanical System (MEMS) compatible technology, including non-destructive peeling technology and in-situ electrodeposition. Specifically, the S-TENG is composed of polydimethylsiloxane (PDMS) as the negative triboelectric layer and electrodeposited nickel as the induction electrode. Theoretically and experimentally, we demonstrate that ferromagnetic Ni electrode could substantially enhance the electrical output of TENG, which is attributed to the generated magnetizing current. The enhancement mechanism proposed in this work follows the law of energy conservation, which utilizes the natural magnetostatic energy inside the ferromagnetic medium and couples the working principle of TENG, thus making great contributions to the output of TENG. Furthermore, the relationship between the magnetic structure and performance is surveyed, and the stability of Ni and Ni-based S-TENG is also investigated. Finally, the proposed strategy for fabricating enhanced and stable Ni electrode is utilized in flexible energy harvester, and shows great prospects in flexible electronics.

The two-phase thermosyphon loop (TPTL) has been widely used in data center cooling. But there is no practical design method and quantitative guidance to ensure a well-designed TPTL. In this paper, the approaching degree to “ideal cycle” ɛ is adopted as the efficiency indicator, based on which, a practical design method for TPTLs used in HVAC is proposed, ensuring the efficiency as well as the capacity of the designed TPTLs. In the design method, the area of the heat exchanger F is determined by the capacity Q, and the filling ratio FR, height difference ΔH, length L and diameter d of the connecting pipes are determined by the efficiency ɛ. The approaching degrees ɛ of TPTLs with filling ratios of 0.05–0.5, height differences of 0.2–5 m, connecting pipe lengths of 1–20 m, and connecting pipe diameters of 6.35–28.6 mm was simulated. For each TPTL, with capacity from 0 W to 10000 W, the recommended diameter, required minimum height difference, and optimal filling ratio were provided quantitatively, and practical design guidance was presented graphically. Additionally, a simple correction method for working fluids was proposed and validated, using five working fluids. Finally, a design example was provided to elaborate how to use the design method.

Heat pumps with rotary compressors operating in low ambient temperature will face heavy performance degradation. A novel vapor injection structure on the blade for the rotary compressor has been proposed in this paper to overcome the back-flowing drawback of the traditional cylinder injection structure and increase the injection area. Based on a verified numerical model, the performance of a rotary compressor with the blade injection structure has been investigated. The results indicate that: the proposed vapor injection structure can eliminate the back-flowing of the injected refrigerant; compared to the traditional injection structure, the new structure can enhance the volumetric efficiency and mass flow-rate by 1.8–2.7% and 26.6–57.2%, respectively; and compared to the traditional injection structure, the new structure can increase the heating capacity and COP by 23.1–48.9% and 3.2–8.0%, respectively.

Two-phase thermosyphon loops (TPTLs) are beginning to be extensively used in the field of air conditioning and heat recovery, where they have quite different flow characteristics compared with the traditional TPTLs used in cooling of electronics. However, in the existing studies, the flow features in the downcomer were ignored, and most researchers simply thought the downcomer was always full of liquid. In this study, a visual experimental setup was established, the flow features in the downcomer were observed and measured. And the influencing factors including temperature difference, liquid charge, height difference, and circulation flow resistance on the liquid head have been identified and investigated experimentally. The results show that, different from the conventional understandings, the downcomer can be partially liquid filled. At this time, the upper part of downcomer is a static saturation gas blockage, surrounded by a layer of liquid film, which does not provide the driving force. The liquid head in the downcomer, which provides the driving force, shows great self-regulation ability with different working conditions. Increasing the refrigerant charge, temperature difference, circulation flow resistance, and decreasing the height difference drives the liquid head to rise, and the downcomer tends to be fully liquid filled.

The two-phase thermosyphon loop (TPTL) is widely applied in air-conditioning systems. It is widely believed that a larger height difference between the condenser and the evaporator leads to a larger driving force and better heat-transfer performance. However, in a previous study, we found that the downcomer can be partially liquid-filled, which means the large height difference may not provide a correspondingly large driving force. In this study, a model considering partially liquid-filled scenario was applied to simulate and analyse a TPTL with large height difference (up to 10 m). The results show: (1) the liquid column essentially provides the driving force, rather than the height difference, and a larger height difference more likely results in a partially liquid-filled downcomer; (2) unlike the conventional understanding, a large height difference may enhance or weaken the performance.

Two-phase thermosyphon loops (TPTLs) are extensively used in industries. Numerical simulation is always applied for their configuration design and performance rating. The conventional models are usually developed under large temperature difference, and the fully liquid-filled downcomer are taken for granted. In this study, a generalized model for TPTL is developed, in which the downcomer can be partially liquid filled or fully liquid filled under different conditions. The model is verified by experiments on an air-to-air finned-tube TPTL. Then, it is used to analyze the flow and heat transfer features of a TPTL, under different refrigerant charge and temperature difference. The results show that the downcomer is partially liquid filled in the case of small temperature difference and refrigerant charge. Additionally, the heat transfer rate first increases then decreases with the increase of refrigerant charge, and first increases linearly then increases relatively slowly with the increase of temperature difference.

With the rapid increase of Internet data centers (IDCs), a large amount of electricity is consumed for IDCs' cooling, and a considerable amount of heat is exhausted directly into the ambient all year round. On the other hand, to efficiently and cleanly heat the room in cold regions a lot of energy is consumed. In this study, a heat recovery system based on a water-cooled integrated air conditioner with thermosyphon is proposed. To analyze the feasibility and energy performance of the system, a steady-state hourly energy model of the system was developed. The energy performance of a typical system located in Beijing was analyzed, and compared with the most common cooling and heating method in China. The results show that the system can substantially reduce the energy consumption of IDCs for cooling and the energy consumption of buildings for heating simultaneously. It should be an alternative heating system making use of potential heating and natural cooling resources.

Data centers and mobile phone base stations (MBS) are consuming more and more energy due to the development of the information communication technology (ICT) industry. The integrated air conditioner with thermosyphon (IACT) was proposed as an efficient cooling device for these spaces, combining vapor compression (VC) refrigeration with a thermosyphon (TS). In this paper, a model to simulate the annual energy consumption of IACT is built and compared with the traditional air conditioner (TAC) in four standard MBSs located in the typical climate zones of China. The results show that the IACT saves 19.1%–28.2% more energy than the TAC, and in hotter regions has a larger energy saving amount but a smaller energy saving rate (ESR). The TS mode works effectively when the mean monthly temperature is 5.4 °C–24.5 °C. The monthly ESR of IACT in the MBSs of Beijing and Guangzhou is 12.2%–48.6% and 12.1%–45.2% respectively.

A hybrid air conditioner combining a thermosyphon cycle with a vapor compression refrigeration cycle has a large energy saving potential compared with a common air conditioner for spaces requiring year-round cooling. The performance of the switch between the vapor compression mode and the thermosyphon mode largely impacts the safety and reliability of hybrid air conditioners. Therefore, a self-operated three-way valve is proposed. A thermodynamic model and a kinetic model are developed in this paper to evaluate the dynamic performance of the switch valve. The effects of the spring force constant, compressor discharging volume, fit clearance and piston length on the dynamic performance of the switch valve are analyzed. In conclusion, the proposed self-operated three-way valve can realize the switch operation accurately.

Spaces with high heat density such as mobile phone base station (MBS) and data center increase fast recently. Huge energy is consumed for the air conditioning because of large indoor heat rejection and year round cooling in these spaces. The integrated air conditioner with thermosyphon (IACT) combines the vapor compression (VC) refrigeration and thermosyphon (TS) in one body, which can save energy and space for these spaces. After analyzing the problems in the existing IACT, a new IACT is proposed by using the novel heat exchangers and the self-operated three-way valve (STV), which works well in both VC and TS modes. The 3HP air cooled IACT is developed and tested in laboratory. The COP of TS mode reaches 9.43 when outdoor temperature is 20 °C lower than indoor, and the COP of VC mode reaches 2.94 under standard condition. This IACT has been applied in MBSs of different cities in China and the energy consumption is tested on-site and compared with the traditional air conditioners (TAC). It is shown that the IACT can save more than 30% energy compared with TAC.