Output Power Conversion Research Articles

間欠運転による双方向絶縁形DC-DCコンバータの低出力領域の変換効率向上

This paper describes a 750-Vdc, 100-kW, 20-kHz bidirectional isolated dual-active-bridge (DAB) dc-dc converter using SiC-MOSFET modules without free-wheeling diodes, focusing on the output voltage ripple and power conversion efficiency. Intermittent operation brings higher conversion efficiencies in a low-power range to the dc-dc converter than does continuous operation because it makes a significant contribution to reducing switching (snubber) and iron losses. Experimental results reveal that the efficiency at 10kW (10% of the rated power) is 95.1% in continuous operation, whereas it is improved to 98.2% in intermittent operation. The combination of intermittent operation with continuous operation makes the efficiencies in a power range from 3 to 100kW higher than 98%, excluding power losses produced by the control and gate-drive circuits. However, the intermittent operation may bring additional voltage ripples to the dc output terminals. Theoretical analysis confirms that the voltage ripples in a power range from 1 to 18kW are lower than 7V in peak-to-peak, which is only 0.9% of 750Vdc. Theoretical results are confirmed by comparison to simulated and experimental ones.

Cooperative control strategy of energy storage systems and micro sources for stabilizing microgrids in different operation modes

The islanding capability of Microgrids (MGs), when a fault happens in the grid, is seen as one major driver in enhancing the reliable of MGs. However, the use of power electronic interfaces based-MGs faces some difficulties in maintaining stability in an islanding mode due to the low capacity of installed DGs as well as the low speed response of MGs to any load changes. In fact, the lack of availability of spinning reserves makes fast response to load changes, as well as balancing between power generation and load demand difficult. This leads to deviation in voltage and frequency from their (pre-determined) permitted values. In this paper, a new MG’s topology along with a novel control strategy is proposed for stabilizing MGs in different operation modes. Battery storage is used to address the slow response problem of micro sources (MSs) to load changes, and to balance load demands and power generation in the islanding mode of MGs. A droop control idea is adopted in the lowest level of a hierarchy controller to enhance cooperation between power electronic inverters, and to improve load dispatch and voltage regulation. The voltage variation of the MG’s main bus is considered as a load change criterion, instead of the output power of converters, to increase the response speed of the control system. The effectiveness of the proposed control strategy is assessed using MATLA/SIMULINK. It is shown that the proposed control strategy improves the operation of MGs in grid connected and islanding modes where soft transient between these two modes are achieved.

Investigation and Improvement of the Electrical Performance of Silicon Solar Cells

The present paper focuses on the investigation and improvement of the electrical performance of photovoltaic mono-crystalline silicon solar cells. Where, temperature effect was studied in details, where cells loss were observed, analyzed, and several trials to reduce its value were investigated applying passive and active aluminum heat sinks. Also, introduction of thermoelectric generators was proved to pronouncedly enhance the cells performances. The dependences of the open-circuit voltage, maximum output power, short-circuit current, and power conversion efficiency on temperature were observed. Where, their values were reported to be in the order of - 2.3 mV/oC, -0.59 mW/oC, + 0.22 mA/oC, and 0.065%/oC, measured for free PV, and cell temperature of 39.0 oC. The maximum output power and conversion efficiency of the cell were shown to be reduced down to - 0.53 mW/oC and -0.058%/oC, measured for the solar cell/thermoelectric hybrid system. Moreover, for solar cell/thermoelectric/passive heat sink hybrid system, their values were shown to be - 0.52 mW/oC and -0.057%/oC. Finally, for the solar cell/thermoelectric/active heat sink hybrid system, the values reach -0.34 mW/oC and 0.037 %/oC, respectively.

Thermoelectric properties of materials near the band crossing line in Mg2Sn–Mg2Ge–Mg2Si system

Thermoelectric properties of materials with compositions on and near the band crossing line connecting Mg2Sn0.765Ge0.22Sb0.015 and Mg2Sn0.685Si0.3Sb0.015 in the Mg2Sn–Mg2Ge–Mg2Si system are investigated. Although ZTs are very similar, power factors are different. On the line, the power factor decreases from Mg2Sn0.765Ge0.22Sb0.015 to Mg2Sn0.685Si0.3Sb0.015, and off the line, the power factor also decreases. The output power and energy conversion efficiency are calculated using engineering power factor (PF)eng and figure of merit (ZT)eng. It is shown that although similar energy conversion efficiency of ∼11% could be achieved for all compositions studied, the output power are different, increasing from ∼9.1 W cm−2 for Mg2Sn0.685Si0.3Sb0.015 to ∼10.3 W cm−2 for Mg2Sn0.765Ge0.22Sb0.015, due to the different power factors.

A comprehensive design method for segmented thermoelectric generator

A comprehensive method for indicating the length ratio of segmented thermoelectric generator (TEG) is proposed to increase the output power and thermoelectric conversion efficiency. It is found that for a segmented TEG, there is an optimal length ratio corresponding to the highest maximum output power or thermoelectric conversion efficiency, which is not only dependent on the material properties but also the heat transfer conditions and geometry structure. The optimal length ratios corresponding to the output power and thermoelectric conversion efficiency are different. This method is also validated, and the error is within a reasonable range, indicating that this method can be used accurately and time-efficiently for the design of segmented TEGs.

Effects of antimony content in MgAg0.97Sbx on output power and energy conversion efficiency

We report the effect of Sb content in MgAg0.97Sbx (x = 0.99, 0.9925, 0.995, and 1.00) on output power and leg efficiency. Due to the doubling of carrier concentration with increasing the Sb content, power factor is enhanced around 20% over the whole temperature range. Simultaneously, the average thermoelectric figure of merit (ZT) is even enhanced a little in spite of the increased corresponding thermal conductivity. We further calculated the engineering power factor (PF)eng, output power, engineering figure of merit (ZT)eng and leg efficiency by taking into account of the Thomson effect. Assuming Tc = 300 K and Th = 548 K, leg length ∼2 mm, an output power of ∼1.77 W cm−2 and leg efficiency η of ∼10.1% are finally obtained for the optimized composition MgAg0.97Sb0.995.

Entransy analyses of thermal processes with variable thermophysical properties

Entransy method is developed for the analyses of thermal processes with variable thermophysical properties. The entransy and enthalpy entransy balance equations for thermal processes with variable thermophysical properties are derived based on the definitions of differential entransy and enthalpy entransy. The definitions of the entransy and enthalpy entransy for an object are presented. The optimization principles for heat transfer and heat-work conversion processes with variable thermophysical properties are discussed with the concept of entransy. Entransy analyses for a two-stream heat exchanger and a closed Brayton cycle with variable thermophysical properties are conducted and compared with the entropy generation analyses. For the heat exchanger, the results show that both the minimum entransy-dissipation-based thermal resistance and the minimum revised entropy generation number correspond to the maximum heat exchanger effectiveness when the inlet condition of the heat exchanger is prescribed, but the minimum entropy generation rate and the minimum entropy generation number do not. For the closed Brayton cycle, the results show that both the maximum entransy loss rate and the minimum entropy generation rate correspond to the maximum output power and both the maximum entransy loss coefficient and the minimum entropy generation number correspond to the maximum heat-work conversion efficiency when the inlet conditions of the streams and the environmental temperature are prescribed. When the inlet temperature of the hot stream increases, larger entransy loss rate and larger entransy loss coefficient still correspond to larger output power and larger heat-work conversion efficiency respectively.

Comparative Analysis of Semiconductor Power Losses of Galvanically Isolated Quasi-Z-Source and Full-Bridge Boost DC-DC Converters

Abstract This paper compares semiconductor losses of the galvanically isolated quasi-Z-source converter and full-bridge boost DC-DC converter with active clamping circuit. Operation principle of both converters is described. Short design guidelines are provided as well. Results of steady state analysis are used to calculate semiconductor power losses for both converters. Analytical expressions are derived for all types of semiconductor power losses present in these converters. The theoretical results were verified by means of numerical simulation performed in the PSIM simulation software. Its add-on module “Thermal module” was used to estimate semiconductor power losses using the datasheet parameters of the selected semiconductor devices. Results of calculations and simulation study were obtained for four operating points with different input voltage and constant input current to compare performance of the converters in renewable applications, like photovoltaic, where input voltage and power can vary significantly. Power loss breakdown is detailed and its dependence on the converter output power is analyzed. Recommendations are given for the use of the converter topologies in applications with low input voltage and relatively high input current.

Steady-State Analysis and Design of Class-DE Inverter at Any Duty Ratio

This paper presents steady-state analytical expressions of the class-D zero-voltage switching inverter at any duty ratio along with a design example. The obtained expressions include stead-state voltage and current waveforms, output power capability, peak switch voltage, peak switch current, output power, and power conversion efficiency as functions of the duty ratio. Additionally, switching-timing allowance due to antiparallel diodes of switching devices can be predicted from the analytical results. The analytical expressions are verified by showing quantitative agreements with PSpice simulations and circuit experiments.

Theoretical and Experimental Investigations on the Quasi-Optical Mode Converter for a Pulsed Terahertz Gyrotron

Theoretical and experimental investigations on the quasi-optical (QO) mode converter for a 0.42 THz, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">26</sub> mode pulsed gyrotron have been carried out. The vector diffraction theory and the multiple iterative algorithms are used to design and optimize the QO mode converter, and the theoretical value for the conversion efficiency is ~88%. The actual conversion efficiency is ~86% by measuring the input and output power of the QO mode converter indirectly with a calorimeter. The field distribution on the output window of the QO mode converter is measured with an infrared video camera, and a well-focused Gaussian-like beam has been observed on the output window. The experimental results agree well with the theoretical predictions.

Reactive Power and Soft-Switching Capability Analysis of Dual-Active-Bridge DC-DC Converters with Dual-Phase-Shift Control

This paper focuses on a systematical and in-depth analysis of the reactive power and soft-switching regions of Dual Active Bridge (DAB) converters with dual-phase-shift (DPS) control to achieve high efficiency in a wide operating range. The key features of the DPS operating modes are characterized and verified by analytical calculation and experimental tests. The mathematical expressions of the reactive power are derived and the reductions of the reactive power are illustrated with respect to a wide range of output power and voltage conversion ratios. The ZVS soft-switching boundary of the DPS is presented and one more leg with ZVS capability is achieved compared with the CPS control. With the selection of the optimal operating mode, the optimal phase-shift pair is determined by performance indices, which include the minimum peak or rms inductor current. All of the theoretical analysis and optimizations are verified by experimental tests. The experimental results with the DPS demonstrate the efficiency improvement for different load conditions and voltage conversion ratios.

A Novel Algorithm for MPPT for PV Application System by use of Direct Control Method

PV) cell converts sunrays into electrical energy, which has non-linear I-V characteristics. For extracting maximum power from photovoltaic (PV) array for varying environmental conditions, it is necessary to use MPPT tracker. The MPPT consists of DC-DC converter and MPPT algorithm, DC-DC converter is inserted between solar panel and load or battery. The task of MPPT tracker is to match input impedance of the converter and it is equals to the equivalent output impedance of the solar panel to achieve unique point (MPP) where maximum power extracted. The used TL598 based DC-DC converter operates with defined MPPT algorithm. The DC-DC converter output power is controlled by varying its duty cycle to operating at knee voltage level. The novel algorithm is used for efficiently extracts the maximum output power from PV panel with the use of direct control method. The sensing of input voltage and current parameters, which are processed based on PO obtain higher efficiency and reliability as compared to standard methods. The MPPT has capability to propose the technique which has been verified experimentally with a 75-W solar panel for different insolation (incident solar radiation) level and under large signal insolation of level changes.

Piezoelectric nanogenerators synthesized using KNbO3nanowires with various crystal structures

KNbO3 (KN) nanowires having a tetragonal structure or a polymorphic phase boundary (PPB) structure, which contains both tetragonal (P4mm) and orthorhombic (Amm2) structures, are formed at low temperatures. The presence of tetragonal and PPB KN nanowires is attributed to the existence of OH− and H2O defects. Further, the tetragonal and PPB KN nanowires change to orthorhombic KN nanowires in the temperature range between 300 and 400 °C owing to desorption of the lattice hydroxyl group. A composite consisting of polydimethylsiloxane (PDMS) and KN nanowires having a PPB structure shows large dielectric constant and low dielectric loss values of 9.2 and 0.5%, respectively, at 100 kHz. Moreover, a nanogenerator (NG) synthesized using the PPB KN nanowires exhibits the largest output voltage and current among NGs synthesized using the tetragonal or orthorhombic KN nanowires. In particular, the NG containing 0.7 g of PPB KN nanowires shows an output voltage of 10.5 V and an output current of 1.3 μA; these values are among the highest ever reported for NGs synthesized using a lead-free composite. In addition, this NG exhibited the maximum output power and energy conversion efficiency, which were 4.5 μW and 0.9%, respectively, for an external load of 1.0 MΩ.

Simulation Analysis of Control Methods for Parallel Multi-Operating System constructed by the Same Output Power Converters

A large-capacity power conversion system constructed by using two or more existing power converters has a lot of flexibility in how the power converters are used. However, at the same time, it has a problem of cross current flows between power converters. The cross current must be suppressed by controlling the system while miniaturizing the combination reactor. This paper focuses on two current control methods of a power conversion system constructed by using two power converters connected in parallel supplying the same power. In order to elucidate the control performance of cross current, each control method which are aimed at controlling cross current and not directly controlling it are examined in simulations.

A power electronics controlled current source based on a double-converter topology

Abstract The paper presents a conception of power electronics voltage controlled current source (VCCS) which is able much more precise mapping of its an output current in a reference signal, compared to a typical converter solution. It can be achieved by means of such interconnection of two separate converters that one of them corrects a total output current towards a reference signal. An output power of auxiliary converter is much smaller than an output power of main one. Thanks to continuous work of this converter also pulse modulation components in this current are minimized. These benefits are paid for by a relatively small increase in the complexity and the cost of the system. This conception of a converter has been called the double-converter topology (DCT). In the author opinion presented solution of the system can find application in many power electronics equipment and, therefore, will be developed. In the paper DCT basics, simulation experiments, and possible practical arrangement of the DCT are presented

Design of a chaos synchronisation‐based maximum power tracking controller for a wind‐energy‐conversion system

This study proposes a maximum power tracking (MPT) controller based on chaos synchronisation (CS) for a wind-energy-conversion system. The output power conversion of a wind generator depends on the wind speed, and therefore the optimal conversion of wind energy can be obtained by a variable-speed variable-frequency model. Based on a sensorless controller, CS can express dynamic behaviours by using an incremental conductance to adjust the terminal voltage to the maximum power point. A voltage detector based on the Sprott system is used to track the desired voltage and to control the duty cycle of a boost converter. For a permanent-magnet synchronous generator, the simulation results demonstrate the effectiveness of the proposed MPT controller.

Nanoscale flexoelectric energy harvesting

One of the most tantalizing applications of piezoelectricity is to harvest energy from ambient mechanical vibrations for powering micro and nano devices. However, piezoelectricity is restricted only to certain materials and is severely compromised at high temperatures. In this article, we examine in detail, the possibility of using the phenomenon of flexoelectricity for energy harvesting. The flexoelectric effect is universally present in all dielectrics and exhibits a strong scaling with size. Using a simple beam-based paradigmatical design, we theoretically and computationally examine flexoelectric energy harvesting under harmonic mechanical excitation. We find that the output power density and conversion efficiency increase significantly when the beam thickness reduces from micro to nanoscale and flexoelectricity-based energy harvesting can be a viable alternative to piezoelectrics. Specifically, the conversion efficiency in flexoelectric transduction at sub-micron thickness levels is observed to increase by two orders of magnitude as the thickness is reduced by an order of magnitude. The flexoelectric energy harvester works even for a single layer beam with a symmetric cross section which is not possible in piezoelectric energy harvesting. Our results also pave the way for exploration of high temperature energy harvesting since unlike piezoelectricity, flexoelectricity persists well beyond the Curie temperatures of the high electromechanical coupling ferroelectrics that are often used.

The Investigation of a 6.5-kV, 1-kA SiC Diode Module for Medium Voltage Converters

This paper introduces a 6.5-kV 1-kA SiC PiN diode module for megawatt-range medium voltage converters. The analysis comprises a short description of the die and module technology and a device characterization. The effects of di/dt and temperature variation, as well as parasitic oscillations are discussed. A comparison of the results with a commercial Si diode (6.5 kV and 1.2 kA) is included. In the last section, an estimation of maximum converter output power, maximum switching frequency, losses and efficiency in a 3L-NPC converter operating with SiC and Si diodes is presented. The analyzed diode module exhibits a very good performance regarding switching loss reduction, which allows an increase of at least 16% in the output power of a 6-MVA converter. Alternatively, the switching frequency can be increased by 46%.

Narrow Linewidth 177.3-nm Nanosecond Laser With High Efficiency and High Power

We demonstrate a high-efficiency high average power vacuum ultraviolet (VUV) 177.3-nm laser with a narrow linewidth generated by second harmonic (SH) generation from a nanosecond (ns) 355-nm laser in the KBe2BO3F2 crystal. The 355-nm pumping source was a frequency-tripled Q-switched 1064-nm Nd:YAG oscillator-amplifier laser system, in which a quartz etalon was used to compress the linewidth of 1064-nm laser. Under a pump power of 5.2 W at 355 nm with a linewidth of 2 pm, a high average power of 146.5 mW at 177.3 nm with a linewidth of ~0.71 pm was obtained, corresponding to an SH conversion efficiency as high as 2.8%. This is, to the best of our knowledge, the highest output power and highest SH conversion efficiency for the ns 177.3-nm lasers. This VUV coherent light can be used as light source for fine precision spectroscopy and semiconductor photolithography owing to its narrow linewidth, high energy resolution, and high average power.

Waveform Equations, Output Power, and Power Conversion Efficiency for Class-E Inverter Outside Nominal Operation

This paper presents analytical expressions for steady-state waveforms, output power, and power conversion efficiency for the class-E inverter outside the class-E zero-voltage switching and zero-derivative switching conditions. The analytical expressions in this paper include the MOSFET-body-diode effect. By carrying out PSpice simulations and circuit experiments, it is shown that the analytical predictions agree with the simulated and experimental results quantitatively, which indicates the validity of the analytical expressions. Additionally, the switching-pattern distribution maps are also given by using the analytical waveform equations.