Conversion System Research Articles

Modulating intramolecular charge transfer via π-d conjugative effect in coordination polymers toward photo-thermo-electric conversion

The prospect of harnessing sunlight to generate cost-effective heat and electricity presents a captivating opportunity to address water scarcity and electricity shortages. Photothermal materials with a broad absorption spectrum are indispensable for effectively harnessing solar energy and converting it into heat/electricity. Herein, we developed an intramolecular charge transfer strategy via conjugated effect to regulate solar absorption and photothermal conversion ability of M-DABDT (M = Fe/Co/Ni/Cu/Zn, DABDT = 2,5-diaminobenzene-1,4-dithiol), overcoming a common limitation of narrow absorption and low photothermal efficiency in traditional metal-organic assemblies. The density functional theory calculations reveal that altering transition metal ions induces the structural torsion of organic linkers, resulting in a significant change in dihedral angle from 89.96° to 2.57°. The structural change significantly facilitates the charge transfer and electron relaxation, ultimately promoting the conversion of light to heat. Under one sun irradiation, the maximum temperature of Fe-DABDT can reach approximately 92.5 ℃. More significantly, the integration of M-DABDT CPs and thermoelectric devices under normal solar irradiation results in an extraordinary open circuit voltage (238 mV) and maximum power density (364.5 μW m−2), processing a peak output voltage density of 76.9 V m−2 at 11:00 a.m. in the presence of outdoor sunlight. This strategy presents an avenue for developing metal-organic solar absorbers for photo-thermo-electric conversion systems.

Anisotropic MXene@polydopamine- and Dialdehyde Carboxymethyl Cellulose-Modified Collagen Aerogel Supported Form-Stable Phase Change Composites for Light-To-Heat Conversion and Energy Storage.

As a renewable alternative heat source, the inherently intermittent feature of solar energy needs to be coordinated by reliable energy conversion and storage systems for utilizing the most abundant solar energy. Phase change materials (PCMs) are supposed to be advanced mediums for storing a great deal of heat generated by solar light. However, PCMs cannot effectively absorb and utilize solar energy due to leakage, low photothermal conversion efficiency, and poor thermal conductivity. Herein, we developed a collagen-based aerogel modified by dialdehyde carboxymethyl cellulose and polydopamine-modified two-dimensional transition-metal carbide/nitride (MXene@PDA) through bidirectional freeze-drying technology for supporting PCMs, which exhibited anisotropy in structure and properties. In particular, the thermal conductivity of the aerogel was 0.0871 W/(m·K) in the axial direction and 0.0504 W/(m·K) in the radial direction, demonstrating its anisotropic thermal insulation performance. Moreover, the final aerogel composite PCMs had been obtained via impregnating the obtained aerogel supporting matrix into polyethylene glycol (PEG) and hydrophobic treatment of polydimethylsiloxane, which exhibited outstanding solar-thermal conversion ability, good thermal storage capacity, advanced leakage-proof property, and antifouling performance. The loading rate of PEG was as high as 92.2%, and the melting enthalpy was 132.6 J/g. Most importantly, the water contact angle was evaluated to be 156.8°, indicating its superior antifouling performance. This material has intensive application prospects in the fields of solar energy collection, conversion, and storage.

High-Switching-Frequency SiC Power Conversion Systems with Improved Finite Control Set Method Prediction Control

High-Switching-Frequency SiC Power Conversion Systems with Improved Finite Control Set Method Prediction Control

Effect of phytohormones on the carbon sequestration performance of CO2 absorption-microalgae conversion system under low light restriction

Microalgae have the potential to fix CO2 into valuable compounds. Low photosynthetic efficiency caused by low light was one of the challenges faced by microalgae carbon sequestration. In this study, Melatonin (MT) and indole-propionic acid (IPA) were used to alleviate the growth inhibition of Spirulina in CAMC system under low light restriction. The results showed that MT and IPA increased biomass and carbon fixation capacity. 10 mg/L IPA group achieved the maximum biomass and carbon fixation capacity, which were 17.11% and 21.46% higher than control. MT and IPA promoted the synthesis of chlorophyll, which in turn captured more light energy for microalgae growth. The increase of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) activities enhanced the resistance of microalgae to low light stress. MT and IPA promoted the secretion of extracellular polymeric substances (EPS) which was benefit to protect cells. The maximum phycocyanin content and yield was found in 10 mg-IPA group, which was 20.67% and 46.67% higher than control. MT and IPA improved the synthesis of carbohydrates and proteins and increased carbohydrates and proteins yield. This indicated that adding phytohormones was an effective method to alleviate the growth of microalgae restricted by low light stress, which provided a theoretical guidance for the application of CAMC system in CO2 capture and resource utilization.

Comparative Analysis and Performance Evaluation of Dual Active Bridge Converter Using Different Modulation Techniques

The development in the renewable energy systems and the necessity of the enhanced grid that includes the conventional energy sources and the renewable energy sources and storage systems have realized the importance of power electronics conversion systems. These interfaces are crucial for enhancing the efficiency as well as the control in bi-directional power flow. The use of HEVs as a means of preserving the future supply of fossil fuels, as well as the need for improving the efficiency of the power electronics interfaces required for efficient power management between the two energy sources of the vehicle, is discussed. Furthermore, the improvements in the Uninterruptible Power Supply (UPS) systems and regenerative power systems also require sophisticated power electronic conversion systems. To address these issues of modern technologies, the Dual Active Bridge (DAB) converter comes in as a potential solution. In this paper, several modulation techniques employed in DAB converters are discussed and compared in detail in order to find the best approach to control the converter’s performance in the whole range of its operation. The paper also presents an enhanced modulation strategy applied to DAB converters and built with FPGAs and MPC to minimize the power loss. This proposed optimization technique is more general and easier than the current ones. The performance of the converter is assessed in terms of efficiency, current stress, and the power of backflow, in addition to the proposed optimization and modulation techniques. The article also looks at the closed-loop performance under step load fluctuations and circuit performance as well as efficiency.

Fe3O4/Au@SiO2 nanocomposites with recyclable and wide spectral photo-thermal conversion for a direct absorption solar collector

The Fe3O4-based photothermal materials have been widely applied for the recycling of heavy metal nanoparticles to avoid secondary pollution in the utilization of solar energy owing to excellent magnetic properties. However, the narrow absorption band of Fe3O4 or metal nanoparticles limits their capacity for solar energy harvesting. In this paper, a recyclable and wide spectral photo-thermal conversion system (Fe3O4/Au@SiO2) based on Fe3O4 nanospheres and SiO2-modified Au nanorods (NRs) is prepared. Owing to the strong coupling effect between Fe3O4 and different Au NRs, the wide and strong absorption band from 400 nm to 1100 nm is obtained. Under the influence of magnetic field, the temperature change of nanofluids (NFs) at different positions can be controlled. When the concentration is 0.0467 vol%, the solar energy absorption fraction reaches 96.50 %. Besides, the photothermal performance of Fe3O4/Au@SiO2 NFs at different flow rates is investigated and the results show that temperature rise decreases as increasing flow rate. Finally, a water evaporator device based Fe3O4/Au@SiO2 NFs is developed. The maximum water evaporation rate of 2.6 kg m−2 h−1 can be reached and the movement of evaporator on water can be operated using magnetic field, showing great photothermal conversion performance and recovery potential in practical applications.

Change in sexual and reproductive health knowledge among young women using the conversational agent “Nthabi” in Lesotho: a clinical trial

BackgroundYoung women worldwide face problems like unwanted pregnancy and sexually transmitted infections. Providing sexual and reproductive health education to young women in low- and middle-income countries is a priority. It is unknown if using digital health interventions to deliver health education is effective in resource-constrained settings.MethodsWe conducted an interventional trial among young women in the low–middle-income country of Lesotho to test the impact of the conversational agent system (“Nthabi”) designed to deliver sexual and reproductive health information on either a smartphone or tablet for up to 6 weeks. We measured participant’s knowledge before and after the discussion of family planning, folic acid use, and healthy eating. The main outcome measure was a change in knowledge among participants in these three topic areas. The number of correct pre- and post-test responses was compared using generalized linear models that directly estimated the proportions and percentages of correct responses.ResultsWe recruited 172 young women (mean age 22.5 years, 91% unmarried, 69% completed high school, 23% unemployed, 66% students) to use Nthabi on a mobile phone (11.6%) or loaned tablet device (88.4%). The mean number of interactions with Nthabi was 8.6. Family planning was chosen to be discussed by 82 of the 172 participants (52.2%), and of those, 49 (31.2%) completed the content on this topic, and 26 (16.6) completed the post-test. For the 11 questions about family planning, the total percent of correct responses increased from 85.5% (796 total responses) on the pre-test to 89.9% (320 responses) on the post-test (p = 0.0233). Folic acid use was chosen to be discussed by 74 (47.1%) of 172 participants, and of those, 27 (17.7%) completed the content on this topic, and all 27 (17.7%) completed the post-test. For the 5 questions about folic acid use, the total percent of correct responses was 45.3% (181 total responses) on the pre-test and 71.6% (111 responses) on the post-test (p < 0.0001).ConclusionsThe Nthabi conversational agent system increased knowledge of family planning methods and folic acid use among young women in Lesotho. Digital health interventions like Nthabi have the potential to offer new ways to deliver reproductive health information.Trial registrationClinicalTrials.gov registration April 20, 2020; ID: NCT04354168.

Ultrablack surface with omnidirectional high absorption.

Black surface plays an important role in solar-thermal conversion systems and space optical systems. Despite the significant efforts in developing a black surface with strong broadband absorption, realizing scalable omnidirectional high absorption ultrablack surfaces remains challenging. Here, we report an ultrablack surface based on a structural black paint (SBP), realized by femtosecond (fs) laser fabrication of high aspect ratio microstructure mold followed by mold transferring on black paint coating. The SBP exhibits extremely low hemispheric reflectance (R < 1.2%) in the solar spectrum at a normal incident angle; even at an 80° incident angle, the SBP also has good anti-reflection performance (R < 9%). Based on such properties, we further show the enhanced solar-thermal performance over commercial black paints. Our approach holds promises for scalable and cost-effective manufacturing of ultrablack surface, with potential applications in solar-thermal conversion efficiency improvement and space optical system stray light suppression.

Hybrid feedback control of PMSG-based WECS with multilevel flying-capacitor inverter enhanced by fractional order extremum seeking

This paper proposes an original hybrid control strategy for a three-phase multilevel flying capacitor inverter (FCI) used in wind energy conversion system (WECS). A state feedback law, computed using a Lyapunov function and an invariance principle, is designed to manage grid current control and capacitor voltage balancing, guaranteeing the asymptotic stability of the closed-loop system. Additionally, a fractional-order extremum seeking control (FOESC) algorithm for Maximum Power Point Tracking (MPPT) is introduced, which optimizes power capture by adjusting turbine speed from the DC side without requiring a detailed system model (model-free approach). The use of fractional-order calculus leads to improved convergence and more seamless performance while preserving the robust characteristics of the system. The proposed control strategies are validated through simulation, demonstrating improved transient response and stability under varying wind conditions and parameter uncertainties. These results highlight the potential of advanced control algorithms to enhance the efficiency and reliability of wind energy systems, contributing to global efforts to achieve sustainable energy goals.

Tailoring Energy Structure of Low‐Toxic Ternary Ag−Bi−S Quantum Dots through Solution‐Phase Synthesis for Quantum‐Dot‐Sensitized Solar Cells

AbstractLow‐toxic multinary semiconductor quantum dots (QDs) showing a photoresponse in a wide wavelength range from visible to near‐IR wavelength regions have been intensively investigated for fabricating efficient solar light conversion systems. Recently, AgBiS2 QDs have attracted much attention for their application to improve the performance of QD solar cells because they have a large absorption coefficient in the visible and near‐IR regions. In this report, we describe solution‐phase preparation of ternary Ag−Bi−S QDs with different sizes and with different Ag/Bi ratios. The average size of resulting spherical Ag−Bi−S QDs was controllable from 2.7 to 8.1 nm by adjusting the reaction temperature from 373 to 473 K, while the Ag fraction of obtained QDs also increased with an increase in particle size. The absorption onset wavelength shifted from 850 to 1200 nm in the near‐IR region as the particle size increased. AgBiS2 QDs with an almost stoichiometric composition, obtained at reaction temperatures of 423 K and 473 K, contained no deep intragap states and exhibited a p‐type semiconductor behavior, while QDs prepared at a reaction temperature of 393 K or lower had non‐stoichiometric Ag‐deficient compositions producing intragap defect states. Sensitized solar cells fabricated with stoichiometric AgBiS2 QDs exhibited a photoresponse in visible and near‐IR wavelength regions, the optimal PCE being 0.74% with AgBiS2 QDs of 6.2 nm in diameter that were prepared at 423 K.

Testing Algorithms for Controlling the Distributed Power Supply System of a Railway Signal Box

Trends in the use of renewable energy sources to power buildings do not bypass objects for which maintaining a power supply is critical. This also applies to railway signal boxes. The aim of the research work was to test the multisource power supply system for a railway signal box with power electronic converter systems and a DC bus, built as part of the research project. The assumption for powering the railway signal box building was to use renewable sources, energy storage devices, and a 3 kV DC traction network as the second required power supply grid. Both power grids were connected by power electronic converters, and the power values of the converters were set based on the calculated power balance values using the values measured at the system nodes and the set constraints. The tests primarily tested the response of the power supply system to changes in load power and power generated by the photovoltaic source, as well as the charge level of the energy storage devices. The correctness of the control algorithm’s operation was assessed based on the recorded power values in the power supply system nodes. The tests were carried out for 60 scenarios that covered all normal and emergency operating conditions. During the tests, delays in response to changes in the power supplied to the converters and the values of circular power flow between the power grid connections were recorded. The recorded delays ranged from 2 to about 50 s and the circular power flows did not exceed 1500 W. Based on the results of the tests, it was found necessary to improve the power measurement system in the power supply system nodes and to improve the quality of communication and the transmission time of measurement data transmission time.

Optimization of methane partial oxidation for hydrogen production with local free space addition: An efficient combination of porous media reactor and thermoelectric conversion system

Thermoelectric technology plays a pivotal role in the efficient conversion and utilization of exhaust heat. Abundant waste heat resources are present in the tail gas of hydrogen production, significantly influenced by combustion characteristics. To investigate the impact of combustion performance on hydrogen yield and power output, an efficient porous media reactor was designed and integrated with a thermoelectric conversion system. The combustion characteristics of a thermoelectric system incorporating unsteady porous media were examined. Furthermore, the effects of the porous media’s structural parameters on temperature distribution, gas concentration, and power generation were analyzed under various operating conditions. The results revealed that radial free space structures exhibited higher hydrogen and methane conversion efficiencies compared to axial free space structures at verge and toroidal positions. However, the height of the radial free space exerted a more substantial influence on temperature distribution and gas production than the size of the free space itself. At a height of 39 mm, hydrogen concentration and yield reached their peak values of 14.5 % and 53 %, respectively. Combustion was found to significantly impact power generation, with a downward shift of the flame resulting in decreased power output. The inlet velocity was observed to affect the thermoelectric generator power, with a maximum steady-state power of 3.95 W achieved at a velocity of 16 cm/s. These findings provide practical guidance for the efficient utilization of industrial waste heat.

Design optimization of stiffening system for ocean thermal energy conversion (OTEC) cold water pipe (CWP)

Ocean thermal energy conversion (OTEC) uses heat from seawater to generate electrical energy by utilizing the temperature difference between the surface and deep ocean layer. Cold water pipe (CWP) attached to transport cold seawater over distances of up to 600 m to the plant is susceptible to failures caused by environmental loads. At this point, the CWP should be strengthened to prevent structural failure by inducing ring stiffeners along the pipe. To ensure its structural integrity, this study conducts a design optimization by investigating how the variation of ring stiffening system parameters such as height, thickness, distance between stiffeners, and shape of stiffeners influence the load-carrying capacity of the pipe using a Finite Element Analysis. Initially, the benchmarking procedure is done to ensure the reliability of the FEM modeling. After being verified, the modeling procedure is used for parametric study analyses. Results state that the higher and thicker stiffener ring increases the structural strength. In the case of the ring stiffener distance, as predicted, reducing the spacing between reinforcements is preferable to increasing the bending strength of CWP OTEC. Analyzing the variation of reinforcement height, thickness, and spacing, it was found that thickness had the most significant influence, followed by reinforcement height, and reinforcement spacing had the least. Additionally, changes in the shape of the reinforcement have minimal impact on the flexural strength of the structure when regions of identical moments of inertia exist.

Construction of Mesoporous Aluminosilicate Thin Films on ITO Electrodes for Immobilizing a Cationic Ru(II) Water Oxidation Catalyst

Aluminosilicate (Al‐SiO2) thin films with vertically aligned mesochannels were successfully synthesized on ITO electrodes and employed for the immobilization of a cationic Ru(II) water oxidation catalyst without requiring linker groups. Optimal synthesis conditions yielded uniform mesoporous Al‐SiO2 films with tunable Al content, high surface area (568 m2/g), 3.94 nm pore size, and 155 nm thickness. Electrochemical studies confirmed the presence of the immobilized Ru complex undergoing diffusion‐controlled Ru(III/II) and Ru(IV/III) electron transfer. The Ru loading reached 4.71 nmol/cm2 at Si/Al = 9.6, with higher Al content enhancing loading amounts via cation exchange. The Ru‐modified electrode exhibited high electrocatalytic water oxidation activity, achieving 75.3% Faradaic efficiency and a turnover number of 298.6 for O2 evolution for 1 hour. This work provides a new approach to construct porous environments on an electrode surface to immobilize positively charged transition‐metal complexes as catalysts, offering potential applications in the development of electrocatalytic systems for energy conversion.

Powerful Electronic Systems for Conversion of Parameters of Electrical Energy with Nonlinear Pulse-Width Regulation of the Output Voltage

Powerful Electronic Systems for Conversion of Parameters of Electrical Energy with Nonlinear Pulse-Width Regulation of the Output Voltage

Multilevel Middle Point Clamped (MMPC) Converter for DC Wind Power Applications

This manuscript introduces a novel multilevel middle point clamped (MMPC) DC-DC converter and its associated switching scheme aimed at maintaining the desired medium-voltage DC (MVDC) collector grid within offshore all-DC wind farms. Building upon previous work by the authors, which proposed an all-DC structure serving as a benchmark system, this study explores the application of the MMPC DC-DC converter within this framework. Within the all-DC wind generation system, a 9-phase hybrid generator (HG) integrated into the wind turbine is linked to the MVDC collector grid through an AC-DC stage, which is a passive rectifier. This passive rectifier offers elevated voltage ratings and protection against back power flow. The conventional neutral point clamped (NPC) converter concept has been thoroughly investigated and expanded upon to develop the proposed MMPC DC-DC converter. The proposed MMPC DC-DC converter integrates boosting capabilities, facilitating the connection of the generator’s rectified voltage to the MVDC collector grid while regulating variable rectified voltage to a fixed MVDC collector grid voltage. The MVDC collector grid is further interconnected with high-voltage DC (HVDC) through a DC-DC converter situated in an offshore substation. This paper further provides a comprehensive overview of the proposed MMPC DC-DC converter, detailing its operational modes and corresponding switching schemes. Through an in-depth examination of operational modes, duty cycles for each switch and mode are defined, subsequently establishing the relationship between rectified input voltage and MVDC output voltage for the MMPC DC-DC converter. Utilizing the middle point clamped architecture, this innovative converter offers several advantages, including low ripple voltage, a modular structure, and reduced switching stress because of the multilevel voltage and the incorporation of a hard point, which also facilitates the capacitor voltage balancing. Finally, the effectiveness of the proposed converter is evaluated via simulation studies of a wind turbine conversion system utilizing two cascaded MMPC DC-DC converters operating under variable input voltage conditions. The simulations confirm its efficacy, supported by promising results, and validating its performance.

A hybrid fuzzy logic-based MPPT algorithm for PMSG-based variable speed wind energy conversion system on a smart grid

Recently, wind power has gained popularity as a sustainable energy source. Wind energy conversion systems (WECSs) can accept fixed speed and variable speed (VS) operations. VS-WECSs are preferable to conventional WECSs because of their higher electricity collection capacity. Maximum power point tracking systems (MPPTs) are essential for maximizing the efficiency of wind energy generation in wind turbine installations linked to power grids. This study introduces a hybrid fuzzy logic controller-based MPPT (FLC-MPPT) for wind turbines (WTs) connected to permanent magnet synchronous generators (PMSGs) to accurately determine the maximum power output of WTs. This study employs a three-phase back-to-back converter to link a PMSG to a utility grid. The reference signals for pulse width modulation controllers comprise two-phase system currents. It constructs a converter that can transfer electrical energy in both directions using insulated-gate bipolar transistor technology and is powered by a battery. The machine-side converter uses model predictive control for the present control loop. Given the generator's susceptibility to changes in wind conditions, this factor is of the utmost importance. A WT simulation was conducted using MATLAB/Simulink and an FLC methodology was employed. The model used a PMSG. Measurements of rotor speed, power, induced voltage, and current were taken in relation to variations in wind speed. The simulation results show that the FLC-MPPT can maximize the output power over a wide range of wind speeds with a higher efficiency of 92.6% and performance of 95.7%.

Heat transfer characterization of SCO2 in non-uniformly heated rectangular cooling tubes under large mass flow and heat flux conditions

A Thermoelectric Conversion System (TECS) with SCO2 as the circulating mass can effectively mitigate the fuel heat sink and electrical energy shortage problems of hypersonic vehicles. However, there is still a lack of research on the use of SCO2 in the harsh thermal conditions in scramjet. In this paper, the heat transfer mechanism of SCO2 in rectangular cooling tubes are numerically analyzed for the first time based on the actual working environment of scramjet. The effects of uneven heat flux (q) axial and circumferential allocations and circumferential and axial tilting of the tube are innovatively considered. Meanwhile, we analyze the heat transfer mechanism of SCO2 under different operating parameters. The results show that the uneven allocation of axial q can suppress the heat transfer deterioration (HTD), and the overall heat transfer coefficient (HTC) can be increased by 11.68 % when the q allocation is Gaussian. The increase of the tube inclination angle can improve its heat transfer performance, and the local HTC can be increased by up to 5.64 % when θ2 = 180°.Increasing the mass flow rate (G) and inlet temperature (Tin) can increase the overall HTC of the tube. This paper can provide guidance for the design and improvement of TECS.

Analysis of the Stability and System Participation Factors of Grid-Connected Converters

When the sources of renewable energy and numerous power electronic gadgets are introduced into the power system, which is related to the negative resistance features of power electronic gadgets, the power system with high power electronics often exhibits weak inertia. Also the stability of multi-converter grid-connected systems is adversely affected. To address these issues, a process of small disturbance stability analysis is proposed in this paper for the grid-connected systems of multiple grid-forming converters based on impedance ratio criterion. The Nyquist trajectory map of the system ratio of impedance in the complex plane is used in the suggested analytical method to estimate the quantity of RHPs in the right half-plane of the system, and the stability at the common points of the system is evaluated against the Nyquist criterion. In addition, the instability of grid-forming converters in grid-connected systems is reproduced by using grid-forming converters state space equation. Unlike the traditional methods of analysis, the method proposed in this paper can be applied to specifically judge the key equipment or control link of instability of the system, which provides guidance on optimizing the parameters of the system. Finally, simulation analysis is conducted to confirm that the model is accurate and that the proposed approach is applicable.

Phytohormones enhanced carbon fixation and biomass production in CO2 absorption-microalgae conversion system under light stress

CO2 absorption-microalgae conversion (CAMC) system can be a sustainable technology to achieve low-energy carbon capture and resource utilization in response to zero carbon. However, light stress could restrict the carbon utilization efficiency of CAMC system. In this study, melatonin (MT) and indole-propionic acid (IPA) as phytohormones, were used to promote the carbon fixation capacity and biomass yield of CAMC system under light stress, and the potential mechanism was also discussed. Both MT and IPA promoted the growth of Spirulina, and 10 mg-MT group obtained the highest biomass and carbon fixation capacity, which were 27.61 % and 30.62 % higher than control. MT and IPA promoted the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR), enhancing antioxidant activity to avoid oxidative damage. The results of transmission electron microscopy (TEM) and cell apoptosis showed MT and IPA reduced cell membrane damage and apoptosis. Phytohormones induced the carbon flux flow to the synthesis of carbohydrate and protein, and increased carbohydrate and protein productivity. Moreover, the maximum phycocyanin content and yield were 13.74 % and 43.25 % higher than the control. Therefore, adding phytohormones was an effective strategy to achieve high-value bioresources of microalgae in CAMC system under light stress.