The space Brayton nuclear reactor system usually adopts the helium−xenon gas mixture (He−Xe) as the working fluid. The flow of He−Xe in the micro channel regenerator of the system is generally laminar. Since the properties of He−Xe are significantly different from those of common pure gases, the impact of this difference on the laminar flow and heat transfer needs to be evaluated. In present study, the numerical simulations of laminar convective heat transfer for helium, nitrogen and He−Xe are conducted by Ansys Fluent. Compared with simulation results, the applicability of existing laminar friction factor (f) and Nusselt number correlations is evaluated. By establishing the functions of property ratios with the temperature ratio and the mixing ratio, a new laminar f correlation for property-variable He−Xe is proposed. Results show that the calculation error of existing f correlations for He−Xe is obviously large, exceeding 13%. With the new f correlation, the predictions of laminar f... [more]

The stability of a hybrid AC-DC microgrid depends mainly upon the bidirectional interlinking converter (BIC), which is responsible for power transfer, power balance, voltage solidity, frequency and transients sanity. The varying generation from renewable resources, fluctuating loads, and bidirectional power flow from the utility grid, charging station, super-capacitor, and batteries produce various stability issues on hybrid microgrids, like net active-reactive power flow on the AC-bus, frequency oscillations, total harmonic distortion (THD), and voltage variations. Therefore, the control of BIC between AC and DC buses in grid-connected hybrid microgrid power systems is of great importance for the quality/smooth operation of power flow, power sharing and stability of the whole power system. In literature, various control schemes are suggested, like conventional droop control, communication-based control, model predictive control, etc., each addressing different stability issues of hybr... [more]

Agriculture activities are completely dependent upon energy production worldwide. This research presents sensorless speed control of a three-phase induction motor aided with an extended Kalman filter (EKF). Although a proportional integral (PI) controller can ensure tracking of the rotor speed, a considerable magnitude of ripples is present in the torque generated by a motor. Adding a simple derivative to have a proportional integral derivative (PID) action can cause a further increase in ripple magnitude, as it allows the addition of high-frequency noise in the system. Therefore, a fractional-order-based PID control is presented. The proposed control scheme is applied in a closed loop with the system, and simulation results are compared with the PID controller. It is evident from the results that the fractional order control not only ensures 20 times faster tracking, but ripple magnitude in torque was also reduced by a factor of 50% compared to that while using PID and ensures the eff... [more]

In this paper, we present AWEbox, a Python toolbox for modeling and optimal control of multi-aircraft systems for airborne wind energy (AWE). AWEbox provides an implementation of optimization-friendly multi-aircraft AWE dynamics for a wide range of system architectures and modeling options. It automatically formulates typical AWE optimal control problems based on these models, and finds a numerical solution in a reliable and efficient fashion. To obtain a high level of reliability and efficiency, the toolbox implements different homotopy methods for initial guess refinement. The first type of method produces a feasible initial guess from an analytic initial guess based on user-provided parameters. The second type implements a warm-start procedure for parametric sweeps. We investigate the software performance in two different case studies. In the first case study, we solve a single-aircraft reference problem for a large number of different initial guesses. The homotopy methods reduce th... [more]

The article presents an overview of experimental layout design solutions and the general operation scheme of combined heat and power systems with a high-temperature solid oxide fuel cell (SOFC). This system is an environmentally friendly and energy-saving way to produce electricity and heat. The use of high-temperature SOFCs makes it possible to obtain an electrical efficiency of 45−55%. Combining the electrochemical and mechanical system can increase the total efficiency by up to 60−65% in a hybrid power plant. This article discusses the structure and relationship between the components of a hybrid power plant and various modification options for efficient power generation. The technological schemes for existing and tested hybrid power plants with an SOFC and gas turbine are presented and described in detail. When designing a hybrid power plant, the key factors are the choice of design, heat source, and fuel-reforming method; the design of a solid oxide fuel cell and the number of mod... [more]

There is increasing interest in migrating to a carbon-neutral power system that relies on renewable energy due to concerns about greenhouse gas emissions, energy shortages, and global warming. However, the increasing share of renewable energy has added volatility and uncertainty to power system operations. Introducing new devices and using flexible resources may help solve the problem, but expanding the domain of the problem can be another solution. Sector coupling, which integrates production, consumption, conversion, and storage by connecting various energy domains, could potentially meet the needs of each energy sector. It can also reduce the generation of surplus energy and unnecessary carbon emissions. As a result, sector coupling, an integrated energy system, increases the acceptance of renewable energy in the traditional power system and makes it carbon neutral. However, difficulties in large-scale integration, low conversion efficiency and economic feasibility remain obstacles.... [more]

The use of regenerative braking systems is an important approach for improving the travel mileage of electric vehicles, and the use of an auxiliary hydraulic braking energy recovery system can improve the efficiency of the braking energy recovery process. In this paper, we present an algorithm for optimizing the energy recovery efficiency of a hydraulic regenerative braking system (HRBS) based on fuzzy Q-Learning (FQL). First, we built a test bench, which was used to verify the accuracy of the hydraulic regenerative braking simulation model. Second, we combined the HRBS with the electric vehicle in ADVISOR. Third, we modified the regenerative braking control strategy by introducing the FQL algorithm and comparing it with a fuzzy-control-based energy recovery strategy. The simulation results showed that the power savings of the vehicle optimized by the FQL algorithm were improved by about 9.62% and 8.91% after 1015 cycles and under urban dynamometer driving schedule (UDDS) cycle conditi... [more]

Grass and other herbaceous biomass are abundant, but often under- or not utilized as a renewable resource. Here, the production of biochar from extensive late-harvest grass via multiple thermochemical conversion technologies was investigated at lab and farm scale for use in soil applications. While biochar is a product with highly diverse potential applications, it has a multitude of benefits for agricultural usage as a soil amendment, if the quality adheres to certain limit values of potentially toxic constituents. The results show that the biochar can adhere to all limit values of the European Biochar Certificate (EBC) for utilization in agriculture. Generally, the contents of heavy metals were well below the proposed EBC limits and very low PAH concentrations in the biochar were achieved. The high ash content in the grass of 7.71 wt resulted in high nutrient concentrations in the biochar, of benefit in soil applications, but the ash also contains chlorine, nitrogen and sulphur, whic... [more]

State estimation (SE) is regarded as an essential tool for achieving the secure and efficient operation of distribution networks, and extensive research on SE has been conducted over the past three decades. Nonetheless, the high penetration of distribution generations (DGs) is accompanied by uncertainties and dynamics, and the extensive application of intelligent electronic devices (IEDs) is associated with data processing issues, all of which raise new challenges, and these issues must be taken care of for further development of SE in smart distribution networks. This paper attempts to present a comprehensive literature review of numerous works that address various issues in SE, examining key technical research issues and future perspectives. Hopefully, it will be able to meet the needs for the development of smart distribution networks.

Asynchronous loads (AL), because of their low negative-sequence resistance, produce the effect of reduced unbalance at their connection points. Therefore, proper modeling of unbalanced load flows in power supply systems requires properly accounting for AL. Adequate models of the induction motor can be realized in the phase frame of reference. The effective use of such models is possible only if accurate data on the parameters of induction motor equivalent circuits for positive and negative sequences are available. Our analysis shows that the techniques used to determine these parameters on the basis of reference data can yield markedly disparate results. It is possible to overcome this difficulty by applying parameter identification methods that use the phase frame of reference. The paper proposes a technique for parameter identification of models of individual induction motors and asynchronous load nodes. The results of computer-aided simulation allow us to conclude that by using para... [more]

While poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) film is an excellent piezoelectric material for mechanical energy harvesting, the piezoelectric output varies considerably with the spin coating conditions. Herein, we reported a systematic evaluation of the structural, electrical, mechanical, and microstructural properties of spin-coated P(VDF-TrFE) films obtained at various distances from the center, as well as under different rotational speeds. With increasing distance, the remnant polarization, dielectric constant, and crystallinity of the films increased, which resulted in enhanced piezoelectric power at the largest distance. With increasing rotational speed, the remnant polarization, dielectric constant, and crystallinity of the films initially increased and then decreased, while the Young’s modulus continuously increased. This resulted in an enhanced piezoelectric power at a given rotational speed. The piezoelectric power is proportional to the remnant polarization... [more]

Nuclear fusion is the gateway to a whole new paradigm of energy and is a strong candidate for the decarbonization of electricity generation on a global scale. With recent developments in high-temperature super-conducting magnets, the race is on to develop sub-systems which will support a commercially viable fusion reactor for use as a thermal power plant. The fusion of lighter elements creates an enormous amount of heat which must be transferred away from the reactor core. These intense conditions require novel approaches to efficiently transfer very high heat loads into useable thermal energy without compromising the structural integrity of the reactor core and the surrounding components. This report outlines the concept of a fundamental approach to solve the heat transfer problem as proposed by Commonwealth Fusion System’s design for a fusion reactor. A literature review was conducted for other applications that could serve as inspirations, as well as material properties and machinin... [more]

Conventional and cyclic unloading tests with different unloading rates were conducted to study the influences of unloading patterns and rates on the deformation characteristics and mechanical properties of tectonic coal. The results demonstrate that, under continuous unloading, a lower unloading rate promotes an increase in the circumferential strain but inhibits increases in axial strain. A lower unloading rate was found to be able to promote volume expansion under the cyclic unloading path, and the axial, circumferential, and volume strains increased stepwise with the unloading levels, but the increment of the strains decreased with the number of cycles in the same unloading level. It was easier for tectonic coal to reach the elastic limit by a low speed unloading rate when the unloading level was small, and volume dilatation was promoted when the unloading level was large. In both unloading patterns, the tangential modulus and the Poisson ratio were proportional to the unloading rat... [more]

The penetration of electric vehicles is becoming more and more widespread and recently electric buses and trains are fetching the attention of researchers and developers. In this review, we examine the charging systems applied to the fast charging of electric vehicles and buses as well as the charging strategies, mainly applied to electric buses. We also briefly delve into power topologies for a more electrified railway system where we discuss their different supply systems as well as their motor drive systems. Problems and charge challenges for electric buses and trains are discussed too.

The new perspectives of the water−energy nexus, water-for-energy and energy-for-water, emphasize the current and future need to find ways to produce as much energy with as low an amount of water as possible and to obtain as much water with as little energy as possible. In order to promote and implement the concept of sustainable development, the understanding of the dynamic and complex relationship between water and energy is crucial, especially in the context of energy transition. This paper presents a comprehensive analysis of the recent approaches regarding water and energy and the interlink during implementation, operation and servicing of various water and energy production systems. This endeavor is placed in the context of current energy transition from fossil fuels to renewable energy sources. A qualitative and quantitative analysis is performed with various literature solutions from water-for-energy and energy-for-water perspectives for a broader view of the impact of implement... [more]

Solar energy utilization for covering and offsetting the heating loads of buildings, is a sustainable way to reduce energy consumption (electricity, gas, etc.) for space heating. As such, a Trombe wall technology is a classic passive solar heating system used in buildings, that can be modified and applied to cold climate regions. This work presents a case study on a Trombe wall’s application in relation to its thermal energy output for space heating purposes at a factory building in central Latvia. The solar radiation and temperature measurements were carried out throughout the months of June to October. The results show that the examined wall has a monthly energy yield of 120 to 290 kWh, suggesting that Trombe wall systems are applicable as a secondary space heating source in cold climate regions such as northern Europe, however, a number of design and structural aspects have to be thoroughly considered.

In order to give full play to the frequency regulation ability of multiple types of resources such as wind power, energy storage, and controllable load in a microgrid, this paper proposes a hierarchical cooperative frequency regulation control strategy of wind-storage-load in a microgrid based on model prediction. Firstly, according to the operation characteristics of each resource in the microgrid, a hierarchical cooperative frequency regulation architecture of wind-storage-load is constructed. On this basis, the frequency regulation control models of wind power, energy storage, and controllable load are established, respectively, and the calculation method of the characteristic index of the system frequency response is proposed. Then, taking the maximum frequency deviation as the stratification index, a hierarchical cooperative frequency regulation control strategy of wind-storage-load based on model prediction is proposed, and a power compensation strategy for connecting the wind tu... [more]

The annual target reliability level for structural components is given as β = 3.3 in the main design standard for wind turbines IEC 61400-1 ed. 4. However, since the same safety factors are used for a range of load cases and limit states, deviations in the obtained reliability level can be expected, and it should be considered how to handle this in relation to the development of the IEC TS 61400-9 on probabilistic design measures. In this paper, structural reliability analyses were performed for components designed using safety factors for a range of extreme load cases, and by using the correlation between limit states for different years, the development of the reliability level over time was calculated. A relative risk-based assessment was applied to assess the optimal target reliability level and safety factors. The risk-based assessment explicitly includes the uncertainties, benefits, and costs and can motivate differentiation of the annual reliability level between load cases. Ann... [more]

This paper describes an energy management strategy for a DC microgrid that utilizes a hybrid renewable energy system (HRES) composed of a photovoltaic (PV) module, a wind turbine based on a permanent magnetic synchronous generator (PMSG), and a battery energy storage system (BESS). The strategy is designed to provide a flexible and reliable system architecture that ensures continuous power supply to loads under all conditions. The control scheme is based on the generation of reference source currents and the management of power flux. To optimize the supply−demand balance and ensure optimal power sharing, the strategy employs artificial intelligence algorithms that use previous data, constantly updated forecasts (such as weather forecasts and local production data), and other factors to control all system components in an optimal manner. A double-input single-output DC−DC converter is used to extract the maximum power point tracking (MPPT) from each source. This allows the converter to... [more]

As tidal current and marine hydro-kinetic energy converters start to be deployed in pre-commercial arrays, it is critical that the design conditions are properly characterised. Turbulence is known to influence fatigue loads and power production, so developers use turbulence models to generate unsteady flows in order to simulate device performance. Most such models construct a synthetic flow field using a combination of measured parameters and theoretical assumptions. The majority in use today are based on atmospheric flow conditions and may have limited applicability in tidal environments. In the present work, we compare key turbulence model assumptions (which are recommended by the tidal turbine standards and are used in design software) to turbulence measurements from two tidal test sites in Scotland and Canada. Here, we show that the two sites have different levels of conformity to theoretical models, with significant variability within nearby locations at the same site. The agreeme... [more]

Geothermal energy is a type of renewable energy that has rich reserves, is clean, environmentally friendly and has been widely used in the heating industry. The single-well closed-loop geothermal system is a technology with the characteristics of “taking heat without taking water” and is mainly used for geothermal energy heating. Although the heating requirements in the cold region of Northeast China are urgent, the traditional heating mode not only has high economic costs but also causes serious damage to the environment. Therefore, it is of important practical significance to change the heating structure and develop and utilize geothermal energy for heating according to local conditions. In this study, the actual operating single-well geothermal system in the Songyuan area of Jilin Province is used as a case study, and a numerical model is established based on the T2WELL simulation program. The flow production temperature and heat extraction response law of the single-well system in... [more]

The concern with the best energy performance of buildings is a current theme, and construction materials that bring improvements to the performance of buildings and their surroundings are in demand. Façades play a crucial role in regulating the temperature within buildings by permitting or obstructing the transfer of heat and also affect the ambient temperature. Light-colored façades help maintain environments with milder temperatures, but pollution, rain, and other degrading agents darken the colors of the façades, reducing their capacity of sunlight reflection. In this scenario, the present study analyzed the addition of different types of titanium dioxide, anatase and rutile, in cement tiles for building façades, combining the ease and speed of assembly with the self-cleaning effects of photocatalysis. The 1 cm thick tiles were produced with a 1:3 mortar ratio (cement:sand/dry aggregate) with a 0.5 water:cement ratio and the addition of 0.3% polypropylene fiber. Different admixture... [more]

In the present paper a new multi-objective optimisation procedure for the design of a shell-and-tube Latent Heat Thermal Energy Storage (LHTES) is proposed. A simple arrangement of a cylindrical shell with multiple vertical tubes has been examined. The optimisation considers, as design variables, the number of tubes, the tube internal radius and the device height-to-diameter ratio, H/D, while the storage volume is kept constant. This analysis aims to detect the set of solutions which optimises the LHTES performances evaluated in terms of charging and discharging times and overall thermal energy capacity. To accomplish the multi-objectives optimal thermal storage design, a simplified mathematical model of the LHTES has been employed. This model can evaluate the prescribed performances for a given set of design variables. The proposed optimisation procedure evaluates new solutions along the most promising directions in the design variables domain, leading to a significant improvement in... [more]

A bottom-up cost analysis for delivering utility-scale PV-generated electricity as hydrogen through pipelines and as electricity through power is undertaken. Techno-economic, generation, and demand data for California are used to calculate the levelized cost of transmitting (LCOT) energy and the levelized cost of electricity (LCOE) prior to distribution. High-voltage levels of 230 kV and 500 kV and 24-inch and 36-inch pipelines for 100 to 700 miles of transmission are considered. At 100 miles of transmission, the cost of transmission between each medium is comparable. At longer distances, the pipeline scenarios become increasingly cheaper at low utilization levels. The all-electric pathways utilizing battery energy storage systems can meet 95% of the load for as low as 356 USD/MWh, whereas when meeting 100% of load with the hydrogen gas turbine and fuel cell pathways, the costs are 278 and 322 USD/MWh, respectively.

Nanostructured materials have gained much attention in recent engineering and material- science research due to their unique structural makeup, which stands them out from their bulk counterparts. Their novel properties of tiny-size structural elements (molecules or crystallites, clusters) of nanoscale dimensions (1 to 100 nm) make them a perfect material for energy applications. The recent keen interest in nanostructured materials research by academia and industrial experts arises from the unique variable characteristics of increased electrical and thermal conductivity. This occurs as nanostructured materials undergo a transient process from infinite-extended solid to a particle of ascertainable numbers of atoms. The commercial and energy sectors are very interested in developing and expanding simple synthetic pathways for nanostructured-electrocatalysts materials to aid in optimizing the number of active regions. Over the decades, various techniques have been put forward to design and... [more]