Turbulent flows around bluff bodies are present in a large number of aeronautical, civil, mechanical, naval and oceanic engineering problems and still need comprehension. This paper provides a detailed investigation of turbulent boundary layer flows past a bluff body. The flows are disturbed by superficial roughness effect, one of the most influencing parameters present in engineering applications. A roughness model, recently developed by the authors, is here employed in order to capture the main features of these complex flows. Starting from subcritical Reynolds number simulations (Re = 1.0 × 105), typical phenomena found on critical and supercritical flow regimes are successfully captured, like non-zero lift force and its direction change, drag crisis followed by a gradual increase on this force, and separation and stagnation points displacement. The main contribution of this paper is to present a wide discussion related with the temporal history of aerodynamic loads of a single roug... [more]

Nowadays, most metro vehicles are equipped with an automatic train operation (ATO) system, and the speed control method, combining cruise speed planning and proportional-integral-derivative (PID) control, is widely used. The automation is achieved, and the energy-efficient can be improved. This paper presents an improved artificial bee colony algorithm for speed profile optimization with coast mode and an adaptive terminal sliding mode method for speed tracking. Specifically, a multi-objective optimization model is established, which considers energy consumption, comfortableness, and punctuality. Then, a novel artificial bee colony algorithm named regional reinforcement artificial bee colony (RR-ABC) is designed, to search the optimal speed profile with coast mode, in which some improvements are made to speed up convergence and to avoid local optimal solutions. For speed-tracking control, the adaptive terminal sliding mode controller (ATSMC) is used to improve the speed error, robustne... [more]

Solar power towers with thermal energy storage based on direct-flow regenerators have the potential to generate cost-effective base-load electricity. An inventory option that opens up further cost-saving potential but has not yet been extensively investigated for this application is slag from electric arc furnace. This use has not only economic advantages, but also serves environmental protection, since a large proportion of this type of slag is currently not used any further, but is disposed of in landfills. In the completed EU project REslag, various subsequent uses of the slag were investigated, including the possibility presented here of using sintered slag pebbles as an inventory for regenerators in solar power towers with air as the heat transfer fluid. The main results from the different phases of the project are presented, with a focus on the investigations not yet published. In addition to results from thermal simulations on different designs and on the partial load and off-de... [more]

This study investigated a double loop network operated with ultra-low supply/return temperatures of 45/25 °C as a novel solution for low heat-density areas in Denmark and compared the proposed concept with a typical tree network and with individual heat pumps to each end-users rather than district networks. It is a pump-driven system, where the separate circulation of supply and return flow increased the flexibility of the system to integrate and displace heating and cooling energy along the network. Despite the increased use of central and local water pumps to operate and control the system, the simulated overall pump energy consumption was 0.9% of the total energy consumption. This was also an advantage at the design stage as the larger pressure gradient, up to 570 Pa/m, allowed minimal pipe diameters. In addition, the authors proposed the installation of electrically heated vacuum-insulated micro tanks of 10 L on the primary side of each building substation as a supplementary heatin... [more]

A saturated iron-core superconducting fault current limiter (SI-SFCL) can significantly limit the magnitude of the fault current and reduce the stress on circuit breakers in direct current (DC) power systems. The SI-SFCL consists of three main parts: one magnetic iron-core, one normal conductive primary coil (CPC), and one superconducting secondary coil (SSC). This paper deals with the design options for the coil system of the SI-SFCL and confirms their operating characteristics through a physical experiment. The electromagnetic characteristics and operational features of the SI-SFCL was analyzed by a 3D finite element method simulation model. The design of the SSC was based on shape, wire types, required fault current limit and protection aspects. In the CPC, the bobbin was designed based on material selection, cost, structural design, and the effects of the SI-SFCL on the fault current limit. Based on these simulation results, a laboratory-scale SI-SFCL was developed, specifically fa... [more]

Producing value-added chemicals from glycerol is imperative for the sustainable future of biodiesel. Despite worldwide efforts, the commercial production of acrylic acid from glycerol faces challenges, both technologically and economically. Based on our new technology using CO2 as a reaction medium in a two-step process to catalytically convert glycerol to acrylic acid, we established computer simulation models to analyze the energy efficiency and estimate production costs at different scenarios. The analysis was conducted in conjunction with published data of a typical, intermediate-sized biodiesel facility, aiming at the feasibility of producing acrylic acid on-site in the context of a circular economy. Variable analysis in response to the market value of glycerol, the source and cost of carbon dioxide recycling, and the changes in process scale and conditions are also presented. Results indicates that a cost-effective supply of CO2 to the acrylic acid plant is key to the further res... [more]

Biogas, a product of anaerobic digestion process that consists mainly of methane and carbon dioxide is a suitable alternative fuel if unwanted impurities are removed as they have a negative impact on the equipment. The most significant technologically troublesome trace compounds that must be removed are siloxanes since they are converted into silica on gas surface engines and turbines resulting in equipment damage. The quality of the gas is certainly improved by reducing the amount of impurities and the end use determines the extent of biogas cleaning needed. The major aim of this study was to compile information that can assist researchers or even designers in selecting a suitable technology to remove siloxanes. Siloxane removal definitely can be achieved using different methods and the effectiveness of each method relies on careful consideration of the characteristics of both biogas and siloxane, as well as the technological aspects of the method. Herein, we review on different clean... [more]

A novel fuzzy rule is proposed to adopt a positive pitch strategy when the error between the measured and rated generator speed becomes large and continues to increase, and to adopt a negative pitch strategy when the error is small. The improved approach is introduced into the normal Fuzzy-Proportional-Integral (Fuzzy-PI) control strategy by dividing the fuzzy rules into four areas and analyzing the design method for each area. Furthermore, a low pass filter is used to reduce the ultimate loads of the pitch driver caused by the novel fuzzy rules. The modeling of the wind turbine load under turbulent wind conditions is conducted in GH Bladed, and MATLAB/Simulink is used to interact with the modeling to verify the novel Fuzzy-PI control. The results show that, compared with normal Fuzzy-PI control, the novel Fuzzy-PI control can greatly reduce the ultimate loads and fatigue loads of the pitch driver. The novel Fuzzy-PI control not only reduces the extremum of power deviation, but also de... [more]

Renewable energy curtailment often occurs to accommodate large amounts of renewable energy sources in power systems while maintaining system stability and reliability. Widely known methods, such as new transmission line construction, the introduction of demand-side resources, and the reduction of conventional generator output, can minimize the occurrence of curtailment; however, there are difficulties in introducing them because of social and economic problems. For these problems, the Jeju power system adopted a battery energy storage system (BESS) resource to mitigate the curtailment and secure frequency stability with the high penetration of renewable energy. The small-size Jeju island power system is operated with reliability must-run (RMR) units and high-voltage direct current (HVDC) lines connected to the mainland. Since the number of RMR units contributes to frequency stability by providing inertia, reducing the number of operating units for curtailment mitigation is difficult. T... [more]

Positive Energy Districts and Neighborhoods (PEDs) are seen as a promising pathway towards sustainable urban areas. Several cities have already taken up such PED-related developments. To support such approaches, European countries joined forces to achieve 100 PEDs until 2025 through a comprehensive research and innovation program. A solid understanding and consideration of cities’ strategies, experiences and project features serve as the basis for developing and designing the PED program. JPI Urban Europe has been collecting information on projects towards sustainable urbanization and the energy transition across Europe. The collected cases are summarized in a PED Booklet whose update was recently published on the JPI Urban Europe website. Results presented in this paper provide insights from the analysis of 61 projects in Europe and offer recommendations for future PED developments.

The tip clearance has an important effect on the performance of an engine compressor. While the impact of tip clearance on a concentric compressor has been widely explored in previous research, the flow field distribution of an eccentric compressor has only been minimally explored. Both the steady and unsteady computational fluid dynamics (CFD) methods have been widely used in the studies of concentric axial-compressors, and they have similar simulation results in terms of flow field. However, they have been rarely applied to axial-compressors with non-uniform tip clearance to investigate their flow field. In this paper, ANSYS CFX is used as CFD software, and both steady and unsteady CFD methods are applied to study a single rotor of ROTOR67 to investigate the compressor characteristic line and flow field under different eccentricity conditions. The results show that non-uniform tip clearance creates a non-uniform flow field at the inlet and tip regions over the whole operating range.... [more]

Besides many benefits deriving from the energy transition process, it is not uncommon for modern power systems to be faced with difficulties in their operation. The issues are dominantly related to the non-dispatchable nature of renewable energy sources (RES) and the mismatching between electricity generation and load demand. As a consequence of a constant peak load growth, this problem is particularly pronounced during the daily peak hours. Therefore, it is of great importance to conduct all necessary activities within the system in order to preserve the system stability and continuity of operation. Energy storage systems have been recognized as a major facilitator of renewable energy, by providing additional operational flexibility. Since pumped hydroelectric energy storage (PHES) accounts for almost 97% of the world’s storage capacity, in this paper, we have investigated the benefits of using pumped-storage hydropower in modern power systems characterized by high penetration of RES... [more]

The article presents results of laboratory tests performed on samples of NOMEX®910 cellulose−aramid insulation impregnated with Nynas Nytro 10× inhibited insulating mineral oil using the polarization and depolarization current analysis method (PDC Method). In the course of the tests, the insulation samples were subjected to a process of accelerated thermal degradation of cellulose macromolecules, as well as weight-controlled dampening, thereby simulating the ageing processes occurring when using the insulation in power transformers. The effects of temperature in the ranges typical of normal transformer operation were also taken into account. On the basis of the obtained data, the activation energy was then fixed together with dominant time constants of cellulose−aramid insulation relaxation processes with respect to the temperature and degree of moisture, as well as thermal degradation of cellulose macromolecules. It was found that the greatest and predictable changes in the activation... [more]

Investigating characteristics for the secondary breakup of dropping liquid is a fundamental scientific and practical problem in multiphase flow. For its solving, it is necessary to consider the features of both the main hydrodynamic and secondary processes during spray granulation and vibration separation of heterogeneous systems. A significant difficulty in modeling the secondary breakup process is that in most technological processes, the breakup of droplets and bubbles occurs through the simultaneous action of several dispersion mechanisms. In this case, the existing mathematical models based on criterion equations do not allow establishing the change over time of the process’s main characteristics. Therefore, the present article aims to solve an urgent scientific and practical problem of studying the nonstationary process of the secondary breakup of liquid droplets under the condition of the vibrational impact of oscillatory elements. Methods of mathematical modeling were used to a... [more]

This paper develops a voltage modulated direct power control (VM-DPC) strategy of a doubly fed induction generator (DFIG) using extended power theory under unbalanced grid voltage conditions. By introducing the modulated voltage of the active and reactive powers and the non-linear VM controller, the proposed VM-DPC strategy enables the generator dynamics to change from the time-varying differential equations into the time-invariant ones. Then, in order to deal with the voltage unbalance, three available power feedbacks, a combination of real active power and extended reactive power, a combination of extended active power and real reactive power, a combination of extended active power and extended reactive power, are developed. Together with a conventional controller (PI + R and feedforwards) and a non-linear VM controller, the power feedbacks are well controlled to track their references with the achievement of the constant active power, the constant reactive power and the balanced sta... [more]

Motivated by the environmental challenges and the increase in energy demand, this review assesses the suitability of nuclear power production as an alternative option to using fossil fuels. First, we assess the competitiveness of nuclear power compared to other power sources considering its economic efficiency, environmental impact and implications for health, and conclude that this is a viable option to serve in addition to and as a backup to renewable sources. Second, we review previous findings in various fields on advantages and disadvantages of nuclear power technology and conclude that there is a gap between reality and perception. Third, we discuss challenges related to nuclear weapons proliferation and misperceived public opinion on nuclear power. We conclude that the gap between perception and reality stems from a lack of consolidated interdisciplinary view, media communications focusing mainly on unilateral assessments.

In this paper, the Greitzer surge model was systematically analysed with the model compressor duct length Lc as the tuning parameter. The surge phenomenon is known to induce a serious risk to centrifugal compressor operation. The two-dimensional Greitzer model is a well-established way of modelling this dangerous instability, but the determination and changes of the model parameters are still being discussed. In this paper an automated procedure determines the Lc value providing the best fit with the experimental data has been presented. The algorithm was tested on five valve positions and revealed that the best fit was obtained for different Lc values following a linear trend against the mass flow rate. The study has also shown that the Greitzer model has two solutions for a given pressure oscillation amplitude: one similar to the deep surge (low Lc) and one similar to the mild surge (low Lc). This suggests that this model can be used to simulate both types of the phenomenon known fro... [more]

In this study, a full-scale three-dimensional trapezoidal thermoelectric cooler model is constructed to study its cooling performance and mechanical reliability using finite element simulation. Temperature dependent material properties are considered in this work. The boundary conditions similar to those in a real experimental environment are applied. The effects of the input electrical current and geometry of the thermoelectric leg on the cooling performance and reliability of a trapezoidal thermoelectric cooler are analyzed, and a comparison is made with a rectangular thermoelectric cooler. The results indicate that increasing the leg height and the variable cross-sectional design of the leg can improve the cooling performance of the trapezoidal thermoelectric cooler. Compared to the original rectangular thermoelectric cooler, the minimum chip temperature was reduced by 0.87% under the trapezoidal thermoelectric cooler with optimized geometry. Furthermore, increasing the leg height e... [more]

It is important to understand the effect of increasing electric vehicles (EV) penetrations on the existing electricity transmission infrastructure and to find ways to mitigate it. While, the easiest solution is to opt for equipment upgrades, the potential for reducing overloading, in terms of voltage drops, and line loading by way of optimization of the locations at which EVs can charge, is significant. To investigate this, a heuristic optimization approach is proposed to optimize EV charging locations within one feeder, while minimizing nodal voltage drops, cable loading and overall cable losses. The optimization approach is compared to typical unoptimized results of a monte-carlo analysis. The results show a reduction in peak line loading in a typical benchmark 0.4 kV by up to 10%. Further results show an increase in voltage available at different nodes by up to 7 V in the worst case and 1.5 V on average. Optimization for a reduction in transmission losses shows insignificant savings... [more]

This paper evaluates the potential energy savings when switchable insulation systems (SIS) are applied to walls of residential buildings located in Belgium and other locations in Europe. The study considers two low-energy prototypical dwellings (an apartment and a detached house) that are representative of post-2010 constructions and renovations in Belgium. Using an 3R2C-based analysis tool, the performance of both dwellings is evaluated with static and dynamic wall insulation systems. First, the switchable insulating system is described along with its associated simple 2-step rule-based control strategy. Then the modeling strategy and simulation analysis tools are presented. In Belgium, it was found that SIS-integrated walls allow energy savings up to 3.7% for space heating and up to 98% for cooling. Moreover, it was found that to further reduce the energy consumption of SIS-integrated buildings in various European climates, thermal mass placement needs to be considered. By optimizing... [more]

In recent decades, the energy market around the world has been reshaped to accommodate the high penetration of renewable energy resources. Although renewable energy sources have brought various benefits, including low operation cost of wind and solar PV power plants, and reducing the environmental risks associated with the conventional power resources, they have imposed a wide range of difficulties in power system planning and operation. Naturally, classical optimal power flow (OPF) is a nonlinear problem. Integrating renewable energy resources with conventional thermal power generators escalates the difficulty of the OPF problem due to the uncertain and intermittent nature of these resources. To address the complexity associated with the process of the integration of renewable energy resources into the classical electric power systems, two probability distribution functions (Weibull and lognormal) are used to forecast the voltaic power output of wind and solar photovoltaic, respective... [more]

The paper is devoted to computer simulations of the distribution and time evolution of the temperature in a wooden house in summer. The goal of simulations was to assess the effect of covering walls inside the house with a PCM (phase change material) on its passive cooling, which prevents the undesired overheating of the house and provides the required thermal comfort for the occupants under warm summer days. Computer simulations were performed by the COMSOL Multiphysics software (COMSOL Inc., Stockholm, Sweden). A model of a house without the PCM coverage was compared with models of houses in which the PCM was located on all walls, except a floor, and on a wall opposite the window. Results of simulations proved that the wood wall thickness and PCMs location influence overheating the wooden house. Under studied conditions, the coverage of a wall opposite the window best eliminated extremes of the air temperature inside the house. The maximum temperature decrease was 3.9 °C (i.e., drop... [more]

The western region of China is rich in mineral resources. The vigorous development of mineral resources has exacerbated the environmental and safety problems in the region. One of the important links to solve this problem is to control the development laws and distribution characteristics of the overburdened cracks in the mining of this area. In this paper, the Xiashijie coal mine 3-2 coal seam and 4-2 coal seam are examples of repeated mining, and are examined as the background, through theoretical analysis to optimize the size of the coal pillars in the lower section, using the 3DEC numerical simulation experiment method and the rise of the cracks in the short-distance coal seam. Repeated mining monitoring and analysis of the development law are used to ascertain distribution characteristics of overburdened cracks caused by the repeated mining process of the working face. The results show that: (1) By establishing a mechanical model of the overlying strata structure under short-dista... [more]

Sustainable business models have become a phenomenon of global interest. Since the beginning of the 21st century, companies have sought to shift from traditional business models towards sustainable ones in order to leverage their sustainable performance and increase their competitive advantage. The aim of this paper is to provide an extensive overview of the academic research on sustainable business models with the focus on inquiry of main trends related to business and economics studies and identification of future research directions. This paper reviews existing literature on sustainable business models in order to map this research stream and identify the most substantial contributors to the advances of research in this field. It contributes to the existing body of knowledge on sustainable business models by: (1) describing the most influential and high-impact researchers/authors, journals, and papers; (2) highlighting main and most important research trends related to sustainable b... [more]

In self-healing grid systems, high utility in the use of greedy algorithms is observed. One of the most popular solutions is based on Prim’s algorithm. In the computation, the power grid is represented as a weighted graph. This paper presents a few possibilities of calculation of the numerical weight of a branch of the graph. The proposition of a modified edge weight calculation based on active power belongs to this group. The other solutions are novel subalgorithms bounded by real power, reactive power, and normalized factor. This factor is a mathematical combination of active and reactive power multiplied by influence coefficients. Requirements necessary for a power system are applied in the considered algorithms. Each of these proposed algorithms includes the power source limits, voltage level at busbars, and power system transmission features, such as transmission lines rated currents and power losses. All mentioned methods were compiled into separate algorithms, which can be used... [more]