This paper explores how water temperature changes in a district heating system (DHS) impact the monitoring of water losses. Water volume in DHS is constantly monitored, recorded, and replenished. The leakage and failure status of the DHS is often monitored through measuring the make-up water flow rate. In this paper, we present the methodology and a simplified model of the dynamics of the heating system operation, which was used to determine the profile of changes in the average temperature and density of water in the system. The mathematical model of the district heating network (DHN) was verified by comparing the results of simulation calculations, i.e., calculated values of the temperature of water returning to the heat source, with the measured values. Fluctuations in water temperature cause changes in the density and volume of water in the DHN, which affect the amount of water supplementing the system. This is particularly noticeable in a DHN with a large water volume. The study r... [more]

Offshore wind power has great development potential, for which the key factors are reliable and economical wind farms and integration systems. This paper proposes a medium-frequency wind farm and MMC-HVDC integration system. In the proposed scheme, the operating frequency of the offshore wind farm and its power collection system is increased from the conventional 50/60 Hz rate to the medium-frequency range, i.e., 100−400 Hz; the offshore wind power is transmitted to the onshore grid via the modular multilevel converter-based high-voltage direct current transmission (MMC-HVDC). First, this paper explains the principles of the proposed scheme in terms of the system topology and control strategy aspects. Then, the impacts of increasing the offshore system operating frequency on the main parameters of the offshore station are discussed. As the frequency increases, it is shown that the actual value of the electrical equipment, such as the transformers, the arm inductors, and the SM capacito... [more]

In horizontal wells, the casing string is affected by the gravity effect, temperature effect, swelling effect, bending effect, friction effect and other mechanical effects. In view of this situation, the mathematical models of casing swelling effect and temperature effect caused by volume fracturing are established. The case analysis shows that the length of the unsealed section in the vertical section has a great influence on the axial shortening of the casing during fracturing. With the increase of the unsealed section length, the axial shortening of the casing increases gradually under the same wellhead pressure. In the process of fracturing, repeated squeezing and pressurization lead to periodic changes of the wellhead pressure, casing deformation and load, which leads to fatigue damage and even fracture of casing. At the same time, a large amount of fracturing fluid is continuously injected through the casing during the fracturing process, which makes the wellbore temperature chan... [more]

The exact nature of thermal processes occurring in an electric motor is often unknown. Thus, the estimation of temperature rise using mathematical models and computational experiments is becoming increasingly important. Thermal analysis is the key design aspect, which has become significant in the design process for electric motors. The thermal analysis of electric motors can be helpful in developing effective thermal monitoring methods. This analysis is crucial for a better understanding of the overall performance and failure prevention for these electrical motors. In this paper, laboratory investigations of thermal processes in low-voltage asynchronous motors are described. The analysis of the results leads to the conclusion that the classic single-exponential models do not match the dynamically changing thermal processes in electric motors especially in the case of intermittent motor operation.

Wind turbine (WT) technology becomes more and more important due to the serious environmental and energy issues. The toothed poles outer rotor doubly salient permanent magnet (DSPM) generator with simple and durable design, high torque and high-power density has a great prospect in wind turbines application. The large diameter makes the construction of such a machine more convenient due to the installation of the turbine blades directly to the outer rotor generator surface. Nevertheless, the size of the generator must be increased to provide larger output power. This increases the generator’s mass. Thus, larger massive DSPM generators are undesirable in wind turbine design. In this paper, an optimization design procedure of the outer rotor doubly salient permanent magnet generator ORDSPMG is proposed for 10 kW WT application. The reduction of the generator weight is demonstrated and proofed. The considered machine version is characterized by having the same effective axial length and o... [more]

Here, we overview the Energies journal special issue that is dedicated to the topic of “Energy Efficiency in Smart Homes and Smart Grids” (https://www [...]

Power distribution networks are increasingly challenged by ageing plant, environmental extremes and previously unforeseen operational factors. The combination of high loading and weather conditions is responsible for large numbers of recurring faults in legacy plants which have an impact on service quality. Owing to their scale and dispersed nature, it is prohibitively expensive to intensively monitor distribution networks to capture the electrical context these disruptions occur in, making it difficult to forestall recurring faults. In this paper, localised weather data are shown to support fault prediction on distribution networks. Operational data are temporally aligned with meteorological observations to identify recurring fault causes with the potentially complex relation between them learned from historical fault records. Five years of data from a UK Distribution Network Operator is used to demonstrate the approach at both HV and LV distribution network levels with results showin... [more]

Mass Impregnated PPLP cable, which is applied to various high-voltage direct current (HVDC) projects due to its excellent dielectric and temperature properties, has a problem wherein voids are formed inside the butt-gap due to cavitation. However, there has been no previous research into technology for void defect identification and insulation diagnosis on HVDC MI-PPLP cables. In this paper, to propose an insulation diagnosis method for void defects in HVDC MI-PPLP cable, the direct current (DC) void discharge patterns were analyzed according to the specimen temperature and the magnitude of applied voltage using the pulse sequence analysis method. In addition, to confirm the pre-symptoms of dielectric breakdown in MI-PPLP cable due to DC void discharge, partial discharge patterns were analyzed continuously until dielectric breakdown occurred. From the experimental results, DC void discharge patterns of the same shape were obtained regardless of the specimen temperature and the magnitud... [more]

This paper proposes a galvanically isolated dc-dc converter that can regulate the input voltage in a wide range. It is based on the series resonance dc-dc converter (SRC) topology and a novel boost rectifier. The proposed topology has a smaller number of semiconductors than its SRC-based existing topologies employing an ac-switch in the boost rectifier. The proposed dc-dc converter comprises only two diodes and one switch at the output side, while the existing solutions use two switches and two diodes to step up the voltage. The proposed converter boosts the input voltage within a single boosting interval in the positive half-cycle of the switching period. In addition, the resonant current in the negative half-cycle is sinusoidal, which could enhance the converter efficiency. The resonant capacitor voltage is clamped at the level of the output voltage. Therefore, the voltage stress of the capacitor could significantly reduce at various input voltage and power levels. This makes it perf... [more]

Accuracy of hydrogeological and geotechnical investigation in place of shaft sinking is a key factor for selection of sinking method and design of the shaft lining. The following work presents the influence of the rising level of accuracy of geological data gathered in the area of shaft sinking in the Silesian Coal Basin and technical projects of shaft lining and technology of its sinking, which have been changing over the years. The initial project of the shaft was repeatedly modified. Each modification eventuated in rising requirements for the shaft lining, such as increasing its thickness or changing concrete class. It has become necessary to use additional methods of reinforcing rock mass around the shaft.

In this paper, a model-based predictive rotor field-oriented angle compensation approach is proposed for induction machine drives. Indirect rotor field-oriented control is widely used in induction machine drives for its simple implementation and low cost. However, the accuracy of the rotor field-oriented angle is affected by variable parameters such as the rotor resistance and inductance. An inaccurate rotor field-oriented angle leads to a degradation of the torque and dynamic performance, especially in the high-speed flux-weakening region. Therefore, the d-axis and q-axis currents in the rotation reference frame are predicted based on the model and compared with the feedback current to correct the rotor field-oriented angle. To improve the stability and robustness, the proposed predictive algorithm is based on the storage current, voltage, and velocity data. The algorithm can be easily realized in real-time. Finally, the simulated and experimental results verify the algorithm’s effect... [more]

The results of numerical investigations of heat transfer and pressure drops in a channel with 30° helical micro-fins are presented. The main aim of the analysis is to examine the influence of the height of the micro-fins on the heat-flow characteristics of the channel. For the tested pipe with a diameter of 12 mm, the micro-fin height varies within the range of 0.05−0.40 mm (with 0.05 mm steps), which is equal to 0.4−3.3% of its diameter. The analysis was performed for a turbulent flow, within the range of Reynolds numbers 10,000−100,000. The working fluid is water with an average temperature of 298 K. For each tested geometry, the characteristics of the friction factor f(Re) and the Nusselt number Nu(Re) are shown in the graphs. The highest values of Nusselt numbers and friction factors were obtained for pipes with the micro-fins H = 0.30 mm and H = 0.35 mm. A large discrepancy is observed in the friction factors f(Re) calculated from the theoretical relationships (for the irregular r... [more]

The introduction of stationary storage systems into the Italian electric network is necessary to accommodate the increasing share of energy from non-programmable renewable sources and to reach progressive decarbonization targets. In this framework, a life cycle assessment is a suitable tool to assess environmental impacts during the entire life cycle of stationary storage systems, i.e., their sustainability. A Li-ion battery (lithium−iron−phosphate (LFP), nickel−manganese−cobalt (NMC) 532, and NMC 622) entire life cycle assessment (LCA) based on primary and literature data was performed. The LCA results showed that energy consumption (predominantly during cell production), battery design (particularly binder choice), inventory accuracy, and data quality are key aspects that can strongly affect results. Regarding the battery construction phase, LFP batteries showed better performance than the NMC ones, but when the end-of-life (EoL) stage was included, NMC cell performance became very c... [more]

In 2016, a law was introduced in Poland that required management bodies of energy companies owned by the state and a municipality to adopt management goals. The remuneration of the board members of these companies depended on the extent to which these goals were implemented. The challenge was that profit became an end in itself. The purpose of this article is to describe the process used to set up strategic goals on the basis of balancing capital within energy companies, which demonstrates a positive impact on the effectiveness of these companies. The solution proposed in this article is an alternative to using profit as the sole measure. The article consists of three parts. The first part is a review of the literature on management goals and how they relate to various economic measures—the main conclusions of this literature review were that, despite many attempts, we were not able to (1) clearly define the purpose of a company and (2) find an indisputable measure of the degree to whi... [more]

The topic of the article considers the functioning of the renewable energy (RE) sector in Poland. This is really important in the context of the energy transition of the national economy because it influences the creation of modern technologies and increases the competitiveness and innovation of the country. Poland is in a process of energy transition where the RE sector has been developing for two decades. The authors aimed to research the RE sector improvement possibilities in Poland, including the influence of this sector on chosen social and economic aspects. Because of this research’s aim a critical situation assessment of RE in Poland was conducted and a survey of a group of experts in this field was also involved. Legal, physical and mental determinants and their influence on RE sector were looked into. In the legal determinant context a necessity to simplify relevant legislation acts in Poland was found. Undoubtedly there is a need to improve several legal acts, including the D... [more]

Energy communities (ECs) are becoming increasingly common entities in power distribution networks. To promote local consumption of renewable energy sources, governments are supporting members of ECs with strong incentives on shared electricity. This policy encourages investments in the residential sector for building retrofit interventions and technical equipment renovations. In this paper, a general EC is modeled as an energy hub, which is deemed as a multi-energy system where different energy carriers are converted or stored to meet the building energy needs. Following the standardized matrix modeling approach, this paper introduces a novel methodology that aims at jointly identifying both optimal investments (planning) and optimal management strategies (operation) to supply the EC’s energy demand in the most convenient way under the current economic framework and policies. Optimal planning and operating results of five refurbishment cases for a real multi-family building are found a... [more]

Thermal barrier coatings (TBCs) have been investigated both experimentally and through simulation for mixing controlled combustion (MCC) concepts as a method for reducing heat transfer losses and increasing cycle efficiency, but it is still a very active research area. Early studies were inconclusive, with different groups discovering obstacles to realizing the theoretical potential. Nuanced papers have shown that coating material properties, thickness, microstructure, and surface morphology/roughness all can impact the efficacy of the thermal barrier coating and must be accounted for. Adding to the complexities, a strong spatial and temporal heat flux inhomogeneity exists for mixing controlled combustion (diesel) imposed onto the surfaces from the impinging flame jets. In support of the United States Department of Energy SuperTruck II program goal to achieve 55% brake thermal efficiency on a heavy-duty diesel engines, this study sought to develop a deeper insight into the inhomogeneou... [more]

In recent years, the automotive sector has been focused on emission reductions using hybrid and electric vehicles. This was mainly caused by political trends promoting “green energy”. However, that does not mean that internal combustion engines (ICEs) should be forgotten. The ICE has still the potential of recovering energy from exhaust gases. One of the promising ways to recover energy is turbocharging. Over the years engine manufacturers have designed very efficient turbocharger systems which have greatly increased the overall engine efficiency. This led to pollutant emission reductions. This paper presents the results of the three-dimensional (3-D) numerical simulations of the two-stage, six-inlet turbocharging system under the influence of unsteady, pulsed-flow conditions. The calculations were carried out for three turbine speeds. The most interesting results of this study were the separation of exhaust gases coming from the six-exhaust pipes and the performance of both stages und... [more]

For decades of wind energy technology developments, much research on the subject has been carried out, and this has given rise to many works encompassing different topics related to it. As a logical consequence of such a research and editorial activity, state-of-the-art review works have also been published, reporting about a wide variety of research proposals. Review works are particularly interesting documents for researchers because they try to gather different research works on the same topic present their achievements to researchers. They act, in a way, as a guidance for researchers to quickly access the most meaningful works. The proposal of this paper consists of going one step further, and to present a review of state-of-the-art review works on wind-energy-related issues. A classification into several main topics in the field of energy research has been done, and review works that can be classified in all these areas have been searched, analyzed, and commented on throughout the... [more]

As renewable energy source integration increases, P2G technology that can store surplus renewable power as methane is expected to expand. The development of a CO2 methanation catalyst, one of the core processes of the P2G concept, is being actively conducted. In this work, low-rank coal (LRC) was used as a catalyst support for CO2 methanation, as it can potentially enhance the diffusion and adsorption behavior by easily controlling the pore structure and composition. It can also improve the process efficiency owing to its simplicity (no pre-reduction step) and high thermal conductivity, compared to conventional metal oxide-supported catalysts. A screening of single metals (Ni, Co, Ru, Rh, and Pd) on LRC was performed, which showed that Ni was the most active. When Ni on the LRC catalyst was doped with a promoter (Ce and Mg), the CO2 conversion percentage increased by >10% compared to that of the single Ni catalyst. When the CO2 methanation activity was compared at 250−500 °C, the Ce-do... [more]

A rotating detonation rocket engine (RDRE) with various convergent nozzles and chamber lengths is investigated. Three hundred hot-fire tests are performed using methane and oxygen ranging from equivalence ratio equaling 0.5−2.5 and total propellant flow up to 0.680 kg/s. For the full-length (76.2 mm) chamber study, three nozzles at contraction ratios ϵc = 1.23, 1.62 and 2.40 are tested. Detonation is exhibited for each geometry at equivalent conditions, with only fuel-rich operability slightly increased for the ϵc = 1.62 and 2.40 nozzles. Despite this, counter-propagation, i.e., opposing wave sets, becomes prevalent with increasing constriction. This is accompanied by higher number of waves, lower wave speed Uwv and higher unsteadiness. Therefore, the most constricted nozzle always has the lowest Uwv. In contrast, engine performance increases with constriction, where thrust and specific impulse linearly increase with ϵc for equivalent conditions, with a 27% maximum increase. Additional... [more]

Solid Oxide Fuel Cells are a promising technology for Solid Oxide Fuel Cells (SOFC) are a promising technology For high-efficiency electrochemical conversion of a vast range of fuel gas mixtures, thigh operating temperature conditions (650−900 °C) represent a challenge both at system level and at laboratory testing level, in terms of material properties and performance dynamics. In this work a detailed procedural analysis is presented for an innovative all-ceramic compact SOFC test rig and first experimental testing results are reported in terms of polarization curves obtained under parametric variation of operating conditions (H2 content, air ratio λ and temperature) and short-term voltage stability test under load (140 h at 0.3 A/cm2). The electrochemical characterization results confirm the validity of the used all-ceramic cell holder, showing excellent cell performances in terms of polarization. H2 content has the most impact on SOFC performance, followed by temperature and finally... [more]

The paper proposes a novel framework for state observer design, in which learning-based observers are incorporated. The aim of the method is to provide a framework, which is able to guarantee the limitation of the observation error, even if the error of the learning-based observer under all scenarios cannot be verified. The framework is based on the robust H∞ design method, which is able to provide guarantees on the resulted observer. Moreover, the observer design process is extended with a controller design, which leads to a joint robust H∞ controller-observer design. In this paper the proposed method is applied on a vehicle control problem, such as lateral path following. In this problem the goal of the observer is to provide an accurate lateral velocity signal for the vehicle, which is used in the controlled system for the generation of front wheel steering angle. The effectiveness of the method is illustrated through simulation examples on high-fidelity vehicle dynamic simulator Ca... [more]

This study assesses the impact of economic and non-economic incentives to induce demand response in private households. The experiment was realized by a three-months residential phase in which two tenants lived in the Energy Smart Home Lab, an experimental lab with the equipment of a modern smart home. The tenants received calls to action (CtAs) on a regular basis, incentivized economically or by moral nudges with a social or environmental background. A mixed-methods approach, consisting of smart meter data analysis, a value scale assessment, surveys and interviews, assessed the tenants’ reactions on their energy consumption behavior towards the CtAs. The smart meter data shows that the tenants performed the majority of CtAs, revealing no significant difference between economic or non-economic incentives. Results from the value scale, the interviews and the surveys indicate that this behavior might be due to the tenants’ high tendency towards biospheric and altruistic values and a high... [more]

In the case of developing countries, it is not clear which crisis management tools will ensure sustainable development in a sustainable and environmentally friendly way, as well as reducing CO2 emissions in addition to ensuring GDP growth. The next analysis discusses the details of this issue. The study explores the connection between per capita GDP, emission of CO2, combustible energy, and waste consumption. The Hausman test ratifies that the regression model with the fixed effect is the proper method for the panel balanced data from 1990 to 2019 in the selected 13 countries of the EU. This study ordered the data into three categories (for 13 selected countries, the top nine EU countries (in GDP), and Visegrad countries (Hungary, Slovakia, Czech Republic and Poland)). The study found a significant positive effect of combustible energy and waste consumption and the negative impact of CO2 emission on GDP per capita. The cointegration test confirms that all three variables are cointegrat... [more]