Several high voltage condenser type OIP (oil impregnated paper) bushings used in the electrical industry are filled with dodecylbenzene, because of its ability to absorb hydrogen formed by corona partial discharges in the thick paper insulation of these pieces of equipment. Some of them form large quantities of ethane, raising the concern of overheating faults in their paper insulation, which may be risky for their safe operation in service. The article presents dissolved gas analysis results of oil samples taken from the bushings with high ethane formation, together with results of laboratory tests of stray gassing of dodecylbenzene performed according to CIGRE procedure. By using Duval Pentagon 2 it is possible to compare patterns in the laboratory and in bushings and evaluate the temperature range of possible defects. Stray gassing/overheating of dodecylbenzene in bushings within the stray gassing temperature range and whatever the possible other causes, is not a concern for their s... [more]

Enhancing forecasting performance in terms of both the expected mean value and variance has been a critical challenging issue for energy industry. In this paper, the novel methodology of finite mixture Generalized AutoRegressive Conditional Heteroskedasticity (GARCH) approach with Expectation−Maximization (EM) algorithm is introduced. The applicability of this methodology is comprehensively evaluated for the forecasting of energy related time series including wind speed, wind power generation, and electricity price. Its forecasting performances are evaluated by various criteria, and also compared with those of the conventional AutoRegressive Moving-Average (ARMA) model and the less conventional ARMA-GARCH model. It is found that the proposed mixture GARCH model outperforms the other two models in terms of volatility modeling for all the energy related time series considered. This is proven to be statistically significant because the p-values of likelihood ratio test are less than 0.000... [more]

Deterministic load flow analyses of power grids do not include the uncertain factors that affect the network elements; hence, their predictions can be very unreliable for distribution system operators and for the decision makers who deal with the expansion planning of the power network. Adding uncertain probability parameters in the deterministic load flow is vital to capture the wide variability of the currents and voltages. This is achieved by probabilistic load flow studies. Photovoltaic systems represent a remarkable source of uncertainty in the distribution network. In this study, we used a Gaussian copula to model the uncertainty in correlated photovoltaic generators. Correlations among photovoltaic generators were also included by exploiting the Gaussian copula technique. The large sets of samples generated with a statistical method (Gaussian copula) were used as the inputs for Monte Carlo simulations. The proposed methodologies were tested on two different networks, i.e., the 1... [more]

A casing treatment using inclined oblique slots (INOS) is proposed to improve the stability of the single-stage transonic axial compressor, NASA Stage 37, during operation. The slots are installed on the casing of the rotor blades. The aerodynamic performance was estimated using three-dimensional steady Reynolds-Averaged Navier-Stokes analysis. The results showed that the slots effectively increased the stall margin of the compressor with slight reductions in the pressure ratio and adiabatic efficiency. Three geometric parameters were tested in a parametric study. A single-objective optimization to maximize the stall margin was carried out using a Genetic Algorithm coupled with a surrogate model created by a radial basis neural network. The optimized design increased the stall margin by 37.1% compared to that of the smooth casing with little impacts on the efficiency and pressure ratio.

When using the explicit dynamic relaxation approach (DRA) to model the quasi-static rock breakage, fragmentation, and flow problems, especially the top-coal caving question, introducing numerical damping into the solution equation is inevitable for reducing the vibration frequency and impact speed of mesh nodes, which is significantly affect the fidelity of the computation results. Although the DRA has been widely adopted to simulate top-coal caving, the reasonable value and calibration method of numerical damping are still open issues. In this study, the calibration process of reasonable numerical damping for modeling top-coal caving is investigated by comparing with the experimental results, in which several geometry parameters of the drawing funnel are selected as the calibration indexes. The result shows that the most reasonable numerical damping value is 0.07 for the numerical modeling of interval top-coal caving in extra-thick coal seams. Finally, the correlation between the nume... [more]

Krakow has an extensive district heating network, which is approximately 900 km long. It is the second largest city in terms of the number of inhabitants in Poland, resulting in a high demand for energy—for both heating and cooling. The district heating of the city is based on coal. The paper presents the conception of using the available renewable sources to integrate them into the city’s heating system, increasing the flexibility of the system and its decentralization. An innovative solution of the use of hybrid, modular heat pumps with power dependent on the needs of customers in a given location and combining them with geothermal waters and photovoltaics is presented. The potential of deep geothermal waters is based on two reservoirs built of carbonate rocks, namely Devonian and Upper Jurassic, which mainly consist of dolomite and limestone. The theoretical potential of water intake equal to the nominal heating capacity of a geothermal installation is estimated at 3.3 and 2.0 MW, r... [more]

It is widely accepted that the market uptake of electric vehicles is essential for the decarbonisation of transport. However, scaling up the roll out of electric vehicles (EV) is challenging considering the lack of charging infrastructure. The latter is, currently, developing in an uneven way across the EU countries. A charging infrastructure with wide coverage addresses range limitations but requires high investment with uncertain returns during the early years of deployment. The aim of this paper is to assess how different policy options affect EV penetration and the involvement of private sector in infrastructure deployment. We propose a mathematical programming model of the decision problem and the interaction between the actors of EV charging ecosystem and apply it to the case of Greece from the time period until 2030. Greece represents a typical example of a country with ambitious targets for EV penetration by 2030 (10% of the total stock) with limited effort made until now. The... [more]

This paper presents an experimental investigation on photovoltaic array (PV array) power output affected by partial shading conditions (PSCs). An experiment setup of a PV array with a series configuration using 2 × 4 photovoltaic modules (PV modules) was built. The power output loss due to the shading effect on the first photovoltaic cells (PV cell) connected with bypass diodes of each photovoltaic module, installed in the PV array in the horizontal direction, was evaluated. Depending on the direction of the sun relative to the PV array configuration, the shading percentage was measured during the test and recorded the current and voltage of the PV array. The performance evaluation of the PV array configurations is referred to with respect to the values of maximum power voltage, the maximum power current, maximum power output, power output losses and fill factor (FF). The experimental results show that 44% shading of the first PV cells affects PV array power output loss by more than 80... [more]

This paper presents a finite element method (FEM)-based model, which describes the magnetic circuit of the BMW i3 traction machine. The model has been reconstructed based on data available in the public domain. The reader is provided with numerical data regarding flux linkage surfaces in d- and q-axes, as well as with all the information needed to develop a space-vector model of the machine in steady-state, taking into consideration the non-linearity of the magnetic circuit. Hence, the data of a highly-saturated machine from a renowned product are provided, which can serve as a reference design for research. After that, torque curve and partial load operation points are calculated. Finally, the machine model is linearized and the calculations are repeated with the simplified linearized model. The results from both models are then compared with each other. This comparison is intended to assess the magnitude of the expected inaccuracies, when simplified analytical tools are applied to hi... [more]

Utilization of natural shale formations for the creation of annular barriers in oil and gas wells is currently discussed as a mean of simplifying cumbersome plugging and abandonment procedures. Shales that are likely to form annular barriers are shales with high content of swelling clays and relatively low content of cementation material (e.g., quartz, carbonates). Shales with large content of quartz and low content of swelling clays will be rather brittle and not easily deformable. In this paper we ask the question whether and to what extent it is possible to modify the mechanical properties of relatively brittle shales by chemically removing some cementation material. To answer this question, we have leached out carbonates from Pierre I shale matrix using hydrochloric acid and we have compared mechanical properties of shale before and after leaching. We have also followed leaching dynamics using X-ray tomography. The results show that removal of around 4−5 wt% of cementation material... [more]

Offshore wind farms are increasingly built in the North Sea and the number of HVDC systems transmitting the wind power to shore increases as well. To connect offshore wind farms to adjacent AC transmission systems, onshore and offshore modular multilevel converters transform the transmitted power from AC to DC and vice versa. Additionally, modern wind farms mainly use wind turbines connected to the offshore point of common coupling via voltage source converters. However, converters and their control systems can cause unwanted interactions, referred to as converter-driven stability problems. The resulting instabilities can be predicted by applying an impedance-based analysis in the frequency domain. Considering that the converter models and system data are often confidential and cannot be exchanged in real systems, this paper proposes an enhanced impedance measurement method suitable for black-box applications to investigate the interactions. A frequency response analysis identifies cou... [more]

Transportation, as one of the most growing industries, is problematic due to environmental pollution. A solution to reduce the environmental burden is stricter emission standards and homologation tests that correspond to the actual conditions of vehicle use. Another solution is the widespread introduction of hybrid vehicles—especially the plug-in type. Due to exhaust emission tests in RDE (real driving emissions) tests, it is possible to determine the real ecological aspects of these vehicles. The authors of this paper used RDE testing of the exhaust emissions of plug-in hybrid vehicles and on this basis evaluated various hybrid vehicles from an ecological point of view. An innovative solution proposed by the authors is to define classes of plug-in hybrid vehicles (classes from A to C) due to exhaust emissions. An innovative way is to determine the extreme results of exhaust gas emission within the range of acceptable scatter of the obtained results. By valuating vehicles, it will be p... [more]

Wearable sensors to monitor vital health are becoming increasingly popular both in our daily lives and in medical diagnostics. The human body being a huge source of thermal energy makes it interesting to harvest this energy to power such wearables. Thermoelectric devices are capable of converting the abundantly available body heat into useful electrical energy using the Seebeck effect. However, high thermal resistance between the skin and the device leads to low-temperature gradients (2−10 K), making it difficult to generate useful power by this device. This study focuses on the design optimization of the micro-thermoelectric generator for such low-temperature applications and investigates the role of structural geometries in enhancing the overall power output. Electroplated p-type bismuth antimony telluride (BiSbTe) and n-type copper telluride (CuTe) materials’ properties are used in this study. All the simulations and design optimizations were completed following microfabrication con... [more]

The research explores the potential of digital-twin-based methods and approaches aimed at achieving an intelligent optimization and automation system for energy management of a residential district through the use of three-dimensional data model integrated with Internet of Things, artificial intelligence and machine learning. The case study is focused on Rinascimento III in Rome, an area consisting of 16 eight-floor buildings with 216 apartment units powered by 70% of self-renewable energy. The combined use of integrated dynamic analysis algorithms has allowed the evaluation of different scenarios of energy efficiency intervention aimed at achieving a virtuous energy management of the complex, keeping the actual internal comfort and climate conditions. Meanwhile, the objective is also to plan and deploy a cost-effective IT (information technology) infrastructure able to provide reliable data using edge-computing paradigm. Therefore, the developed methodology led to the evaluation of th... [more]

In this paper, the effectiveness of lightning overvoltage protection of cables in high voltage overhead cable lines has been analyzed. Because of the high overvoltage level, the cables are protected by surge arresters and by metallic sheath earthing. However, in practice, quite a lot of cases of electricity-evoked damage to the cable outer sheaths are observed, proving that the effectiveness of the protection used is insufficient. As a result, the cables are exposed to environmental factors, especially moisture penetration, which contributes to cable degradation. To explain the causes of this situation, simulation studies were carried out to determine the relevant factors affecting the level of expected overvoltages. The circuit-field model of the overhead cable line in EMTP-ATP, COMSOL and MATLAB software was used for determining overvoltages on the main cable insulation and the outer protective sheath. The studies reveal that the efficiency of the cable insulation overvoltage protect... [more]

Interoperability testing and analysis tools provide a means for achieving and assuring the integrity of multivendor intelligent electronic devices (IEDs) data exchanges. However, the testing and analysis are very time consuming and error prone, and these problems worsen when a substation becomes large and complex during the engineering process, commission, replacement, maintenance, and extension. To address this challenge, this paper presents a virtual digital substation test system (VDSTS) with interoperability analysis tools for assessing and identifying the engineering challenges for the multiple-vendors digital substation. This VDSTS consists of three parts: (i) A virtual digital substation modelling for generating real-time digital substation primary plant operation and fault conditions, (ii) a standard IEC 61850-based substation protection, automation, and control (PAC) system architecture with multivendor IEDs and bay solutions, and (iii) multivendor Substation Configuration des... [more]

Across the world, the impact of increasing electric vehicle (EV) adoption requires a better understanding. The authors hypothesize that the introduction of EV’s will cause significant overloading within low voltage distribution grids. To study this, several low voltage networks were reconstructed based on the literature and modelled using DigSilent Powerfactory, taking into account the stochastic variability of household electricity consumption, EV usage, and solar irradiance. The study incorporates two distinct usage scenarios—residential loads with varying EV penetrations without and with distributed grid tied generation of electricity. The Monte-Carlo simulation took into account population demographics and showed that in urban networks, EV introduction could lead to higher cable loading percentages than allowed, and in rural networks, this could lead to voltage drops beyond the allowed limits. Distributed generation (DG) in the form of solar power could significantly offset both th... [more]

Ocean thermal energy conversion (OTEC) uses a very simple process to convert the thermal energy stored mainly in tropical oceans into electricity. In designs, operations, and evaluations, we need to consider the unique characteristics of OTEC to achieve the best performance or lower the electricity cost of projects. The concept and design constraints of OTEC power generation differ from those of conventional thermal power plants due to the utilization of a low temperature difference. This research theoretically recognizes the unique characteristics of the energy conversion system and summarizes the appropriate performance evaluation methods for OTEC based on finite-time thermodynamics and the equilibrium condition of the heat source. In addition, it presents the concept of normalization of thermal efficiency for OTEC and exergy efficiency based on the available thermal energy in the ocean defined as the transferable thermal energy from the ocean and the equilibrium condition as the dea... [more]

The global demand for electricity is rising due to the increased electrification of multiple sectors of economic activity and an increased focus on sustainable consumption. Simultaneously, the share of cleaner electricity generated by transient, renewable sources such as wind and solar energy is increasing. This has made additional buffer capacities for electrical grids necessary. Battery energy storage systems have been investigated as storage solutions due to their responsiveness, efficiency, and scalability. Storage systems based on the second use of discarded electric vehicle batteries have been identified as cost-efficient and sustainable alternatives to first use battery storage systems. Large quantities of such batteries with a variety of capacities and chemistries are expected to be available in the future, as electric vehicles are more widely adopted. These batteries usually still possess about 80% of their initial capacity and can be used in storage solutions for high-energy... [more]

District energy systems, especially those integrating renewables or low exergy sources, have multiple elements for generating heating and cooling. Some of these elements might be used for both purposes: heating and cooling, either simultaneously or alternatively. This makes it more complex to separate the assessment and have a clear picture on performance of cooling service on one side, and heating services on the other, in terms of energy, environmental, and economic results. However, a correct comparison between different district energy configurations or among district energy and conventional solutions requires split assessment of each service. The paper presents a methodology for calculating different district heating and cooling system key performance indicators (KPIs), distinguishing between heating and cooling ones. A total of eleven indicators are organized under four categories: energy, environment, economy and socio-economy. Each KPI is defined for heating service and for coo... [more]

A mathematical model of the vaporization process in the coil is developed, taking into account the experimental data. To investigate and visualize the evaporation procedure in the coil, a mathematical pattern of the vapor-liquid mixture motion is compiled and reproduced. In the methodology of the study of the movement of the steam-water mixture, correction coefficients are proposed for calculating the velocities of the coolant in non-standard coaxial coils. The parameters were calculated using data sensitivity analysis and data validation was performed by repeated tests; uncertainty was detected when using the instruments, as well as the total extended uncertainty, the upper and lower limit of uncertainty for each measured parameter. In addition, as part of the steam generator set, solar collectors operate in the summer mode. Using the example of the studied steam generator operating in the conditions of an oil and gas field in the subarctic climate, it is shown that it is possible to... [more]

Different prediction models (multiple linear regression, vector support machines, artificial neural networks and random forests) are applied to model the monthly global irradiation (MGI) from different input variables (latitude, longitude and altitude of meteorological station, month, average temperatures, among others) of different areas of Galicia (Spain). The models were trained, validated and queried using data from three stations, and each best model was checked in two independent stations. The results obtained confirmed that the best methodology is the ANN model which presents the lowest RMSE value in the validation and querying phases 1226 kJ/(m2∙day) and 1136 kJ/(m2∙day), respectively, and predict conveniently for independent stations, 2013 kJ/(m2∙day) and 2094 kJ/(m2∙day), respectively. Given the good results obtained, it is convenient to continue with the design of artificial neural networks applied to the analysis of monthly global irradiation.

This paper presents a comparative analysis of six sampling techniques to identify an efficient and accurate sampling technique to be applied to probabilistic voltage stability assessment in large-scale power systems. In this study, six different sampling techniques are investigated and compared to each other in terms of their accuracy and efficiency, including Monte Carlo (MC), three versions of Quasi-Monte Carlo (QMC), i.e., Sobol, Halton, and Latin Hypercube, Markov Chain MC (MCMC), and importance sampling (IS) technique, to evaluate their suitability for application with probabilistic voltage stability analysis in large-scale uncertain power systems. The coefficient of determination (R2) and root mean square error (RMSE) are calculated to measure the accuracy and the efficiency of the sampling techniques compared to each other. All the six sampling techniques provide more than 99% accuracy by producing a large number of wind speed random samples (8760 samples). In terms of efficienc... [more]

The paper presents a developed methodology of short-term forecasting for heat production in combined heat and power (CHP) plants using a big data-driven model. An accurate prediction of an hourly heat load in the day-ahead horizon allows a better planning and optimization of energy and heat production by cogeneration units. The method of training and testing the predictive model with the use of generalized additive model (GAM) was developed and presented. The weather data as an input variables of the model were discussed to show the impact of weather conditions on the quality of predicted heat load. The new approach focuses on an application of the moving window with the learning data set from the last several days in order to adaptively train the model. The influence of the training window size on the accuracy of forecasts was evaluated. Different versions of the model, depending on the set of input variables and GAM parameters were compared. The results presented in the paper were ob... [more]

The article presents the methodology of isolating aperiodic phenomena constituting the basis of the energy balance of vehicles for the analysis of electromobility system indicators. The symptom observation matrix (SOM) and experimental input data are used to analyze periodic phenomena symptoms. The multidimensional nature of the engine efficiency shortage has been well defined and analyzed in terms of errors in the general model using neural networks, singular value decomposition, and principal component analysis. A more difficult task is the analysis of a multidimensional decision-making process. The research used a data fusion method and the concept of symptom reliability, which is applied to the generalized failure symptom obtained by applying the singular value decomposition (SVD). The model research has been based on the gray system theory (GST) and GM forecasting models (1,1). Input data were obtained from the assessment of driving cycles and analysis of the failure frequency for... [more]