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Energies, Volume 14, Issue 8 (April-2 2021) – 310 articles

Cover Story (view full-size image): The purpose of this study was to accurately predict the mutual interference of wind turbine wakes, especially in a large-scale wind farm above the ocean, consisting of multiple wind turbine groups. For this purpose, we proposed the CFD porous disk wake model as an intermediate method between engineering wake models and CFD wake models, including actuator line model. Targeting the vertical wind speed distribution in the near-wake region acquired in the lidar, we tuned the parameters of the CFD porous disk wake model. The CFD porous disk wake model applied to the wind-turbine-swept area was found to be very effective. View this paper.
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Article
Investigating the Converter-Driven Stability of an Offshore HVDC System
Energies 2021, 14(8), 2341; https://doi.org/10.3390/en14082341 - 20 Apr 2021
Cited by 2 | Viewed by 981
Abstract
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 [...] Read 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 coupling currents depending on the control system. The currents are subsequently added to the impedance models to achieve higher accuracy. The proposed method is applied to assess an offshore HVDC system’s converter-driven stability, using impedance measurements of laboratory converters and a wind turbine converter controller replica. The results show that the onshore modular multilevel converter interacts with AC grids of moderate short-circuit ratios. However, no interactions are identified between the offshore converter and the connected wind farm. Full article
(This article belongs to the Special Issue Hybrid AC/DC Grid)
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Article
Plug-in Hybrid Ecological Category in Real Driving Emissions
Energies 2021, 14(8), 2340; https://doi.org/10.3390/en14082340 - 20 Apr 2021
Cited by 5 | Viewed by 631
Abstract
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 [...] Read 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 possible in the future to determine the guidelines useful in designing more environmentally friendly power units in plug-in hybrid vehicles. Full article
(This article belongs to the Special Issue Energy Transfer in Alternative Vehicles)
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Article
Structural Design Optimization of Micro-Thermoelectric Generator for Wearable Biomedical Devices
Energies 2021, 14(8), 2339; https://doi.org/10.3390/en14082339 - 20 Apr 2021
Cited by 4 | Viewed by 1097
Abstract
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 [...] Read 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 constraints along with realistic temperature gradient scenarios. A series of structural optimizations were performed including the thermoelectric pillar geometries, interconnect contact material layers and fill factor of the overall device. The optimized structural design of the micro-thermoelectric device footprint of 4.5 × 3.5 mm2, with 240 thermoelectric leg pairs, showcased a maximum power output of 0.796 mW and 3.18 mW when subjected to the low-temperature gradient of 5 K and 10 K, respectively. These output power values have high potential to pave the way of realizing future wearable devices. Full article
(This article belongs to the Topic Thermoelectric Energy Harvesting)
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Article
Cyber-Physical Systems Improving Building Energy Management: Digital Twin and Artificial Intelligence
Energies 2021, 14(8), 2338; https://doi.org/10.3390/en14082338 - 20 Apr 2021
Cited by 10 | Viewed by 1924
Abstract
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. [...] Read 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 the effectiveness and efficiency of integrative systems for renewable energy production from solar energy necessary to raise the threshold of self-produced energy, meeting the nZEB (near zero energy buildings) requirements. Full article
(This article belongs to the Special Issue Open Data and Models for Energy and Environment)
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Article
Virtual Digital Substation Test System and Interoperability Assessments
Energies 2021, 14(8), 2337; https://doi.org/10.3390/en14082337 - 20 Apr 2021
Cited by 1 | Viewed by 1059
Abstract
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 [...] Read 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 description Language (SCL) tools and in-house built data visualisation tool. The study focuses on the interoperability testing of sampled values (SV), generic object-oriented substation events (GOOSE), and manufacturing message specification (MMS) communication services, as defined in IEC 61850. The main issues identified are compatibility issues of SCL tools, protocol implementation issues, different information models, and application limitations. The outcomes will help utilities to reduce the risks associated with the general rollout of digital substations. Full article
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Article
Performance Evaluation Concept for Ocean Thermal Energy Conversion toward Standardization and Intelligent Design
Energies 2021, 14(8), 2336; https://doi.org/10.3390/en14082336 - 20 Apr 2021
Cited by 4 | Viewed by 861
Abstract
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 [...] Read 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 dead state for exergy. The differences between conventional thermal efficiency and the effectiveness of the evaluation methods are visualized using the various reference design data, and it is ascertained that there is no clear relation between the conventional thermal efficiency and exergy efficiency, whereas the normalized thermal efficiency is definitely proportional to the exergy efficiency. Moreover, the exergy efficiency shows the effectiveness of the staging Rankine, Kalina, and Uehara cycles. Therefore, the normalized thermal efficiency and the exergy efficiency are important to analyze the heat and mass balance as well as improvement of the system. Full article
(This article belongs to the Special Issue Selected Papers from The 8th International OTEC Symposium)
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Article
Stationary, Second Use Battery Energy Storage Systems and Their Applications: A Research Review
Energies 2021, 14(8), 2335; https://doi.org/10.3390/en14082335 - 20 Apr 2021
Cited by 6 | Viewed by 1677
Abstract
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 [...] Read 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 as well as high-power applications, and even hybrid solutions encompassing both. There is, however, no holistic review of current research on this topic. This paper first identifies the potential applications for second use battery energy storage systems making use of decommissioned electric vehicle batteries and the resulting sustainability gains. Subsequently, it reviews ongoing research on second use battery energy storage systems within Europe and compares it to similar activities outside Europe. This review indicates that research in Europe focuses mostly on “behind-the-meter” applications such as minimising the export of self-generated electricity. Asian countries, especially China, use spent batteries for stationary as well as for mobile applications. In developing countries, off-grid applications dominate. Furthermore, the paper identifies economic, environmental, technological, and regulatory obstacles to the incorporation of repurposed batteries in second use battery energy storage systems and lists the developments needed to allow their future uptake. This review thus outlines the technological state-of-the-art and identifies areas of future research on second use battery energy storage systems. Full article
(This article belongs to the Section D1: Advanced Energy Materials)
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Article
Performance Assessment of District Energy Systems with Common Elements for Heating and Cooling
Energies 2021, 14(8), 2334; https://doi.org/10.3390/en14082334 - 20 Apr 2021
Viewed by 613
Abstract
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 [...] Read 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 cooling service. According to this, the methodology proposes a demand-based and an investment-based share factors that facilitate the heating and cooling KPI calculation. Full article
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Article
Development of Methodological Bases of the Processes of Steam Formation in Coil Type Boilers Using Solar Concentrators
Energies 2021, 14(8), 2333; https://doi.org/10.3390/en14082333 - 20 Apr 2021
Cited by 1 | Viewed by 507
Abstract
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 [...] Read 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 use air-type solar collectors for the ventilation system of the production room, as well as water-heating solar collectors for technical systems of hot water supply and chemical water treatment. Full article
(This article belongs to the Special Issue Solar PV, Thermal, Concentrator and Hybrid Power Systems)
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Article
Modelling and Prediction of Monthly Global Irradiation Using Different Prediction Models
Energies 2021, 14(8), 2332; https://doi.org/10.3390/en14082332 - 20 Apr 2021
Cited by 3 | Viewed by 716
Abstract
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 [...] Read 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. Full article
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Article
Forecasting of Heat Production in Combined Heat and Power Plants Using Generalized Additive Models
Energies 2021, 14(8), 2331; https://doi.org/10.3390/en14082331 - 20 Apr 2021
Cited by 3 | Viewed by 759
Abstract
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 [...] Read 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 obtained using a data coming from the real district heating system of a European city. The accuracy of the methods during the different periods of heating season was performed by comparing heat demand forecasts with actual values, coming from a measuring system located in the case study CHP plant. As a result, a model with an averaged percentage error for the analyzed period (November–March) of less than 7% was obtained. Full article
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Article
Simulating the Impacts of Uncontrolled Electric Vehicle Charging in Low Voltage Grids
Energies 2021, 14(8), 2330; https://doi.org/10.3390/en14082330 - 20 Apr 2021
Viewed by 623
Abstract
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 [...] Read 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 these overloading characteristics, as well as the active and reactive power demands of the network, by between 10–50%, depending on the topology of the network. Full article
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Article
Study of the Lightning Overvoltage Protection Effectiveness of High Voltage Mixed Overhead Cable Power Lines
Energies 2021, 14(8), 2329; https://doi.org/10.3390/en14082329 - 20 Apr 2021
Viewed by 807
Abstract
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 [...] Read 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 protection is ensured regardless of the lightning strike location and the crest value of its current. However, the obtained results confirm that no matter the applied protection, the cable outer sheaths may be exposed to overvoltages with higher values than those of the main insulation. Although the analysis was performed for 110 kV lines, the conclusions are general and are also applicable to power lines with higher rated voltages. Full article
(This article belongs to the Section A: Electrical Engineering)
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Article
Identification of Efficient Sampling Techniques for Probabilistic Voltage Stability Analysis of Renewable-Rich Power Systems
Energies 2021, 14(8), 2328; https://doi.org/10.3390/en14082328 - 20 Apr 2021
Cited by 6 | Viewed by 734
Abstract
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 [...] Read more.
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 efficiency, on the other hand, the three versions of QMC are the most efficient sampling techniques, providing more than 96% accuracy with only a small number of generated samples (150 samples) compared to other techniques. Full article
(This article belongs to the Special Issue Voltage Stability Analysis in Power Systems)
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Article
The Use of a Real-Time Simulator for Analysis of Power Grid Operation States with a Wind Turbine
Energies 2021, 14(8), 2327; https://doi.org/10.3390/en14082327 - 20 Apr 2021
Cited by 3 | Viewed by 679
Abstract
The main issue in this paper is the real-time simulator of a part of a power grid with a wind turbine. The simulator is constructed on the basis of a classic PC running under a classic operating system. The proposed solution is expected [...] Read more.
The main issue in this paper is the real-time simulator of a part of a power grid with a wind turbine. The simulator is constructed on the basis of a classic PC running under a classic operating system. The proposed solution is expected and desired by users who intend to manage power microgrids as separate (but not autonomous) areas of common national power systems. The main reason for the decreased interest in real-time simulators solutions built on the basis of PC is the simulation instability. The instability of the simulation is due to not keeping with accurate results when using small integration steps and loss of accuracy or loss of stability when using large integration steps. The second obstacle was due to the lack of a method for integrating differential equations, which gives accurate results with a large integration step. This is the scientific problem that is solved in this paper. A new solution is the use of a new method for integrating differential equations based on average voltage in the integration step (AVIS). This paper shows that the applied AVIS method, compared to other methods proposed in the literature (in the context of real-time simulators), allows to maintain simulation stability and accurate results with the use of large integration steps. A new (in the context of the application of the AVIS method) mathematical model of a power transformer is described in detail, taking into account the nonlinearity of the magnetization characteristics. This model, together with the new doubly-fed induction machine model (described in the authors’ previous article), was implemented in PC-based hardware. In this paper, we present the results of research on the operation states of such a developed real-time simulator over a long period (one week). In this way, the effectiveness of the operation of the real-time simulator proposed in the paper was proved. Full article
(This article belongs to the Special Issue Power System Dynamics and Renewable Energy Integration)
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Article
The Role of Flexibility in Photovoltaic and Battery Optimal Sizing towards a Decarbonized Residential Sector
Energies 2021, 14(8), 2326; https://doi.org/10.3390/en14082326 - 20 Apr 2021
Cited by 1 | Viewed by 974
Abstract
The ambitious environmental goals set by the 2030 Climate Target Plan can be reached with a strong contribution coming from the residential sector and the exploitation of its flexibility, intended as the capacity of a building to shift its consumption to maximize the [...] Read more.
The ambitious environmental goals set by the 2030 Climate Target Plan can be reached with a strong contribution coming from the residential sector and the exploitation of its flexibility, intended as the capacity of a building to shift its consumption to maximize the use of renewable energy. In the literature, the impact of flexibility has been mainly studied for the optimization of the control logic, assuming that the photovoltaic system and the electric storage have already been installed. Conversely, in this work, we adopt a different perspective that analyses the system from the designer point of view. Different scenarios with a variable degree of flexibility have been created and tested in a residential district considering various demand profiles (i.e., home appliances, heat pumps, and electric vehicles consumption). The profiles have been then used as input for an optimization tool that can design the optimal system according to a specific target function. Firstly, the system has been optimized according to economic indicators. However, results suggested that adopting only an economic perspective in the design phase could lead to results that are not in line with the European environmental targets. Thus, the system has been optimized also considering energy indicators to design a system that could give a relevant contribution to the energy transition of the residential sector. Results suggest that demand flexibility coupled with storage can boost the installation of photovoltaic systems due to the improved economic profitability and at the same time guarantee a relevant contribution to the decarbonization of the sector. Full article
(This article belongs to the Special Issue Energy-Flexible Buildings and Districts)
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Article
Model-Free Neural Network-Based Predictive Control for Robust Operation of Power Converters
Energies 2021, 14(8), 2325; https://doi.org/10.3390/en14082325 - 20 Apr 2021
Cited by 6 | Viewed by 1383
Abstract
An accurate definition of a system model significantly affects the performance of model-based control strategies, for example, model predictive control (MPC). In this paper, a model-free predictive control strategy is presented to mitigate all ramifications of the model’s uncertainties and parameter mismatch between [...] Read more.
An accurate definition of a system model significantly affects the performance of model-based control strategies, for example, model predictive control (MPC). In this paper, a model-free predictive control strategy is presented to mitigate all ramifications of the model’s uncertainties and parameter mismatch between the plant and controller for the control of power electronic converters in applications such as microgrids. A specific recurrent neural network structure called state-space neural network (ssNN) is proposed as a model-free current predictive control for a three-phase power converter. In this approach, NN weights are updated through particle swarm optimization (PSO) for faster convergence. After the training process, the proposed ssNN-PSO combined with the predictive controller using a performance criterion overcomes parameter variations in the physical system. A comparison has been carried out between the conventional MPC and the proposed model-free predictive control in different scenarios. The simulation results of the proposed control scheme exhibit more robustness compared to the conventional finite-control-set MPC. Full article
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Review
Assessment of Demand Side Flexibility in European Electricity Markets: A Country Level Review
Energies 2021, 14(8), 2324; https://doi.org/10.3390/en14082324 - 20 Apr 2021
Cited by 9 | Viewed by 1311
Abstract
Power systems in many countries have recently undergone a significant transition towards renewable and carbon-free generation sources. Those sources pose new challenges to the grid operation due to their intermittency and uncertainty. Consequently, advanced policy strategies and technologies offering new flexibility solutions on [...] Read more.
Power systems in many countries have recently undergone a significant transition towards renewable and carbon-free generation sources. Those sources pose new challenges to the grid operation due to their intermittency and uncertainty. Consequently, advanced policy strategies and technologies offering new flexibility solutions on the inelastic demand side are required to maintain the reliability of power systems. Given the diversity of situations, legislation and needs across European countries and the varying nature of distribution system operators, this article reviews the deployment of demand side flexibility at national level to identify best practices and main barriers. The analysis concerns European countries of different progress in solutions that leverage flexibility towards offering electricity grid services. The scope is to explore the operation principles of European electricity markets, to assess the participation of emerging flexible resources, and to propose new approaches that facilitate the integration of flexible assets in the distribution grid. The countries reviewed are the United Kingdom, Belgium, Italy and Greece. These countries were selected owing to their diversity in terms of generation mix and market design. Barriers for market access of flexibility resources are also identified in order to form relevant country-specific recommendations. Full article
(This article belongs to the Special Issue Innovation, Policy, and Regulation in Electricity Markets)
Article
Assessing the Sustainable Development and Renewable Energy Sources Relationship in EU Countries
Energies 2021, 14(8), 2323; https://doi.org/10.3390/en14082323 - 20 Apr 2021
Cited by 19 | Viewed by 1096
Abstract
In the context of the 2030 Agenda for Sustainable Development, by adopting the EU Renewable Energy Directive and the European Green Deal, the European Union aims at an extremely ambitious goal to become climate neutral by 2050. This goal involves a massive investment [...] Read more.
In the context of the 2030 Agenda for Sustainable Development, by adopting the EU Renewable Energy Directive and the European Green Deal, the European Union aims at an extremely ambitious goal to become climate neutral by 2050. This goal involves a massive investment plan to support this initiative, but also to reduce disparities between Member States, in order to transform the Union into a modern, resource-efficient, and competitive economy. The main objective of this paper is to investigate the sustainable development and renewable energy sources relationship in EU countries from a new perspective. Based on Eurostat available data and with the help of hierarchical clustering analysis, the Member States were divided in 2019 into five clusters, highlighting the key characteristics of the selected variables. The results of this research revealed high-performing groups of countries, as well as countries that need increased attention and additional support to become more efficient in achieving their sustainable development goals and renewable energy source targets. Full article
(This article belongs to the Special Issue Exploitation of Renewable Energy Sources for Power Generation)
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Article
A Novel Feature Extraction Method for the Condition Monitoring of Bearings
Energies 2021, 14(8), 2322; https://doi.org/10.3390/en14082322 - 20 Apr 2021
Cited by 3 | Viewed by 743
Abstract
This paper presents an innovative approach to the extraction of an indicator for the monitoring of bearing degradation. This approach is based on the principles of the empirical mode decomposition (EMD) and the Hilbert transform (HT). The proposed approach extracts the temporal components [...] Read more.
This paper presents an innovative approach to the extraction of an indicator for the monitoring of bearing degradation. This approach is based on the principles of the empirical mode decomposition (EMD) and the Hilbert transform (HT). The proposed approach extracts the temporal components of oscillating vibration signals called intrinsic mode functions (IMFs). These components are classified locally from the highest frequencies to the lowest frequencies. By selecting the appropriate components, it is possible to construct a bank of self-adaptive and automatic filters. Combined with the HT, the EMD allows an estimate of the instantaneous frequency of each IMF. A health indicator called the Hilbert marginal spectrum density is then extracted in order to detect and diagnose the degradation of bearings. This approach was validated on two test benches with variable speeds and loads. The obtained results demonstrated the effectiveness of this approach for the monitoring of ball and roller bearings. Full article
(This article belongs to the Special Issue Fault Diagnosis of Electrical Machines and Drives)
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Article
Experimental Validation of the Thermal Processes Modeling in a Solar Still
Energies 2021, 14(8), 2321; https://doi.org/10.3390/en14082321 - 20 Apr 2021
Cited by 2 | Viewed by 629
Abstract
Passive solar distillation is cheap and energy-efficient technology but its main disadvantage is low productivity. Thus, there are many attempts to improve solar stills’ productivity, and one of them is changing the mass of the water. This paper presents the results of validation [...] Read more.
Passive solar distillation is cheap and energy-efficient technology but its main disadvantage is low productivity. Thus, there are many attempts to improve solar stills’ productivity, and one of them is changing the mass of the water. This paper presents the results of validation of the thermal processes modeling in a solar still (SS). In order to validate the model, the experimental studies were conducted in a laboratory to ensure uniform climatic conditions. The studies were carried out for 10 kg, 15 kg, and 20 kg of water under three different solar irradiance conditions. The results show that 10 kg and 20 kg of water ensure the highest and the lowest daily productivity, respectively, independently of solar irradiance. When the water mass is 10 kg, the solar still’s productivity is 800 mL/m2/day, 3732 mL/m2/day, and 9392 mL/m2/day for low, medium, and high solar irradiance, respectively. Additionally, it is found that reducing the water mass from 20 kg to 10 kg can improve solar still’s productivity by a maximum value of 21.6%, which is obtained for low solar irradiance. The proposed mathematical model allows predicting the performance of the SS. The results of the theoretical calculations are in good agreement with the results of the experiments. The minimum and maximum deviation between the actual and theoretical productivity of the SS is 1.1% and 8.3%, respectively. Full article
(This article belongs to the Special Issue Thermal Energy Storage and Solar Thermal Energy Systems)
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Article
The Effects of Ultra-Low Viscosity Engine Oil on Mechanical Efficiency and Fuel Economy
Energies 2021, 14(8), 2320; https://doi.org/10.3390/en14082320 - 20 Apr 2021
Viewed by 676
Abstract
The development of a sustainable powertrain requires improved thermal efficiency. Reducing frictional power losses through the use of ultra-low viscosity oil is one of the most effective and economical ways. To assess the potential for efficiency enhancement in a new generation of future [...] Read more.
The development of a sustainable powertrain requires improved thermal efficiency. Reducing frictional power losses through the use of ultra-low viscosity oil is one of the most effective and economical ways. To assess the potential for efficiency enhancement in a new generation of future engines using low-viscosity oils, a technical analysis was conducted based on numerical simulation and theoretical analysis. This study proposes a numerical method coupling the whole multi-dynamics model and lubrication model under mixed lubrication regimes. Then, load distribution was calculated numerically and verified experimentally. Finally, this paper compares the bearing load and frictional energy loss of the main bearings when using The Society of Automotive Engineers (SAE) 15W40 and SAE 0W20 oil. The results indicate that the application of ultralow-viscosity lubricant can reduce the hydraulic friction loss up to 24%, but the asperity friction loss would increase due to the reduction in load capacity. As a result, the design of a new generation of high efficiency internal combustion engines requires careful calculation and design to balance the trade-off relations between hydraulic friction and asperity friction. Full article
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Article
Analysis of Wind Turbine Aging through Operation Data Calibrated by LiDAR Measurement
Energies 2021, 14(8), 2319; https://doi.org/10.3390/en14082319 - 20 Apr 2021
Cited by 9 | Viewed by 758
Abstract
This study analyzed the performance decline of wind turbine with age using the SCADA (Supervisory Control And Data Acquisition) data and the short-term in situ LiDAR (Light Detection and Ranging) measurements taken at the Shinan wind farm located on the coast of Bigeumdo [...] Read more.
This study analyzed the performance decline of wind turbine with age using the SCADA (Supervisory Control And Data Acquisition) data and the short-term in situ LiDAR (Light Detection and Ranging) measurements taken at the Shinan wind farm located on the coast of Bigeumdo Island in the southwestern sea of South Korea. Existing methods have generally attempted to estimate performance aging through long-term trend analysis of a normalized capacity factor in which wind speed variability is calibrated. However, this study proposes a new method using SCADA data for wind farms whose total operation period is short (less than a decade). That is, the trend of power output deficit between predicted and actual power generation was analyzed in order to estimate performance aging, wherein a theoretically predicted level of power generation was calculated by substituting a free stream wind speed projecting to a wind turbine into its power curve. To calibrate a distorted wind speed measurement in a nacelle anemometer caused by the wake effect resulting from the rotation of wind-turbine blades and the shape of the nacelle, the free stream wind speed was measured using LiDAR remote sensing as the reference data; and the nacelle transfer function, which converts nacelle wind speed into free stream wind speed, was derived. A four-year analysis of the Shinan wind farm showed that the rate of performance aging of the wind turbines was estimated to be −0.52%p/year. Full article
(This article belongs to the Special Issue Wind Turbine Monitoring through Operation Data Analysis)
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Article
Physical Dimensions as a Design Objective in Heat Transfer Equipment: The Case of Plate and Fin Heat Exchangers
Energies 2021, 14(8), 2318; https://doi.org/10.3390/en14082318 - 20 Apr 2021
Cited by 1 | Viewed by 730
Abstract
To incorporate exchanger dimensions as a design objective in plate and fin heat exchangers, a variable that must be taken into consideration is the geometry of the finned surfaces to be used. In this work, a methodology to find the surface geometry that [...] Read more.
To incorporate exchanger dimensions as a design objective in plate and fin heat exchangers, a variable that must be taken into consideration is the geometry of the finned surfaces to be used. In this work, a methodology to find the surface geometry that will produce the required heat transfer coefficient and pressure drop to achieve the design targets was developed. The geometry of secondary surfaces can be specified by the fin density, which represents the number of fins per unit length. All other geometrical features, as well as the thermo-hydraulic performance, can be derived from this parameter. This work showed the way finned surfaces are engineered employing generalised thermo-hydraulic correlations as a part of a design methodology. It also showed that there was a volume space referred to as volume design region (VDR) where heat duty, pressure drop, and dimensions could simultaneously be met. Such a volume design region was problem- and surface-specific; therefore, its limits were determined by the heat duty, the pressure drop, and the type of finned surface chosen in the design. The application of this methodology to a case study showed that a shell and tube heat exchanger of 227.4 m2, with the appropriate fin density using offset strip-fins, could be replaced by a plate and fin exchanger with any combination of height, width, and length in the ranges of 0–0.58 m, 0–0.58 m, and 0–3.59 m. The approach presented in this work indicated that heat exchanger dimensions could be fixed as a design objective, and they could effectively be achieved through surface design. Full article
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Article
Support Vector Machine Based Fault Location Identification in Microgrids Using Interharmonic Injection
Energies 2021, 14(8), 2317; https://doi.org/10.3390/en14082317 - 20 Apr 2021
Cited by 2 | Viewed by 746
Abstract
This paper proposes an algorithm for detection and identification of the location of short circuit faults in islanded AC microgrids (MGs) with meshed topology. Considering the low level of fault current and dependency of the current angle on the control strategies, the legacy [...] Read more.
This paper proposes an algorithm for detection and identification of the location of short circuit faults in islanded AC microgrids (MGs) with meshed topology. Considering the low level of fault current and dependency of the current angle on the control strategies, the legacy overcurrent protection schemes are not effective in in islanded MGs. To overcome this issue, the proposed algorithm detects faults based on the rms voltages of the distributed energy resources (DERs) by means of support vector machine classifiers. Upon detection of a fault, the DER which is electrically closest to the fault injects three interharmonic currents. The faulty zone is identified by comparing the magnitude of the interharmonic currents flowing through each zone. Then, the second DER connected to the faulty zone injects distinctive interharmonic currents and the resulting interharmonic voltages are measured at the terminal of each of these DERs. Using the interharmonic voltages as its features, a multi-class support vector machine identifies the fault location within the faulty zone. Simulations are conducted on a test MG to obtain a dataset comprising scenarios with different fault locations, varying fault impedances, and changing loads. The test results show that the proposed algorithm reliably detects the faults and the precision of fault location identification is above 90%. Full article
(This article belongs to the Section A5: Smart Grids and Microgrids)
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Article
The Use of the Fourier Series to Analyze the Shaping of Thermodynamic Processes in Heat Engines
Energies 2021, 14(8), 2316; https://doi.org/10.3390/en14082316 - 20 Apr 2021
Cited by 1 | Viewed by 644
Abstract
The article presents the application of the Fourier series to theoretical considerations on the method of maximum temperature control in thermodynamic cycles of internal combustion engines equipped with an additional independent kinematic system. The analysis assumes that the processes are zero-dimensional and the [...] Read more.
The article presents the application of the Fourier series to theoretical considerations on the method of maximum temperature control in thermodynamic cycles of internal combustion engines equipped with an additional independent kinematic system. The analysis assumes that the processes are zero-dimensional and the gases consumed in the engine cycles are perfect, simplifying the considerations for temperature control as a function of the two variables, pressure and volume, of which the volume as a geometric quantity can be completely controlled. In view of this fact, a predetermined temperature curve was assumed, ultimately reducing the considerations of specific volume changes, that is to say a kinematic system that could implement these changes. Moreover, in the analysis of volume changes, a cycle not used so far in the description of internal combustion engines was used. In the next step, the cycle was modified using the popular Vibe function, which was replaced in the theoretical cycle by two isochoric and isothermal transformations. Heat exchange was completely omitted in the considerations, in that it is of secondary importance, ultimately bringing the temperature function to the function of one variable, the angle of rotation of the crankshaft. Then, the kinematics was divided into the kinematics of the crank-piston system and the additional system, which was approximated with five words from the Fourier series, which in the technique correspond, for example, to the system of oscillators. At the end of the article we have explained one of the ways of actual technical implementation using a single nonlinear oscillator, the so-called ACC system equivalent to a few words from the mentioned Fourier series. Full article
(This article belongs to the Special Issue Internal Combustion Engine Performance)
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Article
The Investigation into the Tribological Impact of Alternative Fuels on Engines Based on Acoustic Emission
Energies 2021, 14(8), 2315; https://doi.org/10.3390/en14082315 - 20 Apr 2021
Viewed by 509
Abstract
The wide use of different alternative fuels (AL) has led to challenges to the internal combustion (IC) engine tribology. To avoid any unpredicted damages to lubrication joints by using AL fuels, this study aims to accurately evaluate the influences of alternative fuels on [...] Read more.
The wide use of different alternative fuels (AL) has led to challenges to the internal combustion (IC) engine tribology. To avoid any unpredicted damages to lubrication joints by using AL fuels, this study aims to accurately evaluate the influences of alternative fuels on the tribological behavior of IC engines. Recent achievements of the acoustic emission (AE) mechanism in sliding friction provide an opportunity to explain the tribological AE responses on engines. The asperity–asperity–collision (AAC) and fluid–asperity–shearing (FAS) mechanisms were applied to explain the AE responses from the piston ring and cylinder liner system. A new adaptive threshold–wavelet packets transform (WPT) method was developed to extract tribological AE features. Experimental tests were conducted by fueling three fuels: pure diesel (PD), biodiesel (BD), and Fischer–Tropsch (F–T) diesel. The FAS–AE indicators of biodiesel and F–T diesel show a tiny difference compared to the baseline diesel using two types of lubricants. Biodiesel produces more AAC impacts with higher AAC–AE responses than F–T diesel, which occurs at high speeds due to high temperatures and more particles after combustion than diesel. This new algorithm demonstrated the high performance of using AE signals in monitoring the tribological impacts of alternative fuels on engines. Full article
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Article
Methodology for Assessing the Impact of Aperiodic Phenomena on the Energy Balance of Propulsion Engines in Vehicle Electromobility Systems for Given Areas
Energies 2021, 14(8), 2314; https://doi.org/10.3390/en14082314 - 20 Apr 2021
Cited by 22 | Viewed by 903
Abstract
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 [...] Read 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 1200 vehicles and mileage of 150,000 km. Based on this analysis, it can be concluded that with the current infrastructure and operating costs and the frequency of failure of PHEV and BEV drives, ICEV vehicles are unrivaled in terms of their operating costs. Full article
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Article
Development of AI-Based Diagnostic Model for the Prediction of Hydrate in Gas Pipeline
Energies 2021, 14(8), 2313; https://doi.org/10.3390/en14082313 - 20 Apr 2021
Cited by 3 | Viewed by 752
Abstract
For the stable supply of oil and gas resources, industry is pushing for various attempts and technology development to produce not only existing land fields but also deep-sea, where production is difficult. The development of flow assurance technology is necessary because hydrate is [...] Read more.
For the stable supply of oil and gas resources, industry is pushing for various attempts and technology development to produce not only existing land fields but also deep-sea, where production is difficult. The development of flow assurance technology is necessary because hydrate is aggregated in the pipeline and prevent stable production. This study established a system that enables hydrate diagnosis in the gas pipeline from a flow assurance perspective. Learning data were generated using an OLGA simulator, and temperature, pressure, and hydrate volume at each time step were generated. Stacked auto-encoder (SAE) was used as the AI model after analyzing training loss. Hyper-parameter matching and structure optimization were carried out using the greedy layer-wise technique. Through time-series forecast, we determined that AI diagnostic model enables depiction of the growth of hydrate volume. In addition, the average R-square for the maximum hydrate volume was 97%, and that for the formation location was calculated as 99%. This study confirmed that machine learning could be applied to the flow assurance area of gas pipelines and it can predict hydrate formation in real time. Full article
(This article belongs to the Special Issue Volume II: Energy Resource Potential of Gas Hydrates)
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Article
Assessing the Effectiveness of the Energy Storage Rule-Based Control in Reducing the Power Flow Uncertainties Caused by Distributed Photovoltaic Systems
Energies 2021, 14(8), 2312; https://doi.org/10.3390/en14082312 - 20 Apr 2021
Cited by 1 | Viewed by 578
Abstract
Battery energy storage systems (BESSs) are increasingly adopted to mitigate the negative effects caused by the intermittent generation of photovoltaic (PV) systems. The majority of commercial BESSs implement the self-consumption, rule-based approach, which aims at storing the excess of PV production, and then [...] Read more.
Battery energy storage systems (BESSs) are increasingly adopted to mitigate the negative effects caused by the intermittent generation of photovoltaic (PV) systems. The majority of commercial BESSs implement the self-consumption, rule-based approach, which aims at storing the excess of PV production, and then reusing it when the power demand of the loads exceeds the PV power generation. Even though this approach proved to be a valid solution to increase the self-consumption of distributed generators, its ability to reduce the power flow uncertainties caused by PV systems is still debatable. To fill this gap, this study aims at answering this question by proposing a dedicated set of key performance indicators (KPIs). These KPIs are used to evaluate the performance of a 13.8 kWp/25.2 kWh Lithium-Ion BESS coupled with a 64 kWp PV system. The results of the study revealed that the impact of the storage system had almost negligible effects on the uncertainty of the net power flows, while showing better results in terms of the reduction of the absolute power ramps, particularly during the BESS charge stages. These results represent an interesting point of discussion by suggesting that different storage control approaches should be investigated. Full article
(This article belongs to the Section B1: Solar Energy and Photovoltaic Systems)
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