Tidal turbines are located in shallow water depths in comparison to their dimensions (15 m-diameter turbines in 40 m depths, typically). Constrained vertically by the water depth and laterally by neighbouring turbines, the flow within a tidal farm is subjected to blockage effects that influence the performance of individual devices. The Betz limit (which is the maximum power extractable from an unconstrained flow) can, therefore, be exceeded as demonstrated by Garrett and Cummins. Thus, beyond a significant blockage ratio, blockage effects should be considered when assessing the energy production of a tidal farm. The actuator disk method is particularly suited to simulate the flow field within an array of turbines under realistic tidal flow conditions. However, the implementation of actuator disks in coastal numerical models relies on relationships that neglect the blockage effects on the thrust and power of devices. We propose here an actuator disk formulation corrected to integrate t... [more]

The gas diffusion layer (GDL), as the main mass transfer carrier in a hydrogen fuel cell, transports fuel and discharges water, the only by-product of the electrochemical reaction. The dispersion process of water in the pore will hinder the diffusion of gas, thus reducing the concentration of fuel gas at the catalytic site, resulting in the decrease of the electrochemical reaction rate. In this paper, the effect of wettability in the GDL hole on the water transport process is studied. When the pore wall is hydrophilic, the liquid phase is affected by the gas phase eddy current velocity field, and the particles at the center advance to the edge, forming a liquid phase interface with a thin center and thick edge. With the increase of the wall contact angle, the curvature of the three-phase interface increases, the wall adhesion decreases, and the liquid phase is more likely to be discharged. When the contact angle is 130°, the liquid phase almost does not shift in the hole with a radius... [more]

The aim of this work is to propose a methodology to obtain an effectiveness factor for biofilm in a microbial electrolysis cell (MEC) system and use it to reduce a partial differential equation (PDE) biofilm MEC model to an ordinary differential equation (ODE) MEC model. The biofilm mass balances of the different species are considered. In addition, it is considered that all the involved microorganisms are attached to the anodic biological film. Three effectiveness factors are obtained from partial differential equations describing the spatial distributions of potential and substrate in the biofilm. Then, a model reduction is carried out using the global mass balances of the different species in the system. The reduced model with three uncertain but bounded effectiveness factors is evaluated numerically and analyzed in the sense of stability and parametric sensibility to demonstrate its applicability. The reduced ODE model is compared with a validated model taken from the literature, a... [more]

Recently, there has been a growing interest in the use of new types of cheaper raw materials for biodiesel production. There are many prospects for microalgae, which do not compete for land with conventional biodiesel raw materials, are characterized by rapid reproduction and high biomass accumulation, and under certain conditions, some are able to accumulate a large amount of oil. A number of studies have been conducted on the extraction of oil from microalgae cells and transesterification with various acyl receptors. This paper provides an overview of the results of research on microalgal biomass preparation and oil extraction. Indicators of the quality of the oil are presented and its suitability for biodiesel synthesis is analyzed. The homogeneous and heterogeneous catalysts used for oil transesterification are described and the optimal conditions of the process when using various alcohols as acyl receptors are presented. Much attention is paid to the parameters affecting the trans... [more]

Wastewaters generated by crude oil extraction processes, called “produced waters” (PWs), are complex solutions that contain organic compounds, mainly hydrocarbons, and often exhibit high salinity. The large amounts of PWs represent a global issue because of their environmental impact. An approach widely used in the oil industry is the reinjection of this wastewater into the extraction wells after a suitable treatment. The high salt concentration of such solutions may be used in salinity gradient technologies to produce green electricity. Among these technologies, reverse electrodialysis (RED) is one of the most promising. In this work, the application of RED for energy generation from two different real oil industry brines was investigated. An experimental campaign was performed by testing 10 × 10 cm2 units in long-run continuous operations, monitoring the performance for more than 25 days. Fouling phenomena, occurring during the continuous operation, decrease the unit performance and... [more]

Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are unable to prevent the AC side contribution to DC side fault currents, thus necessitating circuit breakers (CBs) for protection. A solution to this problem is using submodules (SMs) that are capable of blocking the flow of current from the AC grid to feed the DC side fault. The full-bridge submodule (FBSM) is one type of fault blocking SM where the presence of two extra switches ensures that in the event of a DC fault, the reverse voltage from the FBSM capacitor is placed in the path of the AC side current feeding the DC side fault through the antiparallel diodes. However, the additional semiconductor switches in the FBSMs increase the converter cost, complexity, and losses. Several SM configurations have been proposed in recent years that provide DC fault blocking capability with lower losses and device counts than those of FBSMs. Besides, many of the proposed hybrid converter configurations that combine d... [more]

Natural ventilation dominates in Polish residential buildings. It is a simple and low-cost system but its performance is affected by varying environmental conditions. Hence, setting up constant ventilation airflow results in errors when calculating heating and cooling energy. In this paper, an attempt to integrate the buoyancy effect in natural ventilation of a residential building at hourly resolution with the hourly simulation method of EN ISO 13790 to obtain energy use for space heating and cooling is presented. The ping-pong coupling algorithm was proposed and applied. Hourly variation of ventilation airflow rate was from −26.8 m3/h (flow from outdoor to the interior of the building) to 87.2 m3/h with 55 m3/h on average. The lack of a cooling system resulted in overheating during summer and indicated the necessity of its application or use of other techniques to reduce solar gains. Application of the cooling system resulted in an hourly ventilation rate from −38.0 m3/h to 87.2 m3/h... [more]

The creep and stress relaxation behaviors of coal are common in coal mining. The unified constitutive model is suitable to describe and predict both the creep and relaxation evolution characteristics of rocks. The generalized Kelvin model is the core element for traditional and improved component models to reflect both the nonlinear creep and relaxation. In this paper, an improved core damage model, which could both reflect the creep and stress relaxation in relation to the damage evolution, was established based on a comparison of the traditional and improved component models, and the responding constitutive equations (creep and stress relaxation equation) at constant stress/strain were deduced. Then, the core damage model was validated to the uniaxial compressive multistage creep and stress relaxation test results of coal, showing that the model curves had great accordance with the experimental data. Moreover, the model comparisons on accuracy, parameter meaning, and popularization a... [more]

Modeling the photovoltaic (PV) energy output with high accuracy is essential for predicting and analyzing the performance of a PV system. In the particular cases of building-integrated and building-attached photovoltaic systems (BIPV and BAPV, respectively) the time-varying partial shading conditions are a relevant added difficulty for modeling the PV power conversion. The availability of laser imaging detection and ranging (LIDAR) data to create very-high-resolution elevation digital models can be effectively used for computing the shading at high resolution. In this work, an artificial neural network (ANN) has been used to model the power generation of different BIPV arrays on a 5 min basis using the meteorological and solar irradiance on-site conditions, as well as the shading patterns estimated from a digital surface model as inputs. The ANN model has been validated using three years of 5-min-basis monitored data showing very high accuracy (6−16% of relative error depending on the... [more]

In order to further evaluate the impact of vehicle-to-grid (V2G) on the distribution network, this paper studies a method to assess the influence of electric vehicles participating in charge and discharge on the voltage quality of the distribution network. First, considering the state of charge of the EV, the participation of the owner and other factors, the charging and discharging model is built. Then, the probabilistic power flow calculation based on Latin hypercube sampling is used to obtain the probability distribution of the voltage amplitude of the charge and discharge load connected to the distribution network, and finally the evaluation index is established to quantify and calculate the voltage quality of the distribution network participating in the V2G process of electric vehicles. Simulation results show that the evaluation method has the advantage of fast calculation speed while ensuring known accuracy, introduces the probability distribution of expected value and variance... [more]

Deflection switched reluctance motors (DSRM) are prone to chattering at low speeds, which always affects the output efficiency of the DSRM and the mechanical loss of the motor. Combining the characteristics of a traditional reluctance motor with the strong nonlinear and high coupling of the DSRM, a control system for a DSRM based on a flexible neural network (FNN) is proposed in this paper. Based on the better robustness and fault tolerance of fuzzy PI control, the given speed signal is adjusted and converted into a torque control signal. As a result, the FNN control module possesses the strong self-learning ability and adaptive adjustment ability necessary to obtain the control voltage signal. Through simulations and experiments, it was verified that the control system can run stably on DSRM and shows good dynamic performance and anti-interference ability.

One of the most critical aspects of integrating renewable energy sources into the smart grid is photovoltaic (PV) power generation forecasting. This ensemble forecasting technique combines several forecasting models to increase the forecasting accuracy of the individual models. This study proposes a regression-based ensemble method for day-ahead PV power forecasting. The general framework consists of three steps: model training, creating the optimal set of weights, and testing the model. In step 1, a Random forest (RF) with different parameters is used for a single forecasting method. Five RF models (RF1, RF2, RF3, RF4, and RF5) and a support vector machine (SVM) for classification are established. The hyperparameters for the regression-based method involve learners (linear regression (LR) or support vector regression (SVR)), regularization (least absolute shrinkage and selection operator (LASSO) or Ridge), and a penalty coefficient for regularization (λ). Bayesian optimization is perf... [more]

Continuous urbanization and modernization have increased the burning of fossil fuels to meet energy needs across the globe, emanating environmental pollution and depleting fossil fuels. Therefore, a shift towards sustainable and renewable energy is necessary. Several techniques to exploit biomass to yield energy are trending, with pyrolysis one of them. Usually, a single feedstock is employed in pyrolysis for anoxygenic generation of biochar together with bio-oil at elevated temperatures (350−600 °C). Bio-oil produced through pyrolysis can be upgraded to crude oil after some modification. However, these modifications of bio-oil are one of the major drawbacks for its large-scale adoption, as upgradation increases the overall cost. Therefore, in recent years the scientific community has been researching co-pyrolysis technology that involves the pyrolysis of lignocellulosic biomass waste with non-biodegradable waste. Co-pyrolysis reduces the need for post-modification of bio-oil, unlike p... [more]

Li-ion batteries are already being used in several applications, from portable devices to the automotive industry, and they represent a promising option also for other critical uses, such as in the storage of energy from renewable sources. However, two of the main concerns that still hinder their massive introduction in these further areas, are their safety and reliability. Depending on cell characteristics and operating conditions, the heat generated within the cell can exceed that dissipated from its surface, and the cell will fail, possibly with catastrophic consequences. To identify the hazardous working conditions of a cell, a simulation model including the main exothermic reactions was set up to investigate the onset of thermal runaway in several Li-ion cell configurations under various operating conditions. The behavior of four different cathodes under thermal abuse and the influence of external factors such as the environmental temperature and the cooling system efficiency were... [more]

Using underground space to store natural gas resources is an important means by which to solve emergency peak shaving of natural gas. Rock salt gas storage is widely recognized due to its high-efficiency peak shaving and environmental protection. Damage and stress concentrations inside the cavern injection during withdrawal operations and throughout the storage facility life have always been among the most important safety issues. Therefore, accurate evaluation of the stability of rock salt gas storage during operation is of paramount significance to field management and safety control. In this study, we used the finite element numerical analysis software Flac3D to numerically simulate large displacement deformations of the cavern wall during gas storage—in addition to the distribution of the plastic zone of the rock around the cavern and the surface settlement—under different working conditions. We found that the maximum surface settlement value occurred near the upper part of the cav... [more]

High-alkali coal is rich in alkali metals, which can cause serious effects such as slagging and corrosion on the heating surface during combustion and utilization. A portable spectral system was utilized to simultaneously measure gaseous Na concentration and temperature in a 20 kW slag-tapping combustor and a slagging boiler furnace of a 300 MW power generation unit by flame spontaneous emission spectroscopy (FES) for simultaneous measuring. The result shows that both ZD-FK and ZD-HSQ (Fukang coal and Hongshaquan coal, Xinjiang Zhundong high-alkali coal) combustion flame temperatures are around 1400 °C at the outlet of the cyclone burner while the latter is slightly higher. The sodium concentration in the gas phase increases with the rising of the initial combustion temperature and unit load for one kind of coal, and the level of sodium concentration has a strong correlation with the Na content for different coal. Most of the sodium in the high temperature zone of the furnace exists in... [more]

The Special Issue “Industry and Tertiary Sectors towards Clean Energy Transition” is focused on technical, financial and policy-related aspects linked to the transition of industrial and services sectors towards energy saving and decarbonisation. These different aspects are interrelated, and as such, they have been analysed with an interdisciplinary approach combining economic and technical information. Collecting and analysing quantitative data would allow researchers to better understand the clean energy transition process, and how the international and national regulatory and policy framework are contributing to it. The papers within this Special Issue focus on energy efficiency and clean energy key technologies, renewable sources, energy management and monitoring systems, energy policies and regulations, and economic and financial aspects.

The grid-connected inverter is a vital power electronic equipment connecting distributed generation (DG) systems to the utility grid. The quality of the grid-connected current is directly related to the safe and stable operation of the grid-connected system. This study successfully constructed a robust control system for a grid-connected inverter through a dynamic recurrent fuzzy-neural-network imitating sliding-mode control (DRFNNISMC) framework. Firstly, the dynamic model considering system uncertainties of the grid-connected inverter is described for the global integral sliding-mode control (GISMC) design. In order to overcome the chattering phenomena and the dependence of the dynamic information in the GISMC, a model-free dynamic recurrent fuzzy-neural-network (DRFNN) is proposed as a major controller to approximate the GISMC law without the extra compensator. In the DRFNN, a Petri net with varied threshold is incorporated to fire the rules, and only the parameters of the fired rul... [more]

This paper introduces a new two-speed planetary gear automatic transmission using an electronically controlled wedge clutch. In order to verify the feasibility of using this transmission in pure electric vehicles, the influencing factors of the two-speed transmission due to the increase in mass and the reduction in transmission efficiency are introduced. The vehicle simulation model was established on the MATLAB/Simulink platform, and the dynamic programming method was used to optimize the transmission ratio and shifting law. The simulation results show that the use of a two-speed automatic transmission can effectively improve the economic performance and dynamic performance of battery electric vehicles.

In the gob-side entry retaining by roof cutting (GERRC) technique, pressure is offloaded via directional roof cutting, and a roadway is automatically formed due to the ground pressure and rock-breaking expansion. To improve the application of the theory and technical system of GERRC in the Karst area in Southwest China, this research studies the key technology of GERRC in a high gas outburst coal seam, based on the engineering background of the 39114 working face of the Honglin coal mine. According to the geological conditions of the 39114 working face, by means of formula calculation, UDEC numerical modeling, and on-site drilling peeping, the optimal roof-cutting parameters suitable for the 39114 working face were determined: the roof cutting height was 7 m, the roof cutting angle was 15°, and the spacing of pre-splitting blasting holes was 600 mm. Additionally, the above roof-cutting parameters have achieved good results in the engineering practices of the 39114 transportation roadwa... [more]

The energy consumption of electric vehicles is closely related to the problems of charging station planning and vehicle route optimization. However, due to various factors, such as vehicle performance, driving habits and environmental conditions, it is difficult to estimate vehicle energy consumption accurately. In this work, a physical and data-driven fusion model was designed for electric bus energy consumption estimation. The basic energy consumption of the electric bus was modeled by a simplified physical model. The effects of rolling drag, brake consumption and air-conditioning consumption are considered in the model. Taking into account the fluctuation in energy consumption caused by multiple factors, a CatBoost decision tree model was constructed. Finally, a fusion model was built. Based on the analysis of electric bus data on the big data platform, the performance of the energy consumption model was verified. The results show that the model has high accuracy with an average rel... [more]

This research aimed to introduce a comprehensive mathematical modeling approach based on the maximization of the power coefficient (Cp) to obtain the regulation in pitch angle and tip speed ratio (TSP), taking into account the detailed power losses at the different stages of the power train of the wind turbine. The model is used to track the optimal power coefficient of the wind turbine power train, considering both direct (without gearbox) and indirect (with gearbox) drive configurations. The result of the direct driveline was validated with a 100 W horizontal-axis wind turbine experimental system. The model estimated the optimal value of Cp at 0.48 for a pitch angle of 0 degrees and a TSR of 8.1, which could be obtained at a wind speed of around 11.2 m/s. The results also revealed that, within the lower wind regime, windage, hysteresis, and eddy current losses dominated, while during higher wind regimes, the copper, stray load, and insulator gate bipolar transistor (IGBT) losses gain... [more]

Data collection is more difficult in distribution network than transmission networks since the structure of distribution networks is more complex. As a result, data could be partly abnormal or missing, which cannot completely describe the operation status of distribution network. In addition, access of distributed generation (DG) to distribution network further aggravates the variability of power flow in distribution network. The traditional deterministic line loss calculation method has some limitations in accurately estimating the line loss of distribution network with DG. A line loss interval calculation method based on power flow calculation and linear optimization is proposed, considering abnormal data collection and distribution network power flow variability. The linear optimization model is established according to sensitivity of line loss to the injected power and sensitivity of transmission power of first branch to the injected power. Introducing the scheduling information in... [more]

The paper investigates the optimization of ecosystem services of podzolized heavy loamy chernozem (black soil) as a result of the cultivation of the perennial energy culture of Miscanthus x giganteus. The research was conducted on an experimental land plot during 2016−2021. No fertilization was applied to the soil during the experiments, and over the years of research, the growing seasons were accompanied by abnormal droughts, but even under such conditions, the plants of Miscanthus x giganteus gradually increased their yield. At the initial stage of research, in the third year of cultivation, dry biomass of Miscanthus x giganteus was obtained at 14.3 t/ha, in the fourth year−18.6 t/ha, and already in the fifth and sixth years, 21.7 and 24.5 t/ha, respectively. That is, energy-wise, the harvest for the last year was equivalent to 15.9 tons of coal or 12,618 m3 of natural gas. Cultivation of Miscanthus x giganteus on black soil for six years has improved the provision of its ecosystem s... [more]

Climate change has brought a compelling need for cooling living spaces to the attention of researchers as well as construction professionals. The problem of overheating enclosures is now exacerbated in traditionally affected areas and is also affecting countries that were previously less prone to the issue. In this paper, we address measurements of thermal comfort and cooling emission efficiency parameters for different devices: ceiling panels, underfloor cooling, fan-assisted radiators, and fan coil. These devices were tested in low and high cooling capacities of up to 40 W/m2 while also featuring heating dummies to imitate internal heat gains. Air temperatures were measured at different heights, allowing to evaluate the thermal stratification with high accuracy. Thermal comfort differences of the tested systems were quantified by measuring both air velocities and operative temperatures at points of occupancy. In summary, the best-performing cooling devices for the studied cooling app... [more]