Energies — Open Access Journal

With the rapid growth of renewable energy and the DC fast charge pile of the electric vehicle, their inherent volatility and randomness increase a power system’s unbalance of instantaneous power. The need for power grid frequency regulation is increasing. The energy storage system (ESS) can be used to assist the thermal power unit so that a better frequency regulation result is obtained without changing the original operating mode of the unit. In this paper, a set of different charging/discharging control strategies of the lithium titanate battery (LTO) is proposed, which are chosen according to the interval of the State of energy (SOE) to improve the utilization rate of the ESS. Finally, the cost-benefit model of the ESS participating in automatic generation control ancillary service is established. Case analysis proves that after a 1.75 MWh ESS is configured for a 600 MW thermal power unit, K_p and D is increased from 1.42 to 6.38 and 2857 to 6895 MW. The net daily income is increased from 20,284 yuan to 199,900 yuan with a repayment period of 93 days. The results show that the control strategies and the energy configuration method can improve the performance and economic return of the system. Full article

This study uses an optimization approach representation and numerical solution for the variable viscosity and non-linear Boussinesq effects on the free convection over a vertical truncated cone in porous media. The surface of the vertical truncated cone is maintained at uniform wall temperature and uniform wall concentration (UWT/UWC). The viscosity of the fluid varies inversely to a linear function of the temperature. The partial differential equation is transformed into a non-similar equation and solved by Keller box method (KBM). Compared with previously published articles, the results are considered to be very consistent. Numerical results for the local Nusselt number and local Sherwood number with the six parameters (1) dimensionless streamwise coordinate ξ, (2) buoyancy ratio N, (3) Lewis number Le, (4) viscosity-variation parameter , (5) non-linear temperature parameter , and (6) non-linear concentration parameter are expressed in figures and tables. The Taguchi method was used to predict the best point of the maxima of the local Nusselt (Sherwood) number of 3.8636 (5.1156), resulting in ξ (4), N (10), Le (0.5), (−2), (2), (2) and ξ (4), N (10), Le (2), (−2), (2), (2), respectively. Full article

The migration of methane through the gas hydrate stability zone (GHSZ) in the marine subsurface is characterized by highly dynamic reactive transport processes coupled to thermodynamic phase transitions between solid gas hydrates, free methane gas, and dissolved methane in the aqueous phase. The marine subsurface is essentially a water-saturated porous medium where the thermodynamic instability of the hydrate phase can cause free gas pockets to appear and disappear locally, causing the model to degenerate. This poses serious convergence issues for the general-purpose nonlinear solvers (e.g., standard Newton), and often leads to extremely small time-step sizes. The convergence problem is particularly severe when the rate of hydrate phase change is much lower than the rate of gas dissolution. In order to overcome this numerical challenge, we have developed an all-at-once Newton scheme tailored to our gas hydrate model, which can handle rate-based hydrate phase change coupled with equilibrium gas dissolution in a mathematically consistent and robust manner. Full article

The eutrophication of surface waters is a natural process; however, anthropogenic activities significantly accelerate degradation processes. Most lakes in Poland and in the world belong to the poor and unsatisfactory water quality class. It is therefore necessary to limit negative anthropogenic impacts and introduce restoration methods, in particular those that are safe for the aquatic ecosystem. One of these is a pulverizing aeration Podsiadłowski method that uses only wind energy. The method allows for the moderate oxygenation of hypolimnion water, which maintains the oxygen conditions in the overlying water zone in the range of 0–1 mg O₂·dm^-1. The purpose of the work was to develop a new method of determining the efficiency of the aerator pulverization unit in the windy conditions of the lake. The method consists in determining the volumetric flow rates of water in the aerator pulverization unit, based on maximum hourly wind speeds. The pulverization efficiency in the conditions of Góreckie Lake was determined based on 6600 maximum hourly wind speeds in 2018. Based on the determined model, the theoretical performance of the machine was calculated, which in the conditions of Góreckie Lake in 2018 amounted to less than 79,000 m³ per year (nine months of the effective aerator operation). Full article

This study proposes novel simulation methods to model the power delivery function of a parallel-connected solid-oxide-fuel-cell stack system. The proposed methods are then used to investigate the possible thermal runaway induced by the performance mismatch between the employed stacks. A challenge in this modeling study is to achieve the same output voltage but different output current for each employed stack. Conventional fuel-cell models cannot be used, because they employ fuel flow rates and stack currents as the input variables. These two variables are unknown in the parallel-connected stack systems. The proposed method solves the aforementioned problems by integrating the fuel supply dynamics with the conventional stack models and then arranging them in a multiple-feedback-loop configuration for conducting simulations. The simulation results indicate that the proposed methods can model the transient response of the parallel-connected stack system. Moreover, for the dynamics of the power distribution, there exists an unstable positive feedback loop between employed stacks when the stack temperatures are low, and a stable negative feedback loop when the stack temperatures are high. A thermal runaway could be initiated when the dynamics of the stack temperature is slower than that of the current distribution. Full article

The development of the renewable energy industry is a priority of economic policies in many countries, since it is viewed as one of the key growth sectors in the economy, playing also a very important role in mitigating climate change. At the international level, renewable energy is an issue of international cooperation but also an area of high trade tensions between countries. The main goal of this paper is to examine the nature and sources of recent trade disputes in the solar photovoltaic sector, which is the most dynamically growing sector in the green energy industry. In particular, the paper explores the links between the contemporary trade disputes and modern protectionism and between protectionist policies and practices and the export competitiveness in the growing sector of the economy. To achieve the aim of the study we explore in detail the WTO trade disputes over photovoltaic (PV) products, which occurred in the years 2007–2018. The products covered by the analysis were solar modules and cells classified under the HS code 854140. In our research we also used measures of descriptive statistics, hierarchical cluster analysis and revealed comparative advantage indexes. Our key results demonstrate the existence of links between protectionist policy causing trade conflicts and the export competitiveness. The research has also allowed us to identify problems of future studies concerning the association between trade protectionism and global value chains in the solar energy sector. Full article

In an era of increasing energy production from renewable sources, the demand for components for renewable energy systems has dramatically increased. Consequently, managers and investors are interested in knowing whether a company associated with the semiconductor and related device manufacturing sector, especially the photovoltaic (PV) systems manufacturers, is a money-making business. We apply a new approach that extends prior research by applying decision trees (DTs) to identify ratios (i.e., indicators), which discriminate between companies within the sector that do (designated as “green”) and do not (“red”) produce elements of PV systems. Our results indicate that on the basis of selected ratios, green companies can be distinguished from the red companies without an in-depth analysis of the product portfolio. We also find that green companies, especially operating in China are characterized by lower financial performance, thus providing a negative (and unexpected) answer to the question posed in the title. Full article

Currently, fuel cells are increasingly used in industrial installations, means of transport, and household applications as a source of electricity and heat. The paper presents the results of experimental tests of a Proton Exchange Membrane Fuel Cell (PEMFC) at variable load, which characterizes the cell’s operation in real installations. A detailed analysis of the power needed for operation fuel cell auxiliary devices (own needs power) was carried out. An analysis of net and gross efficiency was carried out in various operating conditions of the device. The measurements made show changes in the performance of the fuel cell during step changing or smooth changing of an electric load. Load was carried out as a change in the current or a change in the resistance of the receiver. The analysis covered the times of reaching steady states and the efficiency of the fuel cell system taking into account auxiliary devices. In the final part of the article, an analysis was made of the influence of the fuel cell duration of use on obtained parameters. The analysis of the measurement results will allow determination of the possibility of using fuel cells in installations with a rapidly changing load profile and indicate possible solutions to improve the performance of the installation. Full article

While addressing the issue of improving the performance of Photovoltaic (PV) systems, the simulation results are highly influenced by the PV model accuracy. Building the PV module mathematical model is based on its I-V characteristic, which is a highly nonlinear relationship. All the PV cells’ data sheets do not provide full information about their parameters. This leads to a nonlinear mathematical model with several unknown parameters. This paper proposes a new application of the Grasshopper Optimization Algorithm (GOA) for parameter extraction of the three-diode PV model of a PV module. Two commercial PV modules, Kyocera KC200GT and Solarex MSX-60 PV cells are utilized in examining the GOA-based PV model. The simulation results are executed under various temperatures and irradiations. The proposed PV model is evaluated by comparing its results with the experimental results of these commercial PV modules. The efficiency of the GOA-based PV model is tested by making a fair comparison among its numerical results and other optimization method-based PV models. With the GOA, a precise three-diode PV model shall be established. Full article

Modern multiphase electric machines take advantage of additional degrees of freedom for various purposes, including harmonic current injection to increase torque per ampere. This new approach introduces a non-sinusoidal air gap flux density distribution causing additional technical problems and so the conventional assumptions need to be revised. The paper presents a methodology for synthesis of air gap magnetic field generated by a symmetrically distributed multiphase windings including the rotor field reaction due to the machine’s load. The proposed method is suitable either for single-layer or double layer windings and can be adopted either for full-pitched or chorded winding including slots effects. The article analyses the air gap flux density harmonic content and formulates conclusions important to multiphase induction motors. It also discusses effects of time harmonic currents and illustrates the principle of changing number of pole-pairs typical for harmonic currents being injected to increase torque. Full article

The multi-terminal AC/DC system will become one of the important forms of the future power grid. The negative impedance characteristic caused by the constant power load in the DC network will reduce the power transfer capacity between the terminals, especially when a grid fault occurs in AC system at any terminal. Energy storage has played an important role in improving the stability of AC and DC systems. This paper proposes an additional control method based on an energy storage system to improve system power transfer capacity with low cost. The state space model of two-terminal AC/DC system is established, and the feedback laws for additional control are further designed by Lyapunov theory. Furthermore, the additional control strategies based on the energy storage system is built, without changing the existing control system of each control object. Finally, the corresponding system simulation model is established by Matlab/Simulink for analysis and verification. The research results show that the proposed additional control method is effective. The power transfer limitation can be overcome by only adding small damping energy with the stable DC voltages under large disturbances, and the power transfer capacity between the terminals can be significantly improved with low control cost. Full article

Integration of renewable energy and optimization of energy use are key enablers of sustainable energy transitions and mitigating climate change. Modern technologies such the Internet of Things (IoT) offer a wide number of applications in the energy sector, i.e, in energy supply, transmission and distribution, and demand. IoT can be employed for improving energy efficiency, increasing the share of renewable energy, and reducing environmental impacts of the energy use. This paper reviews the existing literature on the application of IoT in in energy systems, in general, and in the context of smart grids particularly. Furthermore, we discuss enabling technologies of IoT, including cloud computing and different platforms for data analysis. Furthermore, we review challenges of deploying IoT in the energy sector, including privacy and security, with some solutions to these challenges such as blockchain technology. This survey provides energy policy-makers, energy economists, and managers with an overview of the role of IoT in optimization of energy systems. Full article

Hydrothermal liquefaction (HTL) is a promising technology for the production of bio-crude. However, some unresolved issues still exist within HTL, which need to be resolved before its promotion on a commercial scale. The management of the aqueous phase is one of the leading challenges related to HTL. In this study, the sewage sludge has been liquefied at 350 °C with and without catalyst (K₂CO₃). Subsequently, aqueous phase recycling was applied to investigate the effect of recycling on bio-crude properties. Obtained results showed that the energy recovery in the form of bio-crude increased by 50% via aqueous phase recirculation, whereas nitrogen content in the bio-crude was approximately doubled after eight rounds of recycling. GCMS characterization of the aqueous phase indicated acetic acid as a major water-soluble compound, which employed as a catalyst (0.56 M), and resulted in a negligible increase in bio-crude yield. ICP-AES highlighted that the majority of the inorganics were transferred to the solid phase, while the higher accumulation of potassium and sodium was found in the aqueous phase via successive rounds of recycling. Full article

In this paper, a simplified efficient method for sensorless finite set current predictive control (FSCPC) for synchronous reluctance motor (SynRM) based on extended Kalman filter (EKF) is proposed. The proposed FSCPC is based on reducing the computation burden of the conventional FSCPC by using the commanded reference currents to directly calculate the reference voltage vector (RVV). Therefore, the cost function is calculated for only three times and the necessity to test all possible voltage vectors will be avoided. For sensorless control, EKF is composed to estimate the position and speed of the rotor. Whereas the performance of the proposed FSCPC essentially necessitates the full knowledge of SynRM parameters and provides an insufficient response under the parameter mismatch between the controller and the motor, online parameter estimation based on EKF is combined in the proposed control strategy to estimate all parameters of the machine. Furthermore, for simplicity, the parameters of PI speed controller and initial values of EKF covariance matrices are tuned offline using Particle Swarm Optimization (PSO). To demonstrate the feasibility of the proposed control, it is implemented in MATLAB/Simulink and tested under different operating conditions. Simulation results show high robustness and reliability of the proposed drive. Full article

Currently, there are challenges with proper disposal of cassava processing wastewater, and a need for sustainable energy in the cassava industry. This study investigated the impact of co-digestion of cassava wastewater (CW) with livestock manure (poultry litter (PL) and dairy manure (DM)), and porous adsorbents (biochar (B-Char) and zeolite (ZEO)) on energy production and treatment efficiency. Batch anaerobic digestion experiments were conducted, with 16 treatments of CW combined with manure and/or porous adsorbents using triplicate reactors for 48 days. The results showed that CW combined with ZEO (3 g/g total solids (TS)) produced the highest cumulative CH₄ (653 mL CH₄/g VS), while CW:PL (1:1) produced the most CH₄ on a mass basis (17.9 mL CH₄/g substrate). The largest reduction in lag phase was observed in the mixture containing CW (1:1), PL (1:1), and B-Char (3 g/g TS), yielding 400 mL CH₄/g volatile solids (VS) after 15 days of digestion, which was 84.8% of the total cumulative CH₄ from the 48-day trial. Co-digesting CW with ZEO, B-Char, or PL provided the necessary buffer needed for digestion of CW, which improved the process stability and resulted in a significant reduction in chemical oxygen demand (COD). Co-digestion could provide a sustainable strategy for treating and valorizing CW. Scale-up calculations showed that a CW input of 1000–2000 L/d co-digested with PL (1:1) could produce 9403 m³ CH₄/yr using a 50 m³ digester, equivalent to 373,327 MJ/yr or 24.9 tons of firewood/year. This system would have a profit of $5642/yr and a $47,805 net present value. Full article

This study examines the performance of Japanese electric power companies from 2003 to 2020. We use an observed data set from 2003 to 2015 and a forecasted data set from 2016 to 2020. The Japanese deregulation of the industry needs to be completed by April 2020. As a method, this study uses data envelopment analysis (DEA) environmental assessment, which measures performance from a holistic perspective. This research adds a new analytical capability to the DEA-based assessment by including an analytical ability to handle an “imprecise” data set. We apply the proposed approach to investigate the performance of these companies before and after the disaster of Fukushima Daiichi nuclear power plant (11 March 2011). All electric power companies have suffered from business damage due to the nuclear disaster. The Japanese government has developed a policy scheme on how to recover from the huge handling costs resulting from the disaster. Nuclear energy has been long considered the most useful approach to handle climate change. However, many industrial nations have changed policy direction since the nuclear disaster. The Japanese government allocates the costs to not only Tokyo Electric Power Company, which produced the nuclear disaster, but also the other incumbent electric power companies that own nuclear power plants. Under the current Japanese scheme, financial conditions have been gradually recovering from the damage due to the managerial efforts and by indirectly allocating the expenditure to consumers and tax payers. Full article

Electric vehicle (EV) powertrains consist of power electronic components as well as electric machines to manage the energy flow between different powertrain subsystems and to deliver the necessary torque and power requirements at the wheels. These power subsystems can generate undesired electrical harmonics on the direct current (DC) bus of the powertrain. This may lead to the on-board battery being subjected to DC current superposed with undesirable high- and low- frequency current oscillations, known as ripples. From real-world measurements, significant current harmonics perturbations within the range of 50 Hz to 4 kHz have been observed on the high voltage DC bus of the EV. In the limited literature, investigations into the impact of these harmonics on the degradation of battery systems have been conducted. In these studies, the battery systems were supplied by superposed current signals i.e., DC superposed by a single frequency alternating current (AC). None of these studies considered applying the entire spectrum of the ripple current measured in the real-world scenario, which is focused on in this research. The preliminary results indicate that there is no difference concerning capacity fade or impedance rise between the cells subjected to just DC current and those subjected additionally to a superposed AC ripple current. Full article

The United States needs to add at least 20 GW of peaking capacity to its grid over the next 10 years, led by large-scale projects in California, Texas and Arizona. Of that, about 60% must be installed between 2023 and 2027, meaning that the energy storage industry has more time to build an economic advantage by lowering costs and improving performance to compete with conventional gas peakers. In this paper, we assess the technical feasibility of utility-scale PV plus battery energy storage (PVS) to provide high capacity factors during summer peak demand periods using a target period capacity factor (TPCF) framework as an alternative to natural gas peakers. Also, a new metric called “Lifetime Cost of Operation” (LCOO) is introduced to provide a metric, focusing on the raw installation and operational costs of PVS technology compared to natural-gas fired peaker plants (simple cycle or conventional combustion turbine) during the target period window. The target period window is the time period during which it is valuable for power plants to provide firm capacity usually during early or late evening peak demand periods in the summer months (from April to September); a framework for which is increasingly being asked for by utilities in recent request for proposals (RFPs). A 50 MW_AC PV system with 60 MW/240 MWh battery storage modelled in California can provide >98% capacity factor over a 7–10 p.m. target period with lower LCOO than a conventional combustion turbine natural gas power plant. Full article

Tidal stream energy is acquiring more attention as a future potential renewable energy source. Considering the harsh submarine environment, the main challenges that face the tidal stream turbine (TST) industry are cost and reliability. Hence, simple and reliable technologies, especially considering the drivetrain, are preferred. The multibrid drivetrain configuration with only a single stage gearbox is one of the promising concepts for TST systems. In this context, this paper proposes the design optimization of a multibrid permanent magnet generator (PMG), the design of a planetary gearbox, and afterwards analyzes the multibrid concept cost-effectiveness for TST applications. Firstly, the system analytical model, which consists of a single-stage gearbox and a medium speed PMG, is presented. The optimization methodology is afterwards highlighted. Lastly, the multibrid system optimization results for different gear ratios including the direct-drive topology are discussed and compared where the suitable gear ratio (topology) is investigated. The achieved results show that the multibrid concept in TST applications seems more attractive than the direct-drive one especially for high power ratings. Full article