The variation of stator winding turns will directly affect the key parameters of a motor, such as winding resistance and winding reactance, which further affect the steady-state performance of the motor. In order to get excellent steady-state performance from line-start permanent magnet synchronous motors (LSPMSMs) under different load powers, taking an 11 kW LSPMSM as an example, the finite element method (FEM), combined with the steady-state phasor diagram and torque angle characteristic, are used in this paper for the optimal design of the stator winding turns of the prototype. The correctness of the model is verified by comparing the experimental data with the calculated data. First, the influences of different stator winding turns on the no-load, back-induced electromotive force (EMF), as well as on inductance and overload ability are studied, and the variation mechanism is obtained. In addition, from the perspective of the torque angle characteristic, the influence of the change... [more]

This paper presents a zero-voltage and zero-current switching (ZVZCS) capacitor-clamped half bridge (HB) three-level dc-dc converter (TLDC), which is well fit for high input voltage dc-dc industrial applications. The maximum voltage stress of the primary switches is limited by the flying capacitor and input capacitors, which is very close to Vin/2. Two unsymmetrical bidirectional switches are used to replace two of the primary switches in a conventional capacitor-clamped HB TLDC, which ensure ZVZCS of the main switches in wide load range. The reverse direction MOSFETs in the unsymmetrical bidirectional switches have low on-state resistance and are controlled with soft-switching mode irrelevant to the load current. Therefore, the additional power loss can be omitted. The current of the flying capacitor is greatly reduced due to ZVZCS operation, which would result in a smaller volume flying capacitor and high system reliability. Furthermore, the current imbalance problem of the power dev... [more]

The cascaded H-bridge static var generator (SVG) has been employed to provide reactive power and regulate grid voltages for many years because of its good modularity, easy scalability, and improved harmonic performance. A novel cluster-balancing power control method combining negative-sequence currents and zero-sequence voltage is proposed to redistribute the unbalanced active powers and eliminate the power oscillation under asymmetrical conditions. Simultaneously, the dynamic performance of the SVG power balance control can be improved under asymmetrical conditions with the zero-sequence voltage expression derived in this paper. On the basis of the proposed method, the fault ride through capability of star-connected SVG under asymmetrical conditions is compared among active power oscillation elimination (APOE), reactive power oscillation elimination (RPOE), and balanced positive sequence current (BPSC) injection references calculation strategies from the perspective of the zero-sequen... [more]

Spent coffee grounds (SCG) are one of the lignocellulosic biomasses that have gained much attention due to their high potential both in valorization and biomethane production. Previous studies have reported single processes that extract either fatty acids/lignin or biogas. In this study, an integrated physicochemical and biological process was investigated, which sequentially recovers lignin, fatty acid methyl esters (FAME) and biogas from the residue of SCG. The determination of optimal conditions for sequential separation was based on central composite design (CCD) and response surface methodology (RSM). Independent variables adopted in this study were reaction temperature (86.1−203.9 °C), concentration of sulfuric acid (0.0−6.4%v/v) and methanol to SCG ratio (1.3−4.7 mL/g). Under determined optimal conditions of 161.0 °C, 3.6% and 4.7 mL/g, lignin and FAME yields were estimated to be 55.5% and 62.4%, respectively. FAME extracted from SCG consisted of 41.7% C16 and 48.16% C18, which... [more]

A smart water network consists of a large number of devices that measure a wide range of parameters present in distribution networks in an automatic and continuous way. Among these data, you can find the flow, pressure, or totalizer measurements that, when processed with appropriate algorithms, allow for leakage detection at an early stage. These algorithms are mainly based on water demand forecasting. Different approaches for the prediction of water demand are available in the literature. Although they present successful results at different levels, they have two main drawbacks: the inclusion of several seasonalities is quite cumbersome, and the fitting horizons are not very large. With the aim of solving these problems, we present the application of pattern similarity-based techniques to the water demand forecasting problem. The use of these techniques removes the need to determine the annual seasonality and, at the same time, extends the horizon of prediction to 24 h. The algorithm... [more]

The European Union is a leading patron for the introduction of renewable energy, having set a target that renewable sources will represent at least 27% of total energy consumption by the year 2030. Presently, the most significant Croatian renewable resource is hydropower, which is presently at its peak and will not develop further because of limited hydro resources. Therefore, the share of electricity generation from onshore wind farms in Croatia during in recent years has grown significantly. However, as the Croatian government has already made most of the concessions for possible locations of wind farms, the aim of the present study is to evaluate a different renewable energy resource, wave energy. An assessment of the offshore wave energy potential in the Croatian part of the Adriatic Sea is performed using data taken from WorldWaves atlas (WWA). WWA is based on satellite measurements, validated against buoy measurements and reanalysed by numerical wave modelling. This assessment wa... [more]

Grid-connected low voltage photovoltaic power plants cover most of the power capacity installed in Italy. They offer an important contribution to the power demand of the utilities connected but, due to the nature of the solar resource, the night-time consumption can be satisfied only withdrawing the energy by the national grid, at the price of the energy distributor. Thanks to the improvement of storage technologies, the installation of a system of battery looks like a promising solution by giving the possibility to increase auto-consumption dramatically. In this paper, a model-based approach to analyze and discuss the performance and the economic feasibility of grid-connected domestic photovoltaic power plants with a storage system is presented. Using as input to the model the historical series (2008−2017) of the main ambient variables, the proposed model, based on Stochastic Hybrid Fault Tree Automaton, allowed us to simulate and compare two alternative technical solutions characteri... [more]

Shale oil and gas resources contribute significantly to the energy production in the U.S. Greenhouse gas emissions come from combustion of fossil fuels from potential sources of power plants, oil refineries, and flaring or venting of produced gas (primarily methane) in oilfields. Economic utilization of greenhouse gases in shale reservoirs not only increases oil or gas recovery, but also contributes to CO2 sequestration. In this paper, the feasibility and efficiency of gas injection approaches, including huff-n-puff injection and gas flooding in shale oil/gas/condensate reservoirs are discussed based on the results of in-situ pilots, and experimental and simulation studies. In each section, one type of shale reservoir is discussed, with the following aspects covered: (1) Experimental and simulation results for different gas injection approaches; (2) mechanisms of different gas injection approaches; and (3) field pilots for gas injection enhanced oil recovery (EOR) and enhanced gas reco... [more]

The main objective of this study is to develop and analyse different harbour grid configurations that can facilitate the charging of batteries for modern vessels and supply onshore power. The use of battery energy storage systems in modern hybrid or entirely electric vessels is rapidly increasing globally in order to reduce emissions, save fuel and increase energy efficiency of ships. To fully utilise their benefits, certain technical issues need to be addressed. One of the most important aspects is to explore alternative ways of charging batteries with high power capacities for modern vessels. The paper presents a comprehensive overview of battery-charging configurations and discusses the technical challenges of each design from the perspective of their practical implementation, both onshore and onboard a vessel. It is found that the proposed models are suitable for vessels operating either entirely on battery storage or having it integrated into the onboard power system. Moreover, th... [more]

For a single-stage variable-pitch axial fan, the aerodynamic performance and through flow with and without blade skewing are examined numerically. Simulated results show that the total pressure rise and efficiency increase by 2.99% and 0.16%, respectively, with the best forward-skewed angle of θ = 3° at the design conditions. At the blade pitch angles of β = 29° and 35°, the total pressure rises and efficiency of the fan with θ = 3.0° under the highest efficiency point change by −0.55%, −0.53% and 1.39%, 2.11%, respectively. At design and off-design conditions, the forward-skewed blades mitigate tip leakage and delay the emergence of separation flow at the blade root, these benefits are higher at the higher blade pitch angle. The θ = 3.0° forward skew effectively raises the stage performance of the impeller and guide vanes.

With the goal of increasing energy efficiency and passenger comfort in an automobile cabin, we developed a regional air-conditioning design to control cold air in specific regions, and an air management strategy that can keep air circulation when the car engine cuts out. According to computational simulations, an air velocity of 2 m/s was adopted, which could form an independent flow field space in the cabin with a temperature gap of 7 °C according to the user’s needs. The designed regional air-conditioning and circulation system could create independent flow field spaces with temperature differences. Inlet air volume demand was also reduced by 60% and blower power by 53 W. In addition, the cabin ventilation system led air exchange rate reached 89% per hour. In 20 min of exposure under sun, the system could lower the cabin temperature by 12.3 °C.

The problem of robust fault-tolerant control for actuators of nonlinear systems with uncertain parameters is studied in this paper. Takagi−Sugeno (T-S) fuzzy model is used to describe the wind energy conversion system (WECS). Fuzzy dedicated observer (FDO) and fuzzy proportional integral observer (FPIO) are established to reconstruct the system state and actuator fault, respectively. Fuzzy Robust Scheduling Fault-Tolerant Controller (FRSFTC) is designed by parallel distributed compensation (PDC) method, so as to realize the purpose of active fault tolerance for actuator faults and ensure the robust stability of the system. The stability of the closed-loop system is proved by Taylor series, Lyapunov function, and Linear Matrix Inequalities (LMIs). Finally, the simulation results verify that the proposed method is feasible and effective applied to WECS with doubly fed induction generators (DFIG).

The Klinkenberg slippage theory has widely been used to obtain gas permeability in low-permeability porous media. However, recent research shows that there is a deviation from the Klinkenberg slippage theory for tight reservoir cores under low-pressure conditions. In this research, a new experimental device was designed to carry out the steady-state gas permeability test with high pressure and low flowrate. The results show that, unlike regular low-permeability cores, the permeability of tight cores is not a constant value, but a variate related to a fluid-dynamic parameter (flowrate). Under high-pressure conditions, the relationship between flowrate and apparent permeability of cores with low permeability is consistent with Klinkenberg slippage theory, while the relationship between flowrate and apparent permeability of tight cores is contrary to Klinkenberg slip theory. The apparent permeability of tight core increases with increasing flowrate under high-pressure conditions, and it i... [more]

A throttle can be installed on the intake pipe of a natural gas (NG)/diesel dual-fuel engine to control the excess air ratio of the air-fuel mixture by adjusting the air intake. Building on a previously proposed NG/diesel dual-fuel supply strategy using the adjustment of excess air ratio, this work further studied the effects of different injection timing schemes on output power, fuel efficiency, and pollutant emissions of a dual-fuel engine under low to medium load conditions. In the experiment, the engine was operated at a speed of 1600 r/min, under either low (27.1 N·m) or medium (50.6 N·m) loads, and the NG substitution rate was either 40%, 60%, or 80%. The effect of different injection timing schemes on the combustion performance of the engine under low to medium load conditions was studied based on in-cylinder pressure changes detected by a pressure sensor. Experimental results showed that under medium-speed low-load conditions and a NG substitution rate of 40%, setting the diese... [more]

The last few decades have seen a significant increase in the number of electric vehicles (EVs) for private and public transportation around the world. This has resulted in high power demands on the electrical grid, especially when fast and ultra-fast or flash (at the bus-stop) charging are required. Consequently, a ground storage should be used in order to mitigate the peak power request period. This paper deals with an innovative and simple fast charging infrastructure based on supercapacitors, used to charge the energy storage system on board electric buses. According to the charging level of the electric bus, the proposed fast charging system is able to provide the maximum power of 180 kW without exceeding 30 s and without using DC−DC converters. In order to limit the maximum charging current, the electric bus is charged in three steps through three different connectors placed between the supercapacitors on board the bus and the fast charging system. The fast charging system has bee... [more]

Phase change material can be used as heat transfer fluid in the solar water heating system, which is the latest way to improve thermal efficiency. In this paper, graphene composite paraffin emulsion is used as heat transfer fluid in a solar water heating-phase change material (SWH-PCM) system. By comparing with the traditional solar water heating (SWH) system, the thermal performance characteristics of SWH-PCM system have been investigated experimentally. The SWH-PCM system has higher heat storage than the SWH system. The heat storage of SWH-PCM system and SWH system all increase with the increase of solar irradiance, while the thermal efficiency has the opposite trend. The flow rate has a greater influence on the thermal efficiency of SWH-PCM system than that of the SWH system. With the flow rate of 200 L/h, the thermal efficiency of SWH-PCM system is 14.21% higher than that of the SWH system. In summary, the SWH-PCM system is a promising solar water heating system with high heat stor... [more]

Climate change is one of the most important issues affecting the future of the planet. Then, a lot of resources are being used to actively work on climate change issues and greenhouse gas reduction. Greenhouse gas (GHG) emissions are monitored by each country and reported yearly to the United Nations Framework Convention on Climate Change (UNFCCC). The Intergovernmental Panel on Climate Change (IPCC) published the document entitled “2006 IPCC Guidelines for National Greenhouse Gas Inventories” to provide the calculation rules and the way to inform the UNFCCC of the national GHG emissions. Currently, this document does not give a procedure to calculate the net carbon dioxide emissions to the atmosphere due to the Portland cement clinker production. The purpose of this paper is to get reliable relationships to better calculate the CO2 uptake by ground granulated blast-furnace slag (GGBFS) mortars. The application of this material cured under controlled conditions could help minimize envi... [more]

Experience shows that under unidirectional wind or certain terrain, the surface of post insulators is non-uniformly polluted between windward and leeward sides, which affects the flashover characteristics. In this paper, a formulation of residual pollution layer resistance was proposed under this non-uniformity and a typical post insulator was taken as an example to analyze and calculate its residual resistance. The theoretical resistance was verified by numerical simulations using COMSOL Multiphysics. The proposed resistance formulation was then implemented in a DC flashover dynamic model to determine the flashover voltage (Ucal), which was validated by artificial flashover tests. Then the factors affecting DC flashover voltage were analyzed. Research results indicate that: the residual resistance formulation agrees well with simulation results, especially when the arc length exceeds 70% of the leakage distance. The good concordance between theoretical and experimental flashover volta... [more]

One of the major challenges facing wind energy at the moment is its dependence on dispatchable energy sources to match power supply to demand and provide an adequate spinning reserve. There is no fundamental impediment for this to be done with wind energy when wind conditions are such that sufficient wind power is available. It is, in fact, common for wind farms to participate in primary and secondary frequency regulation via droop curves, curtailment, synthetic inertia, proportional de-loading, and delta control. However, although the literature presents several approaches to turbine-level control functions of this sort, it is not trivial to extract from it a readily industrializable set of algorithms. Said extraction, focused on delta control and the addition of our own contributions, is the purpose of this paper, where we propose an extension of popular torque and pitch control algorithms, which allows delta control without the wind speed observers used by other authors.

In recent years, municipal solid waste (MSW) management has become a complex problem worldwide. Similarly, Mexico City is facing such a situation for the management and treatment of organic fraction of municipal solid waste (OFMSW). Therefore, in this work, we investigated whether leachate from the composting plant, Bordo Poniente, located in Mexico City can be used as an inoculum for the treatment of OFMSW using thermophilic anaerobic digestion (AD) with a hydraulic retention time of 30 days. We analyzed the physicochemical properties of the leachate and performed a biochemical methane potential test. Archaeal and bacterial diversity was also identified using high throughput DNA sequencing of 16S rDNA libraries. Methane yield was 0.29 m3 CH4/kg VSadded in the positive control and 0.16 m3 CH4/kg VSadded in the treatment group. The phylum, Bacteroidetes, and genus, Methanosarcina, prevailed in the leachate. However, in thermophilic conditions, the microbial communities changed, and the... [more]

Obstacle avoidance systems for autonomous driving vehicles have significant effects on driving safety. The performance of an obstacle avoidance system is affected by the obstacle avoidance path planning approach. To design an obstacle avoidance path planning method, firstly, by analyzing the obstacle avoidance behavior of a human driver, a safety model of obstacle avoidance is constructed. Then, based on the safety model, the artificial potential field method is improved and the repulsive field range of obstacles are rebuilt. Finally, based on the improved artificial potential field, a collision-free path for autonomous driving vehicles is generated. To verify the performance of the proposed algorithm, co-simulation and real vehicle tests are carried out. Results show that the generated path satisfies the constraints of roads, dynamics, and kinematics. The real time performance, effectiveness, and feasibility of the proposed path planning approach for obstacle avoidance scenarios are a... [more]

This paper reports the effects of alternate axis twisted baffle geometric parameters on the heat transfer and flow characteristics within rectangular channels. In our experiments we used modified shapes of alternate axis twisted baffles according to relative pitch ratios (s/w) equal to 2−12 and twist ratios (y/w) equal to 1−5, under conditions where the angle of attack (α) was 90° and the relative blockage height (e/Dh) was at a constant value of 0.095. The results for the Reynolds numbers based on the duct hydraulic diameter ranged from 9000 to 24,000 at a constant Prandtl number, Pr = 0.707, using air as a working fluid. A 0.05 mm thick stainless-steel foil was used as a heater, and a thermochromic liquid crystal technique was used to obtain the local temperature distribution on the heated surfaces. Images were captured in areas with periodic, fully developed regions in the channel. The results show that rectangular channels equipped with alternate axis twisted baffles demonstrated 8... [more]

Parabolic trough collector (PTC) technology is currently the most mature solar technology, which has led to the accumulation of relevant operational experience. The overall performance and efficiency of these plants depends on several components, and the heat transfer fluid (HTF) is one of the most important ones. Using molten salts as HTFs has the advantage of being able to work at higher temperatures, but it also has the disadvantage of the potential freezing of the HTF in pipes and components. This paper models and evaluates two methods of freeze recovery, which is needed for this HTF system design: Heat tracing in pipes and components, and impedance melting in the solar field. The model is used to compare the parasitic consumption in three molten salts mixtures, namely Solar Salt, HiTec, and HiTec XL, and the feasibility of this system in a freezing event. After the investigation of each of these subsystems, it was concluded that freeze recovery for a molten salt plant is possible.

With the increasing complexity of the active distribution network (ADN) due to distributed generation (DG) integration, together with the electricity market evolution, the traditional ADN is divided into multiple areas to operate independently. Due to technical problems or business privacy, the internal network regional control center cannot grasp the changes of the external regional network in time. In order to accurately reflect the distribution network operation state, a multivariable regression equivalent model is proposed in this paper. Firstly, the external network is made equivalent to a multi-port Norton model. The multivariable linear regression model is then derived based on the equivalent distribution network, and the regression model variables are constructed using boundary node information collected by the measurement equipment. Finally, the maximum likelihood estimation (MLE) is used to estimate the parameters of the multivariable linear regression model. Furthermore, cas... [more]

Owing to the necessity of the transformer for the multi-parallel inverters connected to the medium-voltage (MV) grid, the conventional multi-parallel inverter topology can be reconfigured to the dual-inverter fed open-end winding transformer (DI-OEWT) topology to obtain lower output voltage harmonics, which can reduce the requirement of the filter inductance. However, due to the special structure of the DI-OEWT topology, the arrangement scheme of the filter can be more than one kind, and different schemes may affect the filter performance. In this paper, research on the existing two kinds of filters, as well as a proposed one, for the DI-OEWT topology used in photovoltaic grid-tied applications is presented. The equivalent circuits of these filters are derived, and based on this, the harmonic suppression capability of these filters is analyzed and compared. Furthermore, a brief parameter design method of these filters is also introduced, and based on the design examples, the inductance... [more]