In determining the severity of power transformer faults, several approaches have been previously proposed; however, most published studies do not accommodate gas level, gas rate, and Dissolved Gas Analysis (DGA) interpretation in a single approach. To increase the reliability of the faults’ severity assessment of power transformers, a novel approach in the form of fuzzy logic has been proposed as a new solution to determine faults’ severity using the combination of gas level, gas rate, and DGA interpretation from the Duval Pentagon Method (DPM). A four-level typical concentration and rate were established based on the local population. To simplify the assessment of hundreds of power transformer data, a Support Vector Machine (SVM)-based DPM with high agreements to the graphical DPM has been developed. The proposed approach has been implemented to 448 power transformers and further implementation was done to evaluate faults’ severity of power transformers from historical DGA data. This... [more]

This paper presents the results of analysis of energy and economic efficiency of the hierarchical gas-gas engine, with a note that a trigeneration system was analyzed, in which the production of electricity, heat and cold are combined. This solution significantly increases the energy efficiency of the gas and gas system compared to a system without cold production. The analysis includes a system comprising a compressor chiller which is driven by an electric motor in the system, as well as a system applying the mechanical work that is carried out via a rotating shaft of rotor-based machines, i.e., a gas turbine and a turboexpander. The comfort of the regulation of the refrigerating power rather promotes the use of a solution including an electric motor. Analysis contains also a schematic diagram of the system with a absorption chiller, which is driven by low-temperature enthalpy of exhaust gases extracted from a hierarchical gas-gas engine. Application of turboexpander with heat regener... [more]

The temperature distribution and thermal efficiency of a molten salt cavity receiver are investigated by a nonuniform heat transfer model based on thermal resistance analysis. For the cavity receiver MSEE in Sandia National Laboratories, thermal efficiency in this experiment is about 87.5%, and the calculation value of 86.93−87.79% by a present nonuniform model fits very well with the experimental result. Different from the uniform heat transfer model, the receiver surface temperature in the nonuniform heat transfer model is remarkably higher than the backwall temperature. The incident radiation flux plays a primary role in thermal performance of cavity receiver, and thermal efficiency approaches to maximum under optimal incident radiation flux. In order to increase thermal efficiency, various methods are proposed and studied, including heat convection enhancement by an increase of flow velocity or the decrease of the tube diameter and number of tubes in the panel, and heat loss declin... [more]

In load rejection transitional processes in pumped-storage plants (PSPs), the process of closing pump turbines, including guide vane (GVCS) and ball valve closing schemes (BVCS), is crucial for controlling pulsating pressures and water hammer. Extreme pressures generated during the load rejection process may result in fatigue damage to turbines, and cracks or even bursts in the penstocks. In this study, the closing schemes for pump turbine guide vanes and ball valves are optimized to minimize water hammer and pulsating pressures. A model is first developed to simulate water hammer pressures and to estimate pulsating pressures at the spiral case and draft tube of a pump turbine. This is combined with genetic algorithms (GA) or non-dominated sorting genetic algorithm II (NSGA-II) to realize single- or multi-objective optimizations. To increase the applicability of the optimized result to different scenarios, the optimization model is further extended by considering two different load-rej... [more]

In view of the characteristics of the high content of SO42−, Fe2+ and Mn2+ in acid mine drainage (AMD) and low pH value, based on adsorption and biological methods, coal gangue was combined with sulfate-reducing bacteria (SRB). On this basis, four dynamic columns, including Column 1 (SRB combined with spontaneous combustion gangue from the Gaode coal mine), Column 2 (SRB combined with spontaneous combustion gangue from Haizhou), Column 3 (SRB combined with gangue from Haizhou), and Column 4 (SRB combined with gangue from Shanxi), were constructed. The efficacy of four columns was compared by the inflow of AMD with different pollution load. Results showed that the repair effect of four columns was: Column 3 > Column 2 > Column 1 > Column 4. In the second stage of the experiment, the repair effect of Column 3 was the best. The average effluent pH value and oxidation reduction potential (ORP) value were 9.09 and −262.83 mV, the highest removal percentages of chemical oxygen demand (COD) a... [more]

Enhancements in heat transfer, and consequently the thermohydraulic performance of solar air heaters (SAHs), are necessary to widen and optimize their use in many applications such as solar drying or heating buildings. In this investigation, two techniques were used. A novel solar selective coating combined with broken arc ribs roughness was employed with a SAH and the evaluation of the energetic and exergetic performance was applied under four airflow working conditions compared to a smooth absorber SAH coated with the same coating. The results revealed that the Nusselt number of roughened SAH with the new coating exhibited a notable improvement compared to a smooth absorber SAH and a roughened SAH without a coating. Furthermore, the thermal efficiency increased with the increase in the air flow rate and the maximum rise was 18.8% compared to a smooth SAH. The highest increase in exergy was 51.6% with minimum values of exergy destruction and improvement potentials. In brief, the rough... [more]

Many oil fields are full of wind energy. At present, wind power generation technology has catered to oil fields. A larger wind turbine is used to supply power to several pumping units. As a result of the structural characteristics of the pumping unit, the efficiency of the electromotor is very low, which leads to a reduction in the utilization rate of wind energy. At the same time, considering the high cost of large wind turbines, the energy saving effect is not obvious in practical applications. This paper proposes an energy supply model of a pumping unit driven by a small wind turbine and a new wind-motor hybrid structure. Instead of wind power generation technology, wind energy drives the pumping unit directly via a mechanical−hydraulic transmission system. This new mechanical-hydraulic system can optimize the power confluence of wind and electric power. To enhance the efficiency of the motor, a mathematical model and a test station were established. The correctness of the energy co... [more]

The direct injection of natural gas (NG), which is an important research direction in the development of NG engines, has the potential to improve thermal efficiency and emissions. When NG engines operate in low-load conditions, combustion efficiency decreases and hydrocarbon (HC) emissions increase due to lean fuel mixtures and slow flame propagation speeds. The effect of two combustion modes (partially premixed compression ignition (PPCI) and high pressure direct injection (HPDI)) on combustion processes was investigated by CFD (Computational Fluid Dynamics), with a focus on different injection strategies. In the PPCI combustion mode, NG was injected early in the compression stroke and premixed with air, and then the pilot diesel was injected to cause ignition near the top dead center. This combustion mode produced a faster heat release rate, but the HC emissions were higher, and the combustion efficiency was lower. In the HPDI combustion mode, the diesel was injected first and ignite... [more]

The main objective of this study was to develop the supercritical heat transfer correlation applicable for organic fluids when flowing upward in smooth tubes based on the available experimental data. The organic fluids contain R-22, R-134a, R-245fa and Ethanol and the associated heat transfer characteristics were compared with non-organic fluids like water and carbon-dioxide (CO2). It was found that the limit heat flux may result in heat transfer deterioration (HTD) of organic fluid and the corresponding values are much smaller than water or CO2. A new criterion to predict the HTD was developed and this criterion yields the best predictive ability against database. It was found that HTD occurs can be well described by the acceleration parameter evaluated at the wall condition rather than at bulk condition. For estimation of the supercritical heat transfer coefficient (HTC) for organic fluid, the present study proposes a new correlation with a physically based correction factor, which g... [more]

Adsorption refrigeration has become an attractive technology due to the capability to exploit low-grade thermal energy for cooling power generation and the use of environmentally friendly refrigerants. Traditionally, these systems work with pure fluids such as water, ethanol, methanol, and ammonia. Nevertheless, the operating conditions make their commercialization still unfeasible, especially owing to safety and cost issues as a consequence of the working pressures, which are higher or lower than 1 atm. The present work represents the first thermodynamic insight in the use of mixtures for adsorption refrigeration and aims to assess the performance of a binary system of ammonia and ethanol. According to the Gibbs’ phase rule, the addition of a component introduces an additional degree of freedom, which allows to adjust the pressure of the system varying the composition of the mixture. The refrigeration process was simulated with isothermal- isochoric flash calculations to solve the pha... [more]

From the working data of a dual-fuel marine engine, in this paper, we optimized and compared two waste-heat-recovery single-pressure steam plants—the first characterized by a saturated-steam Rankine cycle, the other by a superheated-steam cycle−using suitably developed simulation models. The objective was to improve the recovered heat from the considered engine, running with both heavy fuel oil and natural gas. The comparison was carried out on the basis of energetic and exergetic considerations, concerning various aspects such as the thermodynamic performance of the heat-recovery steam generator and the efficiency of the Rankine cycle and of the combined dual-fuel-engine−waste-heat-recovery plant. Other important issues were also considered in the comparison, particularly the dimensions and weights of the steam generator as a whole and of its components (economizer, evaporator, superheater) in relation to the exchanged thermal powers. We present the comparison results for different en... [more]

The Bolivian government’s concerns that are related to reducing the consumption of diesel fuel, which is imported, subsidized, and provided to isolated electric plants in rural communities, have led to the implementation of hybrid power systems. Therefore, this article presents the performance analysis in terms of energy efficiency, economic feasibility, and environmental sustainability of a photovoltaic (PV)/Stirling battery system. The analysis includes the dynamic start-up and cooling phases of the system, and then compares its performance with a hybrid photovoltaic (PV)/diesel/battery system, whose configuration is usually more common. Both systems were initially optimized in size using the well-known energy optimization software tool, HOMER. An estimated demand for a hypothetical case study of electrification for a rural village of 102 households, called “Tacuaral de Mattos”, was also considered. However, since the characteristics of the proposed systems required a detailed analys... [more]

Solar heat, already captured by vast asphalt fields in urban areas, is potentially a huge energy resource. The vertical soil temperature profile, i.e., low enthalpy geothermal energy, reveals how efficiently the irradiation is absorbed or radiated back to the atmosphere. Measured solar irradiation, heat flux on the asphalt surface and temperature distribution over a range of depths describe the thermal energy from an asphalt surface down to 10 m depth. In this study, those variables were studied by long-term measurements in an open-air platform in Finland. To compensate the nighttime heat loss, the accumulated heat on the surface should be harvested during the sunny daytime periods. A cumulative heat flux over one year from asphalt to the ground was 70% of the cumulative solar irradiance measured during the same period. However, due to the nighttime heat losses, the net heat flux during 5 day period was only 18% of the irradiance in spring, and was negative during autumn, when the soil... [more]

Nowadays, actuator line method (ALM) has become the most potential method in wind turbine simulations, especially in wind farm simulations and fluid-structure interaction simulations. The regularization kernel, which was originally introduced to ALM to avoid numerical singularity, has been found to have great influence on rotor torque predictions and wake simulations. This study focuses on the effect of each parameter used in the standard kernel and the anisotropic kernel. To validate the simulation, the torque and the wake characteristics of a model wind turbine were measured. The result shows that the Gaussian width ϵ (for standard kernel) and the parameter in chord length direction ϵc (for anisotropic kernel) mainly affect the normal velocity of each blade element when using ALM but have little effect on the tangential velocity calculation. Therefore, these parameters have great influence on the attack angle and rotor torque prediction. The thickness parameter ϵ t is the m... [more]

Laminar free convection heat transfer from a heated cylinder and tube arrays is studied numerically to obtain the local and average Nusselt numbers. To verify the numerical simulations, the Nusselt numbers for a single cylinder were compared to other authors for the Rayleigh numbers of 103 and 104. Furthermore, the vertically arranged heated tube arrays 4 × 1 and 4 × 2 with the tube ratio spacing SV/D = 2 were considered, and obtained average Nusselt numbers were compared to the existing correlating equations. A good agreement of the average Nusselt numbers for the single cylinder and the bottom tube of the 4 × 1 tube array is proved. On the other hand, the bottom tubes of the 4 × 2 tube array affect each other, and the Nusselt numbers have a different course compared to the single cylinder. The temperature fields for the tube array 4 × 4 in basic, concave, and convex configurations are studied, and new correlating equations were determined. The simulations were done for the Rayleigh n... [more]

For the first time, a study has been carried out on the pyrolysis of wood residues from Pinus pseudostrobus, Pinus leiophylla and Pinus montezumae, from an area in Western México using TGA analysis to determine the main kinetic parameters (Ea and Z) at different heating rates in a N2 atmosphere. The samples were heated from 25 °C to 800 °C with six different heating rates 5−30 °C min−1. The Ea, was calculated using different widely known mathematical models such as Friedman, Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose. The Ea value of 126.58, 123.22 and 112.72 kJ/mol (P. pseudostrobus), 146.15, 143.24 and 132.76 kJ/mol (P. leiophylla) and 148.12, 151.8 and 141.25 kJ/mol (P. montezumae) respectively, was found for each method. A variation in Ea with respect to conversion was observed with the three models used, revealing that pyrolysis of pines progresses through more complex, multi-stage kinetics. FT-IR spectroscopy was conducted to determine the functional groups present in the thre... [more]

The present work evaluates the application of regenerative braking for energy recovery in diesel-electric freight trains to increase efficiency and to improve decarbonization. The energy from regenerative braking has to be stored onboard when the track is not electrified. Different technologies of energy recovery are presented and discussed. The energy balance of an existing route is presented and simulated for different battery sizes. The analysis is illustrated with experimental data from an important Brazilian railway. Results show that the energy recovery from regenerative brake is a feasible investment and may be recommended to increase the efficiency in transportation and also to improve the low carbon mobility in railway systems.

Post-fracturing well shut-in is traditionally due to the elastic closure of hydraulic fractures and proppant compaction. However, for shale gas wells, the extension of shut-in time may improve the post-fracturing gas production due to formation energy supplements by fracturing-fluid imbibition. This paper presents a methodology using numerical simulation to simulate the hydrodynamic equilibrium phenomenon of a hydraulically fractured shale gas reservoir, including matrix imbibition and fracture network crossflow, and further optimize the post-fracturing shut-in time. A mathematical model, which can describe the fracturing-fluid hydrodynamic transport during the shut-in process, and consider the distinguishing imbibition characteristics of a hydraulically fractured shale reservoir, i.e., hydraulic pressure, capillarity and chemical osmosis, is developed. The key concept, i.e., hydrodynamic equilibrium time, for optimizing the post-fracturing shut-in schedule, is proposed. The fracturing... [more]

Climate change is one of the most important global problems faced by the international community. It is generally believed that increasing the consumption of renewable energy is an effective measure to promote CO2 emissions reduction. Therefore, renewable energy consumption has become one of the best alternative strategies for sustainable development. Based on this, this paper employs the 3SLS model to conduct an empirical study on the relations among real output, renewable energy consumption, and CO2 emissions of BRICS countries (except Russia) in 1999−2014. The empirical results support, for BRICS group, the complete tri-variate relationships (energy-output-emission nexus), and renewable energy had a significant positive impact on the real output, and vice versa. Besides, compared with other countries, Brazil also has the same tri-variate relationships as BRICS group. However, China has no relationship from real output to renewable energy consumption and from real output to CO2 emiss... [more]

The definition of appropriate energy targets for large industrial processes is a difficult task since operability, safety and plant layout aspects represent important limitations to direct process integration. The role of heat exchange limitations in the definition of appropriate energy targets for large process sites was studied in this work. A computational framework was used which allows to estimate the optimal distribution of process stream heat loads in different subsystems and to select and size a site wide utility system. A complex Swedish refinery site is used as a case study. Various system aggregations, representing different patterns of heat exchange limitations between process units and utility configurations were explored to identify trade-offs and bottlenecks for energy saving opportunities. The results show that in spite of the aforementioned limitations direct heat integration still plays a significant role for the refinery energy efficiency. For example, the targeted h... [more]

An experimental study was carried out to explore the heat transfer characteristics on the outside of smooth and enhanced tubes, during evaporation and condensation of R134A in the annulus of a tube-in-tube heat exchanger. The three-dimensional enhanced surface tube consisted of primary enhancement patterns and secondary patterns; results were compared to the performance of an equivalent smooth tube. The equivalent external diameter of the inside horizontal copper tubes used in this study was 19.05 mm, while the outer tube varied in size, allowing a comparison of heat transfer for different annulus dimensions. Tests were conducted with a fixed inlet/outlet vapor quality and a constant saturation temperature for varied mass velocities in the range of 30 to 100 kg/(m2∙s). For condensation, the ratio of heat transfer coefficient enhancement (enhanced tube/ smooth tube) was up to 1.78; this can be attributed to the turbulence increase, as well as liquid film re-distribution, produced from t... [more]

Due to the strengthening of air-quality regulations, researchers have been investigating methods to improve excavator energy efficiency. Many researchers primarily conducted simulation studies employing mathematical models to analyze the energy consumption of excavator systems, which is necessary to examine the fuel efficiency improvement margin and the improvement effect. However, to effectively study the improvement of excavator efficiency, the real-time energy consumption characteristics must be examined through simulations and analyses of actual equipment-based energy consumption. Accordingly, this study establishes an energy flow-down model for the entire excavator system based on actual equipment tests. A measurement system is built to measure the required data, thereby establishing an experimental methodology for modeling each component. This paper presents an excavator system energy flow-down methodology that integrates the energy flow-down model, measurement system, and experi... [more]

In an effort to improve the energy efficiency of existing buildings, it is necessary to first evaluate the energy performance of those buildings. Since it is difficult to obtain detailed information on existing buildings, the challenge is how to conduct reliable energy performance assessments with this limited information. As a result, many countries have adopted evaluation systems based on measured energy consumption data for existing buildings. This study aims to analyze the building energy consumption and characteristics using Korea’s national building database and provide an energy performance benchmark for continuous management of the energy performance of existing buildings. We analyzed the relationship between the basic statistical characteristics of the information collected from the national integrated energy database and energy consumption. The total floor area was found to be closely related to energy consumption, and various regression analysis methods were applied and comp... [more]

For the purpose of collecting solar radiation for energy conversion and utilization and improving the output performance of thermoelectric power-generation components, a new solar thermoelectric conversion device based on an all-glass solar heat transfer pipe and gravity-assisted heat pipe with recycling air cooling and water cooling circuits is designed. The uniqueness of the device lies in the combination of gravity-assisted heat pipes with excellent thermal conductivity and a direct air-cooled mode, a fin-cooled mode, and two solar-driven water-cooling modes with different flow rates. Based on the structure, the device can realize four separate output modes and multiple composite output modes and has practical significance for meeting different load power requirements, such as wireless sensors and electronics. Under a state of regular illumination from 3.14 × 104 lx to 10.04 × 104 lx, with one thermoelectric power generator (TEG) in one mode, the peak output voltage and power values... [more]

This paper aims to present a comparative study into the cascade and series configurations of the organic Rankine cycle based small-scale solar combined cooling, heating and power system for civil application. The energy performance of the systems is studied by developing a thermodynamic model. The simulation model is validated using the literature results. Analyses of the research results indicated that the cascade system can achieve maximum value of the primary energy efficiency of 13.4% for cooling and power generation under solar collecting temperature of 115 °C in cooling mode. The cascade system has more cooling output and less electricity output in cooling mode compared with the series system. In heating mode, the single solar organic Rankine cycle (ORC) operation can achieve highest primary energy efficiency of 19.6% for heating and power generation under solar collecting temperature of 100 °C. Systems with R141b as ORC working fluid show better performance than those with R123... [more]