The rapid growth in energy consumption and environmental pollution have greatly stimulated the exploration and utilization of shale gas. The injection of gases such as CO2, N2, and their mixture is currently regarded as one of the most effective ways to enhance gas recovery from shale reservoirs. In this study, molecular simulations were conducted on a kaolinite−kerogen IID composite shale matrix to explore the displacement characteristics of CH4 using different injection gases, including CO2, N2, and their mixture. The results show that when the injection pressure was lower than 10 MPa, increasing the injection pressure improved the displacement capacity of CH4 by CO2. Correspondingly, an increase of formation temperature also increased the displacement efficiency of CH4, but an increase of pore size slightly increased this displacement efficiency. Moreover, it was found that when the proportion of CO2 and N2 was 1:1, the displacement efficiency of CH4 was the highest, which proved th... [more]

Vertical-axis wind turbines (VAWTs) are compact and efficient and have become increasingly popular for wind energy harvesting. This paper mainly focuses on free and forced vibration analysis of two different types of VAWTs, i.e., an H-type VAWT and a new hybrid VAWT. The H-type VAWT has a lower cost, while the hybrid VAWT has a better self-starting capability at a low wind velocity. Both of them can be used for wind energy harvesting. By using the assumed modes method, the two VAWTs are simplified by a single degree-of-freedom (SDOF) model. By utilizing the method of structural mechanics, a multi-degree-of-freedom (MDOF) model is developed for the two VAWTs and the turbines in them are reasonably simplified. Natural frequency analyses for the SDOF and MDOF models of the two VAWTs are conducted. A beam element model (BEM) of the two VAWTs is created to calculate their natural frequencies and mode shapes and to verify natural frequency results from the SDOF and MDOF models. By using the... [more]

This study investigated the technological developments in the shale petroleum industry by analyzing patent data using a network of technological indices. The technological developments were promoted by the beginning of the shale industry, and after the first five years, it showed a more complex development pattern with the convergence of critical technologies. This paper described progress in the shale petroleum technologies as changes in relatedness networks of technological components. The relatedness represents degree of convergence between technological components, and betweenness centrality of network represents priority of technological components. In the results, the progress of the critical technologies such as directional drilling, increasing permeability, and smart systems, were actively carried out from 2012 to 2016. Especially, unconverged technology of increasing permeability and the converged technology of directional drilling and smart system has been intensively develop... [more]

As concerns regarding the adverse impacts of energy production and consumption on the environment grow, countries across the world are now charged with developing effective strategies that provide energy security and protect the environment. This means that efforts to generate significant investments and business opportunities to boost the growth of renewable energy need to increase rapidly. However, there are limited studies on what will facilitate the increase of renewable energy investment in Africa. The main factor considered in this study relates to the sensitivity to changes in oil prices, gross domestic product (GDP), interest rate and oil price volatility’s impact on the renewable energy investment (REI) in countries with energy security concerns and if there is any significant influence from oil price shocks. With the help of an unrestricted vector retrogressive model and an annual panel data approach that covers the period 1990−2018, this paper examines the link between renew... [more]

This research optimises the mix and structure of Generation Companies (GenCos) in the Sumatra power system, Indonesia. Market power, indicating the ability to raise prices profitably above the competitive level, tends to be a significant problem in the aftermath of electricity market restructuring. In the process of regulatory reform and the development of competitive electricity markets, it is desirable and practical to establish an efficient number of competitor GenCos. Simulations of a power system account for multi-plant mergers of GenCos subject to a regulatory measure of the Residual Supply Index and the influence of direct current load flow and the topology of the system. This study simulates the Sumatra power system in order to determine the following: optimal market structure, efficient GenCo generation mix, and the optimal number of competitive GenCos. Further, this study seeks to empirically optimise the electricity generation mix and electricity market structure of the Suma... [more]

For this work, the specific heat value of a Li-ion cell was determined experimentally as if it were a homogeneous body. The heat absorbed in the cell was compared with the amount of energy contained in the charged cell. It was found that a fully charged cell poses a risk of spontaneous combustion in the event of the release of electrical energy. On the basis of literature research, the combustion process of a lithium cell has been described. The formula was derived for the value of the state of charge that does not pose a risk of self-ignition. In view of the existing threats, the currently used protection against cell damage and tests to demonstrate the safety of cells were analyzed. It has been indicated that currently used battery management systems do not guarantee the safety in a state of developing thermal runaway process. A new active way of protecting cells in a battery has been proposed, consisting in sectoral discharge of cells. The use of this solution would be important for... [more]

Tumbling mills have been widely implemented in many industrial sectors for the grinding of bulk materials. They have been used for decades in the production of fines and in the final stages of ore comminution, where optimal levels for the enrichment particles’ sizes are obtained. Even though these ubiquitous machines of relatively simple construction have been subjected to extensive studies, the industry still struggles with very low energy efficiency of the comminution process. Moreover, obtaining an optimal size for the grinding product particles is crucial for the effectiveness of the following processes and waste production reduction. New, innovative processing methods and machines are being developed to tackle the problem; however, tumbling mills are still most commonly used in all ranges of the industry. Since heavy equipment retrofitting is the most costly approach, process optimization with dedicated models and control systems is the most preferable solution for energy consumpt... [more]

In this paper, a procedure to simulate an electronic power converter for control design and optimization purposes is proposed. For the addressed application, the converter uses SiC-MOSFET technology in bidirectional battery chargers composed of two power stages. The first stage consists of a single-phase AC/DC power factor correction synchronous rectifier. The following stage is a DC/DC dual active bridge. The converter has been modulated using a phase-shift technique which is able to manage bidirectional power flows. The development of a model-based simulation approach is essential to simplify the different design phases. Moreover, it is also important for the final validation of the control algorithm. A suitable tool consisting of a system-level simulation environment has been adopted. The tool is based on a block diagram design method accomplished using the Simulink toolbox in MATLAB™.

In Poland, existing barrages are characterized by relatively high flow and low head, which is challenging for the effective utilization of theoretical watercourse power. The paper presents the impact of the minimum head of the hydro sets on the annual electricity production of small hydropower plants at low-head locations for two types of water turbines: Archimedes and Kaplan turbines. A developed mathematical model was used to simulate energy yield from Archimedes and Kaplan turbines for a given value of the minimum technical head, depending on the number of installed hydro sets. For economic analysis purposes, the levelized cost of electricity (LCOE) and net present value (NPV) indicators were calculated. The conducted research allowed for comparing Archimedes and Kaplan’s turbine operating conditions and how the minimum head parameter influences their electricity production and utilization time. As concluded in the results, the influence of minimum head in energy production is more... [more]

Airtightness is a major issue in architectural design and it has a significant impact on the energy performance of buildings. Moreover, the energy behaviour of built heritage is due, to its singular characteristics, still a great unknown. The aim of this study is to establish a better knowledge of the airtightness of historical buildings, based on an in depth field study using blower-door tests. A set of 37 enclosures were analyzed inside eight buildings located in historical areas of a Spanish city with a significant built heritage. They were constructed between 1882 and 1919 and include diverse construction typologies applied for many building uses such as residential, cultural, educational, administrative and emblematic. The results indicate lower values compared to other previous airtightness studies of historical buildings. The average air change rate was found to be n50 = 9.03 h−1 and the airtightness of the enclosures presented a wide range of between 0.68 and 37.12 h−1. Three m... [more]

The article presents the problem of modelling the charging of a constant-phase bed storage in the first hours of the process. The places of errors in the heat transfer calculations for the packed beds were indicated. Granite in the shape of spheres and crushed rocks, with a characteristic dimension of 50 mm, was used for the experimental tests. The material was subjected to tomographic examination and then used as a storage material. The charging process was carried out for three flow rates: 0.006, 0.008 and 0.010 m3/s. After three hours of testing, the temperature of the outlet air for the granite sphere as the storage material was the same as for the granite crushed rock. However, the biggest differences occurred after 1 h of charging. They were equal to: 40.4% for the flow rate of 0.006 m3/s, 22.0% for the flow rate of 0.008 m3/s, and 18.5% for the flow rate of 0.010 m3/s. The differences were greater than the uncertainty of the measurements. As a result, different temperatures of t... [more]

Microtabs (MTs) are a regularly used flow control device in terms of wind turbine optimization. The present study introduces the application of the novel cell-set model for an MT implementation on a DU91W(2)250 airfoil. The cell-set model is based on the reusability of a mesh to add new geometries on the domain; the matching geometry is located where the user requires, and a set of cells is constructed around the mentioned geometry. Subsequently, wall boundaries are assigned to the generated region. Computational simulations were carried out for fully mesh and cell-set models: MT lengths were set at 1.0%, 1.5% and 2.0% of the airfoil chord length (c) and the MTs were placed at 93% and 95% of c from the leading edge of the airfoil. Resulting data showed that the MT behavior was similar for both models with regard to aerodynamic performance curve representations. A global relative error of 3.784% was obtained for the cell-set model and a maximum relative error of 7.332% was determined. Q... [more]

Solar photovoltaic (PV) energy is one of the most viable renewable energy sources, considered less polluting than fossil energy. However, the average power conversion efficiency of PV systems is between 15% and 20%, and they must operate with high efficiency. Photovoltaic cells have non-linear voltage−current characteristics that are dependent on environmental factors such as solar irradiation and temperature, and have low efficiency. Therefore, it becomes crucial to harvest the maximum power from PV panels. This paper aims to study and analyze the most common and well-known maximum power point tracking (MPPT) algorithms, perturb and observe (P&O) and incremental conductance (IncCond). These algorithms were found to be easy to implement, low-cost techniques suitable for large- and medium-sized photovoltaic applications. The algorithms were tested and compared dynamically using MATLAB/Simulink software. In order to overcome the low performance of the P&O and IncCond methods under time-v... [more]

The article describes a case study when the voltage collapse during lightning impulse tests of new power transformers was noticed and when the repeated tests finished with a positive result. The step-by-step process of reaching the conclusion on the basis of dissolved gas analysis (DGA) as a key method of the investigations was presented. The considerations on the possible source of the analysis showed that the Duval triangle method, used in the analysis of the concentration of gases dissolved in oil samples taken from bushings, more reliably and unambiguously than the ratio method recommended in the IEC 60599 Standard, indicated a phenomenon which was identified in the insulation structure of bushings analyzed. Additionally, the results from DGA were found to be consistent with an internal inspection of bushings, which showed a visible trace of discharge on the inside part of the epoxy housing, as a result of the lightning induced breakdown.

In the present study, the effect of different kinds of impellers with different baffles or no baffle was investigated. Up-pumping pitched blade turbine (PBTU) and Rushton turbine (RT) were the two types of impellers tested. The reactor was equipped with different designs of baffles: full, half and surface baffles or no baffles. Single (PBTU or RT) and dual (PBTU/PBTU or RT/RT) use of impellers with full (FB), half (HB), surface (SB) and no baffle (NB) combinations formed two sets of 16 experiments. There was estimation of rate of hydrate formation, induction time, hydrate productivity, overall power consumption, split fraction and separation factor. In both single and dual impellers, the results showed that RT experiments are better compared to PBTU in rate of hydrate formation. The induction time is almost the same since we are deep in the equilibrium line while hydrate productivity values are higher in PBTU compared to RT experiments. As general view RT experiments consume more energ... [more]

The projected increase in the availability of gaseous fuels by growing popularity of household natural gas (NG) filling stations and the increase in the production of gaseous biogas-derived fuels is conducive to an increase in the use of NG fuel. Currently, natural gas in various forms (compressed natural gas (CNG), liquefied natural gas (LNG)) is popular in maritime, rail and road transport. A new direction of natural gas application may be non-road mobile machines powered by a small spark-ignition engine (SI). The use of these engines in the wood chippers can cause the reduction of machine costs and emissions of harmful exhaust gases. In addition, plant material chippers intended for composting in bio-gas plants can be driven by the gas they are used to produce. The biogas can be purified to bio-methane to meet natural gas quality standards. The article presents the design of the natural gas supply system, which is an upgrade of the Lifan GX 390 combustion engine spark ignition engin... [more]

The main goal of this work was to conduct a comparative analysis of the electrical properties of the silicon implanted with neon ions, depending on the dose of ions and the type of substrate doping, for the possibility of generating additional energy levels by ion implantation in terms of improving the efficiency of photovoltaic cells made on its basis. The article presents the results of research on the capacitance and conductance of silicon samples doped with boron and phosphorus, the structure of which was modified in the implantation process with Ne+ ions with energy E = 100 keV and different doses. The analysis of changes in electrical properties recorded at the annealing temperature of the samples Ta = 298 K, 473 K, 598 K, 673 K, and 873 K, concerned the influence of the test temperature in the range from 203 K to 373 K, as well as the frequency f from 100 Hz to 10 MHz, and voltage U from 0.25 V to 2 V. It was possible to detect intermediate bands in the tested samples and determ... [more]

Vehicle transmission which has discrete gear-stages inevitably produces torque drop during shifting gears. This torque drop should be minimized because it may lead to uncomfortable driving feeling and degradation of acceleration performance. In accordance with the spread of electric-powered vehicle technology, this study proposes novel transmission architecture to eliminate torque drop during shifting gears by using one electric motor and verifies its operating concept through experiments with a test-bench. The proposed transmission, called CGST (clutchless geared smart transmission) can synchronize the gear-shaft to be engaged for the next gear-stage with the output shaft by using a planetary gear-set and an electric motor. The CGST has a dual input gear-box with even and odd gear-stages on different input shafts, and the planetary gear-set and the electric motor control the speeds of each input shafts to smoothly engage the next gear-stage. This idea was verified by the simplified te... [more]

Energy efficiency and management is certainly one of the key drivers of human progress. Thus, the trends in the energy research are a topic of interest for the scientific community. The aim of this study is to highlight global research trends in this field through the analysis of a scientific journal indexed exclusively in the energy and fuels category. For this purpose, a journal has been selected that is in the center of the category considering its impact factor, which is only indexed in this category and of open access, Energies of the publisher MDPI. Therefore, a bibliometric analysis of all the contents of the journal between 2008 and 2020, 13,740 documents published, has been carried out. Analyzing the articles that are linked to each other by their citations, 14 clusters or research topics have been detected: smart grids; climate change−electric energy community; energy storage; bioenergy sources; prediction algorithms applied to power; optimization of the grid link for renewab... [more]

An important trend of Building Integrated Solar Thermal (BIST) system is to improve the aesthetic aspect of the solar collector to meet the requirement of architectural style and energy collection. Painting on the glass cover in an appropriate method is a simple and practical way. In this study, a halftone coating was used to print a red brick wall pattern on the glass cover. A series of comparative experiments were carried out to test the effect of the coating on the thermal behavior of the solar collector. In heat collection processes, compared with the solar collector with blank cover plate, the one with coated cover plate has lower absorber plate temperature and higher cover plate temperature. The lower the solar irradiance, the smaller the effect of color coating on the solar collector. Compared with the uncoated surface, the coated surface is more sensitive to solar irradiation. In the same heat collection process, compared with the solar collector coated on the outer surface of... [more]

Air handling units are usually used to maintain the thermal and humidity parameters in indoor swimming pools that are used for lap swimming and recreation. Their use throughout the year consumes much more energy than, for example, air conditioning in residential or office buildings. Their considerable energy consumption is due to the need for their continuous operation to remove moisture gains from the evaporation of water and to heat the air and cover the heat losses of the pool. It is possible to design ventilation devices in such a way that the operating costs will be reduced, and the environmental impact will be significantly reduced. In this article, six different ventilation units in which different heat sources are used to heat the ventilation air have been evaluated; the selection of which was based on the most commonly used solutions. The results of the analysis are aimed to show which of the available heat sources is the most advantageous in terms of global and operating cost... [more]

In view of the disastrous air pollution in the Silesian−Kraków region, reducing emissions from the combustion of conventional fuels is a particularly important issue. Geothermal energy is among the clean and renewable sources of heat that could be used to this end, for instance for recreation or space heating purposes. However, its use in the region is limited, mainly due to insufficient knowledge of the characteristics of the geothermal complexes present. Mesozoic complexes in the region have much smaller energy resources compared to the Polish Lowlands or Podhale, but Paleozoic ones, mainly the Lower Carboniferous and Devonian, offer realistic prospects for energy extraction. The aim of this study was to identify geothermal conditions within the Lower Carboniferous−Devonian complex, primarily in terms of using the waters present there for space heating purposes. The prospective zones identified and indicated during the research are only forecasts, and their actual utility can only be... [more]

Usually thermal response tests are restricted to big geothermal projects; the high investment makes them less suitable for designing domestic low-enthalpy geothermal energy systems. The work here presented aims to study the influence of time reduction in thermal response tests on their precision. Due to the importance of the correct assessment of the thermal characterization of the ground for any kind of geothermal system, time reduction in this essay could make it more affordable to be implemented in some domestic systems. A thermal response test has been implemented, and several time intervals of the test have been considered in order to obtain different results for the thermal conductivity of the ground. The mentioned results have been then compared and also the domestic geothermal systems designed from them by the use of the geothermal software GES-CAL. Results have shown that, in some cases (our testing borehole has some singular characteristics), a significant time reduction in t... [more]

Metal halide perovskites have been investigated for the next-generation light-emitting materials because of their advantages such as high photoluminescence quantum yield (PLQY), excellent color purity, and facile color tunability. Recently, red- and green-emissive perovskite light-emitting diodes (PeLEDs) have shown an external quantum efficiency (EQE) of over 20%, whereas there is still room for improvement for blue emissive PeLEDs. By controlling the halide compositions of chloride (Cl−) and bromide (Br−), the bandgap of perovskites can be easily tuned for blue emission. However, halide segregation easily occurrs in the mixed-halide perovskite under light irradiation and LED operation because of poor phase stability. Here, we explore the effect of A-site cation engineering on the phase stability of the mixed-halide perovskites and find that a judicious selection of low dipole moment A cation (formamidinium or cesium) suppresses the halide segregation. This enables efficient bandgap t... [more]

MgAl2-xGaxCl8 and MgGa2Cl7Br was synthesized from MgCl2, MgBr2, AlCl3, and GaCl3, and the physical properties were evaluated by XRD, AC conductivity, and X-ray fluorescence. MgAl2-xGaxCl8 has the same crystal structure as MgAl2Cl8, belongs to the space group C2/c, and forms a solid solution in all the synthesized compositions. We attempted to MgGa2Cl8-yBry synthesize in the same manner as MgAl2Cl8, but only obtained MgGa2Cl7Br as a single-phase compound. The AC conductivity of MgAl2-xGaxCl8 was lowest for MgAl2Cl8 and highest for MgGa2Cl8. The conductivity of MgGa2Cl8 at 400 K was 1.7 × 10−6 S/cm. The conductivity of MgGa2Cl7Br was higher than that of MgAl2-xGaxCl8 with a value of 1.6 × 10−5 S/cm at 400 K. The formation of Mg metal was observed at the surface of the cathode electrode in an experiment of flowing a direct current using an electrochemical cell, indicating that this compound is a magnesium ion conductor.