Low-pressure Gas Research Articles

Research on the Multiscale Microscopic Pore Structure of a Coalbed Methane Reservoir

Coal rock pores are the space in which coalbed gas is stored and flows. Accurately characterizing the pore structure of coalbed gas is the foundation of coalbed gas reserve assessment and production forecasting. Traditional experimental methods are unable to characterize the multi-scale pore structure characteristics of coal rock. In this paper, a multi-scale pore structure characterization method is proposed by coupling various experimental methods, including low-pressure nitrogen gas adsorption experiments, X-ray computed tomography (XCT) imaging technology, and scanning electron microscopy (SEM). Using Zhengzhuang coalbed gas as an example, the micro-pore structure of coalbed gas reservoirs is characterized and depicted from a multi-scale perspective. The results indicate that a single experimental approach can only partially reveal the microstructure of coal rock pores. The combined use of multiple methods can accurately reveal the full-scale microstructure of coal rock pores. The pore structure of the experimental coal rock samples exhibits multi-scale characteristics, with a complex variety of pore types, including inorganic pores, organic pores, and fractures. Organic pores are predominant, with a small number of inorganic pores, and their sizes range from 2 nm to 50 μm. Mineral particles and fractures are observed at both the nanoscale and microscale, exhibiting typical multi-scale characteristics, with quartz being the predominant mineral.

Influence of tectonic preservation conditions on the nanopore structure of shale reservoir: A case study of Wufeng-Longmaxi Formation shale in western Hubei area, south China

Tectonism is one of the dominant factors affecting the shale pore structure. However, the control of shale pore structure by tectonic movements is still controversial, which limits the research progress of shale gas accumulation mechanism in the complex tectonic region of southern China. In this study, 34 samples were collected from two exploratory wells located in different tectonic locations. Diverse experiments, e.g., organic geochemistry, XRD analysis, FE-SEM, low-pressure gas adsorption, and high-pressure mercury intrusion, were conducted to fully characterize the shale reservoir. The TOC, Ro, and mineral composition of the shale samples between the two wells are similar, which reflects that the shale samples of the two wells have proximate pores-generating capacity and pores-supporting capacity. However, the pore characteristics of shale samples from two wells are significantly different. Compared with the stabilized zone shale, the porosity, pore volume, and specific surface area of the deformed zone shale were reduced by 60.61%, 64.85%, and 27.81%, respectively. Moreover, the macroscopic and fine pores were reduced by 54.01% and 84.95%, respectively. Fault activity and uplift denudation are not conducive to pore preservation, and the rigid basement of Huangling uplift can promote pore preservation. These three factors are important reasons for controlling the difference in pore structure between two wells shales. We established a conceptual model of shale pores evolution under different tectonic preservation conditions. This study is significant to clarify the scale of shale gas formation and enrichment in complex tectonic regions, and helps in the selection of shale sweet spots.

Fractal Analysis of Coal Pore Structure Based on Low-Pressure Gas Adsorption and Its Influence on Methane Adsorption Capacity: A Perspective from Micropore Filling Model

Fractal Analysis of Coal Pore Structure Based on Low-Pressure Gas Adsorption and Its Influence on Methane Adsorption Capacity: A Perspective from Micropore Filling Model

Deposition of carbon nanolayers by PECVD on ceramic substrates

Graphene layers and nanostructures were deposited on silicon dioxide (SiO2/Si) and silicon carbide (SiC) substrates at low gas pressure (1 – 5 torr) by microwave discharge PECVD (Plasma Enhanced Chemical Vapor Deposition). The advantage of this method is the relatively low temperature (600-700°C) of the substrate in the deposition process. The diffusion processes of hydrocarbon radicals on the surface of the substrates have a significant effect on the homogeneity of deposited structures. The deposited graphene nanotubes on SiC were analyzed by scanning electron microscope (SEM) and Raman spectroscopy is applied for characterization of the graphene layers. The deposited carbon layers on SiO2 were analyzed by atomic force microscope and their thickness (12-20 nm) were determined.

Gain measurements of longitudinally excited CO2 laser using low-pressure gas without helium

The gain characteristics of a CO2 laser pumped by a longitudinal pulsed discharge were investigated to develop a compact He-free CO2 laser operating at a high repetition rate. The longitudinal discharge tube had an inner diameter of 0.8 cm and a length of 45 cm. High-voltage pulses with a discharge starting voltage of 13.4–30.2 kV at a rise time of about 321 ns were applied to the discharge tube. CO2/N2 mixture gas with mixing ratios of 1:0, 1:1, 1:2 and 1:3 was used. The small signal gain coefficient was measured by inputting a seed CO2 laser pulse into the longitudinal discharge tube. The small signal gain coefficient depended on the delay time between the start of the discharge in the longitudinal discharge tube and the input of a seed CO2 laser pulse, the gas mixing ratio, the gas pressure, the repetition rate and the input energy to the longitudinal discharge tube. In this work, in He-free gas, the maximum small signal gain coefficient at a repetition rate of 300 Hz was 2.11%/cm with a delay time of 30.4 μs, a CO2/N2 gas mixing ratio of 1:2, a gas pressure of 2.6 kPa and an input energy of 191 mJ.

The influence of hydrocarbon generation on the sealing capability of mudstone caprock rich in organic matter

To investigate the sealing capability of mudstone caprock during the evolution of organic matter (OM)-rich mudstone, a series of hydrous pyrolysis experiments were first conducted to examine the impact of hydrocarbon generation. The pore type, pore structure, porosity, and gas breakthrough pressure of pyrolytic residual samples were analyzed by field emission scanning electron microscopy, low pressure nitrogen adsorption measurements, porosimetry, and gas breakout core experiments. To model the environment at different depths, these six experiments on hydrous pyrolysis were performed at different temperatures, lithostatic pressures, and hydrodynamic pressures, while other experimental factors such as the original sample, heating time, and rate were kept constant. The results showed that during the thermal evolution process, hydrocarbons were generated from OM in mudstone, resulting in the formation of pores within the OM. Organic acids produced by hydrocarbon generation effectively dissolved minerals, leading to the creation of numerous dissolution pores. Changes in pore type led to changes in pore structure and porosity. The volume of micropores and macropores showed an increasing trend before reaching a Ro value of 1.41%. However, after passing this threshold, they began to decrease. The volume of mesopores showed a decreasing trend before reaching a Ro value of 1.32%. After 1.32%, they began to increase. The porosity was mainly affected by the pore volumes of the mesopores and macropores. The porosity exhibited two peaks: the first occurred at a Ro value of 0.72%, with a porosity level of 4.6%. The second occurred at a Ro value of 1.41% and a porosity level of 10.3%. The breakthrough pressure was a comprehensive reflection of these influences, and its trend exhibited a negative correlation with porosity (R2 = 0.886). For two high values of porosity, the breakthrough pressure corresponded to two low values. Smaller values of the breakthrough pressure indicated a poorer sealing capability of the mudstone caprock. Overall, hydrocarbon generation in the mudstone affected the sealing capability. The mudstone in the studied area exhibited good sealing at Ro below 1.32%. However, once above the 1.32% threshold, the fluctuations of the breakthrough pressure values exhibited considerable variability, requiring a comprehensive evaluation to assess its sealing capability.

Zeolites Derived from Natural Kaolinite for CO2 Adsorption

This manuscript deals with the synthesis of different types of zeolites from natural kaolinite samples for CO2 adsorption. A zeolite A was prepared from kaolinite by means of an alkaline fusion process, followed by hydrothermal treatment, whereas a highly crystalline zeolite X was synthesized by optimizing the previously mentioned synthetic procedure. In detail, the SiO2/Al2O3 molar ratio in the preliminary mixture was modified with the addition of a secondary silicon source (sodium silicate) in order to obtain the one required for zeolites X. The physicochemical properties of the pristine clay and of the different zeolites were investigated by means of a multi-technique approach, including XRPD; SEM-EDX; 23Na, 27Al and 29Si MAS NMR spectroscopy; and N2 physisorption analysis at 77 K. Since the Si and Al molar ratios and reactivities are key parameters for the synthesis of zeolites, these aspects, primarily related to the use of a naturally occurring aluminosilicate as the raw material, have been investigated for their correlation with the physicochemical properties of the synthetic products. Moreover, by means of a custom-built volumetric apparatus, the CO2 adsorption capacity of the resulting zeolites at low gas pressures (<1 bar) and at 25 °C was assessed.

Sputter grown CuO thin films: Impact of growth pressure and annealing temperature on their microstructural architectures

High-quality copper oxide (CuO) thin films were deposited on the silicon (Si) substrate at the room temperature using the physical vapour deposition (PVD) technique named radio frequency (RF) sputtering. The copper-oxide thin-films were single crystalline and of uniform thickness. Subsequently, the influence of growth pressure (low gas pressure - 3 mTorr and high gas pressure - 100 mTorr) and post growth annealing at different temperatures (300 °C to 700 °C) were investigated to understand the microstructural and morphological changes of the thin film. With the influence of growth pressure and post thermal annealing temperature, significant changes in crystallinity, surface roughness, and surface oxidation rate of the CuO thin film were detected, which were adequately analyzed via several characterization techniques. X-ray diffraction (XRD) patterns revealed the phase formation with good crystallinity of the film, which is substantiated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) characterization. Atomic force microscopy (AFM) images disclosed that the surface roughness of the film and grain size. By gaining insights into the structural and surface properties of CuO/Si thin films, this research presents new prospects for tuning of CuO phases, structures, and compositions for multifunctional applications.

Role of moisture content in coal oxidation during the spontaneous combustion latency

The role of moisture content in coal oxidation during the spontaneous combustion latency was studied by an isothermal flow reactor. The physical and chemical changes during the drying and oxidation process were investigated using low-pressure nitrogen gas adsorption (LP-N2GA) experiments, Electron spin resonance (ESR) experiments and X-ray photoelectron spectroscopy (XPS) experiments. Experimental results reveal that moisture acts either as a promoting or an inhibiting agent under different contents. The differences in total temperature rise (TTR) of varying coal samples are the result of a complex set of competing factors. The LP-N2GA experiments reveal no pore collapse until the moisture reaches 3.05 %. Collapse clearly occurs with the micropore and mesoporous size range. When the moisture content is higher 3.05 %, the specific surface and pore volume increases with the removal of moisture. In addition, the evaporation of water also re-exposes the previously covered active sites and increases the concentration of free radicals. However, the removal of moisture makes the generation of peroxides more difficult due to moisture acts as a catalyst in the formation of such peroxide species.

Plasma density distribution and its perturbation by probes in axially symmetrical plasma

An analysis of plasma density distributions at arbitrary ion–atom collisionality for one-dimensional axially symmetrical cylindrical and annular plasmas is presented. Perturbations of plasma densities caused by a cylindrical probe are studied for arbitrary ion–atom collisionality. Analytical expressions for the plasma characteristics near the probe for low collisionality have been obtained. The plasma was modeled by the hydrodynamic neutral plasma equations, taking into account ionization, ion inertia, and a non-linear ion frictional force, which dominates the plasma transport at low gas pressures. Significant plasma density depletion around the probe has been observed for a wide range of ion–atom collisionality. The presented results predict underestimation of plasma density obtained from the classical Langmuir probe procedure and should provide a better understanding of electrostatic, magnetic, and microwave probes inserted into plasmas at low gas pressure.

DC Flashover in Printed Circuit Boards at Low Gas Pressures: Mechanism and Mitigation Recommendations

DC Flashover in Printed Circuit Boards at Low Gas Pressures: Mechanism and Mitigation Recommendations

Technical and economic calculation of a high (medium) pressure dead-end gas network

The article is devoted to the analysis of situations that arose as a result of the explosion of the Nord Stream gas pipelines and Europe’s refusal of Russian gas. Russia is forced to redistribute gas flows. There was a need to increase gas supplies to China. Moreover, the corresponding gas pipelines have already been built. But most importantly, the prospect of providing gas to Russian consumers arose. A strange situation has arisen when Ukraine is 100% supplied with natural gas, and Russia supplies its consumers with 70%. At the same time, Russia has enormous reserves of natural gas. To supply consumers with economical and safe fuel, it is necessary to design the gas distribution system in such a way that, regardless of the mode of gas consumption, a stable pressure is maintained in front of gas appliances. This requirement can be met by using a pressure regulator for each consumer. The designed system is characterized by reduced capital costs, as there is a transition from a ring to a dead-end circuit. But most importantly, the laying of low-pressure gas pipelines is excluded from the gas distribution system. When hydraulically calculating a dead-end gas network of medium and high pressure, the differences between successive sections are redistributed in such a way that the total cost of capital costs, or material characteristics, is significantly reduced. Currently, polyethylene pipes are widely used in the construction of gas networks, which have significant advantages over steel ones, since they do not corrode.

A Study of the Factors Influencing Multipactor to Low-Pressure Gas Discharge

Multipactor and low-pressure gas discharges have a negative impact on the performance and security of microwave components in space. In this manuscript, a 2-dimensional parallel flat plate structural model was designed to simulate the breakdown of multipactor to low-pressure gas discharges. The breakdown levels in the pressure range of 0.1 to 100 torr were verified under the conditions of the Ag material, 4-mm gap, 2.6-GHz frequency, and air as the background gas. Random secondary electron emission and gas molecule collision processes are incorporated, and the effect of their transition section is studied using trajectory tracking and Monte Carlo methods. A comparison of the model to the available experimental data demonstrates the model’s accuracy. On this basis, the variation of gas adsorption in relation to the surface secondary electron yields was considered, and the effect of the gas adsorption issues on breakdown level was calculated by means of numerical simulations based on some assumptions. The calculation reveals that in a finite volume of space, smaller desorption volumes cause negligible pressure changes, but the secondary electron emission capacity of the material decreases, which, in turn, leads to a flat or rising breakdown level. On the contrary, the larger the desorption volume, the more the gas is collisionally ionized and the breakdown level decreases.

Tribological properties and machine learning prediction of FeCoCrNiAlN high entropy coatings

High-entropy nitride ceramic coatings exhibit excellent wear resistance and are regarded as promising coatings in tools. Predicting the coefficients of friction (COF) and related microstrain under friction condition is critical for their application. In this study, nine different FeCoCrNiAlN high-entropy coatings were prepared with varying substrate bias voltage and nitrogen gas pressure using PVD CAIP (Physical Vapor Deposition-cathode arc-ion plating). The bias voltage and nitrogen pressure significantly influenced the wear resistance of the coatings. Low nitrogen gas pressure results in insufficient ceramic phase content, leading to lower hardness and the generation of more grooves. However, coatings with high hardness exhibit excellent wear resistance. Groove wear and debris wear are closely related to the surface particles and mechanical properties of the coating. Alterations in friction load and velocity were also conducted to investigate the tribological performance of various coatings. Results suggest that the COF increased with the friction load and velocity. Furthermore, 53 sets of COF data for machine learning (ML) predictions were obtained. Seven ML regression models, including Multilayer Perceptron (MLP), were employed to predict the COF using input features, including coating deposition process, microstructure features such as elemental composition, and grain size, as well as mechanical features such as nanohardness, H/E (Hardness/Young's modulus), Lc1 (Critical load). Friction conditions include friction load and velocity. Simultaneously, the significance of multiple features such as the coating preparation processes, microstructural, mechanical properties, and friction conditions in affecting the COF of the coatings, was analyzed by the feature importance model. The results indicate that the XGBoost regression algorithm exhibited the highest R2 value and the lowest RMSE, MAE, and MSE values, which were 0.8629, 0.0659, 0.0567, and 0.0047, respectively. This model demonstrated a favorable capability in predicting the COF of FeCoCrNiAlN high-entropy coatings. Furthermore, feature importance analysis revealed that the load, Lc1, velocity and Cr have the most significant impact on the COF of the coatings.

Boosting the Performance of on-Chip Vertically-Aligned Carbon Nanotube Supercapacitors Via Solvent-Free Radiofrequency Nitrogen Plasma Treatment

Supercapacitors are essential components for energy storage devices in many fields, including renewable energy systems, electric vehicles, and portable electronics [1]. Carbon nanotube arrays have shown great promise for use in supercapacitors due to their high surface area and excellent electrical conductivity. However, the specific capacitance of these arrays needs to be increased to improve the overall energy storage capacitance of the supercapacitor.This study presents an innovative approach to enhance the specific capacitance of on-chip vertically-aligned carbon nanotube (VACNT) structures for the development of high-performance supercapacitors. The proposed technique involves employing a solvent-free radiofrequency (RF) nitrogen plasma treatment to dope nitrogen onto VACNT surfaces. The plasma doping process utilizes reactive ion etching-induced coupled plasma (RIE-ICP) to optimize the parameters for nitrogen incorporation [2]. The nitrogen plasma treatment introduces nitrogen-containing functional groups, thereby improving their specific capacitance. The dry process is compatible with semiconductor fabrication and can be integrated into existing manufacturing techniques [1]. This study provides a promising avenue for tailoring the nitrogen doping process to achieve desired performance enhancements for on-chip VACNT supercapacitors.The plasma doping process involves the generation of reactive nitrogen species in the plasma, which subsequently interact with the VACNT surfaces. Under the influence of the RF electric field, nitrogen gas molecules are ionized and can react with the carbon atoms of the VACNTs, resulting in the formation of nitrogen-containing functional groups and the substitution of carbon atoms with nitrogen atoms within the carbon lattice [3].The improved capacitance can be attributed to the incorporation of nitrogen functional groups and to the modification of the electronic properties of the carbon nanotubes, including additional charge carriers, leading to an increase in electrical conductivity [4] and introduction of the pseudocapacitance effect [5]. There are various types of nitrogen species in the carbon lattice, such as pyridinic, pyrrolic, and graphitic nitrogen. Among these, pyridinic and pyrrolic nitrogen are known to be more electrochemically active [5], contributing to the enhanced specific capacitance. The increased electrical conductivity stems from the introduction of nitrogen dopants, which alter the electronic structure of the VACNTs, creating localized states near the Fermi level and thus facilitating charge transport [4].The level of nitrogen doping depends on several process factors, including the plasma power, gas pressure, and treatment duration. Higher plasma power generates a larger number of reactive nitrogen, while lower gas pressure results in a higher ionization efficiency. Furthermore, a longer treatment duration increases the chance of nitrogen incorporation into the VACNT structures [3]. By carefully controlling these parameters during the nitrogen plasma treatment, it is possible to tailor the nitrogen doping process to achieve the desired performance enhancement for on-chip VACNT supercapacitors.The treatment is performed in a chamber with a pressure of 10 Pa, a nitrogen gas flow rate of 10 sccm, an ICP power of 200W, and a bias voltage of -120 V. We report a specific capacitance of modified VACNTs of 18 mF/cm2(Figure 1), nearly twice that of pristine samples (Figure1(B)), for dense arrays of 1012 CNTs/ cm2. XPS result confirms 11.76% nitrogen content and 13% oxygen content in treated samples (N-VACNTs), aligning with previous studies on wet process nitrogen-doped carbon nanotubes [5]. Oxygen content also plays a crucial role in VACNT-based supercapacitor performance, improving surface wettability which will reduce the resistance between electrode and electrolyte [6, 7]. However, excessive oxygen content can negatively affect VACNT supercapacitor lifetime. By finding the trade-off between nitrogen and oxygen content, we optimized the doping process, enhancing specific capacitance, stability (Figure 2), and lifetime for on-chip VACNT structures.In conclusion, our study demonstrates that the solvent-free RF nitrogen plasma treatment can effectively enhance the specific capacitance of on-chip VACNT structures by up to 100%, making them suitable for integration into high-performance on-chip supercapacitor.

Pore Structure and Fractal Characteristics of the Middle and Upper Permian Dalong and Gufeng Shale Reservoirs, Western Hubei Province, South China

The Middle and Upper Permian Dalong and Gufeng Formations in South China have recently been considered as potential gas-producing shales. However, their pore structure characteristics remain poorly understood. To investigate the pore structure and fractal characteristics of the pores in these two formations, a suite of shale samples from the Dalong and Gufeng Formations in the western Hubei Province, South China were analyzed by multiple techniques, namely, TOC content, X-ray diffraction (XRD) mineralogy analysis, optical microscopy observations, major elemental analysis, field emission-scanning electron microscopy (FE-SEM), and low-pressure gas adsorption measurements (N2 and CO2). The identified major shale lithofacies include siliceous mudstone, carbonaceous mudstone, argillaceous-siliceous mixed mudstone, and calcareous-siliceous mixed mudstone. SEM images show that the dominant pore types include the pores between brittle minerals, slit-shaped pores between clay sheets, and secondary organic matter (OM) pores within solid bitumen. The pore size distribution is dominated by micropores and mesopores (<30 nm), which are the major contributors to total pore volume and surface area for the Dalong and Gufeng Formations. Based on the Frenkel–Halsey–Hill (FHH) method, fractal dimensions (D1, D2) calculated from the nitrogen adsorption data have a range of from 2.489 to 2.772 (D1) and from 2.658 to 2.963 (D2), and are higher in the Gufeng Formation (average TOC = 8.3 wt.%) due to a higher TOC content comparing to the Dalong Formation (average TOC = 6.2 wt.%). The pore development and fractal characteristics are primarily controlled by organic matter (OM), carbonate minerals, and clay minerals for both the Dalong and Gufeng Formations. Shale samples with high TOC content, low carbonate content, and high clay content tend to develop more heterogeneous micropores and mesopores, which is ascribed to the generation of clay-related and OM-hosted pores, along with the destruction of primary pores by pore-filling carbonate cements.

Application of Petrographic and Stereological Analyses to Describe the Pore Space of Rocks as a Standard for the Characterization of Pores in Slags and Ashes Generated after the Combustion of Municipal Waste.

Slags and ashes generated in thermal waste treatment facilities require proper management. In line with the principles of the circular economy, new, more economical, and ecological possibilities for utilizing these substances are sought. These waste materials exhibit characteristics similar to rocks in many aspects. This study focuses on analyzing the similarities and differences between nine selected rocks and three samples of slags and ashes obtained from municipal waste incineration. The main research device used was a polarizing microscope, using reflected, transmitted, and fluorescent light. Additionally, low-pressure gas adsorption equipment, as well as helium and quasi-liquid pycnometers, were used for complementary analysis. The parameters analyzed mainly covered broadly defined petrographic properties of rocks and slags, with a particular emphasis on porosity, porous structure, and the spatial distribution of pores and fractures. The conducted analyses confirmed a significant similarity between slags and ashes and clastic sedimentary rocks such as sandstones and mudstones. The research results underscore the importance of petrographic microscopic studies for analyzing materials other than rocks. They also allow for exploring applications for slags and ashes in areas traditionally reserved for natural rock resources. The findings also indicate the necessity of using methods other than microscopic ones to describe the pore space of rocks. The lower measurement range of the optical microscope is limited to approximately 1 μm, covering only macropores. Other methods extend this characteristic to micro- and mesopores. Therefore, a combination of different methods is often employed to obtain a more comprehensive characterization of rock porosity.

Krypton tagging velocimetry in the plume of a plasma hollow cathode

Krypton tagging velocimetry (KTV) is an emerging flow diagnostic that investigates the bulk movement of high-speed, low-pressure gas flows. The present contribution expands the typical KTV method to the lower density (vacuum) conditions of interest in electric propulsion research. Our KTV scheme utilizes a two-photon (Write) excitation at 214.7 nm from a pulsed dye laser, followed by (Read) re-excitation of the ensuing metastable at 769.5 nm with an optical parametric oscillator. For the case of cold krypton gas expanding from a hollow cathode into vacuum, we find a bulk velocity of 452 ± 37 m/s and temperature of 20 ± 16 K. We also study the flow in the plume with the hollow cathode plasma operating for which we find bulk velocity of 1200 ± 130 m/s and temperature of 880 ± 370 K. Measurements are performed at background pressures down to ∼10−5 Torr.

Microgrids for green hydrogen production for fuel cell buses – A techno-economic analysis for Fiji

The study examines the feasibility of producing hydrogen for fuel cell buses in Fiji. The paper focuses on sizing hybrid microgrids comprising solar panels and wind turbines as the primary power source for hydrogen production while considering both off-grid and grid-connected cases. As no fuel cell vehicles exist in Fiji at present, five long-distance buses with daily travel of 380 km are proposed as a pilot scale baseline system, which would operate in the daytime only. Detailed technical and economic modelling was done using the HOMER Pro software with cost inputs derived from literature. A total of three cases for the on-grid and four cases for the off-grid systems are analyzed based on the various combinations of components. HOMER Pro optimized the grid configurations to minimize the Net Present Cost (NPC) values. All seven cases were also modelled with and without the effects of hydrogen gas compressors to emulate the production of low-pressure gas without compression and compressed high-pressure hydrogen required for fuel cell buses. It was found that a grid-connected system with solar and wind hybrid was the most feasible configuration and yielded the lowest NPC of $6 million. The grid-connected case also yielded a Levelized Cost of Electricity (LCOE) of $0.10/kWh and a Levelized Cost of Hydrogen (LCOH) of $9.08/kg with compression to 400 bars. The LCOH stood at $8.73/kg for hydrogen generation without compression. The most feasible off-grid configuration using solar and wind power attained an LCOE of $1.15/kWh while giving a LCOH of $13.00/kg for compressed hydrogen. It was also found that neglecting hydrogen compression load and costs can underestimate the LCOH by as much as 6.92 %. The off-grid best case had no CO2 produced during its operation, leading to a net zero process that offsets 929.9 tons of CO2 compared to the baseline. The grid-connected system does have some emissions but leads to a 93.9 % reduction in CO2 emissions, offsetting 873.3 tons of CO2 compared to the baseline. The study updated the cost inputs to account for inflation, logistics, and hydrogen gas compression, which are not available directly in HOMER Pro software. At the time of writing this paper, no feasibility study existed for hydrogen production in Fiji using renewable energy.

Prediction of Condensation in the Heat Exchangers of Engine Intake Systems according to External Environmental and Operating Conditions

Low-pressure exhaust gas recirculation (LP-EGR) systems are applied to diesel engines because they reduce nitrogen oxide emission by lowering the internal temperature of the cylinder by mixing the oxides with intake air. However, low-temperature ambient conditions include a large amount of vapor in the mixed gas flowing into the intercooler; when heat is exchanged, the water vapor condenses and is adsorbed on the surface of the intercooler fin to form a liquid film. Condensation occurs as the thermal resistance between the vapor and solid surface increases with the thickness of the liquid film and causing a corrosion due to condensation of the surface. In this study, the amount of condensation was predicted through calculations based on thermodynamic studies. Factors that can cause condensation inside the intercooler (fuel, air, and LP-EGR) were selected as variables. A mathematical formula was established to predict the convergence form of condensation or the amount of condensation over time at various temperature and relative humidity conditions. The formula predicted the condensation amount in the intercooler of the diesel engine, compared it to the actual amount of condensation in the test evaluation with an error of less than 4%. Additionally, because the formula can predict the amount of condensation by changing the heat exchange area of the intercooler, the application range of the formula was expanded to predict the condensation in the intercoolers of gasoline vehicles with different heat exchange areas and fuel types. The condensation error was within 2%, indicating a high consistency. Validation of the formula predicts a reliable amount of condensation under various operating and ambient temperature conditions, which means that both the time and cost of the test evaluation require the determination of the cause before solving the actual condensation problem.