Computational Evaluation of Li-doped g-C2N Monolayer as Advanced Hydrogen Storage Media

The emerging 2D g-C₂N obtained increasingly more popularity in functional materials design, and its natural porosity can easily accommodate metal atoms, making itself more suitable for energy gases storage. In this study, we employed DFT computational studies to systematically solve the electronic structure of Li-doped g-C₂N monolayer, and evaluate its performance in hydrogen storage. In our calculations, we found that each pore of g-C₂N can adsorb at most three Li atoms that bind with pyridinic N atoms. We also noticed that considerable amount of charges were transferred from the adsorbed Li to the pristine materials, potentially enhancing its overall conductivity. The change of electronic structure also leads to its improved performance in H₂ adsorption, due to the fact that the electrostatic interactions between the adsorbed H₂ and Li can be largely enhanced. The optimised configurations of the Li-doped g-C₂N with multiple adsorbed H₂ molecules were presented, and the fundamental mechanisms of adsorption were also investigated in details. The highest storage capacity of hydrogen by Li-doped g-C₂N can reach to 7.8 wt%, much higher than the target value of 5.5 wt %, defined by the U.S department of energy (DOE). Moreover, except Li, we also found that the nitrogen atoms or the N-C bonds can also serve as active adsorption sites. The computational explorations conducted in this study actually indicates a promising prospect of alkali metals decorated 2D materials in the area of hydrogen storage; and we believe the performance of these kinds of novel materials can be further enhanced via more decent modifications.     

Computational evaluation of superalkali-decorated graphene nanoribbon as advanced hydrogen storage materials

In this study, we proposed that homo superalkali NM₄ clusters with high tetrahedral geometry, can be applied to develop high-performance hydrogen storage materials. Moreover, their special bonding structures and chemical stability make them ideal units for decoration of different kinds of pristine monolayers. We made a trial to decorate the NLi₄ clusters onto the 1D graphene nanoribbon, and employed density functional theory (DFT) computational studies to solve its electronic structure, and further evaluate its applicability in hydrogen storage. We found that the electronic charges on Li atoms were successfully transferred to the pristine monolayer, thus a partial electronic field around each Li atom was formed. This subsequently leads to the polarization of the adsorbed hydrogen molecules, and further enhances the electrostatic interactions between the Li atoms and hydrogen. Each NLi₄ cluster can adsorb at most 16 hydrogen molecules. For this novel material, its total capacity of hydrogen storage can reach to 11.2 wt %, surpassing the target value of 5.5 wt %, set by the U.S department of energy (DOE) [1], making itself an ideal unit for advanced energy materials design.     

Hydrogen storage on uncharged and positively charged Mg-decorated graphene

The storage of H₂ molecules was studied theoretically on charged and uncharged Mg decorated graphene surfaces using density-functional theory and by incorporating the van der Waals (vdW) interactions. We found that an increase in the number of Mg atoms and H₂ molecule increases the net interaction of the hydrogen molecule with the surface. The Mg-Gr⁺ has the hydrogen storage capacity of up to nine H₂ molecules, with the average adsorption energy of −0.134 eV/H₂. Also, we found that hydrogen molecules play an important role in the interaction between the graphene surface and the Mg atom. The charge density difference analysis showed that electron transfer occurs from H₂ molecules to Mg atom in uncharged system. However, the Bader charge analysis showed that the positive charges in the Mg-Gr⁺ and nH₂-Mg-Gr⁺ systems are concentrated on the Mg atom. When the number of H₂ molecules reaches more than 4, the charge transfer instead occurs from the Mg atom to H₂ molecules as well as to the graphene surface. This results in better interaction between the Mg atom and the Gr⁺ surface.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.     

Computational evaluation of Ca-decorated nanoporous CN monolayers as high capacity and reversible hydrogen storage media

Developing novel materials with high-capacity and reversible properties for storing hydrogen (H₂) is crucial for energy treatments. We here investigated comprehensively the H₂ storage performance of the Ca-decorated g-CN (Ca@CN) monolayers using first-principles calculations. The Ca atoms can be uniformly decorated into the center of the pores of g-CN monolayers without aggregation. The Ca@CN monolayer has an average H₂ adsorption energy of around 0.163–0.228 eV as well as high H₂ storage capacity of 10.1 wt%. The stabilities of the H₂ adsorption systems are confirmed by high hardness and low electrophilicity. The temperature of desorption is anticipated to be near the room temperature and ideal for fuel cell devices. The thermodynamic analysis along with desorption temperature reveal that the Ca@CN monolayer has promising potentials as reversible and high capacity hydrogen storage materials (HSM), which will motivate experimental efforts to synthesize the high-efficient HSM.     

Hydrogen adsorption studies of TiFe surfaces via 3-d transition metal substitution

Hydrogen adsorption over TiFe surface and doped TiFe surface is investigated within density functional theory. Surface energy calculations confirm that TiFe (111) surface has the minimum value among three low index crystallographic surfaces, (100), (111) and (110). The (111) TiFe surface has two different terminations one with Fe and the other with Ti. Here both the (111) surfaces with different terminations are considered for doping with all the 3-d transition metal atoms from Sc to Zn. Furthermore, the molecular hydrogen adsorption over all the doped surfaces is investigated. V was found to be the most suitable element for doping in Fe terminated (111) surface. V doping in Fe terminated surface enhanced E_ads by 0.6 eV from ?3.30 eV (undoped) to ?3.90 eV after doping. Whereas in case of Ti terminated surface Co was found to be the best element for doping as it enhanced E_ads by ～0.5 eV from ?2.64 eV (undoped) to ?3.15 eV after doping. A significant decrease in d-band width from 1.95 eV to 1.22 eV in case of Co substitution in Ti terminated surface and from 2.42 eV to 1.33 eV in case of V substitution in Fe terminated surface enhances the hydrogen adsorption in TiFe (111) surface. Thus, even using a very small amount of dopant can influence the hydrogen adsorption properties of TiFe alloy.     

Thermal stability evaluation of palm oil as energy transport media

The thermal stability of palm oil as energy transport media in a hydraulic system was studied. The oils were aged by circulating the oil in an open loop hydraulic system at an isothermal condition of 55 °C for 600 h. The thermal behavior and kinetic parameters of fresh and degraded palm oil, with and without oxidation inhibitor, were studied using the dynamic heating rate mode of a thermogravimetric analyser (TGA). Viscometric properties, total acid number and iodine value analyses were used to complement the TGA data. The thermodynamic parameter of activation energy of the samples was determined by direct Arrhenius plot and integral methods. The results may have important applications in the development of palm oil based hydraulic fluid. The results were compared with commercial vegetable based hydraulic fluid. The use of F10 and L135 additives was found to suppress significantly the increase of acid level and viscosity of the fluid.     

Clean energy and energy storage research --The 2nd international conference on clean energy sciences

每3年举行1次的国际清洁能源会议(ICCES)旨在促进国际合作和交流,为在清洁能源和能源储存领域工作的国际研究者提供一个讨论清洁能源基础研究和技术革新的论坛.本文总结了2014年4月13~16日在中国青岛召开的第二届国际清洁能源会议的学术报告情况,特别是关于清洁能源和能源存储研究的最新进展及其未来科学发展所面临的挑战和根本问题.材料与纳米技术依然是解决清洁能源利用,转换和储存的关键;具有广泛应用基础的太阳能转化,电化学能量转化与储存,光催化与环境催化依然是研究热点;同时,清洁煤及化石燃料,生物燃料和生物质转化,生物和仿生系统的能源转化正在成为新的研究热点;而氢气制备与储存,二氧化碳捕获储存与使用等体系也引起了大家的广泛兴趣和关注.本文重点评述了清洁能源领域的研究重点,进展和热点问题.     

C7N6 monolayer as high capacity and reversible hydrogen storage media: A DFT study

Searching advanced materials with high capacity and efficient reversibility for hydrogen storage is a key issue for the development of hydrogen energy. In this work, we studied systematically the hydrogen storage properties of the pure C₇N₆ monolayer using density functional theory methods. Our results demonstrate that H₂ molecules are spontaneously adsorbed on the C₇N₆ monolayer with the average adsorption energy in the range of 0.187–0.202 eV. The interactions between H₂ molecules and C₇N₆ monolayer are of electrostatic nature. The gravimetric and volumetric hydrogen storage capacities of the C₇N₆ monolayer are found to be 11.1 wt% and 169 g/L, respectively. High hardness and low electrophilicity provides the stabilities of H₂–C₇N₆ systems. The hydrogenation/dehydrogenation (desorption) temperature is predicted to be 239 K. The desorption temperatures and desorption capacity of H₂ under practical conditions further reveal that the C₇N₆ monolayer could operate as reversible hydrogen storage media. Our results thus indicate that the C₇N₆ monolayer is a promising material with efficient, reversible, and high capacity for H₂ storage under realistic conditions.     

Swirling distributed combustion for clean energy conversion in gas turbine applications

Distributed combustion provides significant performance improvement of gas turbine combustors. Key features of distributed combustion includes uniform thermal field in the entire combustion chamber, thus avoiding hot-spot regions that promote NO_x emissions (from thermal NO_x) and significantly improved pattern factor. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of hot and active species present in the flame and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here for our quest to explore the beneficial aspects of such flows on clean combustion in simulated gas turbine combustion conditions. The goal is to develop high intensity combustor with ultra low emissions of NO and CO, and much improved pattern factor. Experimental results are reported from a cylindrical geometry combustor with different modes of fuel injection and gas exit stream location in the combustor. In all the configurations, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes for the geometries investigated here. Swirling flow configuration, wherein the product gas exits axially resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from traditional diffusion mode to distributed combustion regime. Results showed very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions even at rather high equivalence ratio of 0.7 at a high heat release intensity of 36 MW/m³-atm with non-premixed mode of combustion. Results are also reported on lean stability limit and OH^* chemiluminescence under both premixed and non-premixed conditions for determining the extent of distribution combustion conditions.     

Metal adatoms-decorated silicene as hydrogen storage media

Hydrogen storage properties of 10 different adatom decorated silicene are carried out using density functional theory calculations with long-range van der Waals dispersion correction. It is found that the binding energy between metal adatoms and the silicene is greater than the cohesive energy of bulk metal so that clustering of adatom will not occur once it is bonded with silicene. The adsorption of H₂ on Li, Na, K, Mg, Ca, and Au decorated silicene is a weak physisorption. Differently, a weaker chemisorption is responsible for the adsorption of H₂ on Be, Sc, Ti, and V decorated silicene. In particular, silicene with Na, K, Mg, and Ca decorating on both sides leads to 7.31–9.40 wt% hydrogen storage capacity with desirable adsorption energy, indicating that the metal-decorated silicene can serve as a high capacity hydrogen storage medium.     

Computational simulation of district solar heating systems with seasonal thermal energy storage

A computational simulation model for determining the thermal performance of large-scale community solar heating systems with interseasonal heat storage is described. Special attention has been paid to the mathematical formulation of the storage unit. It comprises an uninsulated stratified hot water tank excavated in rock. The storage capacity of the surrounding ground may also be utilized by vertical heat exchanger pipes. Comparisons of theoretical system performance predictions with recent experimental measurements from a full-scale prototype installation are presented and found to be in reasonable agreement. The simulation program is also used to evaluate the thermal performance of various district solar heating system configurations for northern cold climatic conditions (60°N).     

压缩空气储能系统储气装置研究现状与发展趋势

压缩空气储能被认为是最具发展潜力的大规模物理储能技术,储气装置作为压缩空气储能系统的关键环节,对系统高效、稳定和安全运行具有重要影响.近些年来,随着压缩空气储能技术的快速发展,储气装置的研究备受人们关注.储气装置的特点主要取决于其材料属性,因此本文根据材料不同对储气装置进行分类,并着重论述了天然地下洞穴储气、人造洞室储...     

洁净燃料二甲醚的制取方法

基于二甲醚作为清洁能源代用燃料的目的，对二甲醚的特性进行了介绍，并阐述了二甲醚的合成方法、所涉及的催化剂以及合成反应器，最后提出了生物质高温裂解气作为合成气气源以制取二甲醚的思路。     

Evaluation of conceptual electrolysis-based energy storage systems using gas expanders

In this study, four energy storage systems (Power-to-Gas-to-Power) were analysed that allow electrolysis products to be fully utilized immediately after they are produced. For each option, the electrolysis process was supplied with electricity from a wind farm during the off-peak demand periods. In the first two variants, the produced hydrogen was directed to a natural gas pipeline, while the third and fourth options assumed the use of hydrogen for synthetic natural gas production. All four variants assumed the use of a gas expander powered by high-temperature exhaust gases generated during gas combustion. In the first two options, gas was supplied from a natural gas network, while synthetic natural gas produced during methanation was used in the other two options. A characteristic feature of all systems was the combustion of gaseous fuels within a ballast-free oxidant atmosphere without nitrogen, which is the fundamental component of air in conventional systems. The fifth variant was a reference for the systems equipped with gas expanders and assumed the use of fuel cells for power generation. To evaluate the individual variants, the energy storage efficiency was defined and determined, and the calculated overall efficiency ranged from 17.08 to 23.79%, which may be comparable to fuel cells.     

The promise of clean energy

The world's energy system is at least a 1.5 trillion dollars market dominated by fossil fuels, where small changes can have a large influence on efforts to reach sustainability. Renewable energy sources are key to achieving this goal. Excluding traditional biomass, in 2001 renewables represented 4.4% of primary energy consumption, unevenly distributed between developed and developing countries. Environmental problems at local, regional and global levels, as well as external dependency and security of supply will persist if we rely on an energy future based on fossil fuels. Solutions encompass extending the life of fossil fuel reserves and expanding the share of renewable in the world energy system through top down and bottom up policies, described in this paper.     

Economic analysis of using above ground gas storage devices for compressed air energy storage system

Above ground gas storage devices for compressed air energy storage（CAES） have three types：air storage tanks,gas cylinders,and gas storage pipelines.A cost model of these gas storage devices is established on the basis of whole life cycle cost（LCC） analysis.The optimum parameters of the three types are determined by calculating the theoretical metallic raw material consumption of these three devices and considering the difficulties in manufacture and the influence of gas storage device number.The LCCs of the three types are comprehensively analyzed and compared.The result reveal that the cost of the gas storage pipeline type is lower than that of the other two types.This study may serve as a reference for designing large-scale CAES systems.     

Application of magnetic energy storage unit as load-frequencystabilizer

Fast-acting energy storage devices can effectively damp electromechanical oscillations in a power system because they provide storage capacity in addition to the kinetic energy of the generator rotor, which can share the sudden changes in power requirement. The effectiveness of small-sized magnetic energy storage (MES) units (both superconducting and normal loss types) for this application is shown, and means of best utilizing the small energy storage capacity of such units to improve the load-frequency dynamics of large power areas are suggested. The proposed method of improving the load frequency control of power systems has the advantage that it does not require the governor or any other part of the power system to perform any sophisticated control action. The control logic suggested for this purpose takes the area control error as its input and uses inductor current deviation feedback. In a power system with a SMES (superconducting MES) unit, the optimal setting of the integrator gain is altered to a higher value. With the suggested control measure, SMES units of 4-6 MJ capacity would suffice in reducing the maximum deviations of frequency and tie-line power flow by about 40% in power areas of 1000-2000 MW capacity     

A comprehensive evaluation of energy storage options for better sustainability

Due to ever increasing global energy demand and the limited nature of fossil fuel reserves, there has been tremendous research and development studies in the literature, focusing on alternative and clean energy resources and systems. Renewables are the promising choice when it comes to addressing some critical energy issues such as climate change and energy security. However, renewables have intermittent and discontinuous supplies; hence, they need to be stored in ways that are affordable, reliable, flexible, clean, safe, and efficient. As a result, energy storage is becoming a crucial step to build innovative energy systems for a sustainable future. Energy can be stored in many forms, from electrical to chemical (eg, hydrogen), or electrochemical, thermal, electromagnetic, etc. Each form consists of different technologies, some of which are already commercially mature while others are at early research and development stages. Each of these options can be tailored to meet different end users' needs at different scales. Therefore, this study aims to conduct a comprehensive review on the most recent status of energy storage options, along with the requirements of various end users, and characteristics of smart energy storage systems. The main objective is to summarize the performance evaluation statuses of mechanical, electrochemical, chemical, thermal, and electromagnetic energy storage technologies. The selected performance measures are capacity flexibility, energy arbitrage, system balancing, congestion management, environmental impact, and power quality. In the end, some key recommendations and future directions for energy storage systems are provided.