Storage technologies are critical for the massive integration of renewable sources. Among the most promising storage technologies are those based on Carnot batteries. This work analyses CO2-based electrothermal energy storage coupled with geological storage (CEEGS) under off-design conditions when integrated into a fluctuating system such as a PV system. The integration of simulation models over an annual cycle provides valuable insights into the dynamic behavior of the system, supporting informed decision-making for future research on such innovative technologies. The developed model enables the optimisation of both design and operational parameters, tailored to specific applications and geographic locations. The valuable information obtained has shown a bottleneck in the compressor that leads to a strong effect on the overall performance of the storage system based on technological limits and stored energy losses. In this regard, this work proposes different pathways based on compressor layout that can improve the integration of storage systems in general and, in particular, the system under study when integrated into fluctuating systems. The multi-compressor operating mode leads to an improved efficiency of up to 18% and an average increase in daily operating range of 1.5 h based on improved compressor adjustment. The proposed split flow for identical compressor operating mode shows a daily operating range equal to the multi-compressor operating mode and a slight loss of efficiency with respect to the multi-compressor operating mode because the first adjusts better with constraining system fluctuations. However, the latter is a simpler configuration, and it is highly efficient, 16.3% more efficient than the basic compressor layout.

We present a detailed overview of the calculation of the enthalpy of dissociation of structure I pure carbon dioxide ( $ \ce {CO2} $ CO2) hydrates, focussing primarily on the direct calculation of the enthalpies of all the components that are involved in the hydrate dissociation reaction. Molecular Dynamics simulations are used extensively in order to calculate the enthalpies and molar volumes of water, $ \ce {CO2} $ CO2 and the sI pure hydrate (with variable degree of occupancy) at pressure and temperature conditions along the three-phase (Hydrate-Liquid water-Vapour or Hydrate-Liquid water-Liquid $ \ce {CO2} $ CO2) equilibrium line. Good agreement is found between the obtained MD simulations and literature-reported experimental measurements.

This study presents a machine learning approach for predicting the diffusivity of CO2 in liquid H2O over a wide range of temperatures and pressures. A comprehensive experimental dataset is compiled, including over 300 data points from existing literature, as well as, 75 newly identified diffusivity measurements. These data span a broad spectrum of temperatures and pressures. Various machine learning models namely, Support Vector Machines (SVM), Random Forest (RF), k-Nearest Neighbors (kNN), and Autoencoders, are trained on this enhanced dataset and evaluated for their accuracy in diffusivity prediction. Results show that the Autoencoder model achieves superior performance, accurately predicting CO2 diffusivity even in regions where experimental data is sparse. The model’s ability to generalize across a wide range of temperatures and pressures, demonstrates its potential for use in real-world applications, enabling fast, reliable predictions with minimized computational cost.

This extensive review highlights the central role of classical molecular simulation in advancing hydrogen (H2) technologies. As the transition to a sustainable energy landscape is urgently needed, the optimization of H2 processes, spanning production, purification, transportation, storage, safety, and utilization is essential. To this end, accurate prediction of thermodynamic, transport, structural, and interfacial properties is important for overcoming engineering challenges across the entire H2 value chain. Experimental measurements, despite being the traditional way of obtaining these properties, can be limited by the distinctive nature of H2, harsh operating conditions, safety constraints, and extensive parameter spaces. Free from such limitations, classical molecular simulations, in the general frameworks of Monte Carlo and Molecular Dynamics, provide an optimal balance between computational efficiency and accuracy, bridging the gap between quantum mechanical calculations and macro-scale modeling. This review also systematically covers molecular simulation methods and force fields for computing key properties of H2 systems, such as phase and adsorption equilibria and transport coefficients. Beyond property prediction, we explore how molecular simulation reveals fundamental mechanisms governing hydrate formation and dissociation, membrane permeations, and H2 embrittlement. When possible, data from multiple sources are compared and critically assessed, while effort is put on evaluating the force fields used and methodological approaches followed in the literature. Finally, this review aims at identifying research gaps and future opportunities, emphasizing emerging approaches, such as molecular simulation in the era of artificial intelligence.

The goal to reduce greenhouse gas emissions necessitates the increase in RES utilization. To accomplish this goal, energy storage solutions are required. This study investigates the performance of an electrothermal energy storage system, the CEEGS, which consists of an above-surface energy storage system and a below-surface geological system. The focus is set initially on the analysis of the above-surface system to gain insight into its operation. Then, steady-state optimization is utilized to identify the operating conditions that maximize the system performance, before investigating the below-surface system integration and the effect that the geological conditions have on system performance. For the above-surface system, efficiency (ηR-T) up to 46.89% is calculated. For systems integrated with CO2 geological storage, two case studies are examined, presenting higher ηR-T compared to the above-surface system (Case study 1: 50.37%, Case study 2: 67.39%). The optimal ηR-T for Case study 2 is achieved for higher injection/production pressures and temperatures conditions and minimal ΔP and ΔT between injection and production. In conclusion, it is the selection of the geological storage conditions that contribute the most to the optimal ηR-T; thus, the selection of the appropriate geological storage formation is imperative.

Electrothermal energy storage is a promising technology for high penetration of renewable energy. In recent years, the integration of this energy storage system with geological CO2 storage has been introduced. The system consists of a reversible heat pump formed by transcritical CO2 cycles with thermal storage at two temperature levels, enabling the simultaneous operation of geological CO2 storage and the storage/production of renewable electrical energy. This work focuses on studying high and low-temperature thermal energy storage. Step heating on the high-temperature side allows for better integration of the supercritical and subcritical temperature profiles of the CO2 and the thermal storage fluid. Thermal storage at different temperature levels provides a higher turbine inlet temperature, improving the efficiency of the power production cycle and increasing heating applications such as district heating or domestic hot water. Considering four high-temperature tanks, round-trip efficiency increases from 52.8 to 55.4 %. It presents a thermal demand coverage range of about 20-150 degrees C, with temperature increases of approximately 30 degrees C. The phase change temperature shift on the low-temperature side directly impacts electric power production and enables new cooling applications. The system's efficiency increases as the low-temperature phase change temperature decreases, reaching 58.7 % at -30 degrees C. Using alternative configurations in the transcritical CO2 cycle, such as the recuperative cycle and multi-stage compression and expansion, high-efficiency values can be maintained with lower system requirements.

Microsecond molecular dynamics (MD) simulations are employed in the isobaric-isothermal ensemble for the determination of the growth rate of sI methane hydrate. Statistically meaningful measurements of the growth rate are given for a wide range of pressures and temperatures. The reported growth rates are measured from the MD trajectories with two different methods. The first method is based on the time evolution of the potential energy of the system by correlating the rate of potential energy decrease to the speed of the moving hydrate interface. The second method that was originally introduced in the current study uses Voronoi tessellation to characterize the molecules depending on their neighborhood and thus allows the identification for each time step of the exact position of the interface. Both methods yield practically identical results. At the low pressure regime, the MD results are compared to available experimental data with highly satisfactory agreement. A useful correlation for engineering applications is suggested, which offers prediction of the methane growth rate as a function of the concentration of methane in the aqueous phase between the supersaturated and the isobaric equilibrium conditions. The calculated hydrate growth rates are also compared to reported experimental and MD simulations data.

In this study, we perform an extensive evaluation of a simple model for hydrate equilibrium calculations of binary, ternary, and limited quaternary gas hydrate systems that are of practical interest for separation of gas mixtures. We adopt the model developed by Lipenkov and Istomin and analyze its performance at temperature conditions higher than the lower quadruple point. The model of interest calculates the dissociation pressure of mixed gas hydrate systems using a simple combination rule that involves the hydrate dissociation pressures of the pure gases and the gas mixture composition, which is at equilibrium with the aqueous and hydrate phases. Such an approach has been used extensively and successfully in polar science, as well as research related to space science where the temperatures are very low. However, the particular method has not been examined for cases of higher temperatures (i.e., above the melting point of the pure water). Such temperatures are of interest to practical industrial applications. Gases of interest for this study include eleven chemical components that are related to industrial gas-mixture separations. Calculations using the examined methodology, along with the commercial simulator CSMGem, are compared against experimental measurements, and the range of applicability of the method is delineated. Reasonable agreement (particularly at lower hydrate equilibrium pressures) between experiments and calculations is obtained considering the simplicity of the methodology. Depending on the hydrate-forming mixture considered, the percentage of absolute average deviation in predicting the hydrate equilibrium pressure is found to be in the range 3–91%, with the majority of systems having deviations that are less than 30%.

We present a detailed overview of the calculation of the enthalpy of dissociation of structure II pure propane or mixed methane + propane hydrates, focussing primarily on methods that are based on either the Clausius-Clapeyron equation or the direct calculation of the enthalpies of all the components that are involved in the hydrate dissociation reaction. Molecular dynamics simulations are used extensively in order to calculate the enthalpies and molar volumes of water, methane, propane, and the sII mixed hydrate (with variant degree of occupancy) at pressure and temperature conditions along the three-phase (Hydrate-Liquid water-Vapor or Hydrate-Liquid water-Liquid hydrocarbon) equilibrium line.

An in-depth review of the available experimental and molecular simulation studies of CO2 diffusion in H2O, which is a central property in important industrial and environmental processes, such as carbon capture and storage, enhanced oil recovery, and in the food industry is presented. The cases of both bulk and confined systems are covered. The experimental and molecular simulation data gathered are analyzed, and simple and computationally efficient correlations are devised. These correlations are applicable to conditions from 273 K and 0.1 MPa up to 473 K and 45 MPa. The available experimental data for diffusion coefficients of CO2 in brines are also collected, and their dependency on temperature, pressure, and salinity is examined in detail. Other engineering models and correlations reported in literature are also presented. The review of the simulation studies focuses on the force field combinations, the data for diffusivities at low and high pressures, finite-size effects, and the correlations developed based on the Molecular Dynamics data. Regarding the confined systems, we review the main methods to measure and compute the diffusivity of confined CO2 and discuss the main natural and artificial confining media (i.e., smectites, calcites, silica, MOFs, and carbon materials). Detailed discussion is provided regarding the driving force for diffusion of CO2 and H2O under confinement, and on the role of effects such as H2O adsorption on hydrophilic confining media on the diffusivity of CO2. Finally, an outlook of future research paths for advancing the field of CO2 diffusivity in H2O at the bulk phase and in confinement is laid out.

The use of renewable energy sources as a solution to the energy dependency on fossil fuels requires innovative solutions to the issue of energy storage. Among the solutions suggested in the literature, electrothermal energy storage comprised of a heat pump and a heat engine using transcritical CO2 cycles, water as a thermal energy storage (TES) fluid to store sensible heat and ice as a cold storage medium to store latent heat, appears promising. In this paper, a steady state mathematical model of the system is developed using Aspen Plus V11, validated and compared against results available in the literature. The validated model’s performance is then studied utilizing parametric sensitivity analysis by exploring the effect of different parameters on multiple efficiency metrics, with the best case achieving improvement on the round-trip efficiency (?R-T) of 7.64 %. The hydraulic turbine inlet temperature and the heat engine minimum pressure are found to contribute the most to the ?R-T improvement, with the minimum pressure being the one that can be further decreased by using cold TES mediums with lower freezing points. Finally, the effect of alternative cold TES mediums (with lower freezing temperature than ice) on the performance of the system is evaluated. It was concluded, that the ?R-T of the model decreases as the freezing temperature declines, from 46.90 % at 0 °C to 44.90 % at -20.19 °C. As a result, no benefit related to the ?R-T of the model can be deduced by choosing cold TES mediums with lower freezing p than ice.

Available data from experiments and molecular simulations for the intra-diffusivities of H-2 and O-2 in H2O, and for the self-diffusivity of pure H2O (at pressure and temperature conditions in which the solvent is in the vapour phase) are compared against calculations based on the ChapmanEnskog theory or other semi-empirical/semi-theoretical methods. A novel methodology is proposed to extrapolate the intra-/self-diffusivities data computed from molecular dynamics simulations at low pressures. The extrapolated values are used to further refine the recently-proposed [Tsimpanogiannis et al., J. Chem. Eng. Data, 66, 3226-3244, (2021)], molecular simulation based correlation of intra-/self-diffusivities as a function of pressure and temperature with the solvent being in the vapour phase.

We report the three-phase (hydrate-liquid water-vapor) equilibrium conditions of the hydrogen-water binary system calculated with molecular dynamics simulations via the direct phase coexistence approach. A significant improvement of ∼10.5 K is obtained in the current study, over earlier simulation attempts, by using a combination of modifications related to the hydrogen model that include (i) hydrogen Lennard-Jones parameters that are a function of temperature and (ii) the water-guest energy interaction parameters optimized further by using the Lorentz-Berthelot combining rules, based on an improved description of the solubility of hydrogen in water.

We have performed molecular dynamics simulations to study the adsorption of ten hydrate anti-agglomerants onto a mixed methane-propane sII hydrate surface covered by layers of liquid water of various thickness. As a general trend, we found that the more liquid water that is present on the hydrate surface, the less favorable the adsorption becomes even though there are considerable differences between the individual molecules, indicating that the presence and thickness of this liquid water layer are crucial parameters for anti-agglomerant adsorption studies. Additionally, we found that there exists an optimal thickness of the liquid water layer favoring hydrate growth due to the presence of both liquid water and hydrate-forming guest molecules. For all other cases of liquid water layer thickness, hydrate growth is slower due to the limited availability of hydrate-forming guests close to the hydrate formation front. Finally, we investigated the connection between the thickness of the liquid water layer and the degree of subcooling and found a very good agreement between our molecular dynamics simulations and theoretical predictions.

In this study, all available data from experiments and molecular simulations for the intra-diffusivities of H2 and O2 in H2O, and for the self-diffusivity of pure H2O are analyzed to examine the validity of the Stokes-Einstein relation. This analysis is motivated by the significant amount of work devoted through the years for improving the predictions of intra- and self-diffusivities in binary and multi-component mixtures relevant to chemical and environmental processes. Here, we calculate the slopes s and t corresponding to the ln(D)vs.ln(Tη) and ln(DT)vs.ln(1η) plots, respectively, where D is the intra-diffusivity, η the viscosity, and T the temperature of the systems. Our results show that s and t deviate from unity no matter if the experimental or simulation data are used. This means that the Stokes-Einstein relation is violated for the binary systems of H2 and O2 with H2O, and for pure H2O. Although prior studies mainly focused on re-evaluating the parameter A of the SE-based semi-theoretical/semi-empirical approaches expressed as D=ATη, our results indicate that reliable predictions for the intra- and self-diffusivities can be achieved by improving the accuracy of the prediction of slopes s and t.

We examine the critical pore radius that results in critical gas saturation during pure methane hydrate dissociation within geologic porous media. Critical gas saturation is defined as the fraction of gas volume inside a pore system when the methane gas phase spans the system. Analytical solutions for the critical pore radii are obtained for two, simple pore systems consisting of either a single pore-body or a single pore-body connected with a number of pore-throats. Further, we obtain critical values for pore sizes above which the production of methane gas is possible. Results shown in the current study correspond to the case when the depression of the dissociation temperature (due to the presence of small-sized pores; namely, with a pore radius of less than 100 nm) is considered. The temperature shift due to confinement in porous media is estimated through the well-known Gibbs-Thompson equation. The particular results are of interest to geological media and particularly in the methane production from the dissociation of natural hydrate deposits within off-shore oceanic or on-shore permafrost locations. It is found that the contribution of the depression of the dissociation temperature on the calculated values of the critical pore sizes for gas production is limited to less than 10% when compared to our earlier study where the porous media effects have been ignored.

We use a novel hybrid method to explore the temperature dependence of the solid-liquid interfacial tension of a system that consists of solid methane hydrate and liquid water. The calculated values along the three-phase (hydrate-liquid water-vapor) equilibrium line are obtained through the combination of available experimental measurements and computational results that are based on approaches at the atomistic scale, including molecular dynamics and Monte Carlo. An extensive comparison with available experimental and computational studies is performed, and a critical assessment and re-evaluation of previously reported data is presented.

Molecular dynamics simulations are carried out to compute the intradiffusion coefficients of H2 and O2 in H2O for temperatures ranging from 275.15 to 975.15 K and pressures ranging from 0.1 to 200 MPa. These conditions span vapor, liquid, and supercritical conditions. For the vast majority of the state points examined, experimental data are not available. The accuracy of six H2 and six O2 force fields is tested in reproducing the available experimentally measured densities, self-diffusivities, and shear viscosities of the pure gas and the intradiffusivity of the gas in H2O. Namely, we screen the H2 force fields developed by Buch, Vrabec and co-workers, Hirschfelder et al., Cracknell, a modified Silvera-Goldman, and Marx and Nielaba. For O2, the force fields by Bohn et al., Miyano, Coon et al., Hansen et al., Vrabec et al., and Watanabe are tested. Overall, the force fields by Buch and Bohn for H2 and O2, respectively, were found to perform the best, and combined with the TIP4P/2005 H2O force field are used to compute the intradiffusivities in the entire temperature and pressure range. The new data are used to develop an engineering model that can predict the H2 and O2 intradiffusivity in vapor, liquid, and supercritical H2O. The new model uses 11 parameters and has an accuracy of 4-11%. The model is validated with other available experimental and simulation data for H2 and O2 in H2O and pure H2O. Aside from the extensive collection of new data for the intradiffusivities of H2 and O2 in H2O, we present new data for the densities, shear viscosities, and self-diffusivities of pure TIP4P/2005 H2O in the same wide temperature and pressure range. The new data and the engineering model presented here can be used for the design and optimization of chemical processes, for which the knowledge of H2 and O2 diffusivities in H2O is important.

The dissociation process of spherical sII mixed methane-propane hydrate particles in liquid hydrocarbon was investigated via microsecond-long molecular dynamics simulations. A strong dependence of the melting temperature on the particle size was found. Analysis in the context of the Gibbs-Thomson effect provided insights into the fundamental properties of gas hydrates.

Methane hydrates are encountered in a plethora of industrial and geological or environmental applications. In the current study, we present a novel methodology which is based on molecular dynamics simulations for the calculation of the enthalpy of enclathration of sI methane hydrates. Simulations are performed along the three-phase (Hydrate – Liquid water – Vapour; H–Lw–V) equilibrium line in the temperature range 274–310 K. The methodology takes into account the two different types of cages that are present in the sI methane hydrate and provides results for the enthalpy of enclathration for both types of cages, while it avoids performing calculations with the metastable, completely empty hydrate lattice. The formulation proposed is general and can be also applied to sII hydrates, while it can be modified/extended appropriately for use in the case of sH hydrates. Comparison is provided with available data from the literature and good agreement is observed. GRAPHICAL ABSTRACT