• Energy Research

Abstract This study analyzes the possibility of determining the parameters of an adsorption equilibrium model based on a reduced number of isotherms for the working pair water/zeolite 13X. The employed models rely on the Dubinin-Polanyi theory of micropore adsorption. The reliability of the adsorption equilibrium model based on sparse data is evaluated in terms of the error in the adsorption equilibrium and in terms of the error in loading lift and heat storage density for an adsorption cycle typical for heat storage applications. It is found that as little as three measured adsorption isotherms are sufficient to yield a description of the adsorption equilibrium of zeolite 13X in a wide pressure and temperature range, if the following criteria are obeyed: (i) the measured isotherms should cover the entire range of the characteristic curve and (ii) it is recommended to include isotherms at temperatures close to the working cycle limits. Based on these considerations, temperature ranges for the experimental determination of a reduced set of adsorption isotherms are recommended that yield a reliable description of the adsorption equilibrium in a wide pressure and temperature range. Thereby it is demonstrated that the experimental effort can be reduced significantly while maintaining the predictive capability of the theoretical model.

Abstract Thermochemical reactions can be employed in heat storage devices. The choice of suitable reactive material pairs involves a thorough kinetic characterisation by, e.g., extensive thermogravimetric measurements. Before testing a material on a reactor level, simulations with models based on the Theory of Porous Media can be used to establish its suitability. The extent to which the accuracy of the kinetic model influences the results of such simulations is unknown yet fundamental to the validity of simulations based on chemical models of differing complexity. In this article we therefore compared simulation results on the reactor level based on an advanced kinetic characterisation of a calcium oxide/hydroxide system to those obtained by a simplified kinetic model. Since energy storage is often used for short term load buffering, the internal reactor behaviour is analysed under cyclic partial loading and unloading in addition to full monotonic charge/discharge operation. It was found that the predictions by both models were very similar qualitatively and quantitatively in terms of thermal power characteristics, conversion profiles, temperature output, reaction duration and pumping powers. Major differences were, however, observed for the reaction rate profiles themselves. We conclude that for systems not limited by kinetics the simplified model seems sufficient to estimate the reactor behaviour. The degree of material usage within the reactor was further shown to strongly vary under cyclic loading conditions and should be considered when designing systems for certain operating regimes.

Abstract Thermochemical heat storage devices based on water adsorption on microporous materials are a viable option for heat storage applications. The Dubinin-Polanyi theory of micropore filling has been widely applied for the thermodynamical characterization of various adsorption working pairs. It has been used for the deduction of adsorption enthalpies from adsorption equilibrium data. How well the theory predicts the dependence of storage densities on the storage cycle characteristics remains to be clarified as it is vital for technology assessment and design. This study compares the heat storage densities predicted by the Dubinin-Polanyi theory to experimentally determined data of two granulated zeolite samples, namely a Na-X and a Ca-X, under various humidity conditions.

Abstract Composite materials consisting of a salt-impregnated porous host matrix constitute a way to combine the high energy storage density of hygroscopic salts with the fast kinetics of the carrier material. Depending on its pore structure the carrier can furthermore prevent or inhibit leakage of the salt solution. It has been shown experimentally that by impregnation with CaCl2 the heat storage density of zeolite Ca-X can be increased by 53 % to 270 kWh m−3, which confirms the potential of this material class. In transforming this potential into technical heat storage solutions, numerical simulations can support the design process by bridging the gap between material characterization, process specification and reactor design. Such simulations rest, among others, on suitable constitutive relations. For the equilibria and kinetics of salt/zeolite composite sorbents those relations are still missing in the literature. In this work, we present an axisymmetric model of the mass and heat transport through a packed bed of composite sorbent pellets accounting for radial effects such as increased bed void fraction near the sorption chamber walls. Special focus is laid on the modelling of the sorption equilibria and kinetics of CaCl2/zeolite Ca-X composites of various salt loadings. The developed sorption equilibrium model for arbitrary salt loadings of the CaCl2/zeolite Ca-X is based on isotherm measurements of only one composite sample and one sample of pure zeolite Ca-X thereby enabling reduced experimental effort for the equilibrium characterization. The linear driving force kinetics is calibrated using data from dynamic sorption experiments on zeolite Ca-X and used to predict the dynamic sorption behaviour of CaCl2/zeolite Ca-X composites. We found a good predictive capability of the unmodified kinetics model for high inlet humidities—i.e., the practically most relevant cases where the composite plays its strengths. Contrarily, for low inlet humidities, the used kinetics model strongly overestimates the sorption rate, which indicates the presence of additional kinetic inhibition mechanisms under such conditions.

Abstract The concept of material or configurational forces, albeit not new, is one of those innovations in theoretical mechanics that has struggled to reach the success of wide-spread acceptance, or even familiarity. Perhaps, one reason for this is to be found in the few available introductory examples or in the non-trivial physical-mathematical approach often taken to establish this concept, although by no means more complex than other treatments in non-linear continuum mechanics. With this work we aim at contributing to the dissemination of configurational mechanics concepts by guiding the reader through an introductory analytical example step by step and comparing it to numerically obtained results. The numerical model is solved with OpenGeoSys (OGS-6), an open-source, C++-based, object-oriented finite element platform for the thermo-hydro-mechanical analysis of coupled processes in fractured porous media. In the spirit of the open-source philosophy, and to enable the readers to reproduce the example themselves, both the source code and the input files are available online. The example highlights—in a simple and intuitive manner—several insightful aspects related to configurational mechanics.

Simulating adsorption-based heat storage devices requires knowledge of both the adsorption equilibria and the adsorption enthalpies of the adsorbent materials involved. The Dubinin-Polanyi theory of micropore filling can be used as a tool to reduce the experimental work for the thermodynamical characterization of various adsorption working pairs. In particular it can be used for the deduction of adsorption enthalpies from adsorption equilibrium data. In this work we assess if this theory can be employed to predict the outcome of experiments performed on a lab-scale heat storage device. For that purpose, we present a numerical model of the sorption chamber, which describes the sorption behavior by means of the Dubinin-Polanyi theory. The simulated heat storage densities and water loading lifts are compared to experimentally determined data of two granulated zeolite samples, namely a zeolite Na-X and a zeolite Ca-X, under various humidity conditions.

AbstractContrasting deformation mechanisms precede volcanic eruptions and control precursory signals. Density increase and high uplifts consistent with magma intrusion and pressurization are in contrast with dilatant responses and reduced surface uplifts observed before eruptions. We investigate the impact that the rheology of rocks constituting the volcanic edifice has on the deformation mechanisms preceding eruptions. We propose a model for the pressure and temperature dependent brittle-ductile transition through which we build a strength profile of the shallow crust in two idealized volcanic settings (igneous and sedimentary basement). We have performed finite element analyses in coupled thermo-hydro-mechanical conditions to investigate the influence of static diking on the local brittle-ductile transition. Our results show that in active volcanoes: (i) dilatancy is an appropriate indicator for the brittle-ductile transition; (ii) the predicted depth of the brittle-ductile transition agrees with the observed attenuated seismicity; (iii) seismicity associated with diking is likely to be affected by ductile deformation mode caused by the local temperature increase; (iv) if failure occurs within the edifice, it is likely to be brittle-dilatant with strength and stiffness reduction that blocks stress transfers within the volcanic edifice, ultimately damping surface uplifts.