Modeling thermochemical reactions in thermal energy storage systems

\u3cp\u3eThe focus of this chapter is mainly on molecular modeling techniques for the hydration and dehydration (sorption and desorption) processes occurring in salt hydrates at the nano-scale. Modeling techniques such as density function theory, molecular dynamics and monte carlo are briefly introduced. Some attention is also given to micro- and macro-scale modeling techniques used at larger length scales, such as Mampel's model and the continuum approach. Before introducing all the length (and time) scales involved when modeling a heat storage system, a qualitative description is given of the hydration and dehydration processes on the nano/micro-scale.\u3c/p\u3

Characterization of potassium carbonate salt hydrate for thermochemical energy storage in buildings

Thermochemical heat storage in salt hydrates is a promising method to improve the solar fraction in the built environment. One of the most promising salt hydrates to be used as thermochemical material is potassium carbonate. In this study, the use of potassium carbonate in heat storage applications is investigated experimentally. The most important objective is to form a kinetic model for the de/re-hydration reaction of the material. In order to do so, it is crucial to understand the behavior of the salt when it reacts with water vapor. Reaction kinetics and mechanism are investigated for K2CO3, as one of the most promising materials. Characterization of the materials is carried out with combined Thermo-Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) methods. By employing the experimental results, kinetics models are developed for the hydration and dehydration reactions of the material. The kinetics model can be further used to predict the performance of a heat storage system working with K2CO3. In addition, cyclability and reaction enthalpy are investigated

Kinetic study of LI\u3csub\u3e2\u3c/sub\u3eSO\u3csub\u3e4\u3c/sub\u3e·H\u3csub\u3e2\u3c/sub\u3eO dehydration using microscopy and modeling

\u3cp\u3eA phenomena-based method is presented to study the kinetics of the dehydration reaction of Li\u3csub\u3e2\u3c/sub\u3eSO\u3csub\u3e4\u3c/sub\u3e·H\u3csub\u3e2\u3c/sub\u3eO single crystals. The reaction proceeds by nucleation and growth processes, which are recorded photographically using a camera system. Based on a series of pictures of surface observations under isothermal conditions, an areic nucleation rate and growth rate were estimated. Since only surface information was obtained so far, an experimental study about the speed of growth into the crystal was carried out to quantify the growth rate in-depth. Together with the surface information, a nucleation and growth model was developed and employed to predict the reaction kinetics. The fractional conversion of the dehydration reaction was calculated and compared with experimental results from TGA (thermogravimetric analysis) measurements. A satisfactory prediction was achieved as a function of sample medium and experimental environment.\u3c/p\u3

Shrinking core model for the reaction-diffusion problem in thermo-chemical heat storage

In this work, we develop a kinetic model to study the dehydration reaction of Li2SO4.H2O single particles involving interaction between the intrinsic chemical reaction and the bulk diffusion. The mathematical framework of the model is based on the shrinking core model. A variable-grid, finite-difference method with fully implicit formulation is used for solving the model. It is found that the Damkӧhler number 0 0 (k r ) / (D c ) Da r e plays an important role in determining the nature of the diffusion/reaction dynamics. A very small Da value means that the overall reaction is controlled by the intrinsic chemical reaction at the interface, while a very large Da value means that the overall reaction is controlled by the diffusion of water through the product phase. Moreover, the numerical results of fractional conversion calculated in the model are in good agreement with the theoretical analysis under extreme cases in which either diffusion (large Da) or reaction (small Da) dominates the dehydration process. With consideration of numerical solutions at various Da values, it is concluded that both intrinsic reaction and mass diffusion are important in determining the reaction kinetics within a range of Da values between 0.1 and 10

Three-dimensional transition of a water flow around a heated cylinder at Re=85 and Ri =1.0

The three-dimensional flow transition behind a heated cylinder subjected to a horizontal flow (water is used as the working fluid; Pr=7) at a Reynolds number Re=85 and a Richardson number Ri=1.0, manifests itself in the far-wake as escaping mushroom-type structures from the upper vortices. The origin of the escaping mushroom-type structures lies in the generation of streamwise vorticity in the near-wake, which is described as a cyclic-process. In the presence of a spanwise temperature gradient in the near-wake, streamwise vorticity is generated, which results from baroclinic vorticity production. Due to these streamwise vorticity regions, low-speed flow will move upwards at so-called in-plume positions resulting in high- and low-speed streaks in the upper half of the wake. Next, `transverse' vorticity is generated by the spanwise gradients in the streamwise velocity component, resulting in counter-rotating vortices directly behind the cylinder. These vortices lead to high- and low-temperature regions in spanwise direction and the process repeats itself