Abstract
Sorption materials such as zeolite are intensively investigated for thermochemical heat storage applications. The heat storage process is based on a reversible adsorption-desorption reaction, which is exothermic in one direction (hydration) and endothermic in the reverse direction (dehydration). For evaluating the transport phenomena occurring in a heat storage reactor, a detailed model is needed, considering also the transversal terms. In a cylindrical reactor, these terms appear as radial effects that disturb the plug flow assumption in the packed bed, and hence, a model with only axial terms is insufficient to simulate the bed. The radial effects in a porous medium, created by presence of the wall surrounding it, can be caused by: (i) heat losses to the ambient through the wall, (ii) a higher bed void fraction in the wall region, resulting in flow channelling, and (iii) non-uniform initial state of charge near the wall for the subsequent re/de-hydration (e.g. due to heat loss during dehydration). A 2D model is developed for transport phenomena in a packed bed by doing a literature survey on representative models. The model is validated by experimental results measured in a lab-scale setup by comparing the pressure drop over the bed, velocity profile below the bed and temperature profile inside the bed. In addition, the concentration of adsorbed water is compared with experimental results from MRI (Magnetic Resonance Imaging) experiments. The validated numerical model is employed to understand the significance of the above mentioned effects on the thermal performance of the reactor. An accurate model for the thermal dynamics of an adsorption bed on reactor scale is obtained, which is used to present suggestions to optimize the charging and discharging process times, hence, to improve the performance of the reactor.
Highlights
Climate change and depletion of fossil fuel resources are among the main issues under investigation all over the world
Winterberg et al [17] have re-evaluated the functions and determined the parameters by comparison of the model to the experimental data available from literature. They interpreted the quantities for the heat transfer case as follows: K1;h determines the slope of rise in the effective thermal conductivity with flow velocity; K2;h sets the damping parameter after the coefficient begins to decline towards the wall; the exponent nh determines the curvature of the damping function
The diffusion-controlled kinetics may be satisfactory represented by the Linear Driving Force (LDF) approximation first time introduced by Glueckauf [51]: dq dt where q and qeq are the adsorbed and equilibrium concentrations in the solid phase in moles of water per kilograms of dry zeolite, and kLDF is the mass transfer coefficient between the fluid phase and the solid phase
Summary
Climate change and depletion of fossil fuel resources are among the main issues under investigation all over the world. The adsorption kinetics is frequently modeled by the Linear Driving Force (LDF) model [7] [8] [9], because it is simple, analytical, and physically consistent [10] In the literature, both open and closed systems are investigated for long-term thermal storage of solar energy [11]. A disadvantage of the packed bed reactor concept is the risk of a nonuniform flow leading to a lower energy storage density This can be avoided by consideration of specific measures in the design of the reactor, which require an in-depth understanding of the physical processes inside the reactor which can be obtained by developing a validated numerical model. Suggestions are given in order to optimize the charging and discharging process times, to improve the performance of the reactor
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