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Computational Fluid Dynamics (CFD) simulations are increasingly employed in the development of latent thermal energy storage units. Yet there are often strong deviations between the experiments and numerical results. To unveil the sources of the deviations for the CFD simulation of a vertical shell-and-tube latent thermal energy storage unit, a basic analysis of different uncertainties is undertaken in this paper. Consequently, the effect of a variation of 10 material properties, six initial and boundary conditions, as well as a displacement of the temperature measuring points in the simulation, are examined. The results depict that the influence of the substance data depend on the output variable under consideration. Beside material properties, which have almost no influence, there are some properties that influence the power and the global liquid phase fraction over time, and a third group, which also has an influence on the mean power. Partly in contrast to results found in literature, the highest influence on the mean power occurs for the heat losses (which are varied in an on/off manner), the density, and the melting enthalpy (both varied by ± 10 % ). [ABSTRACT FROM AUTHOR]

Two pilot-scale regenerative heat storage systems have been tested by the French Alternative Energies and Atomic Energy Commission (CEA). The first one is a 1.1-MWh th structured packed bed consisting of ceramic plates forming corrugated channels. The second one is a 1.4-MWh th granular packed bed consisting of basaltic rocks enclosed by refractory walls. The two regenerators were tested over a hundred of thermal cycles between 80 °C and 800 °C with different fluid mass flows. Both systems showed their ability to store heat efficiently and to provide thermal energy at a stable temperature for the most part of the discharge process. The granular packed bed exhibited large transverse thermal heterogeneities due to flow channelling in the corners of the cross section. However, this phenomenon appears not to have degraded significantly the thermal performances, and the average one-dimensional thermal behaviour of the system may be assessed by the surface weighted average of the temperature over the bed cross section. Compared to the granular packed bed, the structured bed showed comparable thermal performances while inhibiting flow heterogeneities and reducing by up to 54% the average pressure drop. Furthermore, at the end of the test campaign, the packed beds were observed and compared from a mechanical point of view. The thermal results were successfully simulated over numerous charge/discharge cycles by using a one-dimensional numerical model. This is significant since the discrepancies between experimental and numerical results are likely to accumulate from a cycle to the other. The model considers the packed beds as continuous and homogeneous porous media but takes the conduction resistances within the solid filler and the walls into account. The pressure drop of the beds was computed using a correlation developed from a previous CFD study for the structured packed bed, and the Ergun equation for the granular packed bed. Compared to experimental data, these correlations enabled to estimate the order of magnitude and the evolution trend of the pressure drop with an average deviation ranging from −7.2% to +61.9%. For the granular packed bed, these deviations are ascribed to the flow heterogeneities and the shape of the rocks which are not taken into account in the Ergun equation. • Experimental testing of two pilot-scale gas/solid packed-bed heat storage systems. • Investigation of the cycling behaviour with various mass flows. • Ability of the tested storages to provide useful heat at constant temperature. • Successful simulation of the results thanks to a one-dimensional numerical model. • Average one-dimensional behaviour of the storage almost unaffected by significant flow channelling. [ABSTRACT FROM AUTHOR]

Abstract Thanks to their versatility and their relatively low cost, packed-bed sensible heat storage systems are promising for various applications like in concentrated solar power plants, adiabatic compressed energy storage and pumped thermal energy storage. A versatile one-dimensional numerical model able to describe many packed-bed configurations is developed and presented. This model is able to treat liquid and gaseous heat transfer fluids, and packed bed with a monomodal or a bimodal particle size repartion, i.e. consisting of a mixture of large and small solid particles (such as rocks and sand). This configuration is commonly encountered in the literature due to the advantages it procures. The model is compared and validated with specific experimental data and results from the literature covering wide ranges of configurations and operating conditions: several heat transfer fluids (molten salts, thermal oil, air), solid materials (rocks, sand, ceramics), fluid velocities, temperature levels and packed bed configurations are successfully tested. This shows the versatility of the developed model. The influence of the fluid velocity on heat losses, thermal diffusion and fluid/solid heat exchange are analysed. It enables to determine the optimal velocity which maximizes the performance of the storage system. Highlights • Description of a versatile numerical model for packed-bed heat storage systems. • Relevant approaches and correlations to compute a priori heat transfer coefficients. • Validation of the model in various configurations with experimental data. • Utilization of the model to investigate the fluid velocity influence. [ABSTRACT FROM AUTHOR]

Solar thermal energy is a clean, climate-friendly and inexhaustible energy resource. It is therefore promising to cope with fossil fuel depletion and climate change. Thermal storage enables to make this intermittent energy resource dispatchable, reliable on demand and more competitive. Nowadays, most of the concentrated solar power plants equipped with integrated thermal storage systems use the two-tank molten salt technology. Despite its relative simplicity and efficiency, this technology is expensive and requires huge amounts of nitrate salts. In the short to medium term, packed-bed thermal energy storage with either liquid or gaseous heat transfer fluid is a promising alternative to reduce storage costs and hence improve the development of solar energy. To design reliable, efficient and cost-effective packed-bed storage systems, this technology, which involves many physical phenomena, has to be better understood. This paper aims to sum up some key aspects about design, operation, and performances of packed-bed storage systems. In the first part, most representative setups and their experience feedback are presented. The controllability of packed-bed storage systems and the special influence of thermal stratification are pointed out. In the second part, the various numerical models used to predict packed-bed storage performances are reviewed. In the last part, some useful correlations enabling to quantify the main physical phenomena involved in packed-bed operation and modeling are presented and compared. The correlations investigated enable to calculate fluid/solid and fluid/wall heat transfer coefficients, effective thermal conductivity and pressure drop in packed beds. [ABSTRACT FROM AUTHOR]