Behavior of confined granular beds under cyclic thermal loading

We investigate the mechanical behavior of a confined granular packing of irregular polyhedral particles under repeated heating and cooling cycles by means of numerical simulations with the Non-Smooth Contact Dynamics method. Assuming a homogeneous temperature distribution as well as constant temperature rate, we study the effect of the container shape, and coefficients of thermal expansions on the pressure buildup at the confining walls and the density evolution. We observe that small changes in the opening angle of the confinement can lead to a drastic peak pressure reduction. Furthermore, the displacement fields over several thermal cycles are obtained and we discover the formation of convection cells inside the granular material having the shape of a torus. The root mean square of the vorticity is then calculated from the displacement fields and a quadratic dependency on the ratio of thermal expansion coefficients is established

Experimental and Numerical Investigation of Combined Sensible/Latent Thermal Energy Storage for High-Temperature Applications

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi12 was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements

Insulin Promotes Glycogen Storage and Cell Proliferation in Primary Human Astrocytes

In the human brain, there are at least as many astrocytes as neurons. Astrocytes are known to modulate neuronal function in several ways. Thus, they may also contribute to cerebral insulin actions. Therefore, we examined whether primary human astrocytes are insulin-responsive and whether their metabolic functions are affected by the hormone.Commercially available Normal Human Astrocytes were grown in the recommended medium. Major players in the insulin signaling pathway were detected by real-time RT-PCR and Western blotting. Phosphorylation events were detected by phospho-specific antibodies. Glucose uptake and glycogen synthesis were assessed using radio-labeled glucose. Glycogen content was assessed by histochemistry. Lactate levels were measured enzymatically. Cell proliferation was assessed by WST-1 assay.We detected expression of key proteins for insulin signaling, such as insulin receptor β-subunit, insulin receptor substrat-1, Akt/protein kinase B and glycogen synthase kinase 3, in human astrocytes. Akt was phosphorylated and PI-3 kinase activity increased following insulin stimulation in a dose-dependent manner. Neither increased glucose uptake nor lactate secretion after insulin stimulation could be evidenced in this cell type. However, we found increased insulin-dependent glucose incorporation into glycogen. Furthermore, cell numbers increased dose-dependently upon insulin treatment.This study demonstrated that human astrocytes are insulin-responsive at the molecular level. We identified glycogen synthesis and cell proliferation as biological responses of insulin signaling in these brain cells. Hence, this cell type may contribute to the effects of insulin in the human brain

On impedance in shock-refraction problems

Publications on shock-refraction problems typically predict wave patterns resulting from the interaction from the acoustic-impedance ratio. In this note, an analysis based on the shock-impedance ratio is used to derive conditions under which the acoustic-impedance ratio predicts the incorrect type of reflected wave. The range of density ratios for which incorrect types of reflected waves are predicted is found to be quite narrow.ISSN:1432-2153ISSN:0938-128

Thermocline control through multi-tank thermal-energy storage systems

Thermal-energy storage systems consisting of multiple tanks allow the implementation of thermocline-control methods, which can reduce the drop in the outflow temperature during discharging and increase the volumetric storage density and utilization factor. Multi-tank systems based on the extraction and mixing thermocline-control methods were assessed using simulations assuming fluvial rocks as storage material and compressed air as heat-transfer fluid. For adiabatic conditions, the simulations showed improved performance for all multi-tank systems, with diminishing improvements as the number of tanks increases. The mixing method performed better than the extraction method. The mixing method delivered an outflow temperature drop of 5.1% using two tanks whose total volume was 2.15 times smaller than that of the single-tank system. For diabatic conditions, more than three tanks were not beneficial. With two tanks, the mixing method attained a temperature drop of 5.8% with a volume that is 2.5 times smaller than that of the single-tank system. The exergy efficiency of the two-tank system was 91.3% compared to 98.1% of the single-tank system. The specific material costs of the two-tank system were 1.5 times lower than those of the single-tank system.ISSN:0306-2619ISSN:1872-911

Toward a new method for the design of combined sensible/latent thermal-energy storage using non-dimensional analysis

Placing an encapsulated phase-change material (PCM) on top of a packed bed of sensible filler material is an effective way of reducing the drop in the heat-transfer fluid (HTF) outflow temperature during discharging associated with a sensible thermal-energy storage (TES). So far, the literature lacks guidelines for the design of a combined sensible/latent TES. This study aims at developing a new method for the design of combined TES based on non-dimensional analysis. The method will provide a designer with non-dimensional plots, produced from quasi-steady-state results of simulations with a one-dimensional model, that relate performance parameters to geometrical, thermophysical, and operational parameters of the combined TES. In this paper, a simplified version of the method is demonstrated that allows the selection of a metallic PCM and its amount such that a specified drop in the HTF outflow temperature is attained during discharging, assuming a fixed sensible section of natural rocks and air as HTF. The plots show that the drop in the outflow temperature during discharging is minimized by selecting a PCM with a melting temperature equal to 98% of the HTF inflow temperature during charging. The plots also show that the heat of fusion, provided it exceeds a threshold, has a subordinate effect on the drop in the outflow temperature. Finally, the plots show that a smaller heat of fusion can be compensated with a larger height of the latent section. The method is illustrated with a specific example.ISSN:0306-2619ISSN:1872-911

A grid-transparent numerical method for compressible viscous flows on mixed unstructured grids

The goal of the present work is the development of a numerical method for compressible viscous flows on mixed unstructured grids. The discretisation is based on a vertex-centred finite-volume method. The concept of grid transparency is developed as a framework for the discretisation on mixed unstructured grids. A grid-transparent method does not require information on the cell types. For this reason, the numerical method developed in the present work can be applied to triangular, quadrilateral, and mixed grids without modification. The inviscid fluxes are discretised using the approximate Riemann solver of Roe. A limited linear-reconstruction method leads to monotonic capturing of shock waves and second-order accuracy in smooth regions of the flow. The discretisation of the viscous fluxes on triangular and quadrilateral grids is first studied by reference to Laplace's equation. A variety of schemes are evaluated against several criteria. The chosen discretisation is then extended to the viscous fluxes in the Navier-Stokes equations. A careful study of the various terms allows a form to be developed which may be regarded as a thin-shear-layer approximation. In contrast to previous implementations, however, the present approximation does not require knowledge of normal and tangential coordinate directions near solid surfaces. The effects of turbulence are modelled through the eddy-viscosity hypothesis and the one-equation model of Spalart and Allmaras. The discrete equations are marched to the steady-state solution by an explicit Runge-Kutta method with local time-stepping. The turbulence-model equation is solved by a point-implicit method. To accelerate the convergence rate, an agglomeration multigrid method is employed. In contrast to previous implementations, the governing equations are entirely rediscretised on the coarse grid levels. The solution method is applied to various inviscid, laminar, and turbulent flows. The performance of the multigrid method is compared for triangular and quadrilateral grids. Care is taken to assess numerical errors through grid-refinement studies or comparisons with analytical solutions or experimental data. The main contributions of the present work are the careful development of a solution method for compressible viscous flows on mixed unstructured grids and the comparison of the impact of triangular, quadrilateral, and mixed grids on convergence rates and solution quality

Analytical modeling of advanced adiabatic compressed air energy storage: Literature review and new models

We review the literature on analytical models of advanced adiabatic compressed air energy storage plants with isochoric reservoirs, with a focus on the insights that can be extracted from the models. The review indicates that models for plants with adiabatic reservoirs, adiabatic turbomachinery, and without throttling is missing from the literature. We proceed to derive such models, assuming that the plant is operating at the quasi-steady state, that air can be treated as a calorically and thermally perfect gas, and that thermal-energy storage units are free of thermal and pressure losses. The models result in closed-form expressions for key performance indicators like the plant efficiency and volumetric energy density in terms of component efficiencies and pressure ratios. The derivation of these expressions rests on approximating integrals involving simultaneous temporal variations of temperature and pressure. The approximation leads to relative errors with magnitudes smaller than 1%. The models show that the compression and expansion work, the plant efficiency, and the maximum process temperature exhibit minima. The models also show that for a given non-dimensional storage capacity and maximum reservoir pressure, the maximum efficiency of plants that minimize the maximum process temperature is approximately equal to the minimum efficiency of plants that maximize the efficiency. For a two-stage plant with a diabatic cavern and diabatic thermal-energy storage units, our analytical model predicts the volumetric energy density to within 4.76%, indicating that it is accurate enough to be used for initial plant design.ISSN:1364-032

An assessment of thermocline-control methods for packed-bed thermal-energy storage in CSP plants, Part 1: Method descriptions

Thermocline thermal-energy storage (TES) suffers from so-called thermocline degradation, which refers to the flattening of temperature gradients in the TES with successive charging-discharging cycles. Thermocline degradation increases the variations of the heat-transfer fluid (HTF) outflow temperatures, decreases storage utilization factors, and increases specific TES material costs. Methods that prevent or reduce thermocline degradation by changing the operation of the storage are called thermocline-control (TCC) methods. The assessment of TCC methods is the main objective of this work. Three TCC methods that were chosen for this assessment are described in this paper. Two methods, based on either extracting or injecting HTF through ports, were derived from previously published methods while the third method, based on mixing multiple HTF streams, one of which is extracted through a port, is novel. In a companion paper (Geissbühler et al., Solar Energy, submitted 2018), the three TCC methods are assessed for air and molten salt as HTF using simulations of stand-alone storages as well as storages integrated into a concentrated solar power plant.ISSN:0038-092XISSN:1471-125