Material Properties Of Components Research Articles

Non-destructive experimentation: Dynamic identification of multi-leaf masonry walls damaged and consolidated

Abstract The dynamic monitoring of multi-leaf masonry wall is here proposed. The results of non-destructive tests carried out in this work are a first part of a wider testing campaign aimed at verifying the structural performance of masonry walls. Multi-leaf masonry walls constitute the composite construction typology most widely adopted in historic buildings. This aspect, together with the intrinsic structural complexity, heterogeneity and irregularity, directs the present research towards a topic not yet sufficiently investigated by the scientific community. In this paper, the case of multi-leaf masonry wall has been investigated, and with the aim of reproducing historical buildings structural elements, three different typologies of multi-leaf masonry walls have been considered: (i) full infill, (ii) damaged infill, (iii) consolidated infill. Several masonry specimens of the above-described typologies have been built and tested in lab. The dynamic parameters, such as frequencies, modal shapes and damping ratios have been identified through the output-only procedure and the data were processed through the Least Square Complex Frequency (LSFC) estimator; the analysis of results allowed to evaluate the structural integrity, the efficacious of consolidating intervention and the potential performance mechanisms of different complex multi-leaf masonry walls. Experimental results are compared with numerical Finite Elements models. Composite multi-leaf masonry is modeled as an equivalent continuum obtained through a full 3D homogenization procedure. The mechanical properties of component materials have been obtained in lab by tests and then used to model the masonry at the micro-scale in order to derive the mechanical properties to be adopted at macro-scale. A parametric natural frequencies analysis is performed in order to calibrate the model by the comparison whit the experimental measures.

Quantification and design of jumping-ski characteristics

Elastic and dynamic characteristics of jumping-skis affect ski performance at all four ski jumping phases. Compared with the number of studies on alpine-skis, there have been few studies on the characteristics of jumping-skis. This article identifies design parameters that have the most influence on jumping-ski characteristics. To identify the elastic and dynamic characteristics of jumping-skis, previous research and testing methods for alpine-skis were modified. Spring constants and bending stiffness distributions for three jumping-skis were measured. Natural frequencies, modal shapes, and damping ratios were also measured. From these results, the bending stiffness distribution was identified as a more elastic characteristic of jumping-skis than the spring constant. The natural frequency and damping ratio were selected as the relevant dynamic characteristics. To determine the effective design parameter for elastic and dynamic characteristics of jumping-skis, a jumping-ski was modeled by finite element method, and the inner structures and material properties of components of a jumping-ski were measured and analyzed. By comparing the simulation with actual test results, the reliability of the finite element method simulation was verified. The geometrical feature of the ski thickness profile is the most significant design parameter for elastic characteristics of jumping-skis. A mechanical property of the face material and wood at the sidewall is a highly influencing design parameter for dynamic characteristics of jumping-skis.

Influence analysis of aging of spinal segment on the mechanical behavior of vertebral cortex

The risk of vertebral cortical shell fracture increases with aging. However, it remains unclear how aging contributes to cortex fracture at present. The aim of this study is to make understanding of the mechanism of how the spinal aging influences the cortical shell strain. Two finite element (FE) models of spinal segments (mildly and fully aged) were created, and then were compared to the FE models of the healthy spinal segment. The FE models of the aged spinal segments were generated by modifying both the geometry of the intervertebral disc (IVD) and the material properties of the spinal components. To find out under which case the cortical shell strain was influenced more, we created two types of FE model comparison methods: one with changes only in the spinal material properties and the other with changes only in the IVD geometry. The results showed that the cortical shell strains increased with aging and that compared to the changes of IVD geometry, the changes of spinal material property have a higher influence on the cortical shell strains. This study may suggest that for the prevention and treatment of vertebral cortex fracture, the augmentation of the vertebral body is a more effective treatment.

Ceramic Additive Manufacturing of Solid State Electrochemical Devices

The additive manufacturing of electro-ceramics is a major technological challenge with many possible applications. The technology can prototype and produce in small volume production, custom dielectric, piezoelectric and solid state ionic devices. Sandia National Laboratories was the birthplace of modern, 3-D ceramic additive manufacturing. The newly developed manufacturing methods are considered to be the foundation of 3-D printing via particulate slurries and our spin off company Robocasting Enterprises, is commercially producing dense ceramic and composite parts. These parts are used in various applications including complex geometry catalyst supports and casting filters. This technique is based on the layer-by-layer deposition of highly loaded colloidal slurries that are extruded through a small nozzle. The rheology of the slurry is crucial in order to maintain the structural integrity of the part being built, as Robocasting does not rely on polymerization reactions or solidification to retain shape after extrusion. Since this process is essentially binderless, a dense ceramic part can typically be freeformed, dried, and sintered in less than 24 hours. Although Robocasting is a highly useful technique, it has been optimized for ceramic and composite materials. More recently there has been some work on expanding this technology for use in printing electrode materials. Printing conductors and other structures for electrochemical and electronics using direct write technology gives the potential to reduce production time and reduce waste of expensive material. We have demonstrated that this direct write manufacturing technique can produce Multi-Chip Modules (MCM) by depositing silver and gold on Low Temperature Co-fired Ceramics (LTCC). We have also applied this additive manufacturing technique to our unique design of electrochemical gas sensors. These multi-component mixed potential sensors consist of solid electrodes and a porous electrolyte. In order to expand the versatility of this manufacturing method to be used in solid state ionic devices, slurries of solid electrolytes and electrode materials with specific rheological properties were developed. Another challenge was controlling the residual stress and shrinkage through the material properties of various components. These devices are capable of detecting NOx, hydrocarbons, and NH3 at concentrations on the order of parts per million. The additive manufacturing technique allows for the rapid prototyping of these devices and low volume production of custom devices optimized for specific applications. Figure 1

Electron emission under uniform magnetic field of materials for fusion and space applications

The power handling capacity of RF components can be limited by a resonant process known as Multipactor effect. Multipactor-induced breakdowns can be critical to microwave systems in space communication payloads or in experimental fusion devices. Multipactor simulations can be used to predict power thresholds but the results highly depend on the electron emission properties of the RF component materials. Moreover, in both space and fusion applications, the RF devices can be subjected to DC magnetic fields. This magnetic field may affect the electron emission properties. In order to improve understanding and predictability of the multipactor effect, the assessment of DC magnetic field on the electron emission properties is required; an experimental setup has been developed to measure the Total Electron Emission Yield (TEEY) under DC magnetic field. The aim of this paper is to describe the newly developed experimental setup and the associated TEEY measurements techniques. The effect of DC magnetic field on the TEEY of copper is investigated.

Stress analysis of fracture of atherosclerotic plaques: crack propagation modeling.

Traditionally, the degree of luminal obstruction has been used to assess the vulnerability of atherosclerotic plaques. However, recent studies have revealed that other factors such as plaque morphology, material properties of lesion components and blood pressure may contribute to the fracture of atherosclerotic plaques. The aim of this study was to investigate the mechanism of fracture of atherosclerotic plaques based on the mechanical stress distribution and fatigue analysis by means of numerical simulation. Realistic models of type V plaques were reconstructed based on histological images. Finite element method was used to determine mechanical stress distribution within the plaque. Assuming that crack propagation initiated at the sites of stress concentration, crack propagation due to pulsatile blood pressure was modeled. Results showed that crack propagation considerably changed the stress field within the plaque and in some cases led to initiation of secondary cracks. The lipid pool stiffness affected the location of crack formation and the rate and direction of crack propagation. Moreover, increasing the mean or pulse pressure decreased the number of cycles to rupture. It is suggested that crack propagation analysis can lead to a better recognition of factors involved in plaque rupture and more accurate determination of vulnerable plaques.

Fabrication of Solid State Ionic Devices Using Additive Manufacturing

The fabrication of solid state ionic devices requires customization of size, shape, geometry, and composition for various components. Traditionally used fabrication methods are tape casting and screen printing. Although these methods are well developed they are difficult, time consuming, and costly to adjust to fit particular device requirements. Sandia National Laboratory has recently developed Robocasting, an additive manufacturing technique that was developed as a method that allows for the adaptation needed to rapidly prototype specialty ceramics in a more economically efficient way. Robocasting is currently used commercially to produce complex ceramic parts that are used in various applications including catalyst supports and casting filters. The technology has not been developed for the fabrication of ceramic electrochemical devices. While this has worked well for these limited specific needs, this technology has been optimized primarily for alumina based ceramic materials. In order to expand the versatility of this method for solid state ionic devices, powder precursors of solid electrolytes and electrode materials with specific properties must be developed. Tailoring the rheology of the pastes and slurries used in this extrusion method is an important aspect that determines the resolution of the resulting ceramic components. Another challenge is controlling the residual stress and shrinkage through the material properties of various components. We have applied this additive manufacturing technique to our unique design of gas sensors. These multi-component sensors consist of a yttria-stabilized zirconia substrate onto which a set of three solid electrodes and a porous YSZ electrolyte are deposited. The production method employed involves the direct printing of a suspended powder of the electrode and electrolyte components using Robocasting technology. This allows for the rapid prototyping of these devices. The key to this process is the use of high temperature co-fired ceramics which enables all components to be fired together. The electrodes consist of a platinum counter electrode and lanthanum-strontium-chromite and gold-palladium working electrodes. By taking advantage of the dissimilar electrochemical catalytic activity of these materials, the difference in mixed potential between two electrodes can be used to quantify the concentration and discriminate the type of gas present. These devices are capable of detecting NOx, hydrocarbons, H2, and NH3 at concentrations on the order of parts per million. Figure 1

The Effect of Cooling Rate on Properties of Intermetallic Phase in a Complex Al-Si Alloy

Abstract The cooling rate is one of the main tools available to the process engineer by means of which it is possible to influence the crystallisation process. Imposing a desired microstructure on a casting as early as in the casting solidification phase widens significantly the scope of technological options at disposal in the process of aluminium-silicon alloy parts design and application. By changing the cooling rate it is possible to influence the course of the crystallisation process and thus also the material properties of individual microstructure components. In the study reported in this paper it has been found that the increase of cooling rate within the range of solidification temperatures of a complex aluminium-silicon alloy resulted in a decrease of values of the instrumented indentation hardness (HIT) and the instrumented indentation elastic modulus (EIT) characterising the intermetallic phase occurring in the form of polygons, rich in aluminium, iron, silicon, manganese, and chromium, containing also copper, nickel, and vanadium. Increased cooling rate resulted in supersaturation of the matrix with alloying elements.

Improvement of predicting mechanical properties from spherical indentation test

Although nowadays the Instrumented Indentation Test (IIT) can be applied to the fields from nano- micro- to macroscopic scale, this paper only addresses its application in the area of life assessment of components in industry, such as power and/or petroleum chemistry plants. In comparison of IIT with other miniature specimen techniques, such as the small punch test, impression test, micro-tensile or small ring tests etc., the relative advantages of the IIT technique are: non-destructive test could be performed on site, no sample machine is needed to cut out material, thus no sample processing is necessary. Therefore, the IIT method has potential for residual life assessment of components in service. A portable IIT indenter has been developed to measure both load and displacement, from which actual material properties of components can be evaluated.In order to develop analytical software for a new portable IIT instrument, authors reviewed several existing analytical methods, such as the representative stress-strain method, dimensional analysis method, and inverse finite element method. This paper first gives an overview of these analytical methods, their advantage and disadvantage, and then put forward ideas to improve them. Finally, the so-called Neural Network (NN) is introduced as the NN method. This method, can deliver evaluation on site without time-consuming finite element analysis.Experimental results for IIT are provided by DNV KEMA. First verifications are carried out for elastic-plastic properties, such as yield strength, tensile strength, Young's modulus and Brinell hardness. A good agreement is found between the conventional test results and the analytical prediction from IIT results. More validation and tests are needed and planned for the near future.

Effect of heat treatment on interfacial and mechanical properties of A6022/A7075/A6022 roll-bonded multi-layer Al alloy sheets

Multi-layered Al alloy sheets can exhibit unique properties by the combination of properties of component materials. A poor corrosion resistance of high strength Al alloys can be complemented by having a protective surface with corrosion resistant Al alloys. Here, a special care should be taken regarding the heat treatment of multi-layered Al alloy sheets because dissimilar Al alloys may exhibit unexpected interfacial reactions upon heat treatment. In the present study, A6022/A7075/A6022 sheets were fabricated by a cold roll-bonding process, and the effect of the heat treatment on the microstructure and mechanical properties was examined. The solution treatment gave rise to the diffusion of Zn, Mg, Cu and Si elements across the core/clad interface. In particular, the pronounced diffusion of Zn, which is a major alloying element (for solid-solution strengthening) of the A7075 core, resulted in a gradual hardness change across the core/clad interface. Mg2Si precipitates and the precipitate free zone were also formed near the interface after the heat treatment. The heat-treated sheet showed high strengths and reasonable elongation without apparent deformation misfit or interfacial delamination during the tensile deformation. The high strength of the sheet was mainly due to the T4 and T6 heat treatment of the A7075 core.

Buckling and large deflection behaviors of radially functionally graded ring-stiffened circular plates with various boundary conditions

The buckling and large deflection behaviors of axis-symmetric radially functionally graded (RFG) ring-stiffened circular plates are investigated by the dynamic relaxation (DR) method combined with the finite difference discretization technique. The material properties of the constituent components of the RFG plate are assumed to vary continuously according to the Mori-Tanaka distribution along the radial direction. The nonlinear governing equations are obtained in the incremental form based on the first-order shear deformation plate theory (FSDT) and the von Karman relations for large deflection. In the buckling analysis, an external in-plane load is applied to the plate incrementally so that, in each load-step, the incremental form of the governing equations can be solved by a numerical code prepared based on the DR method. After converging the DR code in the first increment, the latter load-step is added to the previous one, and the program is repeated again. The critical buckling load is determined from the compressive load-displacement curve obtained by solving the incremental form of the governing equa- tions. Based on the present incremental form of formulation, a bending analysis can also be conducted if the whole load is applied simultaneously. Finally, a detailed parametric study is carried out to investigate the influences of various boundary conditions, grading indices, thickness-to-radius ratios, stiffener’s positions and depths on the critical buckling load, and displacements and stresses resulted from the bending analysis. It is observed that the effect of the stiffener on the results is much greater in the functionally graded plate with higher material grading indices. The results also reveal that, by increasing the depth of the stiffer, the values of ascending the critical buckling load are approximately identical for both simply supported and clamped boundary conditions.

Contribution of material properties of cellular components on the viscoelastic, stress-relaxation response of a cell during AFM indentation

ABSTRACTThe close relationship between the mechanical properties of biological cells, namely, elasticity, viscosity, and the state of its disease condition has been widely investigated using atomic force microscopy (AFM). In this study, computational simulation of the AFM indentation is carried out using a finite element (FE) model of an adherent cell. A parametric evaluation of the material properties of the cellular components on the viscoelastic, stress-relaxation response during AFM indentation is performed. In addition, the loading rate, the size of the nucleus, and the geometry of the cell are varied. From the present study, it is found that when comparing the material properties derived from experimental force-deflection curves, the influence of loading rates should be accommodated. It also provides a framework that can quantify the variation of the mechanical property with various stages of malignancy of the cancer cell, a potential procedure for cancer diagnosis.

(Invited) Pathway to Developing Highly Efficient and Durable Photelectrochemical Device

The photoelectrochemical (PEC) conversion of solar energy to chemical fuels is an attractive and feasible route to a sustainable energy infrastructure. After the first demonstration by Fujishima and Honda in 1972,[1,2] there has been significant progress with notable conversion efficiency records established using III-V semiconductor and earth abundant silicon photocatalysts.[3,4,5,6] However, the current level of efficiency and durability are not yet sufficient for widespread commercial use of this technology. One particular challenge has been exploiting the proximity of charge carrier generation to the catalytically active sites, which is one advantage offered by a PEC device over photovoltaic-driven electrolysis. [7] In this talk, we will review key aspects in realizing high efficiency and durability in conventional III-V PEC photocatalysts and focus on the interfacial properties that are integral to reliable PEC devices. Using ab-initio molecular dynamics simulations, we first show that the hydrogen bond networks at the water-InP interface exchange themselves more freely than those at water-GaP interface, which leads to more facile diffusion of hydrogen at the interface. [8] Interestingly, it was reported that Pt catalysts can activate hydrogen evolution on the surrounding SiO2 surface of Si-based Metal-Insulator-Semiconductor solar-to-hydrogen conversion devices, with the activation suggested to take place due to hydrogen diffusion (H-spillover) at the electrolyte-insulator interface.[9] We relate these results to the III-P PEC systems and discuss a possible general device design strategy that takes advantage of this phenomena. As a next step, we have investigated the electronic structure of water-semiconductor interfaces using many-body perturbation theory (GW approximation) optimized for large systems, so as to develop more comprehensive understanding of interfacial properties (structural, electronic, and chemical) relevant for the PEC device performance.[10] We discuss how this information can be used to engineer surface treatments for fine tuning the band alignment for improved PEC performance. [11] In order to bridge the knowledge obtained from atomistic ab-initio simulations and the behavior of a macroscopic PEC device, we have also been developing highly collaborative surface validation capabilities based upon the state-of-art X-ray spectroscopy.[12] We demonstrate that X-ray spectroscopy, when combined with computational spectroscopy, can provide a detailed atomistic picture of the state of chemical species in semiconductors or on semiconductor surfaces. Such information is crucial in understanding the correlation between microscopic properties of device component materials and the device performance. Lastly, we will discuss an effective design strategy of PEC device component materials, where we leverage a tightly coupled collaboration of theory, synthesis, and characterization. The case study has been performed based on a development of thin film Cu(In,Ga)(S,Se)2 photo absorbers and the suitable buffer.[13] [1] A. Fujishima and K. Honda, Nature 238, 37 (1972). [2] J. Ager, M. Shaner, K. Walczak, I. Sharp, S. Ardo, Energy & Env. Sci. 8, 2811 (2015). [3] O. Khaselev and J. Turner, Science 280, 425 (1998). [4] R. Rocheleau, E. Miller, and A. Misra, Energy & Fuels 12, 3 (1998). [5] R. Fan et al. App. Phys. Lett. 106, 213901 (2015). [6] M. May et al. Nat. Comm. 6, 8286 (2015). [7] A. Nakamura et al. App. Phys. Exp. 8, 107 (2015). [8] B. Wood, E. Schwegler, W.-Ih Choi, T. Ogitsu, J. Am. Chem. Soc. 135, 15774 (2013). [9] D. Esposito, I. Levin, T. Moffat, A. Talin, Nat. Mat. 12, 562 (2013). [10] T. Pham et al. in preparation. [11] B. MacLeod et al. ACS App. Mat. & Int. 7, 11346 (2015). [12] W.-Ih Choi et al. in preparation. [13] J. Varley et al. in preparation.

Machining characteristics of the haversian and plexiform components of bovine cortical bone

This paper investigates the characteristic differences observed while machining the haversian and plexiform components of a bovine cortical femoral bone. To this end micro-milling slotting experiments are performed on both the components by varying both the cutting velocity and the feed-per-tooth values. The scale of machining is chosen specifically to ensure sensitivity to the microstructural variations in the bone. The material properties of the microstructural components and their size-scale relative to the feed-per-tooth values are seen to dictate the failure mechanisms encountered during machining. The cutting force, surface roughness, and tool wear are all uniquely affected by the plexiform and haversian components of the cortical bone. In general, plexiform bone requires a higher cutting force than the haversian bone. While a higher cutting velocity can lower the surface roughness of haversian bone, it typically results in the most surface damage. The cutting force and surface roughness values for both the components show strain rate sensitivity. The tool wear is seen to be the highest while cutting parallel to the lamellar structures seen in the plexiform bone.

Vertical Facing Panel-Joint Gap Analysis for Steel-Reinforced Soil Walls

AbstractThis paper reports the results of a numerical parametric study focused on the prediction of vertical load distribution and vertical gap compression between precast concrete facing panel units in steel-reinforced soil walls ranging in height from 6 to 24 m. The vertical compression was accommodated by polymeric bearing pads placed at the horizontal joints between panels during construction. This paper demonstrates how gap compression and magnitude of vertical load transmitted between horizontal joints are influenced by joint location along the height of the wall, joint compressibility, and backfill and foundation soil stiffness. The summary plots in this study can be used to estimate the number and type (stiffness) of the bearing pads to ensure a target minimum gap thickness at the end of construction, to demonstrate the relative influence of wall height and different material component properties on vertical load levels and gap compression, or as a benchmark to test numerical models used for proje...

Evaluation of Round Window Stimulation Performance in Otosclerosis Using Finite Element Modeling

Round window (RW) stimulation is a new type of middle ear implant's application for treating patients with middle ear disease, such as otosclerosis. However, clinical outcomes show a substantial degree of variability. One source of variability is the variation in the material properties of the ear components caused by the disease. To investigate the influence of the otosclerosis on the performance of the RW stimulation, a human ear finite element model including middle ear and cochlea was established based on a set of microcomputerized tomography section images of a human temporal bone. Three characteristic changes of the otosclerosis in the auditory system were simulated in the FE model: stapedial annular ligament stiffness enlargement, stapedial abnormal bone growth, and partial fixation of the malleus. The FE model was verified by comparing the model-predicted results with published experimental measurements. The equivalent sound pressure (ESP) of RW stimulation was calculated via comparing the differential intracochlear pressure produced by the RW stimulation and the normal eardrum sound stimulation. The results show that the increase of stapedial annular ligament and partial fixation of the malleus decreases RW stimulation's ESP prominently at lower frequencies. In contrast, the stapedial abnormal bone growth deteriorates RW stimulation's ESP severely at higher frequencies.

Inflation and Bi-Axial Tensile Testing of Healthy Porcine Carotid Arteries

Knowledge of the intrinsic material properties of healthy and diseased arterial tissue components is of great importance in diagnostics. This study describes an in vitro comparison of 13 porcine carotid arteries using inflation testing combined with functional ultrasound and bi-axial tensile testing. The measured tissue behavior was described using both a linear, but geometrically non-linear, one-parameter (neo-Hookean) model and a two-parameter non-linear (Demiray) model. The shear modulus estimated using the linear model resulted in good agreement between the ultrasound and tensile testing methods, GUS = 25 ± 5.7 kPa and GTT = 23 ± 5.4 kPa. No significant correspondence was observed for the non-linear model aUS = 1.0 ± 2.7 kPa vs. aTT = 17 ± 8.8 kPa, p ∼ 0); however, the exponential parameters were in correspondence (bUS = 12 ± 4.2 vs. bTT = 10 ± 1.7, p > 0.05). Estimation of more complex models in vivo is cumbersome considering the sensitivity of the model parameters to small changes in measurement data and the absence of intraluminal pressure data, endorsing the use of a simple, linear model in vivo.

Design for Fiber-Reinforced Additive Manufacturing

The continuously decreasing life cycle of modern products leads to new challenges for product development. Additive manufacturing (AM) processes are able to support faster development by rapid production of samples and prototypes. However, the material properties of components produced by common (plastic-) 3D-printers are often insufficient for functional prototyping. A well-established way to improve the properties of plastics is the embedding of reinforcing fibers. Thus, this paper shows a method for fiber-reinforced 3D-printing. Through this combination, several restrictions of conventional composite production can be eased and additional freedoms of design are gained. To support the design of such parts, an adapted design methodology for fiber-reinforced 3D-printing is developed.

A Framework for Local Mechanical Characterization of Atherosclerotic Plaques: Combination of Ultrasound Displacement Imaging and Inverse Finite Element Analysis.

Biomechanical models have the potential to predict plaque rupture. For reliable models, correct material properties of plaque components are a prerequisite. This study presents a new technique, where high resolution ultrasound displacement imaging and inverse finite element (FE) modeling is combined, to estimate material properties of plaque components. Iliac arteries with plaques were excised from 6 atherosclerotic pigs and subjected to an inflation test with pressures ranging from 10 to 120 mmHg. The arteries were imaged with high frequency 40 MHz ultrasound. Deformation maps of the plaques were reconstructed by cross correlation of the ultrasound radiofrequency data. Subsequently, the arteries were perfusion fixed for histology and structural components were identified. The histological data were registered to the ultrasound data to construct FE model of the plaques. Material properties of the arterial wall and the intima of the atherosclerotic plaques were estimated using a grid search method. The computed displacement fields showed good agreement with the measured displacement fields, implying that the FE models were able to capture local inhomogeneities within the plaque. On average, nonlinear stiffening of both the wall and the intima was observed, and the wall of the atheroslcerotic porcine iliac arteries was markedly stiffer than the intima (877 ± 459 vs. 100 ± 68 kPa at 100 mmHg). The large spread in the data further illustrates the wide variation of the material properties. We demonstrated the feasibility of a mixed experimental–numerical framework to determine the material properties of arterial wall and intima of atherosclerotic plaques from intact arteries, and concluded that, due to the observed variation, plaque specific properties are required for accurate stress simulations.

Numerical modeling and optimization of an insulation system for underground thermal energy storage

Underground thermal energy storage (TES) systems require an insulation system to control the heat flux from the TES system into the surrounding rock mass to minimize the adverse effects of the high-temperature storage media on the underground environment. In this study, numerical simulations were performed to investigate the temperature distribution and heat transfer in the insulation system and to determine the optimal design for the TES system. The performance of the mineral wool chosen as an insulation material is gradually degraded as its temperature is increased; thus, the temperature-dependent properties of component materials were applied to prevent the overestimation of the insulation performance. We demonstrated that the component arrangement could affect the performance of the insulation system under transient heat transfer conditions; thus, we compared three types of insulation systems according to the location of the mineral wool layer. An insulation system composed of an inner mineral wool layer, a steel liner, an outer mineral wool layer, and a concrete lining was proposed as the optimal system under the conditions given in this study. We showed that regular emptying of the storage caverns for a long time interval in periods of low energy demand would slow down the temperature increase in the surrounding rock mass, and we proposed that the operation schedule including idle time could also be an alternative to reduce the thickness of the mineral wool layers.