Effect of internal void placement on the heat transfer performance – Encapsulated phase change material for energy storage

The effect of an internal air void on the heat transfer phenomenon within encapsulated phase change material (EPCM) is examined. Heat transfer simulations are conducted on a two dimensional cylindrical capsule using sodium nitrate as the high temperature phase change material (PCM). The effects of thermal expansion of the PCM and the buoyancy driven convection within the fluid media are considered in the present thermal analysis. The melting time of three different initial locations of an internal 20% air void within the EPCM capsule are compared. Latent heat is stored within an EPCM capsule, in addition to sensible heat storage. In general, the solid/liquid interface propagates radially inward during the melting process. The shape of the solid liquid interface as well as the rate at which it moves is affected by the location of the internal air void. The case of an initial void located at the center of the EPCM capsule has the highest heat transfer rate and thus fastest melting time. An EPCM capsule with a void located at the top has the longest melting time. Since the inclusion of a void space is necessary to accommodate the thermal expansion of a PCM upon melting, understanding its effect on the heat transfer within an EPCM capsule is necessary.     

Influence of thermal stresses on thermal diffusion of hydrogen

Thermal diffusion of hydrogen atoms in zirconium taking into account thermal stresses is investigated. As mathematical model the steady-state temperature in the hollow cylinder is considered. The first invariant of the tensor of thermal stresses in the hollow cylinder has a logarithmic dependence on the radial coordinate. Such dependence permits an exact analytical solution of diffusion kinetics problem in view of thermal stresses.     

Numerical Analysis of FG and TBC Systems Based on Thermo-Elasto-Plastic-Damage Model

The model of thermo-elasto-plastic damage functionally graded material is developed and the numerical simulation of the material behaviour under complex thermomechanical loading conditions is performed. Sensitivity of the model response to variation of the basic mechanical moduli is examined for a graphite cast iron characterized by a changing volume fraction of graphite particles. The capability of the model to predict response of the complex (thermal barrier/functionally graded material/substrate material) system under thermal loading is checked by the use of two constituents: cast iron and ceramic, and the spatially changing temperature-dependent properties are governed by the appropriate rule of mixture. The response of the three-layer TBC/FGM/S system is compared to the reference metallic structure to show the benefits of the three-layer system.     

CO2 reduction in a solid oxide electrolysis cell with a ceramic composite cathode: Effect of load and thermal cycling

A solid oxide electrolysis cell (SOEC) powered by a renewable source can convert CO₂ into carbon monoxide, which is a valuable feedstock for a range of fuels and chemical processes. The cathode material of the SOEC is required to possess sufficient catalytic activity for CO₂ reduction, and also sustain the thermal and electrical load cycling to which the SOEC would be subjected when coupled with an intermittent renewable source without an auxiliary electricity or thermal storage system. The operating conditions can become even more challenging if solar or waste heat from exothermic downstream industrial processes is to be embedded in the process. In this study, we evaluated a mixed ionic–electronic conducting composite (La_0·80Sr_0.20Sc_0.05Mn_0·95O_3-δ–Gd_0.20Ce_0·80O_1.95) material as an SOEC cathode. Along with initial electrochemical performance, we investigated the cell's response to accelerated ageing tests, including electrical load cycling and extreme thermal cycling. Factors leading to performance degradation were studied by electrochemical impedance spectroscopy and structural characterisation of the cathode before and after the test. Thermal cycling resulted in more pronounced effect on the cell degradation rate as compared to electrical load cycling.     

AXISYMMETRIC TEMPERATURE DISTRIBUTIONS ON A TRANSVERSELY ISOTROPIC SEMI-INFINITE SOLID

An exact static solution for the axisymmetric boundary value problem of a transversely isotropic semispace subjected to a point heat source is constructed by similarity transformations. The closed-form expressions for the temperature and components of displacements and stresses are derived. In the particular cases of uniform and parabolic-type temperature distributions on a circular area of the surface, the expressions for the displacements and stresses at a distance z beneath the surface are determined. The Mathematica software is used, and the numerical results are presented on graphs depicting the spatial variation of the displacements and stresses in a semispace of cobalt material.     

Development of new analytical solutions for elastic thermal stress components in a hollow cylinder under sinusoidal transient thermal loading

For the analysis of high-cycle thermal fatigue due to striping (such as has been observed due to turbulence at mixing tees of class 1–2–3 piping of nuclear power reactors) it can be necessary to consider the time-dependent temperature gradient within the pipe wall thickness rather than just at the surface. To address this, a set of analytical solutions with several new features has been developed for the temperature field and the associated elastic thermal stress distributions for a hollow circular cylinder subjected to sinusoidal transient thermal loading at the inner surface. The approach uses a finite Hankel transform and some properties of Bessel functions. The analytical predictions have been successfully benchmarked by comparison with results from finite element analysis, and also with some results of independent studies.     

Some fatty acids used for latent heat storage: thermal stability and corrosion of metals with respect to thermal cycling

The present study includes thermal stability of some fatty acids as phase change materials (PCMs). The selected fatty acids were stearic, palmitic, myristic and lauric acid with melting temperatures between 40–63°C and industrial-grade with 90–95 % purity. Latent heat storage capacity and phase transition temperature of the PCMs were determined by Differential Scanning Calorimetry (DSC) technique as a function of after repeated thermal cycles such as 40, 410, 700 and 910. The present work also comprises the investigation of corrosion resistance of some construction materials to the fatty acids over a long period. The containment materials tested were stainless steel (SS 304 L), carbon steel (steel C20), aluminium (Al) and copper (Cu). Gravimetric analysis as mass loss (mg/cm²), corrosion rate (mg/day) and a microscopic or matellographic investigation were performed for corrosion tests after 910 thermal cycles. DSC measurements showed that all fatty acids investigated as PCMs have a good thermal stability as a function of latent heat and phase transition temperature range for an actual middle-term thermal energy storage utility. However, in long-term solar thermal applications, the palmitic acid and myristic acid may be considered more suitable PCMs than the others. From the gravimetric and metallographic results, it can be concluded that stainless steel (SS 304L) with chromium oxide (Cr₂O₃) surface layer and Al with aluminium oxide (Al₂O₃) surface layer are essentially compatible with the investigated fatty acids. Carbon steel (Steel C20) and Cupper (Cu) are only preferantially compatible with PCMs.     

Quasi-Static Thermal Stresses Due to Heat Generation in a Thin Hollow Circular Disk

The present paper deals with the determination of displacement and thermal stresses in a thin hollow circular disk defined by a ≤ r ≤ b due to internal heat generation within it. Time dependent heat flux Q(t) is applied at the outer circular boundary (r = b), whereas inner circular boundary (r = a) is at zero heat flux. Also, initially the circular disk is at arbitrary temperature F(r). The governing heat conduction equation has been solved by the method of integral transform technique. The radial stress function σ_rr is zero at inner and outer circular boundaries (r = a and r = b). The results are obtained in a series form in terms of Bessel's functions. The results for displacement and stresses have been computed numerically and illustrated graphically.     

Theoretical and experimental analysis of the vacuum pressure in a vacuum glazing after extreme thermal cycling

Details of theoretical and experimental studies of the change in vacuum pressure within a vacuum glazing after extreme thermal cycling are presented. The vacuum glazing was fabricated at low temperature using an indium-copper-indium edge seal. It comprised two 4 mm thick 0.4 m by 0.4 m glass panes with low-emittance coatings separated by an array of stainless steel support pillars spaced at 25 mm with a diameter of 0.4 mm and a height of 0.15 mm. Thermal cycling tests were undertaken in which the air temperature on one side of the sample was taken from −30 °C to +50 °C and back to −30 °C 15 times while maintaining an air temperature of 22 °C on the other side. After this test procedure, it was found that the glass to glass heat conductance at the centre glazing area had increased by 10.1% from which the vacuum pressure within the evacuated space was determined to have increased from the negligible level of less than 0.1 Pa to 0.16 Pa using the model of Corrucini. Previous research has shown that if the vacuum pressure is less than 0.1 Pa, the effect of conduction through the residual gas on the total glazing heat transfer is negligible. The degradation of vacuum level determined was corroborated by the change in glass surface temperatures.     

Development of hydrogen absorption–desorption experimental test bench for hydrogen storage material

A volumetric experimental set-up used for measuring hydrogen absorption–desorption characteristics of hydrogen storage material will be presented. Although the experimental set-up is mainly employed to do hydrogen absorption–desorption cycling (including pressure cycling and thermal cycling) measurement automatically, it also can incidentally provide general measurements such as pressure-composition-temperature (P–C–T) curves and kinetics measurements in manual way in the ranges of 0.004–12 MPa and 213–773 K. The experimental set-up can be used to investigate the influence of hydrogen absorption–desorption cycles to hydrogen storage properties of material. The leakage rate of the whole experimental set-up was evaluated systemically. The usability and reliability of the experimental set-up were checked with LaNi₅ and Pd/K (kieselguhr).     

Modeling of thermal stresses and lifetime prediction of planar solid oxide fuel cell under thermal cycling conditions

A typical operating temperature of a solid oxide fuel cell (SOFC) is above 600 °C, which leads to severe thermal stresses caused by the difference in material mechanical properties during thermal cycling. Interfacial shear stress and peeling stress are the two types of thermal stresses that can cause the mechanical failure of the SOFC. Two commonly used SOFC configurations (electrolyte-supported and anode-supported) were considered for this study. The paper developed a mathematical model to estimate the thermal stresses and to predict the lifetime of the cell (Ni/8YSZ-YSZ-LSM). Due to the mismatch of the material mechanical properties of the cell layers, a crack nucleation induced by thermal stresses can be predicted by the crack damage growth rate and the initial damage distribution in the interfacial layer for each thermal cycle. It was found that the interfacial shear stress and peeling stress were more concentrated near the electrode free edge areas. The number of cycles needed for failure decreased with the increase in the porosity of electrode. The number of cycle for failure decreased with increase in electrolyte thickness for both anode- and electrolyte-supported SOFC. The model provides insight into the distribution of interfacial shear stress and peeling stress and can also predict damage evolution in a localized damage area in different SOFC configurations.     

Transient Thermal Stress Analysis of Orthotropic Functionally Graded Materials with a Crack

The transient thermal fracture problem of a crack (perpendicular to the gradient direction) in a graded orthotropic strip is investigated. Most of the materials properties are assumed to vary as an exponential function of thickness direction. The transient two-dimensional temperature problem is analyzed by the methods of Laplace and Fourier transformations. A system of singular integral equations are obtained and solved numerically. Numerical results are figured out to show the variation of the temperature on the crack faces and extended line and stress intensity factors for different material parameters with dimensionless time.     

Functionally graded hollow spheres under non-axisymmetric thermo-mechanical loads

An analytical method is developed to obtain the solution for the two-dimensional (r,θ)$(r,\theta )$ steady state thermal and mechanical stresses in a hollow thick sphere made of functionally graded material. The material properties are assumed to vary through the thickness according to the power law functions. The temperature profile is obtained solving the functionally graded energy equation. The Navier equations are solved analytically using the Legendre polynomials and the system of Euler differential equations. Temperature, displacement components and stresses distributions are obtained and plotted for different power law indices. The results are validated with the known data in literature.