EXPERIMENTAL INVESTIGATION IN THE VARIATION OF SOAPSTONE AND GRANITE ROCKS AS THERMAL ENERGY STORAGE MATERIALS

Description

To determine the suitability of granite and soapstone rocks as materials for thermal energy storage in solar drying and concentrated solar power generation experimental investigation was conducted. Characterization experiments on thermal chemical, thermophysical and mechanical properties were conducted. Whereby, thermal stability was determined using the thermal gravimetric analyzer (TGA); crystalline phases were studied using the X-ray diffraction (XRD); structural analysis was studied through the petrographic imaging. Moreover, chemical composition was determined using the X-ray fluorescence (XRF) and; high temperature test was done in the high temperature furnace. The thermo-physical properties included density and porosity studied using the displacement method; specific and thermal capacity experimented using the differential scanning calorimetry (DSC) and; thermal diffusivity and conductivities determined through the laser flash apparatus (LFA). The thermo-mechanical properties studied were the Young’s modulus obtained using the nano-indentation method. Sample codes depending on rock type and geotectonic setting of origin - Soapstone from Craton geotectonic setting CS - Soapstone from Usagaran geotectonic setting US - Granite from Craton geotectonic setting CG - Granite from Usagaran geotectonic setting UG

Steps to reproduce

Chemical Composition X-ray Fluorescence technique was used. Whereby, 1g of pressed powder pellets of the rock samples were used in the Bruker AXS S4 spectrometer in the presence of P10 detector gas. Structural Analysis Petrographic examination was performed. The rocks were cut to thin section thickness of 0.03 mm. Then using a Carl Zeiss Primotech polarizing microscope equipped with built-in camera and MATSCOPE software. Crystalline Phases Bruker D8 ADVANCE X-ray diffractometer equipment with powdered samples of 150 μm, primary Ge111 monochromator, a LinxEye silicon strip detector and a current of wavelength 1.54059 Å from a Cu-tube were used. The samples were measured in the range 2o to 90o 2θ, at steps of 0.02o and a rate of 4 s per step. Thermal Decomposition The thermo-gravimetric analysis was performed. About 3.5 ± 0.3 mg of <63 µm powdered sample was used in the TG/DTA 6300 machine. The process took place from 40 ℃ to 950 ℃at the rate of 10 ℃/min. Alumina (Al2O3) was used as a reference material. High Temperature Test Samples of 10 x 10 x 10 mm were enclosed in a CBFL518C Cole Palmer Box furnace and exposed to 700 oC and 1000 oC for 6 hours. Specific and Thermal capacity Powder samples of 150 μm were used in NETZSCH DSC 404 F1 Pegasus differential scanning calorimeter against sapphire as calibration standard. Heating was done under air atmosphere at rate of 10 °C/min. Thermal capacity was calculated as a product of density and specific heat capacity as in equation 1. ----Thermal capacity at temperature T (C(T))=(Cp(T))x(ρ(T)) (1) Thermal diffusivity and conductivity Additionally, a laser flash apparatus was used to determine the thermal diffusivity and thermal conductivity. Samples of 10 x 10x 1.5 mm were placed on a graphite holder in the NETZSCH LFA 427 Micro-flash apparatus under argon atmosphere and vacuum of 10-2 mBar. Heating was then done at a rate of 2.5 °C/min from ambient temperature to 950 °C. Bulk density Densities (ρ) of the four rock types was determined according to the standard procedure of ASTM C128-15 by American Society for Testing and Materials for Relative Density of Fine Aggregates. The density at room temperature was then calculated using Equation 2. Density (ρ)= W/(V-50) (2) Densities at higher temperatures (ρ(T)) were calculated from thermal conductivity (λ(T)) thermal diffusivity (α(T)) and specific heat capacity (Cp(T)) at that temperature (Nahhas et al., 2019) using Equation 3. Density at temperature T (ρ(T)) = ((λ(T)))/( (Cp(T))x(α(T))) (3) Relative Porosity Rock porosity was obtained by following the ASTM C128-15 standard procedure for Absorption of Fine Aggregate (ASTM, 2013). The porosities were deducted by using Equation 4. Porosity= ((M-W) x〖 ρ〗_rock)/(W x ρ_water ) (4) Young’s modulus Nano-indentation method with the method by Oliver and Pharr (2011) with the maximum loading of 200 Mn at a loading and unloading rate of 600 Mn/min and a Poisson’s ratio of 0.3.

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