Sustainable Silica Sols from Geothermal Sources and their Potential as Thermal Heat Storage Materials

Abstract

Geothermal energy is a renewable energy source but faces challenges such as silica scaling. This scaling leads to corrosion within process equipment and injection wells, ultimately reducing power generation efficiency and increasing maintenance costs. Therefore, extracting silica from geothermal sources is essential. A company, GeO40 TM, has developed a process for extracting silica as colloidal dispersion, which can be also converted into high value dry products such as composite adsorbents. This study aimed to establish an innovative, cost-effective, practical method for converting geothermal colloidal silica sol, into environmentally friendly and sustainable CaCl2/Silica gel composite adsorbents to be used for thermochemical heat storage (TCHS) applications. Solar energy is also a renewable energy and is abundant during summer but insufficient in winter. TCHS systems can be incorporated into solar energy harvesting systems to address the seasonal energy imbalance by storing excess energy and using it later. Composite adsorbents can be utilised in TCHS systems because they can store heat through exothermic adsorption and endothermic desorption reactions. By storing solar energy in TCHS systems, the need for fossil fuels for heating applications can be mitigated. In this study, composite adsorbents by using CaCl2 and geothermally derived colloidal silica were successfully prepared to be used for TCHS applications. The developed method involves precipitating colloidal silica in an alkaline medium with CaCl2 salt. The ions of the salt change the stability of the silica particles and simultaneously combine with the silica particles to produce composite adsorbents. Furthermore, CaCl2 is a very hygroscopic component that raises the final composite adsorbent's water adsorption capacity (WAC), a desirable property of TCHS materials. The thermochemical characteristics of the composite adsorbents were evaluated at 55% RH between 20 and 150°C using simultaneous TG and DSC analysis. Sample 0.50-SD, with the highest salt concentration (48.2 wt.% CaCl2), exhibited the highest heat of desorption (1207.2J/g) and water adsorption capacity (90 wt.%), without any salt leakage. The volumetric heat storage density (Qv) of the samples was calculated to be 172.7 kWh/m3. Using the imbibition technique, the sample's pore volume (Vp) was determined to be 108 cm3. Ten consecutive cycles of adsorption and desorption were conducted on the sample, showing no significant performance loss. Additionally, the sample was characterized by XRD, SEM, and FTIR. An open-type TCHS prototype was designed and built to assess the sample's TCHS performance on a larger scale. Under the conditions of input air at a 10 L/min flow rate, 60 % RH at 20°C for adsorption, and 150°C for desorption, only 38.674 g of 0.50 SD released 53.434 (± 1.215) kJ of heat. Consequently, the adsorption bed's temperature rose by 21°C, while the exhaust air's temperature increased by 16°C, which could be utilized for space heating. Gravimetric heat storage density (QM) of 1382 (± 27) J/g and Qv of 155 (± 4) kWh/m3 were obtained for the sample, exceeding the minimal QV value of TCHS materials (150 kWh/m3) as suggested by Courbon et al. (2017)1. The study suggests that composite adsorbents prepared using geothermally derived colloidal silica could offer an eco-friendly option for TCHS applications.