Exploring the potential of a potash by-product for thermochemicalheat storage

Abstract

Thermochemical energy storage is an effective method for seasonal heat storage applications, as it stores energy on a long-term basis. This process captures excess heat generated during the summer, whether from solar energy or surplus heat from supply chains, and utilizes it during the winter months. However, the process relies on chemical reactions, which pose various technical challenges. Additionally, it requires a significant amount of materials, leading to increased system costs. In this study, we propose to investigate potassium carnallite as a low-cost thermochemical material (TCM). This material is derived from potash saline deposits located in northeastern Spain. Characterization through chemical analysis revealed that it comprises 86.0% KCl·MgCl2·6 H2O, with NaCl as the main impurity at a concentration of 10%. The dehydration and hydration reactions analyzed involve the loss and retention of 4 molecules of H2O. Importantly, there is no evidence that the hydrolysis decomposition of the material affects the reversibility of these reactions. The study demonstrated a good reversibility of the reaction, with a yield of 81.73%, which decreased to 78.83% by the tenth cycle. These cycles simulate 10 years of seasonal use under specific conditions (PHy = 1.3 kPa, THy = 40 °C, PDe = 4.0 kPa, and TDe = 110 °C). Notably, natural carnallite exhibited 20% higher reversibility compared to synthetic carnallite. However, it was found to be 14% less reversible during the first cycle and 8.4% less reversible by the tenth cycle compared to another carnallite material studied under the same conditions previously. This difference in reversibility may be attributed to variations in the impurity content of both materials, where a higher concentration of NaCl in carnallite may act as a chemical spacing, facilitating water vapor mass transfer and consequently improving cycling stability. We measured an energy density of 0.892 GJ/m3 during the tenth hydration cycle, indicating that the winter energy needs of a household can be met using 9.0 m3 of thermochemical material. These findings suggest that by-products from mining, such as carnallite, are promising candidates for seasonal heat storage applications. However, improvements in the material are needed to increase the energy density at large scale, which would consequently reduce the volume of material required for the application using a reactor system.