Nowadays, there is a growing interest in the use of renewable energies due to the ever-increasing energy demands and the long-term worldwide goal linked to net-zero greenhouse gas emissions. In this context, concentrated solar power (CSP) plants, a renewable energy technology, incorporate thermal energy storage (TES) materials to improve the use and heat storage capacity of CSP, dispatching power even during the night time. In addition, TES would also promote the thermal energy harvesting from industrial processes. Among the different TES, molten salts, liquid-solid phase change materials (PCM) employed in high temperature applications, including CSP, stand out because their excellent energy storage capacities and limited volume changes during the phase transition. However, PCMs commonly present liquid leakage in the molten state, a critical issue that considerably reduces their thermal energy storage efficiencies.

To overcome this issue, we present the development of novel three-dimensional (3D) TES structures (3DTES) based on highly porous 3D printed patterned vermiculite supports, which were further infiltrated with a molten salt. The ceramic supports were additive manufactured by robocasting, a direct ink writing technology, from pseudoplastic aqueous clay-based inks. In order to enhance the total porosity of the scaffolds, the diameter of the printed filaments, the span between the filaments and the use or not of a framed architecture were modified. Besides, some ink formulations contained fugitive materials. The as-printed clay scaffolds were thermally treated at 800 ºC to provide robustness to the supports and remove the fugitive particles. Then, the 3D structures were infiltrated at 350 ºC with a sodium nitrate salt.

The rheological properties of the inks, the microstructure of the 3D printed scaffolds and the PCM infiltration degree were deeply analysed. The thermal performance of 3DTES, including the enthalpy of fusion, thermal stability, energy thermal storage and thermal conductivity, was deeply investigated, jointly with the compressive strength. These novel 3DTES were easy to handle, lightweight (~1.6 g·cm-1), mechanically robust (compressive strength ~68 MPa) and avoided the molten salt leakage. Maximum PCM encapsulations into the patterned support reached values up to 87%, where the PCM fully filled the open channels of the cellular support and also penetrated into the porous clay filaments. 3DTES exhibited outstanding thermal energy storage efficiency (~90%), excellent thermal stability and good thermal conductivity, which would lead to an enhanced charging-discharging ability. In summary, this novel 3DTES would contribute to reduce the current energy and environmental issues through CSP applications and also extend their potential use to other applications, such as nuclear energy or aerospace.