One solution to the high cost of energy and the need of reducing greenhouse gas emissions causing climate change is to switch from fossil fuels to renewable energy sources such as solar energy; therefore, thermal energy storage is required. Thermal energy storage (TES) is a strategy in which energy in the form of heat is temporarily stored in materials (charging process), before being released (discharge process) in the form of heat or other kinds of energy, according to the energy demand. Thermal energy can be stored in the form of sensible heat, latent heat and thermochemical heat of reversible chemical reactions. TES systems store sensible heat energy through temperature changes of the material, requiring modules with high specific heat capacity and thermal conductivity, in addition to a large storage volume capacity. Latent heat storage is defined as an accumulation of thermal energy in phase change materials (PCMs), which can collect and release thermal energy through phase transitions. Latent thermal energy storage with PCMs is a powerful approach to energy conservation needing less volume capacity, enabling storing the excess energy to fill the gap between energy supply and demand. Thermochemical materials (TCMs), comprising hydrated salts (SrBr2·6H2O, SrCl2·6H2O, etc.), are potential materials for low-temperature energy storage systems. Given that salt hydrates are inexpensive and possess energy densities at least 10 times higher than paraffin wax PCMs, they are well suited for storing daily seasonal energy in buildings. Concrete has been considered a promising material for sensible TES systems working at temperatures up to 380°C, but novel and more performing materials are required to reap the full benefits of the technology. A preliminary exploration of the market for this technology would allow to assess whether end-users are willing to pay for its usage in buildings.

This project deals with the fabrication of advanced geopolymer composites for thermal energy storage applications. Specifically, this project will assess the feasibility of impregnation, encasement, and encapsulation of thermochemical materials (hydrated salts TCMs) into geopolymer matrixes.  Lightweight aggregates such as expanded clay, expanded glass granules as well as expanded graphite will be added to increase the reaction surface, thermal conductivity, and heat transfer. Geopolymers offer a low-cost, solid and stable thermal energy storage medium together with appropriate mechanical strength and suitable thermal performance at high working temperatures, providing long service life. In addition, geopolymers have received considerable attention owing to their limited environmental impact in terms of their CO2 fingerprint, which is much lower than that of Portland cement. This project, besides developing geopolymer TES systems, will use additive manufacturing for fabricating large scale geopolymer composites with complex geometries, suitable for building applications. The printer utilizes a printing head with 196 nozzles, enabling the printing of a layer in a few seconds (the building envelope is 60x60x60 cm3). Geopolymer boards and bespoke bricks with complex shapes will be manufactured and tested. The flexibility in shaping functional components for building applications will be coupled with sustainability, using industrial waste as raw material for the geopolymer.