Development of cementitious composite materials for sorption thermal energy storage

The energy transition will deeply transform the current energy system. On the one hand a massive exploitation of various renewable energy sources will be necessary, not only to generate “clean” electricity but also carbon-free heating, cooling and fuels. On the other hand, this shift of production will inevitably increase the sector dependency on weather conditions, with their inherent variability and low predictability. In this context Thermal Energy Storage (TES), together with other forms of storage and flexibility, is expected to play a crucial role to decouple the demand of energy from its generation, ensuring the security of supply while maximizing the exploitation of the installed capacity. For instance, the capability of some TES technologies to store heat on a seasonal basis may be fundamental to increase the share of renewables in space heating: solar energy may be collected and stored during summer to be then utilized during winter. Although still at low technological readiness levels, sorption heat storage has the potential to provide seasonal storage systems with high energy densities and virtually no losses. Belonging to the class of thermochemical storage, this technology is based on a reversible desorption-adsorption endothermic-exothermic reaction between a sorbent and a sorbate, storing energy as the enthalpy of the reaction. However, the lack of a suitable inexpensive and robust sorbent material currently represents a major limitation for this application. Composite materials have been looked at as possible ways to exploit the high energy densities of hygroscopic salts while limiting their deliquescence and loss by entrapping them into a porous matrix, also expected to improve their heat transfer properties. The purpose of this thesis is to develop a new material that may effectively comply with these needs by combining hygroscopic salts with the low cost, porous and widely available cement. More specifically, composite samples will be produced in two ways: by dry impregnation of a salt-saturated solution on a pre-made porous cementitious matrix and by direct mixing of the cement powder with the solution in place of water (in situ production). Magnesium Sulfate (MgSO_4) and Calcium Chloride (CaCl_2) were used as salts, while three families of cements were used with the aim to improve sulfate compatibility: Portland Cement (PC), Calcium Aluminate Cement (CAC) and Calcium Sulfoaluminate Cement (CSA). The last two proved however to be incompatible with CaCl_2, which halted their hardening (if ¬in situ) or lead to the breaking of the samples (if impregnated). In the case of PC, Sepiolite was also included as aggregate as an attempt to improve the matrix porosity. After several trials aimed at optimizing the materials by increasing their porosity and salt content, the more promising samples underwent an adsorption analysis in a climatic chamber at the National Institute of Metrological Research (INRiM), which permitted to test them in “macroscopic” amounts (i.e., 20÷25 g, compared to the < 500 mg that would be tested in a TGA). Their adsorption isotherms have been experimentally obtained at 50°C and 30°C, allowing for the estimation of their isosteric heat, water uptake and energy density. The best samples reached an energy density of about 0.5 GJ/m3 and a cost per storage capacity of around 1.0 €/kWh, calculated through a preliminary economic analysis considering only the raw materials. These constitute very promising results which encourage future research endeavors.