Porous Cementitious Materials for Thermal Energy Storage Applications.

In the last few decades the humankind’s role on global warming and on climate change became undeniable, as recognized by virtually all countries with the signing of the Paris Agreement. The increasing energy demand due to a constantly growing global population, with the consequent carbon dioxide release to the atmosphere by burning of fossil fuels, accounts for the major anthropogenic contribution. In order to pursue a transition to a low-carbon energy system, the heating and cooling sector is crucial as it is responsible for half of all consumed final energy in Europe and the majority of the demand is provided by fossil fuels. A massive reduction in the use of the latters in favour of the renewable energy sources (RESs) is thus one of the fastest way to provide heating and cooling services to reach Europe's sustainability. However, although the path and the goals are clearly designed, the integration of RESs into the heating and cooling sector first requires circumventing the discrepancy problem between surplus and shortage of energy throughout the year. In this context Thermal Energy Storage (TES), together with other forms of storage and flexibility, is expected to play a crucial role in decoupling the demand of energy from its generation, ensuring the security of supply while maximizing RESs exploitation. 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 and mass transport properties. The purpose of this thesis is to develop a new material that may effectively comply with these needs by combining hygroscopic salt, namely magnesium sulfate, 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. 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). In order to enhance the porosity of the composite matrix, a percentage of sepiolite up to 70\% with respect to the mass of cement powder is added. Among the 15 different materials synthesized, the 5 most promising samples underwent an adsorption analysis in a climatic chamber at the National Institute of Metrological Research (INRiM). 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, through the energetic modeling of their potential operating cycles. The best sample reached an energy density of 84.65 MJ per cubic meter and a cost per storage capacity of 9.30 €/kWh if the charging process occurs at 150°C.