A Thermochemical Strategy for Enhanced Low-Temperature Building Comfort: From Novel Salt Hydrate to Thermally Active Mortars

The building sector is a major consumer of energy, accounting for a significant portion of global energy consumption and CO2 emissions. To mitigate the environmental impact of this sector and improve energy efficiency, this research explores the application of an organic salt hydrate as thermochemical materials (TCMs) for low-temperatures thermal energy storage (TES) in buildings. The study focuses on calcium L-Lactate pentahydrate (CL) as a novel TCM and investigates its properties, enhancement through composite materials, and integration into building systems. This thesis work is divided into five chapters, each addressing different aspects of the research. Chapter 1 provides an overview of the energy demand in the building sector and the importance of developing sustainable solutions to reduce energy consumption. It discusses the various factors influencing energy usage in residential and commercial buildings, highlighting the need for improved energy efficiency and the adoption of renewable energy sources. The chapter also reviews different TES technologies, including sensible heat storage (SHS), latent heat storage (LHS), and thermochemical energy storage (TCES). It introduces the concept of TCMs and their potential for low-temperatures applications in buildings, with a particular focus on organic salt hydrates. Chapter 2 delves into the characterization of calcium L-Lactate pentahydrate (CL) as a promising TCM. The research explores its thermal and structural stability, as well as its dehydration/hydration cyclic reversibility. Various techniques, including X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetric (TG-DSC), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric dynamic vapor sorption (DVS) were employed. The results demonstrate that CL offers several advantages over traditional inorganic salt hydrates, such as the absence of deliquescence at high relative humidity levels, because of its low water solubility, no risks of generating harmful by-products, robust performance under a wide range of operating conditions, and no decomposition below 200 °C. Its heat storage capacity (471 kWh m-3) and operational characteristics make it a compelling alternative for low-temperatures TES applications. Chapter 3 focuses on enhancing the thermochemical properties of CL by depositing it onto a sepiolite porous matrix. Sepiolite, a low-cost and abundant mineral, was shown to be an effective support material for CL dispersion. Composites with varying CL content were realized, and characterization revealed an optimal composition for promoting salt dispersion and vapor exchange. The materials were found to be thermally stable for low-temperatures thermochemical heat storage, with sepiolite playing a crucial role in improving the hydration kinetics of CL. The resulting composite materials exhibited improved thermal stability and a good balance between sorption capacity and material density. These findings demonstrate the potential of sepiolite-based composites for TCES, with the possibility of integration into the building sector where sepiolite already has various applications. Chapter 4 and Chapter 5 detail the development and characterization of composite plaster mortars incorporating CL. The addition of CL to the mortars improved heat absorption and delayed the time required for specimens to reach equilibrium temperature, demonstrating the potential for energy savings and enhanced thermal comfort in buildings. The use of natural hydraulic lime (NHL) as an environmentally safe mortar binder contributed to the system's high breathability and crystalline lattice structure. Chapter 5 further explores the reinforcement of these mortars with rock fibers. Particularly, the inclusion of basalt fibers enhanced the mechanical performance of the composite mortars, significantly improving their toughness. This aspect also addresses the possibility of storing energy, considering not only TCES solutions, but also the energy absorption through residual post-crack resistance processes. The enhanced mechanical performance, coupled with the improved thermal characteristics, indicates that CL-modified fiber reinforced mortars can provide a balance between thermal and structural properties. This balance is vital for their effective implementation in building systems, where materials must satisfy both thermal performance and structural integrity requirements. In conclusion, this thesis demonstrates the potential of CL as a viable TCM for low-temperatures building applications. The innovative use of CL, both in its pristine form and when integrated with sepiolite and mortar, offers a range of possibilities for enhancing the energy efficiency and thermal comfort of buildings. The findings contribute to the development of more sustainable building materials and provide a foundation for future research and development in this field.