Development and Evaluation of Phase Change Material (PCM)–Biochar Composites for Sustainable Thermal Energy Storage in Building Materials

: (Alternative Format Thesis)

Abstract

The built environment contributes significantly to global carbon dioxide (CO₂) emissions, accounting for approximately 3 gigatonnes (GtCO₂) in 2019. Space heating and cooling are major energy drivers, highlighting the need for energy-efficient solutions. Phase change materials (PCMs) offer a promising approach by storing and releasing thermal energy, helping to regulate indoor temperatures and reduce energy demand. However, construction applications face challenges, including leakage, durability, thermal stability, and mechanical strength reduction.

This thesis explores PCM-biochar composites in building materials, assessing material selection, mechanical properties, and environmental impacts through experiments and Life Cycle Assessment (LCA). Six candidate PCMs hexadecane, heptadecane, octadecane, RT18HC, RT22HC, and RT25HC underwent long-term thermal cycling (10,000 cycles, simulating ~15 years of use), confirming their stability in phase change temperature and latent heat. Octadecane proved most suitable due to its high latent heat capacity and optimal phase transition temperature. Vacuum impregnation achieved 62% PCM loading in biochar, yielding a PCM-biochar composite with a latent heat capacity of 116.70 J·g⁻¹, a phase change temperature of 22.02 °C, and a volumetric heat capacity of 2.77 MJ·m⁻³·K⁻¹. Characterisation techniques differential scanning calorimetry (DSC), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), and powder X-ray diffraction (PXRD) confirmed successful PCM integration.

Mechanical analysis showed PCM-biochar cement mortars and gypsum samples maintained UK building standards (BS EN 197-1), with compressive strengths of 43 MPa for cement mortar and 2.95 MPa for gypsum. An LCA revealed environmental trade-offs, with PCM-biochar composites (Configuration 1) exhibiting an 87% higher global warming potential (GWP) than the non-PCM alternative (Configuration 2), mainly due to octadecane production. Sensitivity analysis suggested hydrocracking could improve octadecane sustainability.

This research advances PCM integration in construction, emphasizing sustainable production methods to balance environmental costs with energy efficiency and support climate-resilient buildings.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Veronica Ferrandiz-Mas (Supervisor), Adam Squires (Supervisor) & Stephen Allen (Supervisor)