Theoretical Framework for Nanomaterial-Enhanced Thermal Energy Storage in Buildings: Application to Passive Cooling Design for Hot Arid Climate in North East Nigeria

Thermal energy storage in building envelopes offers a passive cooling strategy that is fundamentally aligned with the climate challenges of hot arid North East Nigeria, where peak ambient temperatures exceed 45 degrees Celsius and cooling energy demand is rapidly increasing as urbanisation proceeds, yet rigorous theoretical frameworks for nanomaterial-enhanced thermal storage specifically designed for this climatic context have not been established. This study develops an original theoretical framework for nanomaterial-enhanced phase change material integration into building envelopes for passive cooling in hot arid climates, applied to residential building typologies in Maiduguri, Borno State. The framework integrates three original theoretical contributions: a generalised effective medium theory for thermal conductivity prediction in polydisperse nanoparticle-organic phase change material systems that accounts for interfacial thermal resistance and nanoparticle clustering, a heat transfer scaling analysis identifying dimensionless design parameters governing charging and discharging cycles in building cavity configurations, and a coupled hygrothermal-phase change building energy simulation model implemented in EnergyPlus with custom Python extensions. Nano-phase change material formulations were prepared using beeswax as the matrix incorporating aluminium nitride and boron nitride nanoparticles at 1.0 and 2.5 wt%, and characterised for thermal properties by differential scanning calorimetry, transient plane source thermal conductivity, and accelerated thermal cycling stability. Integration into a 150 mm cavity wall configuration reduced peak interior temperature by 5.8 degrees Celsius and cooling energy demand by 29% in building energy simulations calibrated to Maiduguri climate data. The study establishes the theoretical and experimental foundations for nano-phase change material building integration in Nigeria's hot arid climatic zone. Keywords: phase change material, nanoparticles, passive cooling, building energy, North East Nigeria