Design-Stage Analytical and CFD Assessment of a Coconut Oil–Fly Ash Composite PCM Partition Wall with Fan-Assisted Ventilation for Tropical Residential Cooling

Masonry dwellings in tropical Indonesia often exceed the indoor comfort range on hot afternoons, encouraging reliance on split-unit air conditioning (AC). This study presents a design-stage assessment of a partition wall containing 918 polyethylene spheres (12 cm) filled with a coconut oil–fly ash composite phase change material (PCM; 50 vol% oil, 40 vol% fly ash, 10 vol% air) and assisted by exhaust fans (24 W in total), for two bedrooms of a Type-36 house in Bandung. A Maxwell-Eucken estimate indicates that fly ash raises the composite thermal conductivity from 0.17 to about 0.32 W/m·K (0.24–0.36 W/m·K). A quasi-steady melting analysis shows that capsule charging is limited mainly by surface convection: the latent heat absorbed during a 5-h afternoon (2.0–3.7 kWh for the whole wall) is similar to that of pure-oil capsules, while the composite uses 44% less oil and completes its melting cycle faster. The installed 373.8 kg of oil corresponds to a theoretical air-temperature reduction of up to 4.0 °C in the 49.5 m³ rooms. A steady-state CFD simulation (Butterfly/OpenFOAM) for the EPW design hour (9 December, 14:00; outdoor 29.0 °C), with the PCM surface prescribed at 25.5 °C, gives an indoor air temperature of 26.33 °C against 28.30 °C for brick partitions, within the ASHRAE 55 adaptive comfort limits. Operating electricity and CO₂ emissions are 95–97% lower than for two ½ PK AC units, with a simple payback of about 3.3 years; as the AC provides lower temperatures and dehumidification, these are upper bounds. Transient coupled simulation and prototype testing are required for validation.