Optimizing bio-based phase change materials in operable façade cavities to enhance cooling energy efficiency

Current design practice for operable façades increasingly recognises the role of cavity spaces as thermal buffer zones for external louvre systems. This study experimentally and numerically investigates the integration of a low-cost bio-based phase change material (PCM) into these cavities to enhance the performance of operable building skins. The work focuses on how the number and three-dimensional placement of PCM units affect cavity air temperature and cooling energy demand. A prototype operable double-skin façade with cylindrical bio-PCM components was tested under controlled heating and cooling cycles, and the measured PCM properties were implemented in an EnergyPlus model using the Conduction Finite Difference (ConFD) algorithm within a 3D parametric workflow. Results show that optimal performance is achieved when four PCM are installed in the lower, windward front region of the cavity, coinciding with the operable opening and closest to the solar-exposed outer skin. Among the tested configurations, the module with a 0.3 m × 0.3 m × 0.1 m cavity and four circular PCM units delivered the strongest thermal response, reducing the maximum cavity temperature by up to 2 °C and introducing a pronounced thermal lag. When deployed in a reference office model across five Australian coastal climates, this optimised configuration reduced the average cooling energy use intensity by 20.18% compared with scenarios without PCM. These findings demonstrate that strategically placing bio-based PCM in operable façade cavities can provide meaningful peak-temperature attenuation and cooling energy savings, particularly in warm and hot coastal climates. The results offer practical guidance on PCM quantity and placement for adaptive, energy-efficient façade design. • Operable double skin façade is integrated with bio-based low-cost PCM. • Cooling-energy impacts are quantified across five Australian coastal climates. • Enthalpy-temperature ConFD model is used to quantify PCM in operable façade. • Number and 3D placement of PCM components are optimised in the cavity space. • Four PCM units at the lower windward front achieved optimal performance with 20.18% EUI reduction.