Physics-informed effectiveness indicators for whole-building evaluation of phase change materials in buildings

Phase change materials (PCMs) are a promising passive technology for reducing building heating and cooling energy use, yet their whole-building effectiveness as latent thermal storage remains insufficiently characterized. The present study introduces PCM Effectiveness Indicators (PCMEIs), a set of physics-informed, whole-building metrics that relate daily thermal-load reductions to the installed latent heat capacity. Using these indicators, this study demonstrates that PCM activation is a necessary but not sufficient condition for whole-building effectiveness. The proposed indicators capture dynamic, year-round performance across climates, implicitly accounting for partial phase transitions, fluctuating loads, and interactions with passive design strategies. They can be applied consistently to heating, cooling, and mixed-load analyses. Using EnergyPlus simulations and a global sensitivity analysis, 400 design configurations were evaluated for a prototype building in four ASHRAE climate zones (3A, 4A, 5A, and 6A). Across all climates, envelope characteristics, particularly the window-to-wall ratio, exerted a stronger influence on PCM effectiveness than commonly studied material properties such as melting temperature. These findings highlight the importance of co-optimizing PCMs with envelope design variables, especially at early design stages. Furthermore, PCMs achieved the highest effectiveness in heating-dominated climates and when mitigating cooling loads. Despite substantial PCM-induced load reductions, PCMEI values rarely exceeded 20%, indicating practical limits to whole-building latent-capacity utilization under real climatic variability. A design threshold was identified: maintaining installed latent heat capacity below approximately twice the building’s peak daily thermal load prevents oversizing and maximizes utilization. The proposed PCMEIs provide a transparent and scalable framework for whole-building evaluation of PCMs, supporting benchmark development and performance-based integration in energy-efficient building design.