Saturation of latent heat storage in phase change material-integrated solar stills: a universal sizing criterion

The optimal sizing of latent heat storage in solar thermal systems remains a critical challenge, often relying on empirical rules rather than on physically grounded criteria. This study uncovers a latent heat storage saturation phenomenon in PCM-integrated basin solar stills and introduces a novel dimensionless indicator, the latent heat utilization ratio (Ulat), to quantify it. A fully transient, enthalpy-based TRNSYS model was rigorously validated and used to systematically vary the PCM mass from 0.04 kg to 12 kg under a representative arid climate. Three distinct operating regimes are identified: under-sized, optimal, and saturated. For masses below ∼1.3 kg, the PCM melts completely (U lat ≈ 1), and the daily freshwater productivity reaches a maximum of 5.29 kg·m −2 ·day −1 at only 0.4 kg of PCM. Beyond this critical mass, the peak PCM temperature falls below the melting point, U lat collapses to zero, and the productivity asymptotically declines to that of a conventional still without PCM. These findings demonstrate that excessive PCM acts as an inert thermal mass, challenging the widespread “more PCM is better” assumption. A universal sizing criterion emerges: the optimal PCM mass is the largest mass that still guarantees complete latent activation (U lat ≈ 1). This criterion transforms PCM sizing from an empirical, case-specific exercise into a predictive, physics-based design procedure, directly applicable to latent heat storage optimization in solar thermal systems.