Buoyancy-driven self-purging stills exceeding 1% of thermodynamic limits

Traditional passive solar stills (PSS) are limited by salt accumulation and low heat recovery efficiency, and their energy-water efficiency (η) has long been less than 0.4% of the thermodynamic limit (TL). This study proposes a revolutionary inverted multi - stage solar still (i-MSS). By breaking geometric symmetry, natural convection eddies are triggered to achieve self - cleaning salt rejection and efficient heat recovery. Experiments show that the i-MSS achieves a photo-to-water efficiency (η*) of 850% and a daily water production of 172 L m ^- ² under 10-sun illumination, exceeding 1% of the TL for the first time. Its core innovations include: (1) an density-capillary-coupled salt rejection (DCCSR) that uses the salt concentration gradient to autonomously generate eddies to discharge high-salinity wastewater; (2) a multi - stage nozzle structure that optimizes the steam flow path and improves the latent heat recovery rate; (3) a passive water supply and gravity drainage design that reduces operation and maintenance costs.This work opens up new avenues for large-scale commercialization while advancing our understanding of mass transport of saline in porous media, with far-reaching implications for fields beyond solar desalination.