Comparative evaluation of aggregate based heat storage in solar stills for energy-exergy efficiency, techno-economic viability, and environmental sustainability for remote desalination

Scarcity of fresh, potable water is a major challenge in remote and coastal regions where centralized supply systems are limited. Desalination using solar still provides a simple and sustainable route for decentralized freshwater production however, the wider deployment is restricted by low distillate yield, particularly during off-sunshine periods. Although sensible heat storage materials have been explored to enhance thermal retention, long-term field-based studies that systematically compare low-cost, locally available aggregate storage materials in stepped solar still configurations are limited in the literature. Consequently, this study experimentally evaluated two identical stepped solar stills, each containing black pebbles and concrete grit as aggregate-based sensible heat storage materials under outdoor conditions in Bhavnagar, India (21.76°N, 72.15°E), over six months (September 2023 to March 2024). The two systems were assessed for freshwater productivity and were analyzed using energy, exergy, techno-economic, and environmental indicators. The still with concrete grit consistently showed better performance than the black pebble system, achieving higher energy efficiency (13.86% versus 11.51%) and higher exergy efficiency (0.79% versus 0.64%). The concrete grit system also demonstrated improved economic feasibility with a shorter payback period (1.8 years) compared to black pebbles (2.5 years), along with better environmental benefits through higher carbon mitigation potential and carbon credit earnings. Overall, the findings established concrete grit as a practical aggregate heat storage material for improved stepped solar still desalination.