Techno-Economic and Environmental Assessment of Solar-Driven Hybrid Adsorption Desalination–HDH Using Silica Gel/Cacl₂ Under Saudi Arabian Climate

This study explores a solar-driven hybrid desalination approach developed for Saudi Arabian climatic conditions, combining adsorption desalination (AD) based on a silica gel/CaCl₂ composite with an ejector (EJ) and a HDH system. The proposed integration aims to enhance vapor utilization and reuse condenser heat to generate additional distillate. Two operating modes are examined, including a productivity-focused strategy that activates evaporator/condenser heat recovery (HR) when cooling is not required. Compared to raw silica gel (SG), the composite adsorbent improves adsorption cycle performance, raising the COP from about 0.38–0.43 to 0.55–0.63, and increasing SCP from roughly 130–240 W/kg to 320–675 W/kg. Without HR, the full AD–EJ–HDH system achieves SDWP of 52–100 m³/ton·day with GOR of 2.40–2.75 over the year. In HR-enabled operation, SDWP increases to 81–140 m³/ton·day and GOR rises to 2.7–2.95, reflecting stronger internal heat reuse and improved vapor management. Techno-economic results show that the solar-driven unit cost for AD–EJ–HDH decreases from winter values (2.7–2.9 $/m³) to a minimum around June (1.53 $/m³), while waste heat operation reduces the cost further to 0.49 $/m³ in June (rising to ~0.76–0.80 $/m³ in winter). With HR, the full AD–HR–EJ–HDH reaches around 1.44 $/m³ (solar, June) and 0.38–0.40 $/m³ (waste heat, summer), confirming the advantage of desalination-focused HR operation when cooling is not required. Finally, compared with SWRO, the AD–HR–EJ–HDH configuration delivers an approximately 90% lower carbon footprint on the same environmental assessment basis. The study highlights the environmental benefit of the intensified SG/CaCl₂ hybrid configuration.