High-Productivity Hybrid Adsorption Desalination Using a Sodium Polyacrylate/CaCl₂ Composite with Dual Ejectors and Humidification–Dehumidification Under Saudi Arabian Climate Conditions

This study investigates the utilization of a sodium polyacrylate (SP)/CaCl₂ composite as an adsorbent in a low-grade-heat desalination configuration designed for Saudi Arabian conditions. A dynamic system model was developed and validated for an adsorption desalination (AD) cycle integrated with a dual-ejector and a humidification–dehumidification (HDH) unit. Two operating modes were evaluated, including a production-oriented configuration that applies internal evaporator–condenser heat recovery (HR) when no cooling effect is required. Without HR, the AD–EJ–HDH system achieves 41–56 m³/ton·day SDWP and 2.6–2.9 GOR, with a freshwater cost of 1.8–2.4 $/m³ under solar driving and 0.70–0.90 $/m³ under waste heat. With HR, performance increases to SDWP 95–155 m³/ton·day and GOR 2.9–3.1, while costs decrease to about 1.34 $/m³ (solar) and 0.38 $/m³ (waste heat) in June. The SP/CaCl₂ composite yields about 85% higher freshwater production than silica gel in the same system, highlighting the material’s potential for high-output hybrid adsorption desalination in hot-climate regions.