Assessment and Modeling of Air Gap Membrane Distillation Unit

Technical Summary: Assessment and Modeling of Air Gap Membrane Distillation Unit

Problem Statement
Global freshwater scarcity is intensifying due to population growth and climate change, with conventional desalination technologies like Reverse Osmosis (RO) and thermal distillation (MSF/MED) facing limitations regarding high energy consumption, reliance on fossil fuels, and unsuitability for off-grid applications. While Air Gap Membrane Distillation (AGMD) offers a promising alternative by utilizing low-grade heat and operating at moderate temperatures, current research faces critical gaps. These include a lack of integration with renewable energy sources (specifically solar), the absence of dynamic, sensor-driven control systems to adapt to variable conditions, and insufficient holistic techno-economic assessments that combine thermal efficiency with cost metrics like Specific Thermal Energy Consumption (STEC) and Cost Per Liter (CPL). Furthermore, existing models often lack rigorous experimental validation across diverse operating conditions.

Methodology
This research employs a dual approach combining theoretical modeling and empirical experimentation to assess an AGMD system.

• Theoretical Modeling: A comprehensive thermodynamic and transport model was developed using MATLAB. The model incorporates heat and mass transfer processes, specifically utilizing the Knudsen–molecular diffusion approach for vapor flux calculation. It accounts for membrane resistance ($R_m$) and air gap resistance ($R_a$), considering parameters such as membrane porosity, tortuosity, and pore dimensions. Key performance indicators calculated include permeate flux ($J_p$), Thermal Efficiency (TE), Gained Output Ratio (GOR), and STEC.
• Experimental Setup: A custom-built, solar-integrated AGMD test rig was constructed to validate the model. The system features a flat-plate solar collector for heating and an Arduino-based real-time monitoring system for adaptive control of flow and temperature. The setup utilizes a hydrophobic Polytetrafluoroethylene (PTFE) membrane with a 5 mm air gap, 0.45 μm pore size, and 150 μm thickness.
• Operational Parameters: Experimental simulations varied feed temperatures from 40°C to 70°C and coolant temperatures from 10°C to 25°C, maintaining a steady flow rate of 2 L/min.
• Economic Analysis: A cost analysis was performed to determine the Capital Recovery Factor (CRF), Annual Maintenance Cost (AMC), and final Cost Per Liter (CPL), assuming a 5-year system lifetime and a 12% interest rate.

Key Results
The study yielded the following quantitative findings under optimal conditions (70°C feed temperature, 10°C coolant temperature):

• Permeate Flux: The system achieved a peak permeate flow rate of 12.77 kg/m²·h. The flux demonstrated a nonlinear increase with rising feed temperatures, driven by enhanced vapor pressure differentials and reduced feed viscosity.
• Thermal Efficiency (TE): Thermal efficiency improved significantly with temperature, reaching a peak of 50.62% at 70°C. The study also notes that at a feed temperature of 70°C, a thermal efficiency of 44.49% was attained, which represents a 23.89% improvement over standard on-grid AGMD installations relying on conventional nonrenewable heating sources.
• Energy Consumption: The Specific Thermal Energy Consumption (STEC) was minimized to 29.04 kWh/m³ at the highest feed temperature, indicating efficient energy utilization.
• GOR: The Gained Output Ratio increased by approximately 60% as feed temperature rose from 40°C to 70°C, reaching a maximum of 0.191. The authors note that while this shows improvement, this maximum GOR is much inferior to that of alternative desalination techniques.
• Economic Viability: The system demonstrated a competitive freshwater production cost of $2.52 per cubic meter ($0.00248/L), which is lower than comparable literature values (e.g., $2.77/m³).
• Model Validation: The MATLAB model showed strong alignment with experimental data and previous studies (Koura et al., 2025; El Mokhtar et al., 2019). The model accurately predicted trends in flux, efficiency, GOR, and STEC across varying temperatures, confirming the reliability of the Knudsen–molecular diffusion and heat transfer formulations used.

Significance and Claims
The authors claim that this research establishes a validated framework for optimizing AGMD design, specifically addressing the integration of renewable energy and adaptive control. The study positions the proposed solar-driven AGMD system as a practical, cost-effective solution for decentralized and off-grid desalination in water-scarce regions. By achieving high thermal efficiency and low STEC while maintaining a competitive production cost, the system is presented as a viable alternative to small-scale RO and conventional thermal desalination. The work asserts that its combination of a validated MATLAB modeling tool, a solar-powered prototype with real-time sensor control, and a comprehensive techno-economic evaluation fills critical gaps in current AGMD research, supporting the scalability and robustness of the technology for sustainable freshwater generation.