A Multi-Layer Stacked Photothermal Membrane Distillation Module Enables Effective Latent Heat Recovery

Abstract:

Photothermal membrane distillation (PMD) is an attractive sustainable desalination technique because it harnesses abundant sunlight to produce clean water. Unlike conventional bulk-heating solar desalination, PMD employs an interfacial heating approach, which mitigates the thermal loss, increasing the solar-to-water conversion efficiency. Moreover, PMD can be efficiently operated in a small modular configuration, and with a low initial investment and operating expenses, it makes a promising desalination technique for decentralized communities. However, state-of-the art PMD systems lack proper energy recovery systems, consequently wasting much of the heat from photothermal conversion to the surroundings by conduction and convection. To preserve the photothermal heat in a PMD system, we developed a novel multi-layer stacked airgap membrane module, constructed with 3D printed polymer frames. To convert incident light to heat efficiently in PMD, graphene nanosheets are employed as photothermal material because of their broad light absorption in the solar spectrum and efficient photothermal conversion. In the multi-layer stacked module, photothermally-driven heat facilitates the distillation process across the photothermal membrane, and latent heat generated during condensation is reused multiple times through the stacked layers. Using a four-layer stacked module with optimal airgaps, we achieved a water flux of 1.17 kg/m²/hr under 0.75 kW/m² light density (i.e., 0.75 sunlight), which is equivalent to 105% solar-to-water conversion efficiency. The high solar-to-water conversion efficiency in our multi-layer stacked PMD module highlights the importance of thermal engineering and heat recovery to transform PMD into a sustainable off-grid technique.