Alessandro Alabastri1
Rice University1
Thermal desalination is a flexible choice for water treatment, given its key advantages in robustness and limited salinity dependence. Light can power thermal desalination by carrying electromagnetic energy if efficiently turned into heat [1-2]. Solar-driven photothermal desalination (SDPD) can lead to decentralized water purification, improving accessibility and reducing the environmental impact over conventional, heavy infrastructure-based desalination practices like reverse osmosis. However, today’s best decentralizable SDPD technologies barely surpass 10% of the thermodynamic limit for thermal desalination. A few years ago, we introduced the concept of nanophotonics-enabled solar membrane distillation, where solar-driven localized heat drives the distillation process [1]. Moreover, the desalination efficiency was increased by redistributing the photon flux incident on the membrane, suggesting the possibility of reducing the process footprint by utilizing miniaturized optics instead of more costly and bulky optical concentrators [2]. However, energy recovery is still a challenge.<br/><br/>Two key ingredients are necessary for practical solar desalination: 1) Input power (e.g., from direct sunlight) and 2) energy recovery. Light-heat conversion drives evaporation in solar thermal desalination, and I will show how properly designed large-scale ultrathin (~250nm) TiN-based metasurfaces allow extremely large (~GW/m<sup>3</sup>) and broadband (~90% of solar spectrum) dissipated power densities [3]. More recently, we have shown how thicker (~µm) nanoporous structures uniquely enhance light absorption thanks to deep subwavelength dissipative regions [4]. Such efficient absorbers allow exploiting the vast majority of sunlight as a heat source for desalination.<br/><br/>Heating and evaporating water is only one stage of the desalination process. Water vapor needs to be collected through condensation and latent heat recovered to make the process energy efficient. We have been developing desalination modules based on the concept of Resonant Heat Transfer (RHT), where a combination of conductive and convective heat transfer mechanisms allow the desalination process to be described as an oscillating thermal system where heat is re-utilized, thermal energy is stored, and losses are minimized [5,6]. Such an energy recovery method makes compact modules robust, performing and scalable.<br/><br/>We successfully demonstrated an RHT-based solar desalination module capable of delivering up to 20 L/m<sup>2</sup>/day [7], paving the way to realizing scalable and sustainable light-driven water purification systems. I will provide a perspective on the future opportunities and challenges of modular thermal desalination systems [8].<br/><br/><b>References</b><br/>[1] P. D. Dongare, A. Alabastri et al., “Nanophotonics-Enabled Solar Membrane Distillation for off-Grid Water Purification,” PNAS <b>114</b>, 6936 (2017).<br/>[2] P. D. Dongare, A. Alabastri, et al., “Solar Thermal Desalination as a Nonlinear Optical Process,” PNAS <b>116</b>, 13182 (2019)<br/>[3] L. Mascaretti, A. Schirato et al.,“Solar steam generation on scalable ultrathin thermoplasmonic TiN nanocavity arrays”, Nano Energy, <b>83</b>, (2021)<br/>[4] <i>Submitted</i><br/>[5] A. Alabastri, “Flow-Driven Resonant Energy Systems”, Phys. Rev. App. <b>14</b>, (2020).<br/>[6] Q. Ye, S. Sanders and A. Alabastri, “Resonant Energy Transfer and Storage in Coupled Flow-Driven Heat Oscillators”, PRX Energy, <b>2</b>, (2023).<br/>[7] A. Alabastri, P. D. Dongare, et al., “Resonant Energy Transfer Enhances Solar Thermal Desalination,” Energy Environ. Sci. 13, <b>968</b> (2020).<br/>[8] W. Schmid et al., “Decentralized Solar-Driven Photothermal Desalination: An Interdisciplinary Challenge to Transition Lab-Scale Research to Off-Grid Applications,” ACS Photonics <b>14</b>, 3764 (2022).