Mechanisms of Solar-Thermal and Super Solar-Thermal Evaporation of Water

Abstract

The phase change from liquid water to vapor is fundamentally important for many technologies, including electronic cooling, refrigeration, critical mineral harvesting, and desalination. Solar desalination uses solar energy to evaporate water from brine resources to produce clean water through a distillation process. This technology is particularly attractive for decentralized communities with abundant sunlight due to its low capital costs compared to other desalination technologies. Advances in interfacial solar evaporators using porous materials have enhanced the efficiencies of solar stills by concentrating the solar energy near the evaporating interface.

A critical challenge in the large-scale application of solar desalination technologies is the high latent heat of evaporation of water. The maximum amount of water that can be produced is about 1.45 to 1.49 kg m-2 h-1 using the latent heat, sensible heat, and standard 1 sun intensity incident on the evaporating interface. This value is commonly referred to as the solar-thermal limit of evaporation. The latent heat of evaporation is about 3 orders of magnitude higher than the minimum energy needed to separate water from seawater using a membrane process. These low water production rates make it challenging to compete with other desalination technologies on a large-scale. Surprisingly, numerous works over the last decade have shown that highly microporous materials can exceed the solar-thermal limit of evaporation by 2 to 3 times, which we call super solar-thermal evaporation. These materials seem to violate energy conservation, and there needs to be a physical mechanism to explain these observations. Despite the plethora of experimental works, the field currently lacks critical analysis and understanding of the fundamental mechanisms for why super solar-thermal evaporation occurs. The lack of understanding has stalled the translation of this technology into readily deployable devices that can enhance the water production rates of solar stills immensely.

In this thesis, we conducted a critical analysis using coupled transport phenomena to elucidate the mechanisms behind super solar-thermal evaporation. We first reexamined and disproved the prevalent reduced latent heat hypothesis that is commonly used to explain super solar-thermal evaporation rates through a combination of modeling, simulations, and experimental methods. This work also illustrated that water cluster evaporation is likely the only physical mechanism that can satisfy both the vapor transport and energy transport arguments in super solar-thermal evaporation rates. Next, we elucidate the underlying mechanisms for super solar-thermal evaporation rates from 3D interfacial solar evaporators. We show that the enhanced evaporation rates from these materials are driven by continuous dry air flowing near its sidewalls, leading to greater opportunities in outdoor open environments such as brine pond treatment than in closed environments such as solar still devices. Afterwards using our knowledge of coupled heat and mass transport, we designed and built a high-performing reverse solar still prototype for outdoor usage that is an order of magnitude larger in area than almost all other prototypes in the field with a similar configuration. Our understanding of the underlying coupled transport of these devices has given new insights on the optimization pathways of solar stills. Finally, we did further theoretical work on the photomolecular effect. The photomolecular effect is a new theory proposed by our group on how surface interactions between light and the air-water interface can excite water clusters out, leading to super solar-thermal 2D evaporation. We used simulations to understand how cluster dissociation kinetics can couple into the boundary layers above the evaporating surface. These works represent major steps in understanding both solar-thermal and super solar-thermal evaporation mechanisms.