No abstract

Solar interfacial evaporation for freshwater harvesting has received attention recently due to its high evaporation rate and environmental friendliness. Traditional interfacial evaporation mostly uses black porous polymers to absorb solar radiation and transport water which involve high thermal radiation loss to the environment and heat conduction loss to the bulk water. In addition, the freshwater collection ratio is usually lower than the solar evaporation ratio due to the high temperature of the condensation surface under solar irradiation, and no freshwater can be harvested at night due to the absence of sunlight. Here, we design an all-day freshwater-harvesting device using a solarselective absorber (SSA) and sky radiative cooling. The prepared SSA with a high solar absorptance of 0.92 and a mid-infrared thermal emittance of 0.11 provides a great solar−thermal conversion performance (87.1% vs 51.4% for the black porous polymer at 25 °C) by minimizing the thermal radiation loss, and a hollow structure is also used to reduce the conductive heat loss, resulting in a high solar evaporation rate (1.23 vs 0.79 kg m −2 h −1 for the black porous polymer). In addition, a transparent radiative cooling polymer after plasma treatment is used for freshwater collection by enhancing the solar transmittance (0.92) and mid-infrared thermal emittance (0.91 at 25 °C). A theoretical freshwater collection rate of 0.044 kg m −2 h −1 is achieved at night-time. Outdoor results show that the all-day water harvesting is 0.87 kg m −2 . This strategy to achieve all-day water collection by coupling with the SSA and transparent radiative cooling has potential application in the field of desalination and freshwater harvesting in tropical desert areas.

Solar-driven interfacial evaporation has caught wide attention for water purification due to its green and environment-friendly properties. The key issue is how to effectively utilize solar radiation for evaporation. To fully understand the thermal management of the solar evaporation process, a multiphysics model has been built by the finite element method to clarify the heat transfer process for the improvement of solar evaporation. Simulation results indicate that the evaporation performance can be improved by tuning the thermal loss, local heating, convective mass transfer, and evaporation area. The thermal radiation loss of the evaporation interface and thermal convection loss to the bottom water should be avoided, and local heating is good for evaporation. Convection above the interface can improve evaporation performance, although it would enhance the thermal convective loss. In addition, evaporation also can be improved by increasing the evaporation area from 2D to 3D structures. Experimental results confirm that the solar evaporation ratio can be improved from 0.795 kg m–2 h–1 to 1.122 kg m–2 h–1 at 1 sun with a 3D interface and thermal insulation between the interface and bottom water. These results can provide a design principle for the solar evaporation system based on thermal management.