Reversing heat conduction loss: Extracting energy from bulk water to enhance solar steam generation

Interfacial solar steam generation offers a sustainable and affordable technology for seawater desalination and water treatment. During solar steam generation the temperature of the solar evaporation surface is generally higher than the bulk water, which results in energy loss to the bulk water by heat conduction. While many strategies have been developed to minimize and/or eliminate the conductive heat loss, this study focuses on completely reversing conductive heat loss and turning it into an energy extraction from the bulk water to enhance the evaporation during solar steam generation. This was achieved by introducing a certain area of cold evaporation surface between the solar evaporation surface and the bulk water, which led to the conductive heat loss from the solar evaporation surface being completely absorbed and consumed by the cold evaporation surface before reaching the bulk water. Meanwhile, due to its lower surface temperature, the cold evaporation was also able to extract energy from the bulk water, turning the heat conduction loss from the evaporator to the bulk water into the energy harvest from the bulk water. When the surface area of the cold evaporation surface was increased to a certain point (50.3 cm2 in this work), heat flow was reversed, and energy was extracted from the bulk water by the evaporator to enhance solar evaporation. Theoretical simulations agreed well with the experimental results. In addition, as parasitic effects, the cold evaporation surface was also able to gain energy from the ambient air and lower the temperature of the solar evaporation surface, reducing both radiation and convection energy loss. As a result, the evaporation rate and the light-to-vapor energy efficiency of the evaporator were far beyond the theoretical limits, confirming that this strategy has great potential for further practical applications.