Rethinking Solar-Driven Evaporation
MIE Associate Professor Yi Zheng published his research on “Critical Assessment of Water Enthalpy Characterization Through Dark Environment Evaporation” in Science Advances.
Some solar-driven evaporator materials have been reported to exceed the theoretical limit of water evaporation in recent years, and some researchers have attributed this to evaporator materials “effectively lowering” the vaporization enthalpy of water. This claim has become widespread and is typically defended using two characterization methods, dark environment evaporation testing and differential scanning calorimetry. In this work, Zheng’s research team challenges this widely held view by examining these two characterization methods and identifying fundamental issues with these methods, both analytically and experimentally. They illustrate that claims of “reduced vaporization enthalpy” cannot reliably be drawn from such tests. This research emphasizes that the scientists must revisit their understanding of solar-driven evaporation rates which exceed the theoretical limit.
Abstract:
Comparative evaporation rate testing in a dark environment, commonly used to characterize a reduced vaporization enthalpy in interfacial solar evaporators, requires the assumption of equal energy input between cases. However, this assumption is not generally valid, leading to misleading characterization results. Interfacial evaporators yield larger evaporation rates in dark conditions due to enlarged liquid-vapor surface areas, resulting in increased evaporative cooling and larger environmental temperature differentials. Theoretical and experimental evidence is provided, which shows that these temperature differences invalidate the equal energy input assumption. The results indicate that differences in evaporation rates correspond to energy input variations, without requiring enthalpy to be reduced below theoretical values. These findings offer alternative explanations for previous claims of reduced vaporization enthalpy and contradict enthalpy-related conclusions drawn from differential scanning calorimetry. We conclude that postulating a reduced vaporization enthalpy using the dark environment method is inaccurate and that re-evaluation of vaporization enthalpy reduction is required.