The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman’s formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data. Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.

Summary To relieve the fresh water shortage, bi‐layer systems have been widely applied to give high efficiencies of solar steam generation. In current work, a bi‐layer system (C‐wood) is obtained with the low‐cost carbon particles as the light absorbing layer and the boxwood as the supporting layer. The application of wood could lead to both good water transportation and good local heat management in the system, and carbon particles could supply a high light absorptivity at a broad optical absorption wave bands (mostly larger than 98%), which results from the complex three‐dimensional porous structure formed by the wide‐size‐distribution carbon particles. Because simultaneously possessing a high light absorptivity, good water transportation, and good local heat management, the C‐wood system gives a 65% evaporation efficiency under an illumination power of 1 sun. This system is also durable for a future industry applications. Comparing the energy loss of the C‐wood system in the evaporation process with the ones of other systems prepared in this work, we conclude that a low top‐surface reflection of light of the C‐wood system should be responsible for the high evaporation efficiency. It demonstrates that the carbon particle is a promising material for an application in solar steam generation systems for easily accessible resources, low cost, and effective light absorptivity.

Many efforts have been dedicated to improve the solar steam generation by using a bi-layer structure. In this paper, a two-dimensional mathematical model describing the water evaporation in a bi-layer structure is firstly established and then the finite element method is used to simulate the effects of different influence factors on the evaporation rate. Results turn out that: besides the high solar energy absorptivity of the first-layer, an optimum porosity of the second-layer porous material should be applied and the optimum porosity is about 0.45 in this work. This optimum porosity is determined by the balance between the positive effect of the lowering effective thermal conductivity of the second layer and the negative effect of the reduced vapor diffusivity in the second layer when the porosity is decreased. The influence of the thermal conductivity of the second-layer porous material is negligible because the effective thermal conductivity of the second layer is determined by the porosity while a larger porosity means more water in the second layer. The ambient air velocity could greatly enhance the evaporation rate, and the evaporation rate will decrease linearly with the increase of the air relative humidity. This study is expected to supply some information for developing a more effective bi-layer solar steam generation system.

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