Zhang Erjun, Cao Yang, Caloz Christoph, Skorobogatiy Maksim
Appl Opt. 2020 May 1;59(13):D210-D220. doi: 10.1364/AO.382050.
In this work we first solve the radiative heat transfer problem in one dimension to perform a comparative analysis of the time-averaged performance of the partially transparent radiative windows and radiative coolers. In doing so, we clearly distinguish the design goals for the partially transparent windows and radiative coolers and provide optimal choice for the material parameters to realize these goals. Thus, radiative coolers are normally non-transparent in the visible, and the main goal is to design a cooler with the temperature of its dark side as low as possible relative to that of the atmosphere. For the radiative windows, however, their surfaces are necessarily partially transparent in the visible. In the cooling mode, the main question is rather about the maximal visible light transmission through the window at which the temperature on the window somber side does not exceed that of the atmosphere. We then demonstrate that transmission of the visible light through smart windows can be significantly increased (by as much as a factor of 2) without additional heating of the windows. This is accomplished via coupling the windows to the radiative coolers using transparent cooling liquid that flows inside of the window and radiative cooler structures. We also demonstrate that efficient heat exchange between radiative coolers and smart windows can be realized using small coolant velocities (sub-1 mm/s for ${\sim}{1};{\rm m}$∼1m large windows) or even using a purely passive gravitationally driven coolant flows between a hot smart window and a cold radiative cooler mounted on top of the window with only a minimal temperature differential (sub-1K) between the two. We believe that our simple models complemented with an in-depth comparative analysis of the standalone and coupled smart windows and radiative coolers can be of interest to a broad scientific community pursuing research in these disciplines.
在这项工作中，我们首先求解一维辐射传热问题，以对部分透明辐射窗和辐射冷却器的时间平均性能进行对比分析。在此过程中，我们明确区分了部分透明窗和辐射冷却器的设计目标，并为实现这些目标提供了材料参数的最佳选择。因此，辐射冷却器在可见光范围内通常是不透明的，其主要目标是设计一种冷却器，使其暗面温度相对于大气温度尽可能低。然而，对于辐射窗而言，其表面在可见光范围内必然是部分透明的。在冷却模式下，主要问题在于窗口在其暗面温度不超过大气温度的情况下，可见光的最大透过率。然后我们证明，通过使用在窗口和辐射冷却器结构内部流动的透明冷却液将窗口与辐射冷却器耦合，可以在不额外加热窗口的情况下，显著提高智能窗对可见光的透过率（提高多达两倍）。我们还证明，使用小的冷却液流速（对于约1米大的窗口，流速小于1毫米/秒），甚至使用纯被动的重力驱动冷却液流，在热的智能窗和安装在窗口顶部的冷辐射冷却器之间流动，且两者之间只有最小的温差（小于1K），就可以实现辐射冷却器与智能窗之间的高效热交换。我们相信，我们的简单模型以及对独立和耦合的智能窗与辐射冷却器的深入对比分析，会引起从事这些学科研究的广大科学界的兴趣。
