Gang Tan

Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

The lazy way to keep cool in the sun Passive radiative cooling requires a material that radiates heat away while allowing solar radiation to pass through. Zhai et al. solve this riddle by constructing a metamaterial composed of a polymer layer embedded with microspheres, backed with a thin layer of silver (see the Perspective by Zhang). The result is an easy-to-manufacture material near the theoretical limit for daytime radiative cooling. The translucent and flexible film can be made in large quantities for a variety of energy technology applications. Science, this issue p. 1062; see also p. 1023 A polymer film with embedded microspheres and a thin backing film of silver achieves daytime radiative cooling. Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface requires materials that strongly emit thermal energy and barely absorb sunlight. We embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window. When backed with a silver coating, the metamaterial shows a noontime radiative cooling power of 93 watts per square meter under direct sunshine. More critically, we demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, which is vital for promoting radiative cooling as a viable energy technology.

An Overview of Unsteady Analysis Techniques for Natural Wind Turbulence and its Effects on Natural Ventilation

Abstract Focusing on the turbulence in natural ventilation and its impact on both occupant thermal comfort and building energy consumption, this paper presents a review of existing unsteady natural ventilation envelope flow models, as well as other techniques that have potential application to further our understanding of turbulence in natural ventilation and develop models which capture the dynamics and effects on thermal comfort. Literature provides numerous techniques ranging from quasi-steady temporal inertial theory, to unsteady CFD models, to experimental study, and other statistical and signal processing techniques. The Proper Orthogonal Decomposition and Stochastic Estimate techniques have been paid particular attention as they may be an alternative tool for understanding the turbulence of natural wind and possibly be combined with predictive models in an attempt to quantify the effects on thermal comfort and/or optimization of building energy management system’s operation.

Use of proper orthogonal decomposition and linear stochastic estimation technique to investigate real-time detailed airflows for building ventilation

As energy efficiency concerns grow for building heating, ventilation, and air-conditioning systems, more advanced mathematical models and methods may be needed for implementing more comprehensive building environmental monitoring practices. This work investigates the methodology of applying a coupled proper orthogonal decomposition (POD) and linear stochastic estimation (LSE) technique to provide detailed real-time velocity information for a single-room test environment under either mechanical ventilation or wind-driven cross natural ventilation with the support of limited on-site monitoring measurements. In addition, an example application of the proposed POD–LSE methodology with limited measurements to obtain real-time fluctuating wind-driven ventilation rates using a POD–LSE reconstructed velocity profile across a window opening is presented and shows very good agreement with the measured ventilation rates.

Experimental Investigation of Assumptions Used to Quantify Airflows through a Window under Cross Natural Ventilation

Abstract As natural wind is continuously fluctuating in both speed and direction, quantifying instantaneous wind-driven natural ventilation rate can be difficult, and often an average effect is used to stabilize the fluctuations. This work performs an experimental investigation to assess the validity of assumptions often used in quantifying airflows through an opening under cross natural ventilation condition. In the experiment, the three velocity components are analyzed for flows approaching and through an open window exposed to natural wind. Detailed airflows are also examined across an open window to investigate velocity profiles at the opening. Finally, ventilation rates are calculated to assess the use of constant discharge coefficients for wind-driven natural ventilation, and show for the experimental conditions that the traditional orifice flow rate calculation using a constant discharge coefficient CD=0.65 results in an underestimate by 17% compared to a detailed calculation based on seven airspeed measurements within the window opening.