Built Environment & Energy Laboratory
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Yu, X., Chan, J., Chen, C.* (2021). Review of radiative cooling materials: performance evaluation and design approaches. Nano Energy, 88, 106259. (Radiative cooling)

As the planet warms, keeping cool without releasing greenhouse gases will be a challenge, but radiative cooling technology is poised to meet this goal. Hundreds of radiative cooling materials have been reported in the literature to yield acceptable cooling performance, but there is a lack of guideline for engineers to select the suitable candidates for commercialization. In order to tackle this problem, we gathered information on 55 radiative cooling materials reported in the literature according to our selection criteria and grouped them into four categories: multilayer structure, metamaterial, randomly distributed particle structure, and porous structure. Using a comparison method that objectively evaluated their cooling performance and commercialization potential, we found that the polymer-based porous structure and randomly distributed particle structure without reflective metal layer tend to be more promising for commercialization because of their superior cooling performance, low cost, ease of manufacture, high scalability and compatibility. Furthermore, we proposed an approach for the design and optimization of potential radiative cooling materials. This review will not only provide engineers with guidelines for selecting the best materials in applying and commercializing this technology, but will also enable researchers to propel this technology forward through improved material design in the future.

Dai, H.K., Huang, W., Fu, L., Lin, C.-H., Wei, D., Dong, Z., You, R., Chen, C.* (2021). Investigation of pressure drop in flexible ventilation ducts under different compression ratios and bending angles. Building Simulation, 14, 1251–1261. (Ventilation system)

Due to the large degree of freedom in terms of design and installation, flexible ventilation ducts are commonly used in ventilation systems. However, excessive use of flexible ducts may lead to greater pressure drop and higher energy consumption. This study conducted experimental measurements to characterize the pressure drop in flexible ventilation ducts with different compression ratios and bending angles. This investigation first measured the pressure drop in straight flexible ducts with four compression ratios under various airflow rates. The calculated friction factor for the straight flexible ducts was negatively associated with the compression ratio. Next, the pressure drops in single-bend flexible ducts with various bending angles from 30o to 150o were measured under various airflow rates. The calculated loss coefficient of the bend increased with the bending angle for single-bend flexible ducts. Finally, the influence of the intermediate duct length on the pressure drop across two bends was experimentally investigated. When the length of the intermediate duct was greater than eight times the inner diameter, the pressure drop across a double-bend flexible duct could be calculated from the friction factors and loss coefficients with a relative error less than 1%. The data obtained in this study can be used to calculate the total pressure loss in flexible ventilation ducting systems in buildings.

Yu, X., Chen, C.* (2021). Coupling spectral-dependent radiative cooling with building energy simulation. Building and Environment, 197, 107841. (Radiative cooling)

As a passive cooling strategy without energy consumption, radiative cooling has attracted considerable attention, especially in the building field. Building energy simulations have been conducted to identify the benefits of this technology in buildings. However, in existing studies, constant emissivity was used for radiative cooling materials in the building energy simulation programs, which can cause certain errors. To tackle this problem, this study developed a method to couple the spectral-dependent radiative cooling with building energy simulations in EnergyPlus. Compared with the existing constant-emissivity model, the proposed coupled model can further consider the influence of spectral-dependent emissivity, material surface temperature, and precipitable water on the radiative cooling power in EnergyPlus. Based on the results in a typical strip mall in New York, the radiative cooling power calculated by the proposed spectral-dependent model can be significantly different from that by the existing constant-emissivity model. However, since the energy saving from radiative cooling was relatively small compared with the total energy consumption, the differences in annual cooling electricity and heating natural gas consumption calculated by both models were not significant. Furthermore, case studies in five cities using the proposed model showed that using a broadband radiative cooling roof on a typical strip mall would reduce the annual electricity consumption for cooling, while increasing the annual natural gas consumption for heating. The coupling of spectral-dependent radiative cooling with building energy simulation would improve the accuracy of energy performance assessment for buildings with radiative cooling technology.

2020 Energy