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Title: Development of spectral selectivity-based passive radiative roof cooling model for buildings and radiative sky cooling resources mapping
Authors: Chen, Jianheng
Degree: Ph.D.
Issue Date: 2021
Abstract: Passive radiative sky cooling utilizes atmospheric transparency window (8-13 μm) to discharge heat into outer space and inhibits solar absorption, which is an appealing heat exchange form based on thermal radiation from terrestrial objects to outer space without consuming energy. Due to the poor solar reflective ability of previous cooling materials, radiative cooling technology has been largely limited to nocturnal cooling for several decades. Thanks to the recent successful development of highly efficient spectrally selective thermal emitters intrinsically equipped with excellent solar reflective properties, the daytime radiative cooling to sub-ambient temperature under direct sunlight has been practically fulfilled, which is arousing worldwide research interests. The novel radiative cooling materials are featured by outstanding spectrally selective properties, superior daytime cooling capability and large-scale manufacturability. Thus, integrating these fascinating cooling materials into buildings as a roof strategy can be one of the most prominent radiative cooling methods integrated with buildings. Based on this background, this thesis aims to comprehensively investigate the radiative sky cooling technology integrated with buildings as a rooftop strategy and deeply explore the radiative sky cooling resources of China. Accordingly, a new spectral selectivity-based passive radiative roof cooling model was developed to precisely characterize the radiative sky cooling-based roof thermal and energy performance. Moreover, radiative sky cooling resources mapping across contiguous regions of China was provided. Firstly, an in-depth theoretical investigation on the fundamentals of radiative sky cooling was conducted, where extensive parametric studies on key factors influencing the radiative cooling potential were conducted. Specifically, different types of radiative coolers, atmospheric transmittance and non-radiative heat transfer coefficient were thoroughly investigated. The broadband emitter is superior in generating the largest net cooling power whilst the selective emitter shows the strength in achieving the lowest sub-ambient temperature but the minimum net cooling power. A robust wind shield of high transmittance placed above radiative coolers can strengthen radiative cooling potential. Atmospheric precipitable water vapor exerts noticeable impacts on local atmospheric transmittance. This part of study provides crucial perspectives on internal and external key factors in affecting radiative cooling performance. The results can be used to optimize the design of radiative coolers in response to application scenarios. To quantitatively and accurately analyse the impacts of emerging daytime cooling materials with spectral selectivity on roof cooling performance, a novel spectral selectivity-based passive radiative roof cooling model was developed to predict the roof thermal and energy performance based on the principles of radiative sky cooling, roof heat transmission and atmospheric transmittance models. The developed model is capable of fully considering the local precipitable water vapor contents in affecting the atmospheric radiation, which increases its adaptability to climate variability. Moreover, the model enables to incorporate the full spectral selectivity of emerging daytime radiative cooling materials to accurately evaluate their cooling performance when integrated with buildings as a rooftop strategy. The model was experimentally validated by the field experiments. A satisfactory agreement was obtained between the predicted and experimentally measured results. The mean bias errors (MBEs) and root mean square errors (RMSEs) in the model prediction of roof temperatures are less than 4.6% and 8.2%, respectively, and the index of agreement (d) is over 0.97, verifying the sufficient accuracy of the model in characterizing roof temperature variations. The validated model was used to investigate the potential benefits of radiative cooling as a rooftop strategy in the hot and humid region of Hong Kong. The whole year on-site weather data were measured as the model input. Compared to the baseline coating, a porous polymer radiative cooling coating is able to save the annual roof-induced cooling electricity of 54.7-76.6, 97.4-136.4, and 8.8-12.2 kWh/m2 for residential concrete-based, industrial galvanized steel-based, and standard-compliant commercial multilayered roofs, respectively. The new spectral selectivity-based radiative roof cooling model can be widely employed to evaluate the utilization of emerging cooling materials as a rooftop strategy in improving building thermal performance.
Additionally, comprehensive techno-economic and environmental performance assessments of super-cool roof applications in ten typical cities of China were investigated by using the developed model. The results show that compared to the baseline traditional roofs, the super-cool roof can reduce the daytime maximum and average roof temperatures by 24.8 °C (43.4%) and 10 °C (29%), respectively. Cumulative roof thermal transfer values can be decreased by 63.0-195.8 kWh/m2 (78.5­148.2%) in cooling seasons. Annual electricity saving in hot cities ranges from 42.9 to 97.8 kWh/m2 on average, in response to varying COP values. Super-cool roof induced maximum acceptable incremental cost falls within 34.6-64.7 $/m2 and 55.36-103.52 $/m2 for 5-yr and 8-yr simple payback periods, respectively. Besides, carbon emissions can be averagely reduced by 24.6-56.1 kg/(m2·yr). The above results deeply elucidate the energy efficiency, economic feasibility and carbon neutrality potential of radiative sky cooling-based super-cool roof applications in China. In the aspect of holistic passive design approach towards green building envelopes, the radiative cooling roofs and thermally smart glazing windows were incorporated to investigate the overall building thermal and energy performance. For radiative cooling roofs coupled with traditional clear glass windows, relative to the baseline (β =0.2, ε = 0.9), the achievable building energy savings induced by single radiative cooling roof (β =0.95, ε =0.9) in five typical cities of China are 9.3%, 7.1%, 4.1%, 3.3% and 0.6% for Kunming, Hong Kong, Shanghai, Beijing and Harbin, respectively, indicating radiative cooling roof is conducive to achieving energy-efficient buildings in typical climates of China. The further utilization of thermally insulated glazing windows can noticeably enhance the overall building energy saving rates. Specifically, the combination of the thermally insulated low-e glass with the radiative cooling roof can further increase the building energy saving rates by 2.4%-9.1%. Window-to-wall ratios (WWR) and window orientations can exert great impacts on the effectiveness of the holistic passive design approach. Specifically, when the WWR increases from 0.157 to 0.694, compared to the clear glass, the thermally insulated window can save additional building energy by up to 12.1%, indicating such holistic passive design approach is particularly suitable for buildings with large roof-to-wall and window-to-wall ratios. Finally, regarding the radiative sky cooling resources mapping, the cooling potential across contiguous regions of China was newly investigated based on the fundamentals of radiative sky cooling model and extensive meteorological data retrieval. Temporal and spatial variability of cooling power across seven geographic regions of China was comprehensively analyzed. Particularly, annual, seasonal and daily variations of radiative cooling resources were given in detail. The influence of spectral properties from different radiative coolers on cooling potential was clarified. The annual maximum cooling potential is between 36.7 and 71.9 W/m2, among which the humid region of South China demonstrates the lowest cooling power with an average of 48.8 W/m2. Seasonal radiative cooling resources vary discrepantly. The cooling power during spring, summer, autumn and winter deviates +4.02%, -7.22%, ­0.34% and +3.6% from that of annual average, respectively. Compared with the ideal broadband cooler, a practical radiative cooler with the solar absorption of 4% and an inferior non-blackbody thermal emittance reduces the annual net cooling power of 15.4% on average. Lastly, radiative cooling potential-based regional division maps were newly displayed, which could provide a clear picture for deployment of radiative cooling technology in China. In summary, the main novelty and originality of this thesis can be briefly summarized as follows: First, a novel spectral selectivity-based passive radiative roof cooling model was newly developed and experimentally validated. The model is capable of incorporating the full spectral selectivity of emerging daytime radiative cooling materials and fully considering the precipitable water vapor-induced atmospheric transmittance, increasing its prediction accuracy under various climate patterns. Second, the model was newly employed to comprehensively evaluate the techno-economic and environmental performance of radiative sky cooling-based super-cool roof applications in China. Third, a holistic passive design approach towards green building envelopes by incorporating radiative cooling roofs and thermally smart glazing windows was proposed to improve overall building energy performance. Lastly, radiative sky cooling resource maps of China were explored to provide deep insights into the suitable climate and regions for large-scale deployment of the passive radiative cooling technology in China.
Subjects: Building materials -- Environmental aspects
Buildings -- Energy conservation
Hong Kong Polytechnic University -- Dissertations
Pages: xxxvi, 321 pages : color illustrations
Appears in Collections:Thesis

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