Characterization of Heat Mitigation Strategies in Roof and Landscape to Fight against Urban Heat Island in Hot and Humid Climate

基於屋面及景觀的濕熱地區熱島效應緩解策略特徵化研究

Student thesis: Doctoral Thesis

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Award date2 Jun 2017

Abstract

Urban heat island (UHI) phenomenon has a clear impact on local climate, which results in higher air temperatures in dense urban areas compared to their rural surroundings. Deteriorating thermal environment in cities represents a great challenge for sustainable urban design. Heat mitigation is therefore necessary. High albedo material, greening and water are acknowledged as effective mitigation techniques, and implementation of these techniques in urban design process is scale-dependent. Quantifying the associated heat mitigation potential is crucial for effective design guidance. However, current studies in this area are far from sufficiency. In order to handle this multidisciplinary problem of architecture and building engineering, this study aims at consolidating and characterizing the heat mitigation strategies (HMSs) implemented in horizontal urban surfaces (building roof and ground landscape) to fight against UHI in hot and humid climate. HMSs in horizontal urban surfaces can be categorized at the scale of building: cool roof and green roof; and at the scale of landscape: vegetation, high albedo pavement and water body. Their heat mitigation potential was systematically analysed and evaluated through effective performance indicators. Field measurement, numerical simulation, parametric study and regression analysis were employed as the research methodology.

Performances of the HMSs on roof were investigated experimentally in the hot and humid climate, cool roof and three types of green roof were covered. Through the assessment of holistic vertical temperature profiles across the roof, it was found that performances of the two HMSs on roof were closely related to environmental weather conditions and corresponding physical properties of construction. Compared to conventional roof, cool roof and green roof demonstrated large heat mitigation potential in reducing rooftop surface temperature. Moreover, green roof with denser vegetation cover would contribute larger heat mitigation potential.

Roof affords dynamic interactions between building and environment. It would affect environment directly. It also determines building thermal and energy performances, which would ultimately exert influences on environment indirectly. As a first step of the systematic evaluation, the indirect heat mitigation potential of HMSs on roof was quantified: thermal and energy performances. A series of simulations for parametric study were conducted with the validated models developed on the platform of EnergyPlus V8.6.0. Both cool roof and green roof demonstrated large heat mitigation potential in terms of reducing heat gain rate through roof and cooling energy consumption, but cool roof could achieve better overall indirect mitigation performance. The cooling potential of cool roof and green roof was then characterized by an effective performance indicator, in terms of cooling energy reduction (∆E). Based on statistical approach, mathematical models for prediction of ∆E were developed. Furthermore, the key design parameters that have decisive impact on ∆E of HMSs on roof were identified.

The direct heat mitigation potential of HMSs on roof was then quantified: surface sensible heat flux. The same database was adopted in parametric study. Cool roof and green roof showed large heat mitigation potential in lowering mean daily peak heat flux, as well as mean total daily heat flux. But in some green roof designs, these heat fluxes might exceed the counter part of conventional roof, thus aggravated UHI. Cool roof had larger overall direct heat mitigation potential and showed lower mean daily peak and total daily heat flux. Moreover, the cooling potential of cool and green roofs to combat UHI was characterized by another effective performance indicator, in terms of mean total daily heat flux reduction (∆Ht). Mathematical models were established by statistical approach to predict ∆Ht. Corresponding critical design parameters were found. Finally, an evaluation of integrative heat mitigation potential was proposed, which covers both indirect and direct influences.

Thermal performance of different HMSs in landscape was studied by field measurement in hot and humid climate. A series of measurement locations including ground level greening, different albedo pavement materials and water pools, were categorized into green, grey and blue areas, respectively. The spatial and temporal variations of the environmental conditions demonstrated the existence of local heat island. The green and blue areas showed certain heat mitigation potential. But the grey areas had the highest possibility in uncomfortable thermal environment generation. The close relationship between microclimate and diverse HMSs in landscape was verified.

The heat mitigation potential of HMSs in landscape was quantified from both combating heat island and alleviating human heat stress. Different HMSs for landscape design were investigated, including vegetation (Green-HMS), high albedo pavement (Grey-HMS), water body (Blue-HMS) and hybrid of trees and landscaping surface (Hybrid-HMS). Parametric study was conducted with a validated landscape model developed from the microclimate simulation tool ENVI-met V4. Through effective performance indicators of air temperature reduction (∆Ta) and physiological equivalent temperature reduction (∆PET), it was found that Hybrid-HMS would demonstrate the greatest heat mitigation potential, followed by Green-HMS, Blue-HMS and Grey-HMS. For Grey-HMS or Blue-HMS, the heat mitigation potential evaluated by ∆Ta was found not correlated with that by ∆PET. Consequently, characterization charts of different HMSs were established to quantify the heat mitigation potentials in ∆Ta and ∆PET.

This study provides valuable information regarding quantification and characterization of the heat mitigation potential of HMSs by effective performance indicators. The proposed characterization models/tools link up the HMSs implementation and practical urban design process. The results would contribute to the urban design and planning practice to fight against heat island in hot and humid climate. Meanwhile, it also provides some guidance for the development of efficient design tools to help HMSs implementation in other climate context to combat UHI.