Systematic Numerical Study on the Effect of the Thermal Properties of a Building Surface on Its Temperature and Sensible Heat Flux

Owing to urbanization, natural surfaces replaced by paved roads and buildings alter the surface energy balance, resulting in an increase in the surface temperature (T_s) and sensible heat flux (Q_H) in urban areas. The diurnal uniqueness of the urban surface energy balance contributes to the urban heat island (UHI) in both the surfaces and the air, influencing the habitats’ comfort, raising the mortality rate owing to intense pollution and heat, and increasing building energy demand. Consequently, lowering the surface temperature and the sensible heat flux released can effectively mitigate the UHI effects and reduce such problems. Modifying urban surfaces with high-albedo materials or coatings and their effects on surface cooling, UHI mitigation, and in-/outdoor thermal comfort has been widely studied in recent years, revealing their benefits. The thermal behavior of urban surfaces results from not only the radiation budget on the exterior surfaces but also heat conduction through the surfaces and convective heat transfers at both the exterior and interior surfaces of buildings. The thermal properties of the urban surface (e.g., heat capacity (C_a) and thermal conductivity (λ_c)), influencing all the above-mentioned heat transfers, contribute to its heat exchanged with the inside of a building affecting the energy demand, and with the atmosphere forming the urban climate. However, their effects on the heat exchanged between the building surface and the atmosphere has not yet been studied systematically. To bridge this gap by attaining more universal results, the effects of the thermal properties of building surfaces on the heat exchange with the environment is systematically studied in a city block in Yokohama city, Japan by the numerical simulation.

THERMORender model, a SEB model, and input parameters (meteorological data and physical properties) are introduced and validated with a simulation-to-measurement comparison, resulting in a good agreement with an unsystematic RMSE of 0.78 ºC.

Systematic simulations were performed on a typical summer day to estimate the effects that thermal property modifications of building walls and roofs have on T_s and Q_H, compared with the effects caused by optical property modifications, and to explore the impact of these modifications on the evaluation of a high-albedo strategy applied to a roof. The studied parameters were crucial to the practicality and objectiveness of the results. One important contribution was the appropriate input of the thermophysical properties, based on a database established here to characterize the regular thermal properties of commonly used wall and roof materials. The main findings are as follows.

(1) Compared to albedo modification, the effects of individual modifications of thermal properties were limited for the roof but nonnegligible for wall surfaces. For wall surfaces, the daytime cooling effects of increasing λ_c and C_a within an engineering range were close to and above 50% of those of an albedo increase of 0.3, respectively.

(2) The appropriate combined modification of thermal properties can enlarge the accumulated cooling effects of individual modifications. At noon, the combined cooling effect reached 31%–49% higher than the accumulated effects.

(3) Thermal properties should be considered when evaluating the cooling effect of high-albedo roofs. When a high-albedo roof (with an increased albedo of 0.6) were with various typical combinations of thermal properties, the maximum daily, daytime, and noon Q_H reductions were 14 (15%), 26 (15%), and 76 W m^-2 (23%) higher than the minimum, respectively.

The above simulation results were used to estimate the T_s sensitivity to the thermal property and the Q_H error range caused by the assumption and generalization of the thermal properties in the SEB model. The main findings obtained are as follows.

(4) Even though the simulated temperature is more sensitive to the albedo, its sensitivities to the thermal properties are nonnegligible. For shaded façades, T_s sensitivity to the λ_c and C_a is close to the sensitivity to the albedo; meanwhile, they are up to about 1/3 and 1/5 of the sensitivity to the albedo for the roof, respectively.

(5) Estimated by a scenario with the generalization of the thermal properties of the roof surface, the simulated daily, daytime, and noon RMSE ranges in Q_H were 6%–41%, 7%–50%, and 7%–38%, respectively.

These results indicate the importance of urban surface thermal properties on thermal and radiative behavior; meanwhile, they cannot be ignored in SEB model-based simulations. Currently, owing to the generally poor quality and availability of the documented surface data and the tedious work of collecting in-situ measurements, a time- and labor-saving method to obtain the actual thermal properties of urban surfaces is recommended in order to improve the SEB model performance on a neighborhood or larger scale.