Effectiveness of Cool Walls on Urban Energy and Thermal Comfort: From Semi-Arid to Tropical Climate

Livability of future cities will be determined by how their environmental challenges are addressed and how their resources are managed accordingly. Among these challenges is the Urban Heat Island, which indicates the temperature increase of built environments relative to nearby undeveloped areas, and is a direct outcome of urbanization. The ensuing effects of UHI on urban microclimate is then twofold: 1) with the increase in outdoor temperature during cooling seasons, building energy consumption and consequently energy demand increases, and 2) people’s sensation of the thermal environment, i.e. thermal comfort, is negatively affected, raising concerns about thermal-related vulnerability and human health in cities. Additionally, global climate change can further aggravate the impacts of UHI. Therefore, it is paramount that we investigate the mitigation strategies for UHI, considering both aspects of thermal comfort and energy demand in cities.

Similar to cool roof technologies, high albedo walls can be beneficial for reducing the unwanted solar heat gain and therefore the AC demand during the cooling season. Accordingly, new technologies are emerging that can be used as cool walls, including reflective coating, self-cleaning coatings and claddings.  However, contrary to roof surfaces, wall reflective properties modify the energy balance of the street canyon and affect the solar radiation transmitted through the neighboring buildings, which can have a counteracting effect on the thermal load. For instance, we found that in a medium office-building with 0.33 window-to-wall fraction, the sensitivity of overall energy use to wall albedo variation of 0.3 is less than 2-3%. Accordingly, the effectiveness of cool walls in reducing the thermal load should be evaluated for different window-to-wall fractions, as well as the local climate zones and building types. Additionally, cool walls directly interact with the pedestrians’ radiant exposure and thermal comfort which should be taken into consideration.  To date, the combined effects of these factors and therefore the overall performance of cool walls have not been systematically evaluated, which acts as the key barrier for their further development and use in urban environment.

In this study, we aim to evaluate the energy savings potentials of cool walls and further investigate their influence on thermal comfort of pedestrians in the street canyon. Accordingly, an indoor-outdoor energy balance model (Temperature of Outdoor Facets Indoor-Outdoor Building Energy Simulators [1]) is used in combination with a detailed 3D thermal comfort model [2].  We evaluate the performance of cool walls for different local climates (comparing the tropical city of Singapore with a semi-arid climate zone in California) and different building prototypes (such as offices, residential buildings, or restaurants).  To do so, we perform series of simulations to investigate the sensitivity of thermal load and Standard Effective Temperature at the pedestrian level to the wall albedo and the window-to-wall fraction.  

The main implications of these findings are to a) provide an effective metric for cool wall performance that can further inform the development of the cool-wall technologies, and b) give insight on the potential mitigation strategies of UHI in urban environments, so that urban design can effectively contribute to a livable and sustainable city.

References:

_{[1] Yaghoobian, N., & Kleissl., J. "An indoor–outdoor building energy simulator to study urban modification effects on building energy use–Model description and validation." Energy and Buildings 54 (2012): 407-417.}

_{[2] Nazarian, N., Fan, J., Sin, T. Norford. L., & Kleissl., J.  “Predicting Outdoor Thermal Comfort in Urban Environments: A New Numerical Model for Standard Effective Temperature”, submitted to Urban Climate  (2016c).}