The temperatures in urban environments can be significantly higher than those in surrounding rural areas. This phenomenon is known as the Urban Heat Island (UHI) effect and is most pronounced during the night and when the impact of winds is minor. UHI is caused by human activity and urban infrastructure (e.g., buildings, vehicles, pavements, roads, etc.). People constantly generate heat waste due to everyday activities while common structures act as heat insulators. Moreover, as buildings tend to increase in height to accommodate the increasing population of big cities, UHI is exacerbated as the heat is trapped and cannot rise into the cold atmosphere. In addition to temperature rise, the phenomenon also induces air and water pollution, hence scientists are investigating how to address it.
UHI results in people using significant resources for the internal cooling of buildings but this creates a self-perpetuating cycle that worsens the situation. A new study investigates the performance of applying reflective surfaces to cool the urban environment. Reflective surfaces are designed to reflect much more heat than conventional ones and can replace walls, pavements, roofs, etc. In particular, typical urban surfaces tend to absorb about 80%, thus releasing 20%, of the solar power while reflective surfaces can reflect more than 50%, mitigating the UHI. The technique has been used in large cities (e.g., Los Angeles, California) and has indeed been successful. However, scientists agree that establishing such surfaces on large scales could be financially challenging.
The research team utilized a Computational fluid dynamics (CFD) model to assess the performance of reflective surfaces and decrease their application costs. Based on the fact that covering an entire urban area with cooler surfaces is not always efficient, they used 5 different models: 1. A conventional case in which no reflective surfaces are applied, 2. A case of all surfaces being reflective, 3. A case in which 50% of the surfaces are reflective and situated upstream of the dominant wind direction, 4. A case in which 50% of the surfaces are reflective and situated downstream of the dominant wind direction and 5. A case in which 50% of the surfaces are reflective and situated parallel to the dominant wind direction. For each case, additional models accounting for three different building and meteorological conditions were established.
The results suggest that the distribution of the reflective surfaces within a city is crucial. It was found that the most efficient solution was case 3 in which the surfaces were placed upstream of the dominant wind. Those surfaces create a barrier that cools the air and the rest of the city. Hence, if the surfaces were not "strategically selected", the urban environment would experience a disproportionately lower benefit.
The authors of the study suggest that additional aspects of urban environment features should be studied in the future to better evaluate the efficiency of reflective surfaces. These include the environmental impact of manufacturing the reflective surfaces and the people's thermal comfort.