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The urban heat island effect refers to the phenomenon that the temperature in the city is obviously higher than that in the outer suburbs. On the near-ground temperature map, the temperature change in the suburbs is very small, while the urban area is a high-temperature area, like an island protruding from the sea. Because this island represents a high-temperature urban area, it is vividly called an urban heat island. The urban heat island effect makes the annual average temperature in the city 1°C or higher than that in the suburbs. In summer, the temperature in some parts of the city is sometimes more than 6°C higher than that in the suburbs. In addition, the dense and tall buildings in the city hinder the passage of airflow and reduce the wind speed in the city. Due to the urban heat island effect, a day-night opposite thermal circulation is formed between the city and the suburbs, which aggravates the extent of urban overheating.
In recent years, with the rapid development of urban construction, urban overheating is becoming more and more obvious. The main reasons for urban overheating are as follows. First, it is affected by the characteristics of the urban underlying surface. There are a large number of artificial structures in the city, such as concrete, asphalt pavement, and various building walls, which change the thermal properties of the underlying surface. These artificial structures absorb heat quickly and have small heat capacity. Under the same solar radiation conditions, their temperature rises faster than the natural underlying surface (green space, water surface, etc.), so their surface temperature is obviously higher than that of the natural underlying surface. Another main reason is the influence of artificial heat sources. Factory production, transportation, and residential life all need to burn all kinds of fuel, emitting a lot of heat every day. In addition, the reduction of green space, trees, and water bodies in cities is also a major reason. With the development of urbanization and the increase of urban population, the areas of buildings, squares, and roads in the city increase greatly, but the green space and water body decrease accordingly, and thus the ability to alleviate the heat island effect is weakened. Air pollution in cities is also an important reason. Motor vehicles, industrial production, and residents’ life in the city produce a lot of emissions such as nitrogen oxides, carbon dioxide, and dust. These substances absorb thermal radiation from the underlying surface and produce the Greenhouse Effect, which causes the atmosphere to warm up further. Hot weather caused by urban overheating also has adverse effects on human health. Relevant studies have shown that when the ambient temperature is higher than 28°C, people will feel uncomfortable; if the temperature is consistently higher than 34°C, it can also lead to a series of diseases including the increase of the morbidity and mortality of heart, cerebrovascular, and respiratory diseases. Moreover, the increase in temperature will accelerate the speed of photochemical reaction, increasing the concentration of ozone in the near-surface atmosphere, and affecting human health.
To balance this, several mitigation techniques are available. A common technique is to use reflective materials on buildings and urban structures that can reflect some of the incident solar radiation back into space. Cities can also use dense green plants, such as building-integrated vegetation, as well as evaporation technology, solar control systems, and heat dissipation technology to direct excess heat to the ground or low-temperature radiators in the sky. The large-scale application of these ideas in cities shows that it is possible to reduce the peak temperature by up to 2.5°C. Next-generation mitigation technologies may bring even greater reductions, up to 4°C.
Samira Garshasbi et al. first studied the fluorescent cooling capacity of quantum dots as a new type of nano-fluorescent material with tunable fluorescent cooling potential. In addition, a new algorithm for accurately calculating fluorescent cooling indices is proposed, including re-emission energy (QPL) and fluorescent cooling surface temperature reduction potential. The results were published on Solar Energy in August this year.
They found that quantum dots can be used as additives for advanced cooling coatings (near-infrared high reflective materials) to improve their heat dissipation efficiency and alleviate urban overheating. They also proposed a theoretical method to calculate the reemission energy and temperature decrease by photoluminescence (PL) effect and used CIS/ZnS quantum dots (a kind of nano-sized material with tunable fluorescence properties, purchased from CD Bioparticles) to verify the effectiveness of the model. The results show that the solar absorptivity of CIS/ZnS quantum dots film is equal to 0.52 nm, quantum yield PL peak is 42-56%, indicating that the maximum heat loss of PL is 54.2 W/m2, which is equivalent to 8.5% of the absorbed shortwave solar irradiation (285 to 3000 nm). In Sydney, Australia, under two different boundary conditions, the surface temperature of quantum dots with a temperature decrease due to PL effect is 2°C lower than that of their corresponding non-fluorescent reference samples.
Professor Mat Santamouris said, “the algorithm we developed algorithm could be utilized as a reliable model to optimize fluorescent properties of QDs in the future. The proposed algorithm is a very useful tool for precise calculation of PL effect contribution in urban overheating mitigation for all other fluorescent materials.” CD Bioparticles provides non-toxic I–III–VI2 type CIS(Se), CIGS(Se), CZTS(Se) semiconductor quantum dots with excellent optical property, large absorption coefficient and stable performance. Our quantum dots are ideal for highly sensitive cellular imaging, photovoltaic devices, thin film solar cells, light emitting devices, and biomedical devices.
Reference:
Garshasbi, S., Huang, S., Valenta, J., & Santamouris, M. (2020). Can quantum dots help to mitigate urban overheating? An experimental and modelling study. Solar Energy, 206, 308-316.