This paper investigates the impact of anthropogenic heat on formation of urban heat island (UHI) and also determines which factors can directly affect energy use in the city. It explores literally the conceptual framework of confliction between anthropogenic heat and urban structure, which produced UHI intensity and affected energy consumption balance. It then discusses how these two factors can be affected and gives implication to the city and then focuses on whether actions should be taken for balancing adaptation and mitigation of UHI effects. It will be concluded by making the three important strategies to minimise the impact of UHI on energy consumption: landscaping, using albedo materials on external surfaces of buildings and urban areas, and promoting natural ventilation.
The urban built environment itself is related to global changes in the increase of urban temperatures, the rate of energy consumption, the increased use of raw materials, pollution, and the production of waste, conversion of agricultural to developed land, loss of biodiversity, and water shortages [
With the concentration of anthropogenic activities into urban areas, a climatic environmental problem, the “urban heat island” (UHI), has emerged. A UHI is a climatic phenomenon in which urban areas have higher air temperature than their rural surroundings as a result of anthropogenic modifications of land surfaces, significant energy use, and its consequent generation of waste heat. Thus, this proves to be an unsustainable factor that leads to excessive energy use for cooling and places the urban population at greater risk of increased morbidity and mortality.
According to the above perspective and considering that rapid and huge population growth is expected in the near future, it becomes increasingly important to apply UHI mitigation strategies in order to reduce energy consumption and improve the quality of life with focusing on energy consumption.
Thus, this paper investigates the anthropogenic heat factors that produce the UHI and result in the use of significantly increased use of energy. Then, according to the Oke’s energy balance conceptual model, all of the energy which is absorbed by the surface through radiation or from anthropogenic heat goes somewhere and warms the air above the surface, it is evaporated away with moisture or is stored in the material as heat. For energy saving, therefore, this paper suggests some strategies to provide the best possible energy saving solution.
The majority of cities are sources of heat and pollution, and the thermal structure of the atmosphere above them is affected by the “heat island” effect. A UHI is best visualised as a dome of stagnant warm air over the heavily built-up areas of cities [
Generalized cross-section of a typical UHI [
In metropolitan areas, the urban core shows a final “peak” in the UHI where the urban maximum temperature is found. The difference between this value and the background rural temperature defines the “UHI intensity” (
The UHI phenomenon may occur during the day or during the night. Givoni [
Today, the majority of cities are around 2°C warmer than rural areas, and commercial and high-density residential areas are hotter by 5 to 7°C [
According to Landsberg [
Urban areas are the sources of anthropogenic carbon dioxide emissions from the burning of fossil fuels for heating and cooling; from industrial processes; transportation of people and goods, and the like, [
Gartland [ because of impermeable and watertight urban construction materials, moisture is not available to dissipate the sun’s heat; dark materials in concert with canyon-like configurations of buildings and pavement collect and trap more of the sun's energy. Temperatures of dark, dry surfaces in direct sun can reach up to 88°C during the day while vegetated surfaces with moist soil under the same conditions might reach only 18°C.
Rapid urbanization leads to the development of a UHI; Oke et al. [ anthropogenic heat; air pollution; surface waterproofing; thermal properties of fabric; surface geometry.
(1) Anthropogenic heat discharge in a city also contributes to the UHI effect. Sources of anthropogenic heat include cooling and heating buildings, manufacturing, transportation, and lighting. Human and animal metabolisms are also considered sources of artificial heat [
(2) Air pollution results from emissions of particulates, water vapour, and carbon dioxide from industrial, domestic, and automobile combustion processes. These atmospheric pollutants change the urban net all-wave radiation budget by (1) reducing the incident flux of short-wave (i.e., solar) radiation, (2) re-emitting long-wave (i.e., infrared) radiation from the urban surface downward to where it is retained by the ground, and (3) absorbing long-wave radiation from the urban surface, effectively warming the ambient air [
(3) Surface waterproofing refers to the predominance of impermeable surface in urban areas. Buildings and paved streets quickly shed precipitation into catchment basins, creating an evaporation deficit in the city. Conversely, in rural areas exposed soils and natural vegetation retain water for evaporative cooling. A dry urban surface cover enhances sensible heat transfer and suppresses latent heat flux whereas moist rural surface suppress sensible heat transfer and enhance latent heat flux.
(4) The fourth factor contributing to the formation of UHIs relates to the thermal properties of the urban fabric. The heat capacity, and consequently thermal inertia, of urban construction materials such as concrete and asphalt is greater than that of natural materials found in rural environments. A greater heat capacity means that urban materials absorb and retain more solar radiation than do rural soils and vegetation. Reflection of short-wave solar radiation is also affected by the properties of the urban fabric. Urban albedos are, on average, 5–10 percent lower than rural values [
(5) The complex geometry of urban surfaces influences air temperatures in two ways. First, increased friction created by a rough urban surface (as compared to a smooth rural surface) reduces horizontal airflow in the city. Mean annual wind speeds within cities are approximately 30–40 percent lower than mean annual wind speeds in the countryside [
Anthropogenic heat is generated by human activity and comes from many sources, such as buildings, industrial processes, cars, and even people themselves [
In developed countries where concerted action is being taken on UHIs, the main concern is on the large increase in power consumption in urban areas to cool down buildings, with additional air-conditioners or a heavier usage of existing air-conditioners. Higher air temperatures also mean that the air quality deteriorates as a result of increased ozone and pollution.
As discussed previously, there is no single cause of the UHI. In fact, many factors combine to warm cities. Gartland [ increased anthropogenic heat; reduced evaporation; increased heat storage; increased net radiation; reduced convection.
Urban and suburban characteristics important to UHI formation and their effect on the energy balance of the earth’s surface [
Characteristic contributing to UHI formation | Effect on the energy balance |
---|---|
Lack of vegetation | Reduced evaporation |
Widespread use of impermeable surfaces | Reduced evaporation |
Increased thermal diffusivity of urban materials | Increased heat storage |
Low solar reflectance of urban materials | Increased net radiation |
Urban geometries that trap heat | Increased net radiation |
Urban geometries that slow wind speeds | Reduced convection |
Increased levels of air pollution | Increased net radiation |
Increased energy use | Increased anthropogenic heat |
The anthropogenic heat interacts with its environment in a complex manner. To understand and simplify the complexity, Oke [
As shown below, net radiation encompasses four separate radiation process taking place at the Earth’s surface [
The net all-wave radiation could then be calculated as the difference between the incoming and outgoing parts [
Anthropogenic heat represents the heat generated from stationary and mobile sources of an area. It is reported that the
Oke [
Turbulent heat fluxes are comprised of the sensible and latent heat fluxes. These can be directly derived from eddy correlation, or measured using appropriate equipments. The heavily built urban areas are reported to be responsible for increased sensible heat flux which is reported to vary as per the built surface [
Christen and Vogt [
Net heat advection could be referred to as the inaccurate measurement due to spatial gradient in temperature, humidity, and wind. It is suggested that the effects of advection could be negligible provided that caution has been taken in deciding the measurement height [
The preceding discussion highlights how energy is transformed to and from the Earth’s surface. The energy balance equation is based on the first law of thermodynamics, which states that energy is never lost. For a surface on the Earth, this means that all of the energy absorbed by the surface through radiation or from anthropogenic heat goes somewhere. Either it warms the air above the surface, is evaporated away with moisture, or is stored in the material as heat.
Other statistical data show that the amount of energy consumed by cities for heating and cooling offices and residential buildings has increased significantly in the last two decades. Emmanuel [
Increased anthropogenic heat has a direct effect on building energy consumption that an increase in the production of anthropogenic heat leads to raise the electricity demand for cooling and the production of carbon dioxide and other pollutants.
Therefore, it can be concluded that increasing the production of anthropogenic heat, which leads to raised temperatures and generates a UHI that provides a warm air canopy over the city. Consequently, it causes significantly increased energy consumption to heat and cool buildings. This process is summarised in Figure
This paper has tried to develop a concept to show the conflict between anthropogenic heat and urban structure factors can affect the energy balance.
This paper highlights energy consumption, anthropogenic heat, and urban structure factors as the key components; according to Bridgman et al. [
Compiling the three key components into a concept is meaningful in reducing UHI effects and achieving energy consumption balance. The relationship between the three key components is presented in the conceptual model shown in Figures
In Figure
This process can be described in the following way:
Therefore, by decreasing anthropogenic heat and urban structure factors, mitigation of UHI effects is achievable.
Although decreasing the anthropogenic heat is dependent on urban structure factors, an optimal and realistic solution is to focus on urban structure factors, such as natural ventilation, surface materials and landscape, or vegetation covers to decrease UHI intensity and create energy consumption balance;
Under such circumstances, a balanced urban environment can be created (Figure
Patterns of commercial energy consumption [
Activity | Energy consumption (% of total country needs) | ||
US | U.K. | Sri Lanka | |
Industry | 41.2 | 32.0 | 9.9 |
Transportation | 21.0 | 18.0 | 16.4 |
Building energy needs | 28.0 | 48.0 | 67.0 |
Agriculture/other | 7.7 | 2.0 | 6.7 |
The process of increasing energy consumption.
Conflict between anthropogenic heat and urban structure factors, creation of UHI, and its effect on energy consumption balance.
Mitigation of UHI has direct effect on energy consumption balance.
Achieve energy consumption balance by providing balance with natural ventilation, high-albedo materials, and vegetation covers.
The reduction of the energy consumption of buildings by combining techniques to improve the thermal quality of the ambient urban environment with the use of up-to-date alternative passive heating, cooling, and lighting techniques can partly decrease these kinds of environmental problems.
Asimakopoulos et al. [ The layout of the basic road network with a specific orientation. This layout affects the buildings on either side of the road, giving them an orientation that, in most cases, is not suitable for implementing solar and energy saving techniques. The relationship between the height of a building and the width of the road, which causes overshadowing and thus prevents access to direct sunlight in living spaces. The relationship between plot frontage and depth, which can determine how many internal spaces will have a southern aspect. Densely built urban centres, which result in the obstruction of airflow and sunlight by the walls of tall buildings. A lack of greenery that has been replaced by concrete and tarmac. Overshadowing caused by adjacent buildings and other landscape features, which is difficult to avoid. Building regulations and codes that in most cases determine the dimensions of a building and thus its geometrical form and its position on the plot.
It also must be noted that higher temperature and less intensive winds are causes of UHI effects. In addition, inappropriate orientation, high density, and shading can directly affect UHI formation. If proper interventions are implemented in urban design, better climate conditions will be achieved when serious overheating problems occur. A large number of air-conditioning appliances leads to increased cooling loads during the summer and to overconsumption of electric energy, which also increase peak energy demand and creates failures in the energy transport network. Energy saving techniques that can be applied in a building includes two kinds of strategies that can be divided into urban elements (macro scale) and building elements (micro scale) strategies. The first strategy includes the energy conservation methods, which involve application of some strategies in urban areas, while the second method includes strategies for buildings. This paper focuses on three main strategies in each scale.
The combination of these two categories will provide the best possible energy saving solution. According to the above considerations these strategies are described in the following sections.
Natural ventilation is the most effective passive cooling technique that can provide cooling during both day and night, while night ventilation is a very effective strategy in hot climates [ Natural ventilation by arranging the openings in buildings to face the prevailing wind can provide efficient natural ventilation and create a healthy indoor air quality. Natural ventilation by ventilated roofs eliminates overheating. Variation in building height can create better wind at higher levels if differences in building heights between rows are significant. Building orientation with adequate gaps is useful for good airflow. Increasing building permeability by providing void decks or pilots at ground level or at midspan.
An increase in the surface albedo has a direct impact on the energy balance of a building. Cities and urban areas in general are characterised by a relatively reduced effective albedo as a result of two mechanisms [ Darker buildings and urban surfaces absorb solar radiation. Multiple reflections inside urban canyons significantly reduce the effective albedo.
As Asimakopoulos et al. [
Therefore, this paper recommends using reflective materials on external surface of building to reduce UHI effects and improve the urban environment.
A material with high albedo can reduce the solar heat gain during the daytime. The surface temperature of the material is lower than that of a material with low albedo. Because the urban ambient temperature is associated with the surface temperatures of the building façade, lower surface temperature can obviously decrease the ambient air temperature and eventually contribute to better urban thermal environment.
Asphalt temperature can reach 63°C and white pavements only reach 45°C [
Cool roofs reduce building heat-gain, create saving air conditioning expenditures, enhance the life expectancy of both the roof membrane and the building’s cooling equipment, improve thermal efficiency of the roof insulation, reduce the demand for electric power, reduce resulting air pollution and greenhouse gas emissions, provide energy savings, and mitigate UHI effects.
Providing an appropriate landscape in building can also contribute to energy consumption reduction. The impact of an appropriate landscape around a building on energy consumption and surrounding temperature regime is very important. Landscaping the surrounding area is a basic criterion to improving the external climatic conditions. As mentioned by Asimakopoulos et al. [ Significantly decrease the energy required for cooling. Decrease the rate of heat convection inside buildings because of shaded surfaces that have a lower temperature. Decrease the radiation exchange of the wall with the sky.
Sailor [
Therefore, this paper recommends using green spaces in vertical and horizontal layers.
Green spaces in some parts of buildings that provide natural ventilation or appropriate landscapes in different layers or floors of buildings with a multiuse function can significantly decrease the energy required to cool buildings.
Green spaces on roofs absorb heat, decrease the tendency towards thermal air movement, and filter air movement. Through the daily dew and evaporation cycle, plants on vertical and horizontal surfaces are able to cool cities. In the process of evapotranspiration, plants use heat energy from their surroundings when evaporating water.
There is strong scientific evidence that the average temperature of the earth’s surface is rising because of increased energy consumption. Global warming has a major impact on human life and the built environment. Therefore, an effort must be made to reduce energy use and to promote green energies, particularly in the building sector. Energy balance can be achieved by minimising energy demand, rational energy use, recovering heat, and using more green energies. This paper was a step towards achieving that goal. The adoption of green or sustainable approaches to the way in which society is run is seen as an important strategy in finding a solution to the energy problem. As discussed in this paper, one of the most important factors that increases energy use is the formation of urban heat islands. Therefore, this paper considers the effects of UHI and by recognising them, proposes beneficial solutions that can lead to energy consumption balance.