Urban Form And Environmental Performance Of Modern Cities

Figure 1: New York City's daily carbon dioxide emissions as one-tonne spheres. Image elaborate by ‘Carbon Visual’

Facing the challenge of more sustainable urban environments, the density of the development is still a crucial and debated topic, for its controversial economic, social and environmental consequences.

As a matter of fact, urban areas already host more than half of world population, having overcome this significant threshold since 2008 ( World Bank data ). With the actual trend of growth, this figure is predicted to reach 70% by only 2050. Cities are the focal place of the world economy, but also the main cause of current environmental and energy issues. The excessive greenhouse emissions responsible for global climate change are indeed largely produced in urban areas, where most of the energy consumptions take place, due to transportation and energy needs of buildings.

Cities are thus complex physical systems, whose functioning affects large-scale phenomena, such as global climate change, and local scale phenomena, as urban microclimate and health and quality of life of citizens in buildings and urban spaces. This condition places the cities in the heart of the environmental issue. On the one hand, urban areas have huge environmental footprints and are also the most vulnerable place to climate change impacts; on the other hand, they could afford multiple opportunities to significantly reduce the current rate of carbon emissions at a global level, fostering less resource-intensive models of urban development.

A key variable of the environmental behavior of urban areas is the density and morphology of the urban structure. The physical shape of cities has direct and indirect impacts on the energy behavior of built environments because it affects the consumptions for transportation and the energy needs of buildings. The density of buildings also contributes to local climate modifications, such as the so-called ‘Urban heat island’ effect, which is an increase of urban temperature in comparison to rural environs.

In the many disciplines involved in the study of the built environment, the concept of urban ‘density’ and ‘compactness’ have thus shown the potential of being used as a sustainability indicator. However, the concepts of ‘density’ and ‘compact city’ are also rather ambiguous, since they may assume quantitative or qualitative meanings depending on the aim and the field of the analysis, which can be the social sphere, the citizens’ perception, and wellbeing or the energy and environmental performance of the system.

This study provides a quantitative analysis of the impact of urban compactness on the energy performance of buildings in the Mediterranean context. The research was developed as part of a joint Ph.D. programme between the ‘SOS Urban_Lab’ at Sapienza University of Rome, Italy, and the ‘Architecture Energy and Environment’ research group at the School of Architecture of Barcelona, Spain. The two groups carried out many research activities aimed at fostering an integrated and multidisciplinary knowledge in the practice of urban planning and building design, by providing policy makers, professionals, and students with adequate analysis tools to understand the complex environmental phenomena that take place in urban areas.

Urban compactness and building energy performance in Mediterranean climate

Urban compactness has counteracting effects on building energy performance in urban environments. Compact urban structures enhance the so-called urban heat island (UHI) intensity, causing a significant increase of air temperature in urban areas. Nevertheless, urban compactness determines a decrease of direct solar radiation on buildings façades. These phenomena entail opposite outcomes on the energy performance of buildings in temperate climates, where heating and cooling demands are equally relevant. The presence of the UHI contributes to reducing the heating demand of buildings, but it is very detrimental to the cooling one. On the other hand, the reduction of direct solar radiation allows buildings to keep comfortable temperatures during summer season without the need for mechanical cooling, but it also entails higher energy demand during the winter season. Therefore, the overall impact of urban compactness on the energy performance of buildings must be analyzed considering both effects.

This study shows the variability of building energy performance in a Mediterranean urban context, considering a sample of urban textures of Rome (Italy) and Barcelona (Spain) as study cases. The textures represent the typical range of urban compactness of Mediterranean urban areas. The energy demand of a test apartment was calculated under the corresponding varying conditions of UHI intensity and solar radiation of the different urban structures.

The compactness of the textures was measured using the index ‘Site Coverage Ratio’, given by the ratio of the footprint of the building to the total site area. The variability of the UHI intensity according to the compactness of buildings in the sample areas was analyzed with the climate model ‘Urban Weather Generator’, while the dynamic thermal simulation of the buildings was carried out using the software Design Builder.

Figure 2: Orthographic pictures of the real urban textures, Values of the site coverage ratio and simplified models used to perform the energy simulations.

The energy demand of the test apartment was calculated considering together the solar obstruction determined by the geometry of the different urban textures and the corresponding UHI intensity. The resulting energy loads have been analyzed in relation to the site coverage ratios of the different urban textures.

Results show different relationships between the heating demand, the cooling demand and the compactness of the urban texture. The cooling load has a negative linear correlation with the site coverage ratio of the urban texture, which means that compact urban layouts help to reduce the cooling demand in a Mediterranean urban context, despite the UHI effect. Conversely, the heating demand shows a non-linear correlation with the compactness of the texture; the heating demand decreases when urban compactness increases beyond a threshold value of the site coverage ratio. This means that compact urban textures improve the heating performance of the buildings thanks to the beneficial effect of a strong UHI intensity, in spite of the reduction of solar gains.

This study thus highlights that compact urban structures may help to improve the energy performance of buildings throughout the year in a Mediterranean context. This also means that the dense historic fabric of many city centers in this region are likely to be more energy efficient than sparser and less dense urban patterns, typical of more recent urban developments. The quantitative relationship identified between compactness and energy performance could be of help to urban planners and decision makers for the fast recognition of energy vulnerabilities in urban areas and to implement effective retrofitting strategies at the urban scale.

This study, Effects of urban compactness on the building energy performance in Mediterranean climate was recently published in the journal Energy Procedia.

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