A holistic model for analyzing energy benefits of urban density by relating energy use, building height, and overall city structure
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More than half of the world population live in cities, and the urban population is further expected to almost double within 2050. This opens a rare window of time for realizing energy savings through overall city planning. How the overall city structure influence energy consumption is, however, still poorly understood. A central theme in the sustainable development of urban form is the compact city, and as a key instrument of this densification, tall buildings may prove important. Yet, the overall energy-saving potential of building taller and denser remain largely unclear. Moreover, current studies are described as far from holistic, not capturing the interconnectedness and complexity of the system as a whole. They are mostly qualitative, and methods depend largely on context. There is thus a lack of a clear theoretical framework for understanding energy consumption at the urban scale. The ambition of this thesis is to address this knowledge gap. This thesis develops a holistic optimization model for investigating the extent to which urban density and urban structure influence the energy consumption of the urban system. Energy aspects in land use planning, including the influence of building height, are addressed. The model relates energy costs of building heights of three stories and greater, with transportation and infrastructure energy benefits of building denser. Multiple scenarios of differing climate, population, and other variables have been simulated. Only factors considered to be correlated with urban density are taken into account. Of these, solar irradiation and the urban heat island effect have been left out due to their complex nature. A denser and taller city structure than what is normal in cities today is found to be optimal for low urban energy use. The most influential urban density indicators are embodied energy (most heavily influenced by building lifetime) and floor area per capita. The findings of the research indicate that building heights approximately in the range 7-27 stories are optimal for a given population and building lifetime. For buildings taller than this the increased embodied energy outweighs further reduction potentials of other elements. Energy use per capita in a city with optimal density is increasing slightly with population. Transportation energy is found to be much less important than building energy, especially in dense small area scenarios, but becomes increasingly important for low-density scenarios with large urban areas. Road construction, elevator energy, and vertical water transportation energy does not significantly affect the overall energy budget. An energy saving potential for the urban metabolism of the investigated elements of approximately one-third compared to a low-density scenario is found to be viable. However, energy savings of further densification in areas that already have high-density, close to the optimal, are not significant. The energy expenditure is significantly lower in the dense and tall scenarios - with implications for current and near-future city planning policies on optimizing land use based on city size. These findings improve the basis on which decisions are made for policy-makers and urban planners worldwide, although the significance of solar irradiation and the urban heat island effect should be investigated further. The model is a generalized theoretical abstraction and thus has its limitations. Further development of the model by including more elements as well as reducing uncertainties is needed. Nevertheless, the findings are relevant both for further development of existing cities and for conceptually planned future cities.