Urban Form Optimization for the Energy Performance of Buildings Using Citysim

The energy efficiency of the urban texture relies notably on the buildings’ form, which characterizes its capability to take profit of the solar potential as well as its loss of energy through the envelope. Therefore, the general layout at the district scale has a significant impact on the global energy balance in a dense built environment, where the relative position of the adjacent buildings generate shadowing and inter-reflection on a large part of the incoming solar radiation. In the current work, an urban energy simulator named CitySim is used together with a hybrid evolutionary algorithm. The physical model within CitySim features the computation of shortwave radiation including reflections, longwave radiation and a nodal thermal model for the building energy flows. The complete simulation leads to a rather precise evaluation of the annual heating needs, defined as the objective function to be minimized at the district scale. An existing district in Paris is taken as case study: the Bercy park front. Its buildings are disposed in several blocks surrounding courtyards with openings on the park’s side (southwest). This district is recent (1994-2005) and located in a dense area of seven stories buildings. In its original design, the access to sunlight was taken into account by the planner, thus the initial intuition of the architect is compared with optimized configurations. The paper describes a case of urban form optimization, with the formal description of the variables, constraints and objectives of the problem and the definition of the geometrical case for Bercy Front Park. The optimization variables focus on geometrical properties such as the height of the buildings and glazing ratios. The thermal properties of the building (insulation, glazing type) are set in accordance with the French standards of the thermal regulation (RT2005). The urban layouts resulting of the optimization process are analyzed and discussed.