Influence of cool materials on building energy demand at district scale

Since 2007, more than half of world population lives in urban areas and its activity leads to an increase of building energy demand notably in summer. The temperature rise of densely built areas is mainly due to landscaping and anthropogenic heat fluxes such as air conditioning systems. Acting on urban landscaping, building density, surface albedo or green area can mitigate the urban heat island with direct and indirect improvement of building energy performances.  
In the present study, a new numerical approach was developed to assess the building energy demand including microclimate interactions on buildings. The different physical phenomena are computed at the district scale with different meshes, for the surfaces and the volumes, of the tridimensional numerical mockup. The urban microclimate is assessed thanks to specific models developed for the outdoor airflows, the longwave and shortwave radiative exchanges. The thermal behaviour of buildings is computed by a model developed on the basis of the weighting factor method which saves computation time. Then, a coupling method was implemented in order to match the results from the various meshes. The thermal balances are computed simultaneously for the whole district cells, including indoor and outdoor balances, for each time step. 
A case study was selected and studied: a district located in Nantes, France, named Pin Sec district. A parametric study is carried out considering cool materials. The results are displayed as the cooling demand of buildings for each case studied. The impact of cool material areas on both building energy demand and urban microclimate are clearly shown through the results obtained.