Urban context and climate change impact on the thermal performance and ventilation of residential buildings: a case-study in Athens

Urban settings and climate change both impact energy use, thermal comfort and ventilation of buildings. This is more noticeable in hot urban areas where the urban heat island effect is more pronounced; also, in densely built urban areas where thermal comfort in naturally ventilated buildings is affected by changes in natural ventilation rates because of surrounding obstructions. In some cases, overshadowing might alleviate the impact. This paper presents a study of changes in energy demand in residential buildings considering the overlapping effect of climate change and urban heat island intensity in Athens representing a hot European climate and a dense urban setting. The case study building is a real residential building in an urban canyon in central Athens with data obtained from the PRELUDE H2020 project. The impact of urban parameters on air temperature wind speed and solar overshadowing was considered. Urban air temperature was calculated by using the Urban Weather Generator which includes a number of indices such as site coverage, façade to site ratio and average building height; it also considers the building construction materials as well as anthropogenic heat emissions by the operation of the buildings. Urban wind speed was modified using the URBVENT urban canyon model for the computation of wind speed; this model was validated by its proposers in 2005 by measurements in Athens. Solar overshadowing was calculated for the case-study building considering the surrounding buildings. Current-urban and future-urban weather files were generated, and simulations were run considering energy demand and indoor thermal comfort. The thermal simulation results show that in the hot European climate of Athens with densely built urban areas and for a building within an urban canyon, current weather files which include overshadowing, urban heat island and canyon wind will increase the cooling demand by 24% in comparison to using a typical current weather file. However total energy demand (heating and cooling) increased only 3% for lower floors and 12% for higher floors due to the reduction of heating demand. Simulations using future weather files indicated a 66% increase of cooling demand in comparison to using a typical weather file. Future total energy demand increased by 32% for higher floors and 13% for lower floors. If the building is free floating an adaptive thermal comfort analysis indicated that only 25% of the summertime will be comfortable in comparison to the 50% prediction by the current typical weather file. Therefore, the use a suitable weather file to include urban external conditions in thermal simulations is essential for more accurate predictions of energy demand and internal avoidance of overheating in free-floating buildings.