KELVIN

Reducing urban heat islands by improving the radiation properties of buildings and neighborhoods.

The aim of this exploratory project was to investigate different concepts for reducing urban heat islands (UHIs), and to assess the energy and greenhouse gas emissions savings that may result from a cooler city microclimate. In particular, the impacts of modifying the albedo of roofs, roads, parking areas, and green roofs on the microclimate of the City of Vienna were modelled and the potential reduction in energy use and GHGs were estimated.

The project found that a comprehensive application of highly reflective roof (albedo = 70%) in combination with the full implementation of green roofs on potential roofs can lead to a strong decrease in the number of hot days (days with Tmax  30°C). In comparison to the period 1981 – 2010, the number of hot days in the inner city and near inner-city may be reduced by up to 29% and 20% respectively. Model calculations show that the "theoretical potential" can be achieved by including highly reflective facades to buildings and sealed surfaces. In this case, the number of hot days in areas such as Hohe Warte by up to 45% and in the inner city by up to 53%. This leads to an electricity saving potential of 5,000 - 20,000 MWh per year - at current cooling capacity. This is indirectly decreases GHGs by 600 – 2,000 t CO2 /year. However, there is a greater reduction in equivalent GHGs directly due to the decrease in radiative forcing caused by changing the roof albedo. The decreased radiative forcing is comparable to additional savings of around 75,000 t CO2 / year. Additional effects regarding human comfort and other positive aspects in terms of human health should be considered as well.

Zentralanstalt für Meteorologie und Geodynamik (ZAMG)