KELVIN - Reducing the urban heat island effect via improving the reflective properties of buildings and urban areas
Starting point / motivation
Solar energy is absorbed and stored more in urban areas than in rural landscapes. This is mainly due to the colour and aspect of urban surfaces such as rooftops, streets, parking spaces for cars and other built-up areas and the higher heat storage capacity of urban materials as compared to the texture of the rural landscape. The mitigation of the problem that results, the urban heat island (UHI), is the driver for various urban development measures such as greening of façades and roofs, tree planting on roadways and the expansion of parks and lawns. Climate change is modelled to worsen UHIs and, at the same time, the urban texture is thought to exacerbate climate change (positive-feedback) since:
- increased warming of urban areas especially in the summer months will increase the demand for space cooling of apartments, office buildings and houses. The electrical energy required for this partly comes from fossil fuel fired power plants and hence causes greenhouse gas emissions (GHGs) indirectly; and
- increased albedo of urban areas as compared to rural areas also increases the solar energy absorption and is equivalent to a GHG emission warming potential.
Air conditioning use has increased in recent years in Austria and the energy demand for space cooling and ventilation is expected to grow strongly with climate change. The increased intensity and frequency of heat waves in the summer months could thereby have far-reaching consequences in terms of the climate, energy consumption, health and quality of life in cities.
Contents and Objectives
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.
A literature review was used as an initial assessment of the potential to reduce the urban heat island. This was followed by interviews of Austrian stakeholders (roof tile manufacturers, city planners, government officials, etc.) in which the acceptance of identified strategies was discussed. In parallel, the values of the albedo (surface reflectivity) for the building stock for Vienna were estimated from satellite data from 2000 to 2014. These values were used as input to model the impacts of high albedo and "green" roofs on the UHI. The results of the microclimate modelling exercise were used to estimate the energy savings, and GHGs.
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 a "theoretical maximum potential" can be achieved by including highly reflective facades on buildings and paved-over surfaces. In this case, the number of hot days in areas such as Hohe Warte may be reduced 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 which, 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.
Prospects / suggestions for future research
There are many areas for future research including:
- The practical implementation of the project results;
- The implement of a standardized method for modelling of UHI and conversion to energy and GHG savings to other cities;
- The identification of development of materials that increase albedo but fit within the constraints of Austrian architecture and building codes (e.g. red-tile roofs); and
- The forecast of the impacts and benefits of modifying the urban landscape in future climates.
These research topics could be addressed in follow-up projects funded within the FFG research programmes. For example in the programme "City of the future" or SMART CITIES demo actual case examples (measurements on decorated roof surfaces with high albedo or roofs, plantings of pedestrian zones etc.) in selected Austrian cities are planned (points 1 & 2 above). For point 3 an industrial application may be the development of innovative materials or coatings which improve the optical reflectivity of roof tiles. For example, a possible surface treatment for clay roof tiles may be identified that meets the criteria of higher reflectivity without changing the tile colour.
Additional submissions on the topic of reducing UHI in other cities using "urban forestry", green roofs and facades are envisaged. These investigations should not only be assessed to show possible advantages of the evapotranspiration from trees and plants to provide additional cooling, but also the possible positive influence of green roofs on the air quality (i.e. particulates collection) in cities, and the shading of shopping streets that may also have a positive impact on the purchasing behaviour of consumers. Finally, future research could take advantage of a call in the EU Horizon 2020 programme for demonstrating innovative nature-based solutions in cities.
JOANNEUM RESEARCH Forschungsgesellschaft mbH
RESOURCES - Institute for Water, Energy and Sustainability