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There are new solutions to guarantee acceptable temperatures and humidity levels in town centres, whatever the season. Among them, cool ground surfaces are one of the most promising.
Experiment table with 12 samples - Crédits : Elioth
As part of the redevelopment of the part-Dieu district in Lyon, Egis is carrying out an experiment to test innovative “cool ground” solutions, based on the reflective properties (albedo) of certain coverings and materials. The idea is straightforward: the aim is to invent new ground surface materials with specific heat properties to fight against urban heat islands (an increase in temperature resulting from urban development) and also improve temperature conditions in winter weather conditions.
All the current research concludes that ground surfaces not only play an important part in the urban climate, and therefore in the urban heat island effect, but additionally that they are one of the main contributors to it. Material characteristics will also exercise an effect on the local microclimate through the albedo effect which measures the quantity of sunlight reflected by a surface, but also through their insulation or thermal inertia properties.
To combat the heat island effect, the temperature of ground surfaces must be lowered essentially during the summer. One of the important parameters in these often-highlighted phenomena is the albedo of materials used in the public space, i.e. the proportion of radiation reflected. For example, the zinc roofs of Lyon’s historical city centre reflect approximately 60% of radiation and therefore transmit heat less than tiled roofs, which only reflect about 20%.
During the heatwave in June, dark concrete and asphalt surfaces rose to temperatures of 65°C where tests on variable albedo surfaces recorded values lower than 55%. On the experiment table, the first results display significant differences in thermal performance according to coverings (up to 20°C difference between the tested surfaces). They confirm in particular that the use of granite, planned in the major constructions to be built in the district, is more advantageous (lower than 50°C surface temperature) than more traditional materials used in public spaces such as asphalt and concrete (more than 65°C surface temperature).
Research into increasing albedo is important but only focuses on the problem of reducing ground temperatures. At Egis our ambition is to find a material which has a low surface temperature during the summer, but also, and almost contrary to nature, a higher-than-normal temperature during the winter. This material would help to combat the urban heat island effect in the summer but also mitigate the drawbacks of the cold in the winter. Independently, these two conditions can be achieved, but the innovation resides in the way of combining both conditions in a single material.
A 50-sqm experiment table made up of 12 ground covering samples was installed on Rue Boucut to carry out these tests. Six samples are existing materials which are used as a yardstick (concrete, asphalt, stabilised soil, etc). Six others are prototyped samples of “variable albedo” surfaces, with different characteristics. The objective is to compare the properties of the materials over several seasons and in different climate conditions.
Egis is conducting this experiment as part of the city of Lyon’s Ecocité project, which addresses the redevelopment of several districts in the city. This project sponsored by SPL Lyon Part-Dieu, benefits from funding from the Investments for the Future Programme (PIA), operated by Caisse des Dépôts.