Plenary Lecture

Assessment of Vegetated Envelopes Effect on Urban Microclimates and Building Energy Performance

Professor Rafik Belarbi
Civil Engineering and Mechanical Department
University of La Rochelle

Abstract: Green roofs are considered to be an effective contribution to the resolution of several environmental problems at the building and urban levels. In addition to the creation of a pleasant environment, green roofs offer several benefits in comparison to conventional roofs. They improve storm water management as well as reduce air pollution and noise. Green roofs increase vegetal and animal biodiversity in cities, and they also reduce a city’s carbon footprint by converting carbon dioxide to oxygen through photosynthesis. Green roofs improve building energy efficiency by enhancing the heat transfer through roofs. The reduction of the summer temperature around green roofs improves the efficiency of HVAC systems by providing a local free cooling effect to the fluid before it returns to the chiller. This reduced temperature also improves the efficiency of surrounding photovoltaic panels. Green roofs improve the longevity of roofing membranes by limiting the thermal stress to which they are subjected. Finally, at the city level, green roofs contribute to the mitigation of the urban heat island effect. To assess energy and environment performances effects, a physical model describing the thermal and water transfer mechanisms within the vegetated building envelopes has been developed as well as the thermo-physical properties of the green roof components were characterized. The model’s program has been implemented in a building simulation program. This tool, was use to predict the impact of green roofs and green facades on building energy performance. This approach is extended to the street canyon in order to assess the microclimatic interaction in building simulation. Three of the main physical properties of green roofs were experimentally investigated to determine some of the green roofs’ modeling key parameters. First, the thermo-physical properties of green roofs were characterized by correlating the thermal conductivity of the substrate with the water content for different substrates and maximum water capacities. Next, the moisture storage was characterized using the dynamic vapor sorption technique. Third, the micro-structural properties of green roof substrate were characterized using mercury intrusion porosimetry. In addition to these characterizations, the evapotranspiration term, which is very important in the water balance, was measured. Simulation were performed in order to assess hygrothermal behavior of buildings. Results show that thermal and water transfers are strongly coupled. Hence, the thermal behavior of green roofs and green walls depend on the water availability within the growing medium. In summer and winter, measurements and numerical simulations show that green envelopes improve the energy efficiency of buildings and reduce the urban heat island. The model was experimentally validated according to a green roof platform (scale 1:10) constructed on the site of the University of La Rochelle. Measurements have also been conducted in a full scale building equipped with green roofs. Once the proposed model validated, it has been coupled to a building thermal code (TRNSYS) to evaluate the impact of green roofs on the energy performance of buildings. The results show that the effect of mass transfer in the subtract was very effective in reducing the model errors. Comparisons were undertaken with a roof slab concrete model; a significant difference in temperature (up to 30C) between the outer surfaces of the two roofs was noticed in summer. The heat flux through the roof was also evaluated. The roof passive cooling effect was three times more efficient with the green roof. In the winter, the green roof reduced roof heat losses during cold days; however, it increased these losses during sunny days. With a green roof, the summer indoor air temperature was decreased by 2 °C, and the annual energy demand was reduced by 6% for an oceanic climate such as that of La Rochelle. Finally, the simulations performed for different climates suggest that green roofs are thermally beneficial for hot, temperate, and cold European climates.

Brief Biography of the Speaker: Professor, Dr-Ing. Rafik BELARBI, Received his engineer degree in Building Physics from School of engineers of Poitiers (ESIP) and Master of thermal sciences from University of Poitiers, France in 1993. He obtained his PhD Thesis in civil engineering in La Rochelle University, 1998. In 1999, he joined the LEPTAB research staff laboratory and civil engineering department of La Rochelle University as is, actually, full Professor and Head of Civil engineering and Mechanical Department. His Research field covers wide spectrum and several domains. It cover multi physic and multiscale approaches as: building material for energy and environment applications, urban microclimate modelling and durability aspect; comfort and indoor air quality as well as renewable and energy system. His main expertise is in microstructural, thermal, physical and hydric characterization of porous building material and heat and mass transfers with application in Energy Efficiency in Buildings and Indoor Environment and durability of constructions. Since 1994, he was involved in several National and International projects dealing with Heat and moisture transfer in the building energy conservation. The main projects are: Pascool/Joule and Altener/Sink (passive cooling systems modelling and their impact on the building energy consumption), PDEC/Joule II (Utilisation of Passive Downdraught Evaporative Cooling systems on non-domestics buildings), Joule/Thermie B (Efficient Ventilation Systems for Buildings), Altener/Greencode (Reglementary Frame for Renewable Energy Use in Urban Site Through Vegetation Planting and Strategic Surfacing), Altener/SolVent, (Development of Strategies for Efficient Use of Solar Passive Ventilation in Urban Buildings) and Altener/Cluster (Solar Passive Heating and Cooling), Seventh Framework Programme" Marie Curie (OldMasonryRepair), Erasmus+: programme Capacity Building in Higher Education “Boosting Environmental Protection and Energy Efficient Buildings in Mediterranean Region”. He is author or co-author of more than 140 papers in international journals or international conferences.

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