Plenary Lecture

Experimental and Numerical Investigation of the Impact of Green Roof Inside and Outside Buildings. Integration of a Validated Model in a Transient Building Energy Simulation Program

Professor Rafik Belarbi
Head of Mechanical and Civil Engineering Department
LaSIE: Laboratory of Engineering Science of Environment, UMR 7356 CNRS
University of La Rochelle

Abstract: The urban heat island, observed in city temperature peaks compared with rural areas, is caused mainly by the storage of solar irradiance by city structures and materials, the release of anthropogenic heat, a reduced turbulent transfer of heat from within streets and the lack of water and green spaces, which reduce evapotranspiration. The increase in temperature in cities leads to a decline in thermal comfort of city dwellers, who account for more than 70% of the population in Europe. Green roofs can be valuable for building energy performance and for urban microclimate mitigation. Urban surfaces, especially roof coatings, contribute to the urban heat island effect. Green roofs have direct impacts on the urban environment and reduce building cooling energy demand. A green roof improves the thermal insulation of a building, thereby reducing solar heat gain by approximately 70–90% in the summer and reducing heat loss by approximately 10–30% in the winter. In addition, the durability of the water proof membrane of the roof is increased following the reduction of the peak temperature by approximately 30 °C. Indeed, the leaf temperature remains similar to the ambient air temperature because of the evapotranspiration process, whereas the temperature of paved urban surfaces exposed to the sun can exceed the ambient air temperature by approximately 30 °C.
A dynamic model of transient heat and mass transfer across a green roof component was developed. The thermal behavior of the green roof layers is modeled and coupled to the water balance in the substrate that is determined accounting for evapotranspiration. The water balance variations over time directly impact the physical properties of the substrate and the evapotranspiration intensity. This thermal and hydric model incorporates wind velocity effects within the foliage through a new calculation of the resistance to heat and mass transfer within the leaf canopy. Parametric studies of green roof behavior are presented. A surface temperature difference of up to 25 °C was found among green roofs with a dry growing medium or a saturated growing medium. Furthermore, the thermal inertia effects, which are usually simplified or neglected, are taken into account and shown to affect the temperature and flux results. This study highlights the importance of a coupled evapotranspiration process model for the accurate assessment of the passive cooling effect of green roofs.
The developed model was validated with experimental data from a one-tenthscale green roof located at the University of La Rochelle. The experimental platform provides hydrothermal behavior data for green roofs and their interactions with the building. A detailed description of different sensors implemented in the platform is presented. The albedo of the green roof was measured and implemented in the numerical model.
The validated model was implemented in a transient building simulation program (TRNSYS) in order to investigate its dynamic performances coupled with a multizone building code. Simulations were conducted for the experimental platform studied in a temperate French climate where a comparison was undertaken between green and conventional roofing. A reduction of the maximum roof surface temperature by 20 °C was found in summer due to the green roof. The presence of a green roof protects the roof support from high temperature fluctuations increasing the roof longevity. Green roofing is an effective solution to reduce the cooling demand. However, the impact on heating demand is significant when the climate is cold. Finally, the total energy demand decreases by as much as 6% with green roofs for the oceanic climate studied here (La Rochelle, France).

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 Associate Professor. Actually, he is full Professor.
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 modeling 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). He is author or co-author of more than 100 papers in international journals or international conferences. Since October 2007, he is the Head of the Civil Engineering Department.

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