Numerical Study of Heat and Water Vapour Exchanges Inside a Green Roof Building in a High Irradiation Area for Passive Cooling Purpose




Leaf Area Index, Nusselt number, Sherwood number, passive cooling


Vegetation cover provides shading and protects the soil beneath them from warming.  Vegetation can be used as passive cooling technique that reduces the thermal load of a building. A numerical study has been carried out on laminar double-diffusive mixed convection in a green roof enclosure. The model is equipped with inlet and outlet openings for air removal while the left vertical wall is heated and partially saturated with water for indoor air humidification. The mathematical model is governed by the two-dimensional continuity, momentum, energy and concentration equations. Transfer equations are solved using a finite difference scheme and Thomas algorithm. The model was applied for the simulation of a building with green roof in Togolese climate conditions. Results showed that, the flow structure is a mixed convection type, but the isotherms et iso-concentration distributions reveal a vertical stratification of the temperatures and the relative humidity.To predict heat transfers inside the cavity, a correlation has been established for the estimation of the average Nusselt number as a function of the Leaf Area Index and Reynolds number under solar heat flux of 350 W.m-2, the average in case of Togo. It was found that a larger Leaf Area Index reduces the solar flux penetration and therefore, reduces significantly heat transfer inside the enclosure and then stabilizes it temperature. For the LAI equal to 3, the indoor air fluctuates around 26°C and the relative humidity range is found to be 50% - 60% under solar heat flux of 350 W.m-2.


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ROOFSOL, Building in ROOFSOL, “Roof Solutions for natural cooling,” . Commission of the European Communities. DG XII, Science, Research and Development; (Contract N° JOR3CT960074) 1998.

Nahar N, Sharma P, Purohit M. Studies on solar passive cooling techniques for arid areas. Energy Conversion and Management. 1999 01;40(1):89-95.

Tang R, Etzion Y. On thermal performance of an improved roof pond for cooling buildings. Building and Environment. 2004 02;39(2):201-209.

Jain D. Modeling of solar passive techniques for roof cooling in arid regions. Building and Environment. 2006 03;41(3):277-287.

Fioretti R, Palla A, Lanza L, Principi P. Green roof energy and water related performance in the Mediterranean climate. Building and Environment. 2010 08;45(8):1890-1904.

Abalo Samah H, Banna M. Performance analysis of Thermal Insulation Screens used for Classic Roofs in hot-humid Tropics. International Energy Journal. 2009;10:255-266.

Niachou A, Papakonstantinou K, Santamouris M, Tsangrassoulis A, Mihalakakou G. Analysis of the green roof thermal properties and investigation of its energy performance. Energy and Buildings. 2001 09;33(7):719-729.

Nyuk Hien W, Puay Yok T, Yu C. Study of thermal performance of extensive rooftop greenery systems in the tropical climate. Building and Environment. 2007 01;42(1):25-54.

Barrio EPD. Analysis of the green roofs cooling potential in buildings. Energy and Buildings. 1998 04;27(2):179-193.

Alexandri E, Jones P. Developing a one-dimensional heat and mass transfer algorithm for describing the effect of green roofs on the built environment: Comparison with experimental results. Building and Environment. 2007 08;42(8):2835-2849.

Tabares-Velasco PC, Srebric J. A heat transfer model for assessment of plant based roofing systems in summer conditions. Building and Environment. 2012 03;49:310-323.

Sailor D, Hutchinson D, Bokovoy L. Thermal property measurements for ecoroof soils common in the western U.S.. Energy and Buildings. 2008 01;40(7):1246-1251.

Ouldboukhitine S, Belarbi R, Djedjig R. Characterization of green roof components: Measurements of thermal and hydrological properties. Building and Environment. 2012 Oct;56:78-85.

Jim C, Tsang S. Biophysical properties and thermal performance of an intensive green roof. Building and Environment. 2011 06;46(6):1263-1274.

Hodo-Abalo S, Banna M, Zeghmati B. Performance analysis of a planted roof as a passive cooling technique in hot-humid tropics. Renewable Energy. 2012 03;39(1):140-148.

Saha S, Hasan MN, Khan IA. Double Diffusive Mixed Convection Heat Transfer inside a Vented Square Cavity. Chemical Engineering Research Bulletin. 2009 Oct 05;13(1).