Resumen de Potencial de reducción de la demanda energética en edificios con cubiertas verdes en condiciones climáticas del sur de europa
1. introducción o motivación de la tesis In Europe, after the Kyoto Protocol in 1997, lots of efforts focused on controlling the global warming and reducing emissions into the atmosphere. All European Directives concerning the energy efficiency of buildings promoted the rehabilitation of old buildings and the use of renewable energy to cover the energy needs of cooling and heating of buildings. The rehabilitation of a building consists in improving its energy performance, reducing the thermal flux that is exchanged between the building and the external environment. This may be achieved by active strategies, such as implementing efficient HVAC systems, or passive strategies such as green roofs or ventilated façades, that encompassed both opaque and transparent elements of the building envelope, to insulate it during the winter months and protect it from heat gains during the summer months.
Green roofs are passive construction systems of low environmental impact that have a lot of environmental advantages, such as the absorption of CO2, the thermal insulation capacity and the retention of meteoric water. The objective of this thesis was to study experimentally and numerically the potential of energy demand reduction in buildings with green roofs and eco-roofs, green roofs without plants, under climatic conditions of Southern Europe.
2.contenido de la investigación Firstly, two numerical models of an eco-roof and a traditional gravel ballasted roof were calibrated with experimental data of heat flux, in order to simulate their energy behaviour.
Then, the effects of climatic variables and the volumetric water content of the substrate on the evapotranspiration and on the heat flux of the eco-roof were evaluated. Finally, 20 different irrigation strategies were studied in order to investigate the reduction of heat flux through the roof assembly and to identify the strategy that allowed the greatest reduction of energy demand with the lowest water consumption.
Secondly, it was analysed experimentally the energy behaviour of different substrates of green roofs through a dynamic analysis based on several parameters such as time lag, TL, decrement factor, DF, soil-air temperature, Tsa, and cooling potential, CP. Then it was evaluated the reduction of energy demand of the building with green roofs respect to a traditional gravel ballasted roof.
High reductions of annual energy demand were obtained with the eco-roof, 40% without irrigation strategy and 95.8% with 80 min of irrigation every day in the morning, respect to a traditional gravel ballasted roof. The irrigation strategy that allowed to achieve the highest reduction of annual energy demand with the lowest waste of water was irrigating 10 min in the morning in alternate days, 46.7% with 380 l/m2 year.
The experimental analysis of the green roofs with different substrates showed that the substrate with 100% commercial growing medium, managed to retain more water than the rest of the plots and obtained the highest reduction of annual energy demand, up to 80.7%, respect to the traditional roof. Furthermore, the dynamic variables used for the analysis of the green roofs showed that this type of installation can achieve significant reductions of slab temperature and heat flux between the indoor and outdoor of the building, the reduction of the maximum peak of the internal temperature and the reduction of the maximum daily temperature excursions, compared to the traditional roof.
3.conclusión These results showed that both eco-roof and green roof are passive systems that allow to achieve high reduction of energy demand for the rehabilitation of old buildings. A proper irrigation strategy and the presence of plants can help further to improve the energy performance of the roof.
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