Caracterización mediante modelado y ensayos del comportamiento energético de cubiertas ajardinadas utilizando la metodología PASLINK

Resumo

Within the development of a project carried out by a construction company located in the BasqueCountry which aim is to build dwellings by means of prefabricated modules, a need for evaluating thethermal and hygroscopic behaviour of its flat roofs has been found. During the roof design the thicknessof insulation has been defined to fulfil the most severe climatic conditions considered in the SpanishBuilding Code. This way the roof could be installed in any other location without changing anything inthe production chain.The possibility of changing the roof cover from gravel to vegetation has been analysed by means ofexperiments under real conditions. The CTE does not accept the possible improvements on the coolingand / or heating demand of non traditional building components to be considered unless theseimprovements are proved. The vegetation cover is not considered as a traditional building componentand thus its impact on the reduction on the cooling demands of the building cannot be taken intoconsideration unless it is quantified.This is the main reason of carrying out this research by means of experimental validation of the greenroof thermal behaviour. These experiments have been used to identify and validate two models: one tosimulate the hygrothermal (thermal and hygroscopic) behaviour of the gravel covered roof and thesecond one to simulate the thermal behaviour of the vegetation covered roof. The only differencebetween both roofs is the cover, while the rest of the roof structure has been maintained the same:interior concrete layer (8.5 [cm]), rock wool layer (8 [cm]) and exterior concrete layer (15.5 [cm]).The first set of experiments has been carried out to the gravel covered flat roof test sample. This rooftest sample has been designed to be as similar as the real flat roof. The possibility of analysing thehygrothermal behaviour of the flat roof under one dimensional assumption has been checked by meansof a 3D simulation of the roof test sample. The experimental design of the roof test sample has beenmade under the assumption that the one dimensional analysis is possible. Thus 5 temperatures, acentral heat flux sensor and a relative humidity sensor has been installed in the interface of each of thelayers composing the roof test sample. Some of those sensors have been installed in order to permit tovalidate the results of the 3D simulation on the temperature homogeneity in the interior surface of theroof test sample. Once this fact has been validated the experimental data have been used to identifyand validate the thermal and hygroscopic properties of the different layers of the roof test samplecovered by gravel.Since the tests have been carried out in a PASLINK type test cell the obtained experimental data aredynamic (steady-state conditions are never achieved). A PASLINK test cell has been upgraded and aninnovative system to test roof samples has been designed for this thesis. The innovation is mainlyregarding to the design of a special insulation frame for the roof test sample that minimizes the bordereffects (and thus uncertainties) during the testing period.The experimental data have been treated by means of the LORD software to obtain the thermalcharacteristics of the different layers of the roof test sample. Both, the thermal resistance and thermalcapacity of each of the layers have been obtained. Those values have been used to calibrate a onedimensional model of the roof test sample on the Wufi Pro 4.2 software (this software permits to analyzethe hygroscopic behaviour). This model has also been validated by means of the experimental data.Once the gravel covered roof experiments were finished, the gravel layer was changed by a vegetationcover. The vegetation cover is made up by a drainage layer, a soil layer and the plants or canopy layer.The vegetation covered roof has only been used to identify and validate a model that can simulate thethermal behaviour of the roof test sample covered by vegetation. The same identification and validationprocess has been followed with the vegetation covered roof. But in this case the vegetation covered roofhas been more complicated to be modelled. The common part of the roof has been modelled in a similarway to the gravel covered case and thus it has served as a validation to compare the results of bothcovers. But the soil and drainage layers thermal conductivities are very dependent on their watercontent. The two extreme working conditions for this part have been identified and validated: completelydry situation and completely water saturated situation. This way the thermal conductivity and thermalcapacity of the soil and drainage layer have been obtained for the upper and lower bounds.The calculation of the soil layer external surface temperature is the most important factor regarding tothe improvements of the green covers in the cooling demands associated to the roof. This temperaturedepends on many factors but mainly on: solar radiation, evapotranspiration rate, water content of thesoil plus drainage layer, Leaf Area Index of the plant, long wave radiation heat exchange and theconvective heat transfer to the outdoors air. During this research an innovative model to obtain thistemperature is proposed based on the mass convection theory. Actually, a similar model to the sol-airconcept is obtained. This model has been validated for the following extreme two cases: the soil anddrainage layers are completely water saturated having all the required water for evapotranspirationprocess. This is the optimal working mode of the green roof for hot, dry and sunny summer periods andof course it needs of an irrigation system to maintain those optimal conditions on time. The otherworking mode is the case of a not irrigated green roof during long periods of hot, dry and sunny summerwithout rain. This is the worst working mode for the green roof.Once the two models have been identified and validated, both roofs thermal behaviour have beensimulated for three different locations considered in the CTE: Burgos (climatic zone E1, consideredextremely cold in winter), Almeria (climatic zone A4, considered extremely hot in summer and Zaragoza(climatic zone D3, considered an intermediate case)The heating demands have been compared and no considerable difference has been found in case ofchanging the gravel cover for a green cover.For summer periods the green cover reduces the cooling demand up to a 75% in case the green coveris maintained water saturated by means of an irrigation system. But in case the green cover does nothave an irrigation system it behaves worse than the gravel cover case when it dries completely. Thetested green roof dries completely after 8 days of hot, sunny and dry days. Once the roof is completelydry the evapotranspiration effect disappears. Since the soil and plants absorb much better the solarradiation than the gravel cover, the cooling demand can increase up to a 65% compared to the gravelcovered case. Note that if any rain is happening this percentage is reduced.The hygroscopic behaviour has been studied and no interstitial condensation risk has been found in anyof the locations considered for the simulations. Only, under the worst simulation conditions for theindoors air conditions, a slight risk of mold growth has been found. A water vapour barrier in the hot faceof the insulation layer would improve this situation.