Research On Thermal Performance Evaluation Index Of Green Roof

Green roof is an ideal passive way for energy saving. It can significantly increase the thermal resistance and reduce building energy consumption. It is also an method which can effectively solve the problem of urban "heat island". There is significant difference between the thermal performance of green roof and traditional roof. Green roof is a kind of "ecological" construction. Its thermal performance is synthetically influence by internal and external environmental factors. So it is not complete to use a single fixed value of equivalent thermal resistance index for evaluating thermal performance of green roof. This paper used a modular green roof as an example, and studied equivalent thermal resistance changes of green roof with different indoor set temperature, outdoor meteorological conditions and different structure parameters of green roof in winter and summer. It can provide a reference for engineering practice.First of all, starting from the principle of heat insulation of green roof, the vegetation layer and soil layer, roof structure layer composite thermal effect can be seen as integrated shading and integrated thermal resistance properties. Integrated shading property reflects the ability to obtain solar radiation, and its evaluation indexes are LAI and reflectivity. When LAI is between 3 and 4.6, the reflectivity of the Buddha grass lawn can be value of 0.15. The integrated thermal resistance property of green roof is an equival feature Analogied from general palisade structure, including the vegetation transpiration and soil evaporation, soil thermal conductivity etc, and its evaluation index is mainly the equivalent thermal resistance.Secondly, with the analysis of green roof heat transfer process and the basic theory of thermal resistance, the constitution of the equivalent thermal resistance of green roof was defined. And then through the comparison of methodes for determining the value of equivalent thermal resistance, the calculation formula of the equivalent thermal resistance which comes from steady heat transfer was developed. Outer boundary temperature values were discussed. The factors that influence the equivalent thermal resistance values of green roof were analysied and cooling efficiency was put forward to judge insulation performance of green roof without cooling conditions.Then, the test for green roof and contrast roof wih different indoor air temperatures was finished in winter and summer, in Chongqing. Two kinds of roof heat preservation and heat insulation effect were analysied, and it was concluded that: the equivalent thermal resistance of green roof is significant difference between in winter and in summer. From the test, in the coldest winter day, green roof surface outside minimum, average temperature are higher 0.81 ℃ and 1.79 ℃ than contrast roof respectively, when surface inside lowest, average temperature are higher 1.4 ℃, 1.2 ℃ higher than contrast roof. In the hottest summer day with cooling conditon, green roof surface outside maximum, average temperature are lower 18.3 ℃ and 12.9 ℃ than contrast roof respectively, when surface inside maximum, average temperature are lower 14.3 ℃ and 8.6 ℃ than contrast roof. Different indoor conditions can significantly affect soil layers of green roof, structure layer and indoor air temperature distribution, and less effect on the upper layer of vegetation and soil.Finally, the simulation model was established by the energy consumption simulation software, and was verified with the measured datas. Analyzed the indoor set temperature, outdoor meteorological parameters,green roof construction three factors of planting roof of the equivalent thermal resistance are studied. The indoor setting temperature on the winter planting roof has a little influence on the equivalent thermal resistance of. In summer, green roof integrated equivalent thermal resistance increases with the rising of indoor set temperature, at room temperature is higher than the critical heat transfer temperature, the cooling efficiency evaluation. Radiation from the sun and the temperature influence on planting roof equivalent thermal resistance values are: winter, the significant effects of solar radiation, along with the rising of the daily average solar radiation, under the different temperature, vegetation integrated equivalent thermal resistance is then increased slightly; In the summer, the temperature effect significantly, with the increase of the average daily temperature and under different solar radiation intensity, the vegetation of the integrated equivalent thermal resistance is less. The influence of leaf area index of planting roof in winter can be ignored; In the summer, as the leaf area index increased from 1 to 5, the vegetation of the integrated equivalent thermal resistance significantly increased, and the soil thermal conductivity of thermal resistance decreases with the increase of leaf area index, but the overall comprehensive equivalent thermal resistance of planting roof is still linear increase with the increase of LAI. The thermal resistance of thermal conductivity of soil initial moisture content of soil in winter can be ignored; In summer, the greater the initial moisture content, the smaller the thermal resistance of thermal conductivity of the soil, and the greater the integrated equivalent thermal resistance of vegetation, and there is a critical point of roofing heat flow direction change. When to evaluate the cooling efficiency, cooling efficiency decreases with the increase of initial moisture content.This paper combined theory, experiment and simulation in order to study the thermal performance of green roof. The comprehensive, dynamical rule of green roof equivalent thermal resistance value was developed, and the cooling efficiency was used to evaluatied the thermal performance of green with the situation that equivalent is not appropriate.