Heat Prevention Theory And Thermal Performance Evaluation Of Roof Sheet Overhead Shading

In the southern part of China,the summer is hot and the air conditioning energy consumption is huge.As one of the building envelopes,the roof is the longest exposed to the sun and receives the highest intensity of solar radiation,which plays a major role in the air conditioning energy consumption and indoor thermal comfort of the top floor room.As a kind of economical heat prevention measures,roof shading is of great significance to reduce the energy consumption of building air conditioners and improve the indoor thermal environment.Due to the advantages of the convenient construction,light weight,easy management,it is commonly used in residential buildings or public buildings of the middle and lower floors.Roof sheet overhead shading is an extension of reflective roof,which uses shading sheet to directly reflect solar radiation and avoid roof from sunshine.It is also a kind of ventilated roof,which uses ventilation to reduce the accumulation of heat in the roof.However,people have not thoroughly explored the influencing factors of its thermal reduction properties and heat prevention effects,and evaluated the effects only by subjective feelings.At present,the evaluation index of the roof shading insulation performance is equivalent to the relevant specification and standard with an additional thermal resistance value of 0.3 m_·~2K/W,but the theoretical basis is not specifically explained.How to determine and evaluate the heat prevention effect of roof sheet need to be demonstrated in theory and experiment specifically.In this paper,the roof sheet overhead shading is selected as the research object,mainly doing the following work:Firstly,the theoretical analysis of the heat prevention mechanism of roof sheet overhead shading is carried out.According to the heat balance equation,the calculation formulations of the surface temperature of the overhead shading sheet and the outdoor comprehensive temperature of the roof are established,which provide a theoretical basis for calculating the additional thermal resistance and additional attenuation multiple of sheet overhead shading.Comparing the outdoor comprehensive temperature calculation formulations with or without sheet overhead shading,it can be found that compared with the exposed roof,overhead shading sheet can not reduce the outdoor air temperature,but can greatly reduce the solar equivalent temperature.Secondly,this paper builts a roof overhead shading experimental device in the climate chamber,taking the flat roof metal sheet overhead shading as an example,and carried out related tests.The irradiance of the solar simulation lamp and the temperature uniformity of the concrete roof surface are produced and tested.Testing the heat prevention effect of roof shading with 4 different color sheet and 2 different wind speeds respectively.The irradiance of the solar simulation lamp and the temperature uniformity of the concrete roof surface tests show that the average irradiance of the solar simulator vertical irradiation is 645 W/m2,and the relative error of the solar irradiance and the concrete surface temperature average is less than4%,which are considered as good uniformities.The surface color of the four different thin plates and the cooling effect of the roof shading were tested under the condition of no wind speed.The solar irradiance illuminance and the concrete roof surface temperature uniformity test show that the average irradiance of the solar simulator vertical irradiation is 645 W/m2,and the relative error of the solar irradiance and the concrete surface temperature average is less than 4%.The experimental simulation results show that under the no wind condition,the upper surface temperature of the roof with sheet overhead shading is reduced by 11.9°C when there is no shading and the lower surface temperature of the roof with sheet overhead shading is reduced by7.1°C when there is no shading,which means that the sheet overhead shading can be obviously reducing the temperature of the upper and lower surfaces of the roof and helps to save energy in air-conditioned rooms.The test results of the different colors sheet overhead shading under the windless condition show that the higher the absorption coefficient of the sheet surface,the higher the temperature of the sheet.However,the temperature and amplitude of the upper and lower surfaces under different color sheet shadings are not much different,because the average irradiance radiated by the solar simulator is only 645 W/m2.Under different wind speeds with wind speed less than 2 m/s,the surface temperature of the upper and the lower surface of the roof with sheet overhead shading can be significantly reduced compared with the no-shading condition,but the average temperature and amplitude of the roof surface are not much different.Thirdly,the effectiveness of the sheet transient temperature calculation formulation and the outdoor comprehensive temperature calculation formulation established in this paper are verified,and the evaluation index and calculation method for the comprehensive evaluation of the thermal insulation performance of the sheet overhead shading are proposed.The verification results show that the relative error mean is less than 4%and the maximum relative error is less than 10%under different surface absorption coefficients and wind speeds,which indirectly verifies the validity of the comprehensive temperature calculation model for sheet overhead sheet roof.In addition,this paper uses additional thermal resistance to evaluate the effect of sheet overhead shading on the average value of outdoor comprehensive temperature.At the same time,it is creatively proposed to use the“additional attenuation multiple”to evaluate the attenuation effect of roof sheet overhead shading on the amplitude value of the outdoor comprehensive temperature.A calculation formula for additional thermal resistance and additional attenuation multiple are established and additional attenuation multiple are defined.Finally,this paper selects the climates of three cities in Fuzhou,Guangzhou and Haikou as the typical climate in the hot summer and warm winter regions.Using the outdoor comprehensive temperature calculation model,the thermal resistance and additional attenuation multiple of the sheet overhead shading on the full sunny day and the full cloudy day are calculated.It can judge whether the additional thermal resistance value of 0.3 m~2·K/W specified in the relevant standards is suitable or not for sheet overhead shading.At the same time,providing theoretical basis and reference for the application of value in these areas for additional attenuation multiple.The calculation results are as follows:the additional thermal resistance value and the additional attenuation multiple of the full sunny day/full cloudy day in Fuzhou area are 0.431/0.429 m~2·K/W,4.602/2.467 respectively;the additional thermal resistance value and the additional attenuation multiple of the full sunny day/full cloudy day in Guangzhou area are0.423/0.419 m~2·K/W,5.216/3.907 respectively;the additional thermal resistance value and the additional attenuation multiple of the full sunny day/full cloudy day in Haikou area are 0.557/0.679m~2·K/W,6.493/4.396 respectively.It can be concluded that compared with the lower calculated value of the additional thermal resistance,the additional thermal resistance of the roof shading specified in the specification is 0.3 m2·K/W,which is generally lower for sheet overhead shading in hot summer and warm winter areas,especially large error for the area with large solar radiation in summer;the additional attenuation multiple of roof sheet overhead shading is more than 1;The additional thermal resistance and the additional attenuation factor are primarily related to solar radiation and are also related to outdoor air temperature and wind speed.The greater the amount of solar radiation,the larger the additional thermal resistance and the additional attenuation factor;the higher the outdoor air temperature,the larger the additional thermal resistance and the additional attenuation factor;the higher the wind speed,the larger the additional thermal resistance and the additional attenuation factor.