| The thermal performance of outer opaque envelope of the building,an important factor affecting building energy consumption,affected not only by the thickness of thermal insulation layer and the thermal conductivity of thermal insulation material,but also by the optical radiation characteristics of the exterior surface.There is a great energy demand for space cooling in hot summer and warm winter zones.Under the background of carbon neutrality,it is urgent to further improve the thermal insulation performance of outer opaque envelope to reduce the cooling consumption of building.The high solar reflectance of reflective/radiative coating can reflect most of the solar radiation that irradiates to the envelope and the high long-wave emissivity can dissipate the heat of the envelope to outer space in the form of long-wave electromagnetic wave through the atmospheric window.The excellent passive cooling performance can effectively reduce the heat gain of the building and reduce the cooling consumption of the building.Therefore,this paper selects white reflective/radiative coating and analyzes its influence on building thermal insulation performance and energy consumption through on-site tests based on Guangzhou,a typical city in hot summer and warm winter zones.Through analyzing the mechanism of roof heat transfer,the reflective/radiative roof heat transfer model considering the spectral emissivity properties of coatings and atmospheric transmittance is established,and the influence of solar reflectance and long-wave emissivity on roof heat transfer is studied.First,the optical radiation performance of the selected white reflective/radiative coating was tested before and after natural aging.The results show that after 10 months of natural exposure,the solar reflectance of the reflective/radiative coatings on the roof decreases from 0.846 to 0.552,for wall coatings,the solar reflectance decreases from 0.846 to 0.779,the long-wave emissivity of the coatings has no obvious change.Then,the heat insulation and energy saving effect of the aging coating was tested.Under no air-conditioning conditions in summer,the average daily peak indoor air temperature of the painted room is reduced by 2.2℃compared with traditional rooms.Under the all-day air-conditioning condition in summer,the air-conditioning power consumption of the painted room is reduced by 2.46kW·h/d,the electricity saving rate is 39.1%.Secondly,a reflective/radiative roof heat transfer model considering atmospheric transmittance and the spectral emissivity properties of coatings is established by combining one-dimensional unsteady heat transfer and radiation integral methods.Comparison of experimental and modeling data shows that the average relative error of the rooftop temperature is 3.39%,while the average relative error of the ceiling temperature is 2.16%,which verifies the reliability of the roof heat transfer model.Finally,the heat insulation and energy saving effects of reflective/radiative coatings on typical roofs in hot summer and warm winter areas are studied by using the roof heat transfer model.Compared with traditional finishes,the cooling load of the roof with unaged and aged white reflective/radiative coating is reduced by 77.0kW·h/m~2 and 39.3kW·h/m~2,respectively,and the cooling load of the roof with ideal broadband coating and ideal selective coating is reduced by 97.5kW·h/m~2 and 101.2kW·h/m~2,respectively.Then,the effects of the optical parameters and the thickness of the insulation layer on the energy saving performance of the roof were studied.The results show that the cooling load of the roof decreases with the increase of the solar reflectance and long-wave emissivity.When the solar reflectance and long-wave emissivity are both 0.3,the cooling load of the typical roof is 119.94kW·h/m~2,when the solar reflectance and long-wave emissivity are both 0.9,the cooling load of the typical roof is 9.00kW·h/m~2.Sensitivity analysis shows that among the factors affecting the cooling load of roof,the strength of the factors in order are as follows:solar reflectance,long-wave emissivity,insulation layer thickness.For high reflective/radiative roof,increasing the thickness of insulation layer has no obvious effect on reducing the cooling load of roof.In this paper,the influence of reflective/radiative coating on insulation property of outer envelope and building energy consumption was tested,and the heat transfer process of reflective/radiative roof is revealed,which provides theoretical basis and data support for optimal design of roof.It is of great significance to promote the application and development of reflective/radiative coating in hot summer and warm winter zones and promote building energy conservation. |
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