0 & middot; overview
Solar radiation incident on the surface of non-transparent building envelope is mainly absorbed and reflected, and its absorptivity-rho; and reflectance-gamma; the relationship is: & rho; + & gamma; = 1. According to the calculation formula of outdoor air comprehensive temperature:
It can be seen that the main measures to reduce the thermal insulation performance of the external walls and roofs are: (1) improving the thermal performance and reducing the temperature difference heat transfer into the indoor; (2) improving the reflectivity of the external surface, reducing the temperature difference between the indoor and the outdoor, and reducing the heat transferred into the indoor; (3) improving the emissivity of the external surface, reducing the temperature difference between the indoor and outdoor, and reducing the incoming heat. Indoor heat; (4) Increase the heat capacity of the wall and reduce the heat transferred into the room.
At present, there are three types of thermal insulation coatings on the market according to their principles: thermal insulation coatings, reflective thermal insulation coatings and emission thermal insulation coatings. Reflective heat insulation coatings are functional coatings mainly reflecting solar heat radiation, mainly by means of high reflectivity of coatings to reduce the solar heat gain of the outer envelope structure, so as to reduce the temperature of the outer surface to achieve the purpose of heat insulation. In this paper, the thermal insulation properties of reflective thermal insulation coatings are tested. The reflectance spectra of reflective heat insulation coatings and common coatings used in the test are shown in Fig. 1.
1-middot; test method
According to the action mechanism of thermal insulation coatings, the effective heat transfer coefficient [1-2] of thermal insulation coatings with the same color and common coatings on different base walls was measured by outdoor thermal insulation test bench, and the corrected values of effective heat transfer coefficient of thermal insulation coatings were obtained. The specific test methods are as follows:
The thermal insulation test bench consists of a constant temperature protective hot box with two identical comparing test boxes (length & times; width & times; height = 2000mm & times; 2000mm & times; 2400mm), as shown in figs. 2 and 3. The constant temperature protection hot box during the test can ensure that the temperature of the other five surfaces of the test box is uniform and constant except the solar radiation surface. In order to test the thermal insulation performance of specimens with different orientations, the whole test bench can rotate freely and 360 & deg.
The test of thermal insulation performance is directly related to the outdoor environmental parameters (solar radiation, outdoor dry bulb temperature, relative humidity, wind speed and direction, etc.). The system can collect and store the total solar radiation of Outdoor Horizontal plane, test the total solar radiation of the vertical plane of the box, the solar scattering radiation of the horizontal plane, the outdoor dry bulb temperature, relative humidity, wind speed and wind direction and other environmental parameters hourly. The indoor environment test system collects the heat flux of the inner surface of the test box, the temperature of the inner surface of the test box, the temperature in the test box, the radiation and the refrigeration capacity of the air conditioner.
Under sunny weather conditions, the common coatings and heat insulation coatings brushed on two kinds of base wall surfaces were tested. The experimental steps are as follows:
(1) Thermal insulation specimens are made of the following two materials: brick wall structure, 20 m m plastering mortar + 120 mm hollow clay brick + 20 mm plastering mortar, heat transfer coefficient is 2.49W (/m· 2K); thermal insulation wall, 30 mm EPS external insulation system + 20 mm plastering mortar + 120 mm hollow clay brick + 20 mm plastering mortar, heat transfer coefficient is 0.87W.（ / m2 & middot; K).
(2) The same color thermal insulation coatings and common coatings are applied on the test and reference surfaces of two different base wall materials.
(3) Natural convection was used to control the air temperature in the reference box and the test box, and the test data such as the inner surface temperature, the outer surface temperature, the heat flux, the total radiation of the test surface and the indoor and outdoor air temperature were recorded continuously.
2-middot; test results and discussion
2.1 Brick Wall Structure
Fig. 4 is to test the solar radiation intensity of the vertical wall of the brick wall structure. Fig. 5 and Fig. 6 are the heat transfer capacity and the inner and outer surface temperature of the brick wall with common paint and heat insulation paint, respectively.
As can be seen from figs. 4, 5 and 6, when the maximum outdoor solar radiation intensity is 700W/m2, 500W/m2 and 300W/m2 respectively, the maximum temperature difference of the inner and outer surfaces of the walls with ordinary coatings is about 15, 8 and 5 degrees, while that of the walls with thermal insulation coatings is about 8, 5 and 3 degrees, respectively. This shows that the temperature of the outer surface of the wall decreases obviously, while the temperature of the inner surface decreases, resulting in different degrees of temperature difference between the inner and outer surface of the wall, and consequently the heat transferred into the room decreases accordingly. The heat transfer of the wall decreases by about 40% in sunny days, but hardly in sunless days.
2.2 Insulation Wall
Fig. 7 shows the solar radiation intensity of the vertical wall of the daily insulation wall. Fig. 8 and Fig. 9 show the heat transfer capacity and the inner and outer surface temperature of the ordinary and thermal insulation wall respectively.
It can be seen from figs. 7, 8 and 9 that when the maximum outdoor solar radiation intensity is 300 W/m2, the outer surface of thermal insulation coatings is 5-8 C lower than that of common coatings, and the inner surface is 1-2 C lower. This shows that the temperature difference between the inner and outer surface of the wall decreases to varying degrees, and the heat transferred into the room decreases accordingly. The cumulative heat transfer of thermal insulation wall decreases by about 20% in sunny days after using thermal insulation coating, while it hardly affects in sunless days.
According to formula (1), the correction coefficient of heat transfer coefficient of heat insulation coatings in different base wall surfaces is calculated, which is relative to that of common coatings.
According to the experimental data, the effective heat transfer coefficients (actual heat transfer coefficients) of the coatings are calculated under the actual weather conditions, so that the correction coefficients of the effective heat transfer coefficients can be obtained (see Table 1).
3 & middot; conclusion
Thermal insulation coatings mainly change the thermal performance of the envelope structure by changing the radiation heat transfer on the outer surface, so as to reduce the energy consumption of air conditioning in buildings. The quantification of energy-saving effect of thermal insulation coatings is complex, which is directly related to the local climate, the condition of the base wall and the solar reflectivity of the coatings. In this paper, a simple method for evaluating the effective heat transfer coefficients of thermal insulation coatings is proposed by selecting typical days for outdoor test. The correction coefficients of effective heat transfer coefficients of thermal insulation coatings are calculated by testing the effective heat transfer coefficients. Thermal insulation coatings and ordinary coatings are painted on brick walls respectively.Experiments were carried out on brick walls with external insulation, and the corresponding correction coefficients were obtained, which reflected the energy-saving effect of thermal insulation coatings applied on two kinds of base walls. The use of reflective thermal insulation coatings has a certain impact on building energy consumption in Shanghai, which varies with the type of building. For residential buildings, considering the different use modes of air-conditioning, the building can save about 2% (air-conditioning) to 5% (air-conditioning only in summer) by using thermal insulation paint on its walls.