Nowadays, energy conservation in buildings is especially important for countries where high energy consumption in the residential and tertiary sectors causes the high energy intensity of the whole economy. This paper presents some idea of creating energy efficient building envelopes. The focus is on reduction of heat transfer through it. Different structures of external walls are analyzed. In high latitude countries walls should be characterized by high thermal resistance and high thermal capacity (good storage ability). In the paper traditional multilayer wall structures with specific paints on external wall surfaces are analyzed. Paints are characterized by different solar absorptivity. Two extreme cases of solar absorptivity of external wall paints (0.9 for the black paint and 0.1 for the black paint) have been taken into account to give completely different ability of the external wall surface for photo-thermal conversion (of solar radiation). Results show that in moderate or cold climates, if thick thermal insulation is applied as external layer of a building envelope, then the effect of solar absorptivity of a cover paint of external wall surface is not meaningful. In July, when daily solar irradiation is the highest, in the case of the black paint all day long there are small heat gains (about 2kJm−2 in average) with maximum in the night (3.1kJm−2). In the case of the white paint all day long there are small heat losses (about 2.3kJm−2 in average), with maximum in the middle of a day (2.7kJm−2). The effect of the solar absorptivity of the paint, that covers the external surface of the wall, can be seen, but it is very small and perhaps cannot be noticed by inhabitants. However, in the extreme summer weather conditions, i.e. very high irradiance level and high ambient air temperature, what have happened quite often in high latitude countries recently, the considered effect is more strong. Therefore, it would be good to apply envelope of a building, that can change “the skin” from summer to winter. Also modern structures with the Phase Change Materials (PCMs) are analyzed. In simulation study it was assumed that PCMs are applied in the form of light thermal mass panels that are integrated with the external wall structure. A mathematical model of dynamics of a building has been developed. In the paper stress has been put on energy transfer through walls of different structures. The model takes into account the influence of solar radiation on energy balance, including the availability of solar radiation on walls of different location (inclination and orientation). On the basis of the model developed simulation studies have been performed. Selected results of thermal behavior of some multilayer structures of external walls of a building have been analyzed. Through comparative studies some recommendations for creating building wall structures have been formulated. Results (presented in figures) show, how the temperature of a wall surface can increase because of the high solar radiation absorptivity of the surface paint. The temperature is especially high when a thick insulation is used. Because of the insulation the heat transfer through the wall is reduced significantly. If there is no insulation than the heat gained can be conducted (relatively easily) through the wall to the interior of a building, in consequence there are high heat gains in summer, and the temperature of the external wall surface is not high. It is evident that external surface temperature is not the basic parameter that determines the real heat transfer through a wall. Results show that in high latitude countries a wall structure for a standard building (used all year around) must contain thermal insulation as external layer. This is also true when the PCMs are applied in the form of light thermal mass panels. For high latitude countries the recommended structure of an external wall is following: thermal insulation (e.g. mineral wool) at outside (external layer) of thickness of 25cm and PCM thermal mass panel at inside (internal layer) of thickness of 1cm. Such wall structure assures full reduction of the ambient surrounding influence. An external wall made of a brick and a PCM panel does not have high enough thermal resistance (even if thermal capacity is high) to keep the room temperature at the required level. However, such structure could be recommended for the summer season, e.g. for summer houses in a moderate climate or high latitude countries, or for warm climate.
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Dec 1, 2017
M Faraji 
Open Access
