1 . Introduction
The concept of the adaptive façade refers to a building that is designed to respond to outdoor environmental conditions by adjusting the position of a shade installed on its envelope . One building element widely used to form an adaptive façade is an exterior movable shading device ( EMSD ). Such a device can control the solar radiation entering the façade of a building . The total clear‐sky irradiance that reaches the receiving surface is the sum of three components : the beam component from the solar disc ; diffuse component from the sky dome ; and ground‐reflected component from the ground in front of the surface [ 1 ]. Many studies have been carried out to determine the amount of solar heat gain entering a window with various shading devices installed [ 2 – 6 ], but methodologies for determining the amount of diffuse solar heat gain has been limited to screens such as roll and Venetian blinds and fixed wall components such as overhangs and vertical fins .
Sky diffuse solar radiation enters the normal plane of windows from the opposite hemisphere . To determine diffuse solar heat gain , calculation of the view factor ( VF ) between the surface and the sky is necessary . The VF can be calculated with the geometry of the receiving window plane and the position of the shading devices . An adaptive façade has shading elements of various shapes and movements . Therefore , the change in the VFs , based on the movable shading devices , is important for calculating the sky diffuse solar heat gain through the windows .
This study focused on calculating the VF for various types of movable shading devices . To calculate the diffuse solar heat gain that passed through an adaptive façade , the exposure coefficient ( CC � ) and the VF were proposed as major shading factors . The solid angle method was adopted to calculate the unshaded VF at one point on the window . Then , the average VF was calculated by averaging solid angles at multiple points ( n ) spaced at equal intervals on the exposed glazing area . After this , the two‐directional folding shade was evaluated using the proposed method . The result of the application showed that combining the CC � and VF can reflect the step‐by‐step movement of the dynamic shade and allow calculation of the hourly diffuse solar heat gain through an adaptive façade .
2 . Method of diffuse radiation heat gain
2.1 . Shading factors reflecting diffuse component
Sky diffuse solar radiation enters windows from the opposite side of a hemisphere . When surfaces are not shaded , the glazing solar energy flux ( qq ��� ) caused by diffuse sky radiation ( EE � ) is only affected by the window ’ s diffuse solar heat gain coefficient (〈 SSSSSSSS 〉 � ) ( See Eq . ( 1 )).
qq ��� �EE � 〈 SSSSSSSS 〉 � ( 1 )
An adaptive façade can control the amount of solar radiation entering the windows effectively by adjusting its shading state . To evaluate the reduction of diffuse incident radiation reaching the receiving surfaces , an additional shading factor must be determined to account for the effect of the EMSDs on the sky‐diffuse radiation .
Lee et al . [ 7 ] suggested a shading factor , defined as exposure coefficient ( CC � ) in their study , that distinguishes the exposed and unexposed areas of a window to determine the attenuation of diffuse solar radiation heat gain . The exposure coefficient represents the ratio of the exposed glazing area to the whole glazing area ( see Fig . 1 ( a )), a value between zero and one . The exposure coefficient can reflect the change in the receiving glazing area , based on the movement of the shading device , in the process for calculating diffuse solar heat gain . The exposure coefficient reflects diffuse solar attenuation according to the movement of the shade parallel to the façade . In this study , the VF was applied to reflect the diffuse solar attenuation according to the shade movement normal for the façade ( See Fig . 1 ( b )). The VF represents the fraction of the total radiation emanating from a surface in all possible hemispheric directions , across all possible wavelengths [ 8 ].
Calculation Method for Evaluating Diffuse Solar Heat Gains Through Adaptive Façades 84