Numerical study on specular solar reflectors aimed at increasing solar reflectivity of building envelope

Techniques that improve solar reflectivity of urban surfaces to mitigate urban heat island phenomenon have become widespread with the use of highly- reflective paints (Kondo, 2008). Since these reflective paints have a higher proportion of diffuse reflection, there is an anxiety that if used on the sides of buildings, much of the reflected solar radiation is scattered toward the ground and surrounding buildings. This becomes even more of a problem when used in narrow street canyons. Therefore, we aim to develop reflectors with the ability to direct the reflected solar radiation back to the sky.  Retroreflective surfaces are widely used in order to increase visibility of road signs at night (Woltman, 1982), because they reflect car headlights back in the direction from which they came. However, if the aim is to use retroreflectors on building envelopes for solar radiation reflection, the diurnal apparent motion of the sun makes it difficult to reflect efficiently. There was little knowledge or past research about such reflective performance, therefore we examine it in this paper.  In this research, we use corner cube retroreflectors consisting of four pieces of aluminum reflector panel with a glossy surface. At first, the reflective performance of the retroreflector consisting of ideal mirrors are evaluated by numerical simulation. Then, more realistic reflectors consists of aluminum mirrors are evaluated. The reflectivity of the mirror plate is measured by spectrophotometer and the bidirectional reflectance distribution is measured by goniophotometer. These radiant properties were used in the numerical simulation using a Monte Carlo path tracing method.