Assessing the Daylighting Potential in a Renovation Proposal for the Former Acropolis Museum in Athens, Greece

A new algorithm to predict the light transmission efficiency of cylindrical vertical sun ducts has been developed and is presented. The method is based on a simple ray-tracing model, allowing to follow, hour by hour, the path of the solar radiation through the reflections it encounters in its travel down the duct. To that purpose, a solar radiation data-base is generated, providing hourly values of direct and diffuse radiation on horizontal surface, as well as the solar altitude. The sun duct head area is then divided into a limited number (20) of sub-areas, each receiving the corresponding amount of incident solar radiation, direct and diffuse. The direct rays are followed in their spiral-like path, by generating the equations describing the lines incident and reflected at each “impact”. The first line is assumed to enter the cylinder at the center of the head sub-area (point 0) with a vertical slope equal to the solar altitude. The intersection of this line with the cylinder is thus analytically found (point 1), together with the angle of incidence with respect to the normal, the local normal being directed as the local radius. A second line from this point is then generated, with the same vertical slope, but directed as the specular reflection line. The second point of impact (point 2) is thus found, and so on. In fact, the model is based on projecting the light path on the horizontal plane and finding the horizontal distances, d, between two subsequent impact points. Then, the vertical displacements, z, are found, knowing the vertical slope, i.e., the solar altitude. The number of impact points, that is of reflections, is found by adding the vertical displacements until the summation becomes greater than the duct height. A similar procedure is followed to evaluate the transmission of the diffuse component under a uniform sky assumption. The sky dome has been divided into nine portions, defined by 10 degrees altitude intervals (0-10; 10-20; etc.). The incidence of the radiation on the duct head has been taken considering an altitude equal to the average value in each interval. Thanks to the uniform sky assumption, the azimuth angles do not play any role. The summation of all contributions, direct and diffuse, coming out of the duct is the total radiation transmitted by the duct. The radiation is converted to luminous flux assuming a luminous efficacy equal to 120 lm/W A series of graphs, showing the light transmission efficiency, that is, the ratio of the total flux emerging at the bottom of the duct to the total flux incident on its head, are presented, taking as parameters the reflectance in the solar spectrum of the duct wall, and the ratio L/D, between the length of the duct and its diameter. The method has been used to assess the daylighting potential associated with providing sun ducts to a case-study building. The building is the former Acropolis Museum in Athens, Greece. The new Museum, built at the foot of the Acropolis hill has left the old building in search of a new destination. The proposed new function of this one-story building, beautifully located at the top of the hill, next to the Parthenon and to the other monuments, is a visitors center, with a library, reading halls, lecture and conference room, offices, services, etc. The paper presents the architectural project of the new building, which foresees, in addition to the existing windows, a number of vertical sun ducts providing natural light to the areas located in the inner core of the building, far from the windows. The flat roof, occupied by air conditioning units in a very limited way, can easily accommodate such components. The paper shows the amount of natural light obtainable through these devices, as a function of the hour of the day, and of the day of the year, based on the available monthly average daily solar radiation on horizontal surfaces data in Athens