Venetian blinds as a version of sun-shelves to be used in hot and sunny climate conditions

Light-shelves, as a mean to distribute sun-ray’s flows, can be considered as a minor parts of venetian blinds. Light shelves, acting as light- spreading devices, spread sun light rays, transformed into diffuse light flows into the depth of a promise. The same function possesplanks of venetian blinds, though they have a much less geometry sizes. The aim of the investigations presented is to determine the optimum size of these planks and distance between them in order to ensure a maximum natural lighting of interiors under different sort of lighting, which is associated with different climatic regions or with different seasons of a year. The proposed solutions allow reducing electric power consumption in buildings.

But at the same time, traditional LSDareenormously large in size, and this fact makes the use of these elements not convenient from the ergonomicall point of view.
Hence, the appropriate solution of this problem can be the use of a LSD not in form of single and wide light-shelf, but in form of a multi-shelves devices with much less width and increased number of horizontal planks, what makes them resembling a traditional form of venetian blinds. It is a theoretical suggestion of this article and the content of the work presented is going to prove the rightness of this assumption.

Theoretical background
In former scientific investigations a number of positive characteristics of "SPD" and "LSD" as regards to natural illumination of interiors or lowering overheating of promises due to extra sun radiation penetrated indoors were shown [10,11,16,17,18]. Moreover,in these works, the optimum position and sizes of the devices in question were determined. But, due to the novation of the problem considered, these conclusions cannot be considered as the final and complete ones. The new points to be studied are as follows: * Corresponding author: sergioni1947@gmail.com 1. The ergonomic aspect of LSD structures as regards their sizes and distance of projection indoors or outdoors 2. The lighting ability of LSD due to their reflecting properties; 3. The optimum spacing of a LSD planks which may makes them more similar to "venetian blinds", rather than a traditional "light shelves", which are usually big in sizes and few in number.
As a result of studies, which were conducted recently in the "Design of buildings and structures" Chair of Moscow State University of Civil Engineering, a new design offer was elaborated (see Figures 1 and 2).
The studies were not of universal character, as they took into account only SPD/LSD units, fixed in the window opening.( l.e. "Stationary units"), with southern aspect of orientation. Such a situation is most frequent for sun protection devices under clear sky conditions in regions with hot and sunny climate.
In these studies,that authors took into account the need not only to protect the interiors of building from uncomfortable overheating due to the excessive solar radiation, but also the need to supply the interiors with sufficient amount of anatural light. The first problem is to be solved with sun-protection and the second -with light reflection and spreading of light flows.
Hence, the combined functional use of newly-offered units in form of SPD/LSD is quite essential and efficient. Moreover, thestepped position a SPD/LSD planks, as shown in articles [4,7,8] proved low efficiency as regards to natural light supply into the interiors (see Figures 3 and 4)

Design studies
In the studies presented the examplesof a civil buildings in a southern climatic conditionare examined. The window of the building is of southern aspect of orientation, under which the use of horizontal outdoor SPD are traditional and widely used. We consider the suggested light shelves as a minor type of canopies (awnings). The main characteristics of climate conditions chosen are as follows: i/ -The sky is clear and sun rays influence is straight, i.e. the outdoor natural illumination is not diffuse; ii / -The southern regions, close to equator were not considered. Hence,under a low latitudes the role of windows in the process of insolation and sun radiation gains reduces greatly. The reasonable zone for SPD use is from 20 0 to 40 0 of north or south latitude; iii / -under thelatitudeschoosen, the ordinary sun height in summer period is from 60 0 to 75 0 .
Hence,according to sketch on Figure 1, the geometries of horizontal planking within the given window can be expresser as follows: (1) Where all the members of the formula are presented in key, referenced to figure 1. As one can see,this expression does not depend on the height or width of a window.
Assuming the "t" equal to (30 cm) and H 0 range from 60 0 to 75 0 , we can calculate the spacing of horizontal planks as follows: (2) The formula (1) should be better used with the known thickness of a wall, which dictates the width of the SPD/LSD planks. In real practice, the width of planks is usually from 50% to 100% the thickness of a wall. For the value of H 0 equal to 60 0 , ctg H 0 =0,58 and h 1 =

Conclusions
1. It is stated, that light-reflecting functions of light shelves (LSD) can be efficiently combined with their sun-protective functions. Hence, the best design solution in this case is designwindow set of horizontal planks, as an average solution between venetian blinds and sun-protective canopies (awnings).
2. It is shown, that reflective functions of the light-shelves (LSD) is mostly aimed on the increase of natural indoor illumination in furthermost design points in a premise to satisfy the normative requirements to daylight factor values.
3. To ensure the comfortable contact with the outdoor environment, the minimum spacing between SPD/LSD planks should not be less than the thickness of an external wall. From the other side, the maximum spacing should not exceed twice the thickness of a wall, to ensure proper sun -protective functions of the system.