J. Cent. South Univ. (2012) 19: 755-763 

DOI: 10.1007/s11771-012-1068-4

Primary stage of solar energy use in architecture–Shadow control

M. Tahbaz

School of Architecture and Urban Planning, Shahid Beheshti University, Evin 19834, Tehran, Iran

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: In architecture and urban design, solar energy plays an important role to save fossil energy and preserve environment. Passive or active use of this energy needs especial consideration in the preliminary stage of design to the probable shadows. This work will answer two questions: 1) How is it possible to distinguish shadow need periods during a year in a specific climate? 2) How is it possible to produce shadow in hot periods without being deprived of pleasant sunshine in cold periods? In this regard, it will introduce the usage process and opportunities of “shading mask” of Olgyay as an easy and useful graphical method in six steps. At the end, different ways of using this method in design will be discussed. Several research projects that have been done in past 30 years will support the argument as examples of this method.

Key words: shading mask; sun-path chart; shadow angle protractor; thermal comfort; climatic needs calendar

1 Introduction

In the 21st century, reducing fossil energy use in building sector is one of the most important considerations. Using renewable energy especially solar energy seems to be one of the most appropriate and less harmful answers to this requirement. There are two passive and active strategies for replacing fossil energy with solar energy in building. Passive strategies based on design include site plan, form, mass and space, openings, interior design and characteristics of the materials. Active strategies are based on energy transformation by using recent technologies such as BIPV (Building Integrated Photovoltaic). In both systems, there is one preliminary point that must become into special consideration: “Shadow”.

By shading the solar rays, shadow has the ability to reduce the temperature of environments. This ability will be welcome in hot periods while it is a disaster in cold conditions. It would be desired if the shadow being produced in hot periods does not deprive pleasant sunshine in cold times. By the way, BIPV systems will not reach their best efficiency without controlling the surrounding shadows. Providing appropriate sunshine or shadow in a whole time of a year needs enough knowledge about climate of the region, human thermal comfort conditions and geometry of the sun.

This work will introduce the easy and graphical method of “shading mask” of Olgyay, the way of its use and the wide range of its ability to help solar energy use in architecture and urban planning.

2 Methodology

The importance of making appropriate shadows is discussed and the way of using shading mask method is explained in six steps: 1) Taking the climatic data and analyzing it by using appropriate thermal comfort standards to draw the climatic needs calendar of the region; 2) Determining the important climatic periods according to the use of the constructed area for some activities; 3) Converting the information on the sun-path chart of the region; 4) Overlapping the shadow angle protractor on sun-path chart to determine the shadow need period and draw the shading mask pattern; 5) Designing the sunshade pattern according to shading mask pattern; 6) Designing different alternatives of sunshades according to several elements like aesthetic, material, strength, construction, economy, maintenance and other climatic elements such as wind speed, precipitation, freezing and chill factor.

2.1 Sky dome, sun-path chart and shading mask

Shadow is the dark area behind an opaque matter that obstacles the radiant heat and direct light from radiant source. It will be possible to produce shadow if the location of sun as the solar ray source is known and determined. Sky dome concept (Fig. 1) is a simple graphical method, innovated by Swedish architect GUNNER as a comprehensive series in 1954 [1] and has been used by several architects for different purposes.

Some solar overlays are prepared by sky dome concept which gives architects the opportunity to determine sun location and shadow casting conditions (Fig. 2). The method of “shading mask” proposed by OLGYAY in 1957 [2], uses sun-path chart and shadow angle protractor (Fig. 3) to achieve this aim. Using shading mask method, it is possible to design appropriate shadows for architectural areas in six steps.

Fig. 1 Sky dome concept by fish-eye lens [3]

Fig. 2 Sup-path on sky vault (a) and horizontal surface (b) [4]

Fig. 3 Vertical (a, b) and horizontal (c, d) lines on shadow angle protractor [5]

3 Six steps of using shading mask method

Six steps must be passed to design appropriate shadow in a design project or predict existing shadow in a constructed area in a period of a pattern year.

3.1 Step 1: Analyzing climatic conditions

Designing a responsive climate constructed area, it is necessary to analyze climatic data of the place that is gathered by meteorology station. Analyzing process is done in two parts. The first part is describing the data [6] based on the recognition that is provided by the data itself. The second part is illustrating the data according to human thermal comfort conditions. This part will be done by the help of one of the thermal indices which is provided for indoor or outdoor area. According to the subject of this article, the use of illustration here is to identify the periods of shadow or sunshine needs indoor or outdoor according to the activities of the occupants. For example, Givoni bioclimatic chart [7-8] is a good prediction tool to analyze thermal condition and climatic needs inside the building. For outdoor area, one of the proposed indices, such as UTCI (universal temperature climate index), the latest provided index for analyzing outdoor thermal condition [9], is appropriate. Figures 4 and 5 show the bioclimatic indoor and outdoor charts of Bam, one of the cities in central Iran.

Fig. 4 Indoor Givoni bioclimatic chart of Bam [10]

Fig. 5 Outdoor bioclimatic UTCI of Bam [10-11]

3.2 Step 2: Climatic needs calendar

Drawing thermal condition zones on “climatic needs calendar” [12] shows the needs for using indoor and outdoor areas in different periods of a pattern year: sunshine need, shadow need, inadequate sunshine or shadow, very cold critical conditions in winter and very hot conditions in summer [4].

“Climatic needs calendar” has two perpendicular axes for days and hours which covers all periods of a year, and makes it possible to show climatic conditions (Fig. 6).

Fig. 6 Outdoor UTCI climatic calendar of Bam [4]

“Climatic needs calendar” of hot climates indicates that one of the main needs to provide thermal comfort in warm periods is to produce appropriate shadows. Importance of shadow in each climate depends on the intensity of solar energy in that region. In hot climates of Iran, solar energy has the most intensity while in 2/3 to 3/4 of a year weather is hot and sunshine is unpleasant [13].

In cold climates, solar energy is needed to increase the ambient temperature. Critical periods in very cold conditions will appear when flowing of the winds cooperates with chill factor, harsh rain or snow. Such uncomfortable condition needs to be modified by direct solar energy by passive or active strategies (Fig. 7). In cold and moderate climates of Iran, high intensity of solar energy is provided in cold conditions [13] as a good opportunity. Therefore, undesirable shadows need to be controlled by passive design strategies.

Fig. 7 Outdoor UTCI climatic calendar of Esfahan [13-14]

The question here is that how is it possible to produce shadow in hot periods without being deprived of pleasant sunshine in cold season?

3.3 Step 3: Shadow zones on sun path chart

Transferring the information of climatic needs calendar regarding to sunshine and shadow needs periods on appropriate sun-path chart of the region is the next step of controlling shadows. This chart indicates the periods of a pattern year that shadow or sunshine is needed. Also, it will show the critical periods which are not appropriate because of intensity of cold or hot whether (Fig. 8).

Fig. 8 Sun path chart of Bam [4]

3.4 Step 4: Shading mask pattern

Overlapping shadow angle protractor on sun-path chart of the region and drawing shading mask pattern for the site location is the forth step. Sections of the site will help to find the angles of the visible sky from the case location. According to these angles, the shading mask of the obstructed elements is drawn to show the possible shadows on the site (Fig. 9).

Fig. 9 Shading mask of existing construction [4]

According to the shading mask of the existing elements, the proposed shading mask of the shadow need periods is drawn. This mask must be determined so that the cold periods are not deprived of sunshine while in warm periods appropriate shadows are provided. This mask is the optimum mask that will cover the warm periods without shading the cold periods. Sometime in design process, rotating the case for some few angles (20° toward south east in Fig. 10) will provide much better shading mask pattern.

3.5 Step 5: Sunshade pattern

Designing sunshade pattern according to the shading mask pattern is the fifth step. Sunshade pattern shows only the dimensions of the area that must be shaded by using opaque elements. It is not the main sunshade design. To determine sunshade pattern, it is necessary to draw plan, elevation and section of the case. Using the angles of the shading mask pattern, the shadow need area will be assigned (Fig. 11).

3.6 Step 6: Sunshade design

Designing several alternatives of sunshade according to other elements like aesthetic, material, strength, construction, maintenance, economy and other climatic elements such as wind speed, precipitation, freezing and wind chill factor is the sixth step. It is interesting that with a single sunshade pattern, several sunshades will be designed (Fig. 12). Analyzing benefits and weak points of these sunshades, the optimum alternative will be selected to construct (Fig. 13).

Fig. 10 Proposed shading mask pattern for south facing window (a) and south east window (b) [15]

Fig. 11 Sunshade pattern of design case [16]

Fig. 12 Several alternatives for sunshade design [2]

Fig. 13 Optimum choice of sunshade design [16]

4 Results and discussion

4.1 Advantages of using shading mask method

Designing sunshade by shading mask of OLGYAY has several advantages as follows.

1) It gives the ability to control shadow and sunshine in a period of a year. By giving the architect a full understanding of climate and sun conditions, he/she is able to design a sunshade just for the shadow need periods. So, he/she can avoid making unnecessary shades that imposes only extra expenses and deprived of useful sunshine in cold periods.

2) By using sunshade pattern, it is possible to design different alternative shades. It helps architect to design shadings with similar shadows and different abilities according to design elements, construction and so on. So, he/she has the possibility to choose the optimum alternative with better potential answers.

3) Shading mask method is based on drawing and geometry that will be learned easily. It is a familiar and understandable method for architects.

4) There is an ability to convert the information between sun-path chart and climatic needs calendar with same temporal coordinates (dependent on time). So, it is possible to evaluate and control the sunshade efficiency by other climatic elements such as pleasant and unpleasant wind flows, precipitation, freezing periods and critical hot or cold periods, and make decisions according to all effective climatic elements.

4.2 Usage of shading mask in decision making

Knowledge of controlling shadows by shading mask method will give a wide view of shaded and sunlit surfaces for a whole period of a pattern year. Combination of thermal needs periods and shading probability will give the designer the ability to decide about geometry, shape, density and dimensions of the architecture design from the preliminary stages of the concept making for the latest stages of detail constructions.

For example, in the procedure of a settlement design, making decision about the orientation and density of the site will be made according to the dimension and orientation of the buildings (Fig. 14). Inappropriate shadows will deprive the back building from benefits of solar energy in cold periods (Fig. 15).

In cold climates, there are snow and freezing in some parts of the year. Therefore, providing enough sunlit surfaces and areas will help to increase the sol-air temperature and accelerate the melting process on the microclimate level. In these conditions, undesirable shadows will cause icing and lasting cold weather for a longer period (Fig. 16).

Another important consideration in cold climate is the cold trap. A cold trap is close to the surface layer of the atmosphere that is substantially colder than both the deeper and higher layers and will decrease the temperature and cause cold weather lasting more than normal period. For a designer, it is very important to distinguish the cold trap probability in the special places of a site. Mostly, cold traps will appear in the places that are surrounded by a deep and lasting shadow in a whole time of a cold period. Blowing of chill winds in this condition will accelerate the formation of cold trap. This phenomenon will simply appear in the west/east narrow passages or deep courtyards with tall buildings in the south side (Fig. 17). Settlement layout with south-north/ west-east orientation in moderate and cold climates has a great potential for cold trap occurring.

Fig. 14 Density of a site layout according to orientation [15]

Fig. 15 Inappropriate density and orientation of a site layout with unpleasant shadows [16]

Fig. 16 Undesirable shadows and lasting of ice [16]

Fig. 17 Cold trap in west-east narrow passages and  court yards [15]

In design process, the designer must be aware of this phenomenon and prevent to locate the main entrances in such places. Neighboring life spaces to the cold trap is another design consideration. Arrangement of the indoor spaces according to their orientation and outdoor site plan is very important to prevent any losing energy and low temperature caused by undesirable shadows or cold traps. Form and shape of the buildings is another important consideration. The designer must avoid creating unnecessary gaps toward the potential cold traps (Fig. 18).

In BIPV system, the PV arrays are considered as a part of design elements. They are used in different parts of the building such as roof, facades, windows, and atrium surfaces. Controlling the potential shadows     in all selected places is one of the responsibilities of the designer. It is not possible to locate the PV arrays in shaded places. Predicting the shadows in a period of a year according to the visible sky from the design case is one of the perfect abilities of shading mask method.

Fig. 18 Cold trap in north fa?ade’s gaps of a building [16]

Using PV as a window shade is one of the BIPV requirements. PV shades must be located on the window with the possibility of providing desirable shadow in

warm periods and do not deprive sunshine in cold periods. Designing the shading mask pattern of the window will help to decide about the size, shape and place of the PV shades. For example, shading mask pattern of a south face window shows that the needed shade is a horizontal plate on the top of the window  (Fig. 19), while for a north face window it is a vertical opaque element in one or both sides of the window  (Fig. 20) and for an east or west window it is a vertical opaque element in front of the window (Fig. 13).

Designing outdoor shades is another ability of shading mask method. Using the shading mask pattern will give different alternatives for sunshade design in open spaces for roof tops and pedestrian passages    (Fig. 21).

Fig. 19 Sunshade pattern and one example shade of a south face window [16]

Fig. 20 Sunshade pattern and one example shade of a north face window [16]

Fig. 21 Designing cloth shade for open spaces [4]

Some software programs such as Ecotect are available to help the designer to create the shading masks for the design project (Fig. 22). Like all other softwares, getting correct answers from this software needs enough knowledge and understanding about the shading mask design process. Only in this condition, software will be a great opportunity to speed up the process of shade design.

Fig. 22 Shading mask method in Ecotect software [16]

5 Conclusions

The importance of making appropriate shadows for using solar energy as a renewable source replacement for fossil energy is discussed. It explained the way of using shading mask method of OLGYAY in six steps and showed how it make possible to design different alternatives of sunshades according to several design relevant such as aesthetic, material, strength, construction, economy, maintenance and climatic elements such as wind speed, precipitation, freezing, and chill wind.

The advantages of this method was discussed as the ability to control sunshine and shadow in a period of a year, including: 1) the ability to design several alternative shades using the same shading mask and sunshade pattern, 2) simplicity of this method because of its base on geometrical knowledge, 3) the ability of generalizing the results to climatic needs calendar and 4) evaluating the sunshade efficiency according to other climatic elements.

The usage of shading mask method for making design decisions was discussed including: 1) decide geometry, shape, density and dimensions of the design,  2) predict potential shadows in a pattern year and control undesirable shadow or sunshine in cold and hot periods, 3) provide sunlit surfaces and prevent cold traps occurring in cold climates, 4) provide appropriate design decisions for using BIPV systems, and 5) design sunshades for windows as the most popular use of shading mask that will be used both in passive and active systems.

There are some software provided to shortcut the decision making by producing needed graphs. Ecotect is one of these softwares but it will not give reliable output except the designer has a good understanding and knowledge of the shading mask method procedure and be able to analyze the output.

References

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(Edited by YANG Bing)

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: M. Tahbaz, Assistant Professor, PhD; Tel: +98-912-3114923; E-mail: shtahbaz@yahoo.com