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Theory of shading

 Solar radiation is electromagnetic wave motion with a wave length spectrum, which before its entry to the earths atmosphere resembles a spectrum of an ideal black body with the surface temperature of app. 6000 K. The sun spectrum is usually divided into three main parts:

 

 
Ultraviolet solar radiation with wave lengths shorter than 390 nm, which before its entry to the earths atmosphere constitutes app. 4% of the solar radiation energy and is for the most part absorbed by the atmospheric ozone in the stratosphere,

  Visible solar radiation with wave lengths from 390 nm to 760 nm creating a colour spectrum from vilolet to red. It constitutes app. 45% of the solar radiation energy before the entry to the earths atmosphere,

 

 

 Infra-red solar radiationwith wave lengths longer than 760 nm constitutes before its entry to the atmophere app. 51% of the the solar radiation energy flow.

 

 

 This averall solar radiation is at fall to the (window) pane divided in dependence on technical parameters into the following:

         

 

• Transmission radiation - radiation which is let into the interior through a window, the coefficient of radiation transmission takes the value of 0 to 100% or 0 to 1
• Reflected solar radiation - radiation which is reflected back from the window into the outside area, the coefficient of reflected solar radiation takes the value of 0 to 100% or 0 to 1
• Absorbed radiation - radiation which is absorbed by the window and so increases its temperature, the coefficient of reflected solar radiation takes the value of 0 to 100% or 0 to 1

 

this equation is then always valid as a result

Into the relations for calculation of overall coefficient of solar radiation
energy "g" penetration, another two factors come into play:

q_a - factor of the secondary heat transfer into the exterior
q_i - factor of the secondary heat transfer into the interior

 

 

 

For the above stated case would apply that out of the 100% of fallen solar radiation 58% gets into the interior.
The overall factor of solar radiation energy penetration without shade is then calculated according to equation

 

A sample case of a window without a shade is shown in the picture.

 

 

The value of the g factor for theoretical calculation is mostly given by the window pane manufacturer or window manufacturer and is labeled as SF - The overall factor of solar radiation energy penetration.

If we use a sun shade, the factor of solar radiation energy penetration is called g_total. A sample case of a window with an exterior sun shade is shown in the picture - facade shading.

 

If we want to determine the difference between the factor of solar radiation energy penetration without a shade - g and the factor of solar radiation energy penetration of a window fitted with a shade _total, we need to know the size of the so-called reduction coefficient F_c, which is defined according to DIN 4180. The value of this coefficient can range between 0 (the theoretically best sun protection) and 1 (no protection, in this case g = g)_total). The smaller the coefficient F is,_c the more effective is solar radiation protection, energy penetration is smaller and air conditioning costs ar lower

 

 

The quotient of factors g and g_total is defined as the value F_c, or solar radiation protection reduction coefficient .

 

The overall factor of solar energy penetration with sun shading g _total is then calculated according to the equation

 

Determining the exact value of the coefficient F _c is very difficult because the effectiveness of sun protection depends on many factors. The most influential factors are, for example, the radiation transmission coefficient of the shade itself (depending on the used material), colour finish of the shade etc.

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1. The effectiveness of the shade depends on location

a shade placed between window wings is 3 -4 times more effective than a shade placed in the interior

 

 

a shade placed in the exterior is 7- 10 times more effective than a shade placed in the interior

 

 

Sample values of reduction coefficient F_c for individual shade positions

facade shading features, facade shades, outside roller blinds and Venetian blinds Fc = 0,09 - 0,20

Inter-glaze Venetian blinds have the value Fc = 0,20 - 0,30

Interior Venetian blnds, inside roller blinds have the value Fc = 0,30 - 0,60

 

2. Effectiveness of the shade in dependence on solar radiation ventilation

 

facade shade without a box, very good ventilation value Fc = 0,10	facade shade with a closed box, bad ventilation value Fc = 0,20
inside shade, good ventilation, value Fc = 0,42	inside shade, bad ventilation, value Fc = 0,32

 

3. Effectiveness of the shade in dependence on position in the exterior

 

    

The further is the shade in the exterior from the opening filling, the lower is the value of reduction coefficient F. _c This means that the value of the overall sun energy penetration coefficient is also getting smaller. In other words, the effectiveness of the sun shade proper is getting better.


Example:

The sun shines on a 1m large window². The sun rays have the energy of 600 W/m². The overall sun energy penetration coefficient of a window without a shade has the value of g = 0.58. How much of the solar energy gets into the interior
a)through a window without a shade?
b)through a window with a shade placed in the interior?
c)through a window with a shade placed in the exterior (facade shading)?

a) energy getting into the room = 600 W/m² x 1m² x g = 600 × 1 × 0,58 = 348W
b) energy getting into the room = 600 W/m² x 1m² x _gtotal = 600 × 1 x g x F_c = 600 x 1 x 0,58 x 0,42 = 146,2W
c) energy getting into the room = 600 W/m² x 1m² x _gtotal = 600 × 1 x g x F_c = 600 x 1 x 0,58 x 0,1 = 34,8W

For maximal reduction of heat gains through direct solar radiation, the following rules should be observed:

1.In case of using facade shading, it is advisable to use lighter shade colours becuase they have smaller absorption levels than darker shades. Through this smaller absorption, smaller heat radiation into the area of the shade is ensured and subsequently, the resulting heat transfer into the interior is also limited.
2. Keep the windows open only at times when the outside temperature is lower than the inside temperature.
3. If it is necessary to ventilate at a high outside temperature, ventilate quickly and intensively.
4. When regulating solar radiation, make sure that natural daylighting is preserved

In conclusion

Best effectiveness from the range of sun shades have facade shading features, which prevent solar radiation from coming in through the window to the interior. There is then no heating of individual inside objects which could subsequently cause temperature increase in the interior. Little effective is for this reason the use of interior shading (Venetian blinds, roller blinds, pleating), which absorb solar radiation that got through the window and as a result themselves heat the interior. What is more, in the case of natural ventilation of the space between shade and opening filling, the facade shading features are even more effective. A substantially better temperature comfort in the room is the result.