Horizontal and vertical shading devices  

 

Where is: h, D - geometry of horizontal shading device (see pictures above), α - sun height, Φ - solar azimuth, Ψ - plane azimuth 

Where is: w - geometry of vertical shading device (see pictures above), Φ - solar azimuth, Ψ - plane azimuth 

  Where is: α - sun height, Φ - solar azimuth, Ψ - plane azimuth

Understanding the  motion of the Sun is essential for proper solar systems design and the choice of  proper location for solar collectors or photovoltaic modules. The solar path is  described with the Sun path  diagram. The Sun path diagram is a very useful tool  in the early photovoltaic system design phase for shadowing determining. The  most important geometrical parameters, which describe Earth-Sun relations,  include declination /δ/, Sun height /α/ and Solar azimuth /φ/. They are defined  as follows:

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h = ±15o· ( number of hours from/to solar noon ) 
 

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Sun path diagram, created with SunOrb 1.2 
 

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Solar azimuth 
 

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Sun height 

According to the equations above we can estimate the most proper photovoltaic modules tilt employing the following equation:
 

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Declination 
 

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Tilt angle for different latitudes 

Declination changes throughout the year, therefore, the optimal tilt angle changes as well. The optimal tilt angle of 46° N latitude can be seen in the picture above. 
 

The proper tilt and azimuth angle choice is by far more important for photovoltaic systems design than solar thermal system design. Manual or automatic tilt angle adjustment can increase the total light-electricity conversion up to 30 % and more in locations with high values of solar radiation. Incidence angle should be as close to 90° as possible. Photovoltaic module tilt angle and location choice in general require more care than solar collectors tilt angle and location choice. Shaded locations, including partially shaded, are not suitable for photovoltaic module fixation. Modules should be south oriented. The following general recommendations should be considered, if you design a photovoltaic system:

Yearly average maximum output power - the photovoltaic modules tilt angle should equal local latitude.

Maximum output power in winter - the photovoltaic modules tilt angle should equal local latitude + 15° (max +20°). Such a tilt angle is a good solution in areas, where the winter load is greater than the summer load. The electricity consumption for lighting is greater during winter than summer.

Manual photovoltaic module tilt angle adjustment - in small systems modules should be fixed in a way, which allows manual adjustment of the module tilt angle. In March the tilt angle should be adjusted to equal latitude, in May the tilt angle equals latitude minus 10 degrees, in September the tilt angle equals latitude and in December the tilt angle equals latitude plus 10 degrees. With such an adjustment the maximal efficiency could be obtained throughout the year. 

Accurate and maximum energy output of larger systems should be based on exact calculations, because energy output is influenced by different factors, such as local climatic conditions (solar radiation availability in different seasons, local cloudiness or fogginess in winter, temperature and so on). You will need a long-term solar radiation data for the chosen location.

 Shading losses of photovoltaic systems can not be avoided (if shading occurs), but at least portion of them can be minimised. Right time to consider this issue is the system planning's phase, later it is usually too late. 
 
 Shading of strings - if crystalline modules are mounted on the roof like on the picture below, they should be always mounted horizontally (like on the picture) and never vertically. Reason is quite simple: each crystalline module usually includes two bypass diodes which are active if shading occur. When modules are mounted horizontally the module still operates with some amount of power (50% or less) if the bottom row is shaded, because only one bypass diode is active. But if modules are mounted vertically and if lower row is shaded partially or completely both bypass diodes are active and amount of output power is close to zero. 
 
 Array configuration - In some cases, like example of the church roof on the picture - you can also prevent shading loses with carefully array design. Array on the picture has shape of trapezoid, because of shading of church's bell tower. 
 
 String configuration - modules that are shaded more often than other parts of array should be connected into separate string's) if possible. This will prevent losses of the whole system because of partial shading of only one part of array. 
 
 Inverter configuration - some inverter offer several inputs, for each string its own input - in case of shading of one string, other inputs will still operate in MPP. 
 
 Amorphous modules - in cases where shading can not be avoided use of amorphous modules should be considered. Amorphous modules are far less sensitive on partial shading (in comparison with crystalline modules) so that even in case of partial shading they produce significant amount of power.