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©Denis Lenardic
2001-2008
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Monday
12th May, 2008
18:23

This page was last time updated on
25th Jan, 2008

 

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Should you think this web site needs improvement, don't hesitate to let me know. I'll be happy of any constructive criticism. Any comments on terminology and web site content are appreciated. E-mail them to:

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Solar radiation estimation and site analysis

"Turn your face to the Sun and the shadows fall behind you."
(Maori proverb)

Shadowing decreases yield significantly Shadowing decreases yield significantly Shadowing decreases yield significantly

Vegetation, snow and surrounding objects are the most common causes of shadows
(photo: Denis Lenardic)

The choice of a proper location is the first and the very essential step in solar system design procedure. Even the most carefully planned solar system doesn’t work satisfactory, if the location wasn’t properly chosen. It is critical that the modules are exposed to sunlight without shadowing at least from 9 am to 3 pm; therefore, the properties and values of solar insolation should be studied. The modules have to be fixed with proper tilt angle allowing the system efficient operation.

 

Solar radiation potencial estimation

For estimation of electricity generated for predefined location (country and city) a PV Potencial Estimation Utility is available on IES web pages. Calculations for Europe, Africa and Mediteranian possible. The tool offers many different calculation options:

PV potencial with given inclination and orientation
Calculation of optimal inclination for given orientation
Calculation of optimal inclination and orientation
 

PV potencial estimation tool

PV-Calculator - With PV-Calculator you may calculate yield of particular PV systems. Calculation is based on precise global radiation data for choosen location. Tool is available in german language only.

 


Additional information can be found in:

pdf Webmaster's choice A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation;
pdf Webmaster's choice Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces;
 

Sun path

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:

Declination equation

Sun height equation

Solar azimuth equation

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

Sun path diagram

Sun path diagram, created with SunOrb 1.2

Solar azimuth

Solar azimuth

Sun height

Sun height
 

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


Module tilt - equation 1

Module tilt - equation 2

Declination

Module tilt angles

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.
 

Tilt and azimuth angle of photovoltaic modules

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.
 

Tools for site analysis

 
 
 
 
 
 
 
 
 
 

Solar radiation and site analysis - related web sites

 

Languages: gb de

 

Languages: gb Webmaster's choice
Webmaster's choice - solar radiation terms, glossary, alghoritms, models etc.

 

Languages: gb Webmaster's choice
Webmaster's choice - Sun path diagrams explained in detail - very nice web site about renewable energy use and sustainable design.

 

Languages: fr

 

Languages: gb Webmaster's choice
Webmaster's choice - PV potencial in Europe - estimations, maps, charts and detailed explanations of calculation methods. Excellent site for profesionals!

 

Languages: de gb

 

Languages: de gb Webmaster's choice
Webmaster's choice - a lot of useful documents available for download. Shadowing analysis and simulation explained in detail.

 

Languages: nl

More information and some useful on-line tools

On-line calculator Sun position Calculator.
book Meeus, J.: Astronomical Algorithms; Willman-Bell Inc., Richmond, VI, 1999.
book Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 1: Fundamentals and maps.
book Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 2: Database and exploitation software.