Site Analysis

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.

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:

n - day of the year

δ - declination, L - latitude, ω - hour angle[1]

α - Sun height, L - latitude, δ - declination

Notes

[1] Hour angle is calculated by simple equation h = ± 15°⋅number of hours from/to solar noon.

solar height solar azimuth

FIGURE 1: Sun height and azimuth


declination

FIGURE 2: Declination, calculated by equation above, for details see literature

Sun path chart

Sun-path chart or Sun-path diagram is simple graphical method to present Sun's apparent movement. Depending on projection used following Sun path charts different sun-path charts are used:


sun-path diagram

FIGURE 3: Sun-path chart, equidistant projection, generated by Sun-path Chart Software
courtesy: University of Oregon, SRML

Tilt and Azimuth of Solar 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. According to the equations above we can estimate the most proper photovoltaic modules tilt (β) employing the following equation:

δ - declination, L - latitude

Declination changes throughout the year, therefore, the optimal tilt angle changes as well. The optimal tilt angle - at noon - for some latitudes is presented in the picture below.

incident angle

FIGURE 4: Tilt angle for latitudes 30°, 40°, 50° and 60°

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


Software for site analysis


Additional Information


Sun-Earth Geometry - Books

book
Sayigh, A.A.M., Ed. (1977), Solar Energy Engineering; Academic Press, ISBN 0-12-620850-6.
book
Meeus, J. (1999), Astronomical Algorithms; Willman-Bell Inc., ISBN 0-943396-61-1.
book
Muneer, T. (2004), Solar Radiation and Daylight Models, Elsevier, Butterworth-Heinemann, Oxford, ISBN 0-7506-5974-2.
book
Scharmer, K, Greif, J. (2000), The European Solar Radiation Atlas, Vol. 1: Fundamentals and maps, Presses des Mines, ISBN 2-911762-21-5.
book
Scharmer, K, Greif, J. (2000), The European Solar Radiation Atlas, Vol. 2: Database and exploitation software, Presses des Mines, ISBN 2-911762-22-3.

Sun-Earth Geometry - Papers

paper
Reda, I., Andreas, A. (2004), Solar position algorithm for solar radiation applications; Solar Energy, vol. 76, pp 577-589.
paper
Reda, I., Andreas, A. (2007), Corrigendum to "Solar position algorithm for solar radiation applications"; Solar Energy, vol. 81.
paper
Reda, I., Andreas, A. (2008), Solar Position Algorithm for Solar Radiation Applications; NREL, NREL/TP-560-34302, January 2008.
paper
Blanco-Muriel et al. (2001), Computing the solar vector; Solar Energy, vol. 70, pp 431-441.
paper
Cooper, P. I. (1968), The absorption of radiation in solar stills; Solar Energy, vol. 12, pp 333-345.
paper
Grena, R. (2008), An algorithm for the computation of the solar position; Solar Energy, vol. 82, no. 5, pp 462-470.
paper
Walraven, R. (1978), Calculation the position of the sun; Solar Energy, vol.20, pp 393-397.
paper
Walraven, R. (1979), Erratum. Solar Energy vol.22, pp 195.
paper
Wilkinson, B. J. (1983), The effect of atmospheric refraction on the solar azimuth. Solar Energy, vol.30, pp 295.
paper
Archer, C. B. (1980), Comments on "Calculating the position of the sun". Solar Energy, vol.25, p 91.
paper
Kambezidis, H. D.; Papanikolaou, N. S. (1990), Solar position and atmospheric refraction. Solar Energy, vol. 44, pp 143-144.
paper
Muir, L. R. (1983), Comments on "The effect of atmospheric refraction in the solar azimuth". Solar Energy, Vol. 30, p 295.

Other

www
Calculation of Sun Position, Sunrise and Sunset: Sun Position Calculator.