Shading analysis in solar energy engineering

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Shading analysis in photovoltaics and in solar energy engineering

"Reality is merely an illusion; albeit a very persistent one."
(Albert Einstein)

Forte Carpenedo, Italy, 3.46 kWp BIPV system IKEA, Sweden, 10.9 kWp, PV facade Uster, Switzerland, 2.54 kWp PV facade Woudhuis, The Netherlands, 2.28 kWp BIPV system

Solar energy systems in buildings - shading analysis is one of the essential design steps
(courtesy: IEA PVPS Task 2)

Shading analysis is one of the most essential steps in phase of solar energy system design or analysis. In photovoltaics it is important to analyse shading caused by surrounding objects and/or vegetation. In special cases like analysis or design of BIPV systems, exact analysis of "shadow-voltaic" systems (overhangs, vertical shading fins, awnings etc.) is also very important. Similar analysis is also part of passive house or solar house design - overhangs must also be planned very carefully in such case. Basic calculations can be done by some simple equations - formulas for some typical simple cases you may find below. Some graphical tools like solar path calculator (pilkington) are also available. For analysis of complex objects several computer tools are available. Some of them offer even 3D simulation. Shading is especialy important in photovoltaics. It should be eliminated as much as possible. Even small obstacles like chimneys, telephone poles etc. shouldn't be neglected. To minimise influence of photovoltaic array shading (if shading can not be avoided) different system optimisation techniques can be used. Detailed explanation of such cases you may find on this page below.

Shading calculations

Shading & PV system optimisation

For different simple cases it is in generall not difficult to calculate shadows for particular day and time. Below you will find some formulaes end equations which may help you to calculate shadows for most common particular cases in engineering practice.

Horizontal shading device analysis

Vertical shading device analysis

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

Solarfabrik, Freiburg, transparent modules and shadow-voltaic modules as part of a facade, source Solarfabrik GmbH.

Solar fabrik, Freiburg facade with transparent modules and solar modules as shading devices
(Source/copyright Solar fabrik GmbH).

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 plannings 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 horizontaly (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 diod is active. But if modules are mounted vertically and if lower row is shaded partially or completel 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 losse 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.

Strings on the roof, modules oriented horizontaly, souce SSES

Church's roof covered with photovoltaic roof tiles, optimised array shape, courtesy Pfleiderer Dachziegel GmbH

Orientation of modules in strings on the roof (top), optimisation of array shape in the roof integrated modules (roof tiles) in a heritage building (church) in Germany (Source/copyright SSES - top, Pfleiderer Dachziegel Gmbh - bottom).

Shading analysis - software tools
	Visual Sun Chart - Visual Sun Chart is a graphics program for visualizing solar shading from proposed buildings. Use Visual Sun Chart to determine if a site will have sufficienct access to the Sun's energy. Languages:
	Amethyst ShadowFX - Amethyst ShadowFX is a sun and shadow modeling program for architects and town planners. Amethyst ShadowFX enables you to easily generate shadow profiles cast by buildings and other objects for any latitude, longitude and time of year. Languages:
	Sombrero - A PC-tool to calculate shadows on arbitrarily oriented surfaces. For both, active use of solar energy (domestic hot water, photovoitaics) as well as for passive solar architecture, shading or lighting of planes plays an important role SOMBRERO provides quantitative results for the shading of collectors or windows by buildings, trees, overhangs or the horizon. Languages:
	Sundi - Das Simulationsprogramm zur Solarstrahlungsberechnung und Abschattungsanalyse. Languages:

Literature and more information

Budin, R., Budin, L.: A Mathematical Model for Shading Calculations; Solar Energy, vol.29, Pergamonn Press, 1982.
Burns, P.J.: Building Solar Gain Modelling; Passive Solar Buildings, Balcomb, J.D., editor, MIT Press 1992.
Quaschning, V., Hanitsch, R.: Shade Calculations in Photovoltaic Systems; ISES World Solar Conference - Harare, Zimbabwe, 1995 (73 kB).
Tabb, P.: Solar Energy Planning; McGraw-Hill, 1984.
Quaschning, V., Hanitsch, R.: Der Einfluss von Abschatungen auf Photovoltaikanlagen in der Landwirtschaft; 19.Konferenz CIGR Sektion IV, Stuttgart, 25.-28.9.1995.
Quaschning, V.: Simulation der Abschattungsverluste bei solarelektrischen Systemen; Verlag Dr. Köster Berlin, 1. Auflage September 1996.
Quaschning, V.: Höhere Flächenausbeute durch Optimierung bei aufgeständerten Modulen; 13. Symposium Photovoltaische Solarenergie · Staffelstein · 11.-13. März 1998 (36 kB).
Walraven, R: Calculation the position of the sun. Solar Energy Vol.20, 1978, pp. 393-397.
Walraven, R: Erratum. Solar Energy Vol.22, 1979, p.195.
Wilkinson, B.J.: The effect of atmospheric refraction on the solar azimuth. Solar Energy Vol.30, 1983, p.295.
Archer, C.B.: Comments on "Calculating the position of the sun". Solar Energy Vol.25, 1980, .91.
Kambezidis, H.D.; Papanikolaou, N.S.: Solar position and atmospheric refraction. Solar Energy ol. 44, 1990, pp.143-144.
Muir, Langley R.: Comments on "The effect of atmospheric refraction in the solar azimuth". Solar Energy Vol. 30, 1983, p.295.
Sattler, M.A., Sharples, S., Page, J.K.: The geometry of the shading of buildings by various tree shapes; Solar Energy Vol.38 No.3, pp. 187-201, 1987.