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)

courtesy SMA pictures

Solar energy systems in buildings - shading analysis is one of the essential design steps
(courtesy: SMA pictures)

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 especially important in photovoltaics. It should be eliminated as much as possible. Even small obstacles like chimneys, telephone poles etc. shouldn't be neglected. 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 general not difficult to calculate shadows for particular day and time. Below you will find some formulae's end equations which may help you to calculate shadows for most common particular cases in engineering practice.

Horizontal shading device analysis

Horizontal shading device

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

Vertical shading device analysis

Vertical shading device (top view)

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

Vertical Shadow Angle (VSA): 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.

Strings on the roof, modules oriented horizontaly, souce SSES

Preffered orientation of modules in strings on the roof (courtesy SSES).

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.

Array shape - 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.

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

Optimisation of array shape in the roof integrated solar modules (courtesy Pfleiderer Dachziegel Gmbh).

Shading analysis - software tools
	ECOTECT - is an industry leading building analysis program that allows designers to work easily in 3D and apply all the tools neccesary for an energy efficient and sustainable future. Languages:
	Shadow Analyser - Shadow Analyzer is an advanced parametric CAD tool for professionals in the area of Solar Energy Engineering and Architecture. Languages:
	Shadows - Shadows is a program used to design sundials and astrolabes and it is very useful also in solar energy engineering. Key points of Shadows: Free of charge in its entry level Calculates and prints the drawings at real scale Simulates, displays and animates the shadow of the style Exports drawings as BMP, JPG, EMF and DXF (AutoCAD) Offers complete ephemeris of the Sun and draws the Solar Diagram Supports plane sundials with polar style of any orientation and reclination, analemmatic sundials, cylindrical and bifilar sundials. 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:
	GOSOL - Das solar + energetische Simulationsprogramm für städtebauliche Planungen. Comprehensive tool intended for detailed site analysis, included building shape and vegetation shading analysis. Suitable for urban planning and detailed architecture studies (german language only). Languages:
	METEONORM - Global Meteorological Database for Solar Energy and Applied Meteorology. METEONORM is a comprehensive climatological database for solar energy applications: a meteorological database containing comprehensive climatological data for solar engineering applications at all points of the globe between the polar circles; a computer program for climatological calculations; a data source for engineering design programs in the passive, active and photovoltaic application of solar energy with comprehensive data interfaces; a standardization tool permitting developers and users of engineering design programs access to a comprehensive, uniform data basis. Languages:
	pvPlanner - Simulation tool for planning and optimisation of photovoltaic systems using climate and geographic data at high temporal and spatial resolution and new generation high performance algorithms. pvPlanner also includes horizont shading analysis tool. Languages:
	Panorama master - with "Panorama master" you can make exact matched set of photos - base for panorama picture (see also "Horizon" below). Languages:
	Horizon - software simulates sun path diagrams for arbitrary latitude. 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:

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.