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. 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 devices, general
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
Horizontal shading device, overhang, side view (left)
vertical shading device, vertical fin, top view (right)
h, D - geometry of horizontal shading device (overhang dimension, see picture above)
α - sun height,
Φ - solar azimuth,
Ψ - plane azimuth
w, D - geometry of vertical shading device (vertical fin, see picture above)
Φ - solar azimuth, Ψ - plane azimuth
γ = vertical shadow angle (VSA)
w, D - geometry of vertical shading device (see picture above)
Φ - solar azimuth, Ψ - plane azimuth
Solar-Fabrik, modules used as shading devices and as part of a facade
(courtesy: Solar-Fabrik AG, Freiburg)
SHADING AND PV SYSTEM OPTIMIZATION
Shading of strings
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 0% 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
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, o