Impact of Shading
Shading of photovoltaic modules is one of the most significant causes of losses in a pv system. In fact, the shading of 10% of the area of a system could cause a loss of 50%. This disproportionate effect is caused by string design. If there is one shaded module in the string, the whole string underperforms. There are a number of technical solutions that involve inverters finding a new maximum power point or having one inverter per module. The best solution, however, is to avoid partial shading wherever possible.
In any case, it is essential to undertake a shading analysis of the site, which involves taking the view of the solar module from -180° to +180° and recording the horizon. This should be done for far-away objects (like mountains) as well as objects in the vicinity. Close-by objects can cause dark core-shadows with more pronounced effects. Simulation software should be used to estimate the effect on the system.
Case Study: Solar panel in front of St Paul's Cathedral
If a solar panel were to be installed on the foot of St. Paul's Cathedral in London, how much would it be affected by shading? (The images below have been taken with the help of the Suneye device by Solmetric)
- Create fish-eye view: The fish-eye view is a 360° view that captures the whole surrounding. The cathedral is roughly west-facing. Hence, the module sees the cathedral in the east. As can be seen in this view, the module would be surrounded by a number of houses.
- Calculate sunpaths: Given the lattitude and longitude of the location (51°N, 0°W), the fish-eye view can be superimposed with the sun paths for different months. The cathedral hides the sun until roughly 10am. In January, the module won't even get direct sunlight at midday.
- Draw obstruction path: Using software, the obstruction path can be drawn, again for a variety of months. All areas shaded green in the diagram are obstacles. Here, the cathedral is on the left hand side, obstructing the sun in the early hours of the day.
- Calculate shading impact: Using further software, the impact of shading can be calculated. In this instance, losses due to the surrounding houses are 40% over a whole year!
An interesting aspect is the effect whereby rows of ground-mounted pv arrays cause shading of subsequent rows..
If the rows of tilted modules are too close to each other, one row will cause a shadow on the next, causing additional losses. If the modules are spaced out a lot (i.e. small fill factor), there are fewer shadows, but that comes at the cost of space optimization.
The optimal tilt angle of the modules will depend on the latitude. In countries of medium latitude like Germany or the UK, a simple formula of d = 3w (where d is the distance between rows and w is the width of a module) at an optimal tilt angle of 30° can be applied. This amounts to a fill factor of 33%.