Solar PV - Solar PV Shading - Photovoltaics
Domestic properties in the UK offer a range of potential shading sources. Special attention should be paid to chimneys, TV aerials, lightning conductors, satellite dishes, roof and facade projections, offset building structure, roof superstructures etc.
Solar PV Shading – Initial Assessment
It is important to ensure there are no obstructions that will block the sunlight from reaching the Solar PV array. A small amount of shading can reduce the performance of the whole system, not just that of an individual shaded module. The path of the sun across the sky changes throughout the year. In addition, the height and the position of the sun are constantly changing throughout the year.
Solar PV Shading Affects Performance
Because the Solar PV panels are connected together, any shading on a single module will affect the performance of the whole Solar PV array. A system can tolerate some shading early or late in the day without much reduction of overall electrical output but it should not be shaded between 10 a.m. and 4 p.m. Shading can reduce the output of a Solar PV array considerably and ideally should be avoided.
Solar PV Shading - Obstructions
Shading can occur from obstructions like neighbouring buildings, trees and even distant tall buildings can shade the system. The fact that increased growth of trees, shrubs and other vegetation may shade the system after only a couple of years, must be taken into account at the outset. Overhead cables running over the building also have a negative effect, casting a small but effective moving shadow.
Solar PV Shading - The Building Itself
One of the most commonly overlooked sources of shading is that from the building itself. Terraces of houses quite often have groups of houses set back or forward from the rest terrace to provide visual interest to the row. This can cause the shading of the roofs of adjacent properties. Some shading can be avoided by moving some of the Solar PV panels or the object causing the shading (e.g. a TV aerial). If this is not possible, the impact of the shading can be minimised by taking the problem into account when choosing the wiring system.
Solar PV Installation - Equipment Self Shading
In the case of frame mounting systems, self-shading of the Solar PV modules may be caused by the row of modules at the front. Space requirements and shading losses can be minimised through optimisation of the tilt angles and distances between the module rows. A poorly designed and installed mounting system may also cause micro-shading in sloping roof installations.
Solar PV Shading - Temporary Affect
Temporary shading includes factors such as snow, leaves, bird droppings and other types of soiling. Snow is a significant factor especially in hilly areas. Dust and soot falling in industrial areas or falling leaved in wooded areas, are also significant. The effect will be less if the Solar PV array self-cleans. A tilt angle of about 12 degrees is usually sufficient to achieve this.
Solar PV Shading - By-Pass Diodes
To prevent the entire string of cells failing when one cell under performs the installation can be fitted with ‘by-pass diodes’. These reroute the current around the under performing cells. The disadvantage is that rerouting the current loses not only potential energy from these cells, but also lowers the voltage of the entire string.
Finding a clear solution to the shading problem will help guarantee a reliable power supply for the owners of Solar PV system. By reducing shading induced power losses, installers can potentially increase the available roof area for Solar PV arrays