The optimal output from a photovoltaic array is achieved if its surface is always normal to the sun's radiation. However most arrays are fixed rather than tracking and so compromises must be made. Over much of the UK the annual output is maximised if the array is south-facing and tilted at just over 30 degrees from the horizontal. Fortunately the annual output is not particularly sensitive to orientation - an array oriented anywhere between southeast and southwest and at an angle of 0 to 60 degrees from horizontal will recieve at least 90% of the maximum insolation. This means that other factors may decide the precise orientation, for example the building geometry, overshadowing concerns or the requirement for higher output at a particular time of day or year.
Buildings, trees and other structures, which either intervene directly between the sun and the collector or merely obscure part of the visible sky, decrease the annual solar energy density at the collector's surface. A small degree of obstruction is almost inevitable but significant obstruction is a problem. The diagram to the right gives rough estimates of the effect of level skylines of 15, 30 and 50 degrees on a vertical collector. The lower the tilt, i.e. the more nearly horizontal the collector, the less obstructions will effect the annual energy density.
If the skyline is not approximately level then plotting it on a sunpath diagram is a useful way to make a preliminary assessment of the effect of overshadowing. If the direct sun is blocked by obstructions for a significant proportion of the year then you need to seek further advice or use more sophisticated modelling software.
Plotting the skyline on a sunpath diagram also provides seasonal information about when the direct sun is obstructed.
Photovoltaic modules can be particularly sensitive to overshadowing. Each module is comprised of multiple 'solar cells'. These cells are electrically connected in series into 'strings' which are then connected in parallel to form the module.
Each cell generates electricity at a given voltage. Increasing the irradiation increases the electrical current which flows and thereby the power (which is the product of the voltage and the current). If a cell is shaded then the current through it decreases, but this then limits the current flowing through any cell connected in series with it, i.e. all those in the same string.
The upshot is even the shading from a lamp post, or a leafless tree may significantly decrease the performance of a module. Steps must be taken to avoid this. If at all possible the modules should be placed so that they are not overshadowed. When doing so it is best to consider possible future developments.
If some overshadowing cannot be avoided then careful attention must be paid to the wiring of the cells in the modules. A vertical shadow, such as that from a lamp post, will effect an entire module if the cells are arranged in horizontal strings, but only a single string at a time if they are arranged vertically.
|
|
|