Roof requirement for solar rooftop system installation

In general, solar PV modules can be installed on or in pitched roofs, or on flat roofs attached to or adjacent to buildings (ground-mounted). Prior to solar PV installation, the roof conditions have to be evaluated.

The general roof conditions have to be evaluated with regard to the following aspects:
1. Roof damage:
First, the PV system owner has to check whether the roof is damaged and needs to be restored. As most PV module suppliers offer a 25-year warranty, one has to be sure that the roof will not need to be repaired during this time period. If renewal of the roof becomes necessary during the life time of the solar PV generator, the costs for system removal and reinstallation have to be considered.

2. Size and bearing capacity:
Based on the PV system owner’s needs and on the available solar resources, the sizing of the solar PV system can be defined using one of the simulation computer programmes available on the market. Once the size of the desirable system is determined, it is necessary to verify whether the roof is big enough and able to bear the required amount of PV modules and racking system. As a rule of thumb, a standard crystalline silicon-based solar PV installation weighs around 15 to 20 kilograms per square metre. Additionally, wind and snow loads depending on location and weather conditions have to be considered. Additionally, roof anchors, fasteners and required roof penetrations have to meet the local building code regarding safety and weather resistance of the roof.

3. Orientation and inclination of the roof:
The solar PV-generated electricity depends on how much sunshine the modules receive during the day. This depends heavily on the direction the roof is facing (azimuth angle: between the surface of the solar PV module and north/south facing direction (based on the hemisphere); tilt angle: divergence from horizontal). The optimum orientation is south-facing for the northern hemisphere and north-facing for the southern hemisphere. However, east- and west-facing roofs generate lower but more homogenous output power profiles without noon peaks. Depending on the geographical latitude, there is an optimum inclination for solar PV module installation. As the roof does not always offer the optimum tilt, mounting systems can provide the required tilt angle for the highest possible performance. The optimum tilt angle is approximately equal to the latitude. As an example, the highest annual yield values for a solar PV system installed in central Europe are expected for systems with an inclination between 30° and 40° towards the south, as shown in Figure 6. In any case, a minimum tilt of 10° is advantageous for avoiding build-up of dirt and for facilitating the self-cleaning of solar PV modules.

Orientation and tilt are the most important factors affecting PV system performance. Figure 7 is valid for installations in central Europe and shows that low tilt angles affect the performance significantly. Optimal performance is achieved at 30° to the south, but changing that angle to horizontal (0°), the generated electricity will be only 10% lower. The PV system performance is higher during the summertime when the position of the sun is high, and the amount of electricity produced during the wintertime when the sunlight falls below a low angle is much lower. Thus, the PV system performance will not be significantly affected. The situation changes for higher tilt angles from 30° to 90°, where the electricity production will be 30% lower. If the solar PV modules have to be installed in differently orientated and tilted roof segments, multi-string inverters should be used to avoid mismatching. Panels can also be oriented to meet a predicted demand – for example, if the house or grid has a known peak time, the panels can be oriented to deliver peak output during that time. This reduces overall kWh per year, but can increase value.

4. Ventilation:
Solar PV module performance decreases with increasing temperature. The degree to which this occurs depends on the semiconductor material used for building the solar cells. In the case of crystalline silicon, the temperature co-efficient is 0.4 to 0.5% per degree Celsius, meaning that for temperatures above 25°C the output power of the module decreases 0.4 to 0.5% for each 1°C. In conditions of high irradiation (above 800 W per m2) solar PV modules can reach temperatures of around 70°C depending on both module design and roof configuration. To keep the panel temperatures as low as possible and to improve the overall performance, adequate ventilation is important. As a rule of thumb, an air gap of at least 100 millimetres between the solar PV modules and the roof should be applied.

5. Shading:
In addition to orientation and inclination, the shading of solar PV modules is one of the most important causes of losses in a PV system. Shading of 10% of the system’s area could cause 50% system loss. If the solar PV modules are connected in series to a string and there is one shaded module in the string, the whole string underperforms. Partial shading and near shading could be caused by trees, chimneys, roof windows, masts or surrounding buildings. Further horizontal shading might be caused by faraway buildings or mountains. A detailed shadow analysis can be provided using a sun path indicator or more sophisticated computer tools which are widely available online.

The minimum distance between the rows can be calculated as follows:
Minimum distance = (height of the solar PV module) / (tangent of the sun’s height angle)
Where:
Height of the module = (sine of the angle of inclination) x (width of the solar PV module)