System performance – Factors affecting generation

The output of a solar power system varies from day to day, with less energy produced on cloudy days, and in the winter. The overall performance of the system however is dependent on a variety of factors such as Orientation, Tilt, Temperature and Shading. These factors must be considered when designing a solar power system to maximize energy production and the return on capital investment.

Tilt and Orientation

When exposed to sunlight solar panels heat up and this affects the performance of the panels. Solar panels work by using photon’s in light, not the heat of the sun and in fact heat reduces the performance of a solar panel. A correctly installed solar power system should use a framing system that lifts the panels up off the roof surface of the building, allowing airflow around the panels. The cooler the panels are the better they perform and the more electricity they will generate.

Similarly, the inverter works less efficiently the hotter it gets. An inverter should not be installed on a North or West, facing wall, unless it has some shade during the middle of the day. The inverter should also have at least 30 cm clearance around the top and sides to allow airflow and to assist in cooling.

Shading

It is important to consider any obstructions to the North, West and East of a solar power system that may cast shadows on the solar modules.  Shading from chimneys, trees, neighboring buildings etc. will dramatically affect the output of a solar system. The sunlight is at its strongest between 10 am and 2 pm so obstructions that cause shading during this period, are most disruptive to performance.

Shading on one panel in a solar array will reduce the output of that panel and will reduce the output of all the other panels that are electrically connected to the shaded panel.

As discussed in the section on solar modules, the panels include by pass diodes to reduce the effects of shading, but essentially a group of solar panels performs only at the output of its worst performing panel.

It is therefore essential that the system designer is aware of all potential shading issues and the proposed system is located and configured to avoid or minimize the effects of any shading.

Temperature

For most solar power installations, the tilt and orientation, is determined by the available roof space at the premises. Most systems are tilted at the same pitch as the existing roof and are known as flush mounted systems. The optimum tilt angle for a solar system is roughly equal to the latitude of that location, in degrees. In Sydney for example, this is about 34 degrees, however studies have revealed only a 10 percent difference in out put from a system mounted at 0 degrees and a system at 40 degrees.

The main issue with a completely flat installation is the accumulation of debris, as water does not flow freely off the solar modules when it rains, preventing the panels from self-cleaning. Therefore, for optimum energy yield, a solar system should be installed with a tilt angle between 10 and 40 degrees.

The optimum direction for a solar power system in the Sothern Hemisphere is to face towards the North. Substantial deviations from the optimum direction can be made, however, with relatively small effects on performance. Monitoring by the Meridian team has shown that a north - east or north - west orientation results in about a 5 percent loss in performance, while an east or west facing system results in only 15 percent loss in performance.