Insolation
Insolation refers to the energy in sunlight.
The insolation received by a location over a day can be expressed as energy per unit area (Wh/m2 or kWh/m2) or peak sun hours (hours), which expresses the total energy as an equivalent number of hours of 1 kWh/m2 sunlight encountered in a day.
The sunlight received at a site will be affected by the position of the sun, the length of the day, and the amount of cloudiness and other interference in the sky.
Insolation can be either ‘direct’ or ‘diffuse’. Direct insolation reaches the earth’s surface in an uninterrupted path from the sun, while diffuse insolation comes from scattered light reflected off particles in the atmosphere.
PV modules generally convert direct insolation into electricity more effectively than diffuse, although some photovoltaic materials use diffuse insolation more effectively than others. The resultant (or total) insolation combining direct and diffuse components is called the global insolation.
Not to be confused with ‘insolation’, which refers to received energy from the sun over a given period of time (e.g. in kWh/m2), the term ‘irradiance’ refers to power (e.g. in kW/m2) and is an instantaneous quantity.
Tilt and azimuth
The maximum insolation falls on a PV module when it is facing exactly perpendicular to the incoming radiation. This can be achieved through adjustment of the array tilt angle (relative to the horizontal ground surface) and the array azimuth (its east to west bearing).
Dual-axis tracking arrays continuously change the array tilt and azimuth to keep the PV modules constantly perpendicular to the incoming irradiance.
Single-axis tracking arrays have a fixed tilt angle but continuously change the azimuth to keep the PV modules facing as direct as possible to the incoming irradiance.
For fixed (non-tracking) flat plate PV systems, the tilt and azimuth must achieve some compromise so are generally selected to optimise the energy output over the whole year. The fixed PV arrays at the DKASC, Alice Springs are all set at a tilt of 20° and azimuth of 0° (solar north). The exception to this is the Solar Compass system (Site 16) that also has east-facing (90° azimuth), west-facing (-90° azimuth) and flat (2° tilt) sub-arrays.
Temperature
The output of a PV module tends to decrease as temperatures increase, because the equipment becomes less efficient at elevated temperatures.
High ambient temperatures limit the electricity a PV module can produce, but PV modules also generate their own heat as they produce electricity, so it is relevant to consider not only the ambient temperature but the cell temperature.
Dust and dirt
Dust and dirt tend to block light from reaching the PV modules, reducing their output. Horizontal installations are generally avoided to prevent dust build-up, and wind/rain can assist to blow/wash off accumulated matter on the surface of modules.
At the DKASC, Alice Springs, all PV arrays are cleaned annually to remove dust. The time and dates of this cleaning are posted in Notes on the Data.
Shading
Shade from trees and built structures blocks direct insolation from reaching PV modules.
A small amount of shade can have a large impact on the output of a PV module, as it changes the flow of electricity through the module. This is especially true of silicon crystalline modules, where the PV cells tend to be smaller in size or more compact than those in thin film modules.
At the DKASC, Alice Springs, we aim to ensure that no arrays are subject to shading from trees, buildings or other arrays within the hours of 8.30am and 4.30pm each day.