Based on the principle of converting energy from the sun, solar photovoltaic (PV) systems combine technology and engineering for productive electricity outputs, considering the full range of factors affecting performance in the entire process of designing to connecting solar power systems.
Converting energy from the sun
Solar PV modules, also known as solar panels, convert the energy in sunlight to electrical energy.
This occurs due to the properties of photovoltaic materials, which experience an internal flow of electrons in response to light. Electrons in the structure of these materials – typically a silicon crystal structure – absorb the energy of light particles (photons) hitting their surface, energising the movement of particles through the material.
PV modules are constructed of photovoltaic materials ‘doped’ with positive and negative ions so that the internal electron movement is channelled through the material and into an electric circuit, via the metal contacts on the surface of the module.
Designing solar power systems
Designing a solar power system takes environmental factors – such as the amount of sunlight (irradiance), cloud cover and temperature – and load requirements (how much electricity the users demand at different times of the day) into consideration. A designer must also control or select between different approaches or types of solar power system, such as:
Grid-connect or standalone
Grid-connect systems export electricity to a larger network, while standalone systems supply a local load only, usually with battery backup. Most systems close to an existing supply will be grid-connected, to avoid the high cost associated with batteries or other storage. Remote standalone power supplies often incorporate a second power generator, such as a diesel generator (genset).
Different types of PV module suit different situations. High-efficiency modules, like silicon monocrystallines, will often be chosen when installation space is limited or where frames and installation costs are high. Low-efficiency modules, like some thin films, might be chosen where installation space is plentiful. These are both types of flat plate PV module, as are polycrstyalline modules, but alternative concentrator systems (CPV) can use parabolic mirror to focus sun rays onto the PV cell and increase the energy harnessed.
PV modules on fixed arrays will generally be oriented towards the north in the southern hemisphere. This exposes the modules to the greatest amount of sunlight through the course of a day. East-facing arrays will receive a greater share of morning light, and west facing arrays will receive proportionally more afternoon light. Arrays might be faced east or west so power generation matches the times when loads are expected to be highest.
The tilt of an array, measured as an angle from the horizontal plane, will determine the time of year that the PV modules receive the most sunlight. Positioning an array to the installation site’s angle of latitude – for example, around 20-25° in Alice Springs – will expose the modules to the greatest amount of sunlight over a year. A flatter installation will receive more sunlight in summer when the sun is higher in the sky, and a more extreme (nearer to vertical) angle will receive more sunlight in winter.
Factors affecting performance
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.
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.
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.
Connecting solar power systems
PV modules produce DC electricity. This can be used to charge batteries or power specialist DC appliances, but to use this electricity for standard appliances or supply it to the electricity grid, it has to be converted to AC electricity by an inverter.
Solar arrays are usually connected to inverters that convert the power from PV arrays into an output of 240 V AC power. In a domestic grid-connected installation, this inverter will interact with the grid supply to the house, so that it can supply the house’s power needs and feed power to the grid when there is an excess.
In installations with multiple arrays, like the DKASC, Alice Springs, the output from the inverters is marshalled at a switchboard before connecting to the grid. Usually, a meter is also installed to record electricity flowing in either direction. A transformer often steps up the voltage of the electricity to a higher voltage for transportation after the power is marshalled.