Solar electricity is generated by two main technologies: solar photovoltaic (PV) cells and high temperature solar thermal power systems. In addition, relatively low-grade heat is produced in solar water heaters for domestic use.
Solar PV cells
Solar PV cells convert sunlight directly into low voltage electricity. The supply is direct current (like a battery). An inverter is used to convert this to a higher alternating voltage suitable for supply into the electricity grid.
Solar cells are also used to supply electricity in remote areas where grid connection is not available or would be very costly to install. Remote area power systems use batteries to store some of the electricity generated during the day to provide 24 hour supply. Although the low voltage can be used to supply low voltage appliances and lights directly, an inverter is most often included as part of the system so that standard household appliances can be used.
The average size of a household grid-connected solar PV system is about 1.5 kilowatts which has a PV panel area of about 12 square metres. A system of this size has a cost typically around $19,000. This is an expensive power supply option in comparison to other renewable energy technologies.
The solar panels need to be installed facing north and inclined at an angle to maximise exposure to the sun. If the aim is to maximise output during the grid's afternoon peak demand period the panels can be installed facing more towards the north-west. To increase the total amount of electricity generated the solar panels can be mounted on a tracking unit which ensures that the panels are always facing directly towards the sun throughout the day.
Grid-connected PV systems do not need batteries to maintain supply, as electricity is drawn from the grid when the solar cells are not providing power at night or have reduced output due to cloudy weather.
Concentrating solar PV systems provide a means to reduce the cost of PV power by significantly reducing the area of solar cells required. The mirror concentrators can be trough-shaped to focus on a line of solar cells, or dish-shaped to provide high concentration on a small area of solar cells. A cooling system is required for the solar cells, particularly in the case of dish concentrators, to prevent the cells being damaged by extreme temperatures. Such systems are more suited to larger scale power systems. The waste heat from the cooling system can be used for purposes such as heating water for domestic purposes.
Solar Thermal Power Systems
There is a range of solar concentrator systems which can provide high temperatures for power generation. Large-scale systems generally produce steam at temperatures and pressures suitable for generating electricity using standard steam turbines. Waste heat from the turbines can be used for applications such as industrial processes, heating buildings, operating absorption or desiccant chillers and water desalination. Commercial and demonstration systems are installed in various countries particularly the US and Europe. There are also examples of technologies in Australia.
Solar concentrator designs include parabolic trough, linear Fresnel reflector, parabolic dish and power tower. The solar concentrators focus sunlight onto a receiver through which a fluid is passed to transfer the heat to the generation plant. The heat transfer fluid can be hot oil, molten salts or direct steam. Some form of heat storage may be included to enable power generation to be maintained through cloudy periods and to maximise output during peak demand periods. To provide 24-hour power generation, natural gas is used as a backup heat source.
Smaller scale systems may generate power using Stirling engines supported at the focus of dish solar concentrators.
Parabolic trough plants are the most economic and the most mature technology currently available. The parabolic trough-shaped mirrors are used to concentrate sunlight onto receiver tubes supported at the trough focal line. Heat transfer fluid pumped through the receiver tubes is heated to about 400°C and passed through heat exchangers to produce superheated steam. The largest parabolic trough generation plant is installed in Southern California and has been supplying power to the grid since the 1980s. The plant was installed in stages and has a total capacity of 354 MW which is provided by over two million square metres of trough concentrators. Other large-scale solar thermal plants are under construction or proposed in Europe and the United States.
A small-scale demonstration trough array installed at the National Renewable Energy Centre provides heat at 250°C and is being used to drive a small, high-speed turbine generator designed for use in remote power applications and distributed generation.
The Linear Fresnel Reflector (LFR) system uses long segmented mirrors mounted relatively close to the ground. The angle of each long mirror is automatically adjusted to track the movement of the sun to maintain the focus on a long receiver mounted above the mirrors. Due to its simplicity the LFR may prove to be a lower cost alternative to the trough design. The LFR was developed at the University of Sydney.
A proof of concept LFR array with an area of 1300 square metres is installed next to Macquarie Generation's Liddell power station in NSW. The system successfully proved that steam could be generated at a temperature of 285°C which is necessary for integration with the coal-fired power station. The array is being increased in size to 20,000 square metres and the steam generated is to be supplied to the power station to reduce coal use and save 4000 tonnes of carbon dioxide emissions per year.
Commercial LFR systems are also being installed in the US.
The parabolic dish tracks the sun throughout the day and is capable of very high temperatures due to the high solar concentration that can be achieved with this technology. An example of parabolic dish technology is the Big Dish system developed and demonstrated at the Australian National University since the early 1990s. The technology is now undergoing commercial development in Australia.
The Big Dish development builds on the solar thermal power station project at White Cliffs, undertaken by the ANU from 1979 to 1989. The White Cliffs Project Overview for the period 1979-89 (PDF 8.6MB) was completed in 1991. Please note this document was published by a predecessor agency and may not be compliant with current accessibility standards.
Power towers use a significant area of large individually tracking mirrors known as heliostats in a circular or semi-circular array around a tower. The heliostats track the sun and focus onto a central receiver mounted on the top of the tower. The heat transfer mediums so far demonstrated in this type of system include steam, molten salts, liquid sodium and air. The heat transfer medium is used to generate superheated steam for use in turbine generators.
Power tower technology concentrates sunlight over 500 times, providing temperatures around 500 to 1000°C in utility scale quantities. Other uses proposed for the technology include chemical processing and the destruction of toxic chemical wastes.
A small-scale high concentration tower solar array installed at the National Renewable Energy Centre in Newcastle uses 200 heliostats to generate peak temperatures of over 1000°C. The system is being used to develop a means of transferring and storing solar energy through the conversion of methane and steam to carbon monoxide and hydrogen. This 'solar gas' contains over 25 per cent more energy than the natural gas feed into the process. Hydrogen is considered to be the fuel of the future for a low carbon economy, as the only emissions from its combustion is water.
Installing small scale renewable energy systems in NSW
For further information see the Installing small scale renewable energy systems in NSW page.
What is 'Solar Cities'?
The Solar Cities program is a $75.3 million Commonwealth Government initiative announced in June 2004. Solar Cities showcases a new energy scenario, comprising increased uptake of solar power and energy efficiency measures by households and business and the introduction of interval electricity meters and cost-reflective electricity pricing.
In NSW, Blacktown City has been selected as a Solar City. Blacktown City's project, which will run to 2013, involves a consortium led by BP Solar and also includes the ANZ Banking Group, Integral Energy, Landcom and Big Switch Projects. A range of packages are being offered including solar hot water, energy efficiency, free energy audits, smart meters and innovative financing. Projected savings exceed 22 gigawatt-hours of electricity annually with Blacktown City's greenhouse gas emissions reduced by more than 24,000 tonnes each year and annual savings of $3 million in electricity bills for the Blacktown City community.
Information about the Commonwealth greenhouse program can be found on their home page, at www.climatechange.gov.au.